Internet Engineering Task Force (IETF) R. Barnes
Request for Comments: 9420 Cisco
Category: Standards Track B. Beurdouche
ISSN: 2070-1721 Inria & Mozilla
R. Robert
Phoenix R&D
J. Millican
Meta Platforms
E. Omara
K. Cohn-Gordon
University of Oxford
July 2023
The Messaging Layer Security (MLS) Protocol
Abstract
Messaging applications are increasingly making use of end-to-end
security mechanisms to ensure that messages are only accessible to
the communicating endpoints, and not to any servers involved in
delivering messages. Establishing keys to provide such protections
is challenging for group chat settings, in which more than two
clients need to agree on a key but may not be online at the same
time. In this document, we specify a key establishment protocol that
provides efficient asynchronous group key establishment with forward
secrecy (FS) and post-compromise security (PCS) for groups in size
ranging from two to thousands.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9420.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Terminology
2.1. Presentation Language
2.1.1. Optional Value
2.1.2. Variable-Size Vector Length Headers
3. Protocol Overview
3.1. Cryptographic State and Evolution
3.2. Example Protocol Execution
3.3. External Joins
3.4. Relationships between Epochs
4. Ratchet Tree Concepts
4.1. Ratchet Tree Terminology
4.1.1. Ratchet Tree Nodes
4.1.2. Paths through a Ratchet Tree
4.2. Views of a Ratchet Tree
5. Cryptographic Objects
5.1. Cipher Suites
5.1.1. Public Keys
5.1.2. Signing
5.1.3. Public Key Encryption
5.2. Hash-Based Identifiers
5.3. Credentials
5.3.1. Credential Validation
5.3.2. Credential Expiry and Revocation
5.3.3. Uniquely Identifying Clients
6. Message Framing
6.1. Content Authentication
6.2. Encoding and Decoding a Public Message
6.3. Encoding and Decoding a Private Message
6.3.1. Content Encryption
6.3.2. Sender Data Encryption
7. Ratchet Tree Operations
7.1. Parent Node Contents
7.2. Leaf Node Contents
7.3. Leaf Node Validation
7.4. Ratchet Tree Evolution
7.5. Synchronizing Views of the Tree
7.6. Update Paths
7.7. Adding and Removing Leaves
7.8. Tree Hashes
7.9. Parent Hashes
7.9.1. Using Parent Hashes
7.9.2. Verifying Parent Hashes
8. Key Schedule
8.1. Group Context
8.2. Transcript Hashes
8.3. External Initialization
8.4. Pre-Shared Keys
8.5. Exporters
8.6. Resumption PSK
8.7. Epoch Authenticators
9. Secret Tree
9.1. Encryption Keys
9.2. Deletion Schedule
10. Key Packages
10.1. KeyPackage Validation
11. Group Creation
11.1. Required Capabilities
11.2. Reinitialization
11.3. Subgroup Branching
12. Group Evolution
12.1. Proposals
12.1.1. Add
12.1.2. Update
12.1.3. Remove
12.1.4. PreSharedKey
12.1.5. ReInit
12.1.6. ExternalInit
12.1.7. GroupContextExtensions
12.1.8. External Proposals
12.2. Proposal List Validation
12.3. Applying a Proposal List
12.4. Commit
12.4.1. Creating a Commit
12.4.2. Processing a Commit
12.4.3. Adding Members to the Group
13. Extensibility
13.1. Additional Cipher Suites
13.2. Proposals
13.3. Credential Extensibility
13.4. Extensions
13.5. GREASE
14. Sequencing of State Changes
15. Application Messages
15.1. Padding
15.2. Restrictions
15.3. Delayed and Reordered Application Messages
16. Security Considerations
16.1. Transport Security
16.2. Confidentiality of Group Secrets
16.3. Confidentiality of Sender Data
16.4. Confidentiality of Group Metadata
16.4.1. GroupID, Epoch, and Message Frequency
16.4.2. Group Extensions
16.4.3. Group Membership
16.5. Authentication
16.6. Forward Secrecy and Post-Compromise Security
16.7. Uniqueness of Ratchet Tree Key Pairs
16.8. KeyPackage Reuse
16.9. Delivery Service Compromise
16.10. Authentication Service Compromise
16.11. Additional Policy Enforcement
16.12. Group Fragmentation by Malicious Insiders
17. IANA Considerations
17.1. MLS Cipher Suites
17.2. MLS Wire Formats
17.3. MLS Extension Types
17.4. MLS Proposal Types
17.5. MLS Credential Types
17.6. MLS Signature Labels
17.7. MLS Public Key Encryption Labels
17.8. MLS Exporter Labels
17.9. MLS Designated Expert Pool
17.10. The "message/mls" Media Type
18. References
18.1. Normative References
18.2. Informative References
Appendix A. Protocol Origins of Example Trees
Appendix B. Evolution of Parent Hashes
Appendix C. Array-Based Trees
Appendix D. Link-Based Trees
Contributors
Authors' Addresses
1. Introduction
A group of users who want to send each other encrypted messages needs
a way to derive shared symmetric encryption keys. For two parties,
this problem has been studied thoroughly, with the Double Ratchet
emerging as a common solution [DoubleRatchet] [Signal]. Channels
implementing the Double Ratchet enjoy fine-grained forward secrecy as
well as post-compromise security, but are nonetheless efficient
enough for heavy use over low-bandwidth networks.
For a group of size greater than two, a common strategy is to
distribute symmetric "sender keys" over existing 1:1 secure channels,
and then for each member to send messages to the group encrypted with
their own sender key. On the one hand, using sender keys improves
efficiency relative to pairwise transmission of individual messages,
and it provides forward secrecy (with the addition of a hash
ratchet). On the other hand, it is difficult to achieve post-
compromise security with sender keys, requiring a number of key
update messages that scales as the square of the group size. An
adversary who learns a sender key can often indefinitely and
passively eavesdrop on that member's messages. Generating and
distributing a new sender key provides a form of post-compromise
security with regard to that sender. However, it requires
computation and communications resources that scale linearly with the
size of the group.
In this document, we describe a protocol based on tree structures
that enables asynchronous group keying with forward secrecy and post-
compromise security. Based on earlier work on "asynchronous
ratcheting trees" [ART], the protocol presented here uses an
asynchronous key-encapsulation mechanism for tree structures. This
mechanism allows the members of the group to derive and update shared
keys with costs that scale as the log of the group size.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Client: An agent that uses this protocol to establish shared
cryptographic state with other clients. A client is defined by
the cryptographic keys it holds.
Group: A group represents a logical collection of clients that share
a common secret value at any given time. Its state is represented
as a linear sequence of epochs in which each epoch depends on its
predecessor.
Epoch: A state of a group in which a specific set of authenticated
clients hold shared cryptographic state.
Member: A client that is included in the shared state of a group and
hence has access to the group's secrets.
Key Package: A signed object describing a client's identity and
capabilities, including a hybrid public key encryption (HPKE)
[RFC9180] public key that can be used to encrypt to that client.
Other clients can use a client's KeyPackage to introduce the
client to a new group.
Group Context: An object that summarizes the shared, public state of
the group. The group context is typically distributed in a signed
GroupInfo message, which is provided to new members to help them
join a group.
Signature Key: A signing key pair used to authenticate the sender of
a message.
Proposal: A message that proposes a change to the group, e.g.,
adding or removing a member.
Commit: A message that implements the changes to the group proposed
in a set of Proposals.
PublicMessage: An MLS protocol message that is signed by its sender
and authenticated as coming from a member of the group in a
particular epoch, but not encrypted.
PrivateMessage: An MLS protocol message that is signed by its
sender, authenticated as coming from a member of the group in a
particular epoch, and encrypted so that it is confidential to the
members of the group in that epoch.
Handshake Message: A PublicMessage or PrivateMessage carrying an MLS
Proposal or Commit object, as opposed to application data.
Application Message: A PrivateMessage carrying application data.
Terminology specific to tree computations is described in
Section 4.1.
In general, symmetric values are referred to as "keys" or "secrets"
interchangeably. Either term denotes a value that MUST be kept
confidential to a client. When labeling individual values, we
typically use "secret" to refer to a value that is used to derive
further secret values and "key" to refer to a value that is used with
an algorithm such as Hashed Message Authentication Code (HMAC) or an
Authenticated Encryption with Associated Data (AEAD) algorithm.
The PublicMessage and PrivateMessage formats are defined in
Section 6. Security notions such as forward secrecy and post-
compromise security are defined in Section 16.
As detailed in Section 13.5, MLS uses the "Generate Random Extensions
And Sustain Extensibility" (GREASE) approach to maintaining
extensibility, where senders insert random values into fields in
which receivers are required to ignore unknown values. Specific
"GREASE values" for this purpose are registered in the appropriate
IANA registries.
2.1. Presentation Language
We use the TLS presentation language [RFC8446] to describe the
structure of protocol messages. In addition to the base syntax, we
add two additional features: the ability for fields to be optional
and the ability for vectors to have variable-size length headers.
2.1.1. Optional Value
An optional value is encoded with a presence-signaling octet,
followed by the value itself if present. When decoding, a presence
octet with a value other than 0 or 1 MUST be rejected as malformed.
struct {
uint8 present;
select (present) {
case 0: struct{};
case 1: T value;
};
} optional<T>;
2.1.2. Variable-Size Vector Length Headers
In the TLS presentation language, vectors are encoded as a sequence
of encoded elements prefixed with a length. The length field has a
fixed size set by specifying the minimum and maximum lengths of the
encoded sequence of elements.
In MLS, there are several vectors whose sizes vary over significant
ranges. So instead of using a fixed-size length field, we use a
variable-size length using a variable-length integer encoding based
on the one described in Section 16 of [RFC9000]. They differ only in
that the one here requires a minimum-size encoding. Instead of
presenting min and max values, the vector description simply includes
a V. For example:
struct {
uint32 fixed<0..255>;
opaque variable<V>;
} StructWithVectors;
Such a vector can represent values with length from 0 bytes to 2^30
bytes. The variable-length integer encoding reserves the two most
significant bits of the first byte to encode the base 2 logarithm of
the integer encoding length in bytes. The integer value is encoded
on the remaining bits, so that the overall value is in network byte
order. The encoded value MUST use the smallest number of bits
required to represent the value. When decoding, values using more
bits than necessary MUST be treated as malformed.
This means that integers are encoded in 1, 2, or 4 bytes and can
encode 6-, 14-, or 30-bit values, respectively.
+========+=========+=============+=======+============+
| Prefix | Length | Usable Bits | Min | Max |
+========+=========+=============+=======+============+
| 00 | 1 | 6 | 0 | 63 |
+--------+---------+-------------+-------+------------+
| 01 | 2 | 14 | 64 | 16383 |
+--------+---------+-------------+-------+------------+
| 10 | 4 | 30 | 16384 | 1073741823 |
+--------+---------+-------------+-------+------------+
| 11 | invalid | - | - | - |
+--------+---------+-------------+-------+------------+
Table 1: Summary of Integer Encodings
Vectors that start with the prefix "11" are invalid and MUST be
rejected.
For example:
* The four-byte length value 0x9d7f3e7d decodes to 494878333.
* The two-byte length value 0x7bbd decodes to 15293.
* The single-byte length value 0x25 decodes to 37.
The following figure adapts the pseudocode provided in [RFC9000] to
add a check for minimum-length encoding:
ReadVarint(data):
// The length of variable-length integers is encoded in the
// first two bits of the first byte.
v = data.next_byte()
prefix = v >> 6
if prefix == 3:
raise Exception('invalid variable length integer prefix')
length = 1 << prefix
// Once the length is known, remove these bits and read any
// remaining bytes.
v = v & 0x3f
repeat length-1 times:
v = (v << 8) + data.next_byte()
// Check if the value would fit in half the provided length.
if prefix >= 1 && v < (1 << (8*(length/2) - 2)):
raise Exception('minimum encoding was not used')
return v
The use of variable-size integers for vector lengths allows vectors
to grow very large, up to 2^30 bytes. Implementations should take
care not to allow vectors to overflow available storage. To
facilitate debugging of potential interoperability problems,
implementations SHOULD provide a clear error when such an overflow
condition occurs.
3. Protocol Overview
MLS is designed to operate in the context described in [MLS-ARCH].
In particular, we assume that the following services are provided:
* An Authentication Service (AS) that enables group members to
authenticate the credentials presented by other group members.
* A Delivery Service (DS) that routes MLS messages among the
participants in the protocol.
MLS assumes a trusted AS but a largely untrusted DS. Section 16.10
describes the impact of compromise or misbehavior of an AS. MLS is
designed to protect the confidentiality and integrity of the group
data even in the face of a compromised DS; in general, the DS is only
expected to reliably deliver messages. Section 16.9 describes the
impact of compromise or misbehavior of a DS.
The core functionality of MLS is continuous group authenticated key
exchange (AKE). As with other authenticated key exchange protocols
(such as TLS), the participants in the protocol agree on a common
secret value, and each participant can verify the identity of the
other participants. That secret can then be used to protect messages
sent from one participant in the group to the other participants
using the MLS framing layer or can be exported for use with other
protocols. MLS provides group AKE in the sense that there can be
more than two participants in the protocol, and continuous group AKE
in the sense that the set of participants in the protocol can change
over time.
The core organizing principles of MLS are _groups_ and _epochs_. A
group represents a logical collection of clients that share a common
secret value at any given time. The history of a group is divided
into a linear sequence of epochs. In each epoch, a set of
authenticated _members_ agree on an _epoch secret_ that is known only
to the members of the group in that epoch. The set of members
involved in the group can change from one epoch to the next, and MLS
ensures that only the members in the current epoch have access to the
epoch secret. From the epoch secret, members derive further shared
secrets for message encryption, group membership authentication, and
so on.
The creator of an MLS group creates the group's first epoch
unilaterally, with no protocol interactions. Thereafter, the members
of the group advance their shared cryptographic state from one epoch
to another by exchanging MLS messages.
* A _KeyPackage_ object describes a client's capabilities and
provides keys that can be used to add the client to a group.
* A _Proposal_ message proposes a change to be made in the next
epoch, such as adding or removing a member.
* A _Commit_ message initiates a new epoch by instructing members of
the group to implement a collection of proposals.
* A _Welcome_ message provides a new member to the group with the
information to initialize their state for the epoch in which they
were added or in which they want to add themselves to the group.
KeyPackage and Welcome messages are used to initiate a group or
introduce new members, so they are exchanged between group members
and clients not yet in the group. A client publishes a KeyPackage
via the DS, thus enabling other clients to add it to groups. When a
group member wants to add a new member to a group, it uses the new
member's KeyPackage to add them and constructs a Welcome message with
which the new member can initialize their local state.
Proposal and Commit messages are sent from one member of a group to
the others. MLS provides a common framing layer for sending messages
within a group: A _PublicMessage_ provides sender authentication for
unencrypted Proposal and Commit messages. A _PrivateMessage_
provides encryption and authentication for both Proposal/Commit
messages as well as any application data.
3.1. Cryptographic State and Evolution
The cryptographic state at the core of MLS is divided into three
areas of responsibility:
.- ... -.
| |
| | |
| | | Key Schedule
| V |
| epoch_secret |
. | | | .
|\ Ratchet | | | Secret /|
| \ Tree | | | Tree / |
| \ | | | / |
| \ | V | / |
| +--> commit_secret --> epoch_secret --> encryption_secret -->+ |
| / | | | \ |
| / | | | \ |
| / | | | \ |
|/ | | | \|
' | V | '
| epoch_secret |
| | |
| | |
| V |
| |
'- ... -'
Figure 1: Overview of MLS Group Evolution
* A _ratchet tree_ that represents the membership of the group,
providing group members a way to authenticate each other and
efficiently encrypt messages to subsets of the group. Each epoch
has a distinct ratchet tree. It seeds the _key schedule_.
* A _key schedule_ that describes the chain of key derivations used
to progress from epoch to epoch (mainly using the _init_secret_
and _epoch_secret_), as well as the derivation of a variety of
other secrets (see Table 4). For example:
- An _encryption secret_ that is used to initialize the secret
tree for the epoch.
- An _exporter secret_ that allows other protocols to leverage
MLS as a generic authenticated group key exchange.
- A _resumption secret_ that members can use to prove their
membership in the group, e.g., when creating a subgroup or a
successor group.
* A _secret tree_ derived from the key schedule that represents
shared secrets used by the members of the group for encrypting and
authenticating messages. Each epoch has a distinct secret tree.
Each member of the group maintains a partial view of these components
of the group's state. MLS messages are used to initialize these
views and keep them in sync as the group transitions between epochs.
Each new epoch is initiated with a Commit message. The Commit
instructs existing members of the group to update their views of the
ratchet tree by applying a set of Proposals, and uses the updated
ratchet tree to distribute fresh entropy to the group. This fresh
entropy is provided only to members in the new epoch and not to
members who have been removed. Commits thus maintain the property
that the epoch secret is confidential to the members in the current
epoch.
For each Commit that adds one or more members to the group, there are
one or more corresponding Welcome messages. Each Welcome message
provides new members with the information they need to initialize
their views of the key schedule and ratchet tree, so that these views
align with the views held by other members of the group in this
epoch.
3.2. Example Protocol Execution
There are three major operations in the life of a group:
* Adding a member, initiated by a current member;
* Updating the keys that represent a member in the tree; and
* Removing a member.
Each of these operations is "proposed" by sending a message of the
corresponding type (Add / Update / Remove). The state of the group
is not changed, however, until a Commit message is sent to provide
the group with fresh entropy. In this section, we show each proposal
being committed immediately, but in more advanced deployment cases,
an application might gather several proposals before committing them
all at once. In the illustrations below, we show the Proposal and
Commit messages directly, while in reality they would be sent
encapsulated in PublicMessage or PrivateMessage objects.
Before the initialization of a group, clients publish KeyPackages to
a directory provided by the DS (see Figure 2).
Delivery Service
|
.--------' '--------.
| |
Group
A B C Directory Channel
| | | | |
| KeyPackageA | | | |
+------------------------------------------------->| |
| | | | |
| | KeyPackageB | | |
| +-------------------------------->| |
| | | | |
| | | KeyPackageC | |
| | +--------------->| |
| | | | |
Figure 2: Clients A, B, and C publish KeyPackages to the directory
Figure 3 shows how these pre-published KeyPackages are used to create
a group. When client A wants to establish a group with clients B and
C, it first initializes a group state containing only itself and
downloads KeyPackages for B and C. For each member, A generates an
Add proposal and a Commit message to add that member and then
broadcasts the two messages to the group. Client A also generates a
Welcome message and sends it directly to the new member (there's no
need to send it to the group). Only after A has received its Commit
message back from the Delivery Service does it update its state to
reflect the new member's addition.
Once A has updated its state, the new member has processed the
Welcome, and any other group members have processed the Commit, they
will all have consistent representations of the group state,
including a group secret that is known only to the members the group.
The new member will be able to read and send new messages to the
group, but messages sent before they were added to the group will not
be accessible.
Group
A B C Directory Channel
| | | | |
| KeyPackageB, KeyPackageC | |
|<-------------------------------------------+ |
| | | | |
| | | | Add(A->AB) |
| | | | Commit(Add) |
+--------------------------------------------------------------->|
| | | | |
| Welcome(B) | | | |
+------------->| | | |
| | | | |
| | | | Add(A->AB) |
| | | | Commit(Add) |
|<---------------------------------------------------------------+
| | | | |
| | | | |
| | | | Add(AB->ABC) |
| | | | Commit(Add) |
+--------------------------------------------------------------->|
| | | | |
| | Welcome(C) | | |
+---------------------------->| | |
| | | | |
| | | | Add(AB->ABC) |
| | | | Commit(Add) |
|<---------------------------------------------------------------+
| |<------------------------------------------------+
| | | | |
Figure 3: Client A creates a group with clients B and C
Subsequent additions of group members proceed in the same way. Any
member of the group can download a KeyPackage for a new client,
broadcast Add and Commit messages that the current group will use to
update their state, and send a Welcome message that the new client
can use to initialize its state and join the group.
To enforce the forward secrecy and post-compromise security of
messages, each member periodically updates the keys that represent
them to the group. A member does this by sending a Commit (possibly
with no proposals) or by sending an Update message that is committed
by another member (see Figure 4). Once the other members of the
group have processed these messages, the group's secrets will be
unknown to an attacker that had compromised the secrets corresponding
to the sender's leaf in the tree. At the end of the scenario shown
in Figure 4, the group has post-compromise security with respect to
both A and B.
Update messages SHOULD be sent at regular intervals of time as long
as the group is active, and members that don't update SHOULD
eventually be removed from the group. It's left to the application
to determine an appropriate amount of time between Updates. Since
the purpose of sending an Update is to proactively constrain a
compromise window, the right frequency is usually on the order of
hours or days, not milliseconds. For example, an application might
send an Update each time a member sends an application message after
receiving any message from another member, or daily if no application
messages are sent.
The MLS architecture recommends that MLS be operated over a secure
transport (see Section 7.1 of [MLS-ARCH]). Such transport protocols
will typically provide functions such as congestion control that
manage the impact of an MLS-using application on other applications
sharing the same network. Applications should take care that they do
not send MLS messages at a rate that will cause problems such as
network congestion, especially if they are not following the above
recommendation (e.g., sending MLS directly over UDP instead).
Group
A B ... Z Directory Channel
| | | | |
| | Update(B) | | |
| +------------------------------------------->|
| | | | Update(B) |
|<----------------------------------------------------------+
| |<-------------------------------------------+
| | |<----------------------------+
| | | | |
| Commit(Upd) | | | |
+---------------------------------------------------------->|
| | | | Commit(Upd) |
|<----------------------------------------------------------+
| |<-------------------------------------------+
| | |<----------------------------+
| | | | |
Figure 4: Client B proposes to update its key, and client A
commits the proposal
Members are removed from the group in a similar way, as shown in
Figure 5. Any member of the group can send a Remove proposal
followed by a Commit message. The Commit message provides new
entropy to all members of the group except the removed member. This
new entropy is added to the epoch secret for the new epoch so that it
is not known to the removed member. Note that this does not
necessarily imply that any member is actually allowed to evict other
members; groups can enforce access control policies on top of these
basic mechanisms.
Group
A B ... Z Directory Channel
| | | | |
| | | Remove(B) | |
| | | Commit(Rem) | |
| | +---------------------------->|
| | | | |
| | | | Remove(B) |
| | | | Commit(Rem) |
|<----------------------------------------------------------+
| |<-------------------------------------------+
| | |<----------------------------+
| | | | |
Figure 5: Client Z removes client B from the group
Note that the flows in this section are examples; applications can
arrange message flows in other ways. For example:
* Welcome messages don't necessarily need to be sent directly to new
joiners. Since they are encrypted to new joiners, they could be
distributed more broadly, say if the application only had access
to a broadcast channel for the group.
* Proposal messages don't need to be immediately sent to all group
members. They need to be available to the committer before
generating a Commit, and to other members before processing the
Commit.
* The sender of a Commit doesn't necessarily have to wait to receive
its own Commit back before advancing its state. It only needs to
know that its Commit will be the next one applied by the group,
say based on a promise from an orchestration server.
3.3. External Joins
In addition to the Welcome-based flow for adding a new member to the
group, it is also possible for a new member to join by means of an
"external Commit". This mechanism can be used when the existing
members don't have a KeyPackage for the new member, for example, in
the case of an "open" group that can be joined by new members without
asking permission from existing members.
Figure 6 shows a typical message flow for an external join. To
enable a new member to join the group in this way, a member of the
group (A, B) publishes a GroupInfo object that includes the
GroupContext for the group as well as a public key that can be used
to encrypt a secret to the existing members of the group. When the
new member Z wishes to join, they download the GroupInfo object and
use it to form a Commit of a special form that adds Z to the group
(as detailed in Section 12.4.3.2). The existing members of the group
process this external Commit in a similar way to a normal Commit,
advancing to a new epoch in which Z is now a member of the group.
Group
A B Z Directory Channel
| | | | |
| GroupInfo | | | |
+------------------------------------------->| |
| | | GroupInfo | |
| | |<-------------+ |
| | | | |
| | | Commit(ExtZ) | |
| | +---------------------------->|
| | | | Commit(ExtZ) |
|<----------------------------------------------------------+
| |<-------------------------------------------+
| | |<----------------------------+
| | | | |
Figure 6: Client A publishes a GroupInfo object, and Client Z
uses it to join the group
3.4. Relationships between Epochs
A group has a single linear sequence of epochs. Groups and epochs
are generally independent of one another. However, it can sometimes
be useful to link epochs cryptographically, either within a group or
across groups. MLS derives a resumption pre-shared key (PSK) from
each epoch to allow entropy extracted from one epoch to be injected
into a future epoch. A group member that wishes to inject a PSK
issues a PreSharedKey proposal (Section 12.1.4) describing the PSK to
be injected. When this proposal is committed, the corresponding PSK
will be incorporated into the key schedule as described in
Section 8.4.
Linking epochs in this way guarantees that members entering the new
epoch agree on a key if and only if they were members of the group
during the epoch from which the resumption key was extracted.
MLS supports two ways to tie a new group to an existing group, which
are illustrated in Figures 7 and 8. Reinitialization closes one
group and creates a new group comprising the same members with
different parameters. Branching starts a new group with a subset of
the original group's participants (with no effect on the original
group). In both cases, the new group is linked to the old group via
a resumption PSK.
epoch_A_[n-1]
|
|
|<-- ReInit
|
V
epoch_A_[n] epoch_B_[0]
. |
. PSK(usage=reinit) |
.....................>|
|
V
epoch_B_[1]
Figure 7: Reinitializing a Group
epoch_A_[n] epoch_B_[0]
| |
| PSK(usage=branch) |
|....................>|
| |
V V
epoch_A_[n+1] epoch_B_[1]
Figure 8: Branching a Group
Applications may also choose to use resumption PSKs to link epochs in
other ways. For example, Figure 9 shows a case where a resumption
PSK from epoch n is injected into epoch n+k. This demonstrates that
the members of the group at epoch n+k were also members at epoch n,
irrespective of any changes to these members' keys due to Updates or
Commits.
epoch_A_[n]
|
| PSK(usage=application)
|.....................
| .
| .
