EAP Working Group                                          Bernard Aboba
INTERNET-DRAFT                                                 Dan Simon
Category: Standards Track                                      Microsoft
<draft-ietf-eap-keying-06.txt>                                  J. Arkko
1 April 2005                                                    Ericsson
                                                               P. Eronen
                                                                   Nokia
                                                       H. Levkowetz, Ed.
                                                             ipUnplugged



   Extensible Authentication Protocol (EAP) Key Management Framework

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on November 22, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.

Abstract

   The Extensible Authentication Protocol (EAP), defined in [RFC3748],
   enables extensible network access authentication.  This document
   provides a framework for the generation, transport and usage of
   keying material generated by EAP authentication algorithms, known as
   "methods".  It also specifies the EAP key hierarchy.



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Table of Contents

   1.     Introduction ..........................................    4
      1.1       Requirements Language ...........................    4
      1.2       Terminology .....................................    4
      1.3       Overview ........................................    5
      1.4       EAP Invariants ..................................   11
   2.     Key Derivation ........................................   13
      2.1       Key Terminology .................................   13
      2.2       Key Hierarchy ...................................   15
      2.3       AAA-Key Derivation ..............................   19
      2.4       Key Naming ......................................   20
   3.     Security associations .................................   22
      3.1       EAP Method SA ...................................   23
      3.2       EAP-Key SA ......................................   24
      3.3       AAA SA(s) .......................................   24
      3.4       Service SA(s) ...................................   24
   4.     Key Management ........................................   27
      4.1       Key Caching .....................................   28
      4.2       Parent-Child Relationships ......................   29
      4.3       Local Key Lifetimes .............................   29
      4.4       Exported and Calculated Key Lifetimes ...........   30
      4.5       Key Cache Synchronization .......................   31
      4.6       Key Scope .......................................   32
      4.7       Key Strength ....................................   33
      4.8       Key Wrap ........................................   34
   5.     Handoff Vulnerabilities ...............................   35
      5.1       Authorization ...................................   35
      5.2       Correctness .....................................   36
   6.     Security Considerations  ..............................   39
      6.1       Security Terminology ............................   39
      6.2       Threat Model ....................................   39
      6.3       Security Analysis ...............................   41
      6.4       Man-in-the-middle Attacks .......................   44
      6.5       Denial of Service Attacks .......................   45
      6.6       Impersonation ...................................   45
      6.7       Channel Binding .................................   46














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   7.     Security Requirements .................................   47
      7.1       EAP Method Requirements .........................   47
      7.2       AAA Protocol Requirements .......................   50
      7.3       Secure Association Protocol Requirements ........   51
      7.4       Ciphersuite Requirements ........................   53
   8.     IANA Considerations ...................................   54
   9.     References ............................................   54
      9.1       Normative References ............................   54
      9.2       Informative References ..........................   54
   Acknowledgments ..............................................   58
   Author's Addresses ...........................................   58
   Appendix A - Ciphersuite Keying Requirements .................   60
   Appendix B - Example Transient EAP Key (TEK) Hierarchy .......   61
   Appendix C - EAP-TLS Key Hierarchy ...........................   62
   Appendix D - Example Transient Session Key (TSK) Derivation ..   64
   Appendix E - Key Names and Scope in Existing Methods .........   65
   Appendix F - Security Association Examples ...................   66
   Intellectual Property Statement ..............................   69
   Disclaimer of Validity .......................................   70
   Copyright Statement ..........................................   70































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1.  Introduction

   The Extensible Authentication Protocol (EAP), defined in [RFC3748],
   was designed to enable extensible authentication for network access
   in situations in which the IP protocol is not available.  Originally
   developed for use with PPP [RFC1661], it has subsequently also been
   applied to IEEE 802 wired networks [IEEE-802.1X].

   This document provides a framework for the generation, transport and
   usage of keying material generated by EAP authentication algorithms,
   known as "methods".  In EAP keying material is generated by EAP
   methods.  Part of this keying material may be used by EAP methods
   themselves and part of this material may be exported.  The exported
   keying material may be transported by AAA protocols or transformed by
   Secure Association Protocols into session keys which are used by
   lower layer ciphersuites.  This document describes each of these
   elements and provides a system-level security analysis.  It also
   specifies the EAP key hierarchy.

1.1.  Requirements Language

   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 BCP 14 [RFC2119].

1.2.  Terminology

   This document frequently uses the following terms:

authenticator
     The end of the link initiating EAP authentication.  The term
     Authenticator is used in [IEEE-802.1X], and authenticator has the
     same meaning in this document.

peer The end of the link that responds to the authenticator.  In
     [IEEE-802.1X], this end is known as the Supplicant.

Supplicant
     The end of the link that responds to the authenticator in
     [IEEE-802.1X].  In this document, this end of the link is called
     the peer.

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 [IEEE-802.1X].




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AAA  Authentication, Authorization and Accounting.  AAA protocols with
     EAP support include RADIUS [RFC3579] and Diameter [I-D.ietf-aaa-
     eap].  In this document, the terms "AAA server" and "backend
     authentication server" are used interchangeably.

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.

security association
     A set of policies and cryptographic state used to protect
     information.  Elements of a security association may include
     cryptographic keys, negotiated ciphersuites and other parameters,
     counters, sequence spaces, authorization attributes, etc.

1.3.  Overview

   EAP is typically deployed in order to support extensible network
   access authentication in situations where a peer desires network
   access via one or more authenticators.  Since both the peer and
   authenticator may have more than one physical or logical port, a
   given peer may simultaneously access the network via multiple
   authenticators, or via multiple physical or logical ports on a given
   authenticator.  Similarly, an authenticator may offer network access
   to multiple peers, each via a separate physical or logical port.  The
   situation is illustrated in Figure 1.

   Where authenticators are deployed standalone, the EAP conversation
   occurs between the peer and authenticator, and the authenticator must
   locally implement an EAP method acceptable to the peer.  However, one
   of the advantages of EAP is that it enables deployment of new
   authentication methods without requiring development of new code on
   the authenticator.  While the authenticator may implement some EAP
   methods locally and use those methods to authenticate local users, it
   may at the same time act as a pass-through for other users and
   methods, forwarding EAP packets back and forth between the backend
   authentication server and the peer.

   This is accomplished by encapsulating EAP packets within the
   Authentication, Authorization and Accounting (AAA) protocol, spoken
   between the authenticator and backend authentication server.  AAA
   protocols supporting EAP include RADIUS [RFC3579] and Diameter [I-
   D.ietf-aaa-eap].





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                            +-+-+-+-+
                            |       |
                            | EAP   |
                            | Peer  |
                            |       |
                            +-+-+-+-+
                              | | |  Peer Ports
                             /  |  \
                            /   |   \
                           /    |    \
                          /     |     \
                         /      |      \
                        /       |       \
                       /        |        \
                      /         |         \
                   | | |      | | |      | | |  Authenticator Ports
                 +-+-+-+-+  +-+-+-+-+  +-+-+-+-+
                 |       |  |       |  |       |
                 | Auth. |  | Auth. |  | Auth. |
                 |       |  |       |  |       |
                 +-+-+-+-+  +-+-+-+-+  +-+-+-+-+
                      \         |         /
                       \        |        /
                        \       |       /
          EAP over AAA   \      |      /
            (optional)    \     |     /
                           \    |    /
                            \   |   /
                             \  |  /
                            +-+-+-+-+
                            |       |
                            | AAA   |
                            |Server |
                            |       |
                            +-+-+-+-+

Figure 1:  Relationship between peer, authenticator and backend server

   Where EAP key derivation is supported, the conversation between the
   peer and the authenticator typically takes place in three phases:

      Phase 0: Discovery
      Phase 1: Authentication
               1a: EAP authentication
               1b: AAA-Key Transport (optional)
      Phase 2: Secure Association Establishment
               2a: Unicast Secure Association
               2b: Multicast Secure Association (optional)



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   In the discovery phase (phase 0),  peers locate authenticators and
   discover their capabilities.  For example, a peer may locate an
   authenticator providing access to a particular network, or a peer may
   locate an authenticator behind a bridge with which it desires to
   establish a Secure Association.

   The authentication phase (phase 1) may begin once the peer and
   authenticator discover each other.  This phase always includes EAP
   authentication (phase 1a).  Where the chosen EAP method supports key
   derivation, in phase 1a keying material is derived on both the peer
   and the EAP server.  This keying material may be used for multiple
   purposes, including protection of the EAP conversation and subsequent
   data exchanges.

   An additional step (phase 1b) is required in deployments which
   include a backend authentication server, in order to transport keying
   material (known as the AAA-Key) from the backend authentication
   server to the authenticator.

   A Secure Association exchange (phase 2) then occurs between the peer
   and authenticator in order to manage the creation and deletion of
   unicast (phase 2a) and multicast (phase 2b) security associations
   between the peer and authenticator.

   The conversation phases and relationship between the parties is shown
   in Figure 2.

   EAP peer                   Authenticator               Auth. Server
   --------                   -------------               ------------
    |<----------------------------->|                               |
    |     Discovery (phase 0)       |                               |
    |<----------------------------->|<----------------------------->|
    |   EAP auth (phase 1a)         |  AAA pass-through (optional)  |
    |                               |                               |
    |                               |<----------------------------->|
    |                               |       AAA-Key transport       |
    |                               |      (optional; phase 1b)     |
    |<----------------------------->|                               |
    |  Unicast Secure association   |                               |
    |          (phase 2a)           |                               |
    |                               |                               |
    |<----------------------------->|                               |
    | Multicast Secure association  |                               |
    |     (optional; phase 2b)      |                               |
    |                               |                               |

                  Figure 2: Conversation Overview




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1.3.1.  Discovery Phase

   In the discovery phase (phase 0), the EAP peer and authenticator
   locate each other and discover each other's capabilities. Discovery
   can occur manually or automatically, depending on the lower layer
   over which EAP runs.  Since authenticator discovery is handled
   outside of EAP, there is no need to provide this functionality within
   EAP.

   For example, where EAP runs over PPP, the EAP peer might be
   configured with a phone book providing phone numbers of
   authenticators and associated capabilities such as supported rates,
   authentication protocols or ciphersuites.  In contrast, PPPoE
   [RFC2516] provides support for a Discovery Stage to allow a peer to
   identify the Ethernet MAC address of one or more authenticators and
   establish a PPPoE SESSION_ID.

   IEEE 802.11 [IEEE-802.11] also provides integrated discovery support
   utilizing Beacon and/or Probe Request/Response frames, allowing the
   peer (known as the station or STA) to determine the MAC address and
   capabilities of one or more authenticators (known as Access Point or
   APs).

1.3.2.  Authentication Phase

   Once the peer and authenticator discover each other, they exchange
   EAP packets.  Typically, the peer desires access to the network, and
   the authenticators provide that access.  In such a situation, access
   to the network can be provided by any authenticator attaching to the
   desired network, and the EAP peer is typically willing to send data
   traffic through any authenticator that can demonstrate that it is
   authorized to provide access to the desired network.

   An EAP authenticator may handle the authentication locally, or it may
   act as a pass-through to a backend authentication server.  In the
   latter case the EAP exchange occurs between the EAP peer and a
   backend authenticator server, with the authenticator forwarding EAP
   packets between the two. The entity which terminates EAP
   authentication with the peer is known as the EAP server.  Where pass-
   through is supported, the backend authentication server functions as
   the EAP server; where authentication occurs locally, the EAP server
   is the authenticator.  Where a backend authentication server is
   present, at the successful completion of an authentication exchange,
   the AAA-Key is transported to the authenticator (phase 1b).

   EAP may also be used when it is desired for two network devices (e.g.
   two switches or routers) to authenticate each other, or where two
   peers desire to authenticate each other and set up a secure



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   association suitable for protecting data traffic.

   Some EAP methods exist which only support one-way authentication;
   however, EAP methods deriving keys are required to support mutual
   authentication.  In either case, it can be assumed that the parties
   do not utilize the link to exchange data traffic unless their
   authentication requirements have been met.  For example, a peer
   completing mutual authentication with an EAP server will not send
   data traffic over the link until the EAP server has authenticated
   successfully to the peer, and a Secure Association has been
   negotiated.

   Since EAP is a peer-to-peer protocol, an independent and simultaneous
   authentication may take place in the reverse direction.  Both peers
   may act as authenticators and authenticatees at the same time.

   Successful completion of EAP authentication and key derivation by a
   peer and EAP server does not necessarily imply that the peer is
   committed to joining the network associated with an EAP server.
   Rather, this commitment is implied by the creation of a security
   association between the EAP peer and authenticator, as part of the
   Secure Association Protocol (phase 2).  As a result, EAP may be used
   for "pre-authentication" in situations where it is necessary to pre-
   establish EAP security associations in order to decrease handoff or
   roaming latency.

1.3.3.  Secure Association Phase

   The Secure Association phase (phase 2), if it occurs, begins after
   the completion of EAP authentication (phase 1a) and key transport
   (phase 1b).  A Secure Association Protocol used with EAP typically
   supports the following features:

[1]  Generation of fresh transient session keys (TSKs).  Where AAA-Key
     caching is supported, the EAP peer may initiate a new session using
     a AAA-Key that was used in a previous session.  Were the TSKs to be
     derived from a portion of the AAA-Key,  this would result in reuse
     of the session keys which could expose the underlying ciphersuite
     to attack.

     As a result, where AAA-Key caching is supported, the Secure
     Association Protocol phase is REQUIRED, and MUST provide for
     freshness of the TSKs.  This is typically handled via the exchange
     of nonces or counters, which are then mixed with the AAA-Key in
     order to generate  fresh unicast (phase 2a) and possibly multicast
     (phase 2b) session keys.  By not using the AAA-Key directly to
     protect data, the Secure Association Protocol protects against
     compromise of the AAA-Key.



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[2]  Entity Naming.  A basic feature of a Secure Association Protocol is
     the explicit naming of the parties engaged in the exchange.
     Explicit identification of the parties is critical, since without
     this the parties engaged in the exchange are not identified and the
     scope of the transient session keys (TSKs) generated during the
     exchange is undefined.  As illustrated in Figure 1, both the peer
     and NAS may have more than one physical or virtual port, so that
     port identifiers are NOT RECOMMENDED as a naming mechanism.

[3]  Secure capabilities negotiation.  This includes the secure
     negotiation of usage modes, session parameters (such as key
     lifetimes), ciphersuites and required filters, including
     confirmation of the capabilities discovered during phase 0.  It is
     RECOMMENDED that the Secure Association Protocol support secure
     capabilities negotiation, in order to protect against spoofing
     during the discovery phase, and to ensure agreement between the
     peer and authenticator about how data is to be secured.

[4]  Key management. EAP as defined in [RFC3748] supports key
     derivation, but not key management.  While EAP methods may derive
     keying material, EAP does provide for the management of exported or
     derived keys.  For example, EAP does not support negotiation of the
     key lifetime of exported or derived keys, nor does it support
     rekey.  Although EAP methods may support "fast reconnect" as
     defined in [RFC3748] Section 7.2.1, rekey of exported keys cannot
     occur without reauthentication.  In order to provide method
     independence, key management of exported or derived keys SHOULD NOT
     be provided within EAP methods.

     Since neither EAP nor EAP methods provide key management support,
     it is RECOMMENDED that key management facilities be provided within
     the Secure Association Protocol.  This includes key lifetime
     management (such as via explicit key lifetime negotiation, or
     seamless rekey), as well synchronization of the installation and
     deletion of keys so as to enable recovery from partial or complete
     loss of key state by the peer or authenticator.  Since key
     management requires a key naming scheme, Secure Association
     Protocols supporting key management support MUST also support key
     naming.

