Internet Engineering Task Force (IETF) A. DeKok
Request for Comments: 9427 FreeRADIUS
Updates: 4851, 5281, 7170 June 2023
Category: Standards Track
ISSN: 2070-1721
TLS-Based Extensible Authentication Protocol (EAP) Types for Use with
TLS 1.3
Abstract
The Extensible Authentication Protocol-TLS (EAP-TLS) (RFC 5216) has
been updated for TLS 1.3 in RFC 9190. Many other EAP Types also
depend on TLS, such as EAP-Flexible Authentication via Secure
Tunneling (EAP-FAST) (RFC 4851), EAP-Tunneled TLS (EAP-TTLS) (RFC
5281), the Tunnel Extensible Authentication Protocol (TEAP) (RFC
7170). It is possible that many vendor-specific EAP methods, such as
the Protected Extensible Authentication Protocol (PEAP), depend on
TLS as well. This document updates those methods in order to use the
new key derivation methods available in TLS 1.3. Additional changes
necessitated by TLS 1.3 are also discussed.
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/rfc9427.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
2. Using TLS-Based EAP Methods with TLS 1.3
2.1. Key Derivation
2.2. TEAP
2.2.1. Client Certificates
2.3. EAP-FAST
2.3.1. Client Certificates
2.4. EAP-TTLS
2.4.1. Client Certificates
2.5. PEAP
2.5.1. Client Certificates
3. Application Data
3.1. Identities
4. Resumption
5. Security Considerations
5.1. Handling of TLS NewSessionTicket Messages
5.2. Protected Success and Failure Indications
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Author's Address
1. Introduction
EAP-TLS has been updated for TLS 1.3 in [RFC9190]. Many other EAP
Types also depend on TLS, such as EAP-FAST [RFC4851], EAP-TTLS
[RFC5281], and TEAP [RFC7170]. It is possible that many vendor-
specific EAP methods, such as PEAP [PEAP], depend on TLS as well.
All of these methods use key derivation functions that are no longer
applicable to TLS 1.3; thus, these methods are incompatible with TLS
1.3.
This document updates these methods in order to be used with TLS 1.3.
These changes involve defining new key derivation functions. We also
discuss implementation issues in order to highlight differences
between TLS 1.3 and earlier versions of TLS.
1.1. Requirements Language
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.
2. Using TLS-Based EAP Methods with TLS 1.3
In general, all of the requirements in [RFC9190] apply to other EAP
methods that wish to use TLS 1.3. Unless otherwise required herein,
implementations of EAP methods that wish to use TLS 1.3 MUST follow
the guidelines in [RFC9190].
There remain some differences between EAP-TLS and other TLS-based EAP
methods that are addressed by this document. The main difference is
that [RFC9190] uses the EAP-TLS Type (value 0x0D) in a number of
calculations, whereas other method types will use their own Type
value instead of the EAP-TLS Type value. This topic is discussed
further in Section 2.1.
An additional difference is that [RFC9190], Section 2.5 requires the
EAP server to send a protected success result indication once the
EAP-TLS handshake has completed. This indication is composed of one
octet (0x00) of application data. Other TLS-based EAP methods also
use this result indication, but only during resumption. When other
TLS-based EAP methods use full authentication, the result indication
is not needed or used. This topic is explained in more detail in
Sections 3 and 4.
Finally, this document includes clarifications on how various TLS-
based parameters are calculated when using TLS 1.3. These parameters
are different for each EAP method, so they are discussed separately.
2.1. Key Derivation
The key derivation for TLS-based EAP methods depends on the value of
the EAP Type as defined by [IANA] in the "Extensible Authentication
Protocol (EAP) Registry". The most important definition is of the
Type field, as first defined in [RFC3748], Section 2:
Type = value of the EAP Method type
For the purposes of this specification, when we refer to logical
Type, we mean that the logical Type is defined as one octet for
values smaller than 254 (the value for the Expanded Type). When
Expanded EAP Types are used, the logical Type is defined as the
concatenation of the fields required to define the Expanded Type,
including the Type with value 0xfe, Vendor-Id (in network byte
order), and Vendor-Type fields (in network byte order) defined in
[RFC3748], Section 5.7, as given below:
Type = 0xFE || Vendor-Id || Vendor-Type
This definition does not alter the meaning of Type in [RFC3748] or
change the structure of EAP packets. Instead, this definition allows
us to simplify references to EAP Types by using a logical "Type"
instead of referring to "the Type field or the Type field with value
0xfe, plus the Vendor-ID and Vendor-Type". For example, the value of
Type for PEAP is simply 0x19.
Note that unlike TLS 1.2 and earlier, the calculation of the TLS-
Exporter function depends on the length passed to it. Therefore,
implementations MUST pass the correct length instead of passing a
large length and truncating the output. Any output calculated using
a larger length value, which is then truncated, will be different
from the output that was calculated using the correct length.