... ...
| .
| .
V .
epoch_A_[n+k-1] .
| .
| .
|<....................
|
V
epoch_A_[n+k]
Figure 9: Reinjecting Entropy from an Earlier Epoch
4. Ratchet Tree Concepts
The protocol uses "ratchet trees" for deriving shared secrets among a
group of clients. A ratchet tree is an arrangement of secrets and
key pairs among the members of a group in a way that allows for
secrets to be efficiently updated to reflect changes in the group.
Ratchet trees allow a group to efficiently remove any member by
encrypting new entropy to a subset of the group. A ratchet tree
assigns shared keys to subgroups of the overall group, so that, for
example, encrypting to all but one member of the group requires only
log(N) encryptions to subtrees, instead of the N-1 encryptions that
would be needed to encrypt to each participant individually (where N
is the number of members in the group).
This remove operation allows MLS to efficiently achieve post-
compromise security. In an Update proposal or a full Commit message,
an old (possibly compromised) representation of a member is
efficiently removed from the group and replaced with a freshly
generated instance.
4.1. Ratchet Tree Terminology
Trees consist of _nodes_. A node is a _leaf_ if it has no children;
otherwise, it is a _parent_. All parents in our trees have precisely
two children, a _left_ child and a _right_ child. A node is the
_root_ of a tree if it has no parent, and _intermediate_ if it has
both children and a parent. The _descendants_ of a node are that
node's children, and the descendants of its children. We say a tree
_contains_ a node if that node is a descendant of the root of the
tree, or if the node itself is the root of the tree. Nodes are
_siblings_ if they share the same parent.
A _subtree_ of a tree is the tree given by any node (the _head_ of
the subtree) and its descendants. The _size_ of a tree or subtree is
the number of leaf nodes it contains. For a given parent node, its
_left subtree_ is the subtree with its left child as head and its
_right subtree_ is the subtree with its right child as head.
Every tree used in this protocol is a perfect binary tree, that is, a
complete balanced binary tree with 2^d leaves all at the same depth
d. This structure is unique for a given depth d.
There are multiple ways that an implementation might represent a
ratchet tree in memory. A convenient property of left-balanced
binary trees (including the complete trees used here) is that they
can be represented as an array of nodes, with node relationships
computed based on the nodes' indices in the array. A more
traditional representation based on linked node objects may also be
used. Appendices C and D provide some details on how to implement
the tree operations required for MLS in these representations. MLS
places no requirements on implementations' internal representations
of ratchet trees. An implementation may use any tree representation
and associated algorithms, as long as they produce correct protocol
messages.
4.1.1. Ratchet Tree Nodes
Each leaf node in a ratchet tree is given an _index_ (or _leaf
index_), starting at 0 from the left to 2^d - 1 at the right (for a
tree with 2^d leaves). A tree with 2^d leaves has 2^(d+1) - 1 nodes,
including parent nodes.
Each node in a ratchet tree is either _blank_ (containing no value)
or it holds an HPKE public key with some associated data:
* A public key (for the HPKE scheme in use; see Section 5.1)
* A credential (only for leaf nodes; see Section 5.3)
* An ordered list of "unmerged" leaves (see Section 4.2)
* A hash of certain information about the node's parent, as of the
last time the node was changed (see Section 7.9).
As described in Section 4.2, different members know different subsets
of the set of private keys corresponding to the public keys in nodes
in the tree. The private key corresponding to a parent node is known
only to members at leaf nodes that are descendants of that node. The
private key corresponding to a leaf node is known only to the member
at that leaf node. A leaf node is _unmerged_ relative to one of its
ancestor nodes if the member at the leaf node does not know the
private key corresponding to the ancestor node.
Every node, regardless of whether the node is blank or populated, has
a corresponding _hash_ that summarizes the contents of the subtree
below that node. The rules for computing these hashes are described
in Section 7.8.
The _resolution_ of a node is an ordered list of non-blank nodes that
collectively cover all non-blank descendants of the node. The
resolution of the root contains the set of keys that are collectively
necessary to encrypt to every node in the group. The resolution of a
node is effectively a depth-first, left-first enumeration of the
nearest non-blank nodes below the node:
* The resolution of a non-blank node comprises the node itself,
followed by its list of unmerged leaves, if any.
* The resolution of a blank leaf node is the empty list.
* The resolution of a blank intermediate node is the result of
concatenating the resolution of its left child with the resolution
of its right child, in that order.
For example, consider the following subtree, where the _ character
represents a blank node and unmerged leaves are indicated in square
brackets:
...
/
_
______|______
/ \
X[B] _
__|__ __|__
/ \ / \
_ _ Y _
/ \ / \ / \ / \
A B _ D E F _ H
0 1 2 3 4 5 6 7
Figure 10: A Tree with Blanks and Unmerged Leaves
In this tree, we can see all of the above rules in play:
* The resolution of node X is the list [X, B].
* The resolution of leaf 2 or leaf 6 is the empty list [].
* The resolution of top node is the list [X, B, Y, H].
4.1.2. Paths through a Ratchet Tree
The _direct path_ of a root is the empty list. The direct path of
any other node is the concatenation of that node's parent along with
the parent's direct path.
The _copath_ of a node is the node's sibling concatenated with the
list of siblings of all the nodes in its direct path, excluding the
root.
The _filtered direct path_ of a leaf node L is the node's direct
path, with any node removed whose child on the copath of L has an
empty resolution (keeping in mind that any unmerged leaves of the
copath child count toward its resolution). The removed nodes do not
need their own key pairs because encrypting to the node's key pair
would be equivalent to encrypting to its non-copath child.
For example, consider the following tree (where blank nodes are
indicated with _, but also assigned a label for reference):
W = root
|
.-----+-----.
/ \
_=U Y
| |
.-+-. .-+-.
/ \ / \
T _=V X _=Z
/ \ / \ / \ / \
A B _ _ E F G _=H
0 1 2 3 4 5 6 7
Figure 11: A Complete Tree with Five Members, with Labels for
Blank Parent Nodes
In this tree, the direct paths, copaths, and filtered direct paths
for the leaf nodes are as follows:
+======+=============+=========+======================+
| Node | Direct path | Copath | Filtered Direct Path |
+======+=============+=========+======================+
| A | T, U, W | B, V, Y | T, W |
+------+-------------+---------+----------------------+
| B | T, U, W | A, V, Y | T, W |
+------+-------------+---------+----------------------+
| E | X, Y, W | F, Z, U | X, Y, W |
+------+-------------+---------+----------------------+
| F | X, Y, W | E, Z, U | X, Y, W |
+------+-------------+---------+----------------------+
| G | Z, Y, W | H, X, U | Y, W |
+------+-------------+---------+----------------------+
Table 2
4.2. Views of a Ratchet Tree
We generally assume that each participant maintains a complete and
up-to-date view of the public state of the group's ratchet tree,
including the public keys for all nodes and the credentials
associated with the leaf nodes.
No participant in an MLS group knows the private key associated with
every node in the tree. Instead, each member is assigned to a leaf
of the tree, which determines the subset of private keys it knows.
The credential stored at that leaf is one provided by the member.
In particular, MLS maintains the members' views of the tree in such a
way as to maintain the _tree invariant_:
| The private key for a node in the tree is known to a member of the
| group only if the node's subtree contains that member's leaf.
In other words, if a node is not blank, then it holds a public key.
The corresponding private key is known only to members occupying
leaves below that node.
The reverse implication is not true: A member may not know the
private key of an intermediate node above them. Such a member has an
_unmerged_ leaf at the intermediate node. Encrypting to an
intermediate node requires encrypting to the node's public key, as
well as the public keys of all the unmerged leaves below it. A leaf
is unmerged with regard to all of its ancestors when it is first
added, because the process of adding the leaf does not give it access
to the private keys for all of the nodes above it in the tree.
Leaves are "merged" as they receive the private keys for nodes, as
described in Section 7.4.
For example, consider a four-member group (A, B, C, D) where the node
above the right two members is blank. (This is what it would look
like if A created a group with B, C, and D.) Then the public state
of the tree and the views of the private keys of the tree held by
each participant would be as follows, where _ represents a blank
node, ? represents an unknown private key, and pk(X) represents the
public key corresponding to the private key X:
Public Tree
============================
pk(ABCD)
/ \
pk(AB) _
/ \ / \
pk(A) pk(B) pk(C) pk(D)
Private @ A Private @ B Private @ C Private @ D
============= ============= ============= =============
ABCD ABCD ABCD ABCD
/ \ / \ / \ / \
AB _ AB _ ? _ ? _
/ \ / \ / \ / \ / \ / \ / \ / \
A ? ? ? ? B ? ? ? ? C ? ? ? ? D
Note how the tree invariant applies: Each member knows only their own
leaf, the private key AB is known only to A and B, and the private
key ABCD is known to all four members. This also illustrates another
important point: it is possible for there to be "holes" on the path
from a member's leaf to the root in which the member knows the key
both above and below a given node, but not for that node, as in the
case with D.
5. Cryptographic Objects
5.1. Cipher Suites
Each MLS session uses a single cipher suite that specifies the
following primitives to be used in group key computations:
* HPKE parameters:
- A Key Encapsulation Mechanism (KEM)
- A Key Derivation Function (KDF)
- An Authenticated Encryption with Associated Data (AEAD)
encryption algorithm
* A hash algorithm
* A Message Authentication Code (MAC) algorithm
* A signature algorithm
MLS uses HPKE for public key encryption [RFC9180]. The DeriveKeyPair
function associated to the KEM for the cipher suite maps octet
strings to HPKE key pairs. As in HPKE, MLS assumes that an AEAD
algorithm produces a single ciphertext output from AEAD encryption
(aligning with [RFC5116]), as opposed to a separate ciphertext and
tag.
Cipher suites are represented with the CipherSuite type. The cipher
suites are defined in Section 17.1.
5.1.1. Public Keys
HPKE public keys are opaque values in a format defined by the
underlying protocol (see Section 4 of [RFC9180] for more
information).
opaque HPKEPublicKey<V>;
Signature public keys are likewise represented as opaque values in a
format defined by the cipher suite's signature scheme.
opaque SignaturePublicKey<V>;
For cipher suites using the Edwards-curve Digital Signature Algorithm
(EdDSA) signature schemes (Ed25519 or Ed448), the public key is in
the format specified in [RFC8032].
For cipher suites using the Elliptic Curve Digital Signature
Algorithm (ECDSA) with the NIST curves (P-256, P-384, or P-521), the
public key is represented as an encoded
UncompressedPointRepresentation struct, as defined in [RFC8446].
5.1.2. Signing
The signature algorithm specified in a group's cipher suite is the
mandatory algorithm to be used for signing messages within the group.
It MUST be the same as the signature algorithm specified in the
credentials in the leaves of the tree (including the leaf node
information in KeyPackages used to add new members).
The signatures used in this document are encoded as specified in
[RFC8446]. In particular, ECDSA signatures are DER encoded, and
EdDSA signatures are defined as the concatenation of R and S, as
specified in [RFC8032].
To disambiguate different signatures used in MLS, each signed value
is prefixed by a label as shown below:
SignWithLabel(SignatureKey, Label, Content) =
Signature.Sign(SignatureKey, SignContent)
VerifyWithLabel(VerificationKey, Label, Content, SignatureValue) =
Signature.Verify(VerificationKey, SignContent, SignatureValue)
Where SignContent is specified as:
struct {
opaque label<V>;
opaque content<V>;
} SignContent;
And its fields are set to:
label = "MLS 1.0 " + Label;
content = Content;
The functions Signature.Sign and Signature.Verify are defined by the
signature algorithm. If MLS extensions require signatures by group
members, they should reuse the SignWithLabel construction, using a
distinct label. To avoid collisions in these labels, an IANA
registry is defined in Section 17.6.
5.1.3. Public Key Encryption
As with signing, MLS includes a label and context in encryption
operations to avoid confusion between ciphertexts produced for
different purposes. Encryption and decryption including this label
and context are done as follows:
EncryptWithLabel(PublicKey, Label, Context, Plaintext) =
SealBase(PublicKey, EncryptContext, "", Plaintext)
DecryptWithLabel(PrivateKey, Label, Context, KEMOutput, Ciphertext) =
OpenBase(KEMOutput, PrivateKey, EncryptContext, "", Ciphertext)
Where EncryptContext is specified as:
struct {
opaque label<V>;
opaque context<V>;
} EncryptContext;
And its fields are set to:
label = "MLS 1.0 " + Label;
context = Context;
The functions SealBase and OpenBase are defined in Section 6.1 of
[RFC9180] (with "Base" as the MODE), using the HPKE algorithms
specified by the group's cipher suite. If MLS extensions require
HPKE encryption operations, they should reuse the EncryptWithLabel
construction, using a distinct label. To avoid collisions in these
labels, an IANA registry is defined in Section 17.7.
5.2. Hash-Based Identifiers
Some MLS messages refer to other MLS objects by hash. For example,
Welcome messages refer to KeyPackages for the members being welcomed,
and Commits refer to Proposals they cover. These identifiers are
computed as follows:
opaque HashReference<V>;
HashReference KeyPackageRef;
HashReference ProposalRef;
MakeKeyPackageRef(value)
= RefHash("MLS 1.0 KeyPackage Reference", value)
MakeProposalRef(value)
= RefHash("MLS 1.0 Proposal Reference", value)
RefHash(label, value) = Hash(RefHashInput)
Where RefHashInput is defined as:
struct {
opaque label<V>;
opaque value<V>;
} RefHashInput;
And its fields are set to:
label = label;
value = value;
For a KeyPackageRef, the value input is the encoded KeyPackage, and
the cipher suite specified in the KeyPackage determines the KDF used.
For a ProposalRef, the value input is the AuthenticatedContent
carrying the Proposal. In the latter two cases, the KDF is
determined by the group's cipher suite.
5.3. Credentials
Each member of a group presents a credential that provides one or
more identities for the member and associates them with the member's
signing key. The identities and signing key are verified by the
Authentication Service in use for a group.
It is up to the application to decide which identifiers to use at the
application level. For example, a certificate in an X509Credential
may attest to several domain names or email addresses in its
subjectAltName extension. An application may decide to present all
of these to a user, or if it knows a "desired" domain name or email
address, it can check that the desired identifier is among those
attested. Using the terminology from [RFC6125], a credential
provides "presented identifiers", and it is up to the application to
supply a "reference identifier" for the authenticated client, if any.
// See the "MLS Credential Types" IANA registry for values
uint16 CredentialType;
struct {
opaque cert_data<V>;
} Certificate;
struct {
CredentialType credential_type;
select (Credential.credential_type) {
case basic:
opaque identity<V>;
case x509:
Certificate certificates<V>;
};
} Credential;
A "basic" credential is a bare assertion of an identity, without any
additional information. The format of the encoded identity is
defined by the application.
For an X.509 credential, each entry in the certificates field
represents a single DER-encoded X.509 certificate. The chain is
ordered such that the first entry (certificates[0]) is the end-entity
certificate. The public key encoded in the subjectPublicKeyInfo of
the end-entity certificate MUST be identical to the signature_key in
the LeafNode containing this credential. A chain MAY omit any non-
leaf certificates that supported peers are known to already possess.
5.3.1. Credential Validation
The application using MLS is responsible for specifying which
identifiers it finds acceptable for each member in a group. In other
words, following the model that [RFC6125] describes for TLS, the
application maintains a list of "reference identifiers" for the
members of a group, and the credentials provide "presented
identifiers". A member of a group is authenticated by first
validating that the member's credential legitimately represents some
presented identifiers, and then ensuring that the reference
identifiers for the member are authenticated by those presented
identifiers.
The parts of the system that perform these functions are collectively
referred to as the Authentication Service (AS) [MLS-ARCH]. A
member's credential is said to be _validated with the AS_ when the AS
verifies that the credential's presented identifiers are correctly
associated with the signature_key field in the member's LeafNode, and
that those identifiers match the reference identifiers for the
member.
Whenever a new credential is introduced in the group, it MUST be
validated with the AS. In particular, at the following events in the
protocol:
* When a member receives a KeyPackage that it will use in an Add
proposal to add a new member to the group
* When a member receives a GroupInfo object that it will use to join
a group, either via a Welcome or via an external Commit
* When a member receives an Add proposal adding a member to the
group
* When a member receives an Update proposal whose LeafNode has a new
credential for the member
* When a member receives a Commit with an UpdatePath whose LeafNode
has a new credential for the committer
* When an external_senders extension is added to the group
* When an existing external_senders extension is updated
In cases where a member's credential is being replaced, such as the
Update and Commit cases above, the AS MUST also verify that the set
of presented identifiers in the new credential is valid as a
successor to the set of presented identifiers in the old credential,
according to the application's policy.
5.3.2. Credential Expiry and Revocation
In some credential schemes, a valid credential can "expire" or become
invalid after a certain point in time. For example, each X.509
certificate has a notAfter field, expressing a time after which the
certificate is not valid.
Expired credentials can cause operational problems in light of the
validation requirements of Section 5.3.1. Applications can apply
some operational practices and adaptations to Authentication Service
policies to moderate these impacts.
In general, to avoid operational problems such as new joiners
rejecting expired credentials in a group, applications that use such
credentials should ensure to the extent practical that all of the
credentials in use in a group are valid at all times.
If a member finds that its credential has expired (or will soon), it
should issue an Update or Commit that replaces it with a valid
credential. For this reason, members SHOULD accept Update proposals
and Commits issued by members with expired credentials, if the
credential in the Update or Commit is valid.
Similarly, when a client is processing messages sent some time in the
past (e.g., syncing up with a group after being offline), the client
SHOULD accept signatures from members with expired credentials, since
the credential may have been valid at the time the message was sent.
If a member finds that another member's credential has expired, they
may issue a Remove that removes that member. For example, an
application could require a member preparing to issue a Commit to
check the tree for expired credentials and include Remove proposals
for those members in its Commit. In situations where the group tree
is known to the DS, the DS could also monitor the tree for expired
credentials and issue external Remove proposals.
Some credential schemes also allow credentials to be revoked.
Revocation is similar to expiry in that a previously valid credential
becomes invalid. As such, most of the considerations above also
apply to revoked credentials. However, applications may want to
treat revoked credentials differently, e.g., by removing members with
revoked credentials while allowing members with expired credentials
time to update.
5.3.3. Uniquely Identifying Clients
MLS implementations will presumably provide applications with a way
to request protocol operations with regard to other clients (e.g.,
removing clients). Such functions will need to refer to the other
clients using some identifier. MLS clients have a few types of
identifiers, with different operational properties.
Internally to the protocol, group members are uniquely identified by
their leaf index. However, a leaf index is only valid for referring
to members in a given epoch. The same leaf index may represent a
different member, or no member at all, in a subsequent epoch.
The Credentials presented by the clients in a group authenticate
application-level identifiers for the clients. However, these
identifiers may not uniquely identify clients. For example, if a
user has multiple devices that are all present in an MLS group, then
those devices' clients could all present the user's application-layer
identifiers.
If needed, applications may add application-specific identifiers to
the extensions field of a LeafNode object with the application_id
extension.
opaque application_id<V>;
However, applications MUST NOT rely on the data in an application_id
extension as if it were authenticated by the Authentication Service,
and SHOULD gracefully handle cases where the identifier presented is
not unique.
6. Message Framing
Handshake and application messages use a common framing structure.
This framing provides encryption to ensure confidentiality within the
group, as well as signing to authenticate the sender.
In most of the protocol, messages are handled in the form of
AuthenticatedContent objects. These structures contain the content
of the message itself as well as information to authenticate the
sender (see Section 6.1). The additional protections required to
transmit these messages over an untrusted channel (group membership
authentication or AEAD encryption) are added by encoding the
AuthenticatedContent as a PublicMessage or PrivateMessage message,
which can then be sent as an MLSMessage. Likewise, these protections
are enforced (via membership verification or AEAD decryption) when
decoding a PublicMessage or PrivateMessage into an
AuthenticatedContent object.
PrivateMessage represents a signed and encrypted message, with
protections for both the content of the message and related metadata.
PublicMessage represents a message that is only signed, and not
encrypted. Applications MUST use PrivateMessage to encrypt
application messages and SHOULD use PrivateMessage to encode
handshake messages, but they MAY transmit handshake messages encoded
as PublicMessage objects in cases where it is necessary for the
Delivery Service to examine such messages.
enum {
reserved(0),
mls10(1),
(65535)
} ProtocolVersion;
enum {
reserved(0),
application(1),
proposal(2),
commit(3),
(255)
} ContentType;
enum {
reserved(0),
member(1),
external(2),
new_member_proposal(3),
new_member_commit(4),
(255)
} SenderType;
struct {
SenderType sender_type;
select (Sender.sender_type) {
case member:
uint32 leaf_index;
case external:
uint32 sender_index;
case new_member_commit:
case new_member_proposal:
struct{};
};
} Sender;
// See the "MLS Wire Formats" IANA registry for values
uint16 WireFormat;
struct {
opaque group_id<V>;
uint64 epoch;
Sender sender;
opaque authenticated_data<V>;
ContentType content_type;
select (FramedContent.content_type) {
case application:
opaque application_data<V>;
case proposal:
Proposal proposal;
case commit:
Commit commit;
};
} FramedContent;
struct {
ProtocolVersion version = mls10;
WireFormat wire_format;
select (MLSMessage.wire_format) {
case mls_public_message:
PublicMessage public_message;
case mls_private_message:
PrivateMessage private_message;
case mls_welcome:
Welcome welcome;
case mls_group_info:
GroupInfo group_info;
case mls_key_package:
KeyPackage key_package;
};
} MLSMessage;
Messages from senders that aren't in the group are sent as
PublicMessage. See Sections 12.1.8 and 12.4.3.2 for more details.
The following structure is used to fully describe the data
transmitted in plaintexts or ciphertexts.
struct {
WireFormat wire_format;
FramedContent content;
FramedContentAuthData auth;
} AuthenticatedContent;
The following figure illustrates how the various structures described
in this section relate to each other, and the high-level operations
used to produce and consume them:
Proposal Commit Application Data
| | |
+--------------+--------------+
|
V
FramedContent
| | -.
+--------+ | |
| | |
V | +-- Asymmetric
FramedContentAuthData | | Sign / Verify
| | |
+--------+ | |
| | |
V V -'
AuthenticatedContent
| -.
+--------+--------+ |
| | +-- Symmetric
V V | Protect / Unprotect
PublicMessage PrivateMessage -'
| |
| | Welcome KeyPackage GroupInfo
| | | | |
+-----------------+-----+----------+----------+
|
V
MLSMessage
Figure 12: Relationships among MLS Objects
6.1. Content Authentication
FramedContent is authenticated using the FramedContentAuthData
structure.
struct {
ProtocolVersion version = mls10;
WireFormat wire_format;
FramedContent content;
select (FramedContentTBS.content.sender.sender_type) {
case member:
case new_member_commit:
GroupContext context;
case external:
case new_member_proposal:
struct{};
};
} FramedContentTBS;
opaque MAC<V>;
struct {
/* SignWithLabel(., "FramedContentTBS", FramedContentTBS) */
opaque signature<V>;
select (FramedContent.content_type) {
case commit:
/*
MAC(confirmation_key,
GroupContext.confirmed_transcript_hash)
*/
MAC confirmation_tag;
case application:
case proposal:
struct{};
};
} FramedContentAuthData;
The signature is computed using SignWithLabel with label
"FramedContentTBS" and with a content that covers the message content
and the wire format that will be used for this message. If the
sender's sender_type is member, the content also covers the
GroupContext for the current epoch so that signatures are specific to
a given group and epoch.
The sender MUST use the private key corresponding to the following
signature key depending on the sender's sender_type:
* member: The signature key contained in the LeafNode at the index
indicated by leaf_index in the ratchet tree.
* external: The signature key at the index indicated by sender_index
in the external_senders group context extension (see
Section 12.1.8.1). The content_type of the message MUST be
proposal and the proposal_type MUST be a value that is allowed for
external senders.
* new_member_commit: The signature key in the LeafNode in the
Commit's path (see Section 12.4.3.2). The content_type of the
message MUST be commit.
* new_member_proposal: The signature key in the LeafNode in the
KeyPackage embedded in an external Add proposal. The content_type
of the message MUST be proposal and the proposal_type of the
Proposal MUST be add.
Recipients of an MLSMessage MUST verify the signature with the key
depending on the sender_type of the sender as described above.
The confirmation tag value confirms that the members of the group
have arrived at the same state of the group. A FramedContentAuthData
is said to be valid when both the signature and confirmation_tag
fields are valid.
6.2. Encoding and Decoding a Public Message
Messages that are authenticated but not encrypted are encoded using
the PublicMessage structure.
struct {
FramedContent content;
FramedContentAuthData auth;
select (PublicMessage.content.sender.sender_type) {
case member:
MAC membership_tag;
case external:
case new_member_commit:
case new_member_proposal:
struct{};
};
} PublicMessage;
The membership_tag field in the PublicMessage object authenticates
the sender's membership in the group. For messages sent by members,
it MUST be set to the following value:
struct {
FramedContentTBS content_tbs;
FramedContentAuthData auth;
} AuthenticatedContentTBM;
membership_tag = MAC(membership_key, AuthenticatedContentTBM)
When decoding a PublicMessage into an AuthenticatedContent, the
application MUST check membership_tag and MUST check that the
FramedContentAuthData is valid.
6.3. Encoding and Decoding a Private Message
Authenticated and encrypted messages are encoded using the
PrivateMessage structure.
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<V>;
opaque encrypted_sender_data<V>;
opaque ciphertext<V>;
} PrivateMessage;
encrypted_sender_data and ciphertext are encrypted using the AEAD
function specified by the cipher suite in use, using the SenderData
and PrivateMessageContent structures as input.
