Internet Engineering Task Force (IETF) F. Palombini
Request for Comments: 9594 Ericsson AB
Category: Standards Track M. Tiloca
ISSN: 2070-1721 RISE AB
September 2024
Key Provisioning for Group Communication Using Authentication and
Authorization for Constrained Environments (ACE)
Abstract
This document defines how to use the Authentication and Authorization
for Constrained Environments (ACE) framework to distribute keying
material and configuration parameters for secure group communication.
Candidate group members that act as Clients and are authorized to
join a group can do so by interacting with a Key Distribution Center
(KDC) acting as the Resource Server, from which they obtain the
keying material to communicate with other group members. While
defining general message formats as well as the interface and
operations available at the KDC, this document supports different
approaches and protocols for secure group communication. Therefore,
details are delegated to separate application profiles of this
document as specialized instances that target a particular group
communication approach and define how communications in the group are
protected. Compliance requirements for such application profiles are
also specified.
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/rfc9594.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Terminology
2. Overview
3. Authorization to Join a Group
3.1. Authorization Request
3.2. Authorization Response
3.3. Token Transferring
3.3.1. 'sign_info' Parameter
3.3.2. 'kdcchallenge' Parameter
4. KDC Functionalities
4.1. Interface at the KDC
4.1.1. Operations Supported by Clients
4.1.2. Error Handling
4.2. /ace-group
4.2.1. FETCH Handler
4.2.1.1. Retrieve Group Names
4.3. /ace-group/GROUPNAME
4.3.1. POST Handler
4.3.1.1. Join the Group
4.3.2. GET Handler
4.3.2.1. Retrieve Group Keying Material
4.4. /ace-group/GROUPNAME/creds
4.4.1. FETCH Handler
4.4.1.1. Retrieve a Subset of Authentication Credentials in
the Group
4.4.2. GET Handler
4.4.2.1. Retrieve All Authentication Credentials in the
Group
4.5. /ace-group/GROUPNAME/kdc-cred
4.5.1. GET Handler
4.5.1.1. Retrieve the KDC's Authentication Credential
4.6. /ace-group/GROUPNAME/policies
4.6.1. GET Handler
4.6.1.1. Retrieve the Group Policies
4.7. /ace-group/GROUPNAME/num
4.7.1. GET Handler
4.7.1.1. Retrieve the Keying Material Version
4.8. /ace-group/GROUPNAME/nodes/NODENAME
4.8.1. GET Handler
4.8.1.1. Retrieve Group and Individual Keying Material
4.8.2. POST Handler
4.8.2.1. Request to Change Individual Keying Material
4.8.3. DELETE Handler
4.8.3.1. Leave the Group
4.9. /ace-group/GROUPNAME/nodes/NODENAME/cred
4.9.1. POST Handler
4.9.1.1. Uploading an Authentication Credential
5. Removal of a Group Member
6. Group Rekeying Process
6.1. Point-to-Point Group Rekeying
6.2. One-to-Many Group Rekeying
6.2.1. Protection of Rekeying Messages
6.3. Misalignment of Group Keying Material
7. Extended Scope Format
8. ACE Groupcomm Parameters
9. ACE Groupcomm Error Identifiers
10. Security Considerations
10.1. Secure Communication in the Group
10.2. Update of Group Keying Material
10.3. Block-Wise Considerations
11. IANA Considerations
11.1. Media Type Registrations
11.2. CoAP Content-Formats
11.3. OAuth Parameters
11.4. OAuth Parameters CBOR Mappings
11.5. Interface Description (if=) Link Target Attribute Values
11.6. Custom Problem Detail Keys Registry
11.7. ACE Groupcomm Parameters
11.8. ACE Groupcomm Key Types
11.9. ACE Groupcomm Profiles
11.10. ACE Groupcomm Policies
11.11. Sequence Number Synchronization Methods
11.12. ACE Groupcomm Errors
11.13. ACE Groupcomm Rekeying Schemes
11.14. Expert Review Instructions
12. References
12.1. Normative References
12.2. Informative References
Appendix A. Requirements for Application Profiles
A.1. Mandatory-to-Address Requirements
A.2. Optional-to-Address Requirements
Appendix B. Extensibility for Future COSE Algorithms
B.1. Format of 'sign_info_entry'
Acknowledgments
Authors' Addresses
1. Introduction
This document builds on the Authentication and Authorization for
Constrained Environments (ACE) framework and defines how to request,
distribute, and renew keying material and configuration parameters to
protect message exchanges in a group communication environment.
Candidate group members that act as ACE Clients and are authorized to
join a group can interact with the Key Distribution Center (KDC)
acting as the ACE Resource Server that is responsible for that group
in order to obtain the necessary keying material and parameters to
communicate with other group members.
In particular, this document defines the operations and interface
available at the KDC, as well as general message formats for the
interactions between Clients and the KDC. At the same time,
communications in the group can rely on different approaches, e.g.,
based on multicast [GROUP-CoAP] or publish-subscribe (pub-sub)
messaging [CoAP-PUBSUB], and can be protected in different ways.
Therefore, this document delegates details on the communication and
security approaches used in a group to separate application profiles.
These are specialized instances of this document that target a
particular group communication approach and define how communications
in the group are protected, as well as the specific keying material
and configuration parameters provided to group members.
In order to ensure consistency and aid the development of such
application profiles, Appendix A of this document defines a number of
related compliance requirements. In particular, Appendix A.1
compiles the requirements that application profiles are REQUIRED to
fulfill; these are referred to by an identifier that starts with
"REQ". Instead, Appendix A.2 compiles the requirements that
application profiles MAY fulfill; these are referred to by an
identifier that starts with "OPT".
New keying material is intended to be generated and distributed to
the group upon membership changes (rekeying). If the application
requires backward security (i.e., new group members must be prevented
from accessing communications in the group prior to their joining),
then a rekeying has to occur every time new members join the group.
If the application requires forward security (i.e., former group
members must be prevented from accessing communications in the group
after their leaving), then a rekeying has to occur every time current
members leave or are evicted from the group.
A group rekeying scheme performs the actual distribution of the new
keying material by rekeying the current group members when a new
Client joins the group and rekeying the remaining group members when
a Client leaves the group. This can rely on different approaches,
including efficient group rekeying schemes such as those described in
[RFC2093], [RFC2094], and [RFC2627].
Consistently with what is recommended in the ACE framework, this
document uses Concise Binary Object Representation (CBOR) [RFC8949]
for data encoding. However, using JSON [RFC8259] instead of CBOR is
possible by relying on the conversion method specified in Sections
6.1 and 6.2 of [RFC8949].
1.1. 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.
Readers are expected to be familiar with the following:
* The terms and concepts described in the ACE framework [RFC9200]
and in the Authorization Information Format (AIF) [RFC9237] to
express authorization information. The terminology for entities
in the considered architecture is defined in OAuth 2.0 [RFC6749].
In particular, this includes Client (C), Resource Server (RS), and
Authorization Server (AS).
* The terms and concepts described in the Constrained Application
Protocol (CoAP) [RFC7252]. The term "endpoint" is used here
following its OAuth definition, aimed at denoting resources such
as /token and /introspect at the AS and /authz-info at the RS.
This document does not use the CoAP definition of "endpoint",
which is "An entity participating in the CoAP protocol".
* The terms and concepts described in Concise Data Definition
Language (CDDL) [RFC8610], CBOR [RFC8949], and CBOR Object Signing
and Encryption (COSE) [RFC9052] [RFC9053] [RFC9338].
A node interested in participating in group communication, as well as
one that is already participating as a group member, is
interchangeably denoted as "Client".
This document also uses the following terms.
Group: A set of nodes that share common keying material and security
parameters used to protect their communications with one another.
That is, the term refers to a "security group".
This term is not to be confused with an "application group", which
has relevance at the application level and whose members share a
common pool of resources or content. Examples of application
groups are the set of all nodes deployed in a same physical room
or the set of nodes registered to a pub-sub topic.
This term is also not to be confused with a "multicast group",
which has relevance at the network level and whose members all
listen to a group network address for receiving messages sent to
that group. An example of a multicast group is the set of nodes
that are configured to receive messages that are sent to the
group's associated IP multicast address.
The same security group might be associated with multiple
application groups. Also, the same application group might be
associated with multiple security groups. Further details and
considerations on the mapping between the three types of groups
are out of the scope of this document.
Key Distribution Center (KDC): The entity responsible for managing
one or multiple groups, with particular reference to the group
membership and the keying material to use for protecting group
communications.
Furthermore, this document uses "names" or "identifiers" for groups
and nodes. Their different meanings are summarized below.
Group name: The identifier of a group as a text string encoded as
UTF-8 [RFC3629]. Once established, it is invariant. It is used
in the interactions between the Client, AS, and RS to identify a
group. A group name is always unique among the group names of the
existing groups under the same KDC.
GROUPNAME: The text string used in URIs to identify a group. Once
established, it is invariant. GROUPNAME uniquely maps to the
group name of a group, although they do not necessarily coincide.
Group identifier: The identifier of the group keying material used
in a group. Unlike group name and GROUPNAME, this identifier
changes over time when the group keying material is updated.
Node name: The identifier of a node as a text string encoded as
UTF-8 [RFC3629] and consistent with the semantics of URI path
segments (see Section 3.3 of [RFC3986]). Once established, it is
invariant. It is used in the interactions between the Client and
RS, as well as to identify a member of a group. A node name is
always unique among the node names of the current nodes within a
group.
NODENAME: The text string used in URIs to identify a member of a
group. Once established, it is invariant. Its value coincides
with the node name of the associated group member.
This document additionally uses the following terminology:
Transport profile: A profile of the ACE framework as per
Section 5.8.4.3 of [RFC9200]. A transport profile specifies the
communication protocol and communication security protocol between
an ACE Client and Resource Server, as well as proof-of-possession
methods if it supports proof-of-possession access tokens.
Transport profiles of ACE include, for instance, those described
in [RFC9202], [RFC9203], and [RFC9431].
Application profile: A profile that defines how applications enforce
and use supporting security services they require. These services
may include, for instance, provisioning, revocation, and
distribution of keying material. An application profile may
define specific procedures and message formats.
Authentication credential: The set of information associated with an
entity, including that entity's public key and parameters
associated with the public key. Examples of authentication
credentials are CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs)
[RFC8392], X.509 certificates [RFC5280], and C509 certificates
[C509-CERT].
Individual keying material: Information pertaining exclusively to a
group member, as associated with its group membership and related
to other keying material and parameters used in the group. For
example, this can be an identifier that the secure communication
protocol employs to uniquely identify a node as a group member
(e.g., a cryptographic key identifier uniquely associated with the
group member in question). The specific nature and format of
individual keying material used in a group is defined in the
application profiles of this specification. The individual keying
material of a group member is not related to the secure
association between that group member and the KDC.
Throughout this document, examples for CBOR data items are expressed
in CBOR extended diagnostic notation as defined in Section 8 of
[RFC8949] and Appendix G of [RFC8610] ("diagnostic notation"), unless
noted otherwise. We often use diagnostic notation comments to
provide a textual representation of the parameters' keys and values.
2. Overview
At a high level, the key provisioning process is separated in two
phases: the first one follows the ACE framework between the Client,
AS, and KDC, while the second one is the actual key distribution
between the Client and KDC. After the two phases are completed, the
Client is able to participate in the group communication via a
Dispatcher entity.
.------------. .------------.
| AS | .----->| KDC |
'------------' | '------------'
^ |
| |
v |
.------------. | .-----------.
| Client |<-------' .------------. | .---------+-.
| |<------------->| Dispatcher |<----->| | .---------+-.
'------------' '------------' '-+ | Group |
'-+ members |
'-----------'
Figure 1: Key Distribution Participants
The following participants (see Figure 1) take part in the
authorization and key distribution.
* Client (C): A node that wants to join a group and take part in
group communication with other group members. Within the group,
the Client can have different roles.
* Authorization Server (AS): As per the AS defined in the ACE
framework [RFC9200], it enforces access policies that prescribe
whether a node is allowed to join a given group or not and with
what roles and rights (e.g., write and/or read).
* Key Distribution Center (KDC): An entity that maintains the keying
material to protect group communications and provides it to
Clients authorized to join a given group. During the first phase
of the process (Section 3), the KDC takes the role of the RS in
the ACE framework. During the second phase of the process
(Section 4), which is not based on the ACE framework, the KDC
distributes the keying material. In addition, the KDC provides
the latest keying material to group members when requested or, if
required by the application, when group membership changes.
* Group members: Nodes that have joined a group where they take part
in group communication with one another, protecting it with the
group keying material obtained from the KDC.
* Dispatcher: An entity through which the Clients communicate with
the group when sending a message intended for multiple group
members. That is, the Dispatcher distributes such a one-to-many
message to the group members as intended recipients. The
Dispatcher does not have access to the group keying material. A
single-recipient message intended for only one group member may be
delivered by alternative means, i.e., with no assistance from the
Dispatcher.
Examples of a Dispatcher are: the Broker in a pub-sub setting; a
relayer for group communication that delivers group messages as
multiple unicast messages to all group members; and an implicit
entity as in a multicast communication setting, where messages are
transmitted to a multicast IP address and delivered on the
transport channel.
If it consists of an explicit entity, such as a pub-sub Broker or
a message relayer, the Dispatcher is comparable to an untrusted
on-path intermediary; as such, it is able to see the messages sent
by Clients in the group but not able to decrypt them and read
their plain content.
This document specifies a mechanism for:
* Authorizing a Client to join the group (Section 3) and providing
it with the group keying material to communicate with the other
group members (Section 4),
* Allowing a group member to retrieve group keying material
(Sections 4.3.2.1 and 4.8.1.1),
* Allowing a group member to retrieve authentication credentials of
other group members (Section 4.4.1.1) and to provide an updated
authentication credential (Section 4.9.1.1),
* Allowing a group member to leave the group (Section 4.8.3.1),
* Evicting a group member from the group (Section 5), and
* Renewing and redistributing the group keying material (rekeying),
e.g., upon a membership change in the group (Section 6).
Rekeying the group may result in a temporary misalignment of the
keying material stored by the different group members. Different
situations where this can happen and how they can be handled are
discussed in Section 6.3.
Figure 2 provides a high-level overview of the message flow for a
node joining a group. The message flow can be expanded as follows.
1. The joining node requests an access token from the AS in order to
access one or more group-membership resources at the KDC and
hence join the associated groups.
This exchange between the Client and AS MUST be secured, as
specified by the transport profile of ACE used between the Client
and KDC. Based on the response from the AS, the joining node
will establish or continue using a secure communication
association with the KDC.
2. The joining node transfers authentication and authorization
information to the KDC by transferring the obtained access token.
This is typically achieved by including the access token in a
request sent to the /authz-info endpoint at the KDC.
Once this exchange is completed, the joining node MUST have a
secure communication association established with the KDC before
joining a group under that KDC.
This exchange and the following secure communications between the
Client and the KDC MUST occur in accordance with the transport
profile of ACE used between the Client and KDC, such as the DTLS
transport profile of ACE [RFC9202] or the OSCORE transport
profile of ACE [RFC9203].
3. The joining node starts the joining process to become a member of
the group by sending a request to the related group-membership
resource at the KDC. Based on the application requirements and
policies, the KDC may perform a group rekeying by generating new
group keying material and distributing it to the current group
members through the rekeying scheme used in the group.
At the end of the joining process, the joining node has received
the parameters and group keying material from the KDC to securely
communicate with the other group members. Also, the KDC has
stored the association between the authorization information from
the access token and the secure communication association with
the joining node.
4. The joining node and the KDC maintain the secure communication
association to support possible future communications. These
especially include key management operations, such as the
retrieval of updated keying material or the participation in a
group rekeying process.
5. The joining node can communicate securely with the other group
members by using the keying material obtained in step 3.
C AS KDC Group
| | | Members
/ | | | |
| |--- Authorization Request -->| | |
| | | | |
| |<-- Authorization Response --| | |
(*) < | | | |
| | | | |
| |--- Token Transfer Request ---->| |
| | | |
| |<--- Token Transfer Response-----| |
\ | | | |
| | | |
|--------- Join Request --------->| |
| | | |
| | | -- Group rekeying -->|
| | | (optional) |
|<-------- Join Response ---------| |
| | | |
| | | |
| | | Dispatcher |
| | |
|<======= Secure group communication =========|=========>|
| | |
(*) Defined in the ACE framework
Figure 2: Message Flow upon a New Node's Joining
3. Authorization to Join a Group
This section describes in detail the format of messages exchanged by
the participants when a node requests access to a given group. This
exchange is based on ACE [RFC9200].
As defined in [RFC9200], the Client asks the AS for the authorization
to join the group through the KDC (see Section 3.1). If the request
is approved and authorization is granted, the AS provides the Client
with a proof-of-possession access token and parameters to securely
communicate with the KDC (see Section 3.2).
Communications between the Client and the AS MUST be secured
according to what is defined by the used transport profile of ACE.
The Content-Format used in the message also depends on the used
transport profile of ACE. For example, it can be "application/
ace+cbor" for the first two messages and "application/cwt" for the
third message, which are defined in the ACE framework.
The transport profile of ACE also defines a number of details, such
as the communication and security protocols used with the KDC (see
Appendix C of [RFC9200]).
Figure 3 gives an overview of the exchange described above.
Client AS KDC
| | |
|---- Authorization Request: POST /token ------->| |
| | |
|<--- Authorization Response: 2.01 (Created) ----| |
| | |
|---- Token Transfer Request: POST /authz-info ------->|
| | |
|<--- Token Transfer Response: 2.01 (Created) -------->|
| | |
Figure 3: Message Flow of Join Authorization
3.1. Authorization Request
The Authorization Request sent from the Client to the AS is defined
in Section 5.8.1 of [RFC9200] and MAY contain the following
parameters, which, if included, MUST have the format and value as
specified below.
* 'scope': specifying the names of the groups that the Client
requests to access and optionally the roles that the Client
requests to have in those groups.
This parameter is encoded as a CBOR byte string, which wraps a
CBOR array of scope entries. All the scope entries are specified
according to the same format, i.e., either the Authorization
Information Format (AIF) or the textual format defined below.
- If AIF is used, each scope entry is encoded as per [RFC9237],
i.e., as a CBOR array [Toid, Tperm]. If a scope entry
expresses a set of roles to take in a group as per this
document, the object identifier "Toid" specifies the group name
and MUST be encoded as a CBOR text string, while the permission
set "Tperm" specifies the roles that the Client wishes to take
in the group.
AIF is the default format for application profiles of this
specification and is preferable for those that aim for a
compact encoding of scope. This is especially desirable for
application profiles defining several roles, with the Client
possibly asking for multiple roles combined.
Figure 4 shows an example in CDDL notation [RFC8610] where
scope uses AIF.
- If the textual format is used, each scope entry is a CBOR array
formatted as follows.
o As the first element, the group name, encoded as a CBOR text
string.
o Optionally, as the second element, the role or CBOR array of
roles that the Client wishes to take in the group. This
element is optional since roles may have been pre-assigned
to the Client, as associated with its verifiable identity
credentials. Alternatively, the application may have
defined a single, well-known role for the target resource(s)
and audience(s).
Figure 5 shows an example in CDDL notation where scope uses the
textual format with the group name and role identifiers encoded
as CBOR text strings.
It is REQUIRED for application profiles of this specification to
specify the exact format and encoding of scope (REQ1). This
includes defining the set of possible roles and their identifiers,
as well as the corresponding encoding to use in the scope entries
according to the used scope format.
If the application profile uses AIF, it is also REQUIRED to
register its specific instance of "Toid" and "Tperm", as well as
the corresponding media type and Content-Format, as per the
guidelines in [RFC9237] (REQ2).
If the application profile uses the textual format, it MAY
additionally specify CBOR values to use for abbreviating the role
identifiers (OPT1).
* 'audience': with an identifier of the KDC.
As defined in [RFC9200], other additional parameters can be included
if necessary.
;# include rfc9237
gname = tstr
permissions = uint .bits roles
roles = &(
Requester: 1,
Responder: 2,
Monitor: 3,
Verifier: 4
)
scope_entries = AIF-Generic<gname, permissions>
scope = bstr .cbor scope_entries
Figure 4: Example of scope Using AIF
gname = tstr
role = tstr
scope_entry = [gname, ? ( role / [2* role] )]
scope_entries = [* scope_entry]
scope = bstr .cbor scope_entries
Figure 5: Example of scope Using the Textual Format, with the
Role Identifiers Encoded as Text Strings
3.2. Authorization Response
The AS processes the Authorization Request as defined in
Section 5.8.2 of [RFC9200], especially verifying that the Client is
authorized to access the specified groups with the requested roles or
possibly a subset of those.
In case of successful verification, the Authorization Response sent
from the AS to the Client is also defined in Section 5.8.2 of
[RFC9200]. Note that the 'expires_in' parameter MAY be omitted if
the application defines how the expiration time is communicated to
the Client via other means or if it establishes a default value.
Additionally, when included, the following parameter MUST have the
corresponding values:
* 'scope' has the same format and encoding of 'scope' in the
Authorization Request, as defined in Section 3.1. If this
parameter is not present, the granted scope is equal to the one
requested in Section 3.1.
The proof-of-possession access token in the 'access_token' parameter
MUST contain the following:
* a confirmation claim (for example, see 'cnf' defined in
Section 3.1 of [RFC8747] for CWTs)
* an expiration time claim (for example, see 'exp' defined in
Section 3.1.4 of [RFC8392] for CWTs)
* a scope claim (for example, see 'scope' registered in Section 8.14
of [RFC9200] for CWTs)
If the 'scope' parameter is present in the Authorization Response,
this claim specifies the same access control information as in the
'scope' parameter. Instead, if the 'scope' parameter is not
present in the Authorization Response, this claim specifies the
same access control information as in the 'scope' parameter of the
Authorization Request, if the parameter is present therein, or the
default scope that the AS is granting the Client otherwise.
By default, this claim has the same encoding as the 'scope'
parameter in the Authorization Request, as defined in Section 3.1.
Optionally, an alternative extended format of scope defined in
Section 7 can be used. This format explicitly signals the
semantics used to express the actual access control information,
which has to be parsed. This enables a Resource Server to
correctly process a received access token, also in case:
- The Resource Server implements a KDC that supports multiple
application profiles of this specification using different
scope semantics and/or
- The Resource Server implements further services beyond a KDC
for group communication using different scope semantics.
If the Authorization Server is aware that this applies to the
Resource Server for which the access token is issued, the
Authorization Server SHOULD use the extended format of scope
defined in Section 7.
The access token MAY additionally contain other claims that the
transport profile of ACE or other optional parameters require.
When receiving an Authorization Request from a Client that was
previously authorized and for which the AS still stores a valid non-
expired access token, the AS MAY reply with that token. Note that it
is up to application profiles of ACE to make sure that reposting the
same access token does not cause reuse of keying material between
nodes (for example, that is accomplished with the use of random
nonces in [RFC9203]).