Unless otherwise discussed below, the key derivation functions for
all TLS-based EAP Types are defined in [RFC9190], Section 2.3 and
reproduced here for clarity. These definitions include ones for the
Master Session Key (MSK) and the Extended Master Session Key (EMSK):
Key_Material = TLS-Exporter("EXPORTER_EAP_TLS_Key_Material",
Type, 128)
Method-Id = TLS-Exporter("EXPORTER_EAP_TLS_Method-Id",
Type, 64)
Session-Id = Type || Method-Id
MSK = Key_Material(0, 63)
EMSK = Key_Material(64, 127)
We note that these definitions reuse the EAP-TLS exporter labels and
change the derivation only by adding a dependency on the logical
Type. The reason for this change is simplicity. The inclusion of
the EAP Type makes the derivation method specific. There is no need
to use different labels for different EAP Types as was done earlier.
These definitions apply in their entirety to EAP-TTLS [RFC5281] and
PEAP as defined in [PEAP] and [MSPEAP]. Some definitions apply to
EAP-FAST and TEAP with exceptions as noted below.
It is RECOMMENDED that vendor-defined and TLS-based EAP methods use
the above definitions for TLS 1.3. There is no compelling reason to
use different definitions.
2.2. TEAP
TEAP previously used a Protected Access Credential (PAC), which is
functionally equivalent to session tickets provided by TLS 1.3 that
contain a pre-shared key (PSK) along with other data. As such, the
use of a PAC is deprecated for TEAP in TLS 1.3. PAC provisioning, as
defined in [RFC7170], Section 3.8.1, is also no longer part of TEAP
when TLS 1.3 is used.
[RFC7170], Section 5.2 gives a definition for the Inner Method
Session Key (IMSK), which depends on the TLS Pseudorandom Function
(PRF) (also known as TLS-PRF). When the j'th inner method generates
an EMSK, we update that definition for TLS 1.3 as:
IMSK[j] = TLS-Exporter("TEAPbindkey@ietf.org", secret, 32)
The secret is the EMSK or MSK from the j'th inner method. When an
inner method does not provide an EMSK or MSK, IMSK[j] is 32 octets of
zero.
The other key derivations for TEAP are given here. All derivations
not given here are the same as given above in the previous section.
These derivations are also used for EAP-FAST, but using the EAP-FAST
Type.
The derivation of the IMSKs, Inner Method Compound Keys (IMCKs), and
Compound Session Keys (CMKs) is given below.
session_key_seed = TLS-Exporter("EXPORTER: teap session key seed",
Type, 40)
S-IMCK[0] = session_key_seed
For j = 1 to n-1 do
IMCK[j] = TLS-Exporter("EXPORTER: Inner Methods Compound Keys",
S-IMCK[j-1] || IMSK[j], 60)
S-IMCK[j] = first 40 octets of IMCK[j]
CMK[j] = last 20 octets of IMCK[j]
Note: In these definitions, || denotes concatenation.
In TLS 1.3, the derivation of IMCK[j] uses both a different label and
a different order of concatenating fields than what was used by TEAP
with TLS 1.2. Similarly, the session_key_seed in TLS 1.3 uses the
Type as the context. In TLS 1.2, the context was a zero-length
field.
The outer MSK and EMSK are then derived from the final ("n"th) inner
method, as follows:
MSK = TLS-Exporter(
"EXPORTER: Session Key Generating Function",
S-IMCK[n], 64)
EMSK = TLS-Exporter(
"EXPORTER: Extended Session Key Generating Function",
S-IMCK[n], 64)
The TEAP Compound Message Authentication Code (MAC) defined in
[RFC7170], Section 5.3 remains the same, but the MAC for TLS 1.3 is
computed with the Hashed Message Authentication Code (HMAC) algorithm
negotiated for the HMAC-based Key Derivation Function (HKDF) in the
key schedule, as per [RFC8446], Section 7.1. That is, the MAC used
is the MAC derived from the TLS handshake:
Compound-MAC = MAC( CMK[n], BUFFER )
where we define CMK[n] as the CMK taken from the final ("n"th) inner
method.
For TLS 1.3, the MAC is computed with the HMAC algorithm negotiated
for HKDF in the key schedule, as per [RFC8446], Section 7.1. That
is, the MAC used is the MAC derived from the TLS handshake.
The definition of BUFFER is unchanged from [RFC7170], Section 5.3.
2.2.1. Client Certificates
The use of client certificates is still permitted when using TEAP
with TLS 1.3. However, if the client certificate is accepted, then
the EAP peer MUST proceed with additional authentication of Phase 2,
as per [RFC7170], Section 7.6. If there is no Phase 2 data, then the
EAP server MUST reject the session.