[5]  Mutual proof of possession of the AAA-Key.  The Secure Association
     Protocol MUST demonstrate mutual proof of posession of the AAA-Key,
     in order to show that both the peer and authenticator have been
     authenticated and authorized by the backend authentication server.
     Since mutual proof of possession is not the same as mutual
     authentication, the peer cannot verify authenticator assertions
     (including the authenticator identity) as a result of this
     exchange.



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1.4.  EAP Invariants

   Certain basic characteristics, known as the "EAP Invariants" hold
   true for EAP implementations on all media:

      Media independence
      Method independence
      Ciphersuite independence

1.4.1.  Media Independence

   One of the goals of EAP is to allow EAP methods to function on any
   lower layer meeting the criteria outlined in [RFC3748], Section 3.1.
   For example, as described in [RFC3748], EAP authentication can be run
   over PPP [RFC1661],  IEEE 802 wired networks [IEEE-802.1X], and IEEE
   802.11 wireless LANs [IEEE-802.11i].

   In order to maintain media independence, it is necessary for EAP to
   avoid inclusion of media-specific elements.  For example, EAP methods
   cannot be assumed to have knowledge of the lower layer over which
   they are transported, and cannot utilize identifiers associated with
   a particular usage environment (e.g. MAC addresses).

   The need for media independence has also motivated the development of
   the three phase exchange.  Since discovery is typically media-
   specific, this function is handled outside of EAP, rather than being
   incorporated within it.  Similarly, the Secure Association Protocol
   often contains media dependencies such as negotiation of media-
   specific ciphersuites or session parameters, and as a result this
   functionality also cannot be incorporated within EAP.

   Note that media independence may be retained within EAP methods that
   support channel binding or method-specific identification.  An EAP
   method need not be aware of the content of an identifier in order to
   use it.  This enables an EAP method to use media-specific identifiers
   such as MAC addresses without compromising media independence.  To
   support channel binding, an EAP method can pass binding parameters to
   the AAA server in the form of an opaque blob, and receive
   confirmation of whether the parameters match, without requiring
   media-specific knowledge.

1.4.2.  Method Independence

   By enabling pass-through, authenticators can support any method
   implemented on the peer and server, not just locally implemented
   methods.  This allows the authenticator to avoid implementing code
   for each EAP method required by peers.  In fact, since a pass-through
   authenticator is not required to implement any EAP methods at all, it



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   cannot be assumed to support any EAP method-specific code.

   As a result, as noted in [RFC3748], authenticators must by default be
   capable of supporting any EAP method.  Since the Discovery and Secure
   Association exchanges are also method independent, an authenticator
   can carry out the three phase exchange without having an EAP method
   in common with the peer.

   This is useful where there is no single EAP method that is both
   mandatory-to-implement and offers acceptable security for the media
   in use.  For example, the [RFC3748] mandatory-to-implement EAP method
   (MD5-Challenge) does not provide dictionary attack resistance, mutual
   authentication or key derivation, and as a result is not appropriate
   for use in wireless LAN authentication [RFC4017].  However, despite
   this it is possible for the peer and authenticator to interoperate as
   long as a suitable EAP method is supported on the EAP server.

1.4.3.  Ciphersuite Independence

   While EAP methods may negotiate the ciphersuite used in protection of
   the EAP conversation, the ciphersuite used for the protection of the
   data exchanged after EAP authentication has completed is negotiated
   between the peer and authenticator out-of-band of EAP.  Since
   ciphersuite negotiation is assumed to occur out-of-band, there is no
   need for ciphersuite negotiation within EAP.  Since ciphersuite
   negotiation occurs outside of EAP, EAP methods generate keying
   material that is ciphersuite-independent.

   For example, within PPP, the ciphersuite is negotiated within the
   Encryption Control Protocol (ECP) defined in [RFC1968], after EAP
   authentication is completed.  Within [IEEE-802.11i], the AP
   ciphersuites are advertised in the Beacon and Probe Responses prior
   to EAP authentication, and are securely verified during a 4-way
   handshake exchange after EAP authentication has completed.

   Advantages of ciphersuite-independence include:

Reduced update requirements
     If EAP methods were to specify how to derive transient session keys
     for each ciphersuite, they would need to be updated each time a new
     ciphersuite is developed.  In addition, backend authentication
     servers might not be usable with all EAP-capable authenticators,
     since the backend authentication server would also need to be
     updated each time support for a new ciphersuite is added to the
     authenticator.

Reduced EAP method complexity
     Requiring each EAP method to include ciphersuite-specific code for



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     transient session key derivation would increase method complexity
     and result in duplicated effort.

Simplified configuration
     The ciphersuite is negotiated between the peer and authenticator
     out-of-band of EAP.  The backend authentication server is neither a
     party to this negotiation, nor is it an intermediary in the data
     flow between the EAP peer and authenticator.  The backend
     authentication server may not have knowledge of the ciphersuites
     and negotiation policies implemented by the peer and authenticator,
     or be aware of the ciphersuite negotiated between them.  This
     simplifies the configuration of the backend authentication server.
     For example, since ECP negotiation occurs after authentication,
     when run over PPP, the EAP peer, authenticator and backend
     authentication server may not anticipate the negotiated ciphersuite
     and therefore this information cannot be provided to the EAP
     method.

2.  Key Derivation

2.1.  Key Terminology

   The EAP Key Hierarchy makes use of the following types of keys:

Long Term Credential
     EAP methods frequently make use of long term secrets in order to
     enable authentication between the peer and server.  In the case of
     a method based on pre-shared key authentication, the long term
     credential is the pre-shared key.  In the case of a public-key
     based method, the long term credential is the corresponding private
     key.

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.

Extended Master Session Key (EMSK)
     Additional keying material derived between the peer and server that
     is exported by the EAP method.  The EMSK is at least 64 octets in
     length, and is never shared with a third party.

AAA-Key
     A key derived by the peer and EAP server, used by the peer and
     authenticator in the derivation of Transient Session Keys (TSKs).
     Where a backend authentication server is present, the AAA-Key is
     transported from the backend authentication server to the
     authenticator, wrapped within the AAA-Token; it is therefore known



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     by the peer, authenticator and backend authentication server.
     Despite the name, the AAA-Key is computed regardless of whether a
     backend authentication server is present.  AAA-Key derivation is
     discussed in Section 2.3; in existing implementations the MSK is
     used as the AAA-Key.

AAA-Token
     Where a backend server is present, the AAA-Key and one or more
     attributes is transported between the backend authentication server
     and the authenticator within a package known as the AAA-Token.  The
     format and wrapping of the AAA-Token, which is intended to be
     accessible only to the backend authentication server and
     authenticator, is defined by the AAA protocol.  Examples include
     RADIUS [RFC2548] and Diameter [I-D.ietf-aaa-eap].

Initialization Vector (IV)
     A quantity of at least 64 octets, suitable for use in an
     initialization vector field, that is derived between the peer and
     EAP server.  Since the IV is a known value in methods such as EAP-
     TLS [RFC2716], it cannot be used by itself for computation of any
     quantity that needs to remain secret.  As a result, its use has
     been deprecated and EAP methods are not required to generate it.
     However, when it is generated it MUST be unpredictable.

Pairwise Master Key (PMK)
     The AAA-Key is divided into two halves, the "Peer to Authenticator
     Encryption Key" (Enc-RECV-Key) and "Authenticator to Peer
     Encryption Key" (Enc-SEND-Key) (reception is defined from the point
     of view of the authenticator).  Within [IEEE-802.11i] Octets 0-31
     of the AAA-Key (Enc-RECV-Key) are known as the Pairwise Master Key
     (PMK).  In [IEEE-802.11i] the TKIP and AES CCMP ciphersuites derive
     their Transient Session Keys (TSKs) solely from the PMK, whereas
     the WEP ciphersuite as noted in [RFC3580], derives its TSKs from
     both halves of the AAA-Key.

Transient EAP Keys (TEKs)
     Session keys which are used to establish a protected channel
     between the EAP peer and server during the EAP authentication
     exchange. The TEKs are appropriate for use with the ciphersuite
     negotiated between EAP peer and server for use in protecting the
     EAP conversation.  Note that the ciphersuite used to set up the
     protected channel between the EAP peer and server during EAP
     authentication is unrelated to the ciphersuite used to subsequently
     protect data sent between the EAP peer and authenticator. An
     example TEK key hierarchy is described in Appendix C.

Transient Session Keys (TSKs)
     Session keys used to protect data exchanged between the peer and



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     the authenticator after the EAP authentication has successfully
     completed.  TSKs are appropriate for the lower layer ciphersuite
     negotiated between the EAP peer and authenticator.  Examples of TSK
     derivation are provided in Appendix D.

2.2.  Key Hierarchy

   The EAP Key Hierarchy, illustrated in Figure 3, has at the root the
   long term credential utilized by the selected EAP method.  If
   authentication is based on a pre-shared key, the parties store the
   EAP method to be used and the pre-shared key.  The EAP server also
   stores the peer's identity and/or other information necessary to
   decide whether access to some service should be granted.  The peer
   stores information necessary to choose which secret to use for which
   service.

   If authentication is based on proof of possession of the private key
   corresponding to the public key contained within a certificate, the
   parties store the EAP method to be used and the trust anchors used to
   validate the certificates.  The EAP server also stores the peer's
   identity and/or other information necessary to decide whether access
   to some service should be granted.  The peer stores information
   necessary to choose which certificate to use for which service.

   Based on the long term credential established between the peer and
   the server, EAP derives two types of keys:

    [1] Keys calculated locally by the EAP method but not exported
        by the EAP method, such as the TEKs.
    [2] Keys exported by the EAP method: MSK, EMSK, IV

   From the keys exported by the EAP method, two other types of keys may
   be derived:

    [3] Keys calculated from exported quantities: AAA-Key.
    [4] Keys calculated by the Secure Association Protocol from the
        AAA-Key: TSKs.

   In order to protect the EAP conversation, methods supporting key
   derivation typically negotiate a ciphersuite and derive Transient EAP
   Keys (TEKs) for use with that ciphersuite.  The TEKs are stored
   locally by the EAP method and are not exported.

   As noted in [RFC3748] Section 7.10, EAP methods generating keys are
   required to calculate and export the MSK and EMSK, which must be at
   least 64 octets in length.  EAP methods also may export the IV;
   however, the use of the IV is deprecated.  On both the peer and EAP
   server, the exported MSK is utilized in order to calculate the AAA-



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   Key, as described in Section 2.3.  Where a backend authentication
   server is present, the AAA-Key is transported from the backend
   authentication server to the authenticator within the AAA-Token,
   using the AAA protocol.

   Once EAP authentication completes and is successful, the peer and
   authenticator obtain the AAA-Key and the Secure Association Protocol
   is run between the peer and authenticator in order to securely
   negotiate the ciphersuite, derive fresh TSKs used to protect data,
   and provide mutual proof of possession of the AAA-Key.

   When the authenticator acts as an endpoint of the EAP conversation
   rather than a pass-through, EAP methods are implemented on the
   authenticator as well as the peer.  If the EAP method negotiated
   between the EAP peer and authenticator supports mutual authentication
   and key derivation, the EAP Master Session Key (MSK) and Extended
   Master Session Key (EMSK) are derived on the EAP peer and
   authenticator and exported by the EAP method.  In this case, the MSK
   and EMSK are known only to the peer and authenticator and no other
   parties.  The TEKs and TSKs also reside solely on the peer and
   authenticator.  This is illustrated in Figure 4.  As demonstrated in
   [I-D.ietf-roamops-cert], in this case it is still possible to support
   roaming between providers, using certificate-based authentication.

   Where a backend authentication server is utilized, the situation is
   illustrated in Figure 5.  Here the authenticator acts as a pass-
   through between the EAP peer and a backend authentication server. In
   this model, the authenticator delegates the access control decision
   to the backend authentication server, which acts as a Key
   Distribution Center (KDC).  In this case, the authenticator
   encapsulates EAP packet with a AAA protocol such as RADIUS [RFC3579]
   or Diameter [I-D.ietf-aaa-eap], and forwards packets to and from the
   backend authentication server, which acts as the EAP server.  Since
   the authenticator acts as a pass-through, EAP methods reside only on
   the peer and EAP server As a result, the TEKs, MSK and EMSK are
   derived on the peer and EAP server.

   On completion of EAP authentication, EAP methods on the peer and EAP
   server export the Master Session Key (MSK) and Extended Master
   Session Key (EMSK).  The peer and EAP server then calculate the AAA-
   Key from the MSK and EMSK, and the backend authentication server
   sends an Access-Accept to the authenticator, providing the AAA-Key
   within a protected package known as the AAA-Token.

   The AAA-Key is then used by the peer and authenticator within the
   Secure Association Protocol to derive Transient Session Keys (TSKs)
   required for the negotiated ciphersuite.  The TSKs are known only to
   the peer and authenticator.



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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         ---+
|                                                         |            ^
|                EAP Method                               |            |
|                                                         |            |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+   |            |
| |                                 |   |             |   |            |
| |       EAP Method Key            |<->| Long-Term   |   |            |
| |         Derivation              |   | Credential  |   |            |
| |                                 |   |             |   |            |
| |                                 |   +-+-+-+-+-+-+-+   |  Local to  |
| |                                 |                     |       EAP  |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                     |     Method |
|   |             |               |                       |            |
|   |             |               |                       |            |
|   |             |               |                       |            |
|   |             |               |                       |            |
|   V             |               |                       |            |
| +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ |            |
| |  TEK      | | MSK       | |EMSK       | |IV         | |            |
| |Derivation | |Derivation | |Derivation | |Derivation | |            |
| +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ |            |
|                 |               |                 |     |            |
|                 |               |                 |     |            V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         ---+
                  |               |                 |                  ^
                  |               |                 |                  |
                  | MSK (64B)     | EMSK (64B)      | IV (64B)         |
                  |               |                 |          Exported|
                  |               |                 |              by  |
                  |               V                 V              EAP v
                  |                                                 ---+
                  | AAA-Key                                Transported |
                  |                                             by AAA |
                  |                                           Protocol |
                  V                                                    V
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    ---+
   |                           |                                       ^
   |     TSK  Derivation       |                           Lower layer |
   |     [AAA-Key Cache]       |                              Specific |
   |                           |                                       V
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    ---+

                          Figure 3: EAP Key Hierarchy








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   +-+-+-+-+-+               +-+-+-+-+-+
   |         |               |         |
   |         |               |         |
   | Cipher- |               | Cipher- |
   | Suite   |               | Suite   |
   |         |               |         |
   +-+-+-+-+-+               +-+-+-+-+-+
       ^                         ^
       |                         |
       |                         |
       |                         |
       V                         V
   +-+-+-+-+-+               +-+-+-+-+-+
   |         |               |         |
   |         |===============|         |
   |         |EAP, TEK Deriv.|Authenti-|
   |         |<------------->| cator   |
   |         |               |         |
   |         | Secure Assoc. |         |
   | peer    |<------------->| (EAP    |
   |         |===============| server) |
   |         | Link layer    |         |
   |         | (PPP,IEEE802) |         |
   |         |               |         |
   |MSK,EMSK |               |MSK,EMSK |
   | (TSKs)  |               | (TSKs)  |
   +-+-+-+-+-+               +-+-+-+-+-+
       ^                         ^
       |                         |
       | MSK, EMSK               | MSK, EMSK
       |                         |
       |                         |
   +-+-+-+-+-+               +-+-+-+-+-+
   |         |               |         |
   |  EAP    |               |  EAP    |
   |  Method |               |  Method |
   |         |               |         |
   | (TEKs)  |               | (TEKs)  |
   |         |               |         |
   +-+-+-+-+-+               +-+-+-+-+-+

   Figure 4:  Relationship between EAP peer and authenticator (acting as
   an EAP server), where no backend authentication server is present.