3.3. Token Transferring
The Client sends a Token Transfer Request to the KDC, i.e., a CoAP
POST request including the access token and targeting the /authz-info
endpoint (see Section 5.10.1 of [RFC9200]).
Note that this request deviates from the one defined in [RFC9200],
since it allows asking the KDC for additional information concerning
the authentication credentials used in the group to ensure source
authentication, as well as for possible additional group parameters.
The joining node MAY ask for this information from the KDC through
the same Token Transfer Request. In this case, the message MUST have
Content-Format "application/ace+cbor" registered in Section 8.16 of
[RFC9200], and the message payload MUST be formatted as a CBOR map,
which MUST include the access token. The CBOR map MAY additionally
include the following parameter, which, if included, MUST have the
format and value as specified below.
* 'sign_info': defined in Section 3.3.1, specifying the CBOR simple
value null (0xf6) to request information about the signature
algorithm, the signature algorithm parameters, the signature key
parameters, and the exact format of authentication credentials
used in the groups that the Client has been authorized to join.
Alternatively, such information may be pre-configured on the joining
node or may be retrieved by alternative means. For example, the
joining node may have performed an early group discovery process and
obtained the link to the associated group-membership resource at the
KDC, along with attributes that describe the group configuration
(e.g., see [OSCORE-DISCOVERY]).
After successful verification, the Client is authorized to receive
the group keying material from the KDC and join the group. Hence,
the KDC replies to the Client with a Token Transfer Response, i.e., a
CoAP 2.01 (Created) response.
The Token Transfer Response MUST have Content-Format "application/
ace+cbor", and its payload is a CBOR map. Note that this deviates
from what is defined in the ACE framework, where the response from
the /authz-info endpoint is defined as conveying no payload (see
Section 5.10.1 of [RFC9200]).
If a scope entry in the access token specifies a role that requires
the Client to send its own authentication credential to the KDC when
joining the related group, then the CBOR map MUST include the
'kdcchallenge' parameter defined in Section 3.3.2, specifying a
dedicated challenge N_S generated by the KDC.
Later, when joining the group (see Section 4.3.1.1), the Client uses
the 'kdcchallenge' value and additional information to build a proof-
of-possession (PoP) input. In turn, this is used to compute the PoP
evidence that the Client also provides to the KDC, in order to prove
possession of its own private key (see the 'client_cred_verify'
parameter in Section 4.3.1).
While storing the access token, the KDC MUST store the 'kdcchallenge'
value associated with the Client at least until it receives a Join
Request from the Client (see Section 4.3.1.1) to be able to verify
the PoP evidence provided during the join process and thus that the
Client possesses its own private key. The KDC deletes the
'kdcchallenge' value associated with the Client upon deleting the
access token (e.g., upon its expiration, see Section 5.10.3 of
[RFC9200]).
The same 'kdcchallenge' value MAY be reused several times by the
Client to generate new PoP evidence, e.g., in case the Client
provides the KDC with a new authentication credential while being a
group member (see Section 4.9.1.1) or joins a different group where
it intends to use a different authentication credential. Therefore,
it is RECOMMENDED that the KDC keeps storing the 'kdcchallenge' value
after the first join is processed as well. If, upon receiving a Join
Request from a Client, the KDC has already discarded the
'kdcchallenge' value, that will trigger an error response with a
newly generated 'kdcchallenge' value that the Client can use to
restart the join process, as specified in Section 4.3.1.1.
If 'sign_info' is included in the Token Transfer Request, the KDC
SHOULD include the 'sign_info' parameter in the Token Transfer
Response, as per the format defined in Section 3.3.1. Note that the
field 'id' of each 'sign_info_entry' specifies the name or array of
group names to which that 'sign_info_entry' applies. As an
exception, the KDC MAY omit the 'sign_info' parameter in the Token
Transfer Response even if 'sign_info' is included in the Token
Transfer Request in case none of the groups that the Client is
authorized to join use signatures to achieve source authentication.
Note that the CBOR map specified as payload of the 2.01 (Created)
response may include further parameters, e.g., according to the used
transport profile of ACE. Application profiles of this specification
MAY define additional parameters to use within this exchange (OPT2).
Application profiles of this specification MAY define alternative
specific negotiations of parameter values for the signature algorithm
and signature keys if 'sign_info' is not used (OPT3).
If allowed by the used transport profile of ACE, the Client may
provide the access token to the KDC by other means than the Token
Transfer Request. An example is the DTLS transport profile of ACE,
where the Client can provide the access token to the KDC during the
secure session establishment (see Section 3.3.2 of [RFC9202]).
3.3.1. 'sign_info' Parameter
The 'sign_info' parameter is an OPTIONAL parameter of the request and
response messages exchanged between the Client and the /authz-info
endpoint at the RS (see Section 5.10.1 of [RFC9200]).
This parameter allows the Client and the RS to exchange information
about a signature algorithm and about authentication credentials to
accordingly use for signature verification. Its exact semantics and
content are application specific.
In this specification and in application profiles building on it,
this parameter is used to exchange information about the signature
algorithm and about authentication credentials to be used with it in
the groups indicated by the transferred access token as per its
'scope' claim (see Section 3.2).
When used in the Token Transfer Request sent to the KDC (see
Section 3.3), the 'sign_info' parameter specifies the CBOR simple
value null (0xf6). This is done to ask for information about the
signature algorithm and about the authentication credentials used in
the groups that, as per the granted roles, the Client has been
authorized to join or interact with (e.g., as an external signature
verifier).
When used in the following Token Transfer Response from the KDC (see
Section 3.3), the 'sign_info' parameter is a CBOR array of one or
more elements. The number of elements is at most the number of
groups that the Client has been authorized to join or interact with.
Each element contains information about signing parameters and about
authentication credentials for one or more groups and is formatted as
follows.
* The first element 'id' is a group name or a CBOR array of group
names, which is associated with groups for which the next four
elements apply. Each specified group name is a CBOR text string
and is hereafter referred to as 'gname'.
* The second element 'sign_alg' is a CBOR integer or a text string
that indicates the signature algorithm used in the groups
identified by the 'gname' values. It is REQUIRED for application
profiles to define specific values that this parameter can take
(REQ3), which are selected from the set of signing algorithms of
the "COSE Algorithms" registry [COSE.Algorithms].
* The third element 'sign_parameters' is a CBOR array that indicates
the parameters of the signature algorithm used in the groups
identified by the 'gname' values. Its content depends on the
value of 'sign_alg'. It is REQUIRED for application profiles to
define the possible values and structure for the elements of this
parameter (REQ4).
* The fourth element 'sign_key_parameters' is a CBOR array that
indicates the parameters of the key used with the signature
algorithm in the groups identified by the 'gname' values. Its
content depends on the value of 'sign_alg'. It is REQUIRED for
application profiles to define the possible values and structure
for the elements of this parameter (REQ5).
* The fifth element 'cred_fmt' either is a CBOR integer indicating
the format of authentication credentials used in the groups
identified by the 'gname' values or is the CBOR simple value null
(0xf6), which indicates that the KDC does not act as a repository
of authentication credentials for group members. Its acceptable
integer values are taken from the "Label" column of the "COSE
Header Parameters" registry [COSE.Header.Parameters], with some of
those values also indicating the type of container to use for
exchanging the authentication credentials with the KDC (e.g., a
chain or bag of certificates). It is REQUIRED for application
profiles to define specific values to use for this parameter,
consistently with the acceptable formats of authentication
credentials (REQ6).
The CDDL notation [RFC8610] of the 'sign_info' parameter is given
below.
sign_info = sign_info_req / sign_info_resp
sign_info_req = null ; in the Token Transfer
; Request to the KDC
sign_info_resp = [+ sign_info_entry] ; in the Token Transfer
; Response from the KDC
sign_info_entry =
[
id: gname / [+ gname],
sign_alg: int / tstr,
sign_parameters: [any],
sign_key_parameters: [+ parameter: any],
cred_fmt: int / null
]
gname = tstr
This format is consistent with every signature algorithm currently
defined in [RFC9053], i.e., with algorithms that have only the COSE
key type as their COSE capability. Appendix B describes how the
format of each 'sign_info_entry' can be generalized for possible
future registered algorithms having a different set of COSE
capabilities.
3.3.2. 'kdcchallenge' Parameter
The 'kdcchallenge' parameter is an OPTIONAL parameter of the response
message returned from the /authz-info endpoint at the RS, as defined
in Section 5.10.1 of [RFC9200]. This parameter contains a challenge
generated by the RS and provided to the Client.
In this specification and in application profiles building on it, the
Client can use this challenge to prove possession of its own private
key in the Join Request (see the 'client_cred_verify' parameter in
Section 4.3.1).
4. KDC Functionalities
This section describes the functionalities provided by the KDC, as
related to the provisioning of the keying material as well as to the
group membership management.
In particular, this section defines the interface available at the
KDC, specifies the handlers of each resource provided by the KDC
interface, and describes how Clients interact with those resources to
join a group and to perform additional operations as group members.
A key operation that the Client can perform after transferring the
access token to the KDC is a Join Request-Response exchange with the
KDC. In the Join Request, the Client specifies the group it requests
to join (see Section 4.3.1.1). The KDC will then check the stored
access token associated with the Client and verify that the Client is
accordingly authorized to join the specified group. In case of
successful verification, the KDC provides the Client with the keying
material to securely communicate with the other members of the group.
Later on as a group member, the Client can also rely on the interface
at the KDC to perform additional operations consistent with the roles
it has in the group.
4.1. Interface at the KDC
The KDC provides its interface by hosting the following resources.
Note that the root url-path "ace-group" used hereafter is a default
name; implementations are not required to use this name and can
define their own instead.
If request messages sent to the KDC as well as success response
messages from the KDC include a payload and specify a Content-Format,
those messages MUST have Content-Format "application/ace-
groupcomm+cbor", which is registered in Section 11.2. CBOR map keys
used for the message parameters are defined in Section 8.
* /ace-group : the path of this root resource is invariant once the
resource is established. Its employment indicates that this
specification is used. If other applications run on a KDC
implementing this specification and use this same path, those
applications will collide, and a mechanism will be needed to
differentiate the endpoints.
A Client can access this resource in order to retrieve a set of
group names, each corresponding to one of the specified group
identifiers. This operation is described in Section 4.2.1.1.
Clients may be authorized to access this resource even without
being members of any group managed by the KDC and even if they are
not authorized to become group members (e.g., when authorized to
be external signature verifiers).
The Interface Description (if=) Link Target Attribute value
"ace.groups" is registered in Section 11.5 and can be used to
describe the interface provided by this root resource.
The example below shows an exchange with a KDC with address
2001:db8::ab that hosts the resource /ace-group and returns a link
to such a resource in link-format [RFC6690].
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: ".well-known"
Uri-Path: "core"
Uri-Query: "if=ace.groups"
Response:
Header: Content (Code=2.05)
Content-Format: 40 (application/link-format)
Payload:
<coap://[2001:db8::ab]/ace-group>;if="ace.groups"
* /ace-group/GROUPNAME : one such sub-resource to /ace-group is
hosted for each group with the name GROUPNAME that the KDC
manages. In particular, it is the group-membership resource
associated with that group, and it contains the symmetric group
keying material of that group.
A Client can access this resource in order to join the group with
name GROUPNAME or later as a group member to retrieve the current
group keying material. These operations are described in Sections
4.3.1.1 and 4.3.2.1, respectively.
The Interface Description (if=) Link Target Attribute value
"ace.group" is registered in Section 11.5 and can be used to
describe the interface provided by a group-membership resource.
The example below shows an exchange with a KDC with address
2001:db8::ab that hosts the group-membership resource /ace-group/
gp1 and returns a link to such a resource in link-format
[RFC6690].
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: ".well-known"
Uri-Path: "core"
Uri-Query: "if=ace.group"
Response:
Header: Content (Code=2.05)
Content-Format: 40 (application/link-format)
Payload:
<coap://[2001:db8::ab]/ace-group/gp1>;if="ace.group"
If it is not required that the value of the GROUPNAME URI path and
the group name in the access token scope ('gname' in Section 3.1)
coincide, the KDC MUST implement a mechanism to map the GROUPNAME
value in the URI to the group name in order to refer to the
correct group (REQ7).
* /ace-group/GROUPNAME/creds : the path of this resource is
invariant once the resource is established. This resource
contains the authentication credentials of all the members of the
group with the name GROUPNAME.
This resource is created only in case the KDC acts as a repository
of authentication credentials for group members.
As a group member, a Client can access this resource in order to
retrieve the authentication credentials of other group members.
That is, the Client can retrieve the authentication credentials of
all the current group members or a subset of them by specifying
filter criteria. These operations are described in Sections
4.4.2.1 and 4.4.1.1, respectively.
Clients may be authorized to access this resource even without
being group members, e.g., if authorized to be external signature
verifiers for the group.
* /ace-group/GROUPNAME/kdc-cred : the path of this resource is
invariant once the resource is established. This resource
contains the authentication credential of the KDC for the group
with the name GROUPNAME.
This resource is created only in case the KDC has an associated
authentication credential and this is required for the correct
group operation. It is REQUIRED for application profiles to
define whether the KDC has such an associated authentication
credential (REQ8).
As a group member, a Client can access this resource in order to
retrieve the current authentication credential of the KDC. This
operation is described in Section 4.5.1.1.
Clients may be authorized to access this resource even without
being group members, e.g., if authorized to be external signature
verifiers for the group.
* /ace-group/GROUPNAME/policies : the path of this resource is
invariant once the resource is established. This resource
contains the group policies of the group with the name GROUPNAME.
A Client can access this resource as a group member in order to
retrieve the group policies. This operation is described in
Section 4.6.1.1.
* /ace-group/GROUPNAME/num : the path of this resource is invariant
once the resource is established. This resource contains the
current version number for the symmetric group keying material of
the group with the name GROUPNAME.
A Client can access this resource as a group member in order to
retrieve the version number of the keying material currently used
in the group. This operation is described in Section 4.7.1.1.
* /ace-group/GROUPNAME/nodes/NODENAME : one such sub-resource of
/ace-group/GROUPNAME is hosted for each group member of the group
with the name GROUPNAME. Each such resource is identified by the
node name NODENAME of the associated group member and contains the
group keying material and the individual keying material for that
group member.
A Client as a group member can access this resource in order to
retrieve the current group keying material together with its
individual keying material, request new individual keying material
to use in the group, and leave the group. These operations are
described in Sections 4.8.1.1, 4.8.2.1, and 4.8.3.1, respectively.
* /ace-group/GROUPNAME/nodes/NODENAME/cred : the path of this
resource is invariant once the resource is established. This
resource contains the individual authentication credential for the
node with the name NODENAME as a group member of the group with
the name GROUPNAME.
A Client can access this resource in order to upload at the KDC a
new authentication credential to use in the group. This operation
is described in Section 4.9.1.1.
This resource is not created if the group member does not have an
authentication credential to use in the group or if the KDC does
not store the authentication credentials of group members.
The KDC is expected to fully provide the interface defined above. It
is otherwise REQUIRED for the application profiles of this
specification to indicate which resources are not hosted, i.e., which
parts of the interface defined in this section are not supported by
the KDC (REQ9). Application profiles of this specification MAY
extend the KDC interface by defining additional handlers, as well as
defining additional resources and their handlers.
It is REQUIRED for application profiles of this specification to
register a Resource Type for the group-membership resources (REQ10).
This Resource Type can be used to discover the correct URL for
sending a Join Request to the KDC. This Resource Type can also be
used to indicate which specific application profile of this
specification is used by a specific group-membership resource at the
KDC.
It is REQUIRED for application profiles of this specification to
define what specific actions (e.g., CoAP methods) are allowed on each
resource provided by the KDC interface, depending on whether the
Client is a current group member, the roles that a Client is
authorized to take as per the obtained access token (see
Section 3.1), and the roles that the Client has as current group
member (REQ11).
4.1.1. Operations Supported by Clients
It is expected that a Client minimally supports the following set of
primary operations and corresponding interactions with the KDC.
* FETCH request to /ace-group/ in order to retrieve group names
associated with group identifiers.
* POST and GET requests to /ace-group/GROUPNAME/ in order to join a
group (POST) and later retrieve the current group keying material
as a group member (GET).
* GET and FETCH requests to /ace-group/GROUPNAME/creds in order to
retrieve the authentication credentials of all the other group
members (GET) or only some of them by filtering (FETCH). While
retrieving authentication credentials remains possible by using
GET requests, retrieval by filtering allows Clients to greatly
limit the size of exchanged messages.
* GET request to /ace-group/GROUPNAME/num in order to retrieve the
current version of the group keying material as a group member.
* DELETE request to /ace-group/GROUPNAME/nodes/NODENAME in order to
leave the group.
In addition, some Clients may rather not support the following set of
secondary operations and corresponding interactions with the KDC.
This can be specified, for instance, in compliance documents defining
minimalistic Clients and their capabilities in specific deployments.
In turn, these might also have to consider the used application
profile of this specification.
* GET request to /ace-group/GROUPNAME/kdc-cred in order to retrieve
the current authentication credential of the KDC. This is
relevant only if the KDC has an associated authentication
credential and this is required for the correct group operation.
* GET request to /ace-group/GROUPNAME/policies in order to retrieve
the current group policies as a group member.
* GET request to /ace-group/GROUPNAME/nodes/NODENAME in order to
retrieve the current group keying material and individual keying
material. The former can also be retrieved through a GET request
to /ace-group/GROUPNAME/ (see above).
* POST request to /ace-group/GROUPNAME/nodes/NODENAME in order to
ask for new individual keying material. Alternatively, the Client
could obtain new individual keying material by rejoining the group
through a POST request to /ace-group/GROUPNAME/ (see above).
Furthermore, depending on its roles in the group or on the
application profile of this specification, the Client might simply
not be associated with any individual keying material.
* POST request to /ace-group/GROUPNAME/nodes/NODENAME/cred in order
to provide the KDC with a new authentication credential.
Alternatively, the Client could provide a new authentication
credential by rejoining the group through a POST request to /ace-
group/GROUPNAME/ (see above). Furthermore, depending on its roles
in the group, the Client might simply not have an associated
authentication credential to provide.
It is REQUIRED for application profiles of this specification to
categorize possible newly defined operations for Clients into primary
and secondary operations and to provide accompanying considerations
(REQ12).
4.1.2. Error Handling
Upon receiving a request from a Client, the KDC MUST check that it is
storing a valid access token from that Client. If this is not the
case, the KDC MUST reply with a 4.01 (Unauthorized) error response.
Unless the request targets the /ace-group resource, the KDC MUST
check that it is storing a valid access token for that Client such
that:
* the scope specified in the access token includes a scope entry
related to the group name GROUPNAME associated with the targeted
resource and
* the set of roles specified in that scope entry allows the Client
to perform the requested operation on the targeted resource
(REQ11).
In case the KDC stores a valid access token but the verifications
above fail, the KDC MUST reply with a 4.03 (Forbidden) error
response. This response MAY be an AS Request Creation Hints, as
defined in Section 5.3 of [RFC9200], in which case the Content-Format
MUST be "application/ace+cbor".
If the request is not formatted correctly (e.g., required fields are
not present or are not encoded as expected), the KDC MUST reply with
a 4.00 (Bad Request) error response.
If the request includes unknown or unexpected fields, the KDC MUST
silently ignore them and continue processing the request.
Application profiles of this specification MAY define optional or
mandatory payload formats for specific error cases (OPT4).
Some error responses from the KDC can convey error-specific
information according to the problem-details format defined in
[RFC9290]. Such error responses MUST have Content-Format
"application/concise-problem-details+cbor". The payload of these
error responses MUST be a CBOR map specifying a Concise Problem
Details data item (see Section 2 of [RFC9290]). The CBOR map is
formatted as follows.
* It MUST include the Custom Problem Detail entry 'ace-groupcomm-
error', which is registered in Section 11.6 of this document.
This entry is formatted as a CBOR map including only one field,
namely 'error-id'. The map key for 'error-id' is the CBOR
unsigned integer with value 0. The value of 'error-id' is a CBOR
integer specifying the error that occurred at the KDC. This value
is taken from the "Value" column of the "ACE Groupcomm Errors"
registry defined in Section 11.12 of this document.
The CDDL notation [RFC8610] of the 'ace-groupcomm-error' entry is
given below.
ace-groupcomm-error = {
&(error-id: 0) => int
}
* It MAY include further Standard Problem Detail entries or Custom
Problem Detail entries (see [RFC9290]).
In particular, it can include the Standard Problem Detail entry
'detail' (map key -2), whose value is a CBOR text string that
specifies a human-readable, diagnostic description of the error
occurred at the KDC. The diagnostic text is intended for software
engineers as well as for device and network operators in order to
aid debugging and provide context for possible intervention. The
diagnostic message SHOULD be logged by the KDC. The 'detail'
entry is unlikely relevant in an unattended setup where human
intervention is not expected.
An example of an error response using the problem-details format is
shown in Figure 6.
Response:
Header: Service Unavailable (Code=5.03)
Content-Format: 257 (application/concise-problem-details+cbor)
Payload:
{
/ title / -1: "No available individual keying material",
/ detail / -2: "Things will change after a
group rekeying; try later",
/ ace-groupcomm-error / 0: {
/ error-id / 0: 4 / "No available individual keying material" /
}
}
Figure 6: Example of an Error Response with Problem Details
The problem-details format (in general) and the Custom Problem Detail
entry 'ace-groupcomm-error' (in particular) are OPTIONAL for Clients
to support. A Client supporting the entry 'ace-groupcomm-error' and
that can understand the specified error may use that information to
determine what actions to take next.
Section 9 of this specification defines an initial set of error
identifiers as possible values for the 'error-id' field. Application
profiles of this specification inherit this initial set of error
identifiers and MAY define additional values (OPT5).
4.2. /ace-group
This resource implements the FETCH handler.
4.2.1. FETCH Handler
The FETCH handler receives group identifiers and returns the
corresponding group names and GROUPNAME URIs.
The handler expects a request with the payload formatted as a CBOR
map, which MUST contain the following fields:
* 'gid': its value is encoded as a CBOR array, containing one or
more group identifiers. The exact encoding of the group
identifier MUST be specified by the application profile (REQ13).
The Client indicates that it wishes to receive the group names of
all the groups having these identifiers.
The handler identifies the groups where communications are secured by
using the keying material identified by those group identifiers.
If all verifications succeed, the handler replies with a 2.05
(Content) response, whose payload is formatted as a CBOR map that
MUST contain the following fields:
* 'gid': its value is encoded as a CBOR array, containing zero or
more group identifiers. The handler indicates that those are the
identifiers it is sending group names for. This CBOR array is a
subset of the 'gid' array in the FETCH request.