While [RFC5281], Section 7.6 permits "authentication of the client
via client certificate during phase 1, with no additional
authentication or information exchange required," this practice is
forbidden when TEAP is used with TLS 1.3. If there is a requirement
to use client certificates with no inner tunnel methods, then EAP-TLS
should be used instead of TEAP.
[RFC7170], Section 7.4.1 suggests that client certificates should be
sent in Phase 2 of the TEAP exchange "since TLS client certificates
are sent in the clear". While TLS 1.3 no longer sends client
certificates in the clear, TEAP implementations need to distinguish
identities for both User and Machine using the Identity-Type TLV
(with values 1 and 2, respectively). When a client certificate is
sent outside of the TLS tunnel, it MUST include Identity-Type as an
outer TLV in order to signal the type of identity which that client
certificate is for.
2.3. EAP-FAST
For EAP-FAST, the session_key_seed is also part of the key_block as
defined in [RFC4851], Section 5.1.
The definitions of S-IMCK[n], MSK, and EMSK are the same as given
above for TEAP. We reiterate that the EAP-FAST Type must be used
when deriving the session_key_seed and not the TEAP Type.
Unlike [RFC4851], Section 5.2, the definition of IMCK[j] places the
reference to S-IMCK after the textual label and then concatenates the
IMSK instead of the MSK.
EAP-FAST previously used a PAC that is functionally equivalent to
session tickets provided by TLS 1.3, which contain a PSK along with
other data. As such, the use of a PAC is deprecated for EAP-FAST in
TLS 1.3. PAC provisioning [RFC5422] is also no longer part of EAP-
FAST when TLS 1.3 is used.
The T-PRF given in [RFC4851], Section 5.5 is not used for TLS 1.3.
Instead, it is replaced with the TLS 1.3 TLS-Exporter function.
2.3.1. Client Certificates
The use of client certificates is still permitted when using EAP-FAST
with TLS 1.3. However, if the client certificate is accepted, then
the EAP peer MUST proceed with additional authentication of Phase 2,
as per [RFC4851], Section 7.4.1. If there is no Phase 2 data, then
the EAP server MUST reject the session.
While [RFC4851] implicitly permits the use of client certificates
without proceeding to Phase 2, this practice is forbidden when EAP-
FAST is used with TLS 1.3. If there is a requirement to use client
certificates with no inner tunnel methods, then EAP-TLS should be
used instead of EAP-FAST.
2.4. EAP-TTLS
[RFC5281], Section 11.1 defines an implicit challenge when the inner
methods of the Challenge Handshake Authentication Protocol (CHAP)
[RFC1994], MS-CHAP [RFC2433], or MS-CHAPv2 [RFC2759] are used. The
derivation for TLS 1.3 is instead given as:
EAP-TTLS_challenge = TLS-Exporter("ttls challenge",, n)
There is no "context_value" ([RFC8446], Section 7.5) passed to the
TLS-Exporter function. The value "n" given here is the length of the
data required; [RFC5281] requires it to be 17 octets for CHAP
([RFC5281], Section 11.2.2) and MS-CHAPv2 ([RFC5281],
Section 11.2.4), and 9 octets for MS-CHAP ([RFC5281],
Section 11.2.3).
When the Password Authentication Protocol (PAP), CHAP, or MS-CHAPv1
are used as inner authentication methods, there is no opportunity for
the EAP server to send a protected success indication, as is done in
[RFC9190], Section 2.5. Instead, when TLS session tickets are
disabled, the response from the EAP server MUST be either EAP-Success
or EAP-Failure. These responses are unprotected and can be forged by
a skilled attacker.
Where TLS session tickets are enabled, the response from the EAP
server may also continue TLS negotiation with a TLS NewSessionTicket
message. Since this message is protected by TLS, it can serve as the
protected success indication.
Therefore, it is RECOMMENDED that EAP servers always send a TLS
NewSessionTicket message, even if resumption is not configured. When
the EAP peer attempts to use the ticket, the EAP server can instead
request a full authentication. As noted earlier, implementations
SHOULD NOT send TLS NewSessionTicket messages until the "inner
tunnel" authentication has completed in order to take full advantage
of the message as a protected success indication.
When resumption is not used, the TLS NewSessionTicket message is not
available and some authentication methods will not have a protected
success indication. While we would like to always have a protected
success indication, limitations of the underlying protocols,
implementations, and deployment requirements make that impossible.
EAP peers MUST continue running their EAP state machine until they
receive either an EAP-Success or an EAP-Failure. Receiving a TLS
NewSessionTicket message in response to inner method PAP, CHAP, or
MS-CHAP authentication is normal and MUST NOT be treated as a
failure.
2.4.1. Client Certificates
[RFC5281], Section 7.6 permits "authentication of the client via
client certificate during phase 1, with no additional authentication
or information exchange required." This practice is forbidden when
EAP-TTLS is used with TLS 1.3. If there is a requirement to use
client certificates with no inner tunnel methods, then EAP-TLS should
be used instead of EAP-TTLS.