6.3.1. Content Encryption
Content to be encrypted is encoded in a PrivateMessageContent
structure.
struct {
select (PrivateMessage.content_type) {
case application:
opaque application_data<V>;
case proposal:
Proposal proposal;
case commit:
Commit commit;
};
FramedContentAuthData auth;
opaque padding[length_of_padding];
} PrivateMessageContent;
The padding field is set by the sender, by first encoding the content
(via the select) and the auth field, and then appending the chosen
number of zero bytes. A receiver identifies the padding field in a
plaintext decoded from PrivateMessage.ciphertext by first decoding
the content and the auth field; then the padding field comprises any
remaining octets of plaintext. The padding field MUST be filled with
all zero bytes. A receiver MUST verify that there are no non-zero
bytes in the padding field, and if this check fails, the enclosing
PrivateMessage MUST be rejected as malformed. This check ensures
that the padding process is deterministic, so that, for example,
padding cannot be used as a covert channel.
In the MLS key schedule, the sender creates two distinct key ratchets
for handshake and application messages for each member of the group.
When encrypting a message, the sender looks at the ratchets it
derived for its own member and chooses an unused generation from
either the handshake ratchet or the application ratchet, depending on
the content type of the message. This generation of the ratchet is
used to derive a provisional nonce and key.
Before use in the encryption operation, the nonce is XORed with a
fresh random value to guard against reuse. Because the key schedule
generates nonces deterministically, a client MUST keep persistent
state as to where in the key schedule it is; if this persistent state
is lost or corrupted, a client might reuse a generation that has
already been used, causing reuse of a key/nonce pair.
To avoid this situation, the sender of a message MUST generate a
fresh random four-byte "reuse guard" value and XOR it with the first
four bytes of the nonce from the key schedule before using the nonce
for encryption. The sender MUST include the reuse guard in the
reuse_guard field of the sender data object, so that the recipient of
the message can use it to compute the nonce to be used for
decryption.
+-+-+-+-+---------...---+
| Key Schedule Nonce |
+-+-+-+-+---------...---+
XOR
+-+-+-+-+---------...---+
| Guard | 0 |
+-+-+-+-+---------...---+
===
+-+-+-+-+---------...---+
| Encrypt/Decrypt Nonce |
+-+-+-+-+---------...---+
The Additional Authenticated Data (AAD) input to the encryption
contains an object of the following form, with the values used to
identify the key and nonce:
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<V>;
} PrivateContentAAD;
When decoding a PrivateMessageContent, the application MUST check
that the FramedContentAuthData is valid.
It is up to the application to decide what authenticated_data to
provide and how much padding to add to a given message (if any). The
overall size of the AAD and ciphertext MUST fit within the limits
established for the group's AEAD algorithm in [CFRG-AEAD-LIMITS].
6.3.2. Sender Data Encryption
The "sender data" used to look up the key for content encryption is
encrypted with the cipher suite's AEAD with a key and nonce derived
from both the sender_data_secret and a sample of the encrypted
content. Before being encrypted, the sender data is encoded as an
object of the following form:
struct {
uint32 leaf_index;
uint32 generation;
opaque reuse_guard[4];
} SenderData;
When constructing a SenderData object from a Sender object, the
sender MUST verify Sender.sender_type is member and use
Sender.leaf_index for SenderData.leaf_index.
The reuse_guard field contains a fresh random value used to avoid
nonce reuse in the case of state loss or corruption, as described in
Section 6.3.1.
The key and nonce provided to the AEAD are computed as the KDF of the
first KDF.Nh bytes of the ciphertext generated in the previous
section. If the length of the ciphertext is less than KDF.Nh, the
whole ciphertext is used. In pseudocode, the key and nonce are
derived as:
ciphertext_sample = ciphertext[0..KDF.Nh-1]
sender_data_key = ExpandWithLabel(sender_data_secret, "key",
ciphertext_sample, AEAD.Nk)
sender_data_nonce = ExpandWithLabel(sender_data_secret, "nonce",
ciphertext_sample, AEAD.Nn)
The AAD for the SenderData ciphertext is the first three fields of
PrivateMessage:
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
} SenderDataAAD;
When parsing a SenderData struct as part of message decryption, the
recipient MUST verify that the leaf index indicated in the leaf_index
field identifies a non-blank node.
7. Ratchet Tree Operations
The ratchet tree for an epoch describes the membership of a group in
that epoch, providing public key encryption (HPKE) keys that can be
used to encrypt to subsets of the group as well as information to
authenticate the members. In order to reflect changes to the
membership of the group from one epoch to the next, corresponding
changes are made to the ratchet tree. In this section, we describe
the content of the tree and the required operations.
7.1. Parent Node Contents
As discussed in Section 4.1.1, the nodes of a ratchet tree contain
several types of data describing individual members (for leaf nodes)
or subgroups of the group (for parent nodes). Parent nodes are
simpler:
struct {
HPKEPublicKey encryption_key;
opaque parent_hash<V>;
uint32 unmerged_leaves<V>;
} ParentNode;
The encryption_key field contains an HPKE public key whose private
key is held only by the members at the leaves among its descendants.
The parent_hash field contains a hash of this node's parent node, as
described in Section 7.9. The unmerged_leaves field lists the leaves
under this parent node that are unmerged, according to their indices
among all the leaves in the tree. The entries in the unmerged_leaves
vector MUST be sorted in increasing order.
7.2. Leaf Node Contents
A leaf node in the tree describes all the details of an individual
client's appearance in the group, signed by that client. It is also
used in client KeyPackage objects to store the information that will
be needed to add a client to a group.
enum {
reserved(0),
key_package(1),
update(2),
commit(3),
(255)
} LeafNodeSource;
struct {
ProtocolVersion versions<V>;
CipherSuite cipher_suites<V>;
ExtensionType extensions<V>;
ProposalType proposals<V>;
CredentialType credentials<V>;
} Capabilities;
struct {
uint64 not_before;
uint64 not_after;
} Lifetime;
// See the "MLS Extension Types" IANA registry for values
uint16 ExtensionType;
struct {
ExtensionType extension_type;
opaque extension_data<V>;
} Extension;
struct {
HPKEPublicKey encryption_key;
SignaturePublicKey signature_key;
Credential credential;
Capabilities capabilities;
LeafNodeSource leaf_node_source;
select (LeafNode.leaf_node_source) {
case key_package:
Lifetime lifetime;
case update:
struct{};
case commit:
opaque parent_hash<V>;
};
Extension extensions<V>;
/* SignWithLabel(., "LeafNodeTBS", LeafNodeTBS) */
opaque signature<V>;
} LeafNode;
struct {
HPKEPublicKey encryption_key;
SignaturePublicKey signature_key;
Credential credential;
Capabilities capabilities;
LeafNodeSource leaf_node_source;
select (LeafNodeTBS.leaf_node_source) {
case key_package:
Lifetime lifetime;
case update:
struct{};
case commit:
opaque parent_hash<V>;
};
Extension extensions<V>;
select (LeafNodeTBS.leaf_node_source) {
case key_package:
struct{};
case update:
opaque group_id<V>;
uint32 leaf_index;
case commit:
opaque group_id<V>;
uint32 leaf_index;
};
} LeafNodeTBS;
The encryption_key field contains an HPKE public key whose private
key is held only by the member occupying this leaf (or in the case of
a LeafNode in a KeyPackage object, the issuer of the KeyPackage).
The signature_key field contains the member's public signing key.
The credential field contains information authenticating both the
member's identity and the provided signing key, as described in
Section 5.3.
The capabilities field indicates the protocol features that the
client supports, including protocol versions, cipher suites,
credential types, non-default proposal types, and non-default
extension types. The following proposal and extension types are
considered "default" and MUST NOT be listed:
* Proposal types:
- 0x0001 - add
- 0x0002 - update
- 0x0003 - remove
- 0x0004 - psk
- 0x0005 - reinit
- 0x0006 - external_init
- 0x0007 - group_context_extensions
* Extension types:
- 0x0001 - application_id
- 0x0002 - ratchet_tree
- 0x0003 - required_capabilities
- 0x0004 - external_pub
- 0x0005 - external_senders
There are no default values for the other fields of a capabilities
object. The client MUST list all values for the respective
parameters that it supports.
The types of any non-default extensions that appear in the extensions
field of a LeafNode MUST be included in the extensions field of the
capabilities field, and the credential type used in the LeafNode MUST
be included in the credentials field of the capabilities field.
As discussed in Section 13, unknown values in capabilities MUST be
ignored, and the creator of a capabilities field SHOULD include some
random GREASE values to help ensure that other clients correctly
ignore unknown values.
The leaf_node_source field indicates how this LeafNode came to be
added to the tree. This signal tells other members of the group
whether the leaf node is required to have a lifetime or parent_hash,
and whether the group_id is added as context to the signature. These
fields are included selectively because the client creating a
LeafNode is not always able to compute all of them. For example, a
KeyPackage is created before the client knows which group it will be
used with, so its signature can't bind to a group_id.
In the case where the leaf was added to the tree based on a pre-
published KeyPackage, the lifetime field represents the times between
which clients will consider a LeafNode valid. These times are
represented as absolute times, measured in seconds since the Unix
epoch (1970-01-01T00:00:00Z). Applications MUST define a maximum
total lifetime that is acceptable for a LeafNode, and reject any
LeafNode where the total lifetime is longer than this duration. In
order to avoid disagreements about whether a LeafNode has a valid
lifetime, the clients in a group SHOULD maintain time synchronization
(e.g., using the Network Time Protocol [RFC5905]).
In the case where the leaf node was inserted into the tree via a
Commit message, the parent_hash field contains the parent hash for
this leaf node (see Section 7.9).
The LeafNodeTBS structure covers the fields above the signature in
the LeafNode. In addition, when the leaf node was created in the
context of a group (the update and commit cases), the group ID of the
group is added as context to the signature.
LeafNode objects stored in the group's ratchet tree are updated
according to the evolution of the tree. Each modification of
LeafNode content MUST be reflected by a change in its signature.
This allows other members to verify the validity of the LeafNode at
any time, particularly in the case of a newcomer joining the group.
7.3. Leaf Node Validation
The validity of a LeafNode needs to be verified at the following
stages:
* When a LeafNode is downloaded in a KeyPackage, before it is used
to add the client to the group
* When a LeafNode is received by a group member in an Add, Update,
or Commit message
* When a client validates a ratchet tree, e.g., when joining a group
or after processing a Commit
The client verifies the validity of a LeafNode using the following
steps:
* Verify that the credential in the LeafNode is valid, as described
in Section 5.3.1.
* Verify that the signature on the LeafNode is valid using
signature_key.
* Verify that the LeafNode is compatible with the group's
parameters. If the GroupContext has a required_capabilities
extension, then the required extensions, proposals, and credential
types MUST be listed in the LeafNode's capabilities field.
* Verify that the credential type is supported by all members of the
group, as specified by the capabilities field of each member's
LeafNode, and that the capabilities field of this LeafNode
indicates support for all the credential types currently in use by
other members.
* Verify the lifetime field:
- If the LeafNode appears in a message being sent by the client,
e.g., a Proposal or a Commit, then the client MUST verify that
the current time is within the range of the lifetime field.
- If instead the LeafNode appears in a message being received by
the client, e.g., a Proposal, a Commit, or a ratchet tree of
the group the client is joining, it is RECOMMENDED that the
client verifies that the current time is within the range of
the lifetime field. (This check is not mandatory because the
LeafNode might have expired in the time between when the
message was sent and when it was received.)
* Verify that the extensions in the LeafNode are supported by
checking that the ID for each extension in the extensions field is
listed in the capabilities.extensions field of the LeafNode.
* Verify the leaf_node_source field:
- If the LeafNode appears in a KeyPackage, verify that
leaf_node_source is set to key_package.
- If the LeafNode appears in an Update proposal, verify that
leaf_node_source is set to update and that encryption_key
represents a different public key than the encryption_key in
the leaf node being replaced by the Update proposal.
- If the LeafNode appears in the leaf_node value of the
UpdatePath in a Commit, verify that leaf_node_source is set to
commit.
* Verify that the following fields are unique among the members of
the group:
- signature_key
- encryption_key
7.4. Ratchet Tree Evolution
Whenever a member initiates an epoch change (i.e., commits; see
Section 12.4), they may need to refresh the key pairs of their leaf
and of the nodes on their leaf's direct path in order to maintain
forward secrecy and post-compromise security.
The member initiating the epoch change generates the fresh key pairs
using the following procedure. The procedure is designed in a way
that allows group members to efficiently communicate the fresh secret
keys to other group members, as described in Section 7.6.
A member updates the nodes along its direct path as follows:
* Blank all the nodes on the direct path from the leaf to the root.
* Generate a fresh HPKE key pair for the leaf.
* Generate a sequence of path secrets, one for each node on the
leaf's filtered direct path, as follows. In this setting,
path_secret[0] refers to the first parent node in the filtered
direct path, path_secret[1] to the second parent node, and so on.
path_secret[0] is sampled at random
path_secret[n] = DeriveSecret(path_secret[n-1], "path")
* Compute the sequence of HPKE key pairs (node_priv,node_pub), one
for each node on the leaf's direct path, as follows.
node_secret[n] = DeriveSecret(path_secret[n], "node")
node_priv[n], node_pub[n] = KEM.DeriveKeyPair(node_secret[n])
The node secret is derived as a temporary intermediate secret so that
each secret is only used with one algorithm: The path secret is used
as an input to DeriveSecret, and the node secret is used as an input
to DeriveKeyPair.
For example, suppose there is a group with four members, with C an
unmerged leaf at Z:
Y
|
.-+-.
/ \
X Z[C]
/ \ / \
A B C D
0 1 2 3
Figure 13: A Full Tree with One Unmerged Leaf
If member B subsequently generates an UpdatePath based on a secret
"leaf_secret", then it would generate the following sequence of path
secrets:
path_secret[1] ---> node_secret[1] -------> node_priv[1], node_pub[1]
^
|
|
path_secret[0] ---> node_secret[0] -------> node_priv[0], node_pub[0]
^
|
|
leaf_secret ------> leaf_node_secret --+--> leaf_priv, leaf_pub
| |
'-------. .-------'
|
leaf_node
Figure 14: Derivation of Ratchet Tree Keys along a Direct Path
After applying the UpdatePath, the tree will have the following
structure:
node_priv[1] --------> Y'
|
.-+-.
/ \
node_priv[0] ----> X' Z[C]
/ \ / \
A B C D
^
leaf_priv -----------+
0 1 2 3
Figure 15: Placement of Keys in a Ratchet Tree
7.5. Synchronizing Views of the Tree
After generating fresh key material and applying it to update their
local tree state as described in Section 7.4, the generator
broadcasts this update to other members of the group in a Commit
message, who apply it to keep their local views of the tree in sync
with the sender's. More specifically, when a member commits a change
to the tree (e.g., to add or remove a member), it transmits an
UpdatePath containing a set of public keys and encrypted path secrets
for intermediate nodes in the filtered direct path of its leaf. The
other members of the group use these values to update their view of
the tree, aligning their copy of the tree to the sender's.
An UpdatePath contains the following information for each node in the
filtered direct path of the sender's leaf, including the root:
* The public key for the node
* One or more encrypted copies of the path secret corresponding to
the node
The path secret value for a given node is encrypted to the subtree
rooted at the parent's non-updated child, i.e., the child on the
copath of the sender's leaf node. There is one encryption of the
path secret to each public key in the resolution of the non-updated
child.
A member of the group _updates their direct path_ by computing new
values for their leaf node and the nodes along their filtered direct
path as follows:
1. Blank all nodes along the direct path of the sender's leaf.
2. Compute updated path secrets and public keys for the nodes on the
sender's filtered direct path.
* Generate a sequence of path secrets of the same length as the
filtered direct path, as defined in Section 7.4.
* For each node in the filtered direct path, replace the node's
public key with the node_pub[n] value derived from the
corresponding path secret path_secret[n].
3. Compute the new parent hashes for the nodes along the filtered
direct path and the sender's leaf node.
4. Update the leaf node for the sender.
* Set the leaf_node_source to commit.
* Set the encryption_key to the public key of a freshly sampled
key pair.
* Set the parent hash to the parent hash for the leaf.
* Re-sign the leaf node with its new contents.
Since the new leaf node effectively updates an existing leaf node in
the group, it MUST adhere to the same restrictions as LeafNodes used
in Update proposals (aside from leaf_node_source). The application
MAY specify other changes to the leaf node, e.g., providing a new
signature key, updated capabilities, or different extensions.
The member then _encrypts path secrets to the group_. For each node
in the member's filtered direct path, the member takes the following
steps:
1. Compute the resolution of the node's child that is on the copath
of the sender (the child that is not in the direct path of the
sender). Any new member (from an Add proposal) added in the same
Commit MUST be excluded from this resolution.
2. For each node in the resolution, encrypt the path secret for the
direct path node using the public key of the resolution node, as
defined in Section 7.6.
The recipient of an UpdatePath performs the corresponding steps.
First, the recipient _merges UpdatePath into the tree_:
1. Blank all nodes on the direct path of the sender's leaf.
2. For all nodes on the filtered direct path of the sender's leaf,
* Set the public key to the public key in the UpdatePath.
* Set the list of unmerged leaves to the empty list.
3. Compute parent hashes for the nodes in the sender's filtered
direct path, and verify that the parent_hash field of the leaf
node matches the parent hash for the first node in its filtered
direct path.
* Note that these hashes are computed from root to leaf, so that
each hash incorporates all the non-blank nodes above it. The
root node always has a zero-length hash for its parent hash.
Second, the recipient _decrypts the path secrets_:
1. Identify a node in the filtered direct path for which the
recipient is in the subtree of the non-updated child.
2. Identify a node in the resolution of the copath node for which
the recipient has a private key.
3. Decrypt the path secret for the parent of the copath node using
the private key from the resolution node.
4. Derive path secrets for ancestors of that node in the sender's
filtered direct path using the algorithm described above.
5. Derive the node secrets and node key pairs from the path secrets.
6. Verify that the derived public keys are the same as the
corresponding public keys sent in the UpdatePath.
7. Store the derived private keys in the corresponding ratchet tree
nodes.
For example, in order to communicate the example update described in
Section 7.4, the member at node B would transmit the following
values:
+=============+====================================================+
| Public Key | Ciphertext(s) |
+=============+====================================================+
| node_pub[1] | E(pk(Z), path_secret[1]), E(pk(C), path_secret[1]) |
+-------------+----------------------------------------------------+
| node_pub[0] | E(pk(A), path_secret[0]) |
+-------------+----------------------------------------------------+
Table 3
In this table, the value node_pub[i] represents the public key
derived from node_secret[i], pk(X) represents the current public key
of node X, and E(K, S) represents the public key encryption of the
path secret S to the public key K (using HPKE).
A recipient at node A would decrypt E(pk(A), path_secret\[0\]) to
obtain path_secret\[0\], then use it to derive path_secret[1] and the
resulting node secrets and key pairs. Thus, A would have the private
keys to nodes X' and Y', in accordance with the tree invariant.
Similarly, a recipient at node D would decrypt E(pk(Z),
path_secret[1]) to obtain path_secret[1], then use it to derive the
node secret and key pair for the node Y'. As required to maintain
the tree invariant, node D does not receive the private key for the
node X', since X' is not an ancestor of D.
After processing the update, each recipient MUST delete outdated key
material, specifically:
* The path secrets and node secrets used to derive each updated node
key pair.
* Each outdated node key pair that was replaced by the update.
7.6. Update Paths
As described in Section 12.4, each Commit message may optionally
contain an UpdatePath, with a new LeafNode and set of parent nodes
for the sender's filtered direct path. For each parent node, the
UpdatePath contains a new public key and encrypted path secret. The
parent nodes are kept in the same order as the filtered direct path.
struct {
opaque kem_output<V>;
opaque ciphertext<V>;
} HPKECiphertext;
struct {
HPKEPublicKey encryption_key;
HPKECiphertext encrypted_path_secret<V>;
} UpdatePathNode;
struct {
LeafNode leaf_node;
UpdatePathNode nodes<V>;
} UpdatePath;
For each UpdatePathNode, the resolution of the corresponding copath
node MUST exclude all new leaf nodes added as part of the current
Commit. The length of the encrypted_path_secret vector MUST be equal
to the length of the resolution of the copath node (excluding new
leaf nodes), with each ciphertext being the encryption to the
respective resolution node.
The HPKECiphertext values are encrypted and decrypted as follows:
(kem_output, ciphertext) =
EncryptWithLabel(node_public_key, "UpdatePathNode",
group_context, path_secret)
path_secret =
DecryptWithLabel(node_private_key, "UpdatePathNode",
group_context, kem_output, ciphertext)
Here node_public_key is the public key of the node for which the path
secret is encrypted, group_context is the provisional GroupContext
object for the group, and the EncryptWithLabel function is as defined
in Section 5.1.3.
7.7. Adding and Removing Leaves
In addition to the path-based updates to the tree described above, it
is also necessary to add and remove leaves of the tree in order to
reflect changes to the membership of the group (see Sections 12.1.1
and 12.1.3). Since the tree is always full, adding or removing
leaves corresponds to increasing or decreasing the depth of the tree,
resulting in the number of leaves being doubled or halved. These
operations are also known as _extending_ and _truncating_ the tree.
Leaves are always added and removed at the right edge of the tree.
When the size of the tree needs to be increased, a new blank root
node is added, whose left subtree is the existing tree and right
subtree is a new all-blank subtree. This operation is typically done
when adding a member to the group.
_ <-- new blank root _
__|__ __|__
/ \ / \
X ===> X _ <-- new blank subtree ===> X _
/ \ / \ / \ / \ / \
A B A B _ _ A B C _
^
|
new member --+
Figure 16: Extending the Tree to Make Room for a Third Member
When the right subtree of the tree no longer has any non-blank nodes,
it can be safely removed. The root of the tree and the right subtree
are discarded (whether or not the root node is blank). The left
child of the root becomes the new root node, and the left subtree
becomes the new tree. This operation is typically done after
removing a member from the group.
Y Y
__|__ __|__
/ \ / \
X _ ===> X _ ==> X <-- new root
/ \ / \ / \ / \ / \
A B C _ A B _ _ A B
^
|
removed member --+
Figure 17: Cleaning Up after Removing Member C
Concrete algorithms for these operations on array-based and link-
based trees are provided in Appendices C and D. The concrete
algorithms are non-normative. An implementation may use any
algorithm that produces the correct tree in its internal
representation.
7.8. Tree Hashes
MLS hashes the contents of the tree in two ways to authenticate
different properties of the tree. _Tree hashes_ are defined in this
section, and _parent hashes_ are defined in Section 7.9.
Each node in a ratchet tree has a tree hash that summarizes the
subtree below that node. The tree hash of the root is used in the
GroupContext to confirm that the group agrees on the whole tree.
Tree hashes are computed recursively from the leaves up to the root.
P --> th(P)
^ ^
/ \
/ \
th(L) th(R)
Figure 18: Composition of the Tree Hash
The tree hash of an individual node is the hash of the node's
TreeHashInput object, which may contain either a LeafNodeHashInput or
a ParentNodeHashInput depending on the type of node.
LeafNodeHashInput objects contain the leaf_index and the LeafNode (if
any). ParentNodeHashInput objects contain the ParentNode (if any)
and the tree hash of the node's left and right children. For both
parent and leaf nodes, the optional node value MUST be absent if the
node is blank and present if the node contains a value.
enum {
reserved(0),
leaf(1),
parent(2),
(255)
} NodeType;
struct {
NodeType node_type;
select (TreeHashInput.node_type) {
case leaf: LeafNodeHashInput leaf_node;
case parent: ParentNodeHashInput parent_node;
};
} TreeHashInput;
struct {
uint32 leaf_index;
optional<LeafNode> leaf_node;
} LeafNodeHashInput;
struct {
optional<ParentNode> parent_node;
opaque left_hash<V>;
opaque right_hash<V>;
} ParentNodeHashInput;
The tree hash of an entire tree corresponds to the tree hash of the
root node, which is computed recursively by starting at the leaf
nodes and building up.
7.9. Parent Hashes
While tree hashes summarize the state of a tree at point in time,
parent hashes capture information about how keys in the tree were
populated.
When a client sends a Commit to change a group, it can include an
UpdatePath to assign new keys to the nodes along its filtered direct
path. When a client computes an UpdatePath (as defined in
Section 7.5), it computes and signs a parent hash that summarizes the
state of the tree after the UpdatePath has been applied. These
summaries are constructed in a chain from the root to the member's
leaf so that the part of the chain closer to the root can be
overwritten as nodes set in one UpdatePath are reset by a later
UpdatePath.
ph(Q)
/
/
V
P.public_key --> ph(P)
/ ^
/ \
V \
N.parent_hash th(S)
Figure 19: Inputs to a Parent Hash
As a result, the signature over the parent hash in each member's leaf
effectively signs the subtree of the tree that hasn't been changed
since that leaf was last changed in an UpdatePath. A new member
joining the group uses these parent hashes to verify that the parent
nodes in the tree were set by members of the group, not chosen by an
external attacker. For an example of how this works, see Appendix B.
Consider a ratchet tree with a non-blank parent node P and children D
and S (for "parent", "direct path", and "sibling"), with D and P in
the direct path of a leaf node L (for "leaf"):
...
/
P
__|__
/ \
D S
/ \ / \
... ... ... ...
/
L
Figure 20: Nodes Involved in a Parent Hash Computation
The parent hash of P changes whenever an UpdatePath object is applied
to the ratchet tree along a path from a leaf L traversing node D (and
hence also P). The new "Parent hash of P (with copath child S)" is
obtained by hashing P's ParentHashInput struct.
struct {
HPKEPublicKey encryption_key;
opaque parent_hash<V>;
opaque original_sibling_tree_hash<V>;
} ParentHashInput;
The field encryption_key contains the HPKE public key of P. If P is
the root, then the parent_hash field is set to a zero-length octet
string. Otherwise, parent_hash is the parent hash of the next node
after P on the filtered direct path of the leaf L. This way, P's
parent hash fixes the new HPKE public key of each non-blank node on
the path from P to the root. Note that the path from P to the root
may contain some blank nodes that are not fixed by P's parent hash.
However, for each node that has an HPKE key, this key is fixed by P's
parent hash.
