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Versions: (draft-aboba-pppext-key-problem) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 RFC 5247

EAP Working Group                                          Bernard Aboba
INTERNET-DRAFT                                                 Dan Simon
Category: Informational                                        Microsoft
<draft-ietf-eap-keying-03.txt>                                  J. Arkko
18 July 2004                                                    Ericsson
                                                               P. Eronen
                                                                   Nokia
                                                       H. Levkowetz, Ed.
                                                             ipUnplugged



   Extensible Authentication Protocol (EAP) Key Management Framework

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Copyright Notice

   Copyright (C) The Internet Society (2004).  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".





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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.     EAP Key Hierarchy .....................................   13
      2.1       Key Terminology .................................   13
      2.2       Key Hierarchy ...................................   15
      2.3       Key Lifetimes ...................................   17
      2.4       Key Naming ......................................   24
   3.     Security associations .................................   26
      3.1       EAP Method SA ...................................   26
      3.2       EAP-Key SA ......................................   28
      3.3       AAA SA(s) .......................................   28
      3.4       Service SA(s) ...................................   29
   4.     Handoff Support .......................................   34
      4.1       Key Scope Issues ................................   35
      4.2       Authorization Issues ............................   36
      4.3       Correctness Issues ..............................   38
   5.     Security Considerations  ..............................   40
      5.1       Security Terminology ............................   40
      5.2       Threat Model ....................................   41
      5.3       Security Analysis ...............................   43
      5.4       Man-in-the-middle Attacks .......................   47
      5.5       Denial of Service Attacks .......................   47
      5.6       Impersonation ...................................   48
      5.7       Channel Binding .................................   49
      5.8       Key Strength ....................................   50
      5.9       Key Wrap ........................................   50




















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   6.     Security Requirements .................................   51
      6.1       EAP Method Requirements .........................   51
      6.2       AAA Protocol Requirements .......................   54
      6.3       Secure Association Protocol Requirements ........   55
      6.4       Ciphersuite Requirements ........................   57
   7.     IANA Considerations ...................................   58
   8.     References ............................................   59
      8.1       Normative References ............................   59
      8.2       Informative References ..........................   59
   Acknowledgments ..............................................   63
   Author's Addresses ...........................................   63
   Appendix A - Ciphersuite Keying Requirements .................   65
   Appendix B - Transient EAP Key (TEK) Hierarchy ...............   66
   Appendix C - EAP Key Hierarchy ...............................   67
   Appendix D - Transient Session Key (TSK) Derivation ..........   69
   Appendix E - AAA-Key Derivation ..............................   70
   Appendix F - AMSK Derivation .................................   71
   Intellectual Property Statement ..............................   72
   Full Copyright Statement .....................................   72
































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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 [IEEE8021X].

   This document provides a framework for the generation, transport and
   usage of keying material generated by EAP authentication algorithms,
   known as "methods".  Since in EAP keying material is generated by EAP
   methods, transported by AAA protocols, transformed into session keys
   by Secure Association Protocols and used by lower layer ciphersuites,
   it is necessary to describe each of these elements and provide a
   system-level security analysis.

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].

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



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     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 key(s) used to protect information.  This
     information in the security association is stored by each party of
     the security association and must be consistent among the parties.
     Elements of a security association 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.  The situation is illustrated
   in Figure 1.

   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 peer may be stationary, in which case it may establish
   communications with one or more authenticators while remaining in one
   location.  Alternatively, the peer may be mobile, changing its point
   of attachment from one authenticator to another, or moving between
   points of attachment on a single authenticator.

   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.



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                            +-+-+-+-+
                            |       |
                            | EAP   |
                            | Peer  |
                            |       |
                            +-+-+-+-+
                              | | |  Peer Ports
                             /  |  \
                            /   |   \
 Phase 0: Discovery        /    |    \
 Phase 1: Authentication  /     |     \
 Phase 2: Secure         /      |      \
          Association   /       |       \
                       /        |        \
                      /         |         \
                   | | |      | | |      | | |  Authenticator Ports
                 +-+-+-+-+  +-+-+-+-+  +-+-+-+-+
                 |       |  |       |  |       |
                 | Auth. |  | Auth. |  | Auth. |
                 |       |  |       |  |       |
                 +-+-+-+-+  +-+-+-+-+  +-+-+-+-+
                      \         |         /
                       \        |        /
                        \       |       /
          EAP over AAA   \      |      /
            (optional)    \     |     /
                           \    |    /
                            \   |   /
                             \  |  /
                            +-+-+-+-+
                            |       |
                            | AAA   |
                            |Server |
                            |       |
                            +-+-+-+-+

Figure 1:  Relationship between peer, authenticator and backend server

   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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   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)

   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.














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   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

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 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 [IEEE80211] 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



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   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
   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.