The use of client certificates is still permitted when using EAP-TTLS
with TLS 1.3. However, if the client certificate is accepted, then
the EAP peer MUST proceed with additional authentication of Phase 2,
as per [RFC5281], Section 7.2. If there is no Phase 2 data, then the
EAP server MUST reject the session.
2.5. PEAP
When PEAP uses crypto binding, it uses a different key calculation
defined in [PEAP-MPPE] that consumes inner EAP method keying
material. The PRF+ function used in [PEAP-MPPE] is not taken from
the TLS exporter but is instead calculated via a different method
that is given in [PEAP-PRF]. That derivation remains unchanged in
this specification.
Note that the above derivation uses SHA-1, which may be formally
deprecated in the near future.
However, the PRF+ calculation uses a PEAP Tunnel Key (TK), which is
defined in [PEAP-TK] as:
| ... the TK is the first 60 octets of the Key_Material, as
| specified in [RFC5216]: TLS-PRF-128 (master secret, "client EAP
| encryption", client.random || server.random).
We note that the text in [PEAP-PRF] does not define Key_Material.
Instead, it defines TK as the first octets of Key_Material and gives
a definition of Key_Material that is appropriate for TLS versions
before TLS 1.3.
For TLS 1.3, the TK should be derived from the Key_Material defined
here in Section 2.1 instead of using the TLS-PRF-128 derivation given
in [PEAP-PRF]. The method defined in [PEAP-TK] MUST NOT be used.
2.5.1. Client Certificates
As with EAP-TTLS, [PEAP] permits the use of client certificates in
addition to inner tunnel methods. The practice of using client
certificates with no "inner method" is forbidden when PEAP is used
with TLS 1.3. If there is a requirement to use client certificates
with no inner tunnel methods, then EAP-TLS should be used instead of
PEAP.
The use of client certificates is still permitted when using PEAP
with TLS 1.3. However, if the client certificate is accepted, then
the EAP peer MUST proceed with additional authentication of the inner
tunnel. If there is no inner tunnel authentication data, then the
EAP server MUST reject the session.
3. Application Data
Unlike previous TLS versions, TLS 1.3 can continue negotiation after
the initial TLS handshake has been completed; TLS 1.3 calls this the
"CONNECTED" state. Some implementations use receipt of a Finished
message as an indication that TLS negotiation has completed and that
an "inner tunnel" session can now be negotiated. This assumption is
not always correct with TLS 1.3.
Earlier TLS versions did not send application data along with the
Finished message. It was then possible for implementations to assume
that a receipt of a Finished message also meant that there was no
application data available and that another round trip was required.
This assumption is not true with TLS 1.3, and applications relying on
that behavior will not operate correctly with TLS 1.3.
As a result, implementations MUST check for application data once the
TLS session has been established. This check MUST be performed
before proceeding with another round trip of TLS negotiation. TLS-
based EAP methods, such as EAP-TTLS, PEAP, and EAP-FAST, each have
method-specific application data that MUST be processed according to
the EAP Type.
TLS 1.3 in [RFC8446], Section 4.6.1 also permits NewSessionTicket
messages to be sent after the server has received the client Finished
message, which is a change from earlier TLS versions. This change
can cause implementations to fail in a number of different ways due
to a reliance on implicit behavior seen in earlier TLS versions.
In order to correct this failure, we require that implementations
MUST NOT send or expect to receive application data in the TLS
session if the underlying TLS connection is still performing
negotiation. Implementations MUST delay processing of application
data until such time as the TLS negotiation has finished. If the TLS
negotiation is successful, then the application data can be examined.
If the TLS negotiation is unsuccessful, then the application data is
untrusted; therefore, it MUST be discarded without being examined.
The default for many TLS library implementations is to send a
NewSessionTicket message immediately after or along with the Finished
message. This ticket could be used for resumption, even if the
"inner tunnel" authentication has not been completed. If the ticket
could be used, then it could allow a malicious EAP peer to completely
bypass the "inner tunnel" authentication.
Therefore, the EAP server MUST NOT permit any session ticket to
successfully resume authentication unless the inner tunnel
authentication has completed successfully. The alternative would
allow an attacker to bypass authentication by obtaining a session
ticket, immediately closing the current session, and "resuming" using
the session ticket.
To protect against that attack, implementations SHOULD NOT send
NewSessionTicket messages until the "inner tunnel" authentication has
completed. There is no reason to send session tickets that will
later be invalidated or ignored. However, we recognize that this
suggestion may not always be possible to implement with some
available TLS libraries. As such, EAP servers MUST take care to
either invalidate or discard session tickets that are associated with
sessions that terminate in EAP Failure.
The NewSessionTicket message SHOULD also be sent along with other
application data, if possible. Sending that message alone prolongs
the packet exchange to no benefit. In addition to prolonging the
packet exchange, using a separate NewSessionTicket message can lead
to non-interoperable implementations.