Finally, original_sibling_tree_hash is the tree hash of S in the
ratchet tree modified as follows: For each leaf L in
P.unmerged_leaves, blank L and remove it from the unmerged_leaves
sets of all parent nodes.
Observe that original_sibling_tree_hash does not change between
updates of P. This property is crucial for the correctness of the
protocol.
Note that original_sibling_tree_hash is the tree hash of S, not the
parent hash. The parent_hash field in ParentHashInput captures
information about the nodes above P. the original_sibling_tree_hash
captures information about the subtree under S that is not being
updated (and thus the subtree to which a path secret for P would be
encrypted according to Section 7.5).
For example, in the following tree:
W [F]
______|_____
/ \
U Y [F]
__|__ __|__
/ \ / \
T _ _ _
/ \ / \ / \ / \
A B C D E F G _
Figure 21: A Tree Illustrating Parent Hash Computations
With P = W and S = Y, original_sibling_tree_hash is the tree hash of
the following tree:
Y
__|__
/ \
_ _
/ \ / \
E _ G _
Because W.unmerged_leaves includes F, F is blanked and removed from
Y.unmerged_leaves.
Note that no recomputation is needed if the tree hash of S is
unchanged since the last time P was updated. This is the case for
computing or processing a Commit whose UpdatePath traverses P, since
the Commit itself resets P. (In other words, it is only necessary to
recompute the original sibling tree hash when validating a group's
tree on joining.) More generally, if none of the entries in
P.unmerged_leaves are in the subtree under S (and thus no leaves were
blanked), then the original tree hash at S is the tree hash of S in
the current tree.
If it is necessary to recompute the original tree hash of a node, the
efficiency of recomputation can be improved by caching intermediate
tree hashes, to avoid recomputing over the subtree when the subtree
is included in multiple parent hashes. A subtree hash can be reused
as long as the intersection of the parent's unmerged leaves with the
subtree is the same as in the earlier computation.
7.9.1. Using Parent Hashes
In ParentNode objects and LeafNode objects with leaf_node_source set
to commit, the value of the parent_hash field is the parent hash of
the next non-blank parent node above the node in question (the next
node in the filtered direct path). Using the node labels in
Figure 20, the parent_hash field of D is equal to the parent hash of
P with copath child S. This is the case even when the node D is a
leaf node.
The parent_hash field of a LeafNode is signed by the member. The
signature of such a LeafNode thus attests to which keys the group
member introduced into the ratchet tree and to whom the corresponding
secret keys were sent, in addition to the other contents of the
LeafNode. This prevents malicious insiders from constructing
artificial ratchet trees with a node D whose HPKE secret key is known
to the insider, yet where the insider isn't assigned a leaf in the
subtree rooted at D. Indeed, such a ratchet tree would violate the
tree invariant.
7.9.2. Verifying Parent Hashes
Parent hashes are verified at two points in the protocol: When
joining a group and when processing a Commit.
The parent hash in a node D is valid with respect to a parent node P
if the following criteria hold. Here C and S are the children of P
(for "child" and "sibling"), with C being the child that is on the
direct path of D (possibly D itself) and S being the other child:
* D is a descendant of P in the tree.
* The parent_hash field of D is equal to the parent hash of P with
copath child S.
* D is in the resolution of C, and the intersection of P's
unmerged_leaves with the subtree under C is equal to the
resolution of C with D removed.
These checks verify that D and P were updated at the same time (in
the same UpdatePath), and that they were neighbors in the UpdatePath
because the nodes in between them would have omitted from the
filtered direct path.
A parent node P is "parent-hash valid" if it can be chained back to a
leaf node in this way. That is, if there is leaf node L and a
sequence of parent nodes P_1, ..., P_N such that P_N = P and each
step in the chain is authenticated by a parent hash, then L's parent
hash is valid with respect to P_1, P_1's parent hash is valid with
respect to P_2, and so on.
When joining a group, the new member MUST authenticate that each non-
blank parent node P is parent-hash valid. This can be done "bottom
up" by building chains up from leaves and verifying that all non-
blank parent nodes are covered by exactly one such chain, or "top
down" by verifying that there is exactly one descendant of each non-
blank parent node for which the parent node is parent-hash valid.
When processing a Commit message that includes an UpdatePath, clients
MUST recompute the expected value of parent_hash for the committer's
new leaf and verify that it matches the parent_hash value in the
supplied leaf_node. After being merged into the tree, the nodes in
the UpdatePath form a parent-hash chain from the committer's leaf to
the root.
8. Key Schedule
Group keys are derived using the Extract and Expand functions from
the KDF for the group's cipher suite, as well as the functions
defined below:
ExpandWithLabel(Secret, Label, Context, Length) =
KDF.Expand(Secret, KDFLabel, Length)
DeriveSecret(Secret, Label) =
ExpandWithLabel(Secret, Label, "", KDF.Nh)
Where KDFLabel is specified as:
struct {
uint16 length;
opaque label<V>;
opaque context<V>;
} KDFLabel;
And its fields are set to:
length = Length;
label = "MLS 1.0 " + Label;
context = Context;
The value KDF.Nh is the size of an output from KDF.Extract, in bytes.
In the below diagram:
* KDF.Extract takes its salt argument from the top and its Input
Keying Material (IKM) argument from the left.
* DeriveSecret takes its Secret argument from the incoming arrow.
* 0 represents an all-zero byte string of length KDF.Nh.
When processing a handshake message, a client combines the following
information to derive new epoch secrets:
* The init secret from the previous epoch
* The commit secret for the current epoch
* The GroupContext object for current epoch
Given these inputs, the derivation of secrets for an epoch proceeds
as shown in the following diagram:
init_secret_[n-1]
|
|
V
commit_secret --> KDF.Extract
|
|
V
ExpandWithLabel(., "joiner", GroupContext_[n], KDF.Nh)
|
|
V
joiner_secret
|
|
V
psk_secret (or 0) --> KDF.Extract
|
|
+--> DeriveSecret(., "welcome")
| = welcome_secret
|
V
ExpandWithLabel(., "epoch", GroupContext_[n], KDF.Nh)
|
|
V
epoch_secret
|
|
+--> DeriveSecret(., <label>)
| = <secret>
|
V
DeriveSecret(., "init")
|
|
V
init_secret_[n]
Figure 22: The MLS Key Schedule
A number of values are derived from the epoch secret for different
purposes:
+==================+=====================+=======================+
| Label | Secret | Purpose |
+==================+=====================+=======================+
| "sender data" | sender_data_secret | Deriving keys to |
| | | encrypt sender data |
+------------------+---------------------+-----------------------+
| "encryption" | encryption_secret | Deriving message |
| | | encryption keys (via |
| | | the secret tree) |
+------------------+---------------------+-----------------------+
| "exporter" | exporter_secret | Deriving exported |
| | | secrets |
+------------------+---------------------+-----------------------+
| "external" | external_secret | Deriving the external |
| | | init key |
+------------------+---------------------+-----------------------+
| "confirm" | confirmation_key | Computing the |
| | | confirmation MAC for |
| | | an epoch |
+------------------+---------------------+-----------------------+
| "membership" | membership_key | Computing the |
| | | membership MAC for a |
| | | PublicMessage |
+------------------+---------------------+-----------------------+
| "resumption" | resumption_psk | Proving membership in |
| | | this epoch (via a PSK |
| | | injected later) |
+------------------+---------------------+-----------------------+
| "authentication" | epoch_authenticator | Confirming that two |
| | | clients have the same |
| | | view of the group |
+------------------+---------------------+-----------------------+
Table 4: Epoch-Derived Secrets
The external_secret is used to derive an HPKE key pair whose private
key is held by the entire group:
external_priv, external_pub = KEM.DeriveKeyPair(external_secret)
The public key external_pub can be published as part of the GroupInfo
struct in order to allow non-members to join the group using an
external Commit.
8.1. Group Context
Each member of the group maintains a GroupContext object that
summarizes the state of the group:
struct {
ProtocolVersion version = mls10;
CipherSuite cipher_suite;
opaque group_id<V>;
uint64 epoch;
opaque tree_hash<V>;
opaque confirmed_transcript_hash<V>;
Extension extensions<V>;
} GroupContext;
The fields in this state have the following semantics:
* The cipher_suite is the cipher suite used by the group.
* The group_id field is an application-defined identifier for the
group.
* The epoch field represents the current version of the group.
* The tree_hash field contains a commitment to the contents of the
group's ratchet tree and the credentials for the members of the
group, as described in Section 7.8.
* The confirmed_transcript_hash field contains a running hash over
the messages that led to this state.
* The extensions field contains the details of any protocol
extensions that apply to the group.
When a new member is added to the group, an existing member of the
group provides the new member with a Welcome message. The Welcome
message provides the information the new member needs to initialize
its GroupContext.
Different changes to the group will have different effects on the
group state. These effects are described in their respective
subsections of Section 12.1. The following general rules apply:
* The group_id field is constant.
* The epoch field increments by one for each Commit message that is
processed.
* The tree_hash is updated to represent the current tree and
credentials.
* The confirmed_transcript_hash field is updated with the data for
an AuthenticatedContent encoding a Commit message, as described
below.
* The extensions field changes when a GroupContextExtensions
proposal is committed.
8.2. Transcript Hashes
The transcript hashes computed in MLS represent a running hash over
all Proposal and Commit messages that have ever been sent in a group.
Commit messages are included directly. Proposal messages are
indirectly included via the Commit that applied them. Messages of
both types are included by hashing the AuthenticatedContent object in
which they were sent.
The transcript hash comprises two individual hashes:
* A confirmed_transcript_hash that represents a transcript over the
whole history of Commit messages, up to and including the
signature of the most recent Commit.
* An interim_transcript_hash that covers the confirmed transcript
hash plus the confirmation_tag of the most recent Commit.
New members compute the interim transcript hash using the
confirmation_tag field of the GroupInfo struct, while existing
members can compute it directly.
Each Commit message updates these hashes by way of its enclosing
AuthenticatedContent. The AuthenticatedContent struct is split into
ConfirmedTranscriptHashInput and InterimTranscriptHashInput. The
former is used to update the confirmed transcript hash and the latter
is used to update the interim transcript hash.
struct {
WireFormat wire_format;
FramedContent content; /* with content_type == commit */
opaque signature<V>;
} ConfirmedTranscriptHashInput;
struct {
MAC confirmation_tag;
} InterimTranscriptHashInput;
confirmed_transcript_hash_[0] = ""; /* zero-length octet string */
interim_transcript_hash_[0] = ""; /* zero-length octet string */
confirmed_transcript_hash_[epoch] =
Hash(interim_transcript_hash_[epoch - 1] ||
ConfirmedTranscriptHashInput_[epoch]);
interim_transcript_hash_[epoch] =
Hash(confirmed_transcript_hash_[epoch] ||
InterimTranscriptHashInput_[epoch]);
In this notation, ConfirmedTranscriptHashInput_[epoch] and
InterimTranscriptHashInput_[epoch] are based on the Commit that
initiated the epoch with epoch number epoch. (Note that
theepochfield in this Commit will be set toepoch - 1`, since it is
sent within the previous epoch.)
The transcript hash ConfirmedTranscriptHashInput_[epoch] is used as
the confirmed_transcript_hash input to the confirmation_tag field for
this Commit. Each Commit thus confirms the whole transcript of
Commits up to that point, except for the latest Commit's confirmation
tag.
...
|
|
V
+-----------------+
| interim_[N-1] |
+--------+--------+
|
.--------------. +------------------+ |
| Ratchet Tree | | wire_format | |
| Key Schedule |<-------+ content | |
'-------+------' | epoch = N-1 +------------+
| | commit | |
V | signature | V
+------------------------+ +------------------+ +-----------------+
| confirmation_key_[N] +-->| confirmation_tag |<--+ confirmed_[N] |
+------------------------+ +--------+---------+ +--------+--------+
| |
| V
| +-----------------+
+------------>| interim_[N] |
+--------+--------+
|
.--------------. +------------------+ |
| Ratchet Tree | | wire_format | |
| Key Schedule |<-------+ content | |
'-------+------' | epoch = N +------------+
| | commit | |
V | signature | V
+------------------------+ +------------------+ +-----------------+
| confirmation_key_[N+1] +-->| confirmation_tag |<--+ confirmed_[N+1] |
+------------------------+ +--------+---------+ +--------+--------+
| |
| V
| +-----------------+
+------------>| interim_[N+1] |
+--------+--------+
|
V
...
Figure 23: Evolution of the Transcript Hashes through Two Epoch
Changes
8.3. External Initialization
In addition to initializing a new epoch via KDF invocations as
described above, an MLS group can also initialize a new epoch via an
asymmetric interaction using the external key pair for the previous
epoch. This is done when a new member is joining via an external
commit.
In this process, the joiner sends a new init_secret value to the
group using the HPKE export method. The joiner then uses that
init_secret with information provided in the GroupInfo and an
external Commit to initialize their copy of the key schedule for the
new epoch.
kem_output, context = SetupBaseS(external_pub, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
Members of the group receive the kem_output in an ExternalInit
proposal and perform the corresponding calculation to retrieve the
init_secret value.
context = SetupBaseR(kem_output, external_priv, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
8.4. Pre-Shared Keys
Groups that already have an out-of-band mechanism to generate shared
group secrets can inject them into the MLS key schedule to
incorporate this external entropy in the computation of MLS group
secrets.
Injecting an external PSK can improve security in the case where
having a full run of Updates across members is too expensive, or if
the external group key establishment mechanism provides stronger
security against classical or quantum adversaries.
Note that, as a PSK may have a different lifetime than an Update, it
does not necessarily provide the same forward secrecy or post-
compromise security guarantees as a Commit message. Unlike the key
pairs populated in the tree by an Update or Commit, which are always
freshly generated, PSKs may be pre-distributed and stored. This
creates the risk that a PSK may be compromised in the process of
distribution and storage. The security that the group gets from
injecting a PSK thus depends on both the entropy of the PSK and the
risk of compromise. These factors are outside of the scope of this
document, but they should be considered by application designers
relying on PSKs.
Each PSK in MLS has a type that designates how it was provisioned.
External PSKs are provided by the application, while resumption PSKs
are derived from the MLS key schedule and used in cases where it is
necessary to authenticate a member's participation in a prior epoch.
The injection of one or more PSKs into the key schedule is signaled
in two ways: Existing members are informed via PreSharedKey proposals
covered by a Commit, and new members added in the Commit are informed
by the GroupSecrets object in the Welcome message corresponding to
the Commit. To ensure that existing and new members compute the same
PSK input to the key schedule, the Commit and GroupSecrets objects
MUST indicate the same set of PSKs, in the same order.
enum {
reserved(0),
external(1),
resumption(2),
(255)
} PSKType;
enum {
reserved(0),
application(1),
reinit(2),
branch(3),
(255)
} ResumptionPSKUsage;
struct {
PSKType psktype;
select (PreSharedKeyID.psktype) {
case external:
opaque psk_id<V>;
case resumption:
ResumptionPSKUsage usage;
opaque psk_group_id<V>;
uint64 psk_epoch;
};
opaque psk_nonce<V>;
} PreSharedKeyID;
Each time a client injects a PSK into a group, the psk_nonce of its
PreSharedKeyID MUST be set to a fresh random value of length KDF.Nh,
where KDF is the KDF for the cipher suite of the group into which the
PSK is being injected. This ensures that even when a PSK is used
multiple times, the value used as an input into the key schedule is
different each time.
Upon receiving a Commit with a PreSharedKey proposal or a
GroupSecrets object with the psks field set, the receiving client
includes them in the key schedule in the order listed in the Commit,
or in the psks field, respectively. For resumption PSKs, the PSK is
defined as the resumption_psk of the group and epoch specified in the
PreSharedKeyID object. Specifically, psk_secret is computed as
follows:
struct {
PreSharedKeyID id;
uint16 index;
uint16 count;
} PSKLabel;
psk_extracted_[i] = KDF.Extract(0, psk_[i])
psk_input_[i] = ExpandWithLabel(psk_extracted_[i], "derived psk",
PSKLabel, KDF.Nh)
psk_secret_[0] = 0
psk_secret_[i] = KDF.Extract(psk_input_[i-1], psk_secret_[i-1])
psk_secret = psk_secret_[n]
Here 0 represents the all-zero vector of length KDF.Nh. The index
field in PSKLabel corresponds to the index of the PSK in the psk
array, while the count field contains the total number of PSKs. In
other words, the PSKs are chained together with KDF.Extract
invocations (labeled "Extract" for brevity in the diagram), as
follows:
0 0 = psk_secret_[0]
| |
V V
psk_[0] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[1]
|
0 |
| |
V V
psk_[1] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[2]
|
0 ...
| |
V V
psk_[n-1] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[n]
Figure 24: Computation of a PSK Secret from a Set of PSKs
In particular, if there are no PreSharedKey proposals in a given
Commit, then the resulting psk_secret is psk_secret_[0], the all-zero
vector.
8.5. Exporters
The main MLS key schedule provides an exporter_secret that can be
used by an application to derive new secrets for use outside of MLS.
MLS-Exporter(Label, Context, Length) =
ExpandWithLabel(DeriveSecret(exporter_secret, Label),
"exported", Hash(Context), Length)
Applications SHOULD provide a unique label to MLS-Exporter that
identifies the secret's intended purpose. This is to help prevent
the same secret from being generated and used in two different
places. To help avoid the same label being used in different
applications, an IANA registry for these labels has been defined in
Section 17.8.
The exported values are bound to the group epoch from which the
exporter_secret is derived, and hence reflect a particular state of
the group.
It is RECOMMENDED for the application generating exported values to
refresh those values after a Commit is processed.
8.6. Resumption PSK
The main MLS key schedule provides a resumption_psk that is used as a
PSK to inject entropy from one epoch into another. This
functionality is used in the reinitialization and branching processes
described in Sections 11.2 and 11.3, but it may be used by
applications for other purposes.
Some uses of resumption PSKs might call for the use of PSKs from
historical epochs. The application SHOULD specify an upper limit on
the number of past epochs for which the resumption_psk may be stored.
8.7. Epoch Authenticators
The main MLS key schedule provides a per-epoch epoch_authenticator.
If one member of the group is being impersonated by an active
attacker, the epoch_authenticator computed by their client will
differ from those computed by the other group members.
This property can be used to construct defenses against impersonation
attacks that are effective even if members' signature keys are
compromised. As a trivial example, if the users of the clients in an
MLS group were to meet in person and reliably confirm that their
epoch authenticator values were equal (using some suitable user
interface), then each user would be assured that the others were not
being impersonated in the current epoch. As soon as the epoch
changed, though, they would need to redo this confirmation. The
state of the group would have changed, possibly introducing an
attacker.
More generally, in order for the members of an MLS group to obtain
concrete authentication protections using the epoch_authenticator,
they will need to use it in some secondary protocol (such as the
face-to-face protocol above). The details of that protocol will then
determine the specific authentication protections provided to the MLS
group.
9. Secret Tree
For the generation of encryption keys and nonces, the key schedule
begins with the encryption_secret at the root and derives a tree of
secrets with the same structure as the group's ratchet tree. Each
leaf in the secret tree is associated with the same group member as
the corresponding leaf in the ratchet tree.
If N is a parent node in the secret tree, then the secrets of the
children of N are defined as follows (where left(N) and right(N)
denote the children of N):
tree_node_[N]_secret
|
|
+--> ExpandWithLabel(., "tree", "left", KDF.Nh)
| = tree_node_[left(N)]_secret
|
+--> ExpandWithLabel(., "tree", "right", KDF.Nh)
= tree_node_[right(N)]_secret
Figure 25: Derivation of Secrets from Parent to Children within a
Secret Tree
The secret in the leaf of the secret tree is used to initiate two
symmetric hash ratchets, from which a sequence of single-use keys and
nonces are derived, as described in Section 9.1. The root of each
ratchet is computed as:
tree_node_[N]_secret
|
|
+--> ExpandWithLabel(., "handshake", "", KDF.Nh)
| = handshake_ratchet_secret_[N]_[0]
|
+--> ExpandWithLabel(., "application", "", KDF.Nh)
= application_ratchet_secret_[N]_[0]
Figure 26: Initialization of the Hash Ratchets from the Leaves of
a Secret Tree
9.1. Encryption Keys
As described in Section 6, MLS encrypts three different types of
information:
* Metadata (sender information)
* Handshake messages (Proposal and Commit)
* Application messages
The sender information used to look up the key for content encryption
is encrypted with an AEAD where the key and nonce are derived from
both sender_data_secret and a sample of the encrypted message
content.
For handshake and application messages, a sequence of keys is derived
via a "sender ratchet". Each sender has their own sender ratchet,
and each step along the ratchet is called a "generation".
The following figure shows a secret tree for a four-member group,
with the handshake and application ratchets that member D will use
for sending and the first two application keys and nonces.
G
|
.-+-.
/ \
E F
/ \ / \
A B C D
/ \
HR0 AR0--+--K0
| |
| +--N0
|
AR1--+--K1
| |
| +--N1
|
AR2
Figure 27: Secret Tree for a Four-Member Group
A sender ratchet starts from a per-sender base secret derived from a
Secret Tree, as described in Section 9. The base secret initiates a
symmetric hash ratchet, which generates a sequence of keys and
nonces. The sender uses the j-th key/nonce pair in the sequence to
encrypt (using the AEAD) the j-th message they send during that
epoch. Each key/nonce pair MUST NOT be used to encrypt more than one
message.
Keys, nonces, and the secrets in ratchets are derived using
DeriveTreeSecret. The context in a given call consists of the
current position in the ratchet.
DeriveTreeSecret(Secret, Label, Generation, Length) =
ExpandWithLabel(Secret, Label, Generation, Length)
Where Generation is encoded as a big endian uint32.
ratchet_secret_[N]_[j]
|
+--> DeriveTreeSecret(., "nonce", j, AEAD.Nn)
| = ratchet_nonce_[N]_[j]
|
+--> DeriveTreeSecret(., "key", j, AEAD.Nk)
| = ratchet_key_[N]_[j]
|
V
DeriveTreeSecret(., "secret", j, KDF.Nh)
= ratchet_secret_[N]_[j+1]
Here AEAD.Nn and AEAD.Nk denote the lengths in bytes of the nonce and
key for the AEAD scheme defined by the cipher suite.
9.2. Deletion Schedule
It is important to delete all security-sensitive values as soon as
they are _consumed_. A sensitive value S is said to be _consumed_ if:
* S was used to encrypt or (successfully) decrypt a message, or
* a key, nonce, or secret derived from S has been consumed. (This
goes for values derived via DeriveSecret as well as
ExpandWithLabel.)
Here S may be the init_secret, commit_secret, epoch_secret, or
encryption_secret as well as any secret in a secret tree or one of
the ratchets.
As soon as a group member consumes a value, they MUST immediately
delete (all representations of) that value. This is crucial to
ensuring forward secrecy for past messages. Members MAY keep
unconsumed values around for some reasonable amount of time to handle
out-of-order message delivery.
For example, suppose a group member encrypts or (successfully)
decrypts an application message using the j-th key and nonce in the
ratchet of leaf node L in some epoch n. Then, for that member, at
least the following values have been consumed and MUST be deleted:
* the commit_secret, joiner_secret, epoch_secret, and
encryption_secret of that epoch n as well as the init_secret of
the previous epoch n-1,
* all node secrets in the secret tree on the path from the root to
the leaf with node L,
* the first j secrets in the application data ratchet of node L, and
* application_ratchet_nonce_[L]_[j] and
application_ratchet_key_[L]_[j].
Concretely, consider the secret tree shown in Figure 27. Client A,
B, or C would generate the illustrated values on receiving a message
from D with generation equal to 1, having not received a message with
generation 0 (e.g., due to out-of-order delivery). In such a case,
the following values would be consumed:
* The key K1 and nonce N1 used to decrypt the message
* The application ratchet secrets AR1 and AR0
* The tree secrets D, F, and G (recall that G is the
encryption_secret for the epoch)
* The epoch_secret, commit_secret, psk_secret, and joiner_secret for
the current epoch
Other values may be retained (not consumed):
* K0 and N0 for decryption of an out-of-order message with
generation 0
* AR2 for derivation of further message decryption keys and nonces
* HR0 for protection of handshake messages from D
* E and C for deriving secrets used by senders A, B, and C
10. Key Packages
In order to facilitate the asynchronous addition of clients to a
group, clients can pre-publish KeyPackage objects that provide some
public information about a user. A KeyPackage object specifies:
1. a protocol version and cipher suite that the client supports,
2. a public key that others can use to encrypt a Welcome message to
this client (an "init key"), and
3. the content of the leaf node that should be added to the tree to
represent this client.
KeyPackages are intended to be used only once and SHOULD NOT be
reused except in the case of a "last resort" KeyPackage (see
Section 16.8). Clients MAY generate and publish multiple KeyPackages
to support multiple cipher suites.
The value for init_key MUST be a public key for the asymmetric
encryption scheme defined by cipher_suite, and it MUST be unique
among the set of KeyPackages created by this client. Likewise, the
leaf_node field MUST be valid for the cipher suite, including both
the encryption_key and signature_key fields. The whole structure is
signed using the client's signature key. A KeyPackage object with an
invalid signature field MUST be considered malformed.
The signature is computed by the function SignWithLabel with a label
"KeyPackageTBS" and a Content input comprising all of the fields
except for the signature field.
struct {
ProtocolVersion version;
CipherSuite cipher_suite;
HPKEPublicKey init_key;
LeafNode leaf_node;
Extension extensions<V>;
/* SignWithLabel(., "KeyPackageTBS", KeyPackageTBS) */
opaque signature<V>;
} KeyPackage;
struct {
ProtocolVersion version;
CipherSuite cipher_suite;
HPKEPublicKey init_key;
LeafNode leaf_node;
Extension extensions<V>;
} KeyPackageTBS;
If a client receives a KeyPackage carried within an MLSMessage
object, then it MUST verify that the version field of the KeyPackage
has the same value as the version field of the MLSMessage. The
version field in the KeyPackage provides an explicit signal of the
intended version to the other members of group when they receive the
KeyPackage in an Add proposal.
