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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