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
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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].
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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).
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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.
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[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.
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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."
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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].
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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.
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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.
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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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Full Copyright Statement
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