The field leaf_node.capabilities indicates what protocol versions,
cipher suites, credential types, and non-default proposal/extension
types are supported by the client. (As discussed in Section 7.2,
some proposal and extension types defined in this document are
considered "default" and thus are not listed.) This information
allows MLS session establishment to be safe from downgrade attacks on
the parameters described (as discussed in Section 11), while still
only advertising one version and one cipher suite per KeyPackage.
The field leaf_node.leaf_node_source of the LeafNode in a KeyPackage
MUST be set to key_package.
Extensions included in the extensions or leaf_node.extensions fields
MUST be included in the leaf_node.capabilities field. As discussed
in Section 13, unknown extensions in KeyPackage.extensions MUST be
ignored, and the creator of a KeyPackage object SHOULD include some
random GREASE extensions to help ensure that other clients correctly
ignore unknown extensions.
10.1. KeyPackage Validation
The validity of a KeyPackage needs to be verified at a few stages:
* When a KeyPackage is downloaded by a group member, before it is
used to add the client to the group
* When a KeyPackage is received by a group member in an Add message
The client verifies the validity of a KeyPackage using the following
steps:
* Verify that the cipher suite and protocol version of the
KeyPackage match those in the GroupContext.
* Verify that the leaf_node of the KeyPackage is valid for a
KeyPackage according to Section 7.3.
* Verify that the signature on the KeyPackage is valid using the
public key in leaf_node.credential.
* Verify that the value of leaf_node.encryption_key is different
from the value of the init_key field.
11. Group Creation
A group is always created with a single member, the "creator". Other
members are then added to the group using the usual Add/Commit
mechanism.
The creator of a group is responsible for setting the group ID,
cipher suite, and initial extensions for the group. If the creator
intends to add other members at the time of creation, then it SHOULD
fetch KeyPackages for the members to be added, and select a cipher
suite and extensions according to the capabilities of the members.
To protect against downgrade attacks, the creator MUST use the
capabilities information in these KeyPackages to verify that the
chosen version and cipher suite is the best option supported by all
members.
Group IDs SHOULD be constructed in such a way that there is an
overwhelmingly low probability of honest group creators generating
the same group ID, even without assistance from the Delivery Service.
This can be done, for example, by making the group ID a freshly
generated random value of size KDF.Nh. The Delivery Service MAY
attempt to ensure that group IDs are globally unique by rejecting the
creation of new groups with a previously used ID.
To initialize a group, the creator of the group MUST take the
following steps:
* Initialize a one-member group with the following initial values:
- Ratchet tree: A tree with a single node, a leaf node containing
an HPKE public key and credential for the creator
- Group ID: A value set by the creator
- Epoch: 0
- Tree hash: The root hash of the above ratchet tree
- Confirmed transcript hash: The zero-length octet string
- Epoch secret: A fresh random value of size KDF.Nh
- Extensions: Any values of the creator's choosing
* Calculate the interim transcript hash:
- Derive the confirmation_key for the epoch as described in
Section 8.
- Compute a confirmation_tag over the empty
confirmed_transcript_hash using the confirmation_key as
described in Section 6.1.
- Compute the updated interim_transcript_hash from the
confirmed_transcript_hash and the confirmation_tag as described
in Section 8.2.
At this point, the creator's state represents a one-member group with
a fully initialized key schedule, transcript hashes, etc. Proposals
and Commits can be generated for this group state just like any other
state of the group, such as Add proposals and Commits to add other
members to the group. A GroupInfo object for this group state can
also be published to facilitate external joins.
Members other than the creator join either by being sent a Welcome
message (as described in Section 12.4.3.1) or by sending an external
Commit (see Section 12.4.3.2).
In principle, the above process could be streamlined by having the
creator directly create a tree and choose a random value for first
epoch's epoch secret. We follow the steps above because it removes
unnecessary choices, by which, for example, bad randomness could be
introduced. The only choices the creator makes here are its own
KeyPackage and the leaf secret from which the Commit is built.
11.1. Required Capabilities
The configuration of a group imposes certain requirements on clients
in the group. At a minimum, all members of the group need to support
the cipher suite and protocol version in use. Additional
requirements can be imposed by including a required_capabilities
extension in the GroupContext.
struct {
ExtensionType extension_types<V>;
ProposalType proposal_types<V>;
CredentialType credential_types<V>;
} RequiredCapabilities;
This extension lists the extensions, proposals, and credential types
that must be supported by all members of the group. The "default"
proposal and extension types defined in this document are assumed to
be implemented by all clients, and need not be listed in
RequiredCapabilities in order to be safely used. Note that this is
not true for credential types.
For new members, support for required capabilities is enforced by
existing members during the application of Add commits. Existing
members should of course be in compliance already. In order to
ensure this continues to be the case even as the group's extensions
are updated, a GroupContextExtensions proposal is deemed invalid if
it contains a required_capabilities extension that requires non-
default capabilities not supported by all current members.
11.2. Reinitialization
A group may be reinitialized by creating a new group with the same
membership and different parameters, and linking it to the old group
via a resumption PSK. The members of a group reinitialize it using
the following steps:
1. A member of the old group sends a ReInit proposal (see
Section 12.1.5).
2. A member of the old group sends a Commit covering the ReInit
proposal.
3. A member of the old group creates an initial Commit that sets up
a new group that matches the ReInit and sends a Welcome message:
* The version, cipher_suite, group_id, and extensions fields of
the GroupContext object in the Welcome message MUST be the
same as the corresponding fields in the ReInit proposal. The
epoch in the Welcome message MUST be 1.
* The Welcome message MUST specify a PreSharedKeyID of type
resumption with usage reinit, where the group_id field matches
the old group and the epoch field indicates the epoch after
the Commit covering the ReInit.
Note that these three steps may be done by the same group member or
different members. For example, if a group member sends a Commit
with an inline ReInit proposal (steps 1 and 2) but then goes offline,
another group member may recreate the group instead. This
flexibility avoids situations where a group gets stuck between steps
2 and 3.
Resumption PSKs with usage reinit MUST NOT be used in other contexts.
A PreSharedKey proposal with type resumption and usage reinit MUST be
considered invalid.
11.3. Subgroup Branching
A new group can be formed from a subset of an existing group's
members, using the same parameters as the old group.
A member can create a subgroup by performing the following steps:
1. Fetch a new KeyPackage for each group member that should be
included in the subgroup.
2. Create an initial Commit message that sets up the new group and
contains a PreSharedKey proposal of type resumption with usage
branch. To avoid key reuse, the psk_nonce included in the
PreSharedKeyID object MUST be a randomly sampled nonce of length
KDF.Nh.
3. Send the corresponding Welcome message to the subgroup members.
A client receiving a Welcome message including a PreSharedKey of type
resumption with usage branch MUST verify that the new group reflects
a subgroup branched from the referenced group by checking that:
* The version and cipher_suite values in the Welcome message are the
same as those used by the old group.
* The epoch in the Welcome message MUST be 1.
* Each LeafNode in a new subgroup MUST match some LeafNode in the
original group. In this context, a pair of LeafNodes is said to
"match" if the identifiers presented by their respective
credentials are considered equivalent by the application.
Resumption PSKs with usage branch MUST NOT be used in other contexts.
A PreSharedKey proposal with type resumption and usage branch MUST be
considered invalid.
12. Group Evolution
Over the lifetime of a group, its membership can change, and existing
members might want to change their keys in order to achieve post-
compromise security. In MLS, each such change is accomplished by a
two-step process:
1. A proposal to make the change is broadcast to the group in a
Proposal message.
2. A member of the group or a new member broadcasts a Commit message
that causes one or more proposed changes to enter into effect.
In cases where the Proposal and Commit are sent by the same member,
these two steps can be combined by sending the proposals in the
commit.
The group thus evolves from one cryptographic state to another each
time a Commit message is sent and processed. These states are
referred to as "epochs" and are uniquely identified among states of
the group by eight-octet epoch values. When a new group is
initialized, its initial state epoch is 0x0000000000000000. Each
time a state transition occurs, the epoch number is incremented by
one.
12.1. Proposals
Proposals are included in a FramedContent by way of a Proposal
structure that indicates their type:
// See the "MLS Proposal Types" IANA registry for values
uint16 ProposalType;
struct {
ProposalType proposal_type;
select (Proposal.proposal_type) {
case add: Add;
case update: Update;
case remove: Remove;
case psk: PreSharedKey;
case reinit: ReInit;
case external_init: ExternalInit;
case group_context_extensions: GroupContextExtensions;
};
} Proposal;
On receiving a FramedContent containing a Proposal, a client MUST
verify the signature inside FramedContentAuthData and that the epoch
field of the enclosing FramedContent is equal to the epoch field of
the current GroupContext object. If the verification is successful,
then the Proposal should be cached in such a way that it can be
retrieved by hash (as a ProposalOrRef object) in a later Commit
message.
12.1.1. Add
An Add proposal requests that a client with a specified KeyPackage be
added to the group.
struct {
KeyPackage key_package;
} Add;
An Add proposal is invalid if the KeyPackage is invalid according to
Section 10.1.
An Add is applied after being included in a Commit message. The
position of the Add in the list of proposals determines the leaf node
where the new member will be added. For the first Add in the Commit,
the corresponding new member will be placed in the leftmost empty
leaf in the tree, for the second Add, the next empty leaf to the
right, etc. If no empty leaf exists, the tree is extended to the
right.
* Identify the leaf L for the new member: if there are empty leaves
in the tree, L is the leftmost empty leaf. Otherwise, the tree is
extended to the right as described in Section 7.7, and L is
assigned the leftmost new blank leaf.
* For each non-blank intermediate node along the path from the leaf
L to the root, add L's leaf index to the unmerged_leaves list for
the node.
* Set the leaf node L to a new node containing the LeafNode object
carried in the leaf_node field of the KeyPackage in the Add.
12.1.2. Update
An Update proposal is a similar mechanism to Add with the distinction
that it replaces the sender's LeafNode in the tree instead of adding
a new leaf to the tree.
struct {
LeafNode leaf_node;
} Update;
An Update proposal is invalid if the LeafNode is invalid for an
Update proposal according to Section 7.3.
A member of the group applies an Update message by taking the
following steps:
* Replace the sender's LeafNode with the one contained in the Update
proposal.
* Blank the intermediate nodes along the path from the sender's leaf
to the root.
12.1.3. Remove
A Remove proposal requests that the member with the leaf index
removed be removed from the group.
struct {
uint32 removed;
} Remove;
A Remove proposal is invalid if the removed field does not identify a
non-blank leaf node.
A member of the group applies a Remove message by taking the
following steps:
* Identify the leaf node matching removed. Let L be this leaf node.
* Replace the leaf node L with a blank node.
* Blank the intermediate nodes along the path from L to the root.
* Truncate the tree by removing the right subtree until there is at
least one non-blank leaf node in the right subtree. If the
rightmost non-blank leaf has index L, then this will result in the
tree having 2^d leaves, where d is the smallest value such that
2^d > L.
12.1.4. PreSharedKey
A PreSharedKey proposal can be used to request that a pre-shared key
be injected into the key schedule in the process of advancing the
epoch.
struct {
PreSharedKeyID psk;
} PreSharedKey;
A PreSharedKey proposal is invalid if any of the following is true:
* The PreSharedKey proposal is not being processed as part of a
reinitialization of the group (see Section 11.2), and the
PreSharedKeyID has psktype set to resumption and usage set to
reinit.
* The PreSharedKey proposal is not being processed as part of a
subgroup branching operation (see Section 11.3), and the
PreSharedKeyID has psktype set to resumption and usage set to
branch.
* The psk_nonce is not of length KDF.Nh.
The psk_nonce MUST be randomly sampled. When processing a Commit
message that includes one or more PreSharedKey proposals, group
members derive psk_secret as described in Section 8.4, where the
order of the PSKs corresponds to the order of the PreSharedKey
proposals in the Commit.
12.1.5. ReInit
A ReInit proposal represents a request to reinitialize the group with
different parameters, for example, to increase the version number or
to change the cipher suite. The reinitialization is done by creating
a completely new group and shutting down the old one.
struct {
opaque group_id<V>;
ProtocolVersion version;
CipherSuite cipher_suite;
Extension extensions<V>;
} ReInit;
A ReInit proposal is invalid if the version field is less than the
version for the current group.
A member of the group applies a ReInit proposal by waiting for the
committer to send the Welcome message that matches the ReInit,
according to the criteria in Section 11.2.
12.1.6. ExternalInit
An ExternalInit proposal is used by new members that want to join a
group by using an external commit. This proposal can only be used in
that context.
struct {
opaque kem_output<V>;
} ExternalInit;
A member of the group applies an ExternalInit message by initializing
the next epoch using an init secret computed as described in
Section 8.3. The kem_output field contains the required KEM output.
12.1.7. GroupContextExtensions
A GroupContextExtensions proposal is used to update the list of
extensions in the GroupContext for the group.
struct {
Extension extensions<V>;
} GroupContextExtensions;
A GroupContextExtensions proposal is invalid if it includes a
required_capabilities extension and some members of the group do not
support some of the required capabilities (including those added in
the same Commit, and excluding those removed).
A member of the group applies a GroupContextExtensions proposal with
the following steps:
* Remove all of the existing extensions from the GroupContext object
for the group and replace them with the list of extensions in the
proposal. (This is a wholesale replacement, not a merge. An
extension is only carried over if the sender of the proposal
includes it in the new list.)
Note that once the GroupContext is updated, its inclusion in the
confirmation_tag by way of the key schedule will confirm that all
members of the group agree on the extensions in use.
12.1.8. External Proposals
Proposals can be constructed and sent to the group by a party that is
outside the group in two cases. One case, indicated by the external
SenderType, allows an entity outside the group to submit proposals to
the group. For example, an automated service might propose removing
a member of a group who has been inactive for a long time, or propose
adding a newly hired staff member to a group representing a real-
world team. An external sender might send a ReInit proposal to
enforce a changed policy regarding MLS versions or cipher suites.
The external SenderType requires that signers are pre-provisioned to
the clients within a group and can only be used if the
external_senders extension is present in the group's GroupContext.
The other case, indicated by the new_member_proposal SenderType, is
useful when existing members of the group can independently verify
that an Add proposal sent by the new joiner itself (not an existing
member) is authorized. External proposals that are not authorized
are considered invalid.
An external proposal MUST be sent as a PublicMessage object, since
the sender will not have the keys necessary to construct a
PrivateMessage object.
Proposals of some types cannot be sent by an external sender. Among
the proposal types defined in this document, only the following types
may be sent by an external sender:
* add
* remove
* psk
* reinit
* group_context_extensions
Messages from external senders containing proposal types other than
the above MUST be rejected as malformed. New proposal types defined
in the future MUST define whether they may be sent by external
senders. The "Ext" column in the "MLS Proposal Types" registry
(Section 17.4) reflects this property.
12.1.8.1. External Senders Extension
The external_senders extension is a group context extension that
contains the credentials and signature keys of senders that are
permitted to send external proposals to the group.
struct {
SignaturePublicKey signature_key;
Credential credential;
} ExternalSender;
ExternalSender external_senders<V>;
12.2. Proposal List Validation
A group member creating a Commit and a group member processing a
Commit MUST verify that the list of committed proposals is valid
using one of the following procedures, depending on whether the
Commit is external or not. If the list of proposals is invalid, then
the Commit message MUST be rejected as invalid.
For a regular, i.e., not external, Commit, the list is invalid if any
of the following occurs:
* It contains an individual proposal that is invalid as specified in
Section 12.1.
* It contains an Update proposal generated by the committer.
* It contains a Remove proposal that removes the committer.
* It contains multiple Update and/or Remove proposals that apply to
the same leaf. If the committer has received multiple such
proposals they SHOULD prefer any Remove received, or the most
recent Update if there are no Removes.
* It contains multiple Add proposals that contain KeyPackages that
represent the same client according to the application (for
example, identical signature keys).
* It contains an Add proposal with a KeyPackage that represents a
client already in the group according to the application, unless
there is a Remove proposal in the list removing the matching
client from the group.
* It contains multiple PreSharedKey proposals that reference the
same PreSharedKeyID.
* It contains multiple GroupContextExtensions proposals.
* It contains a ReInit proposal together with any other proposal.
If the committer has received other proposals during the epoch,
they SHOULD prefer them over the ReInit proposal, allowing the
ReInit to be resent and applied in a subsequent epoch.
* It contains an ExternalInit proposal.
* It contains a Proposal with a non-default proposal type that is
not supported by some members of the group that will process the
Commit (i.e., members being added or removed by the Commit do not
need to support the proposal type).
* After processing the Commit the ratchet tree is invalid, in
particular, if it contains any leaf node that is invalid according
to Section 7.3.
An application may extend the above procedure by additional rules,
for example, requiring application-level permissions to add members,
or rules concerning non-default proposal types.
For an external Commit, the list is valid if it contains only the
following proposals (not necessarily in this order):
* Exactly one ExternalInit
* At most one Remove proposal, with which the joiner removes an old
version of themselves. If a Remove proposal is present, then the
LeafNode in the path field of the external Commit MUST meet the
same criteria as would the LeafNode in an Update for the removed
leaf (see Section 12.1.2). In particular, the credential in the
LeafNode MUST present a set of identifiers that is acceptable to
the application for the removed participant.
* Zero or more PreSharedKey proposals
* No other proposals
Proposal types defined in the future may make updates to the above
validation logic to incorporate considerations related to proposals
of the new type.
12.3. Applying a Proposal List
The sections above defining each proposal type describe how each
individual proposal is applied. When creating or processing a
Commit, a client applies a list of proposals to the ratchet tree and
GroupContext. The client MUST apply the proposals in the list in the
following order:
* If there is a GroupContextExtensions proposal, replace the
extensions field of the GroupContext for the group with the
contents of the proposal. The new extensions MUST be used when
evaluating other proposals in this list. For example, if a
GroupContextExtensions proposal adds a required_capabilities
extension, then any Add proposals need to indicate support for
those capabilities.
* Apply any Update proposals to the ratchet tree, in any order.
* Apply any Remove proposals to the ratchet tree, in any order.
* Apply any Add proposals to the ratchet tree, in the order they
appear in the list.
* Look up the PSK secrets for any PreSharedKey proposals, in the
order they appear in the list. These secrets are then used to
advance the key schedule later in Commit processing.
* If there is an ExternalInit proposal, use it to derive the
init_secret for use later in Commit processing.
* If there is a ReInit proposal, note its parameters for application
later in Commit processing.
Proposal types defined in the future MUST specify how the above steps
are to be adjusted to accommodate the application of proposals of the
new type.
12.4. Commit
A Commit message initiates a new epoch for the group, based on a
collection of Proposals. It instructs group members to update their
representation of the state of the group by applying the proposals
and advancing the key schedule.
Each proposal covered by the Commit is included by a ProposalOrRef
value, which identifies the proposal to be applied by value or by
reference. Commits that refer to new Proposals from the committer
can be included by value. Commits for previously sent proposals from
anyone (including the committer) can be sent by reference. Proposals
sent by reference are specified by including the hash of the
AuthenticatedContent object in which the proposal was sent (see
Section 5.2).
enum {
reserved(0),
proposal(1),
reference(2),
(255)
} ProposalOrRefType;
struct {
ProposalOrRefType type;
select (ProposalOrRef.type) {
case proposal: Proposal proposal;
case reference: ProposalRef reference;
};
} ProposalOrRef;
struct {
ProposalOrRef proposals<V>;
optional<UpdatePath> path;
} Commit;
A group member that has observed one or more valid proposals within
an epoch MUST send a Commit message before sending application data.
This ensures, for example, that any members whose removal was
proposed during the epoch are actually removed before any application
data is transmitted.
A sender and a receiver of a Commit MUST verify that the committed
list of proposals is valid as specified in Section 12.2. A list is
invalid if, for example, it includes an Update and a Remove for the
same member, or an Add when the sender does not have the application-
level permission to add new users.
The sender of a Commit SHOULD include all proposals that it has
received during the current epoch that are valid according to the
rules for their proposal types and according to application policy,
as long as this results in a valid proposal list.
Due to the asynchronous nature of proposals, receivers of a Commit
SHOULD NOT enforce that all valid proposals sent within the current
epoch are referenced by the next Commit. In the event that a valid
proposal is omitted from the next Commit, and that proposal is still
valid in the current epoch, the sender of the proposal MAY resend it
after updating it to reflect the current epoch.
A member of the group MAY send a Commit that references no proposals
at all, which would thus have an empty proposals vector. Such a
Commit resets the sender's leaf and the nodes along its direct path,
and provides forward secrecy and post-compromise security with regard
to the sender of the Commit. An Update proposal can be regarded as a
"lazy" version of this operation, where only the leaf changes and
intermediate nodes are blanked out.
By default, the path field of a Commit MUST be populated. The path
field MAY be omitted if (a) it covers at least one proposal and (b)
none of the proposals covered by the Commit are of "path required"
types. A proposal type requires a path if it cannot change the group
membership in a way that requires the forward secrecy and post-
compromise security guarantees that an UpdatePath provides. The only
proposal types defined in this document that do not require a path
are:
* add
* psk
* reinit
New proposal types MUST state whether they require a path. If any
instance of a proposal type requires a path, then the proposal type
requires a path. This attribute of a proposal type is reflected in
the "Path Required" field of the "MLS Proposal Types" registry
defined in Section 17.4.
Update and Remove proposals are the clearest examples of proposals
that require a path. An UpdatePath is required to evict the removed
member or the old appearance of the updated member.
In pseudocode, the logic for validating the path field of a Commit is
as follows:
pathRequiredTypes = [
update,
remove,
external_init,
group_context_extensions
]
pathRequired = false
for proposal in commit.proposals:
pathRequired = pathRequired ||
(proposal.msg_type in pathRequiredTypes)
if len(commit.proposals) == 0 || pathRequired:
assert(commit.path != null)
To summarize, a Commit can have three different configurations, with
different uses:
1. An "empty" Commit that references no proposals, which updates the
committer's contribution to the group and provides PCS with
regard to the committer.
2. A "partial" Commit that references proposals that do not require
a path, and where the path is empty. Such a Commit doesn't
provide PCS with regard to the committer.
3. A "full" Commit that references proposals of any type, which
provides FS with regard to any removed members and PCS for the
committer and any updated members.
12.4.1. Creating a Commit
When creating or processing a Commit, a client updates the ratchet
tree and GroupContext for the group. These values advance from an
"old" state reflecting the current epoch to a "new" state reflecting
the new epoch initiated by the Commit. When the Commit includes an
UpdatePath, a "provisional" group context is constructed that
reflects changes due to the proposals and UpdatePath, but with the
old confirmed transcript hash.
A member of the group creates a Commit message and the corresponding
Welcome message at the same time, by taking the following steps:
* Verify that the list of proposals to be committed is valid as
specified in Section 12.2.
* Construct an initial Commit object with the proposals field
populated from Proposals received during the current epoch, and
with the path field empty.
* Create the new ratchet tree and GroupContext by applying the list
of proposals to the old ratchet tree and GroupContext, as defined
in Section 12.3.
* Decide whether to populate the path field: If the path field is
required based on the proposals that are in the Commit (see
above), then it MUST be populated. Otherwise, the sender MAY omit
the path field at its discretion.
* If populating the path field:
- If this is an external Commit, assign the sender the leftmost
blank leaf node in the new ratchet tree. If there are no blank
leaf nodes in the new ratchet tree, expand the tree to the
right as defined in Section 7.7 and assign the leftmost new
blank leaf to the sender.
- Update the sender's direct path in the ratchet tree as
described in Section 7.5. Define commit_secret as the value
path_secret[n+1] derived from the last path secret value
(path_secret[n]) derived for the UpdatePath.
- Construct a provisional GroupContext object containing the
following values:
o group_id: Same as the old GroupContext
o epoch: The epoch number for the new epoch
o tree_hash: The tree hash of the new ratchet tree
o confirmed_transcript_hash: Same as the old GroupContext
o extensions: The new GroupContext extensions (possibly
updated by a GroupContextExtensions proposal)
- Encrypt the path secrets resulting from the tree update to the
group as described in Section 7.5, using the provisional group
context as the context for HPKE encryption.
- Create an UpdatePath containing the sender's new leaf node and
the new public keys and encrypted path secrets along the
sender's filtered direct path. Assign this UpdatePath to the
path field in the Commit.
* If not populating the path field: Set the path field in the Commit
to the null optional. Define commit_secret as the all-zero vector
of length KDF.Nh (the same length as a path_secret value would
be).
* Derive the psk_secret as specified in Section 8.4, where the order
of PSKs in the derivation corresponds to the order of PreSharedKey
proposals in the proposals vector.
* Construct a FramedContent object containing the Commit object.
Sign the FramedContent using the old GroupContext as context.
- Use the FramedContent to update the confirmed transcript hash
and update the new GroupContext.
- Use the init_secret from the previous epoch, the commit_secret
and psk_secret defined in the previous steps, and the new
GroupContext to compute the new joiner_secret, welcome_secret,
epoch_secret, and derived secrets for the new epoch.
- Use the confirmation_key for the new epoch to compute the
confirmation_tag value.
- Calculate the interim transcript hash using the new confirmed
transcript hash and the confirmation_tag from the
FramedContentAuthData.
* Protect the AuthenticatedContent object using keys from the old
epoch:
- If encoding as PublicMessage, compute the membership_tag value
using the membership_key.
- If encoding as a PrivateMessage, encrypt the message using the
sender_data_secret and the next (key, nonce) pair from the
sender's handshake ratchet.
* Construct a GroupInfo reflecting the new state:
- Set the group_id, epoch, tree, confirmed_transcript_hash,
interim_transcript_hash, and group_context_extensions fields to
reflect the new state.
- Set the confirmation_tag field to the value of the
corresponding field in the FramedContentAuthData object.
- Add any other extensions as defined by the application.
- Optionally derive an external key pair as described in
Section 8. (required for external Commits, see
Section 12.4.3.2).
- Sign the GroupInfo using the member's private signing key.
- Encrypt the GroupInfo using the key and nonce derived from the
joiner_secret. for the new epoch (see Section 12.4.3.1).
