EAP WLAN Reqs: Extensible Authentication Protocol (EAP) Method Requirements for Wireless LANs

Network Working Group                                         D. Stanley
Request for Comments: 4017                                 Agere Systems
Category: Informational                                        J. Walker
                                                       Intel Corporation
                                                                B. Aboba
                                                   Microsoft Corporation
                                                              March 2005


      Extensible Authentication Protocol (EAP) Method Requirements
                           for Wireless LANs

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The IEEE 802.11i MAC Security Enhancements Amendment makes use of
   IEEE 802.1X, which in turn relies on the Extensible Authentication
   Protocol (EAP).  This document defines requirements for EAP methods
   used in IEEE 802.11 wireless LAN deployments.  The material in this
   document has been approved by IEEE 802.11 and is being presented as
   an IETF RFC for informational purposes.

Table of Contents

   1.  Introduction .................................................  2
       1.1.  Requirements Specification .............................  2
       1.2.  Terminology ............................................  2
   2.  Method Requirements ..........................................  3
       2.1.  Credential Types .......................................  3
       2.2.  Mandatory Requirements .................................  4
       2.3.  Recommended Requirements ...............................  5
       2.4.  Optional Features ......................................  5
       2.5.  Non-compliant EAP Authentication Methods ...............  5
   3.  Security Considerations ......................................  6
   4.  References ...................................................  8
   Acknowledgments ..................................................  9
   Authors' Addresses ............................................... 10
   Full Copyright Statement ......................................... 11

1.  Introduction

   The IEEE 802.11i MAC Security Enhancements Amendment [IEEE802.11i]
   makes use of IEEE 802.1X [IEEE802.1X], which in turn relies on the
   Extensible Authentication Protocol (EAP), defined in [RFC3748].

   Today, deployments of IEEE 802.11 wireless LANs are based on EAP and
   use several EAP methods, including EAP-TLS [RFC2716], EAP-TTLS
   [TTLS], PEAP [PEAP], and EAP-SIM [EAPSIM].  These methods support
   authentication credentials that include digital certificates, user-
   names and passwords, secure tokens, and SIM secrets.

   This document defines requirements for EAP methods used in IEEE
   802.11 wireless LAN deployments.  EAP methods claiming conformance to
   the IEEE 802.11 EAP method requirements for wireless LANs must
   complete IETF last call review.

1.1.  Requirements Specification

   In this document, several words are used to signify the requirements
   of the specification.  The key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   An EAP authentication method is not compliant with this specification
   if it fails to satisfy one or more of the MUST or MUST NOT
   requirements.  An EAP authentication method that satisfies all the
   MUST, MUST NOT, SHOULD, and SHOULD NOT requirements is said to be
   "unconditionally compliant"; one that satisfies all the MUST and MUST
   NOT requirements but not all the SHOULD or SHOULD NOT requirements is
   said to be "conditionally compliant".

1.2.  Terminology

   authenticator
      The end of the link initiating EAP authentication.  The term
      authenticator is used in [IEEE802.1X], and authenticator has the
      same meaning in this document.

   peer
      The end of the link that responds to the authenticator.  In
      [IEEE802.1X], this end is known as the supplicant.

   Supplicant
      The end of the link that responds to the authenticator in
      [IEEE802.1X].

   backend authentication server
      A backend authentication server is an entity that provides an
      authentication service to an authenticator.  When used, this
      server typically executes EAP methods for the authenticator.  This
      terminology is also used in [IEEE802.1X].

   EAP server
      The entity that terminates the EAP authentication method with the
      peer.  In the case where no backend authentication server is used,
      the EAP server is part of the authenticator.  In the case where
      the authenticator operates in pass-through mode, the EAP server is
      located on the backend authentication server.

   Master Session Key (MSK)
      Keying material that is derived between the EAP peer and server
      and exported by the EAP method.  The MSK is at least 64 octets in
      length.  In existing implementations, an AAA server acting as an
      EAP server transports the MSK to the authenticator.