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1.3.3.  Secure Association Phase

   The Secure Association phase (phase 2) always occurs after the
   completion of EAP authentication (phase 1a) and key transport (phase
   1b), and typically supports the following features:

[1]  Entity Naming.  A basic feature of a Secure Association Protocol is
     the naming of the parties engaged in the exchange.  As illustrated
     in Figure 1, it is possible for both the peer and NAS to have more
     than one physical or virtual port.  For the purposes of
     identification, it is therefore not possible to identify either
     peers or NAS devices using port identifiers.  Proper identification
     of the parties is critical to the Secure Association phase, 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.

[2]  Secure capabilities negotiation.  This provides for 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.  By
     securely negotiating session parameters, the secure Association
     Protocol protects against spoofing during the discovery phase and
     ensures that the peer and authenticator are in agreement about how
     data is to be secured.

[3]  Generation of fresh transient session keys (TSKs).  The Secure
     Association Protocol typically guarantees the freshness of session
     keys by exchanging nonces between both parties and then mixing the
     nonces with the AAA-Key in order to generate fresh unicast (phase
     2a) and 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, and by guaranteeing the
     freshness of transient session keys, assures that they are not
     reused.

[4]  Key activation and deletion. In order for the peer and
     authenticator to communicate securely, it is necessary for both
     sides to derive the same session keys, and remain in sync with
     respect to key state going forward.  One of the functions of the
     Secure Association Protocol is to synchronize the activation and
     deletion of keys so as to enable seamless rekey, or recovery from
     partial or complete loss of key state by the peer or authenticator.

[5]  Mutual proof of possession of the AAA-Key.  This demonstrates 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



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     peer cannot verify authenticator assertions (including the
     authenticator identity) as a result of this exchange.

1.4.  EAP Invariants

   By utilizing a three phase exchange, the EAP key management framework
   guarantees that certain basic characteristics, known as the "EAP
   Invariants" hold true for all implementations of EAP.  These include:

      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 [IEEE8021X], and IEEE
   802.11 wireless LANs [IEEE80211i].

   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.







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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
   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 [WLANREQ].  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 data
   is negotiated within the Secure Association Protocol, out-of-band of
   EAP.

   The backend authentication server is not 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 even have
   knowledge of the ciphersuites implemented by the peer and
   authenticator, or be aware of the ciphersuite negotiated between
   them, and therefore does not implement ciphersuite-specific code.

   Since ciphersuite negotiation occurs in the Secure Association
   protocol, not in EAP, ciphersuite-specific key generation, if
   implemented within an EAP method, would potentially conflict with the
   transient session key derivation occurring in the Secure Association
   protocol.  As a result, EAP methods generate keying material that is
   ciphersuite-independent.  Additional advantages of ciphersuite-
   independence include:

Update requirements
     If EAP methods were to specify how to derive transient session keys



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     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.

EAP method complexity
     Requiring each EAP method to include ciphersuite-specific code for
     transient session key derivation would increase method complexity
     and result in duplicated effort.

Knowledge of capabilities
     In practice, an EAP method may not have knowledge of the
     ciphersuite that has been negotiated between the peer and
     authenticator, since this negotiation typically occurs within the
     Secure Association Protocol.

     For example, PPP ciphersuite negotiation occurs in the Encryption
     Control Protocol (ECP) [RFC1968].  Since ECP negotiation occurs
     after authentication, unless an EAP method is utilized that
     supports ciphersuite negotiation, the peer, authenticator and
     backend authentication server may not be able to anticipate the
     negotiated ciphersuite and therefore this information cannot be
     provided to the EAP method.  Since ciphersuite negotiation is
     assumed to occur out-of-band, there is no need for ciphersuite
     negotiation within EAP.

     For example, a peer might be pre-configured with policy indicating
     the ciphersuite to be used in communicating with a given
     authenticator.  Within PPP, the ciphersuite is negotiated within
     the Encryption Control Protocol (ECP), after EAP authentication is
     completed.  Within [IEEE80211i], the AP ciphersuites are advertised
     in the Beacon and Probe Responses, and are securely verified during
     a 4-way handshake exchange after EAP authentication has completed.

2.  EAP Key Hierarchy

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



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     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
     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 Appendix E; in existing implementations the MSK is
     used as the AAA-Key.

Application-specific Master Session Keys (AMSKs)
     Keys derived from the EMSK which are cryptographically separate
     from each other and may be subsequently used in the derivation of
     Transient Session Keys (TSKs) for extended uses.  AMSK derivation
     is discussed in Appendix F.

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.




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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 [IEEE80211i] Octets 0-31 of
     the AAA-Key (Enc-RECV-Key) are known as the Pairwise Master Key
     (PMK).  In [IEEE80211i] 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 which are appropriate for the
     ciphersuite negotiated between the EAP peer and authenticator.  The
     TSKs are derived from AAA-Key during the Secure Association
     Protocol.  In the case of [IEEE80211i] the Secure Association
     Protocol consists of the 4-way handshake and group key derivation.
     An example TSK derivation is 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.



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   Based on the long term credential established between the peer and
   the server, EAP derives four 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
    [3] Keys calculated from exported quantities: AAA-Key, AMSKs.
    [4] Keys calculated by the Secure Association Protocol: 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 and EMSK are
   utilized in order to calculate the AAA-Key, as described in Appendix
   E.