* 'gname': its value is encoded as a CBOR array, containing zero or
more group names. The elements of this array are encoded as text
strings. Each element of index i in this CBOR array is associated
with the element of index i in the 'gid' array.
* 'guri': its value is encoded as a CBOR array, containing zero or
more URIs, each indicating a group-membership resource. The
elements of this array are encoded as text strings. Each element
of index i in this CBOR array is associated with the element of
index i in the 'gid' array.
If the KDC does not find any group associated with the specified
group identifiers, the handler returns a response with the payload
formatted as a CBOR byte string of zero length (0x40).
Note that the KDC only verifies that the node is authorized by the AS
to access this resource. Nodes that are not members of the group but
are authorized to do signature verification on the group messages may
be allowed to access this resource if the application needs it.
4.2.1.1. Retrieve Group Names
In case the joining node only knows the group identifier of the group
it wishes to join or about which it wishes to get updated information
from the KDC, the node can contact the KDC to request the
corresponding group name and group-membership resource URI. In
particular, it does so by sending a CoAP FETCH request to the /ace-
group endpoint at the KDC formatted as defined in Section 4.2.1. The
node can specify several group identifiers at once.
Figure 7 gives an overview of the exchanges described above, and
Figure 8 shows an example.
Client KDC
| |
|------------ Group Name and URI Retrieval Request: -------->|
| FETCH /ace-group |
| |
|<-- Group Name and URI Retrieval Response: 2.05 (Content) --|
| |
Figure 7: Message Flow of Group Name and URI Retrieval Request-
Response
Request:
Header: FETCH (Code=0.05)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation):
{
/ gid / 0: [1, 2]
}
Response:
Header: Content (Code=2.05)
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation):
{
/ gid / 0: [1, 2],
/ gname / 1: ["group1", "group2"],
/ guri / 2: ["/ace-group/g1", "/ace-group/g2"]
}
Figure 8: Example of Group Name and URI Retrieval Request-Response
4.3. /ace-group/GROUPNAME
This resource implements the POST and GET handlers.
4.3.1. POST Handler
The POST handler processes the Join Request sent by a Client to join
a group and returns a Join Response as a successful result of the
joining process (see Section 4.3.1.1). At a high level, the POST
handler adds the Client to the list of current group members, adds
the authentication credential of the Client to the list of the group
members' authentication credentials, and returns the symmetric group
keying material for the group identified by GROUPNAME.
The handler expects a request with payload formatted as a CBOR map,
which MAY contain the following fields, which, if included, MUST have
the format and value as specified below.
* 'scope': its value specifies the name of the group that the Client
is attempting to join and the roles that the client wishes to have
in the group. This value is encoded as a CBOR byte string
wrapping one scope entry, as defined in Section 3.1.
* 'get_creds': it is included if the Client wishes to receive the
authentication credentials of the current group members from the
KDC. This parameter may be included in the Join Request if the
KDC stores the authentication credentials of the group members,
while it is not useful to include it if the Client obtains those
authentication credentials through alternative means, e.g., from
the AS. Note that including this parameter might result in a
following Join Response of a large size, which can be inconvenient
for resource-constrained devices.
If the Client wishes to retrieve the authentication credentials of
all the current group members, the 'get_creds' parameter MUST
encode the CBOR simple value null (0xf6). Otherwise, if the
Client wishes to retrieve the authentication credentials of nodes
with specific roles, the 'get_creds' parameter MUST encode a non-
empty CBOR array containing the three elements 'inclusion_flag',
'role_filter', and 'id_filter', as defined below.
- The first element, namely 'inclusion_flag', encodes the CBOR
simple value true (0xf5) if the Client wishes to receive the
authentication credentials of the nodes having their node
identifier specified in 'id_filter' (i.e., selection by
inclusive filtering). Instead, this element encodes the CBOR
simple value false (0xf4) if the Client wishes to receive the
authentication credentials of the nodes that do not have the
node identifiers specified in the third element 'id_filter'
(i.e., selection by exclusive filtering). In the Join Request,
this parameter encodes the CBOR simple value true (0xf5).
- The second element, namely 'role_filter', is a CBOR array.
Each element of the array contains one role or a combination of
roles for the group identified by GROUPNAME. This parameter
indicates that the Client wishes to receive the authentication
credentials of all the group members having any of the
specified roles or combination of roles (i.e., having any of
those single roles or at least all the roles indicated in any
of those combinations of roles).
For example, the array ["role1", "role2+role3"] indicates that
the Client wishes to receive the authentication credentials of
all group members that have at least "role1" or at least both
"role2" and "role3". In the Join Request, this parameter is a
non-empty array.
- The third element, namely 'id_filter', is a CBOR array. Each
element of the array contains a node identifier of a group
member for the group identified by GROUPNAME. This parameter
indicates that the Client wishes to receive the authentication
credentials of the nodes that have or do not have the specified
node identifiers based on the value of 'inclusion_flag' (i.e.,
as a selection by inclusive or exclusive filtering). In the
Join Request, the Client does not filter authentication
credentials based on node identifiers, so this parameter is an
empty array.
In fact, when first joining the group, the Client is not
expected or capable to express a filter based on node
identifiers of other group members. Instead, when already a
group member and sending a Join Request to rejoin, the Client
is not expected to include the 'get_creds' parameter in the
Join Request altogether, since it can rather retrieve
authentication credentials associated with specific group
identifiers as defined in Section 4.4.1.1.
The CDDL definition [RFC8610] of 'get_creds' is given in Figure 9;
as an example, it uses node identifiers encoded as CBOR byte
strings, role identifiers encoded as CBOR text strings, and
combinations of roles encoded as CBOR arrays of role identifiers.
Note that, for this handler, 'inclusion_flag' is always set to
true and the array of roles 'role_filter' is always non-empty,
while the array of node identifiers 'id_filter' is always empty.
However, this is not necessarily the case for other handlers using
the 'get_creds' parameter.
inclusion_flag = bool
role = tstr
comb_role = [2* role]
role_filter = [* ( role / comb_role )]
id = bstr
id_filter = [* id]
get_creds = null / [inclusion_flag, role_filter, id_filter]
Figure 9: CDDL Definition of 'get_creds', Using an Example
Node Identifier Encoded as bstr and Role as tstr
* 'client_cred': encoded as a CBOR byte string, whose value is the
original binary representation of the Client's authentication
credential. This parameter MUST be present if the KDC is managing
(collecting from and distributing to Clients) the authentication
credentials of the group members and the Client's role in the
group will require the Client to send messages to one or more
group members. It is REQUIRED for application profiles to define
the specific formats that are acceptable to use for authentication
credentials in the group (REQ6).
* 'cnonce': encoded as a CBOR byte string, whose value is a
dedicated nonce N_C generated by the Client. This parameter MUST
be present.
* 'client_cred_verify': encoded as a CBOR byte string. This
parameter MUST be present if the 'client_cred' parameter is
present and no authentication credential associated with the
Client's access token can be retrieved for that group.
The value of the CBOR byte string is the proof-of-possession (PoP)
evidence computed by the Client over the following PoP input: the
scope (encoded as a CBOR byte string) concatenated with N_S
(encoded as a CBOR byte string) concatenated with N_C (encoded as
a CBOR byte string), where:
- scope is either specified in the 'scope' parameter above, if
present, or a default scope entry that the handler is expected
to know otherwise;
- N_S is the challenge received from the KDC in the
'kdcchallenge' parameter of the 2.01 (Created) response to the
Token Transfer Request (see Section 3.3), encoded as a CBOR
byte string; and
- N_C is the nonce generated by the Client and specified in the
'cnonce' parameter above, encoded as a CBOR byte string.
An example of PoP input to compute 'client_cred_verify' using CBOR
encoding is given in Figure 10.
A possible type of PoP evidence is a signature that the Client
computes by using its own private key, whose corresponding public
key is specified in the authentication credential carried in the
'client_cred' parameter. Application profiles of this
specification MUST specify the exact approaches used to compute
the PoP evidence to include in 'client_cred_verify' and MUST
specify which of those approaches is used in which case (REQ14).
If the access token was not provided to the KDC through a Token
Transfer Request (e.g., the access token is instead transferred
during the establishment of a secure communication association),
it is REQUIRED of the specific application profile to define how
the challenge N_S is generated (REQ15).
* 'creds_repo': it can be present if the format of the Client's
authentication credential conveyed in the 'client_cred' parameter
is a certificate. In such a case, this parameter has as its value
the URI of the certificate. This parameter is encoded as a CBOR
text string. Alternative specific encodings of this parameter MAY
be defined in application profiles of this specification (OPT6).
* 'control_uri': its value is a full URI, encoded as a CBOR text
string. A default url-path is /ace-group/GROUPNAME/node, although
implementations can use different ones instead. The URI MUST NOT
have url-path /ace-group/GROUPNAME.
If 'control_uri' is specified in the Join Request, the Client acts
as a CoAP server and hosts a resource at this specific URI. The
KDC MAY use this URI to send CoAP requests to the Client (acting
as a CoAP server in this exchange), for example, for one-to-one
provisioning of new group keying material when performing a group
rekeying (see Section 6.1) or to inform the Client of its removal
from the group (see Section 5).
In particular, this resource is intended for communications
exclusively concerning the group identified by GROUPNAME and whose
group name is specified in the 'scope' parameter of the Join
Request, if present therein. If the KDC does not implement
mechanisms using this resource for that group, it can ignore this
parameter. Other additional functionalities of this resource MAY
be defined in application profiles of this specifications (OPT7).
scope, N_S, and N_C expressed in CBOR diagnostic notation:
scope = h'826667726f7570316673656e646572'
N_S = h'018a278f7faab55a'
N_C = h'25a8991cd700ac01'
scope, N_S, and N_C as CBOR encoded byte strings:
scope = 0x4f826667726f7570316673656e646572
N_S = 0x48018a278f7faab55a
N_C = 0x4825a8991cd700ac01
PoP input:
0x4f 826667726f7570316673656e646572
48 018a278f7faab55a 48 25a8991cd700ac01
Figure 10: Example of PoP Input to Compute 'client_cred_verify'
Using CBOR Encoding
If the request does not include the 'scope' parameter, the KDC is
expected to understand what roles the Client is requesting to join
the group with. For example, as per the access token, the Client
might have been granted access to the group with only one role. If
the KDC cannot determine which exact roles should be considered for
the Client, it MUST reply with a 4.00 (Bad Request) error response.
The handler considers the scope specified in the access token
associated with the Client and checks the scope entry related to the
group identified by the GROUPNAME associated with the endpoint. In
particular, the handler checks whether the set of roles specified in
that scope entry includes all the roles that the Client wishes to
have in the group as per the Join Request. If this is not the case,
the KDC MUST reply with a 4.03 (Forbidden) error response.
If the KDC manages the group members' authentication credentials, the
handler checks if one is included in the 'client_cred' parameter. If
so, the KDC retrieves the authentication credential and performs the
following actions.
* If the access token was provided through a Token Transfer Request
(see Section 3.3) but the KDC cannot retrieve the 'kdcchallenge'
associated with this Client (see Section 3.3), the KDC MUST reply
with a 4.00 (Bad Request) error response, which MUST also have
Content-Format "application/ace-groupcomm+cbor". The payload of
the error response is a CBOR map including a newly generated
'kdcchallenge' value, which is specified in the 'kdcchallenge'
parameter. The KDC MUST store the newly generated value as the
'kdcchallenge' value associated with this Client, replacing the
currently stored value (if any).
* The KDC checks the authentication credential to be valid for the
group identified by GROUPNAME. That is, it checks that the
authentication credential has the format used in the group, is
intended for the public key algorithm used in the group, and is
aligned with the possible associated parameters used in the group.
If this verification fails, the handler MUST reply with a 4.00
(Bad Request) error response. The response MUST have Content-
Format "application/concise-problem-details+cbor" and is formatted
as defined in Section 4.1.2. Within the Custom Problem Detail
entry 'ace-groupcomm-error', the value of the 'error-id' field
MUST be set to 2 ("Authentication credential incompatible with the
group configuration").
* The KDC verifies the PoP evidence conveyed in the
'client_cred_verify' parameter. Application profiles of this
specification MUST specify the exact approaches used to verify the
PoP evidence and MUST specify which of those approaches is used in
which case (REQ14).
If the PoP evidence does not pass verification, the handler MUST
reply with a 4.00 (Bad Request) error response. The response MUST
have Content-Format "application/concise-problem-details+cbor" and
is formatted as defined in Section 4.1.2. Within the Custom
Problem Detail entry 'ace-groupcomm-error', the value of the
'error-id' field MUST be set to 3 ("Invalid proof-of-possession
evidence").
If no authentication credential is conveyed in the 'client_cred'
parameter, the handler checks if the KDC currently stores an
authentication credential that is associated with the access token
and with the group identified by GROUPNAME (see also
Section 4.3.1.1). Note that the same joining node may use different
authentication credentials in different groups, and all those
authentication credentials would be associated with the same access
token.
If an eligible authentication credential for the Client is neither
present in the 'client_cred' parameter nor retrieved from the stored
ones at the KDC, it is RECOMMENDED that the handler stops the
processing and replies with a 4.00 (Bad Request) error response.
Application profiles MAY define alternatives (OPT8).
If, regardless of the reason, the KDC replies with a 4.00 (Bad
Request) error response, the payload of the response MAY be a CBOR
map. For instance, the CBOR map can include a 'sign_info' parameter
formatted as 'sign_info_resp' defined in Section 3.3.1, with the
'cred_fmt' element set to the CBOR simple value null (0xf6) if the
Client sent its own authentication credential and the KDC is not set
to store authentication credentials of the group members. When the
response payload is a CBOR map including such parameters, the error
response has Content-Format "application/ace-groupcomm+cbor".
If all the verifications above succeed, the KDC proceeds as follows.
First, only in case the Client is not already a group member, the
handler performs the following actions:
* The handler adds the Client to the list of current members of the
group.
* The handler assigns a name NODENAME to the Client and creates a
sub-resource to /ace-group/GROUPNAME at the KDC, i.e., /ace-
group/GROUPNAME/nodes/NODENAME.
* The handler associates the node identifier NODENAME with the
access token and the secure communication association for the
Client.
Then, the handler performs the following actions.
* If the KDC manages the group members' authentication credentials:
- The handler associates the retrieved Client's authentication
credential with the tuple composed of the node name NODENAME,
the group name GROUPNAME, and the access token.
- The handler adds the retrieved Client's authentication
credential to the list of authentication credentials stored for
the group identified by GROUPNAME. If such a list already
includes an authentication credential for the Client, but a
different authentication credential is specified in the
'client_cred' parameter, then the handler MUST replace the old
authentication credential in the list with the one specified in
the 'client_cred' parameter.
* If backward security is prescribed by application policies
installed at the KDC or by the used application profile of this
specification, then the KDC MUST generate new group keying
material and securely distribute it to the current group members
(see Section 6).
* The handler returns a successful Join Response, as defined below,
which contains the symmetric group keying material, the group
policies, and the authentication credentials of the current
members of the group if the KDC manages those and the Client
requested those.
The Join Response MUST have response code 2.01 (Created) if the
Client has been added to the list of group members in this join
exchange (see above) or 2.04 (Changed) otherwise, i.e., if the Client
is rejoining the group without having left it.
The Join Response message MUST include the Location-Path CoAP
Options, specifying the path to the sub-resource associated with the
Client, i.e., /ace-group/GROUPNAME/nodes/NODENAME.
The Join Response message MUST have Content-Format "application/ace-
groupcomm+cbor". The payload of the response is formatted as a CBOR
map, which MUST contain the following fields with the values
specified below:
* 'gkty': identifying the key type of the keying material specified
in the 'key' parameter. This parameter is encoded as a CBOR
integer or a CBOR text string. Possible values are taken from the
"Key Type Value" column of the "ACE Groupcomm Key Types" registry
defined in Section 11.8 of this specification. Implementations
MUST verify that the key type specified by this parameter matches
the application profile being used and, if applicable, that such
an application profile is listed in the "Profile" column of the
"ACE Groupcomm Key Types" registry for the key type in question.
* 'key': containing the keying material used for securing the group
communication or information required to derive such keying
material.
* 'num': containing the current version number of the group keying
material, encoded as a CBOR integer. The version number has a
value that increases in a strictly monotonic way as the group
keying material changes. The application profile MUST define the
initial value of the version number (REQ16).
The format of the keying material conveyed in the 'key' parameter
MUST be defined in application profiles of this specification
(REQ17), together with corresponding key types to specify as value of
the 'gkty' parameter and that are accepted by the application
(REQ18). Additionally, documents specifying a type of keying
material MUST register an entry in the "ACE Groupcomm Key Types"
registry defined in Section 11.8, including its name, the
corresponding key type to specify as value for the 'gkty' parameter,
and the application profile to be used with.
+==========+================+=========+=============+===========+
| Name | Key Type Value | Profile | Description | Reference |
+==========+================+=========+=============+===========+
| Reserved | 0 | | This value | RFC 9594 |
| | | | is reserved | |
+----------+----------------+---------+-------------+-----------+
Table 1: ACE Groupcomm Key Types
The Join Response SHOULD contain the following fields with the values
specified below:
* 'exp': its value specifies the expiration time of the group keying
material specified in the 'key' parameter, encoded as a CBOR
unsigned integer. The value is the number of seconds from
1970-01-01T00:00:00Z UTC until the specified UTC date/time,
ignoring leap seconds, analogous to what is specified for
NumericDate in Section 2 of [RFC7519]. After the time indicated
in this parameter, group members MUST NOT use the group keying
material specified in the 'key' parameter. The group members can
retrieve the latest group keying material from the KDC.
* 'exi': its value specifies the residual lifetime of the group
keying material, encoded as a CBOR unsigned integer. If the 'exp'
parameter is included, this parameter MUST also be included. The
value represents the residual lifetime of the group keying
material specified in the 'key' parameter, i.e., it is the number
of seconds between the current time at the KDC and the time when
the keying material expires (as specified in the 'exp' parameter,
if present). A Client determines the expiration time of the
keying material by adding the seconds specified in the 'exi'
parameter to its current time upon receiving the Join Response
containing the 'exi' parameter. After such an expiration time,
the Client MUST NOT use the group keying material specified in the
'key' parameter. The Client can retrieve the latest group keying
material from the KDC.
If a Client has a reliable way to synchronize its internal clock with
UTC, and both the 'exp' and 'exi' parameters are present, then the
Client MUST use the 'exp' parameter value as expiration time for the
group keying material. Otherwise, the Client uses the 'exi'
parameter value to determine the expiration time as defined above.
When a Client relies on the 'exi' parameter, the expiration time that
it computes is offset in the future with respect to the actual
expiration time as intended by the KDC and specified in the 'exp'
parameter (if present). Such an offset is the amount of time between
when the KDC sends the response message including the 'exi' parameter
and when the Client receives that message. That is, especially if
the delivery of the response to the Client is delayed, the Client
will believe the keying material to be valid for a longer time than
the KDC actually means. However, before approaching the actual
expiration time, the KDC is expected to rekey the group and
distribute new keying material (see Section 6).
Optionally, the Join Response MAY contain the following parameters,
which, if included, MUST have the format and value as specified
below.
* 'ace_groupcomm_profile': its value is encoded as a CBOR integer
and MUST be used to uniquely identify the application profile for
group communication. Applications of this specification MUST
register an application profile identifier and the related value
for this parameter in the "ACE Groupcomm Profiles" registry
(REQ19).
+==========+========================+============+===========+
| Name | Description | CBOR Value | Reference |
+==========+========================+============+===========+
| Reserved | This value is reserved | 0 | RFC 9594 |
+----------+------------------------+------------+-----------+
Table 2: ACE Groupcomm Profiles
* 'creds': it MUST be present if 'get_creds' was present in the Join
Request; otherwise, it MUST NOT be present. Its value is encoded
as a CBOR array specifying the authentication credentials of the
group members, i.e., of all of them or of the ones selected
according to the 'get_creds' parameter in the Join Request. In
particular, each element of the array is a CBOR byte string, whose
value is the original binary representation of a group member's
authentication credential. It is REQUIRED for application
profiles to define the specific formats of authentication
credentials that are acceptable to use in the group (REQ6).
* 'peer_roles': it SHOULD be present if 'creds' is also present;
otherwise, it MUST NOT be present. Its value is encoded as a CBOR
array of n elements, where n is the number of authentication
credentials included in the 'creds' parameter (at most, the number
of members in the group). The i-th element of the array specifies
the role(s) that the group member associated with the i-th
authentication credential in 'creds' has in the group. In
particular, each array element is encoded like the role element of
a scope entry, which is consistent with the used format (see
Section 3.1).
This parameter MAY be omitted if the Client can rely on other
means to unambiguously gain knowledge of the role of each group
member whose associated authentication credential is specified in
the 'creds' parameter. For example, all such group members may
have the same role in the group joined by the Client, and such a
role can be unambiguously assumed by the Client (e.g., based on
what is defined in the used application profile of this
specification). As another example, each of the authentication
credentials specified in the 'creds' parameter can indicate the
role(s) that the corresponding group member has in the group
joined by the Client.
When receiving the authentication credential of a Client in the
'client_cred' parameter of a Join Request (see Section 4.3.1.1) or
of an Authentication Credential Update Request (see
Section 4.9.1.1), the KDC is not expected to check that the
authentication credential indicates the role(s) that the Client
can have or has in the group in question. When preparing a Join
Response, the KDC can decide whether to include the 'peer_roles'
parameter, depending on the specific set of authentication
credentials specified in the 'creds' parameter of that Join
Response.
* 'peer_identifiers': it MUST be present if 'creds' is also present;
otherwise, it MUST NOT be present. Its value is encoded as a CBOR
array of n elements, where n is the number of authentication
credentials included in the 'creds' parameter (at most, the number
of members in the group). The i-th element of the array specifies
the node identifier that the group member associated with the i-th
authentication credential in 'creds' has in the group. In
particular, the i-th array element is encoded as a CBOR byte
string, whose value is the node identifier of the group member.
The specific format of node identifiers of group members is
specified by the application profile (REQ25).
* 'group_policies': its value is encoded as a CBOR map, whose
elements specify how the group handles specific management
aspects. These include, for instance, approaches to achieve
synchronization of sequence numbers among group members. The
possible elements of the CBOR map are registered in the "ACE
Groupcomm Policies" registry defined in Section 11.10 of this
specification. This specification defines the three elements
"Sequence Number Synchronization Methods", "Key Update Check
Interval", and "Expiration Delta", which are summarized in
Table 3. Application profiles of this specification MUST specify
the format and default values for the entries of the CBOR map
conveyed in the 'group_policies' parameter (REQ20).