   Extended Master Session Key (EMSK)
      Additional keying material derived between the EAP client and
      server that is exported by the EAP method.  The EMSK is at least
      64 octets in length.  The EMSK is not shared with the
      authenticator or any other third party.  The EMSK is reserved for
      future uses that are not yet defined.

   4-Way Handshake
      A pairwise Authentication and Key Management Protocol (AKMP)
      defined in [IEEE802.11i], which confirms mutual possession of a
      Pairwise Master Key by two parties and distributes a Group Key.

2.  Method Requirements

2.1.  Credential Types

   The IEEE 802.11i MAC Security Enhancements Amendment requires that
   EAP authentication methods be available.  Wireless LAN deployments
   are expected to use different credential types, including digital
   certificates, user-names and passwords, existing secure tokens, and
   mobile network credentials (GSM and UMTS secrets).  Other credential
   types that may be used include public/private key (without
   necessarily requiring certificates) and asymmetric credential support
   (such as password on one side, public/private key on the other).

2.2.  Mandatory Requirements

   EAP authentication methods suitable for use in wireless LAN
   authentication MUST satisfy the following criteria:

   [1]  Generation of symmetric keying material.  This corresponds to
        the "Key derivation" security claim defined in [RFC3748],
        Section 7.2.1.

   [2]  Key strength.  An EAP method suitable for use with IEEE 802.11
        MUST be capable of generating keying material with 128-bits of
        effective key strength, as defined in [RFC3748], Section 7.2.1.
        As noted in [RFC3748], Section 7.10, an EAP method supporting
        key derivation MUST export a Master Session Key (MSK) of at
        least 64 octets, and an Extended Master Session Key (EMSK) of at
        least 64 octets.

   [3]  Mutual authentication support.  This corresponds to the "Mutual
        authentication" security claim defined in [RFC3748], Section
        7.2.1.

   [4]  Shared state equivalence.  The shared EAP method state of the
        EAP peer and server must be equivalent when the EAP method is
        successfully completed on both sides.  This includes the
        internal state of the authentication protocol but not the state
        external to the EAP method, such as the negotiation occurring
        prior to initiation of the EAP method.  The exact state
        attributes that are shared may vary from method to method, but
        typically include the method version number, the credentials
        presented and accepted by both parties, the cryptographic keys
        shared, and the EAP method specific attributes negotiated, such
        as ciphersuites and limitations of usage on all protocol state.
        Both parties must be able to distinguish this instance of the
        protocol from all other instances of the protocol, and they must
        share the same view regarding which state attributes are public
        and which are private to the two parties alone.  The server must
        obtain the authenticated peer name, and the peer must obtain the
        authenticated server name (if the authenticated server name is
        available).

   [5]  Resistance to dictionary attacks.  This corresponds to the
        "Dictionary attack resistance" security claim defined in
        [RFC3748], Section 7.2.1.

   [6]  Protection against man-in-the-middle attacks.  This corresponds
        to the "Cryptographic binding", "Integrity protection", "Replay
        protection", and "Session independence" security claims defined
        in [RFC3748], Section 7.2.1.

   [7]  Protected ciphersuite negotiation.  If the method negotiates the
        ciphersuite used to protect the EAP conversation, then it MUST
        support the "Protected ciphersuite negotiation" security claim
        defined in [RFC3748], Section 7.2.1.

2.3.  Recommended Requirements

   EAP authentication methods used for wireless LAN authentication
   SHOULD support the following features:

   [8]  Fragmentation.  This implies support for the "Fragmentation"
        claim defined in [RFC3748], Section 7.2.1.  [RFC3748], Section
        3.1 states:  "EAP methods can assume a minimum EAP MTU of 1020
        octets, in the absence of other information.  EAP methods SHOULD
        include support for fragmentation and reassembly if their
        payloads can be larger than this minimum EAP MTU."