   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-



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   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.

2.3.  Key Lifetimes

   As noted earlier, the EAP Key Management framework includes several
   types of keys.  Key lifetime issues associated with each type of key
   are discussed in the sections that follow.  Challenges include:

[a]  Security.  Where key lifetimes cannot be assumed, it may be
     necessary to negotiate them.  While key lifetimes may be announced
     or negotiated in the clear, a protected lifetime negotiation is
     RECOMMENDED.

[b]  Resource reclamation.  While key lifetimes may be securely
     negotiated, it is possible for the NAS or peer to reboot or reclaim
     resources, and therefore not be able to cache keys for their full
     lifetime.  As a result, lifetime negotiation does not guarantee
     that the key cache will remain synchronized.  It is therefore
     RECOMMENDED for the lower layer to provide a mechanism for key
     state resynchronization.  Note that securing this mechanism may be
     difficult since in this situation one or more of the parties
     initially do not possess a key with which to protect the
     resynchronization exchange.







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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                 V              EAP |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+  Method|
               |          AAA  Key Derivation,     | | Known       |        |
               |          Naming & Binding         | |(Not Secret) |        |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+        V
                       |                                                 ---+
                       |                                        Transported |
                       | AAA-Key                                     by AAA |
                       |                                           Protocol |
                       V                                                    V
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    ---+
        |                           |                                       ^
        |            TSK            |                           Ciphersuite |
        |        Derivation         |                              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 |
        | AAA-Key |               | AAA-Key |
        | (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 |
        |         |               |         |        |         |
        |         | Secure Assoc. |         | AAA-Key|         |
        | peer    |<------------->|Authenti-|<-------|  auth   |
        |         |===============| cator   |========| server  |
        |         |  Link Layer   |         |  AAA   | (EAP    |
        |         | (PPP,IEEE 802)|         |Protocol| server) |
        |MSK,EMSK |               |         |        |         |
        | AAA-Key |               | AAA-Key |        |MSK,EMSK,|
        | (TSKs)  |               |  (TSKs) |        | AAA-Key |
        |         |               |         |        |         |
        +-+-+-+-+-+               +-+-+-+-+-+        +-+-+-+-+-+
            ^                                            ^
            |                                            |
            | MSK, EMSK                                  | MSK, EMSK
            |                                            |
            |                                            |
        +-+-+-+-+-+                                  +-+-+-+-+-+
        |         |                                  |         |
        |  EAP    |                                  |  EAP    |
        |  Method |                                  |  Method |
        |         |                                  |         |
        |  (TEKs) |                                  |  (TEKs) |
        |         |                                  |         |
        +-+-+-+-+-+                                  +-+-+-+-+-+


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







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2.3.1.  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.  They remain valid only for the duration of the EAP
   conversation, and are lost once the EAP conversation completes.

   EAP methods may also implement a cache for other local keying
   material which may persist for multiple EAP conversations.  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.

2.3.2.  Exported Key Lifetimes

   All EAP methods generating keys are required to generate the MSK and
   EMSK, and may optionally generate the IV.  However, although new
   exported keys are generated during re-authentication, the lifetime of
   exported keys is conceptually distinct from the re-authentication
   time, since while re-authentication causes new exported keys to be
   derived, exported keys may be cached on the peer and server after a
   session completes and therefore their lifetime may be greater than
   the re-authentication time.

   Although exported keys are generated by the EAP method, most existing
   EAP methods do not negotiate the lifetime of the exported keys.  EAP,
   defined in [RFC3748], also does not support the negotiation of
   lifetimes for exported keying material such as the MSK, EMSK and IV.

   Several mechanisms exist for managing the lifetime of exported EAP
   keys.  Exported EAP keys may be cached on the EAP server as well as
   on the peer.  On the EAP server, it is RECOMMENDED that the lifetime
   of exported keys be managed as a system parameter.  Where the EAP
   method does not support the negotiation of the exported key lifetime,
   and where a negotiation mechanism is not provided by the lower lower,
   it is RECOMMENDED that the peer assume a default value of the
   exported key lifetime.  A value of 8 hours is suggested.

   Attempting to manage the lifetime of the EAP-Key SA using AAA
   attributes is NOT RECOMMENDED, since this requires the authenticator
   to maintain EAP-Key SA state.  As described in Section 3, EAP-Key SA
   state is typically only maintained on the peer and server, so
   requiring EAP-Key SA state to be maintained on the authenticator
   represents an unnecessary additional burden.






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2.3.3.  Calculated Key Lifetimes

   When keying material exported by EAP methods is replaced, new
   calculated keys are also put in place.  Similarly, when the keying
   material exported by EAP methods expires, so do the calculated keys.
   As a result, the lifetime of keys calculated from key material
   exported by EAP methods can be no larger than the lifetime of the
   exported keying material.

   However, since the lifetime of calculated keys can be less than that
   of the exported keys they are derived from, calculated key lifetimes
   are conceptually distinct from exported key lifetimes and re-
   authentication times, and need to be managed as a separate parameter.