+=================+=======+======+===================+===========+
| Name | CBOR | CBOR | Description | Reference |
| | Label | Type | | |
+=================+=======+======+===================+===========+
| Sequence Number | 0 | int | Method for | RFC 9594 |
| Synchronization | | or | recipient group | |
| Method | | tstr | members to | |
| | | | synchronize with | |
| | | | sequence numbers | |
| | | | of sender group | |
| | | | members. Its | |
| | | | value is taken | |
| | | | from the "Value" | |
| | | | column of the | |
| | | | "Sequence Number | |
| | | | Synchronization | |
| | | | Method" | |
| | | | registry. | |
+-----------------+-------+------+-------------------+-----------+
| Key Update | 1 | int | Polling interval | RFC 9594 |
| Check Interval | | | in seconds, for | |
| | | | group members to | |
| | | | check at the KDC | |
| | | | if the latest | |
| | | | group keying | |
| | | | material is the | |
| | | | one that they | |
| | | | store. | |
+-----------------+-------+------+-------------------+-----------+
| Expiration | 2 | uint | Number of | RFC 9594 |
| Delta | | | seconds from | |
| | | | 'exp' until a | |
| | | | UTC date/time, | |
| | | | after which | |
| | | | group members | |
| | | | MUST stop using | |
| | | | the group keying | |
| | | | material that | |
| | | | they store to | |
| | | | decrypt incoming | |
| | | | messages. | |
+-----------------+-------+------+-------------------+-----------+
Table 3: ACE Groupcomm Policies
* 'kdc_cred': its value is the original binary representation of the
KDC's authentication credential, encoded as a CBOR byte string.
This parameter is used if the KDC has an associated authentication
credential and this is required for the correct group operation.
It is REQUIRED for application profiles to define whether the KDC
has an authentication credential as required for the correct group
operation and if this has to be provided through the 'kdc_cred'
parameter (REQ8).
If the KDC has an authentication credential as required for the
correct group operation, the KDC's authentication credential MUST
have the same format used for the authentication credentials of
the group members. It is REQUIRED for application profiles to
define the specific formats that are acceptable to use for the
authentication credentials in the group (REQ6).
* 'kdc_nonce': its value is a dedicated nonce N_KDC generated by the
KDC, encoded as a CBOR byte string. This parameter MUST be
present if the 'kdc_cred' parameter is present.
* 'kdc_cred_verify': its value is as defined below and is encoded as
a CBOR byte string. This parameter MUST be present if the
'kdc_cred' parameter is present.
The value of this parameter is the proof-of-possession (PoP)
evidence computed by the KDC over the following PoP input: the
nonce N_C (encoded as a CBOR byte string) concatenated with the
nonce N_KDC (encoded as a CBOR byte string), where:
- N_C is the nonce generated by the Client and specified in the
'cnonce' parameter of the Join Request.
- N_KDC is the nonce generated by the KDC and specified in the
'kdc_nonce' parameter.
An example of PoP input to compute 'kdc_cred_verify' using CBOR
encoding is given in Figure 11.
A possible type of PoP evidence is a signature that the KDC
computes by using its own private key, whose corresponding public
key is specified in the authentication credential conveyed in the
'kdc_cred' parameter. Application profiles of this specification
MUST specify the approaches used by the KDC to compute the PoP
evidence to include in 'kdc_cred_verify' and MUST specify which of
those approaches is used in which case (REQ21).
* 'rekeying_scheme': identifying the rekeying scheme that the KDC
uses to provide new group keying material to the group members.
The value of this parameter is encoded as a CBOR integer and is
taken from the "Value" column of the "ACE Groupcomm Rekeying
Schemes" registry defined in Section 11.13 of this specification.
+=======+================+===========================+===========+
| Value | Name | Description | Reference |
+=======+================+===========================+===========+
| 0 | Point-to-Point | The KDC individually | RFC 9594 |
| | | targets each node to | |
| | | rekey, using the | |
| | | pairwise secure | |
| | | communication | |
| | | association with | |
| | | that node | |
+-------+----------------+---------------------------+-----------+
Table 4: ACE Groupcomm Rekeying Schemes
Application profiles of this specification MAY define a default
group rekeying scheme to refer to in case the 'rekeying_scheme'
parameter is not included in the Join Response (OPT9).
* 'mgt_key_material': encoded as a CBOR byte string and containing
the specific administrative keying material that the joining node
requires in order to participate in the group rekeying process
performed by the KDC. This parameter MUST NOT be present if the
'rekeying_scheme' parameter is not present and the application
profile does not specify a default group rekeying scheme to use in
the group. Some simple rekeying schemes may not require specific
administrative keying material to be provided, e.g., the basic
"Point-to-Point" group rekeying scheme (see Section 6.1).
In more advanced group rekeying schemes, the administrative keying
material can be composed of multiple keys organized, for instance,
into a logical tree hierarchy, whose root key is the only
administrative key shared by all the group members. In such a
case, each group member is exclusively associated with one leaf
key in the hierarchy and stores only the administrative keys from
the associated leaf key all the way up along the path to the root
key. That is, different group members can be provided with a
different subset of the overall administrative keying material.
It is expected from separate documents to define how the advanced
group rekeying scheme, possibly indicated in the 'rekeying_scheme'
parameter, is used by an application profile of this
specification. This includes defining the format of the
administrative keying material to specify in 'mgt_key_material'
consistently with the group rekeying scheme and the application
profile in question.
* 'control_group_uri': its value is a full URI, encoded as a CBOR
text string. The URI MUST specify addressing information intended
to reach all the members in the group. For example, this can be a
multicast IP address, optionally together with a port number that,
if omitted, defaults to 5683, i.e., the default port number for
the "coap" URI scheme (see Section 6.1 of [RFC7252]). The URI
MUST include GROUPNAME in the url-path. A default url-path is
/ace-group/GROUPNAME, although implementations can use different
ones instead. The URI MUST NOT have url-path /ace-
group/GROUPNAME/nodes.
If 'control_group_uri' is included in the Join Response, the
Clients supporting this parameter act as CoAP servers, host a
resource at this specific URI, and listen to the specified
addressing information.
The KDC MAY use this URI to send one-to-many CoAP requests to the
Client group members (acting as CoAP servers in this exchange),
for example, for one-to-many provisioning of new group keying
material when performing a group rekeying (see Section 6.2) or to
inform the Clients of their removal from the group (see
Section 5).
In particular, this resource is intended for communications
exclusively concerning the group identified by GROUPNAME and whose
group name was specified in the 'scope' parameter of the Join
Request, if present. If the KDC does not implement mechanisms
using this resource for that group, it can ignore this parameter.
Other additional functionalities of this resource MAY be defined
in application profiles of this specifications (OPT10).
N_C and N_KDC expressed in CBOR diagnostic notation:
N_C = h'25a8991cd700ac01'
N_KDC = h'cef04b2aa791bc6d'
N_C and N_KDC as CBOR encoded byte strings:
N_C = 0x4825a8991cd700ac01
N_KDC = 0x48cef04b2aa791bc6d
PoP input:
0x48 25a8991cd700ac01 48 cef04b2aa791bc6d
Figure 11: Example of PoP Input to Compute 'kdc_cred_verify'
Using CBOR Encoding
After sending the Join Response, if the KDC has an associated
authentication credential as required for the correct group
operation, then the KDC MUST store the N_C value specified in the
'cnonce' parameter of the Join Request as a 'clientchallenge' value
associated with the Client, replacing the currently stored value (if
any). If, as a group member, the Client later sends a GET request to
the /ace-group/GROUPNAME/kdc-cred resource for retrieving the latest
KDC's authentication credential (see Section 4.5.1), then the KDC
uses the stored 'clientchallenge' for computing the PoP evidence to
include in the response sent to the Client, hence proving the
possession of its own private key.
If the Join Response includes the 'kdc_cred_verify' parameter, the
Client verifies the conveyed PoP evidence and considers the group
joining unsuccessful in case of failed verification. Application
profiles of this specification MUST specify the exact approaches used
by the Client to verify the PoP evidence in 'kdc_cred_verify' and
MUST specify which of those approaches is used in which case (REQ21).
Application profiles of this specification MUST specify the
communication protocol that members of the group use to communicate
with each other (REQ22) and the security protocol that they use to
protect the group communication (REQ23).
4.3.1.1. Join the Group
Figure 12 gives an overview of the join exchange between the Client
and the KDC when the Client first joins a group, while Figure 13
shows an example.
Client KDC
| |
|-------- Join Request: POST /ace-group/GROUPNAME ------>|
| |
|<------------ Join Response: 2.01 (Created) ----------- |
| Location-Path = "/ace-group/GROUPNAME/nodes/NODENAME" |
Figure 12: Message Flow of the Join Request-Response
Request:
Header: POST (Code=0.02)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation):
{
/ scope / 3: <<["group1", ["sender", "receiver"]]>>,
/ get_creds / 4: [true, ["sender"], []],
/ client_cred / 5: h'a2026008a101a5010202410a20012158
20bbc34960526ea4d32e940cad2a2341
48ddc21791a12afbcbac93622046dd44
f02258204519e257236b2a0ce2023f09
31f1f386ca7afda64fcde0108c224c51
eabf6072',
/ cnonce / 6: h'25a8991cd700ac01',
/ client_cred_verify / 24: h'66e6d9b0db009f3e105a673f88556117
26caed57f530f8cae9d0b168513ab949
fedc3e80a96ebe94ba08d3f8d3bf8348
7458e2ab4c2f936ff78b50e33c885e35'
}
Response:
Header: Created (Code=2.01)
Content-Format: 261 (application/ace-groupcomm+cbor)
Location-Path: "ace-group"
Location-Path: "g1"
Location-Path: "nodes"
Location-Path: "c101"
Payload (in CBOR diagnostic notation):
{
/ gkty / 7: 65600,
/ key / 8: h'73657373696f6e6b6579',
/ num / 9: 12,
/ exp / 11: 1924992000,
/ exi / 12: 2592000,
/ creds / 13: [h'a2026008a101a5010202410220012158
20cd4177ba62433375ede279b5e18e8b
91bc3ed8f1e174474a26fc0edb44ea53
73225820a0391de29c5c5badda610d4e
301eaaa18422367722289cd18cbe6624
e89b9cfd',
h'a2026008a101a5010202410320012158
20ac75e9ece3e50bfc8ed60399889522
405c47bf16df96660a41298cb4307f7e
b62258206e5de611388a4b8a8211334a
c7d37ecb52a387d257e6db3c2a93df21
ff3affc8'],
/ peer_roles / 14: ["sender", ["sender", "receiver"]],
/ peer_identifiers / 15: [h'01', h'02']
}
Figure 13: Example of the First Join Request-Response for Group
Joining
If not previously established, the Client and the KDC MUST first
establish a pairwise secure communication association (REQ24). This
can be achieved, for instance, by using a transport profile of ACE.
The join exchange MUST occur over that secure communication
association. The Client and the KDC MAY use that same secure
communication association to protect further pairwise communications
that must be protected.
It is REQUIRED that the secure communication association between the
Client and the KDC is established by using the proof-of-possession
key bound to the access token. As a result, the proof of possession
to bind the access token to the Client is performed by using the
proof-of-possession key bound to the access token for establishing
the pairwise secure communication association between the Client and
the KDC.
To join the group, the Client sends a CoAP POST request to the /ace-
group/GROUPNAME endpoint at the KDC, where the group to join is
identified by GROUPNAME. The group name is specified in the scope
entry conveyed by the 'scope' parameter of the request (if present),
formatted as specified in Section 4.3.1. This group name is the same
as in the scope entry corresponding to that group, specified in the
'scope' parameter of the Authorization Request/Response, or it can be
determined from it. Note that, in case of successful joining, the
Location-Path Options in the Join Response provide the Client with
the path of the URI to use for retrieving individual keying material
and for leaving the group.
If the node is joining a group for the first time and the KDC
maintains the authentication credentials of the group members, the
Client is REQUIRED to send its own authentication credential and
proof-of-possession (PoP) evidence in the Join Request (see the
'client_cred' and 'client_cred_verify' parameters in Section 4.3.1).
The request is accepted only if both the authentication credential is
provided and the PoP evidence is successfully verified.
If a node rejoins a group as authorized by the same access token and
using the same authentication credential, it can omit the
authentication credential and the PoP evidence, or just the PoP
evidence, from the Join Request. Then, the KDC will be able to
retrieve the node's authentication credential associated with the
access token for that group. If the authentication credential has
been discarded, the KDC replies with a 4.00 (Bad Request) error
response, as specified in Section 4.3.1. If a node rejoins a group
but wants to update its own authentication credential, it needs to
include both its authentication credential and the PoP evidence in
the Join Request, like when it joined the group for the first time.
4.3.2. GET Handler
The GET handler returns the symmetric group keying material for the
group identified by GROUPNAME.
The handler expects a GET request.
In addition to what is defined in Section 4.1.2, the handler verifies
that the Client is a current member of the group. If the
verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
response. The response MUST have Content-Format "application/
concise-problem-details+cbor" and is formatted as defined in
Section 4.1.2. Within the Custom Problem Detail entry 'ace-
groupcomm-error', the value of the 'error-id' field MUST be set to 0
("Operation permitted only to group members").
If all verifications succeed, the handler replies with a 2.05
(Content) response containing the symmetric group keying material.
The payload of the response is formatted as a CBOR map that MUST
contain the parameters 'gkty', 'key', and 'num', as specified in
Section 4.3.1.
The payload MUST also include the parameters 'rekeying_scheme' and
'mgt_key_material' as specified in Section 4.3.1, if they are
included in the payload of the Join Responses sent for the group.
The payload MAY also include the parameters 'ace_groupcomm_profile',
'exp', and 'exi', as specified in Section 4.3.1. If the 'exp'
parameter is included, the 'exi' parameter MUST also be included. If
the 'exi' parameter is included, its value specifies the residual
lifetime of the group keying material from the current time at the
KDC.
4.3.2.1. Retrieve Group Keying Material
A node in the group can contact the KDC to retrieve the current group
keying material by sending a CoAP GET request to the /ace-group/
GROUPNAME endpoint at the KDC, where the group is identified by
GROUPNAME.
Figure 14 gives an overview of the key distribution exchange between
the Client and the KDC, while Figure 15 shows an example.
Client KDC
| |
|------ Key Distribution Request: GET /ace-group/GROUPNAME ------>|
| |
|<----------- Key Distribution Response: 2.05 (Content) --------- |
| |
Figure 14: Message Flow of Key Distribution Request-Response
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Response:
Header: Content (Code=2.05)
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation):
{
/ gkty / 7: 65600,
/ key / 8: h'73657373696f6e6b6579',
/ num / 9: 12
}
Figure 15: Example of Key Distribution Request-Response
4.4. /ace-group/GROUPNAME/creds
This resource implements the GET and FETCH handlers.
4.4.1. FETCH Handler
The FETCH handler receives identifiers of group members for the group
identified by GROUPNAME and returns the authentication credentials of
such group members.
The handler expects a request with the payload formatted as a CBOR
map, which MUST contain the following field.
* 'get_creds': its value is encoded as in Section 4.3.1, with the
following modifications.
- The arrays 'role_filter' and 'id_filter' MUST NOT both be
empty, i.e., in CDDL notation: [ bool, [ ], [ ] ]. If the
'get_creds' parameter has such a format, the request MUST be
considered malformed, and the KDC MUST reply with a 4.00 (Bad
Request) error response.
Note that a group member can retrieve the authentication
credentials of all the current group members by sending a GET
request to the same KDC resource instead (see Section 4.4.2.1).
- The element 'inclusion_flag' encodes the CBOR simple value true
(0xf5) or false (0xf4), as defined in Section 4.3.1.
- The array 'role_filter' can be empty if the Client does not
wish to filter the requested authentication credentials based
on the roles of the group members.
- The array 'id_filter' contains zero or more node identifiers of
group members for the group identified by GROUPNAME, as defined
in Section 4.3.1. The array may be empty if the Client does
not wish to filter the requested authentication credentials
based on the node identifiers of the group members.
Note that, in case the 'role_filter' array and the 'id_filter' array
are both non-empty:
* If the 'inclusion_flag' encodes the CBOR simple value true (0xf5),
the handler returns the authentication credentials of group
members whose roles match with 'role_filter' and/or have their
node identifier specified in 'id_filter'.
* If the 'inclusion_flag' encodes the CBOR simple value false
(0xf4), the handler returns the authentication credentials of
group members whose roles match with 'role_filter' and, at the
same time, do not have their node identifier specified in
'id_filter'.
The specific format of authentication credentials as well as the
identifiers, roles, and combination of roles of group members MUST be
specified by application profiles of this specification (REQ1, REQ6,
REQ25).
The handler identifies the authentication credentials of the current
group members for which either of the following holds:
* The role identifier matches with one of those indicated in the
request; note that the request can specify a combination of roles,
in which case the handler selects only the group members that have
all the roles included in the combination.
* The node identifier matches with one of those indicated in the
request or does not match with any of those, which is consistent
with the value of the element 'inclusion_flag'.
If all verifications succeed, the handler returns a 2.05 (Content)
message response with the payload formatted as a CBOR map, containing
only the following parameters from Section 4.3.1.
* 'num': encoding the version number of the current group keying
material.
* 'creds': encoding the list of authentication credentials of the
selected group members.
* 'peer_roles': encoding the role(s) that each of the selected group
members has in the group.
This parameter SHOULD be present, and it MAY be omitted according
to the same criteria defined for the Join Response (see
Section 4.3.1).
* 'peer_identifiers': encoding the node identifier that each of the
selected group members has in the group.
The specific format of authentication credentials as well as of node
identifiers of group members is specified by the application profile
(REQ6, REQ25).
If the KDC does not store any authentication credential associated
with the specified node identifiers, the handler returns a response
with the payload formatted as a CBOR byte string of zero length
(0x40).
The handler MAY enforce one of the following policies in order to
handle possible node identifiers that are included in the 'id_filter'
element of the 'get_creds' parameter of the request but are not
associated with any current group member. Such a policy MUST be
specified by application profiles of this specification (REQ26).
* The KDC silently ignores those node identifiers.
* The KDC retains authentication credentials of group members for a
given amount of time after their leaving before discarding them.
As long as such authentication credentials are retained, the KDC
provides them to a requesting Client.
If the KDC adopts this policy, the application profile MUST also
specify the amount of time during which the KDC retains the
authentication credential of a former group member after its
leaving, possibly on a per-member basis.
Note that this resource handler only verifies that the node is
authorized by the AS to access this resource. Nodes that are not
members of the group but are authorized to do signature verifications
on the group messages may be allowed to access this resource if the
application needs it.
4.4.1.1. Retrieve a Subset of Authentication Credentials in the Group
In case the KDC maintains the authentication credentials of group
members, a node in the group can contact the KDC to request the
authentication credentials, roles, and node identifiers of a
specified subset of group members by sending a CoAP FETCH request to
the /ace-group/GROUPNAME/creds endpoint at the KDC, which is
formatted as defined in Section 4.4.1 and where GROUPNAME identifies
the group.
Figure 16 gives an overview of the exchange mentioned above, while
Figure 17 shows an example of such an exchange.
Client KDC
| |
| Authentication Credential Request: |
|-------------------------------------------------------->|
| FETCH /ace-group/GROUPNAME/creds |
| |
|<-- Authentication Credential Response: 2.05 (Content) --|
| |
Figure 16: Message Flow of Authentication Credential Request-
Response to Obtain the Authentication Credentials of Specific
Group Members
Request:
Header: FETCH (Code=0.05)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "creds"
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation):
{
/ get_creds / 4: [true, [], [h'02', h'03']]
}
Response:
Header: Content (Code=2.05)
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation):
{
/ creds / 13: [h'a2026008a101a5010202410320012158
20ac75e9ece3e50bfc8ed60399889522
405c47bf16df96660a41298cb4307f7e
b62258206e5de611388a4b8a8211334a
c7d37ecb52a387d257e6db3c2a93df21
ff3affc8',
h'a2026008a101a5010202410920012158
206f9702a66602d78f5e81bac1e0af01
f8b52810c502e87ebb7c926c07426fd0
2f225820c8d33274c71c9b3ee57d842b
bf2238b8283cb410eca216fb72a78ea7
a870f800'],
/ peer_roles / 14: [["sender", "receiver"], "receiver"],
/ peer_identifiers / 15: [h'02', h'03']
}
Figure 17: Example of Authentication Credential Request-Response
to Obtain the Authentication Credentials of Specific Group
Members
4.4.2. GET Handler
The handler expects a GET request.
If all verifications succeed, the KDC replies with a 2.05 (Content)
response as in the FETCH handler in Section 4.4.1, but its payload
specifies the authentication credentials of all the group members,
together with their roles and node identifiers.
The 'peer_roles' parameter SHOULD be present in the payload of the
response, and it MAY be omitted according to the same criteria
defined for the Join Response (see Section 4.3.1).
4.4.2.1. Retrieve All Authentication Credentials in the Group
In case the KDC maintains the authentication credentials of group
members, a node in the group or an external signature verifier can
contact the KDC to request the authentication credentials, roles, and
node identifiers of all the current group members, by sending a CoAP
GET request to the /ace-group/GROUPNAME/creds endpoint at the KDC,
where the group is identified by GROUPNAME.
Figure 18 gives an overview of the message exchange, while Figure 19
shows an example of such an exchange.
Client KDC
| |
| Authentication Credential Request: |
|-------------------------------------------------------->|
| GET /ace-group/GROUPNAME/creds |
| |
|<-- Authentication Credential Response: 2.05 (Content) --|
| |
Figure 18: Message Flow of Authentication Credential Request-
Response to Obtain the Authentication Credentials of All the
Group Members
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "creds"
Response:
Header: Content (Code=2.05)
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation):
{
/ num / 9: 12,
/ creds / 13: [h'a2026008a101a5010202410220012158
20cd4177ba62433375ede279b5e18e8b
91bc3ed8f1e174474a26fc0edb44ea53
73225820a0391de29c5c5badda610d4e
301eaaa18422367722289cd18cbe6624
e89b9cfd',
h'a2026008a101a5010202410320012158
20ac75e9ece3e50bfc8ed60399889522
405c47bf16df96660a41298cb4307f7e
b62258206e5de611388a4b8a8211334a
c7d37ecb52a387d257e6db3c2a93df21
ff3affc8',
h'a2026008a101a5010202410920012158
206f9702a66602d78f5e81bac1e0af01
f8b52810c502e87ebb7c926c07426fd0
2f225820c8d33274c71c9b3ee57d842b
bf2238b8283cb410eca216fb72a78ea7
a870f800'],
/ peer_roles / 14: ["sender", ["sender", "receiver"],
"receiver"],
/ peer_identifiers / 15: [h'01', h'02', h'03']
}
Figure 19: Example of Authentication Credential Request-Response
to Obtain the Authentication Credentials of All the Group Members
4.5. /ace-group/GROUPNAME/kdc-cred
This resource implements a GET handler.