* For each new member in the group:
- Identify the lowest common ancestor in the tree of the new
member's leaf node and the member sending the Commit.
- If the path field was populated above: Compute the path secret
corresponding to the common ancestor node.
- Compute an EncryptedGroupSecrets object that encapsulates the
init_secret for the current epoch and the path secret (if
present).
* Construct one or more Welcome messages from the encrypted
GroupInfo object, the encrypted key packages, and any PSKs for
which a proposal was included in the Commit. The order of the
psks MUST be the same as the order of PreSharedKey proposals in
the proposals vector. As discussed in Section 12.4.3.1, the
committer is free to choose how many Welcome messages to
construct. However, the set of Welcome messages produced in this
step MUST cover every new member added in the Commit.
* If a ReInit proposal was part of the Commit, the committer MUST
create a new group with the parameters specified in the ReInit
proposal, and with the same members as the original group. The
Welcome message MUST include a PreSharedKeyID with the following
parameters:
- psktype: resumption
- usage: reinit
- group_id: The group ID for the current group
- epoch: The epoch that the group will be in after this Commit
12.4.2. Processing a Commit
A member of the group applies a Commit message by taking the
following steps:
* Verify that the epoch field of the enclosing FramedContent is
equal to the epoch field of the current GroupContext object.
* Unprotect the Commit using the keys from the current epoch:
- If the message is encoded as PublicMessage, verify the
membership MAC using the membership_key.
- If the message is encoded as PrivateMessage, decrypt the
message using the sender_data_secret and the (key, nonce) pair
from the step on the sender's hash ratchet indicated by the
generation field.
* Verify the signature on the FramedContent message as described in
Section 6.1.
* Verify that the proposals vector is valid according to the rules
in Section 12.2.
* Verify that all PreSharedKey proposals in the proposals vector are
available.
* Create the new ratchet tree and GroupContext by applying the list
of proposals to the old ratchet tree and GroupContext, as defined
in Section 12.3.
* Verify that the path value is populated if the proposals vector
contains any Update or Remove proposals, or if it's empty.
Otherwise, the path value MAY be omitted.
* If the path value is populated, validate it and apply it to the
tree:
- If this is an external Commit, assign the sender the leftmost
blank leaf node in the new ratchet tree. If there are no blank
leaf nodes in the new ratchet tree, add a blank leaf to the
right side of the new ratchet tree and assign it to the sender.
- Validate the LeafNode as specified in Section 7.3. The
leaf_node_source field MUST be set to commit.
- Verify that the encryption_key value in the LeafNode is
different from the committer's current leaf node.
- Verify that none of the public keys in the UpdatePath appear in
any node of the new ratchet tree.
- Merge the UpdatePath into the new ratchet tree, as described in
Section 7.5.
- Construct a provisional GroupContext object containing the
following values:
o group_id: Same as the old GroupContext
o epoch: The epoch number for the new epoch
o tree_hash: The tree hash of the new ratchet tree
o confirmed_transcript_hash: Same as the old GroupContext
o extensions: The new GroupContext extensions (possibly
updated by a GroupContextExtensions proposal)
- Decrypt the path secrets for UpdatePath as described in
Section 7.5, using the provisional GroupContext as the context
for HPKE decryption.
- Define commit_secret as the value path_secret[n+1] derived from
the last path secret value (path_secret[n]) derived for the
UpdatePath.
* If the path value is not populated, define commit_secret as the
all-zero vector of length KDF.Nh (the same length as a path_secret
value would be).
* Update the confirmed and interim transcript hashes using the new
Commit, and generate the new GroupContext.
* Derive the psk_secret as specified in Section 8.4, where the order
of PSKs in the derivation corresponds to the order of PreSharedKey
proposals in the proposals vector.
* Use the init_secret from the previous epoch, the commit_secret and
psk_secret defined in the previous steps, and the new GroupContext
to compute the new joiner_secret, welcome_secret, epoch_secret,
and derived secrets for the new epoch.
* Use the confirmation_key for the new epoch to compute the
confirmation tag for this message, as described below, and verify
that it is the same as the confirmation_tag field in the
FramedContentAuthData object.
* If the above checks are successful, consider the new GroupContext
object as the current state of the group.
* If the Commit included a ReInit proposal, the client MUST NOT use
the group to send messages anymore. Instead, it MUST wait for a
Welcome message from the committer meeting the requirements of
Section 11.2.
Note that clients need to be prepared to receive a valid Commit
message that removes them from the group. In this case, the client
cannot send any more messages in the group and SHOULD promptly delete
its group state and secret tree. (A client might keep the secret
tree for a short time to decrypt late messages in the previous
epoch.)
12.4.3. Adding Members to the Group
New members can join the group in two ways: by being added by a group
member or by adding themselves through an external Commit. In both
cases, the new members need information to bootstrap their local
group state.
struct {
GroupContext group_context;
Extension extensions<V>;
MAC confirmation_tag;
uint32 signer;
/* SignWithLabel(., "GroupInfoTBS", GroupInfoTBS) */
opaque signature<V>;
} GroupInfo;
The group_context field represents the current state of the group.
The extensions field allows the sender to provide additional data
that might be useful to new joiners. The confirmation_tag represents
the confirmation tag from the Commit that initiated the current
epoch, or for epoch 0, the confirmation tag computed in the creation
of the group (see Section 11). (In either case, the creator of a
GroupInfo may recompute the confirmation tag as MAC(confirmation_key,
confirmed_transcript_hash).)
As discussed in Section 13, unknown extensions in
GroupInfo.extensions MUST be ignored, and the creator of a GroupInfo
object SHOULD include some random GREASE extensions to help ensure
that other clients correctly ignore unknown extensions. Extensions
in GroupInfo.group_context.extensions, however, MUST be supported by
the new joiner.
New members MUST verify that group_id is unique among the groups they
are currently participating in.
New members also MUST verify the signature using the public key taken
from the leaf node of the ratchet tree with leaf index signer. The
signature covers the following structure, comprising all the fields
in the GroupInfo above signature:
struct {
GroupContext group_context;
Extension extensions<V>;
MAC confirmation_tag;
uint32 signer;
} GroupInfoTBS;
12.4.3.1. Joining via Welcome Message
The sender of a Commit message is responsible for sending a Welcome
message to each new member added via Add proposals. The format of
the Welcome message allows a single Welcome message to be encrypted
for multiple new members. It is up to the committer to decide how
many Welcome messages to create for a given Commit. The committer
could create one Welcome that is encrypted for all new members, a
different Welcome for each new member, or Welcome messages for
batches of new members (according to some batching scheme that works
well for the application). The processes for creating and processing
the Welcome are the same in all cases, aside from the set of new
members for whom a given Welcome is encrypted.
The Welcome message provides the new members with the current state
of the group after the application of the Commit message. The new
members will not be able to decrypt or verify the Commit message, but
they will have the secrets they need to participate in the epoch
initiated by the Commit message.
In order to allow the same Welcome message to be sent to multiple new
members, information describing the group is encrypted with a
symmetric key and nonce derived from the joiner_secret for the new
epoch. The joiner_secret is then encrypted to each new member using
HPKE. In the same encrypted package, the committer transmits the
path secret for the lowest (closest to the leaf) node that is
contained in the direct paths of both the committer and the new
member. This allows the new member to compute private keys for nodes
in its direct path that are being reset by the corresponding Commit.
If the sender of the Welcome message wants the receiving member to
include a PSK in the derivation of the epoch_secret, they can
populate the psks field indicating which PSK to use.
struct {
opaque path_secret<V>;
} PathSecret;
struct {
opaque joiner_secret<V>;
optional<PathSecret> path_secret;
PreSharedKeyID psks<V>;
} GroupSecrets;
struct {
KeyPackageRef new_member;
HPKECiphertext encrypted_group_secrets;
} EncryptedGroupSecrets;
struct {
CipherSuite cipher_suite;
EncryptedGroupSecrets secrets<V>;
opaque encrypted_group_info<V>;
} Welcome;
The client processing a Welcome message will need to have a copy of
the group's ratchet tree. The tree can be provided in the Welcome
message, in an extension of type ratchet_tree. If it is sent
otherwise (e.g., provided by a caching service on the Delivery
Service), then the client MUST download the tree before processing
the Welcome.
On receiving a Welcome message, a client processes it using the
following steps:
* Identify an entry in the secrets array where the new_member value
corresponds to one of this client's KeyPackages, using the hash
indicated by the cipher_suite field. If no such field exists, or
if the cipher suite indicated in the KeyPackage does not match the
one in the Welcome message, return an error.
* Decrypt the encrypted_group_secrets value with the algorithms
indicated by the cipher suite and the private key init_key_priv
corresponding to init_key in the referenced KeyPackage.
encrypted_group_secrets =
EncryptWithLabel(init_key, "Welcome",
encrypted_group_info, group_secrets)
group_secrets =
DecryptWithLabel(init_key_priv, "Welcome",
encrypted_group_info, kem_output, ciphertext)
* If a PreSharedKeyID is part of the GroupSecrets and the client is
not in possession of the corresponding PSK, return an error.
Additionally, if a PreSharedKeyID has type resumption with usage
reinit or branch, verify that it is the only such PSK.
* From the joiner_secret in the decrypted GroupSecrets object and
the PSKs specified in the GroupSecrets, derive the welcome_secret
and then the welcome_key and welcome_nonce. Use the key and nonce
to decrypt the encrypted_group_info field.
welcome_nonce = ExpandWithLabel(welcome_secret, "nonce", "", AEAD.Nn)
welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk)
* Verify the signature on the GroupInfo object. The signature input
comprises all of the fields in the GroupInfo object except the
signature field. The public key is taken from the LeafNode of the
ratchet tree with leaf index signer. If the node is blank or if
signature verification fails, return an error.
* Verify that the group_id is unique among the groups that the
client is currently participating in.
* Verify that the cipher_suite in the GroupInfo matches the
cipher_suite in the KeyPackage.
* Verify the integrity of the ratchet tree.
- Verify that the tree hash of the ratchet tree matches the
tree_hash field in GroupInfo.
- For each non-empty parent node, verify that it is "parent-hash
valid", as described in Section 7.9.2.
- For each non-empty leaf node, validate the LeafNode as
described in Section 7.3.
- For each non-empty parent node and each entry in the node's
unmerged_leaves field:
o Verify that the entry represents a non-blank leaf node that
is a descendant of the parent node.
o Verify that every non-blank intermediate node between the
leaf node and the parent node also has an entry for the leaf
node in its unmerged_leaves.
o Verify that the encryption key in the parent node does not
appear in any other node of the tree.
* Identify a leaf whose LeafNode is identical to the one in the
KeyPackage. If no such field exists, return an error. Let
my_leaf represent this leaf in the tree.
* Construct a new group state using the information in the GroupInfo
object.
- Initialize the GroupContext for the group from the
group_context field from the GroupInfo object.
- Update the leaf my_leaf with the private key corresponding to
the public key in the node, where my_leaf is the new member's
leaf node in the ratchet tree, as defined above.
- If the path_secret value is set in the GroupSecrets object:
Identify the lowest common ancestor of the leaf node my_leaf
and of the node of the member with leaf index GroupInfo.signer.
Set the private key for this node to the private key derived
from the path_secret.
- For each parent of the common ancestor, up to the root of the
tree, derive a new path secret, and set the private key for the
node to the private key derived from the path secret. The
private key MUST be the private key that corresponds to the
public key in the node.
* Use the joiner_secret from the GroupSecrets object to generate the
epoch secret and other derived secrets for the current epoch.
* Set the confirmed transcript hash in the new state to the value of
the confirmed_transcript_hash in the GroupInfo.
* Verify the confirmation tag in the GroupInfo using the derived
confirmation key and the confirmed_transcript_hash from the
GroupInfo.
* Use the confirmed transcript hash and confirmation tag to compute
the interim transcript hash in the new state.
* If a PreSharedKeyID was used that has type resumption with usage
reinit or branch, verify that the epoch field in the GroupInfo is
equal to 1.
- For usage reinit, verify that the last Commit to the referenced
group contains a ReInit proposal and that the group_id,
version, cipher_suite, and group_context.extensions fields of
the GroupInfo match the ReInit proposal. Additionally, verify
that all the members of the old group are also members of the
new group, according to the application.
- For usage branch, verify that the version and cipher_suite of
the new group match those of the old group, and that the
members of the new group compose a subset of the members of the
old group, according to the application.
12.4.3.2. Joining via External Commits
External Commits are a mechanism for new members (external parties
that want to become members of the group) to add themselves to a
group, without requiring that an existing member has to come online
to issue a Commit that references an Add proposal.
Whether existing members of the group will accept or reject an
external Commit follows the same rules that are applied to other
handshake messages.
New members can create and issue an external Commit if they have
access to the following information for the group's current epoch:
* group ID
* epoch ID
* cipher suite
* public tree hash
* confirmed transcript hash
* confirmation tag of the most recent Commit
* group extensions
* external public key
In other words, to join a group via an external Commit, a new member
needs a GroupInfo with an external_pub extension present in its
extensions field.
struct {
HPKEPublicKey external_pub;
} ExternalPub;
Thus, a member of the group can enable new clients to join by making
a GroupInfo object available to them. Note that because a GroupInfo
object is specific to an epoch, it will need to be updated as the
group advances. In particular, each GroupInfo object can be used for
one external join, since that external join will cause the epoch to
change.
Note that the tree_hash field is used the same way as in the Welcome
message. The full tree can be included via the ratchet_tree
extension (see Section 12.4.3.3).
The information in a GroupInfo is not generally public information,
but applications can choose to make it available to new members in
order to allow External Commits.
In principle, external Commits work like regular Commits. However,
their content has to meet a specific set of requirements:
* External Commits MUST contain a path field (and is therefore a
"full" Commit). The joiner is added at the leftmost free leaf
node (just as if they were added with an Add proposal), and the
path is calculated relative to that leaf node.
* The Commit MUST NOT include any proposals by reference, since an
external joiner cannot determine the validity of proposals sent
within the group.
* External Commits MUST be signed by the new member. In particular,
the signature on the enclosing AuthenticatedContent MUST verify
using the public key for the credential in the leaf_node of the
path field.
* When processing a Commit, both existing and new members MUST use
the external init secret as described in Section 8.3.
* The sender type for the AuthenticatedContent encapsulating the
external Commit MUST be new_member_commit.
External Commits come in two "flavors" -- a "join" Commit that adds
the sender to the group or a "resync" Commit that replaces a member's
prior appearance with a new one.
Note that the "resync" operation allows an attacker that has
compromised a member's signature private key to introduce themselves
into the group and remove the prior, legitimate member in a single
Commit. Without resync, this can still be done, but it requires two
operations: the external Commit to join and a second Commit to remove
the old appearance. Applications for whom this distinction is
salient can choose to disallow external commits that contain a
Remove, or to allow such resync commits only if they contain a
"reinit" PSK proposal that demonstrates the joining member's presence
in a prior epoch of the group. With the latter approach, the
attacker would need to compromise the PSK as well as the signing key,
but the application will need to ensure that continuing, non-
resynchronizing members have the required PSK.
12.4.3.3. Ratchet Tree Extension
By default, a GroupInfo message only provides the joiner with a hash
of the group's ratchet tree. In order to process or generate
handshake messages, the joiner will need to get a copy of the ratchet
tree from some other source. (For example, the DS might provide a
cached copy.) The inclusion of the tree hash in the GroupInfo
message means that the source of the ratchet tree need not be trusted
to maintain the integrity of the tree.
In cases where the application does not wish to provide such an
external source, the whole public state of the ratchet tree can be
provided in an extension of type ratchet_tree, containing a
ratchet_tree object of the following form:
struct {
NodeType node_type;
select (Node.node_type) {
case leaf: LeafNode leaf_node;
case parent: ParentNode parent_node;
};
} Node;
optional<Node> ratchet_tree<V>;
Each entry in the ratchet_tree vector provides the value for a node
in the tree, or the null optional for a blank node.
The nodes are listed in the order specified by a left-to-right in-
order traversal of the ratchet tree. Each node is listed between its
left subtree and its right subtree. (This is the same ordering as
specified for the array-based trees outlined in Appendix C.)
If the tree has 2^d leaves, then it has 2^(d+1) - 1 nodes. The
ratchet_tree vector logically has this number of entries, but the
sender MUST NOT include blank nodes after the last non-blank node.
The receiver MUST check that the last node in ratchet_tree is non-
blank, and then extend the tree to the right until it has a length of
the form 2^(d+1) - 1, adding the minimum number of blank values
possible. (Obviously, this may be done "virtually", by synthesizing
blank nodes when required, as opposed to actually changing the
structure in memory.)
The leaves of the tree are stored in even-numbered entries in the
array (the leaf with index L in array position 2*L). The root node
of the tree is at position 2^d - 1 of the array. Intermediate parent
nodes can be identified by performing the same calculation to the
subarrays to the left and right of the root, following something like
the following algorithm:
# Assuming a class Node that has left and right members
def subtree_root(nodes):
# If there is only one node in the array, return it
if len(nodes) == 1:
return Node(nodes[0])
# Otherwise, the length of the array MUST be odd
if len(nodes) % 2 == 0:
raise Exception("Malformed node array {}", len(nodes))
# Identify the root of the subtree
d = 0
while (2**(d+1)) < len(nodes):
d += 1
R = 2**d - 1
root = Node(nodes[R])
root.left = subtree_root(nodes[:R])
root.right = subtree_root(nodes[(R+1):])
return root
(Note that this is the same ordering of nodes as in the array-based
tree representation described in Appendix C. The algorithms in that
section may be used to simplify decoding this extension into other
representations.)
For example, the following tree with six non-blank leaves would be
represented as an array of eleven elements, [A, W, B, X, C, _, D, Y,
E, Z, F]. The above decoding procedure would identify the subtree
roots as follows (using R to represent a subtree root):
Y
|
.-----+-----.
/ \
X _
| |
.-+-. .-+-.
/ \ / \
W _ Z _
/ \ / \ / \ / \
A B C D E F _ _
1
0 1 2 3 4 5 6 7 8 9 0
<-----------> R <----------->
<---> R <---> <---> R <--->
- R - - R - - R - - R -
Figure 28: Left-to-Right In-Order Traversal of a Six-Member Tree
The presence of a ratchet_tree extension in a GroupInfo message does
not result in any changes to the GroupContext extensions for the
group. The ratchet tree provided is simply stored by the client and
used for MLS operations.
If this extension is not provided in a Welcome message, then the
client will need to fetch the ratchet tree over some other channel
before it can generate or process Commit messages. Applications
should ensure that this out-of-band channel is provided with security
protections equivalent to the protections that are afforded to
Proposal and Commit messages. For example, an application that
encrypts Proposal and Commit messages might distribute ratchet trees
encrypted using a key exchanged over the MLS channel.
Regardless of how the client obtains the tree, the client MUST verify
that the root hash of the ratchet tree matches the tree_hash of the
GroupContext before using the tree for MLS operations.
13. Extensibility
The base MLS protocol can be extended in a few ways. New cipher
suites can be added to enable the use of new cryptographic
algorithms. New types of proposals can be used to perform new
actions within an epoch. Extension fields can be used to add
additional information to the protocol. In this section, we discuss
some constraints on these extensibility mechanisms that are necessary
to ensure broad interoperability.
13.1. Additional Cipher Suites
As discussed in Section 5.1, MLS allows the participants in a group
to negotiate the cryptographic algorithms used within the group.
This extensibility is important for maintaining the security of the
protocol over time [RFC7696]. It also creates a risk of
interoperability failure due to clients not supporting a common
cipher suite.
The cipher suite registry defined in Section 17.1 attempts to strike
a balance on this point. On the one hand, the base policy for the
registry is Specification Required, a fairly low bar designed to
avoid the need for standards work in cases where different ciphers
are needed for niche applications. On the other hand, there is a
higher bar (Standards Action) for ciphers to set the Recommended
field in the registry. This higher bar is there in part to ensure
that the interoperability implications of new cipher suites are
considered.
MLS cipher suites are defined independent of MLS versions, so that in
principle, the same cipher suite can be used across versions.
Standards work defining new versions of MLS should consider whether
it is desirable for the new version to be compatible with existing
cipher suites, or whether the new version should rule out some cipher
suites. For example, a new version could follow the example of
HTTP/2, which restricted the set of allowed TLS ciphers (see
Section 9.2.2 of [RFC9113]).
13.2. Proposals
Commit messages do not have an extension field because the set of
proposals is extensible. As discussed in Section 12.4, Proposals
with a non-default proposal type MUST NOT be included in a commit
unless the proposal type is supported by all the members of the group
that will process the Commit.
13.3. Credential Extensibility
In order to ensure that MLS provides meaningful authentication, it is
important that each member is able to authenticate some identity
information for each other member. Identity information is encoded
in Credentials, so this property is provided by ensuring that members
use compatible credential types.
The only types of credential that may be used in a group are those
that all members of the group support, as specified by the
capabilities field of each LeafNode in the ratchet tree. An
application can introduce new credential types by choosing an
unallocated identifier from the registry in Section 17.5 and
indicating support for the credential type in published LeafNodes,
whether in Update proposals to existing groups or KeyPackages that
are added to new groups. Once all members in a group indicate
support for the credential type, members can start using LeafNodes
with the new credential. Application may enforce that certain
credential types always remain supported by adding a
required_capabilities extension to the group's GroupContext, which
would prevent any member from being added to the group that doesn't
support them.
In future extensions to MLS, it may be useful to allow a member to
present more than one credential. For example, such credentials
might present different attributes attested by different authorities.
To be consistent with the general principle stated at the beginning
of this section, such an extension would need to ensure that each
member can authenticate some identity for each other member. For
each pair of members (Alice, Bob), Alice would need to present at
least one credential of a type that Bob supports.
13.4. Extensions
This protocol includes a mechanism for negotiating extension
parameters similar to the one in TLS [RFC8446]. In TLS, extension
negotiation is one-to-one: The client offers extensions in its
ClientHello message, and the server expresses its choices for the
session with extensions in its ServerHello and EncryptedExtensions
messages. In MLS, extensions appear in the following places:
* In KeyPackages, to describe additional information related to the
client
* In LeafNodes, to describe additional information about the client
or its participation in the group (once in the ratchet tree)
* In the GroupInfo, to tell new members of a group what parameters
are being used by the group, and to provide any additional details
required to join the group
* In the GroupContext object, to ensure that all members of the
group have the same view of the parameters in use
In other words, an application can use GroupContext extensions to
ensure that all members of the group agree on a set of parameters.
Clients indicate their support for parameters in the capabilities
field of their LeafNode. New members of a group are informed of the
group's GroupContext extensions via the extensions field in the
group_context field of the GroupInfo object. The extensions field in
a GroupInfo object (outside of the group_context field) can be used
to provide additional parameters to new joiners that are used to join
the group.
This extension mechanism is designed to allow for the secure and
forward-compatible negotiation of extensions. For this to work,
implementations MUST correctly handle extensible fields:
* A client that posts a KeyPackage MUST support all parameters
advertised in it. Otherwise, another client might fail to
interoperate by selecting one of those parameters.
* A client processing a KeyPackage object MUST ignore all
unrecognized values in the capabilities field of the LeafNode and
all unknown extensions in the extensions and leaf_node.extensions
fields. Otherwise, it could fail to interoperate with newer
clients.
* A client processing a GroupInfo object MUST ignore all
unrecognized extensions in the extensions field.
* Any field containing a list of extensions MUST NOT have more than
one extension of any given type.
* A client adding a new member to a group MUST verify that the
LeafNode for the new member is compatible with the group's
extensions. The capabilities field MUST indicate support for each
extension in the GroupContext.
* A client joining a group MUST verify that it supports every
extension in the GroupContext for the group. Otherwise, it MUST
treat the enclosing GroupInfo message as invalid and not join the
group.
Note that the latter two requirements mean that all MLS GroupContext
extensions are mandatory, in the sense that an extension in use by
the group MUST be supported by all members of the group.
The parameters of a group may be changed by sending a
GroupContextExtensions proposal to enable additional extensions
(Section 12.1.7), or by reinitializing the group (Section 11.2).
13.5. GREASE
As described in Section 13.4, clients are required to ignore unknown
values for certain parameters. To help ensure that other clients
implement this behavior, a client can follow the "Generate Random
Extensions And Sustain Extensibility" or GREASE approach described in
[RFC8701]. In the context of MLS, this means that a client
generating a KeyPackage, LeafNode, or GroupInfo object includes
random values in certain fields which would be ignored by a correctly
implemented client processing the message. A client that incorrectly
rejects unknown code points will fail to process such a message,
providing a signal to its implementer that the client needs to be
fixed.
When generating the following fields, an MLS client SHOULD include a
random selection of values chosen from these GREASE values:
* LeafNode.capabilities.cipher_suites
* LeafNode.capabilities.extensions
* LeafNode.capabilities.proposals
* LeafNode.capabilities.credentials
* LeafNode.extensions
* KeyPackage.extensions
* GroupInfo.extensions
For the KeyPackage and GroupInfo extensions, the extension_data for
GREASE extensions MAY have any contents selected by the sender, since
they will be ignored by a correctly implemented receiver. For
example, a sender might populate these extensions with a randomly
sized amount of random data.
Note that any GREASE values added to LeafNode.extensions need to be
reflected in LeafNode.capabilities.extensions, since the LeafNode
validation process described in Section 7.3 requires that these two
fields be consistent.
GREASE values MUST NOT be sent in the following fields, because an
unsupported value in one these fields (including a GREASE value) will
cause the enclosing message to be rejected:
* Proposal.proposal_type
* Credential.credential_type
* GroupContext.extensions
* GroupContextExtensions.extensions
Values reserved for GREASE have been registered in the various
registries in Section 17. This prevents conflict between GREASE and
real future values. The following values are reserved in each
registry: 0x0A0A, 0x1A1A, 0x2A2A, 0x3A3A, 0x4A4A, 0x5A5A, 0x6A6A,
0x7A7A, 0x8A8A, 0x9A9A, 0xAAAA, 0xBABA, 0xCACA, 0xDADA, and 0xEAEA.
