Internet DRAFT - draft-ietf-emu-tls-eap-types
draft-ietf-emu-tls-eap-types
Network Working Group DeKok, Alan
INTERNET-DRAFT FreeRADIUS
Updates: 4851, 5281, 7170 16 February 2023
Category: Standards Track
Expires: August 16, 2023
TLS-based EAP types and TLS 1.3
draft-ietf-emu-tls-eap-types-13.txt
Abstract
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-FAST (RFC 4851), EAP-
TTLS (RFC 5281), TEAP (RFC 7170), and possibly many vendor specific
EAP methods. 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 Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on January 29, 2021.
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
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Table of Contents
1. Introduction ............................................. 4
1.1. Requirements Language ............................... 4
2. Using TLS-based EAP methods with TLS 1.3 ................. 5
2.1. Key Derivation ...................................... 5
2.2. TEAP ................................................ 6
2.2.1. Client Certificates ............................ 8
2.3. EAP-FAST ............................................ 8
2.3.1. Client Certificates ............................ 9
2.4. EAP-TTLS ............................................ 9
2.4.1. Client Certificates ............................ 10
2.5. PEAP ................................................ 10
2.5.1. Client Certificates ............................ 11
3. Application Data ......................................... 11
3.1. Identities .......................................... 13
4. Resumption ............................................... 16
5. Implementation Status .................................... 17
6. Security Considerations .................................. 17
6.1. Handling of TLS NewSessionTicket Messages ........... 17
6.2. Protected Success and Failure indications ........... 19
7. IANA Considerations ...................................... 20
8. References ............................................... 21
8.1. Normative References ................................ 21
8.2. Informative References .............................. 22
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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], TEAP [RFC7170], and possibly many vendor specific EAP
methods such as PEAP [PEAP]. All of these methods use key derivation
functions which are no longer applicable to TLS 1.3. As such, all of
those methods are incompatible with TLS 1.3.
This document updates those 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.
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2. Using TLS-based EAP methods with TLS 1.3
In general, all of the requirements of [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 which 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 below in Section 2.1.
An additional difference is that [RFC9190] Section 2.5 requires that
once the EAP-TLS handshake has completed, the EAP server sends a
protected success result indication. 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, and is not used. This topic is explained
in more detail below, in Section 3 and Section 4.
Finally, the 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 to be 1 octet for
values smaller than 254 (the value for the Expanded Type), and when
Expanded EAP Types are used, the logical Type is defined to be 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
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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 TLS-Exporter
depends on the length passed to it. Implementations therefore MUST
pass the correct length instead of passing a large length and
truncating the output. Any output calculated using a larger length
value, and which is then truncated, will be different from the output
which 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 re-use 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 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 which
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.
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[RFC7170] Section 5.2 gives a definition for the Inner Method Session
Key (IMSK), which depends on the 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 Inner Method Session Keys (IMSK), Inner Method
Compound Keys (IMCK), and Compound Session Keys (CMK) 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]
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 was used by TEAP
with TLS 1.2. Similarly, the session_key_seed in TLS 1.3 uses the
Type as the context, where 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 MAC defined in [RFC7170] Section 5.3 remains the
same, but the message authentication code (MAC) for TLS 1.3 is
computed with the HMAC algorithm negotiated for HKDF in the key
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schedule, as per section 7.1 of RFC 8446. 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 message authentication code (MAC) is computed with
the HMAC algorithm negotiated for HKDF in the key schedule, as per
section 7.1 of RFC 8446. 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.
That is, while [RFC7170] 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 suggest 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 definition 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.
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Unlike [RFC4851] Section 5.2, the definition of IMCK[j] places the
reference to S-IMCK after the textual label, and the concatenates the
IMSK instead of MSK.
EAP-FAST previously used a PAC, which is functionally equivalent to
session tickets provided by TLS 1.3 which contain a pre-shared key
(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.
That is, 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 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, which [RFC5281] requires it to be 17 octets for CHAP
(Section 11.2.2) and MS-CHAPv2 (Section 11.2.4), and to be 9 octets
for MS-CHAP (Section 11.2.3).