4.5.1. GET Handler
The handler expects a GET request.
If all verifications succeed, the handler returns a 2.05 (Content)
message containing the KDC's authentication credential together with
the proof-of-possession (PoP) evidence. The response MUST have
Content-Format "application/ace-groupcomm+cbor". The payload of the
response is a CBOR map, which includes the following fields.
* 'kdc_cred: specifying the KDC's authentication credential. This
parameter is encoded like the 'kdc_cred' parameter in the Join
Response (see Section 4.3.1).
* 'kdc_nonce': specifying a nonce generated by the KDC. This
parameter is encoded like the 'kdc_nonce' parameter in the Join
Response (see Section 4.3.1).
* 'kdc_cred_verify': specifying the PoP evidence computed by the KDC
over the following PoP input: the nonce N_C (encoded as a CBOR
byte string) concatenated with the nonce N_KDC (encoded as a CBOR
byte string), where:
- N_C is the nonce generated by the Client group member such
that: i) the nonce was specified in the 'cnonce' parameter of
the latest Join Request that the Client sent to the KDC in
order to join the group identified by GROUPNAME; and ii) the
KDC stored the nonce as a 'clientchallenge' value associated
with the Client after sending the corresponding Join Response
(see Section 4.3.1).
- N_KDC is the nonce generated by the KDC and specified in the
'kdc_nonce' parameter.
An example of PoP input to compute 'kdc_cred_verify' using CBOR
encoding is given in Figure 20.
The PoP evidence is computed by means of the same method used for
computing the PoP evidence that was included in the Join Response
for this Client (see Section 4.3.1).
Application profiles of this specification MUST specify the exact
approaches used by the KDC to compute the PoP evidence to include
in the 'kdc_cred_verify' parameter and MUST specify which of those
approaches is used in which case (REQ21).
If an application profile supports the presence of external
signature verifiers that send GET requests to this resource, then
the application profile MUST specify how external signature
verifiers provide the KDC with a self-generated nonce to use as
N_C (REQ21).
N_C and N_KDC expressed in CBOR diagnostic notation:
N_C = h'25a8991cd700ac01'
N_KDC = h'0b7db12aaff56da3'
N_C and N_KDC as CBOR encoded byte strings:
N_C = 0x4825a8991cd700ac01
N_KDC = 0x480b7db12aaff56da3
PoP input:
0x48 25a8991cd700ac01 48 0b7db12aaff56da3
Figure 20: Example of PoP Input to Compute 'kdc_cred_verify'
Using CBOR Encoding
4.5.1.1. Retrieve the KDC's Authentication Credential
In case the KDC has an associated authentication credential as
required for the correct group operation, a group member or an
external signature verifier can contact the KDC to request the KDC's
authentication credential by sending a CoAP GET request to the /ace-
group/GROUPNAME/kdc-cred endpoint at the KDC, where GROUPNAME
identifies the group.
Upon receiving the 2.05 (Content) response, the Client retrieves the
KDC's authentication credential from the 'kdc_cred' parameter and
MUST verify the proof-of-possession (PoP) evidence specified in the
'kdc_cred_verify' parameter. In case of successful verification of
the PoP evidence, the Client MUST store the obtained KDC's
authentication credential and replace the currently stored one.
The PoP evidence is verified by means of the same method used when
processing the Join Response (see Section 4.3.1). Application
profiles of this specification MUST specify the exact approaches used
by the Client to verify the PoP evidence in 'kdc_cred_verify' and
MUST specify which of those approaches is used in which case (REQ21).
Figure 21 gives an overview of the exchange described above, while
Figure 22 shows an example.
Group
Member KDC
| |
| KDC Authentication Credential Request |
|------------------------------------------------------------>|
| GET /ace-group/GROUPNAME/kdc-cred |
| |
|<-- KDC Authentication Credential Response: 2.05 (Content) --|
| |
Figure 21: Message Flow of KDC Authentication Credential Request-
Response to Obtain the Authentication Credential of the KDC
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "kdc-cred"
Response:
Header: Content (Code=2.05)
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation):
{
/ kdc_cred / 17: h'a2026008a101a5010202419920012158
2065eda5a12577c2bae829437fe33870
1a10aaa375e1bb5b5de108de439c0855
1d2258201e52ed75701163f7f9e40ddf
9f341b3dc9ba860af7e0ca7ca7e9eecd
0084d19c',
/ kdc_nonce / 18: h'0b7db12aaff56da3',
/ kdc_cred_verify / 19: h'3fc54702aa56e1b2cb20284294c9106a
63f91bac658d69351210a031d8fc7c5f
f3e4be39445b1a3e83e1510d1aca2f2e
8a7c081c7645042b18aba9d1fad1bd9c'
}
Figure 22: Example of KDC Authentication Credential Request-
Response to Obtain the Authentication Credential of the KDC
4.6. /ace-group/GROUPNAME/policies
This resource implements the GET handler.
4.6.1. GET Handler
The handler expects a GET request.
In addition to what is defined in Section 4.1.2, the handler verifies
that the Client is a current member of the group. If the
verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
response. The response MUST have Content-Format "application/
concise-problem-details+cbor" and is formatted as defined in
Section 4.1.2. Within the Custom Problem Detail entry 'ace-
groupcomm-error', the value of the 'error-id' field MUST be set to 0
("Operation permitted only to group members").
If all verifications succeed, the handler replies with a 2.05
(Content) response containing the list of policies for the group
identified by GROUPNAME. The payload of the response is formatted as
a CBOR map including only the 'group_policies' parameter defined in
Section 4.3.1 and specifying the current policies in the group. If
the KDC does not store any policy, the payload is formatted as a CBOR
byte string of zero length (0x40).
The specific format and meaning of group policies MUST be specified
in application profiles of this specification (REQ20).
4.6.1.1. Retrieve the Group Policies
A node in the group can contact the KDC to retrieve the current group
policies by sending a CoAP GET request to the /ace-group/GROUPNAME/
policies endpoint at the KDC, which is formatted as defined in
Section 4.6.1 and where GROUPNAME identifies the group.
Figure 23 gives an overview of the exchange described above, while
Figure 24 shows an example.
Client KDC
| |
|-- Policies Request: GET /ace-group/GROUPNAME/policies -->|
| |
|<----------- Policies Response: 2.05 (Content) -----------|
| |
Figure 23: Message Flow of Policies Request-Response
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "policies"
Response:
Header: Content (Code=2.05)
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload(in CBOR diagnostic notation):
{
/ group_policies / 16: {
/ Expiration Delta / 2: 120
}
}
Figure 24: Example of Policies Request-Response
4.7. /ace-group/GROUPNAME/num
This resource implements the GET handler.
4.7.1. GET Handler
The handler expects a GET request.
In addition to what is defined in Section 4.1.2, the handler verifies
that the Client is a current member of the group. If the
verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
response. The response MUST have Content-Format "application/
concise-problem-details+cbor" and is formatted as defined in
Section 4.1.2. Within the Custom Problem Detail entry 'ace-
groupcomm-error', the value of the 'error-id' field MUST be set to 0
("Operation permitted only to group members").
If all verifications succeed, the handler returns a 2.05 (Content)
message containing an integer that represents the version number of
the symmetric group keying material. This number is incremented on
the KDC every time the KDC updates the symmetric group keying
material before the new keying material is distributed. This number
is stored in persistent storage.
The payload of the response is formatted as a CBOR integer.
4.7.1.1. Retrieve the Keying Material Version
A node in the group can contact the KDC to request information about
the version number of the symmetric group keying material by sending
a CoAP GET request to the /ace-group/GROUPNAME/num endpoint at the
KDC, which is formatted as defined in Section 4.7.1 and where
GROUPNAME identifies the group. In particular, the version is
incremented by the KDC every time the group keying material is
renewed before it is distributed to the group members.
Figure 25 gives an overview of the exchange described above, while
Figure 26 shows an example.
Client KDC
| |
|---- Version Request: GET /ace-group/GROUPNAME/num ---->|
| |
|<---------- Version Response: 2.05 (Content) -----------|
| |
Figure 25: Message Flow of Version Request-Response
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "num"
Response:
Header: Content (Code=2.05)
Content-Format: 60 (application/cbor)
Payload (in CBOR diagnostic notation):
13
Figure 26: Example of Version Request-Response
4.8. /ace-group/GROUPNAME/nodes/NODENAME
This resource implements the GET, POST, and DELETE handlers.
In addition to what is defined in Section 4.1.2, each of the handlers
performs the following two verifications.
* The handler verifies that the Client is a current member of the
group. If the verification fails, the KDC MUST reply with a 4.03
(Forbidden) error response. The response MUST have Content-Format
"application/concise-problem-details+cbor" and is formatted as
defined in Section 4.1.2. Within the Custom Problem Detail entry
'ace-groupcomm-error', the value of the 'error-id' field MUST be
set to 0 ("Operation permitted only to group members").
* The handler verifies that the node name of the Client is equal to
NODENAME used in the url-path. If the verification fails, the
handler replies with a 4.03 (Forbidden) error response.
4.8.1. GET Handler
The handler expects a GET request.
If all verifications succeed, the handler replies with a 2.05
(Content) response containing both the group keying material and the
individual keying material for the Client or information enabling the
Client to derive it.
The payload of the response is formatted as a CBOR map, which
includes the same fields of the response defined in Section 4.3.2.
In particular, the format for the group keying material is the same
as defined in the response of Section 4.3.2. If the 'exp' parameter
is included, the 'exi' parameter MUST also be included. If the
parameter 'exi' is included, its value specifies the residual
lifetime of the group keying material from the current time at the
KDC.
The CBOR map can include additional parameters that specify the
individual keying material for the Client. The specific format of
individual keying material for group members or of the information to
derive such keying material MUST be defined in application profiles
of this specification (REQ27), together with the corresponding CBOR
map key that has to be registered in the "ACE Groupcomm Parameters"
registry defined in Section 11.7.
Optionally, the KDC can make the sub-resource at /ace-
group/GROUPNAME/nodes/NODENAME also observable [RFC7641] for the
associated node. In case the KDC removes that node from the group
without having been explicitly asked for it, this allows the KDC to
send an unsolicited 4.04 (Not Found) response to the node as a
notification of eviction from the group (see Section 5).
Note that the node could have also been observing the resource at
/ace-group/GROUPNAME in order to be informed of changes in the group
keying material. In such a case, this method would result in largely
overlapping notifications received for the resource at /ace-group/
GROUPNAME and the sub-resource at /ace-group/GROUPNAME/nodes/
NODENAME.
In order to mitigate this, a node that supports the CoAP No-Response
Option [RFC7967] can use it when starting the observation of the sub-
resource at /ace-group/GROUPNAME/nodes/NODENAME. In particular, the
GET observation request can also include the No-Response option, with
value set to 2 (Not interested in 2.xx responses).
4.8.1.1. Retrieve Group and Individual Keying Material
When any of the following happens, a node MUST stop using the stored
group keying material to protect outgoing messages and SHOULD stop
using it to decrypt and verify incoming messages.
* Upon expiration of the keying material, according to what is
indicated by the KDC through the 'exp' and/or 'exi' parameter
(e.g., in a Join Response) or to a pre-configured value.
* Upon receiving a notification of revoked/renewed keying material
from the KDC, possibly as part of an update of the keying material
(rekeying) triggered by the KDC.
* Upon receiving messages from other group members without being
able to retrieve the keying material to correctly decrypt them.
This may be due to rekeying messages previously sent by the KDC
that the Client was not able to receive or decrypt.
In either case, if it wants to continue participating in the group
communication, the Client has to request the latest keying material
from the KDC. To this end, the Client sends a CoAP GET request to
the /ace-group/GROUPNAME/nodes/NODENAME endpoint at the KDC,
formatted as specified in Section 4.8.1. The Client can request the
latest keying material from the KDC before the currently stored, old
keying material reaches its expiration time.
Note that policies can be set up so that the Client sends a Key
Distribution Request to the KDC only after a given number of received
messages could not be decrypted (because of failed decryption
processing or the inability to retrieve the necessary keying
material).
It is application dependent and pertaining to the used secure message
exchange (e.g., [GROUP-OSCORE]) to set up these policies for
instructing Clients to retain incoming messages and for how long
(OPT11). This allows Clients to possibly decrypt such messages after
getting updated keying material, rather than just consider them
invalid messages to discard right away.
After having failed to decrypt messages from another group member and
having sent a Key Distribution Request to the KDC, the Client might
end up retrieving the same, latest group keying material that it
already stores. In such a case, multiple failed decryptions might be
due to the message sender and/or the KDC that have changed their
authentication credential. Hence, the Client can retrieve such
latest authentication credentials by sending to the KDC an
Authentication Credential Request (see Sections 4.4.1.1 and 4.4.2.1)
and a KDC Authentication Credential Request (see Section 4.5.1.1),
respectively.
The Client can also send to the KDC a Key Distribution Request
without having been triggered by a failed decryption of a message
from another group member, if the Client wants to be sure that it
currently stores the latest group keying material. If that is the
case, the Client will receive from the KDC the same group keying
material it already stores.
Figure 27 gives an overview of the exchange described above, while
Figure 28 shows an example.
Client KDC
| |
|------------------ Key Distribution Request: --------------->|
| GET /ace-group/GROUPNAME/nodes/NODENAME |
| |
|<-------- Key Distribution Response: 2.05 (Content) ---------|
| |
Figure 27: Message Flow of Key Distribution Request-Response
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "nodes"
Uri-Path: "c101"
Response:
Header: Content (Code=2.05)
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation, with "ind-key" being the
profile-specified label for individual keying material):
{
/ gkty / 7: 65600,
/ key / 8: h'73657373696f6e6b6579',
/ num / 9: 12,
"ind-key": h'fcae9023'
}
Figure 28: Example of Key Distribution Request-Response
4.8.2. POST Handler
The POST handler processes requests from a Client that asks for new
individual keying material, as required to process messages exchanged
in the group.
The handler expects a POST request with an empty payload.
In addition to what is defined in Section 4.1.2 and at the beginning
of Section 4.8, the handler verifies that this operation is
consistent with the set of roles that the Client has in the group
(REQ11). If the verification fails, the KDC MUST reply with a 4.00
(Bad Request) error response. The response MUST have Content-Format
"application/concise-problem-details+cbor" and is formatted as
defined in Section 4.1.2. Within the Custom Problem Detail entry
'ace-groupcomm-error', the value of the 'error-id' field MUST be set
to 1 ("Request inconsistent with the current roles").
If the KDC is currently not able to serve this request, i.e., to
generate new individual keying material for the requesting Client,
the KDC MUST reply with a 5.03 (Service unavailable) error response.
The response MUST have Content-Format "application/concise-problem-
details+cbor" and is formatted as defined in Section 4.1.2. Within
the Custom Problem Detail entry 'ace-groupcomm-error', the value of
the 'error-id' field MUST be set to 4 ("No available individual
keying material").
If all verifications succeed, the handler replies with a 2.04
(Changed) response containing newly generated individual keying
material for the Client. The payload of the response is formatted as
a CBOR map. The specific format of newly generated individual keying
material for group members or of the information to derive such
keying material MUST be defined in application profiles of this
specification (REQ27), together with the corresponding CBOR map key
that has to be registered in the "ACE Groupcomm Parameters" registry
defined in Section 11.7.
The typical successful outcome consists in replying with newly
generated individual keying material for the Client, as defined
above. However, application profiles of this specification MAY also
extend this handler in order to achieve different akin outcomes
(OPT12), for instance:
* Not providing the Client with newly generated individual keying
material, but rather rekeying the whole group, i.e., providing all
the current group members with newly generated group keying
material.
* Both providing the Client with newly generated individual keying
material, as well as rekeying the whole group, i.e., providing all
the current group members with newly generated group keying
material.
In either case, the handler may specify the new group keying material
as part of the 2.04 (Changed) response.
Note that this handler is not intended to accommodate requests from a
group member to trigger a group rekeying, whose scheduling and
execution is an exclusive prerogative of the KDC (also see related
security considerations in Section 10.2).
4.8.2.1. Request to Change Individual Keying Material
A Client may ask the KDC for new individual keying material. For
instance, this can be due to the expiration of such individual keying
material or to the exhaustion of Authenticated Encryption with
Associated Data (AEAD) nonces if an AEAD encryption algorithm is used
for protecting communications in the group. An example of individual
keying material can simply be an individual encryption key associated
with the Client. Hence, the Client may ask for a new individual
encryption key or for new input material to derive it.
To this end, the Client performs a Key Renewal Request-Response
exchange with the KDC, i.e., it sends a CoAP POST request to the
/ace-group/GROUPNAME/nodes/NODENAME endpoint at the KDC, which is
formatted as defined in Section 4.8.1, where GROUPNAME identifies the
group and NODENAME is the node name of the Client.
Figure 29 gives an overview of the exchange described above, while
Figure 30 shows an example.
Client KDC
| |
|---------------- Key Renewal Request: ---------------->|
| POST /ace-group/GROUPNAME/nodes/NODENAME |
| |
|<-------- Key Renewal Response: 2.04 (Changed) --------|
| |
Figure 29: Message Flow of Key Renewal Request-Response
Request:
Header: POST (Code=0.02)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "nodes"
Uri-Path: "c101"
Response:
Header: Changed (Code=2.04)
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation, with "ind-key" being the
profile-specified label for individual keying material):
{
"ind-key": h'b71acc28'
}
Figure 30: Example of Key Renewal Request-Response
Note that there is a difference between the Key Renewal Request in
this section and the Key Distribution Request in Section 4.8.1.1.
The former asks the KDC for new individual keying material, while the
latter asks the KDC for the current group keying material together
with the current individual keying material.
As discussed in Section 4.8.2, application profiles of this
specification may define alternative outcomes for the Key Renewal
Request-Response exchange (OPT12), where the provisioning of new
individual keying material is replaced by or combined with the
execution of a whole group rekeying.
4.8.3. DELETE Handler
The DELETE handler removes the node identified by NODENAME from the
group identified by GROUPNAME.
The handler expects a DELETE request with an empty payload.
In addition to what is defined in Section 4.1.2, the handler verifies
that the Client is a current member of the group. If the
verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
response. The response MUST have Content-Format "application/
concise-problem-details+cbor" and is formatted as defined in
Section 4.1.2. Within the Custom Problem Detail entry 'ace-
groupcomm-error', the value of the 'error-id' field MUST be set to 0
("Operation permitted only to group members").
If all verification succeeds, the handler performs the actions
defined in Section 5 and replies with a 2.02 (Deleted) response with
an empty payload.
4.8.3.1. Leave the Group
A Client can actively request to leave the group. In this case, the
Client sends a CoAP DELETE request to the endpoint /ace-
group/GROUPNAME/nodes/NODENAME at the KDC, where GROUPNAME identifies
the group and NODENAME is the Client's node name.
Note that, after having left the group, the Client may wish to join
it again. Then, as long as the Client is still authorized to join
the group, i.e., the associated access token is still valid, the
Client can request to rejoin the group directly to the KDC (see
Section 4.3.1.1) without having to retrieve a new access token from
the AS.
4.9. /ace-group/GROUPNAME/nodes/NODENAME/cred
This resource implements the POST handler.
4.9.1. POST Handler
The POST handler is used to replace the stored authentication
credential of this Client (identified by NODENAME) with the one
specified in the request at the KDC for the group identified by
GROUPNAME.
The handler expects a POST request with the payload as specified in
Section 4.3.1, with the difference that the payload includes only the
parameters 'client_cred', 'cnonce', and 'client_cred_verify'.
The PoP evidence included in the 'client_cred_verify' parameter is
computed in the same way considered in Section 4.3.1 and defined by
the specific application profile (REQ14) by using the following to
build the PoP input: i) the same scope entry specified by the Client
in the 'scope' parameter of the latest Join Request that the Client
sent to the KDC in order to join the group identified by GROUPNAME;
ii) the latest N_S value stored by the Client; and iii) a new N_C
nonce generated by the Client and specified in the parameter 'cnonce'
of this request.
An example of PoP input to compute 'client_cred_verify' using CBOR
encoding is given in Figure 31.
It is REQUIRED for application profiles to define the specific
formats of authentication credentials that are acceptable to use in
the group (REQ6).
In addition to what is defined in Section 4.1.2 and at the beginning
of Section 4.8, the handler verifies that this operation is
consistent with the set of roles that the node has in the group. If
the verification fails, the KDC MUST reply with a 4.00 (Bad Request)
error response. The response MUST have Content-Format "application/
concise-problem-details+cbor" and is formatted as defined in
Section 4.1.2. Within the Custom Problem Detail entry 'ace-
groupcomm-error', the value of the 'error-id' field MUST be set to 1
("Request inconsistent with the current roles").
If the KDC cannot retrieve the 'kdcchallenge' associated with this
Client (see Section 3.3), the KDC MUST reply with a 4.00 (Bad
Request) error response, which MUST also have Content-Format
"application/ace-groupcomm+cbor". The payload of the error response
is a CBOR map including the 'kdcchallenge' parameter, which specifies
a newly generated 'kdcchallenge' value. In such a case, the KDC MUST
store the newly generated value as the 'kdcchallenge' value
associated with this Client, replacing the currently stored value (if
any).
Otherwise, the handler checks that the authentication credential
specified in the 'client_cred' field is valid for the group
identified by GROUPNAME. That is, the handler checks that the
authentication credential is encoded according to the format used in
the group, is intended for the public key algorithm used in the
group, and is aligned with the possible associated parameters used in
the group. If that cannot be successfully verified, the handler MUST
reply with a 4.00 (Bad Request) error response. The response MUST
have Content-Format "application/concise-problem-details+cbor" and is
formatted as defined in Section 4.1.2. Within the Custom Problem
Detail entry 'ace-groupcomm-error', the value of the 'error-id' field
MUST be set to 2 ("Authentication credential incompatible with the
group configuration").
Otherwise, the handler verifies the PoP evidence conveyed in the
'client_cred_verify' parameter of the request, by using the
authentication credential specified in the 'client_cred' parameter as
well as the same way considered in Section 4.3.1 and defined by the
specific application profile (REQ14). If the PoP evidence does not
pass verification, the handler MUST reply with a 4.00 (Bad Request)
error response. The response MUST have Content-Format "application/
concise-problem-details+cbor" and is formatted as defined in
Section 4.1.2. Within the Custom Problem Detail entry 'ace-
groupcomm-error', the value of the 'error-id' field MUST be set to 3
("Invalid proof-of-possession evidence").
If all verifications succeed, the handler performs the following
actions.
* The handler associates the authentication credential from the
'client_cred' parameter of the request with the node identifier
NODENAME, as well as with the access token associated with the
node identified by NODENAME.
* In the stored list of group members' authentication credentials
for the group identified by GROUPNAME, the handler replaces the
authentication credential of the node identified by NODENAME with
the authentication credential specified in the 'client_cred'
parameter of the request.
