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TLS-based EAP types and TLS 1.3
draft-ietf-emu-tls-eap-types-05

Document Type Active Internet-Draft (emu WG)
Author Alan DeKok
Last updated 2022-03-05
Replaces draft-dekok-emu-tls-eap-types
Stream Internet Engineering Task Force (IETF)
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Nov 2019
WG adopts draft providing guidance for use of TLS 1.3 with TLS based EAP methods
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draft-ietf-emu-tls-eap-types-05
Network Working Group                                        DeKok, Alan
INTERNET-DRAFT                                                FreeRADIUS
Updates: 5247, 5281, 7170                                   5 March 2022
Category: Standards Track
Expires: September 05, 2022

                    TLS-based EAP types and TLS 1.3
                  draft-ietf-emu-tls-eap-types-05.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 FAST (RFC 4851), 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) 2022 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified 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
   2.3.  FAST ................................................    7
   2.4.  TTLS ................................................    8
      2.4.1.  Client Certificates ............................    8
   2.5.  PEAP ................................................    9
      2.5.1.  Client Certificates ............................    9
3.  Application Data .........................................    9
   3.1.  Identities ..........................................   11
4.  Resumption ...............................................   13
5.  Implementation Status ....................................   14
6.  Security Considerations ..................................   14
   6.1.  Protected Success and Failure indicators ............   15
7.  IANA Considerations ......................................   16
8.  References ...............................................   17
   8.1.  Normative References ................................   17
   8.2.  Informative References ..............................   18

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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 FAST [RFC4851], 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 necessitates 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.

   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 indicator, but only during resumption.  When other TLS-
   based EAP methods use full authentication, the indicator 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 just 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.

   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:

      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.  There does not
   appear to be compelling reasons to make the labels method-specific,
   when they can just include the logical Type in the key derivation.

   These definitions apply in their entirety to TTLS [RFC5281] and PEAP
   as defined in [PEAP] and [MSPEAP].  Some definitions apply to 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

   [RFC7170] Section 5.2 gives a definition for the Inner Method Session
   Key (IMSK), which depends on the TLS-PRF.  When the inner methods
   generates an EMSK, we update that definition for TLS 1.3 as:

      IMSK = TLS-Exporter("TEAPbindkey@ietf.org", EMSK, 32)

   If an inner method does not support export of an Extended Master
   Session Key (EMSK), then IMSK is the MSK of the inner method as per
   [RFC7170] Section 5.2.

   For MSK and EMSK, TEAP [RFC7170] uses an inner tunnel EMSK to
   calculate the outer EMSK.  As such, those key derivations cannot use
   the above derivation.

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   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 FAST, but using the FAST Type.

      session_key_seed = TLS-Exporter("EXPORTER: 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]

   Where | denotes concatenation.  The outer MSK and EMSK are then
   derived from the above definitions, as:

      MSK  = TLS-Exporter("EXPORTER: Session Key Generating Function",
                           S-IMCK[j], 64)

      EMSK = TLS-Exporter("EXPORTER: Extended Session Key Generating Function",
                           S-IMCK[j], 64)

   The TEAP Compound MAC defined in [RFC7170] Section 5.3 is updated to
   use the definition of CMK[j] given above, which then leads to the
   following definition

      CMK = CMK[j]

      Compound-MAC = MAC( CMK, BUFFER )

   where j is the number of the last successfully executed inner EAP
   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.  The definition of BUFFER
   is unchanged from [RFC7170] Section 5.3

2.3.  FAST

   For 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.

   Unlike [RFC4851] Section 5.2, the definition of IMCK[j] places the
   reference to S-IMCK after the textual label, and the concatenates the

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   IMSK instead of MSK.

   EAP-FAST previously used a PAC, which is a session ticket that
   contains a pre-shared key (PSK) along with other data.  As TLS 1.3
   allows session resumptuion using a PSK, 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.

2.4.  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-CHAP-V2 (Section 11.2.4), and to be 9 octets
   for Ms-CHAP (Section 11.2.3).

   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.

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
   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 TTLS.