   Note that just as the re-authentication time and the exported key
   lifetime are conceptually distinct parameters, so too are calculated
   key lifetimes conceptually distinct from the re-authentication time.

   AAA protocols such as RADIUS [RFC2865] support the Session-Timeout
   attribute.  As described in [RFC3580], this may be used to determine
   the maximum session time prior to re-authentication.  Since re-
   authentication results in the derivation of new exported keys and the
   transport of a new AAA-Key, while a session is in progress the
   maximum session time prior to re-authentication places an upper bound
   on the AAA-Key lifetime.

   However, after the session has terminated, it is possible for the
   AAA-Key to be cached on the authenticator.  Therefore the AAA-Key
   lifetime may be larger than the re-authentication time.  As a result,
   the AAA-Key lifetime needs to be managed as a separate parameter.

   Since the lifetime of the AAA-Key within the authenticator key cache
   is in part determined by authenticator resources, the AAA-Key
   lifetime is often managed as a system parameter on the authenticator.
   Since the authenticator may have fewer resources than either the EAP
   peer or server, it is possible that AAA-Key lifetime on the
   authenticator may be less than exported key lifetime maintained by
   the server, or that the authenticator may need to reclaim AAA-Key
   resources prior to expiration of the AAA-Key lifetime.  As a result,
   knowledge of the AAA-Key lifetime may not be sufficient for the peer
   to determine whether a particular AAA-Key exists within the key cache
   of a given authenticator.

   Issues arise when attempting to manage synchronization of calculated
   key lifetimes between the AAA server and the authenticator using AAA
   attributes.

   Failure to mutually prove possession of the AAA-Key during the Secure



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   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.

   One problem is that the AAA protocol cannot guarantee synchronization
   of the peer and authenticator with respect to calculated key
   lifetimes.  While this synchronization could be provided by the
   Secure Association Protocol, in situations in which this protocol is
   not run immediately after EAP authentication, the calculated key
   lifetime will be undefined during the hiatus between the two
   protocols.  This can lead to problems with respect to key cache
   management.

   For example, where the AAA-key lifetime is negotiated between the
   authenticator and the peer within the Secure Association Protocol,
   this may be used by the peer to manage the lifetime of the AAA-Key
   once the Secure Association Protocol has completed.  However, where
   EAP pre-authentication is used, a hiatus may exist between the
   completion of the EAP method and the initiation of the Secure
   Association Protocol, during which peer cannot determine the lifetime
   of the AAA-Key.

   As a result, unless the AAA-Key lifetime is negotiated within the EAP
   method or the lower layer, the peer will not be able to determine a
   session-specific AAA-Key lifetime until it attempts to negotiate the
   Secure Association Protocol, which could fail due to AAA-Key lifetime
   expiration.

   One solution is to simplify management of the AAA-Key lifetime by
   treating it as a  system parameter of the peer, authenticator and
   server.  This enables a wider range of solutions.  For example, the
   lower layer may utilize Discovery mechanisms to ensure AAA-Key cache
   synchronization between the peer and authenticator.

   If 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.

2.3.4.  TSK Key Lifetimes

   Since the TSKs depend on the AAA-Key, replacement of the AAA-Key
   typically results in replacement of the TSKs.  However, deletion of
   the AAA-Key does not necessarily imply deletion of the corresponding
   TSKs.  Replacement or deletion of TSKs only implies replacement of
   the AAA-Key when the TSKs are taken from a portion of the AAA-Key.




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   While the lifetime of the TSKs may be shorter than or equal to the
   AAA-Key lifetime, the TSK lifetime cannot exceed the AAA-Key
   lifetime.  Where a Secure Association Protocol exists, it is possible
   for TSKs to be refreshed prior to re-authentication, and so the TSK
   Key Lifetime may also be shorter than or equal to the re-
   authentication timeout.  It is RECOMMENDED that the TSK Key lifetime
   be managed as a parameter distinct from the re-authentication timeout
   and the AAA-Key lifetime (except where the TSK is taken from the AAA-
   Key).

   Where TSKs are established as the result of a Secure Association
   Protocol exchange, it is RECOMMENDED that the Secure Association
   Protocol include secure negotiation of the TSK lifetime between the
   peer and authenticator.  Where the TSK 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.

   As described in Section 3, TSKs are part of Service SAs which reside
   on the peer and authenticator and as with the AAA-Key lifetime, the
   TSK lifetime is often determined by authenticator resources.  As a
   result, 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.

2.4.  Key Naming

   MSK Name

      This key is created between the EAP peer and EAP server, and is
      uniquely named by the concatenation of the string "MSK", EAP
      Method Type, EAP peer name, EAP server name, EAP peer nonce, and
      the EAP server nonce.  Here the EAP peer name and EAP server name
      are the identifiers securely exchanged within the EAP method.
      Since multiple MSKs may exist between an EAP peer and EAP server,
      the EAP peer nonce and EAP server nonce allow MSKs to be
      differentiated; at least one of these nonces is necessary. The
      inclusion of the Method Type in the name ensures that each EAP
      method has a distinct name space.