Then, the handler replies with a 2.04 (Changed) response, which does
not include a payload.
scope, N_S, and N_C expressed in CBOR diagnostic notation:
scope = h'826667726f7570316673656e646572'
N_S = h'018a278f7faab55a'
N_C = h'0446baefc56111bf'
scope, N_S, and N_C as CBOR encoded byte strings:
scope = 0x4f826667726F7570316673656E646572
N_S = 0x48018a278f7faab55a
N_C = 0x480446baefc56111bf
PoP input:
0x4f 826667726f7570316673656e646572
48 018a278f7faab55a 48 0446baefc56111bf
Figure 31: Example of PoP Input to Compute 'client_cred_verify'
Using CBOR Encoding
4.9.1.1. Uploading an Authentication Credential
In case the KDC maintains the authentication credentials of group
members, a node in the group can contact the KDC to upload a new
authentication credential to use in the group and to replace the
currently stored one.
To this end, the Client performs an Authentication Credential Update
Request-Response exchange with the KDC, i.e., it sends a CoAP POST
request to the /ace-group/GROUPNAME/nodes/NODENAME/cred endpoint at
the KDC, where GROUPNAME identifies the group and NODENAME is the
Client's node name.
The request is formatted as specified in Section 4.9.1.
Figure 32 gives an overview of the exchange described above, while
Figure 33 shows an example.
Client KDC
| |
|----------- Authentication Credential Update Request: --------->|
| POST /ace-group/GROUPNAME/nodes/NODENAME/cred |
| |
|<-- Authentication Credential Update Response: 2.04 (Changed) --|
| |
Figure 32: Message Flow of Authentication Credential Update
Request-Response
Request:
Header: POST (Code=0.02)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "nodes"
Uri-Path: "c101"
Uri-Path: "cred"
Content-Format: 261 (application/ace-groupcomm+cbor)
Payload (in CBOR diagnostic notation):
{
/ client_cred / 5: h'a2026008a101a501020241fc20012158
20bac5b11cad8f99f9c72b05cf4b9e26
d244dc189f745228255a219a86d6a09e
ff22582020138bf82dc1b6d562be0fa5
4ab7804a3a64b6d72ccfed6b6fb6ed28
bbfc117e',
/ cnonce / 6: h'0446baefc56111bf',
/ client_cred_verify / 24: h'e2aeafd40d69d19dfe6e52077c5d7ff4
e408282cbefb5d06cbf414af2e19d982
ac45ac98b8544c908b4507de1e90b717
c3d34816fe926a2b98f53afd2fa0f30a'
}
Response:
Header: Changed (Code=2.04)
Figure 33: Example of Authentication Credential Update Request-
Response
Additionally, after updating its own authentication credential, a
group member MAY send to the group a number of requests, including an
identifier of the updated authentication credential, to notify other
group members that they have to retrieve it. How this is done
depends on the group communication protocol used and therefore is
application profile specific (OPT13).
5. Removal of a Group Member
A Client identified by NODENAME may be removed from a group
identified by GROUPNAME where it is a member, for example, due to the
following reasons.
1. The Client explicitly asks to leave the group, as defined in
Section 4.8.3.1.
2. The node has been found compromised or is suspected so. The KDC
is expected to determine that a group member has to be evicted
either through its own means or based on information that it
obtains from a trusted source (e.g., an Intrusion Detection
System or an issuer of authentication credentials). Additional
mechanics, protocols, and interfaces at the KDC that can support
this are out of the scope of this document.
3. The Client's authorization to be a group member with the current
roles is not valid anymore, i.e., the access token has expired or
has been revoked. If the AS provides token introspection (see
Section 5.9 of [RFC9200]), the KDC can optionally use it and
check whether the Client is still authorized.
In all cases, the KDC performs the following actions.
* The KDC removes the Client from the list of current members of the
group. When doing so, the KDC deletes the currently stored value
of 'clientchallenge' for that Client, which was specified in the
latest Join Request that the Client sent to the KDC in order to
join the group (see Section 4.3.1).
* In case of forced eviction, i.e., for cases 2 and 3 above, the KDC
deletes the authentication credential of the removed Client if it
acts as a repository of authentication credentials for group
members.
* If the removed Client is registered as an observer of the group-
membership resource at /ace-group/GROUPNAME, the KDC removes the
Client from the list of observers of that resource.
* If the sub-resource /nodes/NODENAME was created for the removed
Client, the KDC deletes that sub-resource.
In case of forced eviction, i.e., for cases 2 and 3 above, the KDC
MAY explicitly inform the removed Client by means of the following
methods.
- If the evicted Client implements the 'control_uri' resource
(see Section 4.3.1), the KDC sends a DELETE request to that
resource, targeting the URI specified in the 'control_uri'
parameter of the Join Request (see Section 4.3.1).
- If the evicted Client is observing its associated sub-resource
at /ace-group/GROUPNAME/nodes/NODENAME (see Section 4.8.1), the
KDC sends an unsolicited 4.04 (Not Found) error response, which
does not include the Observe Option and indicates that the
observed resource has been deleted (see Section 3.2 of
[RFC7641]).
The response MUST have Content-Format "application/concise-
problem-details+cbor" and is formatted as defined in
Section 4.1.2. Within the Custom Problem Detail entry 'ace-
groupcomm-error', the value of the 'error-id' field MUST be set
to 5 ("Group membership terminated").
* If forward security is prescribed by application policies
installed at the KDC or by the used application profile of this
specification, then the KDC MUST generate new group keying
material and securely distribute it to all the current group
members except the leaving node (see Section 6).
6. Group Rekeying Process
A group rekeying is started and driven by the KDC. The KDC is not
intended to accommodate explicit requests from group members to
trigger a group rekeying. That is, the scheduling and execution of a
group rekeying is an exclusive prerogative of the KDC. Some reasons
that can trigger a group rekeying include a change in the group
membership, the current group keying material approaching its
expiration time, or a regularly scheduled update of the group keying
material.
The KDC can perform a group rekeying before the current group keying
material expires, unless it is acceptable or there are reasons to
temporarily pause secure communications in the group, following the
expiration of the current keying material. For example, a pause in
the group communication might have been scheduled to start anyway
when the group keying material expires, e.g., to allow maintenance
operations on the group members. As another example, the KDC might
be carrying out a verification that some group members are seemingly
compromised and to be evicted, and this needs to be completed in
order to appropriately define and schedule the exact rekeying process
to perform. As a result, the KDC could delay the execution of the
group rekeying.
The KDC MUST increment the version number NUM of the current keying
material before distributing the newly generated keying material with
version number NUM+1 to the group. Once the group rekeying is
completed, the KDC MUST delete the old keying material and SHOULD
store the newly distributed keying material in persistent storage.
Distributing the new group keying material requires the KDC to send
multiple rekeying messages to the group members. Depending on the
rekeying scheme used in the group and the reason that has triggered
the rekeying process, each rekeying message can be intended for one
or multiple group members, hereafter referred to as target group
members. The KDC MUST support at least the "Point-to-Point" group
rekeying scheme described in Section 6.1 and MAY support additional
ones.
Each rekeying message MUST have Content-Format "application/ace-
groupcomm+cbor" and its payload is formatted as a CBOR map, which
MUST include at least the information specified in the Key
Distribution Response message (see Section 4.3.2), i.e., the
parameters 'gkty', 'key', and 'num' defined in Section 4.3.1. The
CBOR map SHOULD also include the parameters 'exp' and 'exi'. If the
'exp' parameter is included, the 'exi' parameter MUST also be
included. The CBOR map MAY include the parameter 'mgt_key_material'
to specify new administrative keying material for the target group
members if it is relevant for the used rekeying scheme.
A rekeying message may include additional information, depending on
the rekeying scheme used in the group, the reason that has triggered
the rekeying process, and the specific target group members. In
particular, if the group rekeying is performed due to one or multiple
Clients that have joined the group and the KDC acts as a repository
of authentication credentials of the group members, then a rekeying
message MAY also include the authentication credentials that those
Clients use in the group, together with the roles and node identifier
that each of such Clients has in the group. It is RECOMMENDED to
specify this information by means of the parameters 'creds',
'peer_roles', and 'peer_identifiers', like it is done in the Join
Response message (see Section 4.3.1).
The complete format of a rekeying message, including the encoding and
content of the 'mgt_key_material' parameter, has to be defined in
separate specifications aimed at profiling the used rekeying scheme
in the context of the used application profile of this specification.
As a particular case, an application profile of this specification
MAY define additional information to include in rekeying messages for
the "Point-to-Point" group rekeying scheme defined in Section 6.1
(OPT14).
Consistently with the used group rekeying scheme, the actual delivery
of rekeying messages can occur through different approaches, as
discussed in Sections 6.1 and 6.2.
The possible, temporary misalignment of the keying material stored by
the different group members due to a group rekeying is discussed in
Section 6.3. Further security considerations related to the group
rekeying process are compiled in Section 10.2.
6.1. Point-to-Point Group Rekeying
A point-to-point group rekeying consists in the KDC sending one
individual rekeying message to each target group member. In
particular, the rekeying message is protected by means of the secure
communication association between the KDC and the target group member
in question, as per the used application profile of this
specification and the used transport profile of ACE.
This is the approach taken by the basic "Point-to-Point" group
rekeying scheme, which the KDC can explicitly indicate in the Join
Response (see Section 4.3.1), through the 'rekeying_scheme' parameter
specifying the value 0.
When taking this approach in the group identified by GROUPNAME, the
KDC can practically deliver the rekeying messages to the target group
members in different, coexisting ways.
* The KDC SHOULD make the /ace-group/GROUPNAME resource observable
[RFC7641]. Thus, upon performing a group rekeying, the KDC can
distribute the new group keying material through individual
notification responses sent to the target group members that are
also observing that resource.
In case the KDC deletes the group (and thus deletes the /ace-
group/GROUPNAME resource), relying on CoAP Observe as discussed
above also allows the KDC to send an unsolicited 4.04 (Not Found)
response to each observer group member as a notification of group
termination. The response MUST have Content-Format "application/
concise-problem-details+cbor" and is formatted as defined in
Section 4.1.2. Within the Custom Problem Detail entry 'ace-
groupcomm-error', the value of the 'error-id' field MUST be set to
6 ("Group deleted").
* If a target group member specified a URI in the 'control_uri'
parameter of the Join Request upon joining the group (see
Section 4.3.1), the KDC can provide that group member with the new
group keying material by sending a unicast POST request to that
URI.
A Client that does not plan to observe the /ace-group/GROUPNAME
resource at the KDC SHOULD specify a URI in the 'control_uri'
parameter of the Join Request upon joining the group.
If the KDC has to send a rekeying message to a target group member,
but this did not include the 'control_uri' parameter in the Join
Request and is not a registered observer for the /ace-group/GROUPNAME
resource, then that target group member will not be able to
participate in the group rekeying. Later on, after having repeatedly
failed to successfully exchange secure messages in the group, that
group member can retrieve the current group keying material from the
KDC, by sending a GET request to the /ace-group/GROUPNAME or /ace-
group/GROUPNAME/nodes/NODENAME resource at the KDC (see Sections
4.3.2 and 4.8.1, respectively).
Figure 34 provides an example of point-to-point group rekeying. In
particular, the example makes the following assumptions:
* The group currently consists of four group members, namely C1, C2,
C3, and C4.
* Each group member, when joining the group, provided the KDC with a
URI in the 'control_uri' parameter with url-path "grp-rek".
* Before the group rekeying is performed, the keying material used
in the group has version number num=5.
* The KDC performs the group rekeying in such a way to evict the
group member C3, which has been found to be compromised.
In the example, the KDC individually rekeys the group members
intended to remain in the group (i.e., C1, C2, and C4) by means of
one rekeying message each.
.----------------------------------------------------------------.
| KDC |
'----------------------------------------------------------------'
| | |
Group | Group | Group |
keying | keying | keying |
material | material | material |
(num=6) | (num=6) | (num=6) |
| | |
| | |
| | |
v v v
/grp-rek /grp-rek /grp-rek /grp-rek
.--------. .--------. .--------. .--------.
| C1 | | C2 | | C3 | | C4 |
'--------' '--------' '--------' '--------'
[TO BE EVICTED]
| |
\____________ Stored group keying material (num=5) _____________/
Figure 34: Example of Message Exchanges for a Point-to-Point
Group Rekeying
6.2. One-to-Many Group Rekeying
This section provides high-level recommendations on how the KDC can
rekey a group by means of a more efficient and scalable group
rekeying scheme, e.g., [RFC2093], [RFC2094], and [RFC2627]. That is,
each rekeying message might be, and likely is, intended for multiple
target group members, and thus can be delivered to the whole group,
although possible to decrypt only for the actual target group
members.
This yields an overall lower number of rekeying messages, thus
potentially reducing the overall time required to rekey the group.
On the other hand, it requires the KDC to provide and use additional
administrative keying material to protect the rekeying messages and
to additionally sign them to ensure source authentication (see
Section 6.2.1).
Compared to a group rekeying performed in a point-to-point fashion
(see Section 6.1), a one-to-many group rekeying typically pays off in
large-scale groups due to the reduced time for completing the
rekeying, a more efficient utilization of network resources, and a
reduced performance overhead at the KDC. To different extents, it
also requires individual group members to locally perform additional
operations in order to handle the administrative keying material and
verify source authentication of rekeying messages. Therefore, one-
to-many group rekeying schemes and their employment ought to ensure
that the experienced performance overhead on the group members also
remains bearable for resource-constrained devices.
The exact set of rekeying messages to send, their content and format,
the administrative keying material to use to protect them, as well as
the set of target group members depend on the specific group rekeying
scheme and are typically affected by the reason that has triggered
the group rekeying. Details about the data content and format of
rekeying messages have to be defined by separate documents profiling
the use of the group rekeying scheme in the context of the used
application profile of this specification.
When one of these group rekeying schemes is used, the KDC provides
related information to a Client joining the group in the Join
Response message (see Section 4.3.1). In particular, the
'rekeying_scheme' parameter indicates the rekeying scheme used in the
group (if no default scheme can be assumed); the 'control_group_uri'
parameter, if present, specifies a URI whose addressing information
is, e.g., a multicast IP address where the KDC will send the rekeying
messages for that group as intended to reach all the group members;
and the 'mgt_key_material' parameter specifies a subset of the
administrative keying material intended for that particular joining
Client to have, as used to protect the rekeying messages sent to the
group when also intended for that joining Client.
Rekeying messages can be protected at the application layer by using
COSE [RFC9052] and the administrative keying material as prescribed
by the specific group rekeying scheme (see Section 6.2.1). After
that, the delivery of protected rekeying messages to the intended
target group members can occur in different ways, such as the
following ones.
Over multicast - In this case, the KDC simply sends a rekeying
message as a CoAP request addressed to the URI specified in the
'control_group_uri' parameter of the Join Response (see
Section 4.3.1).
If a particular rekeying message is intended for a single target
group member, the KDC may alternatively protect the message using
the secure communication association with that group member and
deliver the message like when using the "Point-to-Point" group
rekeying scheme (see Section 6.1).
Through a pub-sub communication model - In this case, the KDC acts
as a publisher and publishes each rekeying message to a specific
"rekeying topic", which is associated with the group and is hosted
at a Broker server. Following their group joining, the group
members subscribe to the rekeying topic at the Broker, thus
receiving the group rekeying messages as they are published by the
KDC.
In order to make such message delivery more efficient, the
rekeying topic associated with a group can be further organized
into subtopics. For instance, the KDC can use a particular
subtopic to address a particular set of target group members
during the rekeying process as possibly aligned to a similar
organization of the administrative keying material (e.g., a key
hierarchy).
The setup of rekeying topics at the Broker as well as the
discovery of the topics at the Broker for group members are
application specific. A possible way is for the KDC to provide
such information in the Join Response message (see Section 4.3.1)
by means of a new parameter analogous to 'control_group_uri' and
specifying the URI(s) of the rekeying topic(s) that a group member
has to subscribe to at the Broker.
Regardless of the specifically used delivery method, the group
rekeying scheme can perform a possible rollover of the administrative
keying material through the same sent rekeying messages. Actually,
such a rollover occurs every time a group rekeying is performed upon
the leaving of group members, which have to be excluded from future
communications in the group.
From a high-level point of view, each group member stores only a
subset of the overall administrative keying material, which is
obtained upon joining the group. Then, when a group rekeying occurs:
* Each rekeying message is protected by using a (most convenient)
key from the administrative keying material such that: i) the used
key is not stored by any node leaving the group, i.e., the key is
safe to use and does not have to be renewed; and ii) the used key
is stored by all the target group members that indeed have to be
provided with new group keying material to protect communications
in the group.
* Each rekeying message includes not only the new group keying
material intended for all the rekeyed group members but also any
new administrative keys that: i) are pertaining to and supposed to
be stored by the target group members; and ii) had to be updated
because leaving group members do store the previous version.
Further details depend on the specific rekeying scheme used in the
group.
Figure 35 provides an example of a one-to-many group rekeying over
multicast. In particular, the example makes the following
assumptions:
* The group currently consists of four group members, namely C1, C2,
C3, and C4.
* Each group member, when joining the group, provided the KDC with a
URI in the 'control_uri' parameter with url-path "grp-rek".
* Each group member, when joining the group, received from the KDC a
URI in the 'control_group_uri' parameter, specifying the multicast
address MULT_ADDR and url-path "grp-mrek".
* Before the group rekeying is performed, the keying material used
in the group has version number num=5.
* The KDC performs the group rekeying in such a way to evict the
group member C3, which has been found to be compromised.
In the example, the KDC determines that the most convenient way to
perform a group rekeying that evicts C3 is as follows.
First, the KDC sends one rekeying message over multicast to the
multicast address MULT_ADDR and the url-path "grp-mrek". In the
figure, the message is denoted with solid arrows. The message is
protected with a non-compromised key from the administrative keying
material that only C1 and C2 store. Therefore, even though all the
group members receive this message, only C1 and C2 are able to
decrypt it. The message includes: the new group keying material with
version number num=6 and new keys from the administrative keying
material to replace those stored by the group members C1, C2, and C3.
After that, the KDC sends one rekeying message addressed individually
to C4 and with url-path "grp-rek". In the figure, the message is
denoted with a dotted arrow. The message is protected with the
secure association shared between C4 and the KDC. The message
includes: the new group keying material with version number num=6 and
new keys from the administrative keying material to replace those
stored by both C4 and C3.
.---------------------------------------------------------------------.
| KDC |
'---------------------------------------------------------------------'
| :
* Group keying material (num=6) | * Group keying :
* Updated administrative | material (num=6) :
keying material for C1 and C2 | * Updated administrative :
| keying material for C4 :
| :
| :
+------------+-------------+--------------+ :
| | | | :
| | | | :
v v v v v
/grp-mrek /grp-mrek /grp-mrek /grp-mrek /grp-rek
.--------. .--------. .-----------. .---------------------------.
| C1 | | C2 | | C3 | | C4 |
'--------' '--------' '-----------' '---------------------------'
[TO BE EVICTED]
| |
\_______________ Stored group keying material (num=5) ________________/
Figure 35: Example of Message Exchanges for a One-to-Many Group
Rekeying
6.2.1. Protection of Rekeying Messages
When using a group rekeying scheme relying on one-to-many rekeying
messages, the actual data content of each rekeying message is
prepared according to what the rekeying scheme prescribes.
The following describes one possible method for the KDC to protect
the rekeying messages when using the administrative keying material.
The method assumes that the following holds for the administrative
keying material specified in the 'mgt_key_material' parameter of the
Join Response (see Section 4.3.1).
* The encryption algorithm SHOULD be the same one used to protect
communications in the group.
* The included symmetric encryption keys are accompanied by a
corresponding and unique key identifier assigned by the KDC.
* A Base IV is also included with the same size of the AEAD nonce
considered by the encryption algorithm to use.
First, the KDC computes a COSE_Encrypt0 object as follows.
* The encryption key to use is selected from the administrative
keying material, as defined by the rekeying scheme used in the
group.
* The plaintext is the actual data content of the current rekeying
message.
* The Additional Authenticated Data (AAD) is empty unless otherwise
specified by separate documents profiling the use of the group
rekeying scheme.
* Since the KDC is the only sender of rekeying messages, the AEAD
nonce can be computed as follows, where NONCE_SIZE is the size in
bytes of the AEAD nonce. Separate documents profiling the use of
the group rekeying scheme may define alternative ways to compute
the AEAD nonce.
The KDC considers the following values.
- COUNT: as a 2-byte unsigned integer associated with the used
encryption key. Its value is set to 0 when starting to perform
a new group rekeying instance and is incremented after each use
of the encryption key.
- NEW_NUM: as the version number of the new group keying material
to distribute in this rekeying instance, left-padded with zeros
to exactly NONCE_SIZE - 2 bytes.
Then, the KDC computes a Partial IV as the byte string
concatenation of COUNT and NEW_NUM in this order. Finally, the
AEAD nonce is computed as the XOR between the Base IV and the
Partial IV.
In order to comply with the security requirements of AEAD
encryption algorithms, the KDC MUST NOT reuse the same pair (AEAD
encryption key, AEAD nonce). For example, this includes not using
the same encryption key from the administrative keying material
more than 2^16 times during the same rekeying instance.
* The protected header of the COSE_Encrypt0 object MUST include the
following parameters.
- 'alg': specifying the used encryption algorithm.
- 'kid': specifying the identifier of the encryption key from the
administrative keying material used to protect the current
rekeying message.
* The unprotected header of the COSE_Encrypt0 object MUST include
the 'Partial IV' parameter with the value of the Partial IV
computed above.
In order to ensure source authentication, each rekeying message
protected with the administrative keying material MUST be signed by
the KDC. To this end, the KDC computes a countersignature of the
COSE_Encrypt0 object, as described in Sections 3.2 and 3.3 of
[RFC9338]. In particular, the following applies when computing the
countersignature.
* The Countersign_structure contains the context text string
"CounterSignature0".
* The private key of the KDC is used as the signing key.
* The payload is the ciphertext of the COSE_Encrypt0 object.
* The Additional Authenticated Data (AAD) is empty, unless otherwise
specified by separate documents profiling the use of a group
rekeying scheme.
* The protected header of the signing object MUST include the
parameter 'alg', which specifies the used signature algorithm.
If the source authentication of messages exchanged in the group is
also ensured by means of signatures, then rekeying messages MUST be
signed using the same signature algorithm and related parameters.
Also, the KDC's authentication credential including the public key to
use for signature verification MUST be provided in the Join Response
through the 'kdc_cred' parameter, together with the corresponding
proof-of-possession (PoP) evidence in the 'kdc_cred_verify'
parameter.
