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TLS-Based Extensible Authentication Protocol (EAP) Types for Use with TLS 1.3
draft-ietf-emu-tls-eap-types-13

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9427.
Author Alan DeKok
Last updated 2023-06-27 (Latest revision 2023-02-16)
Replaces draft-dekok-emu-tls-eap-types
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Stream WG state Submitted to IESG for Publication
Associated WG milestone
Nov 2019
WG adopts draft providing guidance for use of TLS 1.3 with TLS based EAP methods
Document shepherd Joseph A. Salowey
Shepherd write-up Show Last changed 2022-10-08
IESG IESG state Became RFC 9427 (Proposed Standard)
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(None)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Paul Wouters
Send notices to jsalowey@gmail.com
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
IANA expert review state Expert Reviews OK
draft-ietf-emu-tls-eap-types-13
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-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   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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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info/) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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.

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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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