(The value 0xFAFA falls within the private use range.) These values
MUST only appear in the fields listed above, and not, for example, in
the proposal_type field of a Proposal. Clients MUST NOT implement
any special processing rules for how to handle these values when
receiving them, since this negates their utility for detecting
extensibility failures.
GREASE values MUST be handled using normal logic for processing
unsupported values. When comparing lists of capabilities to identify
mutually supported capabilities, clients MUST represent their own
capabilities with a list containing only the capabilities actually
supported, without any GREASE values. In other words, lists
including GREASE values are only sent to other clients;
representations of a client's own capabilities MUST NOT contain
GREASE values.
14. Sequencing of State Changes
Each Commit message is premised on a given starting state, indicated
by the epoch field of the enclosing FramedContent. If the changes
implied by a Commit message are made starting from a different state,
the results will be incorrect.
This need for sequencing is not a problem as long as each time a
group member sends a Commit message, it is based on the most current
state of the group. In practice, however, there is a risk that two
members will generate Commit messages simultaneously based on the
same state.
Applications MUST have an established way to resolve conflicting
Commit messages for the same epoch. They can do this either by
preventing conflicting messages from occurring in the first place, or
by developing rules for deciding which Commit out of several sent in
an epoch will be canonical. The approach chosen MUST minimize the
amount of time that forked or previous group states are kept in
memory, and promptly delete them once they're no longer necessary to
ensure forward secrecy.
The generation of Commit messages MUST NOT modify a client's state,
since the client doesn't know at that time whether the changes
implied by the Commit message will conflict with another Commit or
not. Similarly, the Welcome message corresponding to a Commit MUST
NOT be delivered to a new joiner until it's clear that the Commit has
been accepted.
Regardless of how messages are kept in sequence, there is a risk that
in a sufficiently busy group, a given member may never be able to
send a Commit message because they always lose to other members. The
degree to which this is a practical problem will depend on the
dynamics of the application.
15. Application Messages
The primary purpose of handshake messages is to provide an
authenticated group key exchange to clients. In order to protect
application messages sent among the members of a group, the
encryption_secret provided by the key schedule is used to derive a
sequence of nonces and keys for message encryption. Every epoch
moves the key schedule forward, which triggers the creation of a new
secret tree, as described in Section 9, along with a new set of
symmetric ratchets of nonces and keys for each member.
Each client maintains their own local copy of the key schedule for
each epoch during which they are a group member. They derive new
keys, nonces, and secrets as needed while deleting old ones as soon
as they have been used.
The group identifier and epoch allow a recipient to know which group
secrets should be used and from which epoch_secret to start computing
other secrets. The sender identifier and content type are used to
identify which symmetric ratchet to use from the secret tree. The
generation counter determines how far into the ratchet to iterate in
order to produce the required nonce and key for encryption or
decryption.
15.1. Padding
Application messages MAY be padded to provide some resistance against
traffic analysis techniques over encrypted traffic [CLINIC] [HCJ16].
While MLS might deliver the same payload less frequently across a lot
of ciphertexts than traditional web servers, it might still provide
the attacker enough information to mount an attack. If Alice asks
Bob "When are we going to the movie?", then the answer "Wednesday"
could be leaked to an adversary solely by the ciphertext length.
The length of the padding field in PrivateMessageContent can be
chosen by the sender at the time of message encryption. Senders may
use padding to reduce the ability of attackers outside the group to
infer the size of the encrypted content. Note, however, that the
transports used to carry MLS messages may have maximum message sizes,
so padding schemes SHOULD avoid increasing message size beyond any
such limits that exist in a given deployment scenario.
15.2. Restrictions
During each epoch, senders MUST NOT encrypt more data than permitted
by the security bounds of the AEAD scheme used [CFRG-AEAD-LIMITS].
Note that each change to the group through a handshake message will
also set a new encryption_secret. Hence this change MUST be applied
before encrypting any new application message. This is required both
to ensure that any users removed from the group can no longer receive
messages and to (potentially) recover confidentiality and
authenticity for future messages despite a past state compromise.
15.3. Delayed and Reordered Application Messages
Since each application message contains the group identifier, the
epoch, and a generation counter, a client can receive messages out of
order. When messages are received out of order, the client moves the
sender ratchet forward to match the received generation counter. Any
unused nonce and key pairs from the ratchet are potentially stored so
that they can be used to decrypt the messages that were delayed or
reordered.
Applications SHOULD define a policy on how long to keep unused nonce
and key pairs for a sender, and the maximum number to keep. This is
in addition to ensuring that these secrets are deleted according to
the deletion schedule defined in Section 9.2. Applications SHOULD
also define a policy limiting the maximum number of steps that
clients will move the ratchet forward in response to a new message.
Messages received with a generation counter that is too much higher
than the last message received would then be rejected. This avoids
causing a denial-of-service attack by requiring the recipient to
perform an excessive number of key derivations. For example, a
malicious group member could send a message with generation =
0xffffffff at the beginning of a new epoch, forcing recipients to
perform billions of key derivations unless they apply limits of the
type discussed above.
16. Security Considerations
The security goals of MLS are described in [MLS-ARCH]. We describe
here how the protocol achieves its goals at a high level, though a
complete security analysis is outside of the scope of this document.
The Security Considerations section of [MLS-ARCH] provides some
citations to detailed security analyses.
16.1. Transport Security
Because MLS messages are protected at the message level, the
confidentiality and integrity of the group state do not depend on
those messages being protected in transit. However, an attacker who
can observe those messages in transit will be able to learn about the
group state, including potentially the group membership (see
Section 16.4.3 below). Such an attacker might also be able to mount
denial-of-service attacks on the group or exclude new members by
selectively removing messages in transit. In order to prevent this
form of attack, it is RECOMMENDED that all MLS messages be carried
over a secure transport such as TLS [RFC8446] or QUIC [RFC9000].
16.2. Confidentiality of Group Secrets
Group secrets are partly derived from the output of a ratchet tree.
Ratchet trees work by assigning each member of the group to a leaf in
the tree and maintaining the following property: the private key of a
node in the tree is known only to members of the group that are
assigned a leaf in the node's subtree. This is called the _tree
invariant_, and it makes it possible to encrypt to all group members
except one, with a number of ciphertexts that is logarithmic in the
number of group members.
The ability to efficiently encrypt to all members except one allows
members to be securely removed from a group. It also allows a member
to rotate their key pair such that the old private key can no longer
be used to decrypt new messages.
16.3. Confidentiality of Sender Data
The PrivateMessage framing encrypts "sender data" that identifies
which group member sent an encrypted message, as described in
Section 6.3.2. As with the QUIC header protection scheme [RFC9001],
Section 5.4, this scheme is a variant of the HN1 construction
analyzed in [NAN]. A sample of the ciphertext is combined with a
sender_data_secret to derive a key and nonce that are used for AEAD
encryption of the sender data.
(key, nonce) = PRF(sender_data_secret, sample)
encrypted_sender_data =
AEAD.Seal(key, nonce, sender_data_aad, sender_data)
The only differences between this construction and HN1 as described
in [NAN] are that it (1) uses authenticated encryption instead of
unauthenticated encryption and (2) protects information used to
derive a nonce instead of the nonce itself.
Since the sender_data_secret is distinct from the content encryption
key, it follows that the sender data encryption scheme achieves AE2
security as defined in [NAN], and therefore guarantees the
confidentiality of the sender data.
Use of the same sender_data_secret and ciphertext sample more than
once risks compromising sender data protection by reusing an AEAD
(key, nonce) pair. For example, in many AEAD schemes, reusing a key
and nonce reveals the exclusive OR of the two plaintexts. Assuming
the ciphertext output of the AEAD algorithm is indistinguishable from
random data (i.e., the AEAD is AE1-secure in the phrasing of [NAN]),
the odds of two ciphertext samples being identical is roughly
2^(-L/2), i.e., the birthday bound.
The AEAD algorithms for cipher suites defined in this document all
provide this property. The size of the sample depends on the cipher
suite's hash function, but in all cases, the probability of collision
is no more than 2^-128. Any future cipher suite MUST use an
AE1-secure AEAD algorithm.
16.4. Confidentiality of Group Metadata
MLS does not provide confidentiality protection to some messages and
fields within messages:
* KeyPackage messages
* GroupInfo messages
* The unencrypted portion of a Welcome message
* Any Proposal or Commit messages sent as PublicMessage messages
* The unencrypted header fields in PrivateMessage messages
* The lengths of encrypted Welcome and PrivateMessage messages
The only mechanism MLS provides for confidentially distributing a
group's ratchet tree to new members is to send it in a Welcome
message as a ratchet_tree extension. If an application distributes
the tree in some other way, its security will depend on that
application mechanism.
A party observing these fields might be able to infer certain
properties of the group:
* Group ID
* Current epoch and frequency of epoch changes
* Frequency of messages within an epoch
* Group extensions
* Group membership
The amount of metadata exposed to parties outside the group, and thus
the ability of these parties to infer the group's properties, depends
on several aspects of the DS design, such as:
* How KeyPackages are distributed
* How the ratchet tree is distributed
* How prospective external joiners get a GroupInfo object for the
group
* Whether Proposal and Commit messages are sent as PublicMessage or
PrivateMessage
In the remainder of this section, we note the ways that the above
properties of the group are reflected in unprotected group messages,
as a guide to understanding how they might be exposed or protected in
a given application.
16.4.1. GroupID, Epoch, and Message Frequency
MLS provides no mechanism to protect the group ID and epoch of a
message from the DS, so the group ID and the frequency of messages
and epoch changes are not protected against inspection by the DS.
However, any modifications to these will cause decryption failure.
16.4.2. Group Extensions
A group's extensions are first set by the group's creator and then
updated by GroupContextExtensions proposals. A
GroupContextExtensions proposal sent as a PublicMessage leaks the
group's extensions.
A new member learns the group's extensions via a GroupInfo object.
When the new member joins via a Welcome message, the Welcome
message's encryption protects the GroupInfo message. When the new
member joins via an external join, they must be provided with a
GroupInfo object. Protection of this GroupInfo object is up to the
application -- if it is transmitted over a channel that is not
confidential to the group and the new joiner, then it will leak the
group's extensions.
16.4.3. Group Membership
The group's membership is represented directly by its ratchet tree,
since each member's LeafNode contains members' cryptographic keys, a
credential that contains information about the member's identity, and
possibly other identifiers. Applications that expose the group's
ratchet tree outside the group also leak the group's membership.
Changes to the group's membership are made by means of Add and Remove
proposals. If these proposals are sent as PublicMessage, then
information will be leaked about the corresponding changes to the
group's membership. A party that sees all of these changes can
reconstruct the group membership.
Welcome messages contain a hash of each KeyPackage for which the
Welcome message is encrypted. If a party has access to a pool of
KeyPackages and observes a Welcome message, then they can identify
the KeyPackage representing the new member. If the party can also
associate the Welcome with a group, then the party can infer that the
identified new member was added to that group.
Note that these information leaks reveal the group's membership only
to the degree that membership is revealed by the contents of a
member's LeafNode in the ratchet tree. In some cases, this may be
quite direct, e.g., due to credentials attesting to identifiers such
as email addresses. An application could construct a member's leaf
node to be less identifying, e.g., by using a pseudonymous credential
and frequently rotating encryption and signature keys.
16.5. Authentication
The first form of authentication we provide is that group members can
verify a message originated from one of the members of the group.
For encrypted messages, this is guaranteed because messages are
encrypted with an AEAD under a key derived from the group secrets.
For plaintext messages, this is guaranteed by the use of a
membership_tag, which constitutes a MAC over the message, under a key
derived from the group secrets.
The second form of authentication is that group members can verify a
message originated from a particular member of the group. This is
guaranteed by a digital signature on each message from the sender's
signature key.
The signature keys held by group members are critical to the security
of MLS against active attacks. If a member's signature key is
compromised, then an attacker can create LeafNodes and KeyPackages
impersonating the member; depending on the application, this can then
allow the attacker to join the group with the compromised member's
identity. For example, if a group has enabled external parties to
join via external commits, then an attacker that has compromised a
member's signature key could use an external Commit to insert
themselves into the group -- even using a "resync"-style external
Commit to replace the compromised member in the group.
Applications can mitigate the risks of signature key compromise using
pre-shared keys. If a group requires joiners to know a PSK in
addition to authenticating with a credential, then in order to mount
an impersonation attack, the attacker would need to compromise the
relevant PSK as well as the victim's signature key. The cost of this
mitigation is that the application needs some external arrangement
that ensures that the legitimate members of the group have the
required PSKs.
16.6. Forward Secrecy and Post-Compromise Security
Forward secrecy and post-compromise security are important security
notions for long-lived MLS groups. Forward secrecy means that
messages sent at a certain point in time are secure in the face of
later compromise of a group member. Post-compromise security means
that messages are secure even if a group member was compromised at
some point in the past.
Compromise
|
|
| V |
------------------|---------|------------------------->
| | Time
<-----------------| |---------------->
Forward Secrecy | | Post-Compromise
| | Security
Figure 29: Forward Secrecy and Post-Compromise Security
Post-compromise security is provided between epochs by members
regularly updating their leaf key in the ratchet tree. Updating
their leaf key prevents group secrets from continuing to be encrypted
to public keys whose private keys had previously been compromised.
Note that sending an Update proposal does not achieve PCS until
another member includes it in a Commit. Members can achieve
immediate PCS by sending their own Commit and populating the path
field, as described in Section 12.4. To be clear, in all these
cases, the PCS guarantees come into effect when the members of the
group process the relevant Commit, not when the sender creates it.
Forward secrecy between epochs is provided by deleting private keys
from past versions of the ratchet tree, as this prevents old group
secrets from being re-derived. Forward secrecy _within_ an epoch is
provided by deleting message encryption keys once they've been used
to encrypt or decrypt a message. Note that group secrets and message
encryption keys are shared by the group. There is thus a risk to
forward secrecy as long as any member has not deleted these keys.
This is a particular risk if a member is offline for a long period of
time. Applications SHOULD have mechanisms for evicting group members
that are offline for too long (i.e., have not changed their key
within some period).
New groups are also at risk of using previously compromised keys (as
with post-compromise security) if a member is added to a new group
via an old KeyPackage whose corresponding private key has been
compromised. This risk can be mitigated by having clients regularly
generate new KeyPackages and upload them to the Delivery Service.
This way, the key material used to add a member to a new group is
more likely to be fresh and less likely to be compromised.
16.7. Uniqueness of Ratchet Tree Key Pairs
The encryption and signature keys stored in the encryption_key and
signature_key fields of ratchet tree nodes MUST be distinct from one
another. If two members' leaf nodes have the same signature key, for
example, then the data origin authentication properties afforded by
signatures within the group are degraded.
Uniqueness of keys in leaf nodes is assured by explicitly checking
each leaf node as it is added to the tree, whether in an Add
proposal, in an Update proposal, or in the path field of a Commit.
Details can be found in Sections 7.3, 12.2, and 12.4.2. Uniqueness
of encryption keys in parent nodes is assured by checking that the
keys in an UpdatePath are not found elsewhere in the tree (see
Section 12.4.2).
16.8. KeyPackage Reuse
KeyPackages are intended to be used only once. That is, once a
KeyPackage has been used to introduce the corresponding client to a
group, it SHOULD be deleted from the KeyPackage publication system.
Reuse of KeyPackages can lead to replay attacks.
An application MAY allow for reuse of a "last resort" KeyPackage in
order to prevent denial-of-service attacks. Since a KeyPackage is
needed to add a client to a new group, an attacker could prevent a
client from being added to new groups by exhausting all available
KeyPackages. To prevent such a denial-of-service attack, the
KeyPackage publication system SHOULD rate-limit KeyPackage requests,
especially if not authenticated.
16.9. Delivery Service Compromise
MLS is designed to protect the confidentiality and integrity of the
group data even in the face of a compromised DS. However, a
compromised DS can still mount some attacks. While it cannot forge
messages, it can selectively delay or remove them. In some cases,
this can be observed by detecting gaps in the per-sender generation
counter, though it may not always be possible to distinguish an
attack from message loss. In addition, the DS can permanently block
messages to and from a group member. This will not always be
detectable by other members. If an application uses the DS to
resolve conflicts between simultaneous Commits (see Section 14), it
is also possible for the DS to influence which Commit is applied,
even to the point of preventing a member from ever having its Commits
applied.
When put together, these abilities potentially allow a DS to collude
with an attacker who has compromised a member's state to defeat PCS
by suppressing the valid Update and Commit messages from the member
that would lock out the attacker and update the member's leaf to a
new, uncompromised state. Aside from the SenderData.generation
value, MLS leaves loss detection up to the application.
16.10. Authentication Service Compromise
Authentication Service compromise is much more serious than
compromise of the Delivery Service. A compromised AS can assert a
binding for a signature key and identity pair of its choice, thus
allowing impersonation of a given user. This ability is sufficient
to allow the AS to join new groups as if it were that user.
Depending on the application architecture, it may also be sufficient
to allow the compromised AS to join the group as an existing user,
for instance, as if it were a new device associated with the same
user. If the application uses a transparency mechanism such as
CONIKS [CONIKS] or Key Transparency [KT], then it may be possible for
end users to detect this kind of misbehavior by the AS. It is also
possible to construct schemes in which the various clients owned by a
user vouch for each other, e.g., by signing each others' keys.
16.11. Additional Policy Enforcement
The DS and AS may also apply additional policies to MLS operations to
obtain additional security properties. For example, MLS enables any
participant to add or remove members of a group; a DS could enforce a
policy that only certain members are allowed to perform these
operations. MLS authenticates all members of a group; a DS could
help ensure that only clients with certain types of credentials are
admitted. MLS provides no inherent protection against denial of
service; a DS could also enforce rate limits in order to mitigate
these risks.
16.12. Group Fragmentation by Malicious Insiders
It is possible for a malicious member of a group to "fragment" the
group by crafting an invalid UpdatePath. Recall that an UpdatePath
encrypts a sequence of path secrets to different subtrees of the
group's ratchet trees. These path secrets should be derived in a
sequence as described in Section 7.4, but the UpdatePath syntax
allows the sender to encrypt arbitrary, unrelated secrets. The
syntax also does not guarantee that the encrypted path secret for a
given node corresponds to the public key provided for that node.
Both of these types of corruption will cause processing of a Commit
to fail for some members of the group. If the public key for a node
does not match the path secret, then the members that decrypt that
path secret will reject the Commit based on this mismatch. If the
path secret sequence is incorrect at some point, then members that
can decrypt nodes before that point will compute a different public
key for the mismatched node than the one in the UpdatePath, which
also causes the Commit to fail. Applications SHOULD provide
mechanisms for failed commits to be reported, so that group members
who were not able to recognize the error themselves can reinitialize
the group if necessary.
Even with such an error reporting mechanism in place, however, it is
still possible for members to get locked out of the group by a
malformed Commit. Since malformed Commits can only be recognized by
certain members of the group, in an asynchronous application, it may
be the case that all members that could detect a fault in a Commit
are offline. In such a case, the Commit will be accepted by the
group, and the resulting state will possibly be used as the basis for
further Commits. When the affected members come back online, they
will reject the first Commit, and thus be unable to catch up with the
group. These members will need to either add themselves back with an
external Commit or reinitialize the group from scratch.
Applications can address this risk by requiring certain members of
the group to acknowledge successful processing of a Commit before the
group regards the Commit as accepted. The minimum set of
acknowledgements necessary to verify that a Commit is well-formed
comprises an acknowledgement from one member per node in the
UpdatePath, that is, one member from each subtree rooted in the
copath node corresponding to the node in the UpdatePath. MLS does
not provide a built-in mechanism for such acknowledgements, but they
can be added at the application layer.
17. IANA Considerations
IANA has created the following registries:
* MLS Cipher Suites (Section 17.1)
* MLS Wire Formats (Section 17.2)
* MLS Extension Types (Section 17.3)
* MLS Proposal Types (Section 17.4)
* MLS Credential Types (Section 17.5)
* MLS Signature Labels (Section 17.6)
* MLS Public Key Encryption Labels (Section 17.7)
* MLS Exporter Labels (Section 17.8)
All of these registries are under the "Messaging Layer Security"
group registry heading, and assignments are made via the
Specification Required policy [RFC8126]. See Section 17.9 for
additional information about the MLS Designated Experts (DEs).
17.1. MLS Cipher Suites
A cipher suite is a combination of a protocol version and the set of
cryptographic algorithms that should be used.
Cipher suite names follow the naming convention:
CipherSuite MLS_LVL_KEM_AEAD_HASH_SIG = VALUE;
Where VALUE is represented as a 16-bit integer:
uint16 CipherSuite;
+===========+==================================+
| Component | Contents |
+===========+==================================+
| LVL | The security level (in bits) |
+-----------+----------------------------------+
| KEM | The KEM algorithm used for HPKE |
| | in ratchet tree operations |
+-----------+----------------------------------+
| AEAD | The AEAD algorithm used for HPKE |
| | and message protection |
+-----------+----------------------------------+
| HASH | The hash algorithm used for HPKE |
| | and the MLS transcript hash |
+-----------+----------------------------------+
| SIG | The signature algorithm used for |
| | message authentication |
+-----------+----------------------------------+
Table 5
The columns in the registry are as follows:
* Value: The numeric value of the cipher suite
* Name: The name of the cipher suite
* Recommended: Whether support for this cipher suite is recommended
by the IETF. Valid values are "Y", "N", and "D", as described
below. The default value of the "Recommended" column is "N".
Setting the Recommended item to "Y" or "D", or changing an item
whose current value is "Y" or "D", requires Standards Action
[RFC8126].
- Y: Indicates that the IETF has consensus that the item is
RECOMMENDED. This only means that the associated mechanism is
fit for the purpose for which it was defined. Careful reading
of the documentation for the mechanism is necessary to
understand the applicability of that mechanism. The IETF could
recommend mechanisms that have limited applicability, but it
will provide applicability statements that describe any
limitations of the mechanism or necessary constraints on its
use.
- N: Indicates that the item has not been evaluated by the IETF
and that the IETF has made no statement about the suitability
of the associated mechanism. This does not necessarily mean
that the mechanism is flawed, only that no consensus exists.
The IETF might have consensus to leave an item marked as "N" on
the basis of it having limited applicability or usage
constraints.
- D: Indicates that the item is discouraged and SHOULD NOT or
MUST NOT be used. This marking could be used to identify
mechanisms that might result in problems if they are used, such
as a weak cryptographic algorithm or a mechanism that might
cause interoperability problems in deployment.
* Reference: The document where this cipher suite is defined
Initial contents:
+========+===================================================+=+====+
| Value |Name |R|Ref |
+========+===================================================+=+====+
| 0x0000 |RESERVED |-|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x0001 |MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519 |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x0002 |MLS_128_DHKEMP256_AES128GCM_SHA256_P256 |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x0003 |MLS_128_DHKEMX25519_CHACHA20POLY1305_SHA256_Ed25519|Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x0004 |MLS_256_DHKEMX448_AES256GCM_SHA512_Ed448 |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x0005 |MLS_256_DHKEMP521_AES256GCM_SHA512_P521 |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x0006 |MLS_256_DHKEMX448_CHACHA20POLY1305_SHA512_Ed448 |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x0007 |MLS_256_DHKEMP384_AES256GCM_SHA384_P384 |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x0A0A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x1A1A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x2A2A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x3A3A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x4A4A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x5A5A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x6A6A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x7A7A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x8A8A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0x9A9A |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0xAAAA |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0xBABA |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0xCACA |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0xDADA |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0xEAEA |GREASE |Y|RFC |
| | | |9420|
+--------+---------------------------------------------------+-+----+
| 0xF000 |Reserved for Private Use |-|RFC |
| - | | |9420|
| 0xFFFF | | | |
+--------+---------------------------------------------------+-+----+
Table 6: MLS Extension Types Registry
All of the non-GREASE cipher suites use HMAC [RFC2104] as their MAC
function, with different hashes per cipher suite. The mapping of
cipher suites to HPKE primitives [RFC9180], HMAC hash functions, and
TLS signature schemes [RFC8446] is as follows:
+======+======+========+========+========+========================+
|Value |KEM | KDF | AEAD | Hash | Signature |
+======+======+========+========+========+========================+
|0x0001|0x0020| 0x0001 | 0x0001 | SHA256 | ed25519 |
+------+------+--------+--------+--------+------------------------+
|0x0002|0x0010| 0x0001 | 0x0001 | SHA256 | ecdsa_secp256r1_sha256 |
+------+------+--------+--------+--------+------------------------+
|0x0003|0x0020| 0x0001 | 0x0003 | SHA256 | ed25519 |
+------+------+--------+--------+--------+------------------------+
|0x0004|0x0021| 0x0003 | 0x0002 | SHA512 | ed448 |
+------+------+--------+--------+--------+------------------------+
|0x0005|0x0012| 0x0003 | 0x0002 | SHA512 | ecdsa_secp521r1_sha512 |
+------+------+--------+--------+--------+------------------------+
|0x0006|0x0021| 0x0003 | 0x0003 | SHA512 | ed448 |
+------+------+--------+--------+--------+------------------------+
|0x0007|0x0011| 0x0002 | 0x0002 | SHA384 | ecdsa_secp384r1_sha384 |
+------+------+--------+--------+--------+------------------------+
Table 7
The hash used for the MLS transcript hash is the one referenced in
the cipher suite name. In the cipher suites defined above, "SHA256",
"SHA384", and "SHA512" refer, respectively, to the SHA-256, SHA-384,
and SHA-512 functions defined in [SHS].
In addition to the general requirements of Section 13.1, future
cipher suites MUST meet the requirements of Section 16.3.
It is advisable to keep the number of cipher suites low to increase
the likelihood that clients can interoperate in a federated
environment. The cipher suites therefore include only modern, yet
well-established algorithms. Depending on their requirements,
clients can choose between two security levels (roughly 128-bit and
256-bit). Within the security levels, clients can choose between
faster X25519/X448 curves and curves compliant with FIPS 140-2 for
Diffie-Hellman key negotiations. Clients may also choose
ChaCha20Poly1305 or AES-GCM, e.g., for performance reasons. Since
ChaCha20Poly1305 is not listed by FIPS 140-2, it is not paired with
curves compliant with FIPS 140-2. The security level of symmetric
encryption algorithms and hash functions is paired with the security
level of the curves.