When 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.
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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.
It is therefore 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 and following. 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] which consumes inner EAP method keying
material. The pseudo-random function (PRF+) used in [PEAP-MPPE] is
not taken from the TLS exporter, but is instead calculated via a
different method which is given in [PEAP-PRF]. That derivation
remains unchanged in this specification.
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Note that the above derivation uses SHA-1, which may be formally
deprecated in the near future.
However, the pseudo-random function (PRF+) calculation uses a PEAP
Tunnel Key which is defined in [PEAP-PRF] 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 which 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, which TLS 1.3 calls 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.
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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 which 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 if the underlying
TLS connection is still performing negotiation, then implementations
MUST NOT send, or expect to receive application data in the TLS
session. 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, and therefore 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, and then 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 which 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 which are associated
with sessions that terminate in EAP Failure.
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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 which use
"inner tunnel" methods MUST instead begin their "inner tunnel"
negotiation by sending Type-specific application data.
3.1. Identities
For EAP-TLS, [RFC9190] Sections 2.1.3 and 2.1.7 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. This subject must, however, 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 an 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 which
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 which 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. We therefore have a few recommendations on the inner and
outer identities, along with their relationship to each other.
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The outer identity SHOULD use an anonymous NAI realm, which allows
for both user privacy, and for the EAP session to be routed in an 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 which 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 which can both be used as inner identities.
Using only one public realm both keeps internal information private,
and also simplifies realm management for external entities by
minimizing the number of realms which 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".
Where 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 authentication users from a different
(and independent) organization.
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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 which 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:
user name plus realm of the organization. This practice may result
in successful authentications, but it has practical difficulties.
For example, an organization may host their own AAA servers, but use
a "cloud" identity provider to hold user accounts. In that
situation, the organizations could see try to use their own realm as
the outer (routing) identity, 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 which 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. That 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,
in that 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
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that practice. We hope that the discussion above shows not only why
such practices are problematic, but also that it shows 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, 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 the protected success result indication (one octet of
0x00), and cause authentication to fail if that octet is not
received. If either peer or server instead initiates an inner tunnel
method, 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
which 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.
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5. Implementation Status
RFC Editor: Please remove this section before publication.
EAP-TTLS and PEAP are implemented and tested to be interoperable with
wpa_supplicant 2.10 and Windows 11 as EAP peers, and FreeRADIUS
3.0.26 and Radiator as RADIUS / EAP servers.
The wpa_supplicant implementation requires that a configuration flag
be set "tls_disable_tlsv1_3=0", and describes the flag as "enable
TLSv1.3 (experimental - disabled by default)". However,
interoperability testing shows that PEAP and EAP-TTLS both work with
Radiator and FreeRADIUS.
Implementors have demonstrated significant interest in getting PEAP
and EAP-TTLS working for TLS 1.3, but less interest in EAP-FAST and
TEAP. As such, there is no implementation experience with EAP-FAST
or TEAP. However, we believe that the definitions described above
are correct, and are workable.
6. 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.
6.1. 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,
[RFC84346] Section 4.6.1 allows 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"
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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 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 which
still force the inner authentication methods to be run. For example,
the users password may have expired in the time interval between
first authenticaction, and session resumption.
The guidelines given here therefore describe situations where an EAP
server is permitted to allow session resumption, not where it 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 simple 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 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 grant only 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
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able to make the above guarantees. Where the EAP server is unable to
determine the users 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 which do not use the TLS layer to
authenticate
6.2. Protected Success and Failure indications
[RFC9190] provides for protected success and failure indications as
discussed in Section 4.1.1 of [RFC4137]. These result indications
are provided for both full authentication, and for 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 EAP-Failure sent. This behavior is the same as for previous TLS
versions, and therefore 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 which 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
which do not provide protected success and failure indications when
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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, then implementations SHOULD cause
authentication to succeed for the entire session. There MAY be
additional protocol exchanges which 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 Section 2, item 4 of [RFC3748].
Even though both parties have already determined the final
authentication status, the full EAP state machine must still be
followed.
7. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the TLS-
based EAP methods for TLS 1.3 protocol in accordance with [RFC8126].
This memo requires IANA to add 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.
The labels below need to be added to the "TLS Exporter Labels"
registry as "Value", with this specification as "Reference". For all
of these labels the "DTLS-OK" field should be "N", and the
"Recommended" field should be "Y".
These labels are used only for TEAP.
* EXPORTER: teap session key seed
* EXPORTER: Inner Methods Compound Keys
* EXPORTER: Session Key Generating Function
* EXPORTER: Extended Session Key Generating Function
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* TEAPbindkey@ietf.org
8. References
8.1. Normative References
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997, <http://www.rfc-
editor.org/info/rfc2119>.
[RFC3748]
Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748,
June 2004.
[RFC5216]
Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS Authentication
Protocol", RFC 5216, March 2008
[RFC5705]
Rescorla, E., "Keying Material Exporters for Transport Layer
Security (TLS)", RFC 5705, March 2010
[RFC7170]
Zhou, H., et al., "Tunnel Extensible Authentication Protocol (TEAP)
Version 1", RFC 7170, May 2014.
[RFC8126]
Cotton, M., et al., "Guidelines for Writing an IANA Considerations
Section in RFCs", RC 8126, June 2017.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key
Words", RFC 8174, May 2017, <http://www.rfc-
editor.org/info/rfc8174>.
[RFC8446]
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version
1.3", RFC 8446, August 2018.
[RFC9190]
Mattsson, J., and Sethi, M., "Using EAP-TLS with TLS 1.3", RFC
9190, July 2021.
[IANA]
https://www.iana.org/assignments/eap-numbers/eap-numbers.xhtml#eap-
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numbers-4
8.2. Informative References
[MSPEAP]
https://msdn.microsoft.com/en-us/library/cc238354.aspx
[PEAP]
Palekar, A. et al., "Protected EAP Protocol (PEAP)", draft-
josefsson-pppext-eap-tls-eap-10.txt, October 2004.
[PEAP-MPPE]
"PEAP Key Management", https ://docs.microsoft.com/en-
us/openspecs/windows_protocols/MS-
PEAP/e75b0385-915a-4fc3-a549-fd3d06b995b0
[PEAP-PRF]
"PEAP Intermediate PEAP MAC Key (IPMK) and Compound MAC Key (CMK)"
https://docs.microsoft.com/en-us/openspecs/windows_protocols/MS-
PEAP/0de54161-0bd3-424a-9b1a-854b4040a6df
[PEAP-TK]
"PEAP Tunnel Key (TK)" 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, August 1996.
[RFC2433]
Zorn, G. and Cobb, S., "Microsoft PPP CHAP Extensions", RFC 2433,
October 1998.
[RFC2759]
Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC 2759,
January 2000.
[RFC4137]
Vollbrecht, J., et al., "State Machines for Extensible
Authentication Protocol (EAP) Peer and Authenticator ", RFC 4137,
August 2005.
[RFC4851]
Cam-Winget, N., et al., "The Flexible Authentication via Secure
Tunneling Extensible Authentication Protocol Method (EAP-FAST)",
RFC 4851, May 2007.
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[RFC5281]
Funk, P., and Blake-Wilson, S., "Extensible Authentication Protocol
Tunneled Transport Layer Security Authenticated Protocol Version 0
(EAP-TTLS,v0)", RFC 5281, August 2008.
[RFC5422]
Cam-Winget, N., et al., "Dynamic Provisioning Using Flexible
Authentication via Secure Tunneling Extensible Authentication
Protocol (EAP-FAST)", RFC 5422, March 2009.
[RFC7542]
DeKoK, A, "The Network Access Identifier", RFC 7542, May 2015.
[RFC7585]
Winter, S, and McCauley, M., "Dynamic Peer Discovery for RADIUS/TLS
and RADIUS/DTLS Based on the Network Access Identifier (NAI)", RFC
7585, October 2015.
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.
Authors' Addresses
Alan DeKok
The FreeRADIUS Server Project
Email: aland@freeradius.org
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