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   +-+-+-+-+-+               +-+-+-+-+-+
   |         |               |         |
   |         |               |         |
   | Cipher- |               | Cipher- |
   | Suite   |               | Suite   |
   |         |               |         |
   +-+-+-+-+-+               +-+-+-+-+-+
       ^                         ^
       |                         |
       |                         |
       |                         |
       V                         V
   +-+-+-+-+-+               +-+-+-+-+-+        +-+-+-+-+-+
   |         |===============|         |========|         |
   |         |EAP, TEK Deriv.|         |        |         |
   |         |<-------------------------------->| backend |
   |         |               |         |AAA-Key/|         |
   |         | Secure Assoc. |         | Name   |         |
   | peer    |<------------->|Authenti-|<-------|  auth   |
   |         |===============| cator   |========| server  |
   |         |  Link Layer   |         |  AAA   | (EAP    |
   |         | (PPP,IEEE 802)|         |Protocol| server) |
   |         |               |         |        |         |
   |MSK,EMSK |               |  MSK    |        |MSK,EMSK |
   | (TSKs)  |               |  (TSKs) |        |         |
   +-+-+-+-+-+               +-+-+-+-+-+        +-+-+-+-+-+
       ^                                            ^
       |                                            |
       | MSK, EMSK                                  | MSK, EMSK
       |                                            |
       |                                            |
   +-+-+-+-+-+                                  +-+-+-+-+-+
   |         |                                  |         |
   |  EAP    |                                  |  EAP    |
   |  Method |                                  |  Method |
   |         |                                  |         |
   |  (TEKs) |                                  |  (TEKs) |
   |         |                                  |         |
   +-+-+-+-+-+                                  +-+-+-+-+-+


   Figure 5: Pass-through relationship between EAP peer, authenticator
   and backend authentication server.

2.3.  AAA-Key Derivation

   In existing usage, where a AAA-Key is generated as the result of a
   successful EAP authentication with the authenticator, the AAA-Key is



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   based on the MSK:  AAA-Key = MSK(0,63).

2.4.  Key Naming

   Each key created within the EAP key management framework has a name
   (the identifier by which the key can be identified), as well as a
   scope (the parties to whom the key is available).  This section
   describes how keys are named, and the scope within which that name
   applies.

Session-Id

   EAP methods supporting key naming MUST specify a temporally unique
   method identifier known as the EAP Method-Id, which is typically
   constructed from nonces or counters used within the exchange.  Since
   multiple EAP sessions may exist between an EAP peer and EAP server,
   the Method-Id allows MSKs to be differentiated.

   The concatenation of the EAP Type (expressed in ASCII text), ":" and
   the Method-Id (also expressed in ASCII text) is known as the EAP
   Session-Id.  The inclusion of the Type in the EAP Session-Id ensures
   that each EAP method has a distinct name space.

   The EAP Session-Id uniquely identifies the EAP session to the EAP
   peer and server terminating the EAP conversation.  However, suitable
   EAP peer and server names may not always be available.  As described
   in [RFC3748] Section 7.3, the identity provided in the EAP-
   Response/Identity, may be different from the identity authenticated
   by the EAP method, and as a result the EAP-Response/Identity is
   unsuitable for determination of the peer identity.  As a result, the
   Session-Id scope is defined by the EAP peer name (if securely
   exchanged within the method) concatenated with the EAP server name
   (also only if securely exchanged).  Where a peer or server name is
   missing the null string is used.  Since an EAP session is not bound
   to a particular authentication or specific ports on the peer and
   authenticator, the authenticator port or identity are not included in
   the Session-Id scope.

   The EAP Session-Id is exported by the EAP method along with the
   Session-Id scope, if available, and is used to construct names for
   other EAP keys.  Note that the EAP Session-Id and scope are only
   known by the EAP method.  As a result, the format of the EAP Session-
   Id and the definition of the Session-Id scope needs to be specified
   within the method.  Appendix E defines the EAP Session-Id and scope
   provided by existing methods.

MSK Name




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   This key is created between the EAP peer and EAP server, and can be
   referred to using the string "MSK:", concatenated with the EAP
   Session-Id.  As with the EAP Session-Id, the MSK scope is defined by
   the EAP peer name (if securely exchanged within the method) and the
   EAP server name (also only if securely exchanged).  Where a peer or
   server name is missing the null string is used.

EMSK Name

   The EMSK can be referred to using the string "EMSK:", concatenated
   with the EAP Session-Id.

   As with the EAP Session-Id, the EMSK scope is defined by the EAP peer
   name (if securely exchanged within the method) and the EAP server
   name (also only if securely exchanged).  Where a peer or server name
   is missing the null string is used.

AAA-Key Name

   In existing usage, the AAA-Key is always derived from the MSK so can
   be referred to using the MSK name.

   The AAA-Key scope is provided by the concatenation of the EAP peer
   name (if securely provided to the authenticator), and the
   authenticator name (if securely provided to the peer).

   For the purpose of identifying the authenticator to the peer, the
   value of the NAS-Identifier attribute is recommended.  The
   authenticator may include the NAS-Identifier attribute to the AAA
   server in an Access-Request, and the authenticator may provide the
   NAS-Identifier to the EAP peer.  Mechanisms for this include use of
   the EAP-Request/Identity (unsecured) or a lower layer mechanism (such
   as the 802.11 Beacon/Probe Response).  Where the NAS-Identifier is
   provided by the authenticator to the peer a secure mechanism is
   RECOMMENDED.

   For the purpose of identifying the peer to the authenticator, the EAP
   peer identifier provided within the EAP method is recommended.  It
   cannot be assumed that the authenticator is aware of the EAP peer
   name used within the method.  Therefore alternatives mechanisms need
   to be used to provide the EAP peer name to the authenticator.  For
   example, the AAA server may include the EAP peer name in the User-
   Name attribute of the Access-Accept or the peer may provide the
   authenticator with its name via a lower layer mechanism.

   Absent an explicit binding step within the Secure Association
   Protocol, the AAA-Key is not bound to a specific peer or
   authenticator port.  As a result, the peer or authenticator port over



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   which the EAP conversation takes place is not included in the AAA-Key
   scope.

PMK Name

   This document does not specify a naming scheme for the PMK.  The PMK
   is only identified by the AAA-Key from which it is derived.
   Similarly, the PMK scope is the same as the AAA-Key scope.

   Note: IEEE 802.11i names the PMKID for the purposes of being able to
   refer to it in the Secure Association protocol; this naming is based
   on a hash of the PMK itself as well as some other parameters (see
   Section 8.5.1.2 [IEEE-802.11i]).

TEKs

   The TEKs may or may not be named. Their naming is specified in the
   EAP method.  Since the TEKs are only known by the EAP peer and
   server, the TEK scope is the same as the Session-Id scope.

TSKs

   The TSKs are typically named. Their naming is specified in the Secure
   Association (phase 2) protocol, so that the correct set of transient
   session keys can be identified for processing a given packet.  The
   scope of the TSKs is negotiated within the Secure Association
   Protocol.

   TSK creation and deletion operations are typically supported so that
   establishment and re-establishment of TSKs can be synchronized
   between the parties.

   In order to avoid confusion in the case where an EAP peer has more
   than one AAA-Key (phase 1b) applicable to establishment of a phase 2
   security association, the secure Association protocol needs to
   utilize the AAA-Key name so that the appropriate phase 1b keying
   material can be identified for use in the Secure Association Protocol
   exchange.

3.  Security Associations

   During EAP authentication and subsequent exchanges, four types of
   security associations (SAs) are created:

[1]  EAP method SA.  This SA is between the peer and EAP server.  It
     stores state that can be used for "fast reconnect" or other
     functionality in some EAP methods.  Not all EAP methods create such
     an SA.



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[2]  EAP-Key SA.  This is an SA between the peer and EAP server, which
     is used to store the keying material exported by the EAP method.
     Current EAP server implementations do not retain this SA after the
     EAP conversation completes.

[3]  AAA SA(s).  These SAs are between the authenticator and the backend
     authentication server.  They permit the parties to mutually
     authenticate each other and protect the communications between
     them.

[4]  Service SA(s). These SAs are between the peer and authenticator,
     and they are created as a result of phases 1-2 of the conversation
     (see Section 1.3).

   Examples of security associations are provided in Appendix F.

3.1.  EAP Method SA (peer - EAP server)

   An EAP method may store some state on the peer and EAP server even
   after phase 1a has completed.

   Typically, this is used for "fast reconnect": the peer and EAP server
   can confirm that they are still talking to the same party, perhaps
   using fewer round-trips or less computational power. In this case,
   the EAP method SA is essentially a cache for performance
   optimization, and either party may remove the SA from its cache at
   any point.

   An EAP method may also keep state in order to support pseudonym-based
   identity protection. This is typically a cache as well (the
   information can be recreated if the original EAP method SA is lost),
   but may be stored for longer periods of time.

   The EAP method SA is not restricted to a particular service or
   authenticator and is most useful when the peer accesses many
   different authenticators.  An EAP method is responsible for
   specifying how the parties select if an existing EAP method SA should
   be used, and if so, which one.  Where multiple backend authentication
   servers are used, EAP method SAs are not typically synchronized
   between them.

   EAP method implementations should consider the appropriate lifetime
   for the EAP method SA. "Fast reconnect" assumes that the information
   required (primarily the keys in the EAP method SA) hasn't been
   compromised. In case the original authentication was carried out
   using, for instance, a smart card, it may be easier to compromise the
   EAP method SA (stored on the PC, for instance), so typically the EAP
   method SAs have a limited lifetime.



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   Contents:

      o  Implicitly, the EAP method this SA refers to
      o  Internal (non-exported) cryptographic state
      o  EAP method SA name
      o  SA lifetime

3.2.  EAP-Key SA

   This is an SA between the peer and EAP server, which is used to store
   the keying material exported by the EAP method.  Current EAP server
   implementations do not retain this SA after the EAP conversation
   completes.  As a result, all keys exported by the EAP method
   (including the MSK, EMSK and IV) on the AAA server are discarded and
   are not cached.  Calculated keys (such as the AAA-Key) are also
   discarded and not cached.

3.3.  AAA SA(s) (authenticator - backend authentication server)

   In order for the authenticator and backend authentication server to
   authenticate each other, they need to store some information.

   In case the authenticator and backend authentication server are
   colocated, and they communicate using local procedure calls or shared
   memory, this SA need not necessarily contain any information.

3.4.  Service SA(s) (peer - authenticator)

   The service SAs store information about the service being provided.
   These include the Root service SA and derived unicast and multicast
   service SAs.

   The Root service SA is established as the result of the completion of
   EAP authentication (phase 1a) and AAA-Key derivation or transport
   (phase 1b).  It includes:

      o  Service parameters (or at least those parameters
         that are still needed)
      o  On the authenticator, service authorization
         information received from the backend authentication
         server (or necessary parts of it)
      o  On the peer, usually locally configured service
         authorization information.
      o  The AAA-Key, if it can be needed again (to refresh
         and/or resynchronize other keys or for another reason)
      o  AAA-Key lifetime

   Unicast and (optionally) multicast service SAs are derived from the



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   Root service SA, via the Secure Association Protocol.  In order for
   unicast and multicast service SAs and associated TSKs to be
   established, it is not necessary for EAP authentication (phase 1a) to
   be rerun each time.  Instead, the Secure Association Protocol can be
   used to mutually prove possession of the AAA-Key and create
   associated unicast (phase 2a) and multicast (phase 2b) service SAs
   and TSKs, enabling the EAP exchange to be bypassed.  Unicast and
   multicast service SAs include:

      o Service parameters negotiated by the Secure Association Protocol.
      o Endpoint identifiers.
      o Transient Session Keys used to protect the communication.
      o Transient Session Key lifetime.

   One function of the Secure Association Protocol is to bind the the
   unicast and multicast service SAs and TSKs to endpoint identifiers.
   For example, within [IEEE802.11i], the 4-way handshake binds the TSKs
   to the MAC addresses of the endpoints; in [IKEv2], the TSKs are bound
   to the IP addresses of the endpoints and the negotiated SPI.

   It is possible for more than one unicast or multicast service SA to
   be derived from a single Root service SA.  However, a unicast or
   multicast service SA is always descended from only one Root service
   SA.  Unicast or multicast service SAs descended from the same Root
   service SA may utilize the same security parameters (e.g. mode,
   ciphersuite, etc.) or they may utilize different parameters.

   An EAP peer may be able to negotiate multiple service SAs with a
   given authenticator, or may be able to maintain one or more service
   SAs with multiple authenticators, depending on the properties of the
   media.

   Except where explicitly specified by the Secure Association Protocol,
   it should not be assumed that the installation of new service SAs
   implies deletion of old service SAs.  It is possible for multicast
   Root service SAs to between the same EAP peer and authenticator;
   during a re-key of a unicast or multicast service SA it is possible
   for two service SAs to exist during the period between when the new
   service SA and corresponding TSKs are calculated and when they are
   installed.

   Similarly, deletion or creation of a unicast or multicast service SA
   does not necessarily imply deletion or creation of related unicast or
   multicast service SAs, unless specified by the Secure Association
   protocol.  For example, a unicast service SA may be rekeyed without
   implying a rekey of the multicast service SA.

   The deletion of the Root service SA does not necessarily imply the



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   deletion of the derived unicast and multicast service SAs and
   associated TSKs.  Failure to mutually prove possession of the AAA-Key
   during the Secure Association Protocol exchange need not be grounds
   for deletion of the AAA-Key by both parties; the action to be taken
   is defined by the Secure Association Protocol.

3.4.1.  Sharing service SAs

   A single service may be provided by multiple logical or physical
   service elements.  Each service is responsible for specifying how
   changing service elements is handled. Some approaches include:

Transparent sharing
     If the service parameters visible to the other party (either peer
     or authenticator) do not change, the service can be moved without
     requiring cooperation from the other party.

     Whether such a move should be supported or used depends on
     implementation and administrative considerations. For instance, an
     administrator may decide to configure a group of IKEv2/IPsec
     gateways in a cluster for high-availability purposes, if the
     implementation used supports this. The peer does not necessarily
     have any way of knowing when the change occurs.

No sharing
     If the service parameters require changing, some changes may
     require terminating the old service, and starting a new
     conversation from phase 0. This approach is used by all services
     for at least some parameters, and it doesn't require any protocol
     for transferring the service SA between the service elements.

     The service may support keeping the old service element active
     while the new conversation takes phase, to decrease the time the
     service is not available.

Some sharing
     The service may allow changing some parameters by simply agreeing
     about the new values. This may involve a similar exchange as in
     phase 2, or perhaps a shorter conversation.

     This option usually requires some protocol for transferring the
     service SA between the elements. An administrator may decide not to
     enable this feature at all, and typically the sharing is restricted
     to some particular service elements (defined either by a service
     parameter, or simple administrative decision). If the old and new
     service element do not support such "context transfer", this
     approach falls back to the previous option (no transfer).




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     Services supporting this feature should also consider what changes
     require new authorization from the backend authentication server
     (see Section 4.2).

     Note that these considerations are not limited to service
     parameters related to the authenticator--they apply to peer
     parameters as well.