      Note that the components of the MSK Name are only known by the EAP
      method. As a result, the MSK Name is exported from the method, and
      no detailed format of the MSK Name can be specified without a
      reference to a particular method.

   EMSK Name

      The EMSK is named similarly to the above. Its name is the



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      concatenation of the string "EMSK", the EAP Method Type, EAP peer
      name, EAP server name, EAP peer nonce, and the EAP server nonce.


      Note that neither the MSK nor EMSK names include the authenticator
      identity or the peer or authenticator port over which the EAP
      conversation took place.  This is because the MSK and EMSK are not
      bound to an authenticator, or to specific ports on the peer or
      authenticator.

   AMSK Name

      AMSKs, if any, may be named by the concatenation of the string
      "AMSK", key label, application data (see Appendix F), and EMSK
      Name.

   AAA-Key Name

      The AAA-Key is named by the concatenation of the string "AAA-Key",
      the authenticator name (since the AAA-Key is bound to a particular
      authenticator), and the name of the key from which the AAA-Key is
      derived (MSK or AMSK Name).  For the purpose of identifying the
      authenticator, the contents of the NAS-Identifier attribute is
      recommended.  In order to ensure that all parties can agree on the
      authenticator name this requires the authenticator to advertise
      its name (typically using a lower layer mechanism, such as the
      802.11 Beacon/Probe Response).

      Note that the AAA-Key name does not include the peer or
      authenticator port over which the EAP conversation took place.
      This is because the AAA-Key is not bound to a specific peer or
      authenticator port.

   PMK Name

      The PMK has no name of its own, and is only identified by the AAA-
      Key from which it is derived.

   TEKs

      The TEKs may or may not be named. Their naming is specified in the
      EAP method.

   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



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      packet.  Explicit creation and deletion operations are also
      typically supported so that establishment and re-establishment of
      transient session keys 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
      name the AAA-Key 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 resume" or other
     functionality in some EAP methods.  Not all EAP methods create such
     an SA.

[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, but proposals such as [IEEE-03-084] and
     [I-D.irtf-aaaarch-handoff] use this SA for purposes such as pre-
     emptive key distribution.

[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).

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 resume": 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.



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   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 resume" 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.

   Contents:

      o  Implicitly, the EAP method this SA refers to
      o  One or more internal (non-exported) keys
      o  EAP method SA name
      o  SA lifetime

3.1.1.  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 or MK)
      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



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   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).

3.1.2.  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.

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, but future implementations could use this SA for pre-
   emptive key distribution.

   Contents:

      o  MSK and EMSK names
      o  MSK and EMSK
      o  SA lifetime

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.



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3.3.1.  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 [RFC2548]).

   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.)

3.3.2.  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.).

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
   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



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   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 IKE [RFC2409], 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
   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



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   for deletion of the AAA-Key by both parties; the action to be taken
   is defined by the Secure Association Protocol.

3.4.1.  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.  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.
        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.  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)



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         - 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.  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

3.4.2.  Example: IKEv2/IPsec

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

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



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   - 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.

3.4.3.  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



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     approach falls back to the previous option (no transfer).

     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's
     parameters as well.

4.  Handoff Support

   Within EAP, a number of mechanisms may 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.

   "Fast Handoff" is defined as a conversation in which EAP exchange
   (phase 1a) and associated AAA pass-through is bypassed, so as to
   reduce latency.  Unlike EAP pre-authentication, "Fast  Handoff"
   mechanisms do not result in additional AAA server load.  Fast handoff
   mechanisms include:

[a]  Pre-emptive handoff.  In this technique, the AAA server pre-
     establishes key state on the authenticator prior to arrival of the
     peer, without completion of EAP authentication.  As described in
     [IEEE-03-084] and [I.D.irtf-aaaarch-handoff], this technique
     includes conventional AAA-Key transport, but without an EAP
     authentication.

[b]  Context transfer.  In this technique, the old authenticator
     transfers the session text to the new authenticator, either prior
     to, or after the arrival of the peer.  As a result, AAA-Key
     transport (phase 1b) is bypassed.

   Regardless of how the AAA-Key is provisioned on a given
   authenticator, AAA-Key caching may be utilized in order to enable a
   peer to quickly re-establish a session with an authenticator.

   Where key caching is supported, once the AAA-Key is derived and/or
   transported to the authenticator, it may remain cached on the peer
   and authenticator, even after a subsequent session terminates.  To
   initiate a subsequent session with the same authenticator, the peer
   may utilize the Secure Association Protocol to confirm mutual
   possession of the AAA-Key by the peer and authenticator, thereby re-
   activating the AAA-Key for use in a subsequent session.

   The introduction of handoff support introduces new security



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   vulnerabilities as well as requirements for the secure handling of
   authorization context.  These issues are discussed in the sections
   that follow.

4.1.  Key Scope Issues

   As described in Appendix E, the AAA-Key is calculated from the EMSK
   and 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
   is restricted to use 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).  In the process of defining
   the scope of the AAA-Key, 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



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   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.

   Several measures are recommended to address these issues: peers and
   authenticators may have multiple ports.

