Network Working Group                                        DeKok, Alan
INTERNET-DRAFT                                                FreeRADIUS
Updates: 5247, 5281, 7170
Category: Standards Track
19 April 2020

                    TLS-based EAP types and TLS 1.3


   EAP-TLS [RFC5216] is being updated for TLS 1.3 in [EAPTLS].  Many
   other EAP [RFC3748] and [RFC5247] types also depend on TLS, such as
   FAST [RFC4851], TTLS [RFC5281], TEAP [RFC7170], 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

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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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.  FAST and TEAP .......................................    6
3.  Application Data .........................................    7
4.  Security Considerations ..................................    7
5.  IANA Considerations ......................................    8
6.  References ...............................................    8
   6.1.  Normative References ................................    8
   6.2.  Informative References ..............................    9

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1.  Introduction

   EAP-TLS is being updated for TLS 1.3 in [EAPTLS].  Many other EAP
   types also depend on TLS, such as FAST [RFC4851], TTLS [RFC5281],
   TEAP [RFC7170], and possibly many vendor specific EAP methods.  All
   of these methods use key derivation functions that rely on the
   information which is no longer available in TLS 1.3.  As such, all of
   those methods are incompatible with TLS 1.3.

   We wish to enable the use of TLS 1.3 in the wider Internet community.
   As such, it is necessary to update the above EAP types.  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",
   "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 [EAPTLS] apply to other EAP
   methods that wish to use TLS 1.3.  Implementations of other methods
   that wish to use TLS 1.3 MUST follow the guidelines in [EAPTLS].

   There are, however a few key differences between EAP-TLS and other
   TLS-based EAP methods that necessitate this document.  The simplest
   difference is that [EAPTLS] uses the EAP-TLS type ID (0x0D) in a
   number of calculations.  That value should change for other method

   More complex differences include derivation of additional keying
   material, as in FAST [RFC4851].

2.1.  Key Derivation

   The key derivation for TLS-based EAP methods depends on the value of
   the Type-Code as defined by [IANA].  The most important definition is
   of the Type-Code:

      Type-Code  = EAP Method type

   The Type-Code is defined to be 1 octet for values smaller than 255.
   Where expanded EAP Type Codes are used, the Type-Code is defined to
   be the Expanded Type Code (including the Type, Vendor-Id (in network
   byte order) and Vendor-Type fields (in network byte order) defined in
   [RFC3748] Section 5.7).

      Type-Code  = 0xFE || Vendor-Id || Vendor-Type

   Unless otherewise discussed below, the key derivation functions for
   all TLS-based EAP types are defined as follows:

      Key_Material = TLS-Exporter("EXPORTER_EAP_TLS_Key_Material",
                                   Type-Code, 128)
      IV           = TLS-Exporter("EXPORTER_EAP_TLS_IV", Type-Code, 64)
      Method-Id    = TLS-Exporter("EXPORTER_EAP_TLS_Method-Id",
                                   Type-Code, 64)
      Session-Id   = Type-Code || Method-Id
      MSK          = Key_Material(0, 63)
      EMSK         = Key_Material(64, 127)
      Enc-RECV-Key = MSK(0, 31)
      Enc-SEND-Key = MSK(32, 63)
      RECV-IV      = IV(0, 31)
      SEND-IV      = IV(32, 63)

   We note that these definitions re-use the EAP-TLS exporter labels,

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   and change the derivation only by adding a dependency on Type-Code.
   The reason for this change is simplicity.  There does not appear to
   be compelling reasons to make the labels method-specific, when we can
   just include the Type-Code 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.

2.2.  FAST and TEAP

   EAP-FAST [RFC4851] and TEAP [RFC7170] cannot use the above
   derivation.  Those methods use an inner tunnel EMSK to calculate the
   outer EMSK.  As such, those key derivations cannot use the above

   EAP-FAST previously used a PAC, which is a type of pre-shared key
   (PSK).  Such uses are deprecated in TLS 1.3.  As such, PAC
   provisioning is no longer part of EAP-FAST when TLS 1.3 is used.