If source authentication of messages exchanged in the group is not
ensured by means of signatures, then the administrative keying
material conveyed in the 'mgt_key_material' parameter of the Join
Response sent by KDC (see Section 4.3.1) MUST also comprise a KDC's
authentication credential including the public key to use for
signature verification, together with the corresponding PoP evidence.
Within the 'mgt_key_material' parameter, it is RECOMMENDED to specify
this information by using the same format and encoding used for the
parameters 'kdc_cred', 'kdc_nonce', and 'kdc_cred_verify' in the Join
Response. It is up to separate documents profiling the use of the
group rekeying scheme to specify such details.
After that, the KDC specifies the computed countersignature in the
'Countersignature0 version 2' header parameter of the COSE_Encrypt0
object.
Finally, the KDC specifies the COSE_Encrypt0 object as payload of a
CoAP request, which is sent to the target group members as per the
used message delivery method.
6.3. Misalignment of Group Keying Material
A group member can receive a message shortly after the group has been
rekeyed and new keying material has been distributed by the KDC. In
the following two cases, this may result in misaligned keying
material between the group members.
In the first case, the sender protects a message using the old group
keying material. However, the recipient receives the message after
having received the new group keying material, hence it is not able
to correctly process the message. A possible way to limit the impact
of this issue is to preserve the old, retained group keying material
for a maximum amount of time defined by the application, during which
such group keying material is used solely for processing incoming
messages. By doing so, the recipient can still temporarily process
received messages also by using the old, retained group keying
material. Note that a former (compromised) group member can take
advantage of this by sending messages protected with the old,
retained group keying material. Therefore, a conservative
application policy should not admit the storage of old group keying
material. Eventually, the sender will have obtained the new group
keying material too and can possibly resend the message protected
with such keying material.
In the second case, the sender protects a message using the new group
keying material, but the recipient receives that message before
having received the new group keying material. Therefore, the
recipient will not be able to correctly process the message and hence
will discard it. If the recipient receives the new group keying
material shortly after that and the application at the sender
endpoint performs retransmissions, the former will still be able to
receive and correctly process the message. In any case, the
recipient should actively ask the KDC for the latest group keying
material according to an application-defined policy, for instance,
after a given number of unsuccessfully decrypted incoming messages.
7. Extended Scope Format
This section defines an extended format of binary-encoded scope,
which additionally specifies the semantics used to express the same
access control information from the corresponding original scope.
As also discussed in Section 3.2, this enables a Resource Server to
unambiguously process a received access token, also in case the
Resource Server runs multiple applications or application profiles
that involve different scope semantics.
The extended format is intended only for the 'scope' claim of access
tokens for the cases where the claim takes a CBOR byte string as the
value. That is, the extended format does not apply to the 'scope'
parameter included in ACE messages, i.e., the Authorization Request
and Authorization Response exchanged between the Client and the
Authorization Server (see Sections 5.8.1 and 5.8.2 of [RFC9200]), the
AS Request Creation Hints message from the Resource Server (see
Section 5.3 of [RFC9200]), and the Introspection Response from the
Authorization Server (see Section 5.9.2 of [RFC9200]).
The value of the 'scope' claim following the extended format is
composed as follows. Given the original scope using semantics SEM
and encoded as a CBOR byte string, the corresponding extended scope
consists of the same CBOR byte string enclosed by a CBOR tag
[RFC8949], whose tag number identifies the semantics SEM.
The resulting tagged CBOR byte string is used as the value of the
'scope' claim of the access token.
Figures 36 and 37 build on the examples in Section 3.1 and show the
corresponding extended scopes.
;# include rfc9237
gname = tstr
permissions = uint .bits roles
roles = &(
Requester: 1,
Responder: 2,
Monitor: 3,
Verifier: 4
)
scope_entries = AIF-Generic<gname, permissions>
scope = bstr .cbor scope_entries
extended_scope = #6.<TAG_FOR_THIS_SEMANTICS>(scope)
TAG_FOR_THIS_SEMANTICS = uint
Figure 36: Example of Extended scope Using AIF
gname = tstr
role = tstr
scope_entry = [ gname , ? ( role / [ 2*role ] ) ]
scope_entries = [ * scope_entry ]
scope = bstr .cbor scope_entries
extended_scope = #6.<TAG_FOR_THIS_SEMANTICS>(scope)
TAG_FOR_THIS_SEMANTICS = uint
Figure 37: Example of Extended scope Using the Textual Format,
with the Role Identifiers Encoded as Text Strings
The usage of the extended scope format is not limited to application
profiles of this specification or to applications based on group
communication. Rather, it is generally applicable to any application
and application profile where access control information in the
access token is expressed as a binary-encoded scope.
Applications and application profiles using the extended format of
scope have to specify which CBOR tag from [CBOR.Tags] is used for
identifying the scope semantics or to register a new CBOR tag if a
suitable one does not exist already (REQ28). In case there is an
already existing, suitable CBOR tag, a new CBOR tag should not be
registered in order to avoid code point squatting.
If the binary-encoded scope uses semantics associated with a
registered CoAP Content-Format [RFC7252] [CoAP.Content.Formats], then
a suitable CBOR tag associated with that CoAP Content-Format would
already be registered, as defined in Section 4.3 of [RFC9277].
This is especially relevant when the binary encoded scope uses AIF.
That is, it is expected that the definition of an AIF-specific data
model comes together with the registration of CoAP Content-Formats
for the relevant combinations of its Toid and Tperm values. As
discussed above, this yields the automatic registration of the CBOR
tags associated with those CoAP Content-Formats.
8. ACE Groupcomm Parameters
This specification defines a number of parameters used during the
second phase of the key provisioning process, i.e., after the
exchange after the exchange of Token Transfer Request and Response.
The table below summarizes them and specifies the CBOR map keys to
use instead of the full descriptive names.
Note that the media type "application/ace-groupcomm+cbor" MUST be
used when these parameters are transported in the respective CBOR map
entries.
+=======================+======+========================+===========+
| Name | CBOR | CBOR Type | Reference |
| | Key | | |
+=======================+======+========================+===========+
| gid | 0 | array | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| gname | 1 | array of tstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| guri | 2 | array of tstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| scope | 3 | bstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| get_creds | 4 | Null or array | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| client_cred | 5 | bstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| cnonce | 6 | bstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| gkty | 7 | int or tstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| key | 8 | See the "ACE | RFC 9594 |
| | | Groupcomm Key | |
| | | Types" registry | |
+-----------------------+------+------------------------+-----------+
| num | 9 | int | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| ace_groupcomm_profile | 10 | int | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| exp | 11 | uint | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| exi | 12 | uint | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| creds | 13 | array | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| peer_roles | 14 | array | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| peer_identifiers | 15 | array | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| group_policies | 16 | map | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| kdc_cred | 17 | bstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| kdc_nonce | 18 | bstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| kdc_cred_verify | 19 | bstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| rekeying_scheme | 20 | int | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| client_cred_verify | 24 | bstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| creds_repo | 25 | tstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| control_uri | 26 | tstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| mgt_key_material | 27 | bstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| control_group_uri | 28 | tstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| sign_info | 29 | Null or array | RFC 9594 |
+-----------------------+------+------------------------+-----------+
| kdcchallenge | 30 | bstr | RFC 9594 |
+-----------------------+------+------------------------+-----------+
Table 5: ACE Groupcomm Parameters
The KDC is expected to support all the parameters above. Instead, a
Client can support only a subset of such parameters, depending on the
roles it expects to take in the joined groups or on other conditions
defined in application profiles of this specification.
In the following, the parameters are categorized according to the
support expected by Clients. That is, a Client that supports a
parameter is able to: i) use and specify it in a request message to
the KDC; and ii) understand and process it if specified in a response
message from the KDC. It is REQUIRED of application profiles of this
specification to sort their newly defined parameters according to the
same categorization (REQ29).
Note that the actual use of a parameter and its inclusion in a
message depends on the specific exchange, the specific Client and
group involved, as well as what is defined in the used application
profile of this specification.
A Client MUST support the following parameters.
* 'scope'
* 'cnonce'
* 'gkty'
* 'key'
* 'num'
* 'exp'
* 'exi'
* 'gid'
* 'gname'
* 'guri'
* 'creds'
* 'peer_identifiers'
* 'ace_groupcomm_profile'
* 'control_uri'
* 'rekeying_scheme'
A Client SHOULD support the following parameter.
* 'get_creds': That is, not supporting this parameter would yield
the inconvenient and undesirable behavior where: i) the Client
does not ask for the other group members' authentication
credentials upon joining the group (see Section 4.3.1.1); and ii)
later on as a group member, the Client only retrieves the
authentication credentials of all group members (see
Section 4.4.2.1).
The following conditional parameters are relevant only if specific
conditions hold. It is REQUIRED of application profiles of this
specification to define whether Clients must, should, or may support
these parameters and under which circumstances (REQ30).
* 'client_cred' and 'client_cred_verify': These parameters are
relevant for a Client that has an authentication credential to use
in a joined group.
* 'kdcchallenge': This parameter is relevant for a Client that has
an authentication credential to use in a joined group and that
provides the access token to the KDC through a Token Transfer
Request (see Section 3.3).
* 'creds_repo': This parameter is relevant for a Client that has an
authentication credential to use in a joined group and that makes
it available from a key repository different than the KDC.
* 'group_policies': This parameter is relevant for a Client that is
interested in the specific policies used in a group, but it does
not know them or cannot become aware of them before joining that
group.
* 'peer_roles': This parameter is relevant for a Client that has to
know about the roles of other group members, especially when
retrieving and handling their corresponding authentication
credentials.
* 'kdc_nonce', 'kdc_cred', and 'kdc_cred_verify': These parameters
are relevant for a Client that joins a group for which, as per the
used application profile of this specification, the KDC has an
associated authentication credential and this is required for the
correct group operation.
* 'mgt_key_material': This parameter is relevant for a Client that
supports an advanced rekeying scheme possibly used in the group,
such as based on one-to-many rekeying messages sent over IP
multicast.
* 'control_group_uri': This parameter is relevant for a Client that
also acts as a CoAP server supporting: i) the hosting of a
dedicated resource for each group that the Client is interested to
be a part of; and ii) the reception of one-to-many requests sent
to those resources by the KDC (e.g., over IP multicast), as
targeting multiple members of the corresponding group. Examples
of related management operations that the KDC can perform by this
means are the eviction of group members and the execution of a
group rekeying process through an advanced rekeying scheme, such
as based on one-to-many rekeying messages.
9. ACE Groupcomm Error Identifiers
This specification defines a number of values that the KDC can use as
error identifiers. These are used in error responses with Content-
Format "application/concise-problem-details+cbor", as values of the
'error-id' field within the Custom Problem Detail entry 'ace-
groupcomm-error' (see Section 4.1.2).
+=======+=============================================+
| Value | Description |
+=======+=============================================+
| 0 | Operation permitted only to group members |
+-------+---------------------------------------------+
| 1 | Request inconsistent with the current roles |
+-------+---------------------------------------------+
| 2 | Authentication credential incompatible with |
| | the group configuration |
+-------+---------------------------------------------+
| 3 | Invalid proof-of-possession evidence |
+-------+---------------------------------------------+
| 4 | No available individual keying material |
+-------+---------------------------------------------+
| 5 | Group membership terminated |
+-------+---------------------------------------------+
| 6 | Group deleted |
+-------+---------------------------------------------+
Table 6: ACE Groupcomm Error Identifiers
If a Client supports the problem-details format [RFC9290] and the
Custom Problem Detail entry 'ace-groupcomm-error' defined in
Section 4.1.2 of this document and is able to understand the error
specified in the 'error-id' field therein, then the Client can use
that information to determine what actions to take next. If the
Concise Problem Details data item specified in the error response
includes the 'detail' entry and the Client supports it, such an entry
may provide additional context.
In particular, the following guidelines apply, and application
profiles of this specification can define more detailed actions for
the Client to take when learning that a specific error has occurred.
* In case of error 0, the Client should stop sending the request in
question to the KDC. Rather, the Client should first join the
targeted group. If it has not happened already, this first
requires the Client to obtain an appropriate access token
authorizing access to the group and provide it to the KDC.
* In case of error 1, the Client as a group member should rejoin the
group with all the roles needed to perform the operation in
question. This might require the Client to first obtain a new
access token and provide it to the KDC, if the current access
token does not authorize the Client to take those roles in the
group. For operations admitted to a Client that is not a group
member (e.g., an external signature verifier), the Client should
first obtain a new access token authorizing to also have the
missing roles.
* In case of error 2, the Client has to obtain or self-generate a
different asymmetric key pair, as aligned to the public key
algorithm and parameters used in the targeted group. After that,
the Client should provide the KDC with its new authentication
credential, which is consistent with the format used in the
targeted group and including the new public key.
* In case of error 3, the Client should ensure to compute its proof-
of-possession evidence by correctly using the parameters and
procedures defined in the used application profile of this
specification. In an unattended setup, it might not be possible
for a Client to autonomously diagnose the error and take an
effective next action to address it.
* In case of error 4, the Client should wait for a certain (pre-
configured) amount of time before trying to resend its request to
the KDC.
* In case of error 5, the Client may try joining the group again.
This might require the Client to first obtain a new access token
and provide it to the KDC, e.g., if the current access token has
expired.
* In case of error 6, the Client should clean up its state regarding
the group, just like if it has left the group with no intention to
rejoin it.
10. Security Considerations
Security considerations are inherited from the ACE framework
[RFC9200] and from the specific transport profile of ACE used between
the Clients and the KDC, e.g., [RFC9202] and [RFC9203].
When using the problem-details format defined in [RFC9290] for error
responses, then the privacy and security considerations from Sections
4 and 5 of [RFC9290] also apply.
Furthermore, the following security considerations apply.
10.1. Secure Communication in the Group
When a group member receives a message from a certain sender for the
first time since joining the group, it needs to have a mechanism in
place to avoid replayed messages and to assert their freshness, e.g.,
as described in Appendix B.1.2 of [RFC8613] or Section 10 of
[GROUP-OSCORE]. Such a mechanism also aids the recipient group
member in case it has rebooted and lost the security state used to
protect previous group communications with that sender.
By its nature, the KDC is invested with a large amount of trust,
since it acts as a generator and provider of the symmetric keying
material used to protect communications in each of its groups. While
details depend on the specific communication and security protocols
used in the group, the KDC is in the position to decrypt messages
exchanged in the group as if it was also a group member, as long as
those are protected through commonly shared group keying material.
A compromised KDC would thus put the attacker in the same position,
which also means that:
* The attacker can generate and control new group keying material,
hence possibly rekeying the group and evicting certain group
members as part of a broader attack.
* The attacker can actively participate in communications in a
group, even without having been authorized to join it, and can
allow further unauthorized entities to do so.
* The attacker can build erroneous associations between node
identifiers and group members' authentication credentials.
On the other hand, as long as the security protocol used in the group
ensures source authentication of messages (e.g., by means of
signatures), the KDC is not able to impersonate group members since
it does not have their private keys.
Further security considerations are specific to the communication and
security protocols used in the group, and thus have to be provided by
those protocols and complemented by the application profiles of this
specification using them.
10.2. Update of Group Keying Material
The KDC can generate new group keying material and provide it to the
group members (rekeying) through the rekeying scheme used in the
group, as discussed in Section 6.
In particular, the KDC must renew the latest group keying material
upon its expiration. Before then, the KDC MAY also renew the group
keying material on a regular or periodical fashion.
Unless otherwise defined by an application profile of this
specification, the KDC SHOULD renew the group keying material upon a
group membership change. As a possible exception, the KDC may not
rekey the group upon the joining of a new group member if the
application does not require backward security. As another possible
exception discussed more in detail later in this section, the KDC may
rely on a rekeying policy that reasonably takes into account the
expected rate of group membership changes and the duration of a group
rekeying.
Since the minimum number of group members is one, the KDC SHOULD
provide even a Client joining an empty group with new keying material
never used before in that group. Similarly, the KDC SHOULD also
provide new group keying material to a Client that remains the only
member in the group after the leaving of other group members.
Note that the considerations in Section 10.1 about dealing with
replayed messages still hold, even in case the KDC rekeys the group
upon every single joining of a new group member. However, if the KDC
has renewed the group keying material upon a group member's joining
and the time interval between the end of the rekeying process and
that member's joining is sufficiently small, then that group member
is also on the safe side, since it would not accept replayed messages
protected with the old group keying material previous to its joining.
Once a joining node has obtained the new, latest keying material
through a Join Response from the KDC (see Section 4.3.1.1), the
joining node becomes able to read any message that was exchanged in
the group and protected with that keying material. This is the case
if the KDC provides the current group members with the new, latest
keying material before completing the joining procedure. However,
the joining node is not able to read messages exchanged in the group
and protected with keying material older than the one provided in the
Join Response, i.e., having a strictly lower version number NUM.
A node that has left the group should not expect any of its outgoing
messages to be successfully processed if received by other nodes in
the group after its leaving due to a possible group rekeying
occurring before the message reception.
The KDC may enforce a rekeying policy that takes into account the
overall time required to rekey the group, as well as the expected
rate of changes in the group membership. That is, the KDC may not
rekey the group at each and every group membership change, for
instance, if members' joining and leaving occur frequently and
performing a group rekeying takes too long. Instead, the KDC might
rekey the group after a minimum number of group members have joined
or left within a given time interval, after a maximum amount of time
since the last group rekeying was completed, or yet during
predictable network inactivity periods.
However, this would result in the KDC not constantly preserving
backward and forward security in the group. That is:
* Newly joining group members would be able to access the keying
material used before their joining, and thus they could access
past group communications if they have recorded old exchanged
messages. This might still be acceptable for some applications
and in situations where the new group members are freshly deployed
through strictly controlled procedures.
* The leaving group members would remain able to access upcoming
group communications, as protected with the current keying
material that has not been updated. This is typically
undesirable, especially if the leaving group member is compromised
or suspected to be, and it might impact or compromise the security
properties of the protocols used in the group to protect messages
exchanged among the group members.
The KDC should renew the group keying material in case it has
rebooted, even if it stores the whole group keying material in
persistent storage. This assumes that the secure communication
associations with the current group members as well as any
administrative keying material required to rekey the group are also
stored in persistent storage.
However, if the KDC relies on Observe notifications to distribute the
new group keying material, the KDC would have lost all the current
ongoing Observations with the group members after rebooting, and the
group members would continue using the old group keying material.
Therefore, the KDC will rely on each group member asking for the new
group keying material (see Sections 4.3.2.1 and 4.8.1.1) or perform a
group rekeying by actively sending rekeying messages to group members
as discussed in Section 6.
The KDC needs to have a mechanism in place to detect DoS attacks from
nodes repeatedly performing actions that might trigger a group
rekeying. Such actions can include leaving and/or rejoining the
group at high rates or often asking the KDC for new individual keying
material. Ultimately, the KDC can resort to removing these nodes
from the group and (temporarily) preventing them from joining the
group again.
The KDC also needs to have a congestion control mechanism in place in
order to avoid network congestion upon distributing new group keying
material. For example, CoAP and Observe give guidance on such
mechanisms, see Section 4.7 of [RFC7252] and Section 4.5.1 of
[RFC7641].
10.3. Block-Wise Considerations
If the Block-Wise CoAP options [RFC7959] are used and the keying
material is updated in the middle of a Block-Wise transfer, the
sender of the blocks just changes the group keying material to the
updated one and continues the transfer. As long as both sides get
the new group keying material, updating the group keying material in
the middle of a transfer will not cause any issue. Otherwise, the
sender will have to transmit the message again when receiving an
error message from the recipient.
Compared to a scenario where the transfer does not use Block-Wise,
and depending on how fast the group keying material is changed, the
group members might consume a larger amount of the network bandwidth
by repeatedly resending the same blocks, which might be problematic.
11. IANA Considerations
Per this document, IANA has completed the following actions.
11.1. Media Type Registrations
This specification has registered the "application/ace-
groupcomm+cbor" media type for messages of the protocols defined in
this document following the ACE exchange and carrying parameters
encoded in CBOR. This registration follows the procedures specified
in [RFC6838].
Type name: application
Subtype name: ace-groupcomm+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Must be encoded as a CBOR map containing
the parameters defined in RFC 9594.
Security considerations: See Section 10 of RFC 9594.
Interoperability considerations: N/A
Published specification: RFC 9594
Applications that use this media type: The type is used by
Authorization Servers, Clients, and Resource Servers that support
the ACE groupcomm framework as specified in RFC 9594.
Fragment identifier considerations: N/A
Additional information: N/A
Person & email address to contact for further information: ACE WG
mailing list (ace@ietf.org) or IETF Applications and Real-Time
Area (art@ietf.org)
Intended usage: COMMON
Restrictions on usage: None
Author/Change controller: IETF
Provisional registration: No
11.2. CoAP Content-Formats
IANA has registered the following entry in the "CoAP Content-Formats"
registry within the "CoRE Parameters" registry group.
Content Type: application/ace-groupcomm+cbor
Content Coding: -
ID: 261
Reference: RFC 9594
11.3. OAuth Parameters
IANA has registered the following entries in the "OAuth Parameters"
registry, following the procedure specified in Section 11.2 of
[RFC6749].
Name: sign_info
Parameter Usage Location: client-rs request, rs-client response
Change Controller: IETF
Reference: RFC 9594
Name: kdcchallenge
Parameter Usage Location: rs-client response
Change Controller: IETF
Reference: RFC 9594
11.4. OAuth Parameters CBOR Mappings
IANA has registered the following entries in the "OAuth Parameters
CBOR Mappings" registry, following the procedure specified in
Section 8.10 of [RFC9200].
Name: sign_info
CBOR Key: 45
Value Type: Null or array
Reference: RFC 9594
Name: kdcchallenge
CBOR Key: 46
Value Type: byte string
Reference: RFC 9594
11.5. Interface Description (if=) Link Target Attribute Values
IANA has registered the following entry in the "Interface Description
(if=) Link Target Attribute Values" registry within the "Constrained
RESTful Environments (CoRE) Parameters" registry group.
Value: ace.groups
Description: The KDC interface at the parent resource of group-
membership resources is used to retrieve names of security groups
using the ACE framework.
Reference: Section 4.1 of RFC 9594
Value: ace.group
Description: The KDC interface at a group-membership resource is
used to provision keying material and related information and
policies to members of the corresponding security group using the
ACE framework.
Reference: Section 4.1 of RFC 9594
11.6. Custom Problem Detail Keys Registry
IANA has registered the following entry in the "Custom Problem Detail
Keys" registry within the "Constrained RESTful Environments (CoRE)
Parameters" registry group.
Key Value: 0
Name: ace-groupcomm-error
Brief Description: Carry RFC 9594 problem details in a Concise
Problem Details data item.
Change Controller: IETF
Reference: RFC 9594, Section 4.1.2
11.7. ACE Groupcomm Parameters
This specification has established the "ACE Groupcomm Parameters"
IANA registry within the "Authentication and Authorization for
Constrained Environments (ACE)" registry group.
