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 © 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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