   [9]  End-user identity hiding.  This corresponds to the
        "Confidentiality" security claim defined in [RFC3748], Section
        7.2.1.

2.4.  Optional Features

   EAP authentication methods used for wireless LAN authentication MAY
   support the following features:

   [10] Channel binding.  This corresponds to the "Channel binding"
        security claim defined in [RFC3748], Section 7.2.1.

   [11] Fast reconnect.  This corresponds to the "Fast reconnect"
        security claim defined in [RFC3748], Section 7.2.1.

2.5.  Non-compliant EAP Authentication Methods

   EAP-MD5-Challenge (the current mandatory-to-implement EAP
   authentication method), is defined in [RFC3748], Section 5.4.  As
   defined in [RFC3748], EAP-MD5-Challenge, One-Time Password (Section
   5.5), and Generic Token Card (Section 5.6) are non-compliant with the
   requirements specified in this document.  As noted in [RFC3748],
   these methods do not support any of the mandatory requirements
   defined in Section 2.2, including key derivation and mutual
   authentication.  In addition, these methods do not support any of the
   recommended features defined in Section 2.3 or any of the optional
   features defined in Section 2.4.

3.  Security Considerations

   Within [IEEE802.11i], EAP is used for both authentication and key
   exchange between the EAP peer and server.  Given that wireless local
   area networks provide ready access to an attacker within range, EAP
   usage within [IEEE802.11i] is subject to the threats outlined in
   [RFC3748], Section 7.1.  Security considerations relating to EAP are
   discussed in [RFC3748], Sections 7; where an authentication server is
   utilized, the security considerations described in [RFC3579], Section
   4, will apply.

   The system security properties required to address the threats
   described in [RFC3748], Section 7.1, are noted in [Housley56].  In
   the material below, the requirements articulated in [Housley56] are
   listed, along with the corresponding recommendations.

   Algorithm independence
      Requirement: "Wherever cryptographic algorithms are chosen, the
      algorithms must be negotiable, in order to provide resilience
      against compromise of a particular cryptographic algorithm."

      This issue is addressed by mandatory requirement [7] in Section
      2.2.  Algorithm independence is one of the EAP invariants
      described in [KEYFRAME].

   Strong, fresh session keys
      Requirement: "Session keys must be demonstrated to be strong and
      fresh in all circumstances, while at the same time retaining
      algorithm independence."

      Key strength is addressed by mandatory requirement [2] in Section
      2.2.  Recommendations for ensuring the Freshness of keys derived
      by EAP methods are discussed in [RFC3748], Section 7.10.

   Replay protection
      Requirement: "All protocol exchanges must be replay protected."

      This is addressed by mandatory requirement [6] in Section 2.2.

   Authentication
      Requirements: "All parties need to be authenticated.  The
      confidentiality of the authenticator must be maintained.  No
      plaintext passwords are allowed."

      Mutual authentication is required as part of mandatory requirement
      [3] in Section 2.2.  Identity protection is a recommended
      capability, described in requirement [9] in Section 2.3.  EAP does
      not support plaintext passwords, as noted in [RFC3748], Section
      7.14.

   Authorization
      Requirement: "EAP peer and authenticator authorization must be
      performed."

      Authorization issues are discussed in [RFC3748], Sections 1.2 and
      7.16.  Authentication, Authorization, and Accounting (AAA)
      protocols such as RADIUS [RFC2865][RFC3579] may be used to enable
      authorization of EAP peers by a central authority.  AAA
      authorization issues are discussed in [RFC3579], Sections 2.6.3
      and 4.3.7.

   Session keys
      Requirement: "Confidentiality of session keys must be maintained."

      Issues relating to Key Derivation are described in [RFC3748],
      Section 7.10, as well as in [KEYFRAME].

   Ciphersuite negotiation
      Requirement: "The selection of the "best" ciphersuite must be
      securely confirmed."

      This is addressed in mandatory requirement [7] in Section 2.2.

   Unique naming
      Requirement: "Session keys must be uniquely named."