Values in this registry are covered by different registration
policies as indicated below. Some policies require Expert Review;
guidelines are provided in Section 11.14
The columns of this registry are:
Name: This is a descriptive name that enables easier reference to
the item. The name MUST be unique. It is not used in the
encoding.
CBOR Key: This is the value used as the CBOR map key of the item.
These values MUST be unique. The value can be a positive integer,
a negative integer, or a text string. Different ranges of values
use different registration policies [RFC8126]. Integer values
from -256 to 255 as well as text strings of length 1 are
designated as "Standards Action With Expert Review". Integer
values from -65536 to -257 and from 256 to 65535 as well as text
strings of length 2 are designated as "Specification Required".
Integer values greater than 65535 as well as text strings of
length greater than 2 are designated as "Expert Review". Integer
values less than -65536 are marked as "Private Use".
CBOR Type: This field contains the CBOR type of the item or a
pointer to the registry that defines its type when that depends on
another item.
Reference: This field contains a pointer to the public specification
for the item.
This registry has been initially populated with the values in
Table 5.
11.8. ACE Groupcomm Key Types
This specification establishes the "ACE Groupcomm Key Types" IANA
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group.
Values in this registry are covered by different registration
policies as indicated below. Some policies require Expert Review;
guidelines are provided in Section 11.14.
The columns of this registry are:
Name: This is a descriptive name that enables easier reference to
the item. The name MUST be unique. It is not used in the
encoding.
Key Type Value: This is the value used to identify the keying
material. These values MUST be unique. The value can be a
positive integer, a negative integer, or a text string. Different
ranges of values use different registration policies [RFC8126].
Integer values from -256 to 255 as well as text strings of length
1 are designated as "Standards Action With Expert Review".
Integer values from -65536 to -257 and from 256 to 65535 as well
as text strings of length 2 are designated as "Specification
Required". Integer values greater than 65535 as well as text
strings of length greater than 2 are designated as "Expert
Review". Integer values less than -65536 are marked as "Private
Use".
Profile: This field may contain one or more descriptive strings of
application profiles to be used with this item. The values should
be taken from the "Name" column of the "ACE Groupcomm Profiles"
registry.
Description: This field contains a brief description of the keying
material.
Reference: This field contains a pointer to the public specification
for the format of the keying material, if one exists.
This registry has been initially populated with the value in Table 1.
11.9. ACE Groupcomm Profiles
This specification establishes the "ACE Groupcomm Profiles" IANA
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group.
Values in this registry are covered by different registration
policies as indicated below. Some policies require Expert Review;
guidelines are provided in Section 11.14.
The columns of this registry are:
Name: The name of the application profile.
Description: Text giving an overview of the application profile and
the context it is developed for.
CBOR Value: CBOR abbreviation for the name of this application
profile. These values MUST be unique. The value can be a
positive integer or a negative integer. Different ranges of
values use different registration policies [RFC8126]. Integer
values from -256 to 255 are designated as "Standards Action With
Expert Review". Integer values from -65536 to -257 and from 256
to 65535 are designated as "Specification Required". Integer
values greater than 65535 are designated as "Expert Review".
Integer values less than -65536 are marked as "Private Use".
Reference: This field contains a pointer to the public specification
for this application profile, if one exists.
This registry has been initially populated with the value in Table 2.
11.10. ACE Groupcomm Policies
This specification establishes the "ACE Groupcomm Policies" IANA
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group.
Values in this registry are covered by different registration
policies as indicated below. Some policies require Expert Review;
guidelines are provided in Section 11.14.
The columns of this registry are:
Name: The name of the group communication policy.
CBOR Label: The value to be used to identify this group
communication policy. These values MUST be unique. The value can
be a positive integer, a negative integer, or a text string.
Different ranges of values use different registration policies
[RFC8126]. Integer values from -256 to 255 as well as text
strings of length 1 are designated as "Standards Action With
Expert Review". Integer values from -65536 to -257 and from 256
to 65535 as well as text strings of length 2 are designated as
"Specification Required". Integer values greater than 65535 as
well as text strings of length greater than 2 are designated as
"Expert Review". Integer values less than -65536 are marked as
"Private Use".
CBOR Type: The CBOR type used to encode the value of this group
communication policy.
Description: This field contains a brief description for this group
communication policy.
Reference: This field contains a pointer to the public specification
for this group communication policy and its format, if one exists.
This registry has been initially populated with the values in
Table 3.
11.11. Sequence Number Synchronization Methods
This specification establishes the "Sequence Number Synchronization
Methods" IANA registry within the "Authentication and Authorization
for Constrained Environments (ACE)" registry group.
Values in this registry are covered by different registration
policies as indicated below. Some policies require Expert Review;
guidelines are provided in Section 11.14.
The columns of this registry are:
Name: The name of the sequence number synchronization method.
Value: The value to be used to identify this sequence number
synchronization method. These values MUST be unique. The value
can be a positive integer, a negative integer, or a text string.
Different ranges of values use different registration policies
[RFC8126]. Integer values from -256 to 255 as well as text
strings of length 1 are designated as "Standards Action With
Expert Review". Integer values from -65536 to -257 and from 256
to 65535 as well as text strings of length 2 are designated as
"Specification Required". Integer values greater than 65535 as
well as text strings of length greater than 2 are designated as
"Expert Review". Integer values less than -65536 are marked as
"Private Use".
Description: This field contains a brief description for this
sequence number synchronization method.
Reference: This field contains a pointer to the public specification
describing the sequence number synchronization method.
11.12. ACE Groupcomm Errors
This specification establishes the "ACE Groupcomm Errors" IANA
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group.
Values in this registry are covered by different registration
policies as indicated below. Some policies require Expert Review;
guidelines are provided in Section 11.14.
The columns of this registry are:
Value: The value to be used to identify the error. These values
MUST be unique. The value can be a positive integer or a negative
integer. Different ranges of values use different registration
policies [RFC8126]. Integer values from -256 to 255 are
designated as "Standards Action With Expert Review". Integer
values from -65536 to -257 and from 256 to 65535 are designated as
"Specification Required". Integer values greater than 65535 are
designated as "Expert Review". Integer values less than -65536
are marked as "Private Use".
Description: This field contains a brief description of the error.
Reference: This field contains a pointer to the public specification
defining the error, if one exists.
This registry has been initially populated with the values in
Table 6. The "Reference" column for all of these entries refers to
this document.
11.13. ACE Groupcomm Rekeying Schemes
This specification establishes the "ACE Groupcomm Rekeying Schemes"
IANA registry within the "Authentication and Authorization for
Constrained Environments (ACE)" registry group.
Values in this registry are covered by different registration
policies as indicated below. Some policies require Expert Review;
guidelines are provided in Section 11.14.
The columns of this registry are:
Value: The value to be used to identify the group rekeying scheme.
These values MUST be unique. The value can be a positive integer
or a negative integer. Different ranges of values use different
registration policies [RFC8126]. Integer values from -256 to 255
are designated as "Standards Action With Expert Review". Integer
values from -65536 to -257 and from 256 to 65535 are designated as
"Specification Required". Integer values greater than 65535 are
designated as "Expert Review". Integer values less than -65536
are marked as "Private Use".
Name: The name of the group rekeying scheme.
Description: This field contains a brief description of the group
rekeying scheme.
Reference: This field contains a pointer to the public specification
defining the group rekeying scheme, if one exists.
This registry has been initially populated with the value in Table 4.
11.14. Expert Review Instructions
The IANA registries established in this document are defined as
Expert Review. This section gives some general guidelines for what
the experts should be looking for, but they are being designated as
experts for a reason so they should be given substantial latitude.
Expert Reviewers should take into consideration the following points:
* Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate one that is already
registered and that the point is likely to be used in deployments.
The zones tagged as "Private Use" are intended for testing
purposes and closed environments; code points in other ranges
should not be assigned for testing.
* Specifications are required for the Standards Track range of point
assignment. Specifications should exist for Specification
Required ranges, but early assignment before a specification is
available is considered to be permissible. When specifications
are not provided, the description provided needs to have
sufficient information to identify what the point is being used
for.
* Experts should take into account the expected usage of fields when
approving point assignments. The fact that there is a range for
Standards Track documents does not mean that a Standards Track
document cannot have points assigned outside of that range. The
length of the encoded value should be weighed against how many
code points of that length are left, the size of the device it
will be used on, and the number of code points left that encode to
that size.
12. References
12.1. Normative References
[CBOR.Tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<https://www.iana.org/assignments/cbor-tags/>.
[CoAP.Content.Formats]
IANA, "CoAP Content-Formats",
<https://www.iana.org/assignments/core-parameters/>.
[COSE.Algorithms]
IANA, "COSE Algorithms",
<https://www.iana.org/assignments/cose/>.
[COSE.Header.Parameters]
IANA, "COSE Header Parameters",
<https://www.iana.org/assignments/cose/>.
[COSE.Key.Types]
IANA, "COSE Key Types",
<https://www.iana.org/assignments/cose/>.
[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>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/info/rfc7967>.
[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>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/info/rfc9052>.
[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/info/rfc9053>.
[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/info/rfc9200>.
[RFC9237] Bormann, C., "An Authorization Information Format (AIF)
for Authentication and Authorization for Constrained
Environments (ACE)", RFC 9237, DOI 10.17487/RFC9237,
August 2022, <https://www.rfc-editor.org/info/rfc9237>.
[RFC9290] Fossati, T. and C. Bormann, "Concise Problem Details for
Constrained Application Protocol (CoAP) APIs", RFC 9290,
DOI 10.17487/RFC9290, October 2022,
<https://www.rfc-editor.org/info/rfc9290>.
[RFC9338] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Countersignatures", STD 96, RFC 9338,
DOI 10.17487/RFC9338, December 2022,
<https://www.rfc-editor.org/info/rfc9338>.
12.2. Informative References
[C509-CERT]
Preuß Mattsson, J., Selander, G., Raza, S., Höglund, J.,
and M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", Work in Progress, Internet-Draft, draft-
ietf-cose-cbor-encoded-cert-11, 8 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
cbor-encoded-cert-11>.
[CoAP-PUBSUB]
Jimenez, J., Koster, M., and A. Keränen, "A publish-
subscribe architecture for the Constrained Application
Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
ietf-core-coap-pubsub-14, 18 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-pubsub-14>.
[GROUP-CoAP]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", Work in
Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
11, 24 April 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-groupcomm-bis-11>.
[GROUP-OSCORE]
Tiloca, M., Selander, G., Palombini, F., Preuß Mattsson,
J., and R. Höglund, "Group Object Security for Constrained
RESTful Environments (Group OSCORE)", Work in Progress,
Internet-Draft, draft-ietf-core-oscore-groupcomm-22, 28
August 2024, <https://datatracker.ietf.org/doc/html/draft-
ietf-core-oscore-groupcomm-22>.
[OSCORE-DISCOVERY]
Tiloca, M., Amsüss, C., and P. Van der Stok, "Discovery of
OSCORE Groups with the CoRE Resource Directory", Work in
Progress, Internet-Draft, draft-tiloca-core-oscore-
discovery-16, 4 September 2024,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-discovery-16>.
[RFC2093] Harney, H. and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Specification", RFC 2093,
DOI 10.17487/RFC2093, July 1997,
<https://www.rfc-editor.org/info/rfc2093>.
[RFC2094] Harney, H. and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Architecture", RFC 2094,
DOI 10.17487/RFC2094, July 1997,
<https://www.rfc-editor.org/info/rfc2094>.
[RFC2627] Wallner, D., Harder, E., and R. Agee, "Key Management for
Multicast: Issues and Architectures", RFC 2627,
DOI 10.17487/RFC2627, June 1999,
<https://www.rfc-editor.org/info/rfc2627>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
[RFC9202] Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
L. Seitz, "Datagram Transport Layer Security (DTLS)
Profile for Authentication and Authorization for
Constrained Environments (ACE)", RFC 9202,
DOI 10.17487/RFC9202, August 2022,
<https://www.rfc-editor.org/info/rfc9202>.
[RFC9203] Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
"The Object Security for Constrained RESTful Environments
(OSCORE) Profile of the Authentication and Authorization
for Constrained Environments (ACE) Framework", RFC 9203,
DOI 10.17487/RFC9203, August 2022,
<https://www.rfc-editor.org/info/rfc9203>.
[RFC9277] Richardson, M. and C. Bormann, "On Stable Storage for
Items in Concise Binary Object Representation (CBOR)",
RFC 9277, DOI 10.17487/RFC9277, August 2022,
<https://www.rfc-editor.org/info/rfc9277>.
[RFC9431] Sengul, C. and A. Kirby, "Message Queuing Telemetry
Transport (MQTT) and Transport Layer Security (TLS)
Profile of Authentication and Authorization for
Constrained Environments (ACE) Framework", RFC 9431,
DOI 10.17487/RFC9431, July 2023,
<https://www.rfc-editor.org/info/rfc9431>.
Appendix A. Requirements for Application Profiles
This section lists the requirements for application profiles of this
specification for the convenience of application profile designers.
A.1. Mandatory-to-Address Requirements
REQ1: Specify the format and encoding of scope. This includes
defining the set of possible roles and their identifiers, as
well as the corresponding encoding to use in the scope
entries according to the used scope format (see Section 3.1).
REQ2: If scope uses AIF, register its specific instance of "Toid"
and "Tperm" as media type parameters and a corresponding
Content-Format, as per the guidelines in [RFC9237].
REQ3: If used, specify the acceptable values for the 'sign_alg'
parameter (see Section 3.3.1).
REQ4: If used, specify the acceptable values and structure for the
'sign_parameters' parameter (see Section 3.3.1).
REQ5: If used, specify the acceptable values and structure for the
'sign_key_parameters' parameter (see Section 3.3.1).
REQ6: Specify the acceptable formats for authentication credentials
and, if applicable, the acceptable values for the 'cred_fmt'
parameter (see Section 3.3.1).
REQ7: If the value of the GROUPNAME URI path and the group name in
the access token scope ('gname' in Section 3.1) are not
required to coincide, specify the mechanism to map the
GROUPNAME value in the URI to the group name (see
Section 4.1).
REQ8: Define whether the KDC has an authentication credential as
required for the correct group operation and if this has to
be provided through the 'kdc_cred' parameter (see Sections
4.1 and 4.3.1).
REQ9: Specify if any part of the KDC interface as defined in this
document is not supported by the KDC (see Section 4.1).
REQ10: Register a Resource Type for the group-membership resources,
which is used to discover the correct URL for sending a Join
Request to the KDC (see Section 4.1).
REQ11: Define what specific actions (e.g., CoAP methods) are allowed
on each resource that are accessible through the KDC
interface, depending on: whether the Client is a current
group member; the roles that a Client is authorized to take
as per the obtained access token (see Section 3.1); and the
roles that the Client has as a current group member.
REQ12: Categorize possible newly defined operations for Clients into
primary operations expected to be minimally supported and
secondary operations, and provide accompanying considerations
(see Section 4.1.1).
REQ13: Specify the encoding of group identifiers (see
Section 4.2.1).
REQ14: Specify the approaches used to compute and verify the PoP
evidence to include in the 'client_cred_verify' parameter and
which of those approaches is used in which case (see
Section 4.3.1).
REQ15: Specify how the nonce N_S is generated, if the access token
is not provided to the KDC through the Token Transfer Request
sent to the /authz-info endpoint (e.g., the access token is
instead transferred during the establishment of a secure
communication association).
REQ16: Define the initial value of the version number for the group
keying material (see Section 4.3.1).
REQ17: Specify the format of the group keying material that is
conveyed in the 'key' parameter (see Section 4.3.1).
REQ18: Specify the acceptable values of the 'gkty' parameter (see
Section 4.3.1). For each of them, register a corresponding
entry in the "ACE Groupcomm Key Types" IANA registry if such
an entry does not exist already.
REQ19: Specify and register the application profile identifier (see
Section 4.3.1).
REQ20: If used, specify the format and default values of the entries
of the CBOR map to include in the 'group_policies' parameter
(see Section 4.3.1).
REQ21: Specify the approaches used to compute and verify the PoP
evidence to include in the 'kdc_cred_verify' parameter and
which of those approaches is used in which case (see Sections
4.3.1 and 4.5.1). If external signature verifiers are
supported, specify how those provide a nonce to the KDC to be
used for computing the PoP evidence (see Section 4.5.1).
REQ22: Specify the communication protocol that members of the group
use to communicate with each other (e.g., CoAP for group
communication).
REQ23: Specify the security protocol that members of the group use
to protect the group communication (e.g., Group OSCORE).
This must provide encryption, integrity, and replay
protection.
REQ24: Specify how the communication is secured between the Client
and the KDC. Optionally, specify a transport profile of ACE
[RFC9200] to use between the Client and the KDC (see
Section 4.3.1.1).
REQ25: Specify the format of the node identifiers of group members
(see Sections 4.3.1 and 4.4.1).
REQ26: Specify policies at the KDC to handle node identifiers that
are included in the 'get_creds' parameter but are not
associated with any current group member (see Section 4.4.1).
REQ27: Specify the format of (newly generated) individual keying
material for group members or of the information to derive
such keying material, as well as the corresponding CBOR map
key that has to be registered in the "ACE Groupcomm
Parameters" registry (see Sections 4.8.1 and 4.8.2).
REQ28: Specify which CBOR tag is used for identifying the semantics
of binary scopes, or register a new CBOR tag if a suitable
one does not exist already (see Section 7).
REQ29: Categorize newly defined parameters according to the same
criteria of Section 8.
REQ30: Define whether Clients must, should, or may support the
conditional parameters defined in Section 8 and under which
circumstances.
A.2. Optional-to-Address Requirements
OPT1: Optionally, if the textual format of scope is used, specify
CBOR values to use for abbreviating the role identifiers in
the group (see Section 3.1).
OPT2: Optionally, specify the additional parameters used in the
exchange of Token Transfer Request and Response (see
Section 3.3).
OPT3: Optionally, specify the negotiation of parameter values for
signature algorithm and signature keys, if the 'sign_info'
parameter is not used (see Section 3.3).
OPT4: Optionally, specify possible or required payload formats for
specific error cases (see Section 4.1.2).
OPT5: Optionally, specify additional identifiers of error types as
values of the 'error-id' field within the Custom Problem
Detail entry 'ace-groupcomm-error' (see Section 4.1.2).
OPT6: Optionally, specify the encoding of the 'creds_repo'
parameter if the default one is not used (see Section 4.3.1).
OPT7: Optionally, specify the functionalities implemented at the
resource hosted by the Client at the URI indicated in the
'control_uri' parameter, including the encoding of exchanged
messages and other details (see Section 4.3.1).
OPT8: Optionally, specify the behavior of the POST handler of
group-membership resources, for the case when it fails to
retrieve an authentication credential for the specific Client
(see Section 4.3.1).
OPT9: Optionally, define a default group rekeying scheme to refer
to in case the 'rekeying_scheme' parameter is not included in
the Join Response (see Section 4.3.1).
OPT10: Optionally, specify the functionalities implemented at the
resource hosted by the Client at the URI indicated in the
'control_group_uri' parameter, including the encoding of
exchanged messages and other details (see Section 4.3.1).
OPT11: Optionally, specify policies that instruct Clients to retain
messages and for how long, if those are unsuccessfully
decrypted (see Section 4.8.1.1). This makes it possible for
Clients to decrypt such messages after obtaining updated
keying material.
OPT12: Optionally, specify for the KDC to perform a group rekeying
when receiving a Key Renewal Request, together with or
instead of renewing individual keying material (see
Section 4.8.2.1).
OPT13: Optionally, specify how the identifier of a group member's
authentication credential is included in requests sent to
other group members (see Section 4.9.1.1).
OPT14: Optionally, specify additional information to include in
rekeying messages for the "Point-to-Point" group rekeying
scheme (see Section 6).
Appendix B. Extensibility for Future COSE Algorithms
As defined in Section 8.1 of [RFC9053], future algorithms can be
registered in the "COSE Algorithms" registry [COSE.Algorithms] as
specifying none or multiple COSE capabilities.
To enable the seamless use of such future registered algorithms, this
section defines a general, agile format for each 'sign_info_entry' of
the 'sign_info' parameter in the Token Transfer Response; see
Section 3.3.1.
If any of the currently registered COSE algorithms are considered,
using this general format yields the same structure of
'sign_info_entry' defined in this document, thus ensuring backward
compatibility.
B.1. Format of 'sign_info_entry'
The format of each 'sign_info_entry' (see Section 3.3.1) is
generalized as follows.
* 'sign_parameters' includes N >= 0 elements, each of which is a
COSE capability of the signature algorithm indicated in
'sign_alg'.
In particular, 'sign_parameters' has the same format and value of
the COSE capabilities array for the signature algorithm indicated
in 'sign_alg', as specified for that algorithm in the
"Capabilities" column of the "COSE Algorithms" registry
[COSE.Algorithms].
* 'sign_key_parameters' is replaced by N elements 'sign_capab', each
of which is a CBOR array.
The i-th 'sign_capab' array (i = 0, ..., N-1) is the array of COSE
capabilities for the algorithm capability specified in
'sign_parameters'[i].
In particular, each 'sign_capab' array has the same format and
value of the COSE capabilities array for the algorithm capability
specified in 'sign_parameters'[i].
Such a COSE capabilities array is currently defined for the
algorithm capability COSE key type in the "Capabilities" column of
the "COSE Key Types" registry [COSE.Key.Types].
sign_info_entry =
[
id : gname / [+ gname],
sign_alg : int / tstr,
sign_parameters : [* alg_capab : any],
* sign_capab : [* capab : any],
cred_fmt : int / null
]
gname = tstr
Figure 38: 'sign_info_entry' with a General Format
Acknowledgments
The following individuals were helpful in shaping this document:
Christian Amsüss, Carsten Bormann, Roman Danyliw, Martin Duke, Thomas
Fossati, Vidhi Goel, Rikard Höglund, Ben Kaduk, Erik Kline, Warren
Kumari, Watson Ladd, Daniel Migault, John Preuß Mattsson,
Zaheduzzaman Sarker, Jim Schaad, Ludwig Seitz, Göran Selander, Cigdem
Sengul, Dave Thaler, Henry Thompson, Peter van der Stok, and Paul
Wouters.
The work on this document has been partly supported by the Sweden's
Innovation Agency VINNOVA and the Celtic-Next project CRITISEC, by
the H2020 project SIFIS-Home (Grant agreement 952652), and by the
EIT-Digital High Impact Initiative ACTIVE.
Authors' Addresses
Francesca Palombini
Ericsson AB
Torshamnsgatan 23
SE-164 40 Kista
Sweden
Email: francesca.palombini@ericsson.com
Marco Tiloca
RISE AB
Isafjordsgatan 22
SE-164 40 Kista
Sweden
Email: marco.tiloca@ri.se