[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 is typically accomplished using
     the Secure Association Protocol,  but also can be accomplished via
     the EAP method or the lower layer.

4.2.  Authorization Issues

   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



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   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.
   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?




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4.3.  Correctness Issues

   Bypassing all or portions of the AAA conversation creates challenges
   in ensuring that authorization is properly handled. These include:

[a]  Consistent application of session time limits.  A fast handoff
     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.  A fast handoff 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.

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

   A fast handoff mechanism capable of addressing these concerns is said
   to be "correct".  One condition for correctness is as follows: For a
   fast 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 fast handoff scheme will only succeed if it is
   "correct" in this way.  If a successful fast 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 fast
   handoff mechanism that bypasses AAA.  Backend authentication servers



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   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 fast
   handoff mechanism bypasses AAA, then the fast 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 [IEEE8021Q].  If a fast
   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
   factors, unless the restrictions are encoded within the
   authorizations, or a partial AAA conversation is included, then a
   fast 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 fast handoffs within such mechanisms.
   However, where the supported services differ between devices, the
   fast 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."




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   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 fast 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 fast 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 fast
   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 fast 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
   between a authenticator providing confidentiality and another
   authenticator that does not support this service.  The correct result
   of such a fast 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].

5.  Security Considerations

5.1.  Security Terminology

Cryptographic binding
     The demonstration of the EAP peer to the EAP server that a single
     entity has acted as the EAP peer for all methods executed within a
     tunnel method.  Binding MAY also imply that the EAP server



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     demonstrates to the peer that a single entity has acted as the EAP
     server for all methods executed within a tunnel method.  If
     executed correctly, binding serves to mitigate man-in-the-middle
     vulnerabilities.

Cryptographic separation
     Two keys (x and y) are "cryptographically separate" if an adversary
     that knows all messages exchanged in the protocol cannot compute x
     from y or y from x without "breaking" some cryptographic
     assumption.  In particular, this definition allows that the
     adversary has the knowledge of all nonces sent in cleartext as well
     as all predictable counter values used in the protocol.  Breaking a
     cryptographic assumption would typically require inverting a one-
     way function or predicting the outcome of a cryptographic pseudo-
     random number generator without knowledge of the secret state.  In
     other words, if the keys are cryptographically separate, there is
     no shortcut to compute x from y or y from x, but the work an
     adversary must do to perform this computation is equivalent to
     performing exhaustive search for the secret state value.

Key strength
     If the effective key strength is N bits, the best currently known
     methods to recover the key (with non-negligible probability)
     require on average an effort comparable to 2^(N-1) operations of a
     typical block cipher.

Mutual authentication
     This refers to an EAP method in which, within an interlocked
     exchange, the authenticator authenticates the peer and the peer
     authenticates the authenticator.  Two independent one-way methods,
     running in opposite directions do not provide mutual authentication
     as defined here.

5.2.  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 [WLANREQ].

   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.




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   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.

[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.




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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.

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

5.3.  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



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   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
      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



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   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 [IEEE80211i].  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
   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



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   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 [IEEE80211], 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
   [IEEE80211i] 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 [IEEE80211i], so as to address the threat of rogue devices,
   and provide keying material to bind the initial 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.





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5.4.  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
   derived from the EMSK, such as fast handoff keys, discussed in
   Appendix E.

5.5.  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.





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5.6.  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
   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



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   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].

5.7.  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 a AAA or 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
   [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.




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5.8.  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 necessary for EAP methods
   utilizing public key cryptography to 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:

        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

5.9.  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



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   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
   where key separation is implemented, compromise of the AAA-Key does
   not enable an attacker to impersonate the peer to another
   authenticator, since that requires possession of the MK or EMSK,
   which are not transported by the AAA protocol.  This vulnerability
   may be mitigated by implementation of redirect functionality, as
   provided in [RFC3588].

6.  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.

6.1.  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
   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
   Appendix E 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.




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   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.

   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 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 remain on the EAP peer and EAP server where it is
   derived; it MUST NOT be transported to, or shared with, additional



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   parties, or used to derive any other keys.

   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 key derivation (such as those specified
   in IKE [RFC2409] or TLS [RFC2246]), rather than inventing new ones.
   EAP methods SHOULD also utilize well established and analyzed
   mechanisms for MSK and EMSK derivation.

6.1.1.  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 be maintained within the EAP server.
        Only keys (AMSKs) derived according to this specification
        may be exported from the EAP server.

      o The EMSK MUST be unique for each session.

      o The EAP mechanism SHOULD provide a way of naming the EMSK.

   Implementations of EAP frameworks on the EAP-Peer and EAP-Server
   SHOULD provide an interface to obtain AMSKs.  The implementation MAY
   restrict which callers can obtain which keys.

6.1.2.  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



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        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 in any other way except to
        derive Application Master Session Keys (AMSKs) using the
        key derivation specified in Appendix F.  It MUST NOT
        use the EMSK directly for cryptographic protection of data,
        and SHOULD provide only the AMSKs to applications.

      o Applications MUST define distinct key labels, application
        specific data, and the length of derived key material used in the key
        derivation described in Appendix F.

      o Applications MUST define how they use their AMSK to derive TSKs
        for their use.