[RFC9190], Section 2.5 requires a protected result indication, which
indicates that TLS negotiation has finished. Methods that use "inner
tunnel" methods MUST instead begin their "inner tunnel" negotiation
by sending Type-specific application data.
3.1. Identities
For EAP-TLS, Sections 2.1.3 and 2.1.7 of [RFC9190] recommend the use
of anonymous Network Access Identifiers (NAIs) [RFC7542] in the EAP
Response/Identity packet. However, as EAP-TLS does not send
application data inside of the TLS tunnel, that specification does
not address the subject of "inner" identities in tunneled EAP
methods. However, this subject must be addressed for the tunneled
methods.
Using an anonymous NAI for the outer identity as per [RFC7542],
Section 2.4 has a few benefits. An NAI allows the EAP session to be
routed in a AAA framework as described in [RFC7542], Section 3.
Using an anonymous realm also ensures that user identifiers are kept
private.
As for the inner identity, we define it generically as the
identification information carried inside of the TLS tunnel. For
PEAP, that identity may be an EAP Response/Identity. For EAP-TTLS,
it may be the User-Name attribute. Vendor-specific EAP methods that
use TLS will generally also have an inner identity. This identity is
carried inside of the TLS tunnel and is therefore both routed to the
correct destination by the outer identity and kept private by the use
of TLS.
In other words, we can view the outer TLS layer of tunneled EAP
methods as a secure transport layer that is responsible for getting
the actual (inner) authentication credentials securely from the EAP
peer to the EAP server. The EAP server then uses the inner identity
and inner authentication data to identify and authenticate a
particular user.
As the authentication data is routed to the correct destination,
there is little reason for the inner identity to also contain a
realm. Therefore, we have a few recommendations on the inner and
outer identities, along with their relationship to each other.
The outer identity SHOULD use an anonymous NAI realm that allows for
both user privacy and for the EAP session to be routed in a AAA
framework as described in [RFC7542], Section 3. Where NAI realms are
not used, packets will not be routable outside of the local
organization.
The inner identity MUST NOT use an anonymous NAI realm. If anonymous
network access is desired, EAP peers MUST use EAP-TLS without peer
authentication, as per [RFC9190], Section 2.1.5. EAP servers MUST
cause authentication to fail if an EAP peer uses an anonymous "inner"
identity for any TLS-based EAP method.
Implementations SHOULD NOT use inner identities that contain an NAI
realm. Many organizations typically use only one realm for all user
accounts.
However, there are situations where it is useful for an inner
identity to contain a realm. For example, an organization may have
multiple independent sub-organizations, each with a different and
unique realm. These realms may be independent of one another, or the
realms may be a subdomain (or subdomains) of the public outer realm.
In that case, an organization can configure one public "routing"
realm and multiple separate "inner" realms. This separation of
realms also allows an organization to split users into logical groups
by realm, where the "user" portion of the NAI may otherwise conflict.
For example, "user@example.com" and "user@example.org" are different
NAIs that can both be used as inner identities.
Using only one public realm both keeps internal information private
and simplifies realm management for external entities by minimizing
the number of realms that have to be tracked by them.
In most situations, routing identifiers should be associated with the
authentication data that they are routing. For example, if a user
has an inner identity of "user@example.com", then it generally makes
little sense to have an outer identity of "@example.org". The
authentication request would then be routed to the "example.org"
domain, which may have no idea what to do with the credentials for
"user@example.com". At best, the authentication request would be
discarded. At worst, the "example.org" domain could harvest user
credentials for later use in attacks on "example.com".
When an EAP server receives an inner identity for a realm which it is
not authoritative, it MUST reject the authentication. There is no
reason for one organization to authenticate users from a different
(and independent) organization.
In addition, associating inner/outer identities from different
organizations in the same EAP authentication session means that
otherwise unrelated realms are tied together, which can make networks
more fragile.
For example, an organization that uses a "hosted" AAA provider may
choose to use the realm of the AAA provider as the outer identity for
user authentication. The inner identity can then be fully qualified:
username plus realm of the organization. This practice may result in
successful authentications, but it has practical difficulties.
Additionally, an organization may host their own AAA servers but use
a "cloud" identity provider to hold user accounts. In that
situation, the organizations could try to use their own realm as the
outer (routing) identity and then use an identity from the "cloud"
provider as the inner identity.
This practice is NOT RECOMMENDED. User accounts for an organization
should be qualified as belonging to that organization and not to an
unrelated third party. There is no reason to tie the configuration
of user systems to public realm routing; that configuration more
properly belongs in the network.
Both of these practices mean that changing "cloud" providers is
difficult. When such a change happens, each individual EAP peer must
be updated with a different outer identity that points to the new
"cloud" provider. This process can be expensive, and some EAP peers
may not be online when this changeover happens. The result could be
devices or users who are unable to obtain network access, even if all
relevant network systems are online and functional.