      Key naming issues are addressed in [KEYFRAME].

   Domino effect
      Requirement: "Compromise of a single authenticator cannot
      compromise any other part of the system, including session keys
      and long-term secrets."

      This issue is addressed by mandatory requirement [6] in Section
      2.2.

   Key binding
      Requirement: "The key must be bound to the appropriate context."

      This issue is addressed in optional requirement [10] in Section
      2.4.  Channel binding is also discussed in Section 7.15 of
      [RFC3748] and Section 4.3.7 of [RFC3579].

4.  References

4.1.  Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2865]     Rigney, C., Willens, S., Rubens, A., and W. Simpson,
                 "Remote Authentication Dial In User Service (RADIUS)",
                 RFC 2865, June 2000.

   [RFC3579]     Aboba, B. and P. Calhoun, "RADIUS (Remote
                 Authentication Dial In User Service) Support For
                 Extensible Authentication Protocol (EAP)", RFC 3579,
                 September 2003.

   [RFC3748]     Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
                 H. Levkowetz, "Extensible Authentication Protocol
                 (EAP)", RFC 3748, June 2004.

   [802.11]      Information technology - Telecommunications and
                 information exchange between systems - Local and
                 metropolitan area networks - Specific Requirements Part
                 11:  Wireless LAN Medium Access Control (MAC) and
                 Physical Layer (PHY) Specifications, IEEE Std. 802.11-
                 2003, 2003.

   [IEEE802.1X]  IEEE Standards for Local and Metropolitan Area
                 Networks: Port based Network Access Control, IEEE Std
                 802.1X-2004,  December 2004.

   [IEEE802.11i] Institute of Electrical and Electronics Engineers,
                 "Supplement to Standard for Telecommunications and
                 Information Exchange Between Systems - LAN/MAN Specific
                 Requirements - Part 11:  Wireless LAN Medium Access
                 Control (MAC) and Physical Layer (PHY) Specifications:
                 Specification for Enhanced Security", IEEE 802.11i,
                 July 2004.

4.2.  Informative References

   [Housley56]   Housley, R., "Key Management in AAA", Presentation to
                 the AAA WG at IETF 56,
                 http://www.ietf.org/proceedings/03mar/slides/aaa-
                 5/index.html, March 2003.

   [RFC2716]     Aboba, B. and D. Simon, "PPP EAP TLS Authentication
                 Protocol", RFC 2716, October 1999.

   [PEAP]        Palekar, A., et al., "Protected EAP Protocol (PEAP)",
                 Work in Progress, July 2004.

   [TTLS]        Funk, P. and S. Blake-Wilson, "EAP Tunneled TLS
                 Authentication Protocol (EAP-TTLS)", Work in Progress,
                 August 2004.

   [EAPSIM]      Haverinen, H. and J. Salowey, "EAP SIM Authentication",
                 Work in Progress, April 2004.

   [KEYFRAME]    Aboba, B., et al., "EAP Key Management Framework", Work
                 in Progress, July 2004.

Acknowledgements

   The authors would like to acknowledge contributions to this document
   from members of the IEEE 802.11i Task Group, including Russ Housley
   of Vigil Security, David Nelson of Enterasys Networks and Clint
   Chaplin of Symbol Technologies, as well as members of the EAP WG
   including Joe Salowey of Cisco Systems, Pasi Eronen of Nokia, Jari
   Arkko of Ericsson, and Florent Bersani of France Telecom.

Authors' Addresses

   Dorothy Stanley
   Agere Systems
   2000 North Naperville Rd.
   Naperville, IL 60566

   Phone: +1 630 979 1572
   EMail: dstanley@agere.com


   Jesse R. Walker
   Intel Corporation
   2111 N.E. 25th Avenue
   Hillsboro, OR  97214

   EMail: jesse.walker@intel.com


   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 818 4011
   Fax:   +1 425 936 7329
   EMail: bernarda@microsoft.com

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