6.2.  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-



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     Address, NAS-IPv6-Address, NAS-Identifier in RADIUS).  For details,
     see [RFC3579] Section 4.3.7.

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].

6.3.  Secure Association Protocol Requirements

   The Secure Association Protocol supports the following:





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

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



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     authentication server is not fresh.  This is typically supported by
     including an exchange of nonces within the Secure Association
     Protocol.

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.

6.4.  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



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     MUST be cryptographically separate from the TEKs.

7.  IANA Considerations

   This section provides guidance to the Internet Assigned Numbers
   Authority (IANA) regarding registration of values related to EAP key
   management, in accordance with BCP 26, [RFC2434].

   The following terms are used here with the meanings defined in BCP
   26: "name space", "assigned value", "registration".

   The following policies are used here with the meanings defined in BCP
   26: "Private Use", "First Come First Served", "Expert Review",
   "Specification Required", "IETF Consensus", "Standards Action".

   For registration requests where a Designated Expert should be
   consulted, the responsible IESG area director should appoint the
   Designated Expert.  The intention is that any allocation will be
   accompanied by a published RFC.  But in order to allow for the
   allocation of values prior to the RFC being approved for publication,
   the Designated Expert can approve allocations once it seems clear
   that an RFC will be published.  The Designated expert will post a
   request to the EAP WG mailing list (or a successor designated by the
   Area Director) for comment and review, including an Internet-Draft.
   Before a period of 30 days has passed, the Designated Expert will
   either approve or deny the registration request and publish a notice
   of the decision to the EAP WG mailing list or its successor, as well
   as informing IANA.  A denial notice must be justified by an
   explanation and, in the cases where it is possible, concrete
   suggestions on how the request can be modified so as to become
   acceptable.

   This document introduces a new name space for "key labels".  Key
   labels are ASCII strings and are assigned via IETF Consensus.  It is
   expected that key label specifications will include the following
   information:

        o A description of the application
        o The key label to be used
        o How TSKs will be derived from the AMSK and how they will be used
        o If application specific data is used, what it is and how it is
           maintained
        o Where the AMSKs or TSKs will be used and how they are
          communicated if necessary.







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8.  References

8.1.  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.

8.2.  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.

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





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[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.

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

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

[IEEE802] Institute of Electrical and Electronics Engineers, "IEEE
          Standards for Local and Metropolitan Area Networks: Overview



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          and Architecture", ANSI/IEEE Standard 802, 1990.

[IEEE80211]
          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 802.11-1999, 1999.

[IEEE8021X]
          Institute of Electrical and Electronics Engineers, "Local and
          Metropolitan Area Networks: Port-Based Network Access
          Control", IEEE Standard 802.1X-2004, September 2004.

[IEEE8021Q]
          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.

[IEEE80211F]
          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.

[IEEE80211i]
          Institute of Electrical and Electronics Engineers, "Draft
          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 Draft 802.11I/ D8, February 2004.

[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.



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[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-08 (work in progress), June 2004.

[I-D.irtf-aaaarch-handoff]
          Arbaugh, W. and B. Aboba, "Handoff Extension to RADIUS",
          draft-irtf-aaaarch-handoff-04 (work in progress), October
          2003.

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

[I-D.aboba-802-context]
          Aboba, B. and T. Moore, "A Model for Context Transfer in IEEE
          802", draft-aboba-802-context-03 (work in progress), October
          2003.

[I-D.arkko-pppext-eap-aka]
          Arkko, J. and H. Haverinen, "EAP AKA Authentication", draft-
          arkko-pppext-eap-aka-11 (work in progress), October 2003.

[IKEv2]   Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", draft-
          ietf-ipsec-ikev2-14 (work in progress), June 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.

[WLANREQ] Stanley, D., Walker, J. and B. Aboba, "EAP Method Requirements
          for Wireless LANs", draft-walker-ieee802-req-02.txt (work in



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          progress), July 2004.

[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, 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 [IEEE80211], 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
   [IEEE80211], 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 [IEEE80211i]. These include TKIP,
   which requires a single 128-bit encryption key and a 128-bit
   authentication key (used in both directions); AES CCMP, which
   requires a single 128-bit key (used in both directions) in order to
   authenticate and encrypt data; and WRAP, which requires a single
   128-bit key (used in both directions).

   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.
   master_secret = TLS term for the MK.

          |                       |                           |
          |                       | pre_master_secret         |
    server|                       |                           | client
    Random|                       V                           | Random
          |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |
          |     |                                     |       |
          |     |                                     |       |
          +---->|             master_secret           |<------+
          |     |               (MK)                  |       |
          |     |                                     |       |
          |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |
          |                       |                           |
          |                       |                           |
          |                       |                           |
          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 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 MK via a one-
   way function. This ensures that the MK 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 MK, if the MK is compromised then
   the MSK is also compromised.