4.  Key Management

   EAP supports key derivation, but not key management.  As a result,
   key management functionality needs to be provided by the Secure
   Association Protocol.  This includes:

[a]  Generation of fresh transient session keys (TSKs).  Where AAA-Key
     caching is supported, the EAP peer may initiate a new session using
     a AAA-Key that was used in a previous session.  Were the TSKs to be
     derived from a portion of the AAA-Key,  this would result in reuse
     of the session keys which could expose the underlying ciphersuite
     to attack.  As a result, where AAA-Key caching is supported, the
     Secure Association Protocol phase is REQUIRED, and MUST provide for
     freshness of the TSKs.

[b]  Key lifetime determination.  EAP does not support negotiation of
     key lifetimes, nor does it support rekey without reauthentication.
     As a result, the Secure Association Protocol may handle rekey and
     determination of the key lifetime.  Where key caching is supported,
     secure negotiation of key lifetimes is RECOMMENDED.  Lower layers
     that support rekey, but not key caching, may not require key
     lifetime negotiation.  To take an example from IKE, the difference
     between IKEv1 and IKEv2 is that in IKEv1 SA lifetimes were
     negotiated. In IKEv2, each end of the SA is responsible for
     enforcing its own lifetime policy on the SA and rekeying the SA
     when necessary.

[c]  Key resynchronization.  It is possible for the peer or
     authenticator to reboot or reclaim resources, clearing portions or
     all of the key cache.  Therefore, key lifetime negotiation cannot
     guarantee that the key cache will remain synchronized, and the peer
     may not be able to determine before attempting to use a AAA-Key
     whether it exists within the authenticator cache.  It is therefore
     RECOMMENDED for the Secure Association Protocol to provide a
     mechanism for key state resynchronization.  Since in this situation
     one or more of the parties initially do not possess a key with
     which to protect the resynchronization exchange, securing this
     mechanism may be difficult.





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[d]  Key selection.  Where key caching is supported, it may be possible
     for the EAP peer and authenticator to share more than one key of a
     given type.  As a result, the Secure Association Protocol needs to
     support key selection, using the EAP Key Naming scheme described in
     this document.

[e]  Key scope determination.  Since the Discovery phase is handled out-
     of-band, EAP does not provide a mechanism by which the peer can
     determine the authenticator identity.  As a result, where the
     authenticator has multiple ports and AAA-Key caching is supported,
     the EAP peer may not be able to determine the scope of validity of
     a AAA-Key.  Similarly, where the EAP peer has multiple ports, the
     authenticator may not be able to determine whether a peer has
     authorization to use a particular AAA-Key.  To allow key scope
     determination, the lower layer SHOULD provide a mechanism by which
     the peer can determine the scope of the AAA-Key cache on each
     authenticator, and by which the authenticator can determine the
     scope of the AAA-Key cache on a peer.

4.1.  Key Caching

   In existing implementations, key caching may be supported on the EAP
   peer and authenticator but not on the backend server.  Where
   explicitly supported by the lower layer, the EAP peer and
   authenticator MAY cache the AAA-Key and/or TSKs.  The structure of
   the key cache on the peer and authenticator is defined by the lower
   layer.  Unless specified by the lower layer, the EAP peer and
   authenticator MUST assume that peers and authenticators do not cache
   the AAA-Key or TSKs.

   In existing AAA server implementations, all keys exported by EAP
   methods (including the MSK, EMSK and IV) and calculated keys (e.g.
   AAA-Key) are not cached and are lost after EAP authentication
   completes:

[1]  In order to avoid key reuse, on the EAP server, transported keys
     are deleted once they are sent.  An EAP server MUST NOT retain keys
     that it has previously sent to the authenticator.  For example, an
     EAP server that has transported a AAA-Key based on the MSK MUST
     delete the MSK, and no keys may be derived from the MSK from that
     point forward by the server.

[2]  Keys which are not transported, such as the EMSK, are also deleted
     by existing implementations.







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4.2.  Parent-Child Relationships

   When keying material exported by EAP methods expires,  all keying
   material derived from the exported keying material expires, including
   the AAA-Key and TSKs.

   When an EAP reauthentication takes place, new keying material is
   derived and exported by the EAP method, which eventually results in
   replacement of calculated keys, including the AAA-Key and TSKs.

   As a result,  while the lifetime of calculated keys can be less than
   or equal that of the exported keys they are derived from, it cannot
   be greater.  For example, TSK rekey may occur prior to EAP
   reauthentication.

   Failure to mutually prove possession of the AAA-Key during the Secure
   Association Protocol exchange need not be grounds for deletion of the
   AAA-Key by both parties; rate-limiting Secure Association Protocol
   exchanges could be used to prevent a brute force attack.

4.3.  Local Key Lifetimes

   The Transient EAP Keys (TEKs) are session keys used to protect the
   EAP conversation.  The TEKs are internal to the EAP method and are
   not exported.  TEKs are typically created during an EAP conversation,
   used until the end of the conversation and then discarded.  However,
   methods may rekey TEKs during a conversation.

   When using TEKs within an EAP conversation or across conversations,
   it is necessary to ensure that replay protection and key separation
   requirements are fulfilled.  For instance, if a replay counter is
   used, TEK rekey MUST occur prior to wrapping of the counter.
   Similarly, TSKs MUST remain cryptographically separate from TEKs
   despite TEK rekeying or caching. This prevents TEK compromise from
   leading directly to compromise of the TSKs and vice versa.

   EAP methods may cache local keying material which may persist for
   multiple EAP conversations when fast reconnect is used [RFC 3748].
   For example, EAP methods based on TLS (such as EAP-TLS [RFC2716])
   derive and cache the TLS Master Secret, typically for substantial
   time periods.  The lifetime of other local keying material calculated
   within the EAP method is defined by the method.  Note that in
   general, when using fast reconnect, there is no guarantee to that the
   original long-term credentials are still in the possession of the
   peer.  For instance, a card hold holding the private key for EAP-TLS
   may have been removed. EAP servers SHOULD also verify that the long-
   term credentials are still valid, such as by checking that
   certificate used in the original authentication has not yet expired.



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4.4.  Exported and Calculated Key Lifetimes

   All EAP methods generating keys are required to generate the MSK and
   EMSK, and may optionally generate the IV.  However, EAP, defined in
   [RFC3748], does not support the negotiation of lifetimes for exported
   keying material such as the MSK, EMSK and IV.

   Several mechanisms exist for managing key lifetimes:

[a]  AAA attributes.  AAA protocols such as RADIUS [RFC2865] and
     Diameter [I-D.ietf-aaa-eap] support the Session-Timeout attribute.
     The Session-Timeout value represents the maximum lifetime of the
     exported keys, and all keys calculated from it, on the
     authenticator.  Since existing AAA servers do not cache keys
     exported by EAP methods, or keys calculated from exported keys, the
     value of the Session-Timeout attribute has no bearing on the key
     lifetime within the AAA server.

     On the authenticator,  where EAP is used for authentication, the
     Session-Timeout value represents the maximum session time prior to
     re-authentication, as described in [RFC3580].  Where EAP is used
     for pre-authentication, the session may not start until some future
     time, or may never occur.  Nevertheless, the Session-Timeout value
     represents the time after which the AAA-Key, and all keys
     calculated from it, will have expired on the authenticator.  If the
     session subsequently starts, re-authentication will be initiated
     once the Session-Time has expired.  If the session never started,
     or started and ended, the AAA-Key and all keys calculated from it
     will be expired by the authenticator prior to the future time
     indicated by Session-Timeout.

     Since the TSK lifetime is often determined by authenticator
     resources, the AAA server has no insight into the TSK derivation
     process, and by the principle of ciphersuite independence, it is
     not appropriate for the AAA server to manage any aspect of the TSK
     derivation process, including the TSK lifetime.

[b]  Lower layer mechanisms.  While AAA attributes can communicate the
     maximum exported key lifetime, this only serves to synchronize the
     key lifetime between the backend authentication server and the
     authenticator.  Lower layer mechanisms such as the Secure
     Association Protocol can then be used to enable the lifetime of
     exported and calculated keys to be negotiated between the peer and
     authenticator.

     Where TSKs are established as the result of a Secure Association
     Protocol exchange, it is RECOMMENDED that the Secure Association
     Protocol include support for TSK resynchronization. Where the TSK



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     is taken from the AAA-Key, there is no need to manage the TSK
     lifetime as a separate parameter, since the TSK lifetime and AAA-
     Key lifetime are identical.

[c]  System defaults.  Where the EAP method does not support the
     negotiation of the exported key lifetime, and a key lifetime
     negotiation mechanism is not provided by the lower lower, there may
     be no way for the peer to learn the exported key lifetime.  In this
     case it is RECOMMENDED that the peer assume a default value of the
     exported key lifetime; 8 hours is recommended.  Similarly, the
     lifetime of calculated keys can also be managed as a system
     parameter on the authenticator.

[d]  Method specific negotiation within EAP. While EAP itself does not
     support lifetime negotiation, it would be possible to specify
     methods that do.  However, systems that rely on such negotiation
     for exported keys would only function with these methods. As a
     result, it is NOT RECOMMENDED to use this approach as the sole way
     to determine key lifetimes.

4.5.  Key cache synchronization

   Issues arise when attempting to synchronize the key cache on the peer
   and authenticator.  Lifetime negotiation alone cannot guarantee key
   cache synchronization.

   One problem is that the AAA protocol cannot guarantee synchronization
   of key lifetimes between the peer and authenticator.  Where the
   Secure Association Protocol is not run immediately after EAP
   authentication, the exported and calculated key lifetimes will not be
   known by the peer during the hiatus.  Where EAP pre-authentication
   occurs, this can leave the peer uncertain whether a subsequent
   attempt to use the exported keys will prove successful.

   However, even where the Secure Association Protocol is run
   immediately after EAP, it is still possible for the authenticator to
   reclaim resources if the created key state is not immediately
   utilized.

   The lower layer may utilize Discovery mechanisms to assist in this.
   For example, the authenticator manages the AAA-Key cache by deleting
   the oldest AAA-Key first (LIFO), the relative creation time of the
   last AAA-Key to be deleted could be advertised with the Discovery
   phase, enabling the peer to determine whether a given AAA-Key had
   been expired from the authenticator key cache prematurely.






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4.6.  Key Scope

   As described in Section 2.3, in existing applications the AAA-Key is
   derived from the MSK by the EAP peer and server, and is used as the
   root of the ciphersuite-specific key hierarchy.  Where a backend
   authentication server is present, the AAA-Key is transported from the
   EAP server to the authenticator; where it is not present, the AAA-Key
   is calculated on the authenticator.

   Regardless of how many sessions are initiated using it, the AAA-Key
   scope is between the EAP peer that calculates it, and the
   authenticator that either calculates it (where no backend
   authenticator is present) or receives it from the server (where a
   backend authenticator server is present).

   It should be understood that an authenticator or peer:

   [a] may contain multiple physical ports;
   [b] may advertise itself as multiple "virtual" authenticators
       or peers;
   [c] may utilize multiple CPUs;
   [d] may support clustering services for load balancing or failover.

   As illustrated in Figure 1, an EAP peer with multiple ports may be
   attached to one or more authenticators, each with multiple ports.
   Where the peer and authenticator identify themselves using a port
   identifier such as a link layer address, it may not be obvious to the
   peer which authenticator ports are associated with which
   authenticators.  Similarly, it may not be obvious to the
   authenticator which peer ports are associated with which peers.  As a
   result, the peer and authenticator may not be able to determine the
   scope of the AAA-Key.

   When a single physical authenticator advertises itself as multiple
   "virtual authenticators", the EAP peer and authenticator also may not
   be able to agree on the scope of the AAA-Key, creating a security
   vulnerability.  For example, the peer may assume that the "virtual
   authenticators" are distinct and do not share a key cache, whereas,
   depending on the architecture of the physical AP, a shared key cache
   may or may not be implemented.

   Where the AAA-Key is shared between "virtual authenticators" an
   attacker acting as a peer could authenticate with the "Guest"
   "virtual authenticator" and derive a AAA-Key.  If the virtual
   authenticators share a key cache, then the peer can utilize the AAA-
   Key derived for the "Guest" network to obtain access to the
   "Corporate Intranet" virtual authenticator.




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   Several measures are recommended to address these issues:

[a]  Authenticators are REQUIRED to cache associated authorizations
     along with the AAA-Key and apply authorizations consistently.  This
     ensures that an attacker cannot obtain elevated privileges even
     where the AAA-Key cache is shared between "virtual authenticators".

[b]  It is RECOMMENDED that physical authenticators maintain separate
     AAA-Key caches for each "virtual authenticator".

[c]  It is RECOMMENDED that each "virtual authenticator" identify itself
     distinctly to the AAA server, such as by utilizing a distinct NAS-
     identifier attribute.  This enables the AAA server to utilize a
     separate credential to authenticate each "virtual authenticator".

[d]  It is RECOMMENDED that Secure Association Protocols identify peers
     and authenticators unambiguously, without incorporating implicit
     assumptions about peer and authenticator architectures.  Using
     port-specific MAC addresses as identifiers is NOT RECOMMENDED where
     peers and authenticators may support multiple ports.

[e]  The AAA server and authenticator MAY implement additional
     attributes in order to further restrict the AAA-Key scope.  For
     example, in 802.11, the AAA server may provide the authenticator
     with a list of authorized Called or Calling-Station-Ids and/or
     SSIDs for which the  AAA-Key is valid.

[f]  Where the AAA server provides attributes restricting the key scope,
     it is RECOMMENDED that restrictions be securely communicated by the
     authenticator to the peer.  This can be accomplished using the
     Secure Association Protocol,  but also can be accomplished via the
     EAP method or the lower layer.

4.7.  Key Strength

   In order to guard against brute force attacks, EAP methods deriving
   keys need to be capable of generating keys with an appropriate
   effective symmetric key strength.  In order to ensure that key
   generation is not the weakest link, it is RECOMMENDED that EAP
   methods utilizing public key cryptography choose a public key that
   has a cryptographic strength meeting the symmetric key strength
   requirement.

   As noted in [RFC3766] Section 5, this results in the following
   required RSA or DH module and DSA subgroup size in bits, for a given
   level of attack resistance in bits:





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        Attack Resistance     RSA or DH Modulus     DSA subgroup
           (bits)              size (bits)          size (bits)
        -----------------     -----------------     ------------
        70                          947                 128
        80                         1228                 145
        90                         1553                 153
        100                        1926                 184
        150                        4575                 279
        200                        8719                 373
        250                       14596                 475

4.8.  Key Wrap

   As described in [RFC3579] Section 4.3, known problems exist in the
   key wrap specified in [RFC2548].  Where the same RADIUS shared secret
   is used by a PAP authenticator and an EAP authenticator, there is a
   vulnerability to known plaintext attack.  Since RADIUS uses the
   shared secret for multiple purposes, including per-packet
   authentication, attribute hiding, considerable information is exposed
   about the shared secret with each packet. This exposes the shared
   secret to dictionary attacks.  MD5 is used both to compute the RADIUS
   Response Authenticator and the Message-Authenticator attribute, and
   some concerns exist relating to the security of this hash
   [MD5Attack].

   As discussed in [RFC3579] Section 4.3, the security vulnerabilities
   of RADIUS are extensive, and therefore development of an alternative
   key wrap technique based on the RADIUS shared secret would not
   substantially improve security.  As a result, [RFC3759] Section 4.2
   recommends running RADIUS over IPsec.  The same approach is taken in
   Diameter EAP [I-D.ietf-aaa-eap], which defines cleartext key
   attributes, to be protected by IPsec or TLS.