Further, standards such as [RFC7585] allow for dynamic discovery of
home servers for authentication. This specification has been widely
deployed and means that there is minimal cost to routing
authentication to a particular domain. The authentication can also
be routed to a particular identity provider and changed at will with
no loss of functionality. That specification is also scalable since
it does not require changes to many systems when a domain updates its
configuration. Instead, only one thing has to change: the
configuration of that domain. Everything else is discovered
dynamically.
That is, changing the configuration for one domain is significantly
simpler and more scalable than changing the configuration for
potentially millions of end-user devices.
We recognize that there may be existing use cases where the inner and
outer identities use different realms. As such, we cannot forbid
that practice. We hope that the discussion above shows not only why
such practices are problematic, but how alternative methods are more
flexible, more scalable, and are easier to manage.
4. Resumption
[RFC9190], Section 2.1.3 defines the process for resumption. This
process is the same for all TLS-based EAP Types. The only practical
difference is that the value of the Type field is different. The
requirements on identities, use of TLS cipher suites, resumption,
etc. remain unchanged from that document.
Note that if resumption is performed, then the EAP server MUST send
the protected success result indication (one octet of 0x00) inside
the TLS tunnel, as per [RFC9190]. The EAP peer MUST in turn check
for the existence of the protected success result indication (one
octet of 0x00) and cause authentication to fail if that octet is not
received. If either the peer or the server initiates an inner tunnel
method instead, then that method MUST be followed, and inner
authentication MUST NOT be skipped.
All TLS-based EAP methods support resumption, as it is a property of
the underlying TLS protocol. All EAP servers and peers MUST support
resumption for all TLS-based EAP methods. We note that EAP servers
and peers can still choose to not resume any particular session. For
example, EAP servers may forbid resumption for administrative or
other policy reasons.
It is RECOMMENDED that EAP servers and peers enable resumption and
use it where possible. The use of resumption decreases the number of
round trips used for authentication. This decrease leads to lower
latency for authentications and less load on the EAP server.
Resumption can also lower load on external systems, such as databases
that contain user credentials.
As the packet flows for resumption are essentially identical across
all TLS-based EAP Types, it is technically possible to authenticate
using EAP-TLS (Type 13) and then perform resumption using another EAP
Type, such as with EAP-TTLS (Type 21). However, there is no
practical benefit to doing so. It is also not clear what this
behavior would mean or what (if any) security issues there may be
with it. As a result, this behavior is forbidden.
EAP servers therefore MUST NOT resume sessions across different EAP
Types, and EAP servers MUST reject resumptions in which the EAP Type
value is different from the original authentication.
5. Security Considerations
[RFC9190], Section 5 is included here by reference.
Updating the above EAP methods to use TLS 1.3 is of high importance
for the Internet community. Using the most recent security protocols
can significantly improve security and privacy of a network.
For PEAP, some derivations use HMAC-SHA1 [PEAP-MPPE]. In the
interests of interoperability and minimal changes, we do not change
that derivation, as there are no known security issues with HMAC-
SHA1. Further, the data derived from the HMAC-SHA1 calculations is
exchanged inside of the TLS tunnel and is visible only to users who
have already successfully authenticated. As such, the security risks
are minimal.
5.1. Handling of TLS NewSessionTicket Messages
In some cases, client certificates are not used for TLS-based EAP
methods. In those cases, the user is authenticated only after
successful completion of the inner tunnel authentication. However,
[RFC8446], Section 4.6.1 states that "at any time after the server
has received the client Finished message, it MAY send a
NewSessionTicket message." This message is sent by the server before
the inner authentication method has been run and therefore before the
user has been authenticated.
This separation of data allows for a "time of use, time of check"
security issue. Malicious clients can begin a session and receive a
NewSessionTicket message. The malicious client can then abort the
authentication session and use the obtained NewSessionTicket to
"resume" the previous session. If the server allows the session to
resume without verifying that the user had first been authenticated,
the malicious client can then obtain network access without ever
being authenticated.
As a result, EAP servers MUST NOT assume that a user has been
authenticated simply because a TLS session is being resumed. Even if
a session is being resumed, an EAP server MAY have policies that
still force the inner authentication methods to be run. For example,
the user's password may have expired in the time interval between
first authentication and session resumption.
Therefore, the guidelines given here describe situations where an EAP
server is permitted to allow session resumption rather than where an
EAP server is required to allow session resumption. An EAP server
could simply refuse to issue session tickets or could run the full
inner authentication, even if a session was resumed.
Where session tickets are used, the EAP server SHOULD track the
successful completion of an inner authentication and associate that
status with any session tickets issued for that session. This
requirement can be met in a number of different ways.