The mandatory-to-implement cipher suite for MLS 1.0 is
MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519, which uses Curve25519
for key exchange, AES-128-GCM for HPKE, HKDF over SHA2-256, and
Ed25519 for signatures. MLS clients MUST implement this cipher
suite.
17.2. MLS Wire Formats
The "MLS Wire Formats" registry lists identifiers for the types of
messages that can be sent in MLS. The wire format field is two bytes
wide, so the valid wire format values are in the range 0x0000 to
0xFFFF.
Template:
* Value: The numeric value of the wire format
* Name: The name of the wire format
* Recommended: Same as in Section 17.1
* Reference: The document where this wire format is defined
Initial contents:
+=================+==========================+===+==========+
| Value | Name | R | Ref |
+=================+==========================+===+==========+
| 0x0000 | RESERVED | - | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x0001 | mls_public_message | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x0002 | mls_private_message | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x0003 | mls_welcome | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x0004 | mls_group_info | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x0005 | mls_key_package | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0xF000 - 0xFFFF | Reserved for Private Use | - | RFC 9420 |
+-----------------+--------------------------+---+----------+
Table 8: MLS Wire Formats Registry
17.3. MLS Extension Types
The "MLS Extension Types" registry lists identifiers for extensions
to the MLS protocol. The extension type field is two bytes wide, so
valid extension type values are in the range 0x0000 to 0xFFFF.
Template:
* Value: The numeric value of the extension type
* Name: The name of the extension type
* Message(s): The messages in which the extension may appear, drawn
from the following list:
- KP: KeyPackage objects
- LN: LeafNode objects
- GC: GroupContext objects
- GI: GroupInfo objects
* Recommended: Same as in Section 17.1
* Reference: The document where this extension is defined
Initial contents:
+==========+=======================+============+===+==========+
| Value | Name | Message(s) | R | Ref |
+==========+=======================+============+===+==========+
| 0x0000 | RESERVED | N/A | - | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x0001 | application_id | LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x0002 | ratchet_tree | GI | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x0003 | required_capabilities | GC | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x0004 | external_pub | GI | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x0005 | external_senders | GC | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x0A0A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x1A1A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x2A2A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x3A3A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x4A4A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x5A5A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x6A6A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x7A7A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x8A8A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0x9A9A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0xAAAA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0xBABA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0xCACA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0xDADA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0xEAEA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+
| 0xF000 - | Reserved for Private | N/A | - | RFC 9420 |
| 0xFFFF | Use | | | |
+----------+-----------------------+------------+---+----------+
Table 9: MLS Extension Types Registry
17.4. MLS Proposal Types
The "MLS Proposal Types" registry lists identifiers for types of
proposals that can be made for changes to an MLS group. The
extension type field is two bytes wide, so valid extension type
values are in the range 0x0000 to 0xFFFF.
Template:
* Value: The numeric value of the proposal type
* Name: The name of the proposal type
* Recommended: Same as in Section 17.1
* External: Whether a proposal of this type may be sent by an
external sender (see Section 12.1.8)
* Path Required: Whether a Commit covering a proposal of this type
is required to have its path field populated (see Section 12.4)
* Reference: The document where this extension is defined
Initial contents:
+==========+==========================+===+=====+======+==========+
| Value | Name | R | Ext | Path | Ref |
+==========+==========================+===+=====+======+==========+
| 0x0000 | RESERVED | - | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x0001 | add | Y | Y | N | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x0002 | update | Y | N | Y | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x0003 | remove | Y | Y | Y | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x0004 | psk | Y | Y | N | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x0005 | reinit | Y | Y | N | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x0006 | external_init | Y | N | Y | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x0007 | group_context_extensions | Y | Y | Y | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x0A0A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x1A1A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x2A2A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x3A3A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x4A4A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x5A5A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x6A6A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x7A7A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x8A8A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0x9A9A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0xAAAA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0xBABA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0xCACA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0xDADA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0xEAEA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+
| 0xF000 - | Reserved for Private Use | - | - | - | RFC 9420 |
| 0xFFFF | | | | | |
+----------+--------------------------+---+-----+------+----------+
Table 10: MLS Proposal Types Registry
17.5. MLS Credential Types
The "MLS Credential Types" registry lists identifiers for types of
credentials that can be used for authentication in the MLS protocol.
The credential type field is two bytes wide, so valid credential type
values are in the range 0x0000 to 0xFFFF.
Template:
* Value: The numeric value of the credential type
* Name: The name of the credential type
* Recommended: Same as in Section 17.1
* Reference: The document where this credential is defined
Initial contents:
+=================+==========================+===+==========+
| Value | Name | R | Ref |
+=================+==========================+===+==========+
| 0x0000 | RESERVED | - | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x0001 | basic | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x0002 | x509 | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x0A0A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x1A1A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x2A2A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x3A3A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x4A4A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x5A5A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x6A6A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x7A7A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x8A8A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0x9A9A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0xAAAA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0xBABA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0xCACA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0xDADA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0xEAEA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+
| 0xF000 - 0xFFFF | Reserved for Private Use | - | RFC 9420 |
+-----------------+--------------------------+---+----------+
Table 11: MLS Credential Types Registry
17.6. MLS Signature Labels
The SignWithLabel function defined in Section 5.1.2 avoids the risk
of confusion between signatures in different contexts. Each context
is assigned a distinct label that is incorporated into the signature.
The "MLS Signature Labels" registry records the labels defined in
this document and allows additional labels to be registered in case
extensions add other types of signatures using the same signature
keys used elsewhere in MLS.
Template:
* Label: The string to be used as the Label parameter to
SignWithLabel
* Recommended: Same as in Section 17.1
* Reference: The document where this label is defined
Initial contents:
+====================+===+==========+
| Label | R | Ref |
+====================+===+==========+
| "FramedContentTBS" | Y | RFC 9420 |
+--------------------+---+----------+
| "LeafNodeTBS" | Y | RFC 9420 |
+--------------------+---+----------+
| "KeyPackageTBS" | Y | RFC 9420 |
+--------------------+---+----------+
| "GroupInfoTBS" | Y | RFC 9420 |
+--------------------+---+----------+
Table 12: MLS Signature Labels
Registry
17.7. MLS Public Key Encryption Labels
The EncryptWithLabel function defined in Section 5.1.3 avoids the
risk of confusion between ciphertexts produced for different purposes
in different contexts. Each context is assigned a distinct label
that is incorporated into the signature. The "MLS Public Key
Encryption Labels" registry records the labels defined in this
document and allows additional labels to be registered in case
extensions add other types of public key encryption using the same
HPKE keys used elsewhere in MLS.
Template:
* Label: The string to be used as the Label parameter to
EncryptWithLabel
* Recommended: Same as in Section 17.1
* Reference: The document where this label is defined
Initial contents:
+==================+===+==========+
| Label | R | Ref |
+==================+===+==========+
| "UpdatePathNode" | Y | RFC 9420 |
+------------------+---+----------+
| "Welcome" | Y | RFC 9420 |
+------------------+---+----------+
Table 13: MLS Public Key
Encryption Labels Registry
17.8. MLS Exporter Labels
The exporter function defined in Section 8.5 allows applications to
derive key material from the MLS key schedule. Like the TLS exporter
[RFC8446], the MLS exporter uses a label to distinguish between
different applications' use of the exporter. The "MLS Exporter
Labels" registry allows applications to register their usage to avoid
collisions.
Template:
* Label: The string to be used as the Label parameter to MLS-
Exporter
* Recommended: Same as in Section 17.1
* Reference: The document where this label is defined
The registry has no initial contents, since it is intended to be used
by applications, not the core protocol. The table below is intended
only to show the column layout of the registry.
+=======+=============+===========+
| Label | Recommended | Reference |
+=======+=============+===========+
| (N/A) | (N/A) | (N/A) |
+-------+-------------+-----------+
Table 14: MLS Exporter Labels
Registry
17.9. MLS Designated Expert Pool
Specification Required [RFC8126] registry requests are registered
after a three-week review period on the MLS Designated Expert (DE)
mailing list <mailto:mls-reg-review@ietf.org> on the advice of one or
more of the MLS DEs. However, to allow for the allocation of values
prior to publication, the MLS DEs may approve registration once they
are satisfied that such a specification will be published.
Registration requests sent to the MLS DEs' mailing list for review
SHOULD use an appropriate subject (e.g., "Request to register value
in MLS Bar registry").
Within the review period, the MLS DEs will either approve or deny the
registration request, communicating this decision to the MLS DEs'
mailing list and IANA. Denials SHOULD include an explanation and, if
applicable, suggestions as to how to make the request successful.
Registration requests that are undetermined for a period longer than
21 days can be brought to the IESG's attention for resolution using
the <mailto:iesg@ietf.org> mailing list.
Criteria that SHOULD be applied by the MLS DEs includes determining
whether the proposed registration duplicates existing functionality,
whether it is likely to be of general applicability or useful only
for a single application, and whether the registration description is
clear. For example, for cipher suite registrations, the MLS DEs will
apply the advisory found in Section 17.1.
IANA MUST only accept registry updates from the MLS DEs and SHOULD
direct all requests for registration to the MLS DEs' mailing list.
It is suggested that multiple MLS DEs who are able to represent the
perspectives of different applications using this specification be
appointed, in order to enable a broadly informed review of
registration decisions. In cases where a registration decision could
be perceived as creating a conflict of interest for a particular MLS
DE, that MLS DE SHOULD defer to the judgment of the other MLS DEs.
17.10. The "message/mls" Media Type
This document registers the "message/mls" media type in the "message"
registry in order to allow other protocols (e.g., HTTP [RFC9113]) to
convey MLS messages.
Type name: message
Subtype name: mls
Required parameters: none
Optional parameters: version
version: The MLS protocol version expressed as
a string <major>.<minor>. If omitted, the version is "1.0",
which corresponds to MLS ProtocolVersion mls10. If for some
reason the version number in the media type parameter differs
from the ProtocolVersion embedded in the protocol, the protocol
takes precedence.
Encoding considerations: MLS messages are represented using the TLS
presentation language [RFC8446]. Therefore, MLS messages need to
be treated as binary data.
Security considerations: MLS is an encrypted messaging layer
designed to be transmitted over arbitrary lower-layer protocols.
The security considerations in this document (RFC 9420) also
apply.
Interoperability considerations: N/A
Published specification: RFC 9420
Applications that use this media type: MLS-based messaging
applications
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person & email address to contact for further information: IETF MLS
Working Group <mailto:mls@ietf.org>
Intended usage: COMMON
Restrictions on usage: N/A
Author: IETF MLS Working Group
Change controller: IETF
18. References
18.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC9180] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
February 2022, <https://www.rfc-editor.org/info/rfc9180>.
18.2. Informative References
[ART] Cohn-Gordon, K., Cremers, C., Garratt, L., Millican, J.,
and K. Milner, "On Ends-to-Ends Encryption: Asynchronous
Group Messaging with Strong Security Guarantees", Version
2.3, DOI 10.1145/3243734.3243747, March 2020,
<https://eprint.iacr.org/2017/666.pdf>.
[CFRG-AEAD-LIMITS]
Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
AEAD Algorithms", Work in Progress, Internet-Draft, draft-
irtf-cfrg-aead-limits-07, 31 May 2023,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
aead-limits-07>.
[CLINIC] Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know
Why You Went to the Clinic: Risks and Realization of HTTPS
Traffic Analysis", Privacy Enhancing Technologies, pp.
143-163, DOI 10.1007/978-3-319-08506-7_8, 2014,
<https://doi.org/10.1007/978-3-319-08506-7_8>.
[CONIKS] Melara, M. S., Blankstein, A., Bonneau, J., Felten, E. W.,
and M. J. Freedman, "CONIKS: Bringing Key Transparency to
End Users", Proceedings of the 24th USENIX Security
Symposium, ISBN 978-1-939133-11-3, August 2015,
<https://www.usenix.org/system/files/conference/
usenixsecurity15/sec15-paper-melara.pdf>.
[DoubleRatchet]
Cohn-Gordon, K., Cremers, C., Dowling, B., Garratt, L.,
and D. Stebila, "A Formal Security Analysis of the Signal
Messaging Protocol", 2017 IEEE European Symposium on
Security and Privacy (EuroS&P),
DOI 10.1109/eurosp.2017.27, April 2017,
<https://doi.org/10.1109/eurosp.2017.27>.
[HCJ16] Husák, M., Äermák, M., Jirsík, T., and P. Äeleda, "HTTPS
traffic analysis and client identification using passive
SSL/TLS fingerprinting", EURASIP Journal on Information
Security, Vol. 2016, Issue 1,
DOI 10.1186/s13635-016-0030-7, February 2016,
<https://doi.org/10.1186/s13635-016-0030-7>.
[KT] "Key Transparency Design Doc", commit fb0f87f, June 2020,
<https://github.com/google/keytransparency/blob/master/
docs/design.md>.
[MLS-ARCH] Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., and
A. Duric, "The Messaging Layer Security (MLS)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-mls-architecture-10, 16 December 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
architecture-10>.
[NAN] Bellare, M., Ng, R., and B. Tackmann, "Nonces Are Noticed:
AEAD Revisited", Advances in Cryptology - CRYPTO 2019, pp.
235-265, DOI 10.1007/978-3-030-26948-7_9, August 2019,
<https://doi.org/10.1007/978-3-030-26948-7_9>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/info/rfc7696>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.
[RFC8701] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility",
RFC 8701, DOI 10.17487/RFC8701, January 2020,
<https://www.rfc-editor.org/info/rfc8701>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.
[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.
[SHS] National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015,
<https://doi.org/10.6028/NIST.FIPS.180-4>.
[Signal] Perrin(ed), T. and M. Marlinspike, "The Double Ratchet
Algorithm", Revision 1, November 2016,
<https://www.signal.org/docs/specifications/
doubleratchet/>.
Appendix A. Protocol Origins of Example Trees
Protocol operations in MLS give rise to specific forms of ratchet
tree, typically affecting a whole direct path at once. In this
section, we describe the protocol operations that could have given
rise to the various example trees in this document.
To construct the tree in Figure 11:
* A creates a group with B, ..., G
* F sends an empty Commit, setting X, Y, and W
* G removes C and D, blanking V, U, and setting Y and W
* B sends an empty Commit, setting T and W
To construct the tree in Figure 10:
* A creates a group with B, ..., H, as well as some members outside
this subtree
* F sends an empty Commit, setting Y and its ancestors
* D removes B and C, with the following effects:
- Blank the direct paths of B and C
- Set X, the top node, and any further nodes in the direct path
of D
* Someone outside this subtree removes G, blanking the direct path
of G
* A adds a new member at B with a partial Commit, adding B as
unmerged at X
To construct the tree in Figure 13:
* A creates a group with B, C, and D
* B sends a full Commit, setting X and Y
* D removes C, setting Z and Y
* B adds a new member at C with a full Commit
- The Add proposal adds C as unmerged at Z and Y
- The path in the Commit resets X and Y, clearing Y's unmerged
leaves
To construct the tree in Figure 21:
* A creates a group with B, ..., G
* A removes F in a full Commit, setting T, U, and W
* E sends an empty Commit, setting Y and W
* A adds a new member at F in a partial Commit, adding F as unmerged
at Y and W
Appendix B. Evolution of Parent Hashes
To better understand how parent hashes are maintained, let's look in
detail at how they evolve in a small group. Consider the following
sequence of operations:
1. A initializes a new group
2. A adds B to the group with a full Commit
3. B adds C and D to the group with a full Commit
4. C sends an empty Commit
Y Y'
| |
.-+-. .-+-.
==> ==> / \ ==> / \
X X' _=Z X' Z'
/ \ / \ / \ / \ / \
A A B A B C D A B C D
Figure 30: Building a Four-Member Tree to Illustrate Parent Hashes
Then the parent hashes associated to the nodes will be updated as
follows (where we use the shorthand ph for parent hash, th for tree
hash, and osth for original sibling tree hash):
1. A adds B: set X
* A.parent_hash = ph(X) = H(X, ph="", osth=th(B))
2. B adds C, D: set B', X', and Y
* X'.parent_hash = ph(Y) = H(Y, ph="", osth=th(Z)), where th(Z)
covers (C, _, D)
* B'.parent_hash = ph(X') = H(X', ph=X'.parent_hash, osth=th(A))
3. C sends empty Commit: set C', Z', Y'
* Z'.parent_hash = ph(Y') = H(Y', ph="", osth=th(X')), where
th(X') covers (A, X', B')
* C'.parent_hash = ph(Z') = H(Z', ph=Z'.parent_hash, osth=th(D))
When a new member joins, they will receive a tree that has the
following parent hash values and compute the indicated parent hash
validity relationships:
+======+======================================+=====================+
| Node | Parent Hash Value | Valid? |
+======+======================================+=====================+
| A | H(X, ph="", osth=th(B)) | No, B changed |
+------+--------------------------------------+---------------------+
| B' | H(X', ph=X'.parent_hash, osth=th(A)) | Yes |
+------+--------------------------------------+---------------------+
| C' | H(Z', ph=Z'.parent_hash, osth=th(D)) | Yes |
+------+--------------------------------------+---------------------+
| D | (none, never sent an UpdatePath) | N/A |
+------+--------------------------------------+---------------------+
| X' | H(Y, ph="", osth=th(Z)) | No, Y and Z |
| | | changed |
+------+--------------------------------------+---------------------+
| Z' | H(Y', ph="", osth=th(X')) | Yes |
+------+--------------------------------------+---------------------+
Table 15
In other words, the joiner will find the following path-hash links in
the tree:
Y'
|
+-.
\
X' Z'
\ /
A B' C' D
Figure 31: Parent-hash links connect all non-empty parent nodes
to leaves
Since these chains collectively cover all non-blank parent nodes in
the tree, the tree is parent-hash valid.
Note that this tree, though valid, contains invalid parent-hash
links. If a client were checking parent hashes top-down from Y', for
example, they would find that X' has an invalid parent hash relative
to Y', but that Z' has a valid parent hash. Likewise, if the client
were checking bottom-up, they would find that the chain from B' ends
in an invalid link from X' to Y'. These invalid links are the
natural result of multiple clients having committed.
Note also the way the tree hash and the parent hash interact. The
parent hash of node C' includes the tree hash of node D. The parent
hash of node Z' includes the tree hash of X', which covers nodes A
and B' (including the parent hash of B'). Although the tree hash and
the parent hash depend on each other, the dependency relationships
are structured so that there is never a circular dependency.
In the particular case where a new member first receives the tree for
a group (e.g., in a ratchet tree GroupInfo extension
Section 12.4.3.3), the parent hashes will be expressed in the tree
representation, but the tree hash need not be. Instead, the new
member will recompute the tree hashes for all the nodes in the tree,
verifying that this matches the tree hash in the GroupInfo object.
If the tree is valid, then the subtree hashes computed in this way
will align with the inputs needed for parent hash validation (except
where recomputation is needed to account for unmerged leaves).
Appendix C. Array-Based Trees
One benefit of using complete balanced trees is that they admit a
simple flat array representation. In this representation, leaf nodes
are even-numbered nodes, with the n-th leaf at 2*n. Intermediate
nodes are held in odd-numbered nodes. For example, the tree with 8
leaves has the following structure:
X
|
.---------+---------.
/ \
X X
| |
.---+---. .---+---.
/ \ / \
X X X X
/ \ / \ / \ / \
/ \ / \ / \ / \
X X X X X X X X
Node: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Leaf: 0 1 2 3 4 5 6 7
Figure 32: An Eight-Member Tree Represented as an Array
This allows us to compute relationships between tree nodes simply by
manipulating indices, rather than having to maintain complicated
structures in memory. The basic rule is that the high-order bits of
parent and child nodes indices have the following relation (where x
is an arbitrary bit string):
parent=01x => left=00x, right=10x
Since node relationships are implicit, the algorithms for adding and
removing nodes at the right edge of the tree are quite simple. If
there are N nodes in the array:
* Add: Append N + 1 blank values to the end of the array.
* Remove: Truncate the array to its first (N-1) / 2 entries.
The following python code demonstrates the tree computations
necessary to use an array-based tree for MLS.
# The exponent of the largest power of 2 less than x. Equivalent to:
# int(math.floor(math.log(x, 2)))
def log2(x):
if x == 0:
return 0
k = 0
while (x >> k) > 0:
k += 1
return k-1
# The level of a node in the tree. Leaves are level 0, their parents
# are level 1, etc. If a node's children are at different levels,
# then its level is the max level of its children plus one.
def level(x):
if x & 0x01 == 0:
return 0
k = 0
while ((x >> k) & 0x01) == 1:
k += 1
return k
# The number of nodes needed to represent a tree with n leaves.
def node_width(n):
if n == 0:
return 0
else:
return 2*(n - 1) + 1
# The index of the root node of a tree with n leaves.
def root(n):
w = node_width(n)
return (1 << log2(w)) - 1
# The left child of an intermediate node.
def left(x):
k = level(x)
if k == 0:
raise Exception('leaf node has no children')
return x ^ (0x01 << (k - 1))
# The right child of an intermediate node.
def right(x):
k = level(x)
if k == 0:
raise Exception('leaf node has no children')
return x ^ (0x03 << (k - 1))
# The parent of a node.
def parent(x, n):
if x == root(n):
raise Exception('root node has no parent')
k = level(x)
b = (x >> (k + 1)) & 0x01
return (x | (1 << k)) ^ (b << (k + 1))
# The other child of the node's parent.
def sibling(x, n):
p = parent(x, n)
if x < p:
return right(p)
else:
return left(p)
# The direct path of a node, ordered from leaf to root.
def direct_path(x, n):
r = root(n)
if x == r:
return []
d = []
while x != r:
x = parent(x, n)
d.append(x)
return d
# The copath of a node, ordered from leaf to root.
def copath(x, n):
if x == root(n):
return []
d = direct_path(x, n)
d.insert(0, x)
d.pop()
return [sibling(y, n) for y in d]
# The common ancestor of two nodes is the lowest node that is in the
# direct paths of both leaves.
def common_ancestor_semantic(x, y, n):
dx = set([x]) | set(direct_path(x, n))
dy = set([y]) | set(direct_path(y, n))
dxy = dx & dy
if len(dxy) == 0:
raise Exception('failed to find common ancestor')
return min(dxy, key=level)
# The common ancestor of two nodes is the lowest node that is in the
# direct paths of both leaves.
def common_ancestor_direct(x, y, _):
# Handle cases where one is an ancestor of the other
lx, ly = level(x)+1, level(y)+1
if (lx <= ly) and (x>>ly == y>>ly):
return y
elif (ly <= lx) and (x>>lx == y>>lx):
return x
# Handle other cases
xn, yn = x, y
k = 0
while xn != yn:
xn, yn = xn >> 1, yn >> 1
k += 1
return (xn << k) + (1 << (k-1)) - 1
Appendix D. Link-Based Trees
An implementation may choose to store ratchet trees in a "link-based"
representation, where each node stores references to its parents and/
or children (as opposed to the array-based representation suggested
above, where these relationships are computed from relationships
between nodes' indices in the array). Such an implementation needs
to update these links to maintain the balanced structure of the tree
as the tree is extended to add new members or truncated when members
are removed.
The following code snippet shows how these algorithms could be
implemented in Python.
class Node:
def __init__(self, value, left=None, right=None):
self.value = value # Value of the node
self.left = left # Left child node
self.right = right # Right child node
@staticmethod
def blank_subtree(depth):
if depth == 1:
return Node(None)
L = Node.blank_subtree(depth-1)
R = Node.blank_subtree(depth-1)
return Node(None, left=L, right=R)
def empty(self):
L_empty = (self.left == None) or self.left.empty()
R_empty = (self.right == None) or self.right.empty()
return (self.value == None) and L_empty and R_empty
class Tree:
def __init__(self):
self.depth = 0 # Depth of the tree
self.root = None # Root node of the tree, initially empty
# Add a blank subtree to the right
def extend(self):
if self.depth == 0:
self.depth = 1
self.root = Node(None)
L = self.root
R = Node.blank_subtree(self.depth)
self.root = Node(None, left=L, right=R)
self.depth += 1
# Truncate the right subtree
def truncate(self):
if self.root == None:
return
if not self.root.right.empty():
raise Exception("Cannot truncate non-blank subtree")
self.depth -= 1
self.root = self.root.left
Contributors
Joel Alwen
Amazon
Email: alwenjo@amazon.com
Karthikeyan Bhargavan
Inria
Email: karthikeyan.bhargavan@inria.fr
Cas Cremers
CISPA
Email: cremers@cispa.de
Alan Duric
Wire
Email: alan@wire.com
Britta Hale
Naval Postgraduate School
Email: britta.hale@nps.edu
Srinivas Inguva
Email: singuva@yahoo.com
Konrad Kohbrok
Phoenix R&D
Email: konrad.kohbrok@datashrine.de
Albert Kwon
MIT
Email: kwonal@mit.edu
Tom Leavy
Amazon
Email: tomleavy@amazon.com
Brendan McMillion
Email: brendanmcmillion@gmail.com
Marta Mularczyk
Amazon
Email: mulmarta@amazon.com
Eric Rescorla
Mozilla
Email: ekr@rtfm.com
Michael Rosenberg
Trail of Bits
Email: michael.rosenberg@trailofbits.com
Théophile Wallez
Inria
Email: theophile.wallez@inria.fr
Thyla van der Merwe
Royal Holloway, University of London
Email: tjvdmerwe@gmail.com
Authors' Addresses
Richard Barnes
Cisco
Email: rlb@ipv.sx
Benjamin Beurdouche
Inria & Mozilla
Email: ietf@beurdouche.com
Raphael Robert
Phoenix R&D
Email: ietf@raphaelrobert.com
Jon Millican
Meta Platforms
Email: jmillican@meta.com
Emad Omara
Email: emad.omara@gmail.com
Katriel Cohn-Gordon
University of Oxford
Email: me@katriel.co.uk