   As described in [RFC2716], the formula for the derivation of the MSK,
   EMSK and IV from the MK is as follows:

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


   AAA-Key(W,Z)  = Octets W through Z includes of the AAA-Key.



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   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.
   MK            = 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 MK. Note that in [RFC2716],
   the MK is referred to as the "TLS Master Secret".

                                                                       ---+
                                 |                                        ^
                                 | TLS Master Secret (MK)                 |
                                 |                                        |
                                 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 - 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 [IEEE80211i],  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 [IEEE80211i].





















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Appendix E - AAA-Key Derivation

   Where a AAA-Key is generated as the result of a successful EAP
   authentication, the AAA-Key is set to MSK(0,63).

   As discussed in [I-D.irtf-aaaarch-handoff], [IEEE-02-758],
   [IEEE-03-084], and [8021XHandoff], keying material may be required
   for use in fast handoff between authenticators. Where the backend
   authentication server provides keying material to multiple
   authenticators in order to facilitate fast handoff, it is highly
   desirable for the keying material used on different authenticators to
   be cryptographically separate, so that if one authenticator is
   compromised, it does not lead to the compromise of other
   authenticators. Where keying material is provided by the backend
   authentication server, a key hierarchy derived from the EMSK, can be
   used to provide cryptographically separate keying material for use in
   fast handoff:

   AAA-Key-A = MSK(0,63)
   AAA-Key-B = PRF(EMSK(0,63),"EAP AAA-Key derivation for
               multiple attachments", AAA-Key-A,B-Called-Station-Id,
               Calling-Station-Id,length)

   AAA-Key-E = PRF(EMSK(0,63),"EAP AAA-Key derivation for
               multiple attachments",AAA-Key-A,E-Called-Station-Id,
               Calling-Station-Id, length)

   Where:
   Calling-Station-Id  = STA MAC address
   B-Called-Station-Id = AP B MAC address
   E-Called-Station-Id = AP E MAC address
   PRF = Some suitable pseudo-random function
   length = length of derived key material

   Here AAA-Key-A is the AAA-Key derived during the initial EAP
   authentication between the peer and authenticator A. Based on this
   initial EAP authentication, the EMSK is also derived, which can be
   used to derive AAA-Keys for fast authentication between the EAP peer
   and authenticators B and E.  Since the EMSK is cryptographically
   separate from the MSK, each of these AAA-Keys is cryptographically
   separate from each other, and are guaranteed to be unique between the
   EAP peer (also known as the STA) and the authenticator (also known as
   the AP).








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Appendix F - AMSK Key Derivation

   The EAP AMSK key derivation function (KDF) derives an AMSK from the
   Extended Master Session Key (EMSK), an application key label,
   optional application data, and output length.

   AMSK = KDF(EMSK, key label, optional application data, length)

   The key labels are printable ASCII strings unique for each
   application (see Section 7 for IANA Considerations).

   Additional ciphering keys (TSKs) can be derived from the AMSK using
   an application specific key derivation mechanism. In many cases, this
   AMSK->TSK derivation can simply split the AMSK to pieces of correct
   length. In particular, it is not necessary to use a cryptographic
   one-way function. Note that the length of the AMSK must be specified
   by the application.

F.1 The EAP AMSK Key Derivation Function

   The EAP key derivation function is taken from the PRF+ key expansion
   PRF from [IKEv2].  This KDF takes 4 parameters as input: secret,
   label, application data, and output length.  It is only defined for
   255 iterations so it may produce up to 5100 bytes of key material.

   For the purposes of this specification the secret is taken as the
   EMSK, the label is the key label described above concatenated with a
   NUL byte, the application data is also described above and the output
   length is two bytes.  The application data is optional and may not be
   used by some applications.  The KDF is based on HMAC-SHA1 [RFC2104]
   [SHA1]. For this specification we have:

      KDF (K,L,D,O) = T1 | T2 | T3 | T4 | ...

      where:
      T1 = prf (K, S | 0x01)
      T2 = prf (K, T1 | S | 0x02)
      T3 = prf (K, T2 | S | 0x03)
      T4 = prf (K, T3 | S | 0x04)

      prf = HMAC-SHA1
      K = EMSK
      L = key label
      D = application data
      O = OutputLength (2 bytes)
      S = L | " " | D | O

   The prf+ construction was chosen because of its simplicity and



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   efficiency over other PRFs such as those used in [TLS].  The
   motivation for the design of this PRF is described in [SIGMA].

   The NUL byte after the key label is used to avoid collisions if one
   key label is a prefix of another label (e.g. "foobar" and
   "foobarExtendedV2"). This is considered a simpler solution than
   requiring a key label assignment policy that prevents prefixes from
   occurring.

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   will not be revoked by the Internet Society or its successors or
   assigns.  This document and the information contained herein is
   provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
   INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
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Open Issues

   Open issues relating to this specification are tracked on the
   following web site:

   http://www.drizzle.com/~aboba/EAP/eapissues.html

Expiration Date

   This memo is filed as <draft-ietf-eap-keying-03.txt>,  and  expires
   January 5, 2005.
































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