One way is for the EAP server to simply not send any TLS
NewSessionTicket messages until the inner authentication has
completed successfully. The EAP server then knows that the existence
of a session ticket is proof that a user was authenticated, and the
session can be resumed.
Another way is for the EAP server to simply discard or invalidate any
session tickets until after the inner authentication has completed
successfully. When the user is authenticated, a new TLS
NewSessionTicket message can be sent to the client, and the new
ticket can be cached and/or validated.
Another way is for the EAP server to associate the inner
authentication status with each session ticket. When a session
ticket is used, the authentication status is checked. When a session
ticket shows that the inner authentication did not succeed, the EAP
server MUST run the inner authentication method(s) in the resumed
tunnel and only grant access based on the success or failure of those
inner methods.
However, the interaction between EAP implementations and any
underlying TLS library may be complex, and the EAP server may not be
able to make the above guarantees. Where the EAP server is unable to
determine the user's authentication status from the session ticket,
it MUST assume that inner authentication has not completed, and it
MUST run the inner authentication method(s) successfully in the
resumed tunnel before granting access.
This issue is not relevant for EAP-TLS, which only uses client
certificates for authentication in the TLS handshake. It is only
relevant for TLS-based EAP methods that do not use the TLS layer to
authenticate.
5.2. Protected Success and Failure Indications
[RFC9190] provides for protected success and failure indications as
discussed in [RFC4137], Section 4.1.1. These result indications are
provided for both full authentication and resumption.
Other TLS-based EAP methods provide these result indications only for
resumption.
For full authentication, the other TLS-based EAP methods do not
provide for protected success and failure indications as part of the
outer TLS exchange. That is, the protected result indication is not
used, and there is no TLS-layer alert sent when the inner
authentication fails. Instead, there is simply either an EAP-Success
or an EAP-Failure sent. This behavior is the same as for previous
TLS versions; therefore, it introduces no new security issues.
We note that most TLS-based EAP methods provide for success and
failure indications as part of the authentication exchange performed
inside of the TLS tunnel. These result indications are therefore
protected, as they cannot be modified or forged.
However, some inner methods do not provide for success or failure
indications. For example, the use of EAP-TTLS with inner PAP, CHAP,
or MS-CHAP. Those methods send authentication credentials to the EAP
server via the inner tunnel with no method to signal success or
failure inside of the tunnel.
There are functionally equivalent authentication methods that can be
used to provide protected result indications. PAP can often be
replaced with EAP-GTC, CHAP with EAP-MD5, and MS-CHAPv1 with MS-
CHAPv2 or EAP-MSCHAPv2. All of the replacement methods provide for
similar functionality and have protected success and failure
indication. The main cost to this change is additional round trips.
It is RECOMMENDED that implementations deprecate inner tunnel methods
that do not provide protected success and failure indications when
TLS session tickets cannot be used. Implementations SHOULD use EAP-
GTC instead of PAP and EAP-MD5 instead of CHAP. Implementations
SHOULD use MS-CHAPv2 or EAP-MSCHAPv2 instead of MS-CHAPv1. New TLS-
based EAP methods MUST provide protected success and failure
indications inside of the TLS tunnel.
When the inner authentication protocol indicates that authentication
has failed, then implementations MUST fail authentication for the
entire session. There may be additional protocol exchanges in order
to exchange more detailed failure indications, but the final result
MUST be a failed authentication. As noted earlier, any session
tickets for this failed authentication MUST be either invalidated or
discarded.
Similarly, when the inner authentication protocol indicates that
authentication has succeeded, implementations SHOULD cause
authentication to succeed for the entire session. There MAY be
additional protocol exchanges that could still cause failure, so we
cannot mandate sending success on successful authentication.
In both of these cases, the EAP server MUST send an EAP-Failure or
EAP-Success message, as indicated by Step 4 in Section 2 of
[RFC3748]. Even though both parties have already determined the
final authentication status, the full EAP state machine must still be
followed.
6. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding the registration of values related to the
TLS-based EAP methods for the TLS 1.3 protocol in accordance with
[RFC8126].
IANA has added the following labels to the "TLS Exporter Label"
registry defined by [RFC5705]. These labels are used in the
derivation of Key_Material and Method-Id as defined above in
Section 2, and they are used only for TEAP.
+============================+=========+=============+===========+
| Value | DTLS-OK | Recommended | Reference |
+============================+=========+=============+===========+
| EXPORTER: teap session key | N | Y | RFC 9427 |
| seed | | | |
+----------------------------+---------+-------------+-----------+
| EXPORTER: Inner Methods | N | Y | RFC 9427 |
| Compound Keys | | | |
+----------------------------+---------+-------------+-----------+
| EXPORTER: Session Key | N | Y | RFC 9427 |
| Generating Function | | | |
+----------------------------+---------+-------------+-----------+
| EXPORTER: Extended Session | N | Y | RFC 9427 |
| Key Generating Function | | | |
+----------------------------+---------+-------------+-----------+
| TEAPbindkey@ietf.org | N | Y | RFC 9427 |
+----------------------------+---------+-------------+-----------+
Table 1: TLS Exporter Labels Registry
7. References
7.1. Normative References
[IANA] IANA, "Method Types",
<https://www.iana.org/assignments/eap-numbers/>.