   TBD: Is this true?  Comments from EAP-FAST people are useful here.

   The key derivation for FAST and TEAP are similar enough that they
   gave be given together here.  The only difference is the Type-Code.
   All derivations not given here are the same as given above in the
   previous section.

   session_key_seed = TLS-Exporter("EXPORTER: session key seed", Type-
   Code, 40)

   For FAST, the session_key_seed is also used as the key_block, as
   defined in [RFC4851] Section 5.1.

   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.

   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)

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3.  Application Data

   Unlike previous TLS version, TLS 1.3 continues negotiation after the
   TLS session has been initialized.  Some implementations use the TLS
   "Finished" state as a signal that application data is now available,
   and an "inner tunnel" session can now be negotiated.  As noted in
   [RFC8446], TLS 1.3 may include a "NewSessionTicket" after the
   "Finished" state.  This change can cause many implementations to

   In order to correct this failure, implementations MUST also check if
   "Application Data" is available for a TLS connection.  If the
   underlying TLS connection is still performing negotiations, then
   implementations MUST NOT send, or expect to receive application data
   in the TLS session.

   We note that some TLS Application Programming Interfaces (APIs)
   signal the availability of application data by returning zero octets
   of application data, where they previously had returned an error
   which signalled that negotiation should continue.  For those APIs,
   implementations SHOULD treat the combination of the "Finished" state
   and the availability of zero octets of application data as a signal
   that TLS negotiation has completed, and that the tunneled process can

   [EAPTLS] uses an empty application record to indicate that
   negotiation has finished.  Methods which use "inner tunnel" methods
   should instead begin their "inner tunnel" negotiation by sending
   type-specific application data.

4.  Security Considerations

   [EAPTLS] 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 privace 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 sends a NewSessionTicket after receiving the TLS
   Finished message from the client, and therefore before the user is

   This separation of data allows for a "time of use, time of check"
   security issue.  Malicious clients can begin a session and receive

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   the NewSessionTicket.  Then prior to authentication, the malicious
   client can abort the authentication session.  The malicious client
   can then use the obtained NewSessionTicket to "resume" the previous

   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.

5.  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 derivation of Key_Material, IV and Method-Id as defined above in
   Section ?

   The labels above need to be added to the "TLS Exporter Labels"

   * TBD

6.  References

6.1.  Normative References

     Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", RFC 2119, March, 1997,  <http://www.rfc->.

     Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
     Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748,
     June 2004.

     Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS Authentication
     Protocol", RFC 5216, March 2008

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     Aboba, B., Simon, D., and P. Eronen, "Extensible Authentication
     Protocol (EAP) Key Management Framework", RFC 5247, August 2008,

     Rescorla, E., "Keying Material Exporters for Transport Layer
     Security (TLS)", RFC 5705, March 2010

     Zhou, H., et al., "Tunnel Extensible Authentication Protocol (TEAP)
     Version 1", RFC 7170, May 2014.

     Cotton, M., et al, "Guidelines for Writing an IANA Considerations
     Section in RFCs", RC 8126, June 2017.

     Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key
     Words", RFC 8174, May 2017, <http://www.rfc->.

     Rescorla, E., "The Transport Layer Security (TLS) Protocol Version
     1.3", RFC 8446, August 2018.

     Mattsson, J., and Sethi, M., "Using EAP-TLS with TLS 1.3", draft-
     ietf-emu-eap-tls13-03, November 2018.


6.2.  Informative References

     Palekar, A. et al, "Protected EAP Protocol (PEAP)", draft-
     josefsson-pppext-eap-tls-eap-06.txt, March 2003.


     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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     Funk, P., and Blake-Wilson, S., "Extensible Authentication Protocol
     Tunneled Transport Layer Security Authenticated Protocol Version 0
     (EAP-TTLSv0)", RFC 5281, August 2008.

Authors' Addresses

   Alan DeKok
   The FreeRADIUS Server Project


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