   The use of client certificates is still permitted when using 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.

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2.5.  PEAP

   When PEAP uses crypto binding, it uses a different key calculation
   defined in [PEAP-MPPE] which consumes inner method keying material.
   The pseudo-random function (PRF+) used here 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.

   However, the key calculation uses a PEAP Tunnel Key [PEAP-TK] which
   is defined 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 this text 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
   above in Section 2.1, instead of using the TLS-PRF-128 derivation
   given above.

2.5.1.  Client Certificates

   As with TTLS, [PEAP] permits the use of client certificates in
   addition to inner tunnel methods.

   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 a "TLS finished"
   determination 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 always send application data along with
   the "TLS finished" method.  It was then possible for implementations
   to assume that a transition to "TLS finished" also meant that there

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   was no application data available, and that another round trip was
   required.  This assumption is not true with TLS 1.3, and applications
   relying on that behavior will not operate correctly with TLS 1.3.

   As a result, implementations MUST check for application data once the
   TLS session has been established.  This check MUST be performed
   before proceeding with another round trip of TLS negotiation.  If
   application data is available, it MUST be processed according to the
   relevant resumption and/or EAP type.

   TLS 1.3 also permits NewSessionTicket messages to be sent before the
   TLS "Finished", and after application data is sent.  This change can
   cause many 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 TLS
   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.

   The NewSessionTicket message SHOULD also be sent along with other

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   application data, if possible.  Sending that message alone prolongs
   the packet exchange to no benefit.

   [RFC9190] Section 2.5 requires a protected result indicator 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

   [RFC9190] Sections 2.1.3 and 2.1.7 recommend the use of anonymous
   Network Access Identifiers (NAIs) [RFC7542] in the EAP Identity
   Response 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 as per [RFC7542] Section 2.4 has two benefits.
   First, an anonymous identity makes it more difficult to track users.
   Second, an NAI allows the EAP session to be routed in an AAA
   framework as described in [RFC7542] Section 3.

   For the purposes of tunneled EAP methods, we can therefore view the
   outer TLS layer as being mainly a secure transport layer.  That
   transport layer is responsible for getting the actual (inner)
   authentication credentials securely from the EAP peer to the EAP
   server.  As the outer identity is often used as an anonymous routing
   identifier for AAA ([RFC7542] Section 3), there is little reason for
   it to be the same as the inner identity.  We therefore have a few
   recommendations on the inner identity, and its relationship to the
   outer identity.

   For the purpose of this section, we define the inner identity as the
   identification information carried inside of the TLS tunnel.  For
   PEAP, that identity may be an EAP Response Identity.  For TTLS, it
   may be the User-Name attribute.  Vendor-specific EAP methods which
   use TLS will generally also have an inner identity.

   Implementations MUST NOT use anonymous identities for the inner
   identity.  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.  The outer identity contains an NAI realm, which ensures that
   the inner authentication method is routed to the correct destination.
   As such, any NAI realm in the inner identity is almost always

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   redundant.

   However, if the inner identity does contain an NAI realm, the inner
   realm SHOULD be either an exact copy of the outer realm, or be a
   subdomain of the outer realm.  The inner realm SHOULD NOT be from a
   different realm than the outer realm.  There are very few reasons for
   those realms to be different.

   In general, routing identifiers should be associated with 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
   no 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".

   In addition, associating disparate inner/outer identities 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.
   The inner identity can then be fully qualified: user name plus realm
   of the organization.  This practice can result in successful
   authentications, but it has difficulties.

   Other organizations may host their own AAA servers, but use a "cloud"
   identity provider to hold user accounts.  In that situation, the
   organizations may 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.

   Both of these practices mean that changing "cloud" providers is
   difficult.  When such a change happens, each individual supplicant
   must be updated with a different outer identity which points to the
   new "cloud" provider.  This process can be expensive, and some
   supplicants 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

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   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
   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.