[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>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<https://www.rfc-editor.org/info/rfc3748>.
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <https://www.rfc-editor.org/info/rfc5216>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
"Tunnel Extensible Authentication Protocol (TEAP) Version
1", RFC 7170, DOI 10.17487/RFC7170, May 2014,
<https://www.rfc-editor.org/info/rfc7170>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC9190] Preuß Mattsson, J. and M. Sethi, "EAP-TLS 1.3: Using the
Extensible Authentication Protocol with TLS 1.3",
RFC 9190, DOI 10.17487/RFC9190, February 2022,
<https://www.rfc-editor.org/info/rfc9190>.
7.2. Informative References
[MSPEAP] Microsoft Corporation, "[MS-PEAP]: Protected Extensible
Authentication Protocol (PEAP)", Protocol Revision 31.0,
June 2021,
<https://msdn.microsoft.com/en-us/library/cc238354.aspx>.
[PEAP] Palekar, A., Josefsson, S., Simon, D., Zorn, G., Salowey,
J., and H. Zhou, "Protected EAP Protocol (PEAP) Version
2", Work in Progress, Internet-Draft, draft-josefsson-
pppext-eap-tls-eap-10, 15 October 2004,
<https://datatracker.ietf.org/doc/html/draft-josefsson-
pppext-eap-tls-eap-10>.
[PEAP-MPPE]
Microsoft Corporation, "Key Management", Section 3.1.5.7,
October 2020, <https://learn.microsoft.com/en-
us/openspecs/windows_protocols/ms-peap/e75b0385-915a-
4fc3-a549-fd3d06b995b0>.
[PEAP-PRF] Microsoft Corporation, "Intermediate PEAP MAC Key (IPMK)
and Compound MAC Key (CMK)", Section 3.1.5.5.2.2, February
2019, <https://docs.microsoft.com/en-
us/openspecs/windows_protocols/MS-PEAP/0de54161-0bd3-424a-
9b1a-854b4040a6df>.
[PEAP-TK] Microsoft Corporation, "PEAP Tunnel Key (TK)",
Section 3.1.5.5.2.1, April 2021,
<https://docs.microsoft.com/en-
us/openspecs/windows_protocols/MS-PEAP/41288c09-3d7d-482f-
a57f-e83691d4d246>.
[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, DOI 10.17487/RFC1994, August
1996, <https://www.rfc-editor.org/info/rfc1994>.
[RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions",
RFC 2433, DOI 10.17487/RFC2433, October 1998,
<https://www.rfc-editor.org/info/rfc2433>.
[RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2",
RFC 2759, DOI 10.17487/RFC2759, January 2000,
<https://www.rfc-editor.org/info/rfc2759>.
[RFC4137] Vollbrecht, J., Eronen, P., Petroni, N., and Y. Ohba,
"State Machines for Extensible Authentication Protocol
(EAP) Peer and Authenticator", RFC 4137,
DOI 10.17487/RFC4137, August 2005,
<https://www.rfc-editor.org/info/rfc4137>.
[RFC4851] Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "The
Flexible Authentication via Secure Tunneling Extensible
Authentication Protocol Method (EAP-FAST)", RFC 4851,
DOI 10.17487/RFC4851, May 2007,
<https://www.rfc-editor.org/info/rfc4851>.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281,
DOI 10.17487/RFC5281, August 2008,
<https://www.rfc-editor.org/info/rfc5281>.
[RFC5422] Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou,
"Dynamic Provisioning Using Flexible Authentication via
Secure Tunneling Extensible Authentication Protocol (EAP-
FAST)", RFC 5422, DOI 10.17487/RFC5422, March 2009,
<https://www.rfc-editor.org/info/rfc5422>.
[RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542,
DOI 10.17487/RFC7542, May 2015,
<https://www.rfc-editor.org/info/rfc7542>.
[RFC7585] Winter, S. and M. McCauley, "Dynamic Peer Discovery for
RADIUS/TLS and RADIUS/DTLS Based on the Network Access
Identifier (NAI)", RFC 7585, DOI 10.17487/RFC7585, October
2015, <https://www.rfc-editor.org/info/rfc7585>.
Acknowledgments
Thanks to Jorge Vergara for a detailed review of the requirements for
various EAP Types.
Thanks to Jorge Vergara, Bruno Periera Vidal, Alexander Clouter,
Karri Huhtanen, and Heikki Vatiainen for reviews of this document and
for assistance with interoperability testing.
Author's Address
Alan DeKok
The FreeRADIUS Server Project
Email: aland@freeradius.org