   Where an untrusted AAA intermediary is present (such as a RADIUS
   proxy or a Diameter agent), and data object security is not used, the
   AAA-Key may be recovered by an attacker in control of the untrusted
   intermediary.  Possession of the AAA-Key enables decryption of data
   traffic sent between the peer and a specific authenticator.  However,
   as long as a AAA-Key or keys derived from it is only utilized by a
   single authenticator, compromise of the AAA-Key does not enable an
   attacker to impersonate the peer to another authenticator.
   Vulnerability to an untrusted AAA intermediary can be mitigated by
   implementation of redirect functionality, as described in [RFC3588]
   and [I-D.ietf-aaa-eap].







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5.  Handoff Vulnerabilities

   With EAP, a number of mechanisms are be utilized in order to reduce
   the latency of handoff between authenticators.  One such mechanism is
   EAP pre-authentication, in which EAP is utilized to pre-establish a
   AAA-Key on an authenticator prior to arrival of the peer.  Another
   such mechanism is AAA-Key caching, in which an EAP peer can re-attach
   to an authenticator without having to re-authenticate using EAP.  Yet
   another mechanism is context transfer, such as is defined in
   [IEEE-802.11F] and [CTP].  These mechanisms introduce new security
   vulnerabilities, as discussed in the sections that follow.

5.1.  Authorization

   In a typical network access scenario (dial-in, wireless LAN, etc.)
   access control mechanisms are typically applied. These mechanisms
   include user authentication as well as authorization for the offered
   service.

   As a part of the authentication process, the AAA network determines
   the user's authorization profile.  The user authorizations are
   transmitted by the backend authentication server to the EAP
   authenticator (also known as the Network Access Server or
   authenticator) included with the AAA-Token, which also contains the
   AAA-Key, in Phase 1b of the EAP conversation.  Typically, the profile
   is determined based on the user identity, but a certificate presented
   by the user may also provide authorization information.

   The backend authentication server is responsible for making a user
   authorization decision, answering the following questions:

[a]  Is this a legitimate user for this particular network?

[b]  Is this user allowed the type of access he or she is requesting?

[c]  Are there any specific parameters (mandatory tunneling, bandwidth,
     filters, and so on) that the access network should be aware of for
     this user?

[d]  Is this user within the subscription rules regarding time of day?

[e]  Is this user within his limits for concurrent sessions?

[f]  Are there any fraud, credit limit, or other concerns that indicate
     that access should be denied?

   While the authorization decision is in principle simple, the process
   is complicated by the distributed nature of AAA decision making.



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   Where brokering entities or proxies are involved, all of the AAA
   devices in the chain from the authenticator to the home AAA server
   are involved in the decision.  For instance, a broker can disallow
   access even if the home AAA server would allow it, or a proxy can add
   authorizations (e.g., bandwidth limits).

   Decisions can be based on static policy definitions and profiles as
   well as dynamic state (e.g. time of day or limits on the number of
   concurrent sessions).  In addition to the Accept/Reject decision made
   by the AAA chain, parameters or constraints can be communicated to
   the authenticator.

   The criteria for Accept/Reject decisions or the reasons for choosing
   particular authorizations are typically not communicated to the
   authenticator, only the final result.  As a result, the authenticator
   has no way to know what the decision was based on.  Was a set of
   authorization parameters sent because this service is always provided
   to the user, or was the decision based on the time/day and the
   capabilities of the requesting authenticator device?

5.2.  Correctness

   When the AAA exchange is bypassed via use of techniques such as AAA-
   Key caching, this creates challenges in ensuring that authorization
   is properly handled. These include:

[a]  Consistent application of session time limits.  Bypassing AAA
     should not automatically increase the available session time,
     allowing a user to endlessly extend their network access by
     changing the point of attachment.

[b]  Avoidance of privilege elevation.   Bypassing AAA should not result
     in a user being granted access to services which they are not
     entitled to.

[c]  Consideration of dynamic state.  In situations in which dynamic
     state is involved in the access decision (day/time, simultaneous
     session limit) it should be possible to take this state into
     account either before or after access is granted. Note that
     consideration of network-wide state such as simultaneous session
     limits can typically only be taken into account by the backend
     authentication server.

[d]  Encoding of restrictions.  Since a authenticator may not be aware
     of the criteria considered by a backend authentication server when
     allowing access, in order to ensure consistent authorization during
     a fast handoff it may be necessary to explicitly encode the
     restrictions within the authorizations provided in the AAA-Token.



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[e]  State validity.  The introduction of fast handoff should not render
     the authentication server incapable of keeping track of network-
     wide state.

   A handoff mechanism capable of addressing these concerns is said to
   be "correct".  One condition for correctness is as follows: For a
   handoff to be "correct" it MUST establish on the new device the same
   context as would have been created had the new device completed a AAA
   conversation with the authentication server.

   A properly designed handoff scheme will only succeed if it is
   "correct" in this way.  If a successful handoff would establish
   "incorrect" state, it is preferable for it to fail, in order to avoid
   creation of incorrect context.

   Some backend authentication server and authenticator configurations
   are incapable of meeting this definition of "correctness".  For
   example, if the old and new device differ in their capabilities, it
   may be difficult to meet this definition of correctness in a handoff
   mechanism that bypasses AAA.  Backend authentication servers often
   perform conditional evaluation, in which the authorizations returned
   in an Access-Accept message are contingent on the authenticator or on
   dynamic state such as the time of day or number of simultaneous
   sessions.  For example, in a heterogeneous deployment, the backend
   authentication server might return different authorizations depending
   on the authenticator making the request, in order to make sure that
   the requested service is consistent with the authenticator
   capabilities.

   If differences between the new and old device would result in the
   backend authentication server sending a different set of messages to
   the new device than were sent to the old device, then if the handoff
   mechanism bypasses AAA, then the handoff cannot be carried out
   correctly.

   For example, if some authenticator devices within a deployment
   support dynamic VLANs while others do not, then attributes present in
   the Access-Request (such as the authenticator-IP-Address,
   authenticator-Identifier, Vendor-Identifier, etc.) could be examined
   to determine when VLAN attributes will be returned, as described in
   [RFC3580].   VLAN support is defined in [IEEE-802.1Q].  If a handoff
   bypassing the backend authentication server were to occur between a
   authenticator supporting dynamic VLANs and another authenticator
   which does not, then a guest user with access restricted to a guest
   VLAN could be given unrestricted access to the network.

   Similarly, in a network where access is restricted based on the day
   and time, Service Set Identifier (SSID), Calling-Station-Id or other



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   factors, unless the restrictions are encoded within the
   authorizations, or a partial AAA conversation is included, then a
   handoff could result in the user bypassing the restrictions.

   In practice, these considerations limit the situations in which fast
   handoff mechanisms bypassing AAA can be expected to be successful.
   Where the deployed devices implement the same set of services, it may
   be possible to do successful handoffs within such mechanisms.
   However, where the supported services differ between devices, the
   handoff may not succeed.  For example, [RFC2865] section 1.1 states:

      "A authenticator that does not implement a given service MUST NOT
      implement the RADIUS attributes for that service.  For example, a
      authenticator that is unable to offer ARAP service MUST NOT
      implement the RADIUS attributes for ARAP.  A authenticator MUST
      treat a RADIUS access-accept authorizing an unavailable service as
      an access-reject instead."

   Note that this behavior only applies to attributes that are known,
   but not implemented.  For attributes that are unknown, [RFC2865]
   Section 5 states:

      "A RADIUS server MAY ignore Attributes with an unknown Type.  A
      RADIUS client MAY ignore Attributes with an unknown Type."

   In order to perform a correct handoff, if a new device is provided
   with RADIUS context for a known but unavailable service, then it MUST
   process this context the same way it would handle a RADIUS Access-
   Accept requesting an unavailable service.  This MUST cause the
   handoff to fail.  However, if a new device is provided with RADIUS
   context that indicates an unknown attribute, then this attribute MAY
   be ignored.

   Although it may seem somewhat counter-intuitive, failure is indeed
   the "correct" result where a known but unsupported service is
   requested. Presumably a correctly configured backend authentication
   server would not request that a device carry out a service that it
   does not implement.  This implies that if the new device were to
   complete a AAA conversation that it would be likely to receive
   different service instructions.  In such a case, failure of the
   handoff is the desired result.  This will cause the new device to go
   back to the AAA server in order to receive the appropriate service
   definition.

   In practice, this implies that handoff mechanisms which bypass AAA
   are most likely to be successful within a homogeneous device
   deployment within a single administrative domain. For example, it
   would not be advisable to carry out a fast handoff bypassing AAA



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   between a authenticator providing confidentiality and another
   authenticator that does not support this service.  The correct result
   of such a handoff would be a failure, since if the handoff were
   blindly carried out, then the user would be moved from a secure to an
   insecure channel without permission from the backend authentication
   server.  Thus the definition of a "known but unsupported service"
   MUST encompass requests for unavailable security services.  This
   includes vendor-specific attributes related to security, such as
   those described in [RFC2548].

6.  Security Considerations

6.1.  Security Terminology

   "Cryptographic binding", "Cryptographic separation", "Key strength"
   and "Mutual authentication" are defined in [RFC3748] and are used
   with the same meaning here.

6.2.  Threat Model

   The EAP threat model is described in [RFC3748] Section 7.1.  In order
   to address these threats, EAP relies on the security properties of
   EAP methods (known as "security claims", described in [RFC3784]
   Section 7.2.1).  EAP method requirements for application such as
   Wireless LAN authentication are described in [RFC4017].

   The RADIUS threat model is described in [RFC3579] Section 4.1, and
   responses to these threats are described in [RFC3579] Sections 4.2
   and 4.3.  Among other things, [RFC3579] Section 4.2 recommends the
   use of IPsec ESP with non-null transform to provide per-packet
   authentication and confidentiality, integrity and replay protection
   for RADIUS/EAP.

   Given the existing documentation of EAP and AAA threat models and
   responses, there is no need to duplicate that material here.
   However, there are many other system-level threats no covered in
   these document which have not been described or analyzed elsewhere.
   These include:

[1]  An attacker may try to modify or spoof Secure Association Protocol
     packets.

[2]  An attacker compromising an authenticator may provide incorrect
     information to the EAP peer and/or server via out-of-band
     mechanisms (such as via a AAA or lower layer protocol).  This
     includes impersonating another authenticator, or providing
     inconsistent information to the peer and EAP server.




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[3]  An attacker may attempt to perform downgrading attacks on the
     ciphersuite negotiation within the Secure Association Protocol in
     order to ensure that a weaker ciphersuite is used to protect data.

   Depending on the lower layer, these attacks may be carried out
   without requiring physical proximity.

   In order to address these threats, [Housley56] describes the
   mandatory system security properties:

Algorithm independence
     Wherever cryptographic algorithms are chosen, the algorithms must
     be negotiable, in order to provide resilient against compromise of
     a particular algorithm.  Algorithm independence must be
     demonstrated within all aspects of the system, including within
     EAP, AAA and the Secure Association Protocol.  However, for
     interoperability, at least one suite of algorithms MUST be
     implemented.

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

Replay protection
     All protocol exchanges must be replay protected.  This includes
     exchanges within EAP, AAA, and the Secure Association Protocol.

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

Authorization
     EAP peer and authenticator authorization must be performed.

Session keys
     Confidentiality of session keys must be maintained.

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

Unique naming
     Session keys must be uniquely named.

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



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

6.3.  Security Analysis

   Figure 6 illustrates the relationship between the peer, authenticator
   and backend authentication server.

                               EAP peer
                                 /\
                                /  \
            Protocol: EAP      /    \    Protocol: Secure Association
            Auth: Mutual      /      \   Auth: Mutual
            Unique keys:     /        \  Unique keys: TSKs
            TEKs,EMSK       /          \
                           /            \
              EAP server  +--------------+ Authenticator
                            Protocol: AAA
                            Auth: Mutual
                            Unique key: AAA session key

    Figure 6: Relationship between peer, authenticator and auth. server

   The peer and EAP server communicate using EAP [RFC3748].  The
   security properties of this communication are largely determined by
   the chosen EAP method.  Method security claims are described in
   [RFC3748] Section 7.2.  These include the  key strength, protected
   ciphersuite negotiation, mutual authentication, integrity protection,
   replay protection, confidentiality, key derivation, key strength,
   dictionary attack resistance, fast reconnect, cryptographic binding,
   session independence, fragmentation and channel binding claims.  At a
   minimum, methods claiming to support key derivation must also support
   mutual authentication.  As noted in [RFC3748] Section 7.10:

      EAP Methods deriving keys MUST provide for mutual authentication
      between the EAP peer and the EAP Server.

   Ciphersuite independence is also required:

      Keying material exported by EAP methods MUST be independent of the
      ciphersuite negotiated to protect data.

   In terms of key strength and freshness, [RFC3748] Section 10 says:

      EAP methods SHOULD ensure the freshness of the MSK and EMSK even
      in cases where one party may not have a high quality random number
      generator.... In order to preserve algorithm independence, EAP
      methods deriving keys SHOULD support (and document) the protected



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      negotiation of the ciphersuite used to protect the EAP
      conversation between the peer and server...  In order to enable
      deployments requiring strong keys, EAP methods supporting key
      derivation SHOULD be capable of generating an MSK and EMSK, each
      with an effective key strength of at least 128 bits.

   The authenticator and backend authentication server communicate using
   a AAA protocol such as RADIUS [RFC3579] or Diameter [I-D.ietf-aaa-
   eap].  As noted in [RFC3588] Section 13, Diameter must be protected
   by either IPsec ESP with non-null transform or TLS.  As a result,
   Diameter requires per-packet integrity and confidentiality.  Replay
   protection must be supported.  For RADIUS, [RFC3579] Section 4.2
   recommends that RADIUS be protected by IPsec ESP with a non-null
   transform, and where IPsec is implemented replay protection must be
   supported.

   The peer and authenticator communicate using the Secure Association
   Protocol.

   As noted in the figure, each party in the exchange mutually
   authenticates with each of the other parties, and derives a unique
   key.  All parties in the diagram have access to the AAA-Key.

   The EAP peer and backend authentication server mutually authenticate
   via the EAP method, and derive the TEKs and EMSK which are known only
   to them. The TEKs are used to protect some or all of the EAP
   conversation between the peer and authenticator, so as to guard
   against modification or insertion of EAP packets by an attacker.  The
   degree of protection afforded by the TEKs is determined by the EAP
   method; some methods may protect the entire EAP packet, including the
   EAP header, while other methods may only protect the contents of the
   Type-Data field, defined in [RFC3748].

   Since EAP is spoken only between the EAP peer and server, if a
   backend authentication server is present then the EAP conversation
   does not provide mutual authentication between the peer and
   authenticator, only between the EAP peer and EAP server (backend
   authentication server).  As a result, mutual authentication between
   the peer and authenticator only occurs where a Secure Association
   protocol is used, such the unicast and group key derivation handshake
   supported in [IEEE-802.11i].  This means that absent use of a secure
   Association Protocol, from the point of view of the peer, EAP mutual
   authentication only proves that the authenticator is trusted by the
   backend authentication server; the identity of the authenticator is
   not confirmed.

   Utilizing the AAA protocol, the authenticator and backend
   authentication server mutually authenticate and derive session keys



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   known only to them, used to provide per-packet integrity and replay
   protection, authentication and confidentiality.  The AAA-Key is
   distributed by the backend authentication server to the authenticator
   over this channel, bound to attributes constraining its usage, as
   part of the AAA-Token.  The binding of attributes to the AAA-Key
   within a protected package is important so the authenticator
   receiving the AAA-Token can determine that it has not been
   compromised, and that the keying material has not been replayed, or
   mis-directed in some way.

   The security properties of the EAP exchange are dependent on each leg
   of the triangle: the selected EAP method, AAA protocol and the Secure
   Association Protocol.