   Note that if resumption is performed, then the EAP server MUST send
   the protected success result indicator (one octet of 0x00) inside the
   TLS tunnel as per [RFC9190].  If either peer or server instead
   initiates an inner tunnel method, then that method MUST be followed,
   and resumption MUST NOT be used.  The EAP peer MUST in turn check for
   the existence the protected success result indicator (one octet of
   0x00), and cause authentication to fail if that octet is not
   received.

   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.

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   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, just as EAP-TTLS (Type 21).  However, there is no practical
   benefit to doing so.  It is also not clear what this behavior would
   mean, or what (if any) security issues there may be with it.  As a
   result, this behavior is forbidden.

   EAP servers therefore MUST NOT resume sessions across different EAP
   Types, and EAP servers MUST reject resumptions in which the EAP Type
   value is different from the original authentication.

5.  Implementation Status

   TTLS and PEAP are implemented and tested to be inter-operable with
   wpa_supplicant 2.10 and Windows 11 as clients, and FreeRADIUS 3.0.26
   and Radiator as RADIUS 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 TTLS both work with
   Radiator and FreeRADIUS.

   Implementors have demonstrated significant interest in getting PEAP
   and 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.

   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,
   the TLS protocol may send one or more NewSessionTicket after
   receiving the TLS Finished message from the client, and therefore
   before the user is authenticated.

   This separation of data allows for a "time of use, time of check"
   security issue.  Malicious clients can begin a session and receive a
   NewSessionTicket.  The malicious client can then abort the

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   authentication session, and the obtained NewSessionTicket to "resume"
   the previous session.

   As a result, EAP servers MUST NOT permit sessions to be resumed until
   after authentication has successfully completed.  This requirement
   may be met in a number of ways.  For example, by not caching the
   session ticket until after authentication has completed, or by
   marking up the cached session ticket with a flag stating whether or
   not authentication has completed.

   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.  Protected Success and Failure indicators

   [RFC9190] provides for protected success and failure indicators as
   discussed in Section 4.1.1 of [RFC4137].  These indicators are
   provided for both full authentication, and for resumption.

   Other TLS-based EAP methods provide these indicators only for
   resumption.

   For full authentication, the other TLS-based EAP methods do not
   provide for protected success and failure indicators as part of the
   outer TLS exchange.  That is, the protected result indicator 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 indicators as part of the authentication exchange performed
   inside of the TLS tunnel.  These indicators are therefore protected,
   as they cannot be modified or forged.

   However, some inner methods do not provide for success or failure
   indicators.  For example, the use of TTLS with inner PAP or CHAP.
   Those methods send authentication credentials to the 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 indicators.  PAP can often be replaced with

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   EAP-GTC, and CHAP with EAP-MD5.  Both replacement methods provide for
   similar functionality, and have protected success and failure
   indicator.  The main cost to this change is additional round trips.

   It is RECOMMENDED that implementations deprecate inner tunnel methods
   which do not provided protected success and failure indicators.
   Implementations SHOULD use EAP-GTC instead of PAP, and EAP-MD5
   instead of CHAP.  New TLS-based EAP methods MUST provide protected
   success and failure indicators 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 indicators, 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 succeed, then implementations SHOULD cause
   authentication to succeed for the entire session.  There MAY be
   additional protocol exchanges in order which could cause other
   failures, so success is not required here.

   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.  These labels are used only for TEAP.

   * EXPORTER: 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

[RFC5247]
     Aboba, B., Simon, D., and P. Eronen, "Extensible Authentication
     Protocol (EAP) Key Management Framework", RFC 5247, August 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", draft-

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     ietf-emu-eap-tls13-18, July 2021.

[IANA]
     https://www.iana.org/assignments/eap-numbers/eap-numbers.xhtml#eap-
     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-06.txt, May 2003.

[PEAP-MPPE]
     https://docs.microsoft.com/en-us/openspecs/windows_protocols/MS-
     PEAP/e75b0385-915a-4fc3-a549-fd3d06b995b0

[PEAP-PRF]
     https://docs.microsoft.com/en-us/openspecs/windows_protocols/MS-
     PEAP/0de54161-0bd3-424a-9b1a-854b4040a6df

[PEAP-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-TTLSv0)", 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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