   Assuming that the AAA protocol provides protection against rogue
   authenticators forging their identity, then the AAA-Token can be
   assumed to be sent to the correct authenticator, and where it is
   wrapped appropriately, it can be assumed to be immune to compromise
   by a snooping attacker.

   Where an untrusted AAA intermediary is present,  the AAA-Token must
   not be provided to the intermediary so as to avoid compromise of the
   AAA-Token.  This can be avoided by use of re-direct as defined in
   [RFC3588].

   When EAP is used for authentication on PPP or wired IEEE 802
   networks, it is typically assumed that the link is physically secure,
   so that an attacker cannot gain access to the link, or insert a rogue
   device. EAP methods defined in [RFC3748] reflect this usage model.
   These include EAP MD5, as well as One-Time Password (OTP) and Generic
   Token Card.  These methods support one-way authentication (from EAP
   peer to authenticator) but not mutual authentication or key
   derivation.  As a result, these methods do not bind the initial
   authentication and subsequent data traffic, even when the the
   ciphersuite used to protect data supports per-packet authentication
   and integrity protection. As a result, EAP methods not supporting
   mutual authentication are vulnerable to session hijacking as well as
   attacks by rogue devices.

   On wireless networks such as IEEE 802.11 [IEEE-802.11], these attacks
   become easy to mount, since any attacker within range can access the
   wireless medium, or act as an access point.  As a result, new
   ciphersuites have been proposed for use with wireless LANs
   [IEEE-802.11i] which provide per-packet authentication, integrity and
   replay protection.  In addition, mutual authentication and key
   derivation, provided by methods such as EAP-TLS [RFC2716] are
   required [IEEE-802.11i], so as to address the threat of rogue
   devices, and provide keying material to bind the initial



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   authentication to subsequent data traffic.

   If the selected EAP method does not support mutual authentication,
   then the peer will be vulnerable to attack by rogue authenticators
   and backend authentication servers. If the EAP method does not derive
   keys, then TSKs will not be available for use with a negotiated
   ciphersuite, and there will be no binding between the initial EAP
   authentication and subsequent data traffic, leaving the session
   vulnerable to hijack.

   If the backend authentication server does not protect against
   authenticator masquerade, or provide the proper binding of the AAA-
   Key to the session within the AAA-Token, then one or more AAA-Keys
   may be sent to an unauthorized party, and an attacker may be able to
   gain access to the network.  If the AAA-Token is provided to an
   untrusted AAA intermediary, then that intermediary may be able to
   modify the AAA-Key, or the attributes associated with it, as
   described in [RFC2607].

   If the Secure Association Protocol does not provide mutual proof of
   possession of the AAA-Key material, then the peer will not have
   assurance that it is connected to the correct authenticator, only
   that the authenticator and backend authentication server share a
   trust relationship (since AAA protocols support mutual
   authentication).  This distinction can become important when multiple
   authenticators receive AAA-Keys from the backend authentication
   server, such as where fast handoff is supported.  If the TSK
   derivation does not provide for protected ciphersuite and
   capabilities negotiation, then downgrade attacks are possible.

6.4.  Man-in-the-middle Attacks

   As described in [I-D.puthenkulam-eap-binding], EAP method sequences
   and compound authentication mechanisms may be subject to man-in-the-
   middle attacks.  When such attacks are successfully carried out, the
   attacker acts as an intermediary between a victim and a legitimate
   authenticator.  This allows the attacker to authenticate successfully
   to the authenticator, as well as to obtain access to the network.

   In order to prevent these attacks, [I-D.puthenkulam-eap-binding]
   recommends derivation of a compound key by which the EAP peer and
   server can prove that they have participated in the entire EAP
   exchange.  Since the compound key must not be known to an attacker
   posing as an authenticator, and yet must be derived from quantities
   that are exported by EAP methods, it may be desirable to derive the
   compound key from a portion of the EMSK.  In order to provide proper
   key hygiene, it is recommended that the compound key used for man-in-
   the-middle protection be cryptographically separate from other keys



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   derived from the EMSK, such as fast handoff keys, discussed in
   Section 2.3.

6.5.  Denial of Service Attacks

   The caching of security associations may result in vulnerability to
   denial of service attacks.  Since an EAP peer may derive multiple EAP
   SAs with a given EAP server, and creation of a new EAP SA does not
   implicitly delete a previous EAP SA, EAP methods that result in
   creation of persistent state may be vulnerable to denial of service
   attacks by a rogue EAP peer.

   As a result, EAP methods creating persistent state may wish to limit
   the number of cached EAP SAs (Phase 1a) corresponding to an EAP peer.
   For example, an EAP server may choose to only retain a few EAP SAs
   for each peer.  This prevents a rogue peer from denying access to
   other peers.

   Similarly, an authenticator may have multiple AAA-Key SAs
   corresponding to a given EAP peer; to conserve resources an
   authenticator may choose to limit the number of cached AAA-Key (Phase
   1 b) SAs for each peer.

   Depending on the media, creation of a new unicast Secure Association
   SA may or may not imply deletion of a previous unicast secure
   association SA.  Where there is no implied deletion, the
   authenticator may choose to limit Phase 2 (unicast and multicast)
   Secure Association SAs for each peer.

6.6.  Impersonation

   Both the RADIUS and Diameter protocols are potentially vulnerable to
   impersonation by a rogue authenticator.

   While AAA protocols such as RADIUS [RFC2865] or Diameter [RFC3588]
   support mutual authentication between the authenticator (known as the
   AAA client) and the backend authentication server (known as the AAA
   server), the security mechanisms vary according to the AAA protocol.

   In RADIUS, the shared secret used for authentication is determined by
   the source address of the RADIUS packet.  As noted in [RFC3579]
   Section 4.3.7, it is highly desirable that the source address be
   checked against one or more NAS identification attributes so as to
   detect and prevent impersonation attacks.

   When RADIUS requests are forwarded by a proxy, the NAS-IP-Address or
   NAS-IPv6-Address attributes may not correspond to the source address.
   Since the NAS-Identifier attribute need not contain an FQDN, it also



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   may not correspond to the source address, even indirectly.  [RFC2865]
   Section 3 states:

         A RADIUS server MUST use the source IP address of the RADIUS
         UDP packet to decide which shared secret to use, so that
         RADIUS requests can be proxied.

   This implies that it is possible for a rogue authenticator to forge
   NAS-IP-Address, NAS-IPv6-Address or NAS-Identifier attributes within
   a RADIUS Access-Request in order to impersonate another
   authenticator.  Among other things, this can result in messages (and
   MSKs) being sent to the wrong authenticator. Since the rogue
   authenticator is authenticated by the RADIUS proxy or server purely
   based on the source address, other mechanisms are required to detect
   the forgery.  In addition, it is possible for attributes such as the
   Called-Station-Id and Calling-Station-Id to be forged as well.

   As recommended in [RFC3579], this vulnerability can be mitigated by
   having RADIUS proxies check authenticator identification attributes
   against the source address.

   To allow verification of session parameters such as the Called-
   Station- Id and Calling-Station-Id, these can be sent by the EAP peer
   to the server, protected by the TEKs. The RADIUS server can then
   check the parameters sent by the EAP peer against those claimed by
   the authenticator.  If a discrepancy is found, an error can be
   logged.

   While [RFC3588] requires use of the Route-Record AVP, this utilizes
   FQDNs, so that impersonation detection requires DNS A/AAAA and PTR
   RRs to be properly configured.  As a result, it appears that Diameter
   is as vulnerable to this attack as RADIUS, if not more so. To address
   this vulnerability, it is necessary to allow the backend
   authentication server to communicate with the authenticator directly,
   such as via the redirect functionality supported in [RFC3588].

6.7.  Channel binding

   It is possible for a compromised or poorly implemented EAP
   authenticator to communicate incorrect information to the EAP peer
   and/or server. This may enable an authenticator to impersonate
   another authenticator or communicate incorrect information via out-
   of-band mechanisms (such as via AAA or the lower layer protocol).

   Where EAP is used in pass-through mode, the EAP peer typically does
   not verify the identity of the pass-through authenticator, it only
   verifies that the pass-through authenticator is trusted by the EAP
   server. This creates a potential security vulnerability, described in



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   [RFC3748] Section 7.15.

   [RFC3579] Section 4.3.7 describes how an EAP pass-through
   authenticator acting as a AAA client can be detected if it attempts
   to impersonate another authenticator (such by sending incorrect NAS-
   Identifier [RFC2865], NAS-IP-Address [RFC2865] or NAS-IPv6-Address
   [RFC3162] attributes via the AAA protocol).  However, it is possible
   for a pass-through authenticator acting as a AAA client to provide
   correct information to the AAA server while communicating misleading
   information to the EAP peer via a lower layer protocol.

   For example, it is possible for a compromised authenticator to
   utilize another authenticator's Called-Station-Id or NAS-Identifier
   in communicating with the EAP peer via a lower layer protocol, or for
   a pass-through authenticator acting as a AAA client to provide an
   incorrect peer Calling-Station-Id [RFC2865][RFC3580] to the AAA
   server via the AAA protocol.

   As noted in [RFC3748] Section 7.15, this vulnerability can be
   addressed by use of EAP methods that support a protected exchange of
   channel properties such as endpoint identifiers, including (but not
   limited to): Called-Station-Id [RFC2865][RFC3580], Calling-Station-Id
   [RFC2865][RFC3580], NAS-Identifier [RFC2865], NAS-IP-Address
   [RFC2865], and NAS-IPv6-Address [RFC3162].

   Using such a protected exchange, it is possible to match the channel
   properties provided by the authenticator via out-of-band mechanisms
   against those exchanged within the EAP method.  For example, see
   [ServiceIdent].

7.  Security Requirements

   This section summarizes the security requirements that must be met by
   EAP methods, AAA protocols,  Secure Association Protocols and
   Ciphersuites in order to address the security threats described in
   this document. These requirements MUST be met by specifications
   requesting publication as an RFC.  Each requirement provides a
   pointer to the sections of this document describing the threat that
   it mitigates.

7.1.  EAP Method Requirements

   It is possible for the peer and EAP server to mutually authenticate
   and derive keys.  In order to provide keying material for use in a
   subsequently negotiated ciphersuite, 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.  EAP Methods deriving keys MUST provide for mutual



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   authentication between the EAP peer and the EAP Server.

   The MSK and EMSK MUST NOT be used directly to protect data; however,
   they are of sufficient size to enable derivation of a AAA-Key
   subsequently used to derive Transient Session Keys (TSKs) for use
   with the selected ciphersuite.  Each ciphersuite is responsible for
   specifying how to derive the TSKs from the AAA-Key.

   The AAA-Key is derived from the keying material exported by the EAP
   method (MSK and EMSK).  This derivation occurs on the AAA server.  In
   many existing protocols that use EAP, the AAA-Key and MSK are
   equivalent, but more complicated mechanisms are possible (see Section
   2.3 for details).

   EAP methods SHOULD ensure the freshness of the MSK and EMSK even in
   cases where one party may not have a high quality random number
   generator.  A RECOMMENDED method is for each party to provide a nonce
   of at least 128 bits, used in the derivation of the MSK and EMSK.

   EAP methods export the MSK and EMSK and not Transient Session Keys so
   as to allow EAP methods to be ciphersuite and media independent.
   Keying material exported by EAP methods MUST be independent of the
   ciphersuite negotiated to protect data.

   Depending on the lower layer, EAP methods may run before or after
   ciphersuite negotiation, so that the selected ciphersuite may not be
   known to the EAP method.  By providing keying material usable with
   any ciphersuite, EAP methods can used with a wide range of
   ciphersuites and media.

   It is RECOMMENDED that methods providing integrity protection of EAP
   packets include coverage of all the EAP header fields, including the
   Code, Identifier, Length, Type and Type-Data fields.

   In order to preserve algorithm independence, EAP methods deriving
   keys SHOULD support (and document) the protected negotiation of the
   ciphersuite used to protect the EAP conversation between the peer and
   server.  This is distinct from the ciphersuite negotiated between the
   peer and authenticator, used to protect data.

   The strength of Transient Session Keys (TSKs) used to protect data is
   ultimately dependent on the strength of keys generated by the EAP
   method.  If an EAP method cannot produce keying material of
   sufficient strength, then the TSKs may be subject to brute force
   attack.  In order to enable deployments requiring strong keys, EAP
   methods supporting key derivation SHOULD be capable of generating an
   MSK and EMSK, each with an effective key strength of at least 128
   bits.



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   Methods supporting key derivation MUST demonstrate cryptographic
   separation between the MSK and EMSK branches of the EAP key
   hierarchy.  Without violating a fundamental cryptographic assumption
   (such as the non-invertibility of a one-way function) an attacker
   recovering the MSK or EMSK MUST NOT be able to recover the other
   quantity with a level of effort less than brute force.

   Non-overlapping substrings of the MSK MUST be cryptographically
   separate from each other.  That is, knowledge of one substring MUST
   NOT help in recovering some other non-overlapping substring without
   breaking some hard cryptographic assumption.  This is required
   because some existing ciphersuites form TSKs by simply splitting the
   AAA-Key to pieces of appropriate length.  Likewise, non-overlapping
   substrings of the EMSK MUST be cryptographically separate from each
   other, and from substrings of the MSK.  The EMSK MUST NOT be
   transported to, or shared with, additional parties.

   Since EAP does not provide for explicit key lifetime negotiation, EAP
   peers, authenticators and authentication servers MUST be prepared for
   situations in which one of the parties discards key state which
   remains valid on another party.

   The development and validation of key derivation algorithms is
   difficult, and as a result EAP methods SHOULD reuse well established
   and analyzed mechanisms for MSK and EMSK key derivation (such as
   those specified in IKE [RFC2409] or TLS [RFC2246]), rather than
   inventing new ones.

7.1.1.  Requirements for EAP methods

   In order for an EAP method to meet the guidelines for EMSK usage it
   must meet the following requirements:

      o It MUST specify how to derive the EMSK

      o The key material used for the EMSK MUST be
        computationally independent of the MSK and TEKs.

      o The EMSK MUST NOT be used for any other purpose than the key
        derivation described in this document.

      o The EMSK MUST be secret and not known to someone observing
        the authentication mechanism protocol exchange.

      o The EMSK MUST NOT be exported from the EAP server.

      o The EMSK MUST be unique for each session.




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      o The EAP mechanism SHOULD a unique identifier suitable for naming the EMSK.

7.1.2.  Requirements for EAP applications

   In order for an application to meet the guidelines for EMSK usage it
   must meet the following requirements:

      o New applications following this specification SHOULD NOT use the
        MSK.  If more than one application uses the MSK, then the
        cryptographic separation is not achieved.  Implementations SHOULD
        prevent such combinations.

      o A peer MUST NOT use the EMSK directly for cryptographic
        protection of data.

7.2.  AAA Protocol Requirements

   AAA protocols suitable for use in transporting EAP MUST provide the
   following facilities:

Security services
     AAA protocols used for transport of EAP keying material MUST
     implement and SHOULD use per-packet integrity and authentication,
     replay protection and confidentiality.  These requirements are met
     by Diameter EAP [I-D.ietf-aaa-eap], as well as RADIUS over IPsec
     [RFC3579].

Session Keys
     AAA protocols used for transport of EAP keying material MUST
     implement and SHOULD use dynamic key management in order to derive
     fresh session keys, as in Diameter EAP [I-D.ietf-aaa-eap] and
     RADIUS over IPsec [RFC3579], rather than using a static key, as
     originally defined in RADIUS [RFC2865].

Mutual authentication
     AAA protocols used for transport of EAP keying material MUST
     provide for mutual authentication between the authenticator and
     backend authentication server.  These requirements are met by
     Diameter EAP [I-D.ietf-aaa-eap] as well as by RADIUS EAP [RFC3579].

Authorization
     AAA protocols used for transport of EAP keying material SHOULD
     provide protection against rogue authenticators masquerading as
     other authenticators.  This can be accomplished, for example, by
     requiring that AAA agents check the source address of packets
     against the origin attributes (Origin-Host AVP in Diameter, NAS-IP-
     Address, NAS-IPv6-Address, NAS-Identifier in RADIUS).  For details,
     see [RFC3579] Section 4.3.7.



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Key transport
     Since EAP methods do not export Transient Session Keys (TSKs) in
     order to maintain media and ciphersuite independence, the AAA
     server MUST NOT transport TSKs from the backend authentication
     server to authenticator.

Key transport specification
     In order to enable backend authentication servers to provide keying
     material to the authenticator in a well defined format, AAA
     protocols suitable for use with EAP MUST define the format and
     wrapping of the AAA-Token.

EMSK transport
     Since the EMSK is a secret known only to the backend authentication
     server and peer, the AAA-Token MUST NOT transport the EMSK from the
     backend authentication server to the authenticator.

AAA-Token protection
     To ensure against compromise, the AAA-Token MUST be integrity
     protected, authenticated, replay protected and encrypted in
     transit, using well-established cryptographic algorithms.

Session Keys
     The AAA-Token SHOULD be protected with session keys as in Diameter
     [RFC3588] or RADIUS over IPsec [RFC3579] rather than static keys,
     as in [RFC2548].

Key naming
     In order to ensure against confusion between the appropriate keying
     material to be used in a given Secure Association Protocol
     exchange, the AAA-Token SHOULD include explicit key names and
     context appropriate for informing the authenticator how the keying
     material is to be used.

Key Compromise
     Where untrusted intermediaries are present, the AAA-Token SHOULD
     NOT be provided to the intermediaries.  In Diameter, handling of
     keys by intermediaries can be avoided using Redirect functionality
     [RFC3588].

7.3.  Secure Association Protocol Requirements

   The Secure Association Protocol supports the following:

Entity Naming
     The peer and authenticator SHOULD identify themselves in a manner
     that is independent of their attached ports.




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Mutual proof of possession
     The peer and authenticator MUST each demonstrate possession of the
     keying material transported between the backend authentication
     server and authenticator (AAA-Key).

Key Naming
     The Secure Association Protocol MUST explicitly name the keys used
     in the proof of possession exchange, so as to prevent confusion
     when more than one set of keying material could potentially be used
     as the basis for the exchange.

Creation and Deletion
     In order to support the correct processing of phase 2 security
     associations, the Secure Association (phase 2) protocol MUST
     support the naming of phase 2 security associations and associated
     transient session keys, so that the correct set of transient
     session keys can be identified for processing a given packet.  The
     phase 2 Secure Association Protocol also MUST support transient
     session key activation and SHOULD support deletion, so that
     establishment and re-establishment of transient session keys can be
     synchronized between the parties.

Integrity and Replay Protection
     The Secure Association Protocol MUST support integrity and replay
     protection of all messages.

Direct operation
     Since the phase 2 Secure Association Protocol is concerned with the
     establishment of security associations between the EAP peer and
     authenticator, including the derivation of transient session keys,
     only those parties have "a need to know" the transient session
     keys. The Secure Association Protocol MUST operate directly between
     the peer and authenticator, and MUST NOT be passed-through to the
     backend authentication server, or include additional parties.

Derivation of transient session keys
     The Secure Association Protocol negotiation MUST support derivation
     of unicast and multicast transient session keys suitable for use
     with the negotiated ciphersuite.

TSK freshness
     The Secure Association (phase 2) Protocol MUST support the
     derivation of fresh unicast and multicast transient session keys,
     even when the keying material provided by the backend
     authentication server is not fresh.  This is typically supported by
     including an exchange of nonces within the Secure Association
     Protocol.




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Bi-directional operation
     While some ciphersuites only require a single set of transient
     session keys to protect traffic in both directions, other
     ciphersuites require a unique set of transient session keys in each
     direction. The phase 2 Secure Association Protocol SHOULD provide
     for the derivation of unicast and multicast keys in each direction,
     so as not to require two separate phase 2 exchanges in order to
     create a bi-directional phase 2 security association.

Secure capabilities negotiation
     The Secure Association Protocol MUST support secure capabilities
     negotiation.  This includes security parameters such as the
     security association identifier (SAID) and ciphersuites, as well as
     negotiation of the lifetime of the TSKs, AAA-Key and exported EAP
     keys.  Secure capabilities negotiation also includes confirmation
     of the capabilities discovered during the discovery phase (phase
     0), so as to ensure that the announced capabilities have not been
     forged.

Key Scoping
     The Secure Association Protocol MUST ensure the synchronization of
     key scope between the peer and authenticator.  This includes
     negotiation of restrictions on key usage.

7.4.  Ciphersuite Requirements

   Ciphersuites suitable for keying by EAP methods MUST provide the
   following facilities:

TSK derivation
     In order to allow a ciphersuite to be usable within the EAP keying
     framework, a specification MUST be provided describing how
     transient session keys suitable for use with the ciphersuite are
     derived from the AAA-Key.

EAP method independence
     Algorithms for deriving transient session keys from the AAA-Key
     MUST NOT depend on the EAP method.  However, algorithms for
     deriving TEKs MAY be specific to the EAP method.

Cryptographic separation
     The TSKs derived from the AAA-Key MUST be cryptographically
     separate from each other.  Similarly, TEKs MUST be
     cryptographically separate from each other.  In addition, the TSKs
     MUST be cryptographically separate from the TEKs.






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8.  IANA Considerations

   This document does not create any new name spaces nor does it
   allocate any protocol parameters.

9.  References

9.1.  Normative References

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

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
          Considerations Section in RFCs", BCP 26, RFC 2434, October
          1998.

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

9.2.  Informative References

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
          September 1981.

[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
          1661, July 1994.

[RFC1968] Meyer, G. and K. Fox, "The PPP Encryption Control Protocol
          (ECP)", RFC 1968, June 1996.

[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
          for Message Authentication", RFC 2104, February 1997.

[RFC2246] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A.
          and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246,
          January 1999.

[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
          Internet Protocol", RFC 2401, November 1998.

[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
          RFC 2409, November 1998.

[RFC2419] Sklower, K. and G. Meyer, "The PPP DES Encryption Protocol,
          Version 2 (DESE-bis)", RFC 2419, September 1998.





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[RFC2420] Kummert, H., "The PPP Triple-DES Encryption Protocol (3DESE)",
          RFC 2420, September 1998.

[RFC2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.  and
          R. Wheeler, "A Method for Transmitting PPP Over Ethernet
          (PPPoE)", RFC 2516, February 1999.

[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC
          2548, March 1999.

[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
          Implementation in Roaming", RFC 2607, June 1999.

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

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

[RFC3078] Pall, G. and G. Zorn, "Microsoft Point-To-Point Encryption
          (MPPE) Protocol", RFC 3078, March 2001.

[RFC3079] Zorn, G., "Deriving Keys for use with Microsoft Point-to-Point
          Encryption (MPPE)", RFC 3079, March 2001.

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

[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G. and J. Roese,
          "IEEE 802.1X Remote Authentication Dial In User Service
          (RADIUS) Usage Guidelines", RFC 3580, September 2003.

[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J.
          Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public
          Keys Used For Exchanging Symmetric  Keys", RFC 3766, April
          2004.

[RFC4017] Stanley, D., Walker, J. and B. Aboba, "EAP Method Requirements
          for Wireless LANs", RFC 4017, March 2005.

[CTP]     Loughney, J., Nakhjiri, M., Perkins, C. and R. Koodli,
          "Context Transfer Protocol", draft-ietf-seamoby-ctp-11.txt,
          Internet draft (work in progress), August 2004.




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[DESMODES]
          National Institute of Standards and Technology, "DES Modes of
          Operation", FIPS PUB 81, December 1980, <http://
          www.itl.nist.gov/fipspubs/fip81.htm>.

[FIPSDES] National Institute of Standards and Technology, "Data
          Encryption Standard", FIPS PUB 46, January 1977.

[IEEE-802]
          Institute of Electrical and Electronics Engineers, "IEEE
          Standards for Local and Metropolitan Area Networks: Overview
          and Architecture", ANSI/IEEE Standard 802, 1990.

[IEEE-802.11]
          Institute of Electrical and Electronics Engineers,
          "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 IEEE Standard 802.11-2003, 2003.

[IEEE-802.1X]
          Institute of Electrical and Electronics Engineers, "Local and
          Metropolitan Area Networks: Port-Based Network Access
          Control", IEEE Standard 802.1X-2004, December 2004.

[IEEE-802.1Q]
          Institute of Electrical and Electronics Engineers, "IEEE
          Standards for Local and Metropolitan Area Networks: Draft
          Standard for Virtual Bridged Local Area Networks", IEEE
          Standard 802.1Q/D8, January 1998.

[IEEE-802.11i]
          Institute of Electrical and Electronics Engineers, "Supplement
          to STANDARD FOR Telecommunications and Information Exchange
          between Systems - LAN/MAN Specific Requirements - Part 11:
          Wireless Medium Access Control (MAC) and physical layer (PHY)
          specifications: Specification for Enhanced Security", IEEE
          802.11i, December 2004.

[IEEE-802.11F]
          Institute of Electrical and Electronics Engineers,
          "Recommended Practice for Multi-Vendor Access Point
          Interoperability via an Inter-Access Point Protocol Across
          Distribution Systems Supporting IEEE 802.11 Operation", IEEE
          802.11F, July 2003.





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[IEEE-02-758]
          Mishra, A., Shin, M., Arbaugh, W., Lee, I. and K. Jang,
          "Proactive Caching Strategies for IAPP Latency Improvement
          during 802.11 Handoff", IEEE 802.11 Working Group,
          IEEE-02-758r1-F Draft 802.11I/D5.0, November 2002.

[IEEE-03-084]
          Mishra, A., Shin, M., Arbaugh, W., Lee, I. and K. Jang,
          "Proactive Key Distribution to support fast and secure
          roaming", IEEE 802.11 Working Group, IEEE-03-084r1-I,
          http://www.ieee802.org/11/Documents/DocumentHolder/ 3-084.zip,
          January 2003.

[IEEE-03-155]
          Aboba, B., "Fast Handoff Issues", IEEE 802.11 Working Group,
          IEEE-03-155r0-I,  http://www.ieee802.org/11/
          Documents/DocumentHolder/3-155.zip, March 2003.

[I-D.ietf-roamops-cert]
          Aboba, B., "Certificate-Based Roaming", draft-ietf-roamops-
          cert-02 (work in progress), April 1999.

[I-D.ietf-aaa-eap]
          Eronen, P., Hiller, T. and G. Zorn, "Diameter Extensible
          Authentication Protocol (EAP) Application", draft-ietf-aaa-
          eap-10 (work in progress), November 2004.

[I-D.puthenkulam-eap-binding]
          Puthenkulam, J., "The Compound Authentication Binding
          Problem", draft-puthenkulam-eap-binding-04 (work in progress),
          October 2003.

[I-D.arkko-pppext-eap-aka]
          Arkko, J. and H. Haverinen, "EAP AKA Authentication", draft-
          arkko-pppext-eap-aka-15.txt (work in progress), December 2004.

[IKEv2]   Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", draft-
          ietf-ipsec-ikev2-17 (work in progress), September 2004.

[8021XHandoff]
          Pack, S. and Y. Choi, "Pre-Authenticated Fast Handoff in a
          Public Wireless LAN Based on IEEE 802.1X Model", School of
          Computer Science and Engineering, Seoul National University,
          Seoul, Korea, 2002.

[MD5Attack]
          Dobbertin, H., "The Status of MD5 After a Recent Attack",
          CryptoBytes, Vol.2 No.2, 1996.



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[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.

Acknowledgments

   Thanks to Arun Ayyagari, Ashwin Palekar, and Tim Moore of Microsoft,
   Dorothy Stanley of Agere, Bob Moskowitz of TruSecure, Jesse Walker of
   Intel, Joe Salowey of Cisco and Russ Housley of Vigil Security for
   useful feedback.

Author Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: bernarda@microsoft.com
   Phone: +1 425 706 6605
   Fax:   +1 425 936 7329

   Dan Simon
   Microsoft Research
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: dansimon@microsoft.com
   Phone: +1 425 706 6711
   Fax:   +1 425 936 7329

   Jari Arkko
   Ericsson
   Jorvas 02420
   Finland

   Phone:
   EMail: jari.arkko@ericsson.com

   Pasi Eronen
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group
   Finland




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   EMail: pasi.eronen@nokia.com

   Henrik Levkowetz (editor)
   ipUnplugged AB
   Arenavagen 27
   Stockholm  S-121 28
   SWEDEN

   Phone: +46 708 32 16 08
   EMail: henrik@levkowetz.com









































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Appendix A - Ciphersuite Keying Requirements

   To date, PPP and IEEE 802.11 ciphersuites are suitable for keying by
   EAP.  This Appendix describes the keying requirements of common PPP
   and 802.11 ciphersuites.

   PPP ciphersuites include DESEbis [RFC2419], 3DES [RFC2420], and MPPE
   [RFC3078].  The DES algorithm is described in [FIPSDES], and DES
   modes (such as CBC, used in [RFC2419] and DES-EDE3-CBC, used in
   [RFC2420]) are described in [DESMODES].  For PPP DESEbis, a single
   56-bit encryption key is required, used in both directions. For PPP
   3DES, a 168-bit encryption key is needed, used in both directions. As
   described in [RFC2419] for DESEbis and [RFC2420] for 3DES, the IV,
   which is different in each direction, is "deduced from an explicit
   64-bit nonce, which is exchanged in the clear during the [ECP]
   negotiation phase."  There is therefore no need for the IV to be
   provided by EAP.

   For MPPE, 40-bit, 56-bit or 128-bit encryption keys are required in
   each direction, as described in [RFC3078]. No initialization vector
   is required.

   While these PPP ciphersuites provide encryption, they do not provide
   per-packet authentication or integrity protection, so an
   authentication key is not required in either direction.

   Within [IEEE-802.11], Transient Session Keys (TSKs) are required both
   for unicast traffic as well as for multicast traffic, and therefore
   separate key hierarchies are required for unicast keys and multicast
   keys. IEEE 802.11 ciphersuites include WEP-40, described in
   [IEEE-802.11], which requires a 40-bit encryption key, the same in
   either direction; and WEP-128, which requires a 104-bit encryption
   key, the same in either direction.  These ciphersuites also do not
   support per-packet authentication and integrity protection.  In
   addition to these unicast keys, authentication and encryption keys
   are required to wrap the multicast encryption key.

   Recently, new ciphersuites have been proposed for use with IEEE
   802.11 that provide per-packet authentication and integrity
   protection as well as encryption [IEEE-802.11i].  These include TKIP,
   which requires a single 128-bit encryption key and two 64-bit
   authentication keys (one for each direction); and AES CCMP, which
   requires a single 128-bit key (used in both directions) in order to
   authenticate and encrypt data.

   As with WEP, authentication and encryption keys are also required to
   wrap the multicast encryption (and possibly, authentication) keys.




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Appendix B - Transient EAP Key (TEK) Hierarchy

   Figure B-1 illustrates the TEK key hierarchy for EAP-TLS [RFC2716],
   which is based on the TLS key hierarchy described in [RFC2246].  The
   TLS-negotiated ciphersuite is used to set up a protected channel for
   use in protecting the EAP conversation,  keyed by the derived TEKs.
   The TEK derivation proceeds as follows:

   master_secret = TLS-PRF-48(pre_master_secret, "master secret",
                   client.random || server.random)
   TEK           = TLS-PRF-X(master_secret, "key expansion",
                   server.random || client.random)
   Where:
   TLS-PRF-X =     TLS pseudo-random function defined in [RFC2246],
                   computed to X octets.

          |                       |                           |
          |                       | pre_master_secret         |
    server|                       |                           | client
    Random|                       V                           | Random
          |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |
          |     |                                     |       |
          |     |                                     |       |
          +---->|             master_secret           |<------+
          |     |               (TMS)                 |       |
          |     |                                     |       |
          |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |
          |                       |                           |
          |                       |                           |
          |                       |                           |
          V                       V                           V
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                         Key Block                             |
    |                          (TEKs)                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           |           |           |           |           |
      | client    | server    | client    | server    | client    | server
      | MAC       | MAC       | write     | write     | IV        | IV
      |           |           |           |           |           |
      V           V           V           V           V           V

   Figure B-1 - TLS [RFC2246] Key Hierarchy






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Appendix C - EAP-TLS Key Hierarchy

   In EAP-TLS [RFC2716], the MSK is divided into two halves,
   corresponding to the "Peer to Authenticator Encryption Key" (Enc-
   RECV-Key, 32 octets, also known as the PMK) and "Authenticator to
   Peer Encryption Key" (Enc-SEND-Key, 32 octets).  In [RFC2548], the
   Enc-RECV-Key (the PMK) is transported in the MS-MPPE-Recv-Key
   attribute, and the Enc-SEND-Key is transported in the MS-MPPE-Send-
   Key attribute.

   The EMSK is also divided into two halves, corresponding to the "Peer
   to Authenticator Authentication Key" (Auth-RECV-Key, 32 octets) and
   "Authenticator to Peer Authentication Key" (Auth-SEND-Key, 32
   octets).  The IV is a 64 octet quantity that is a known value; octets
   0-31 are known as the "Peer to Authenticator IV" or RECV-IV, and
   Octets 32-63 are known as the "Authenticator to Peer IV", or SEND-IV.

   In EAP-TLS, the MSK, EMSK and IV are derived from the TLS master
   secret via a one-way function. This ensures that the TLS master
   secret cannot be derived from the MSK, EMSK or IV unless the one-way
   function (TLS PRF) is broken.  Since the MSK is derived from the the
   TLS master secret, if the TLS master secret is compromised then the
   MSK is also compromised.

   The key derivation scheme specified in RFC 2716 that was specified
   prior to the introduction of the terminology MSK and EMSK MUST be
   interpreted as follows:

   MSK           = TLS-PRF-64(TMS, "client EAP encryption",
                      client.random || server.random)
   EMSK          = second 64 octets of:
                   TLS-PRF-128(TMS, "client EAP encryption",
                      client.random || server.random)
   IV            = TLS-PRF-64("", "client EAP encryption",
                      client.random || server.random)

   AAA-Key(0,31) = Peer to Authenticator Encryption Key (Enc-RECV-Key)
                   (MS-MPPE-Recv-Key in [RFC2548]).  Also known as the
                   PMK.
   AAA-Key(32,63)= Authenticator to Peer Encryption Key (Enc-SEND-Key)
                   (MS-MPPE-Send-Key in [RFC2548])
   EMSK(0,31)    = Peer to Authenticator Authentication Key (Auth-RECV-Key)
   EMSK(32,63)   = Authenticator to Peer Authentication Key (Auth-Send-Key)
   IV(0,31)      = Peer to Authenticator Initialization Vector (RECV-IV)
   IV(32,63)     = Authenticator to Peer Initialization vector (SEND-IV)

   Where:




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   AAA-Key(W,Z)  = Octets W through Z includes of the AAA-Key.
   IV(W,Z)       = Octets W through Z inclusive of the IV.
   MSK(W,Z)      = Octets W through Z inclusive of the MSK.
   EMSK(W,Z)     = Octets W through Z inclusive of the EMSK.
   TMS           = TLS master_secret
   TLS-PRF-X     = TLS PRF function defined in [RFC2246] computed to X octets
   client.random = Nonce generated by the TLS client.
   server.random = Nonce generated by the TLS server.

   Figure C-1 describes the process by which the MSK,EMSK,IV and
   ultimately the TSKs, are derived from the TLS Master Secret.

                                                                       ---+
                                 |                                        ^
                                 | TLS Master Secret (TMS)                |
                                 |                                        |
                                 V                                        |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                    |
               |                                     |            EAP     |
               |       Master Session Key (MSK)      |           Method   |
               |              Derivation             |                    |
               |                                     |                    V
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+             EAP ---+
                 |               |                 |               API    ^
                 | MSK           | EMSK            | IV                   |
                 |               |                 |                      |
                 V               V                 V                      v
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     ---+
   |                                                             |        |
   |                                                             |        |
   |             backend authentication server                   |        |
   |                                                             |        |
   |                                                             |        V
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     ---+
     |                 |                                                  ^
     | AAA-Key(0,31)   | AAA-Key(32,63)                                       |
     | (PMK)           |                                     Transported  |
     |                 |                                        via AAA   |
     |                 |                                                  |
     V                 V                                                  V
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   ---+
   |                                                               |      ^
   |                Ciphersuite-Specific Transient Session         | Auth.|
   |                       Key Derivation                          |      |
   |                                                               |      V
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   ---+

   Figure C-1 - EAP TLS [RFC2716] Key hierarchy



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Appendix D - Example Transient Session Key (TSK) Derivation

   Within IEEE 802.11 RSN, the Pairwise Transient Key (PTK), a transient
   session key used to protect unicast traffic, is derived from the PMK
   (octets 0-31 of the MSK), known in [RFC2716] as the Peer to
   Authenticator Encryption Key.  In [IEEE-802.11i],  the PTK is derived
   from the PMK via the following formula:

   PTK = EAPOL-PRF-X(PMK, "Pairwise key expansion", Min(AA,SA) ||
         Max(AA, SA) || Min(ANonce,SNonce) || Max(ANonce,SNonce))

   Where:

   PMK             = AAA-Key(0,31)
   SA              = Station MAC address (Calling-Station-Id)
   AA              = Access Point MAC address (Called-Station-Id)
   ANonce          = Access Point Nonce
   SNonce          = Station Nonce
   EAPOL-PRF-X     = Pseudo-Random Function based on HMAC-SHA1, generating
                     a PTK of size X octets.

   TKIP uses X = 64, while CCMP, WRAP, and WEP use X = 48.

   The EAPOL-Key Confirmation Key (KCK) is used to provide data origin
   authenticity in the TSK derivation. It utilizes the first 128 bits
   (bits 0-127) of the PTK.  The EAPOL-Key Encryption Key (KEK) provides
   confidentiality in the TSK derivation.  It utilizes bits 128-255 of
   the PTK. Bits 256-383 of the PTK are used by Temporal Key 1, and Bits
   384-511 are used by Temporal Key 2.  Usage of TK1 and TK2 is
   ciphersuite specific. Details are available in [IEEE-802.11i].





















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Appendix E - Key Names and Scope in Existing Methods

   This appendix specifies the key names and scope in methods that have
   been published prior to the publication of this RFC.  What is needed
   in addition to the rules in Section 2.4 is the definition of what EAP
   peer and server names are used, what Method-Id is used, and how these
   are encoded.

EAP-TLS

   The EAP-TLS Method-Id is provided by the concatenation of the peer
   and server nonces.

   Where certificates are used, the Session-Id scope is determined via
   the EAP peer and server names, deduced from the altSubjectName in the
   peer and server certificates.

   Issue: What happens if a pre-shaked key ciphersuite is negotiated?
   How are the EAP peer and server names determined?

EAP-AKA

   The EAP-AKA Method-Id is the contents of the RAND field from the
   AT_RAND attribute, followed by the contents of the AUTN field in the
   AT_AUTN attribute.

   The EAP peer name is the contents of the Identity field from the
   AT_IDENTITY attribute, using only the Actual Identity Length octets
   from the beginning, however.  Note that the contents are used as they
   are transmitted, regardless of whether the transmitted identity was a
   permanent, pseudonym, or fast reauthentication identity.  The EAP
   server name is an empty string.

EAP-SIM

   The Method-Id is the contents of the RAND field from the AT_RAND
   attribute, followed by the contents of the NONCE_MT field in the
   AT_NONCE_MT attribute.

   The EAP peer name is the contents of the Identity field from the
   AT_IDENTITY attribute, using only the Actual Identity Length octets
   from the beginning, however.  Note that the contents are used as they
   are transmitted, regardless of whether the transmitted identity was a
   permanent, pseudonym, or fast reauthentication identity.  The EAP
   server name is an empty string.






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Appendix F - Security Association Examples

EAP Method SA Example: EAP-TLS

   In EAP-TLS [RFC2716], after the EAP authentication the client (peer)
   and server can store the following information:

      o  Implicitly, the EAP method this SA refers to (EAP-TLS)
      o  Session identifier (a value selected by the server)
      o  Certificate of the other party (server stores the client's
         certificate and vice versa)
      o  Ciphersuite and compression method
      o  TLS Master secret (known as the EAP-TLS Master Key)
      o  SA lifetime (ensuring that the SA is not stored forever)
      o  If the client has multiple different credentials (certificates
         and corresponding private keys), a pointer to those credentials

   When the server initiates EAP-TLS, the client can look up the EAP-TLS
   SA based on the credentials it was going to use (certificate and
   private key), and the expected credentials (certificate or name) of
   the server. If an EAP-TLS SA exists, and it is not too old, the
   client informs the server about the existence of this SA by including
   its Session-Id in the TLS ClientHello message. The server then looks
   up the correct SA based on the Session-Id (or detects that it doesn't
   yet have one).

EAP Method SA Example: EAP-AKA

   In EAP-AKA [I-D.arkko-pppext-eap-aka], after EAP authentication the
   client and server can store the following information:

      o  Implicitly, the EAP method this SA refers to (EAP-AKA)
      o  A re-authentication pseudonym
      o  The client's permanent identity (IMSI)
      o  Replay protection counter
      o  Authentication key (K_aut)
      o  Encryption key (K_encr)
      o  Original Master Key (MK)
      o  SA lifetime (ensuring that the SA is not stored forever)

   When the server initiates EAP-AKA, the client can look up the EAP-AKA
   SA based on the credentials it was going to use (permanent identity).
   If an EAP-AKA SA exists, and it is not too old, the client informs
   the server about the existence of this SA by sending its re-
   authentication pseudonym as its identity in EAP Identity Response
   message, instead of its permanent identity. The server then looks up
   the correct SA based on this identity.




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AAA SA Example: RADIUS

   In RADIUS, where shared secret authentication is used, the client and
   server store each other's IP address and the shared secret, which is
   used to calculate the Response Authenticator [RFC2865] and Message-
   Authenticator [RFC3579] values, and to encrypt some attributes (such
   as the AAA-Key, see [RFC3580] Section 3.16).

   Where IPsec is used to protect RADIUS [RFC3579] and IKE is used for
   key management, the parties store information necessary to
   authenticate and authorize the other party (e.g. certificates, trust
   anchors and names). The IKE exchange results in IKE Phase 1 and Phase
   2 SAs containing information used to protect the conversation
   (session keys, selected ciphersuite, etc.)

AAA SA Example: Diameter with TLS

   When using Diameter protected by TLS, the parties store information
   necessary to authenticate and authorize the other party (e.g.
   certificates, trust anchors and names). The TLS handshake results in
   a short-term TLS SA that contains information used to protect the
   actual communications (session keys, selected TLS ciphersuite, etc.).

Service SA Example: 802.11i

   [IEEE802.11i] Section 8.4.1.1 defines the security associations used
   within IEEE 802.11.  A summary follows; the standard should be
   consulted for details.

   o Pairwise Master Key Security Association (PMKSA)

      The PMKSA is a bi-directional SA, used by both parties for sending
      and receiving.  The PMKSA is the Root Service SA.  It is created
      on the peer when EAP authentication completes successfully or a
      pre-shared key is configured.  The PMKSA is created on the
      authenticator when the PMK is received or created on the
      authenticator or a pre-shared key is configured.  The PMKSA is
      used to create the PTKSA.  PMKSAs are cached for their lifetimes.
      The PMKSA consists of the following elements:

      - PMKID (security association identifier)
      - Authenticator MAC address
      - PMK
      - Lifetime
      - Authenticated Key Management Protocol (AKMP)
      - Authorization parameters specified by the AAA server or
        by local configuration.  This can include
        parameters such as the peer's authorized SSID.



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        On the peer, this information can be locally
        configured.
      - Key replay counters (for EAPOL-Key messages)
      - Reference to PTKSA (if any), needed to:
          o delete it (e.g. AAA server-initiated disconnect)
          o replace it when a new four-way handshake is done
      - Reference to accounting context, the details of which depend
        on the accounting protocol used, the implementation
        and administrative details. In RADIUS, this could include
        (e.g. packet and octet counters, and Acct-Multi-Session-Id).

   o Pairwise Transient Key Security Association (PTKSA)

      The PTKSA is a bi-directional SA created as the result of a
      successful four-way handshake.  The PTKSA is a unicast service SA.
      There may only be one PTKSA between a pair of peer and
      authenticator MAC addresses.  PTKSAs are cached for the lifetime
      of the PMKSA.  Since the PTKSA is tied to the PMKSA, it only has
      the additional information from the 4-way handshake.  The PTKSA
      consists of the following:

         - Key (PTK)
         - Selected ciphersuite
         - MAC addresses of the parties
         - Replay counters, and ciphersuite specific state
         - Reference to PMKSA: This is needed when:
            o A new four-way handshake is needed (lifetime, TKIP
              countermeasures), and we need to know which PMKSA to use

   o Group Transient Key Security Association (GTKSA)

      The GTKSA is a uni-directional SA created based on the four-way
      handshake or the group key handshake. The GTKSA is a multicast
      service SA.  A GTKSA consists of the following:

         - Direction vector (whether the GTK is used for transmit or receive)
         - Group cipher suite selector
         - Key (GTK)
         - Authenticator MAC address
         - Via reference to PMKSA, or copied here:
           o Authorization parameters
           o Reference to accounting context

   Service SA Example: IKEv2/IPsec

      Note that this example is intended to be informative, and it does
      not necessarily include all information stored.




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   o IKEv2 SA

      - Protocol version
      - Identities of the parties
      - IKEv2 SPIs
      - Selected ciphersuite
      - Replay protection counters (Message ID)
      - Keys for protecting IKEv2 messages (SK_ai/SK_ar/SK_ei/SK_er)
      - Key for deriving keys for IPsec SAs (SK_d)
      - Lifetime information
      - On the authenticator, service authorization information
        received from the backend authentication server.

   When processing an incoming message, the correct SA is looked up
   based on the SPIs.

   o IPsec SAs/SPD

      - Traffic selectors
      - Replay protection counters
      - Selected ciphersuite
      - IPsec SPI
      - Keys
      - Lifetime information
      - Protocol mode (tunnel or transport)

      The correct SA is looked up based on SPI (for inbound packets), or
      SPD traffic selectors (for outbound traffic).  A separate IPsec SA
      exists for each direction.

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   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary



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   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
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   This document and the information contained herein are provided on an
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Open Issues

   Open issues relating to this specification are tracked on the
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   http://www.drizzle.com/~aboba/EAP/eapissues.html

























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