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RFC 5247 - Extensible Authentication Protocol (EAP) Key Manageme


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Network Working Group                                           B. Aboba
Request for Comments: 5247                                      D. Simon
Updates: 3748                                      Microsoft Corporation
Category: Standards Track                                      P. Eronen
                                                                   Nokia
                                                             August 2008

   Extensible Authentication Protocol (EAP) Key Management Framework

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   The Extensible Authentication Protocol (EAP), defined in RFC 3748,
   enables extensible network access authentication.  This document
   specifies the EAP key hierarchy and provides a framework for the
   transport and usage of keying material and parameters generated by
   EAP authentication algorithms, known as "methods".  It also provides
   a detailed system-level security analysis, describing the conditions
   under which the key management guidelines described in RFC 4962 can
   be satisfied.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Language ......................................3
      1.2. Terminology ................................................3
      1.3. Overview ...................................................7
      1.4. EAP Key Hierarchy .........................................10
      1.5. Security Goals ............................................15
      1.6. EAP Invariants ............................................16
   2. Lower-Layer Operation ..........................................20
      2.1. Transient Session Keys ....................................20
      2.2. Authenticator and Peer Architecture .......................22
      2.3. Authenticator Identification ..............................23
      2.4. Peer Identification .......................................27
      2.5. Server Identification .....................................29
   3. Security Association Management ................................31
      3.1. Secure Association Protocol ...............................32
      3.2. Key Scope .................................................35
      3.3. Parent-Child Relationships ................................35
      3.4. Local Key Lifetimes .......................................37
      3.5. Exported and Calculated Key Lifetimes .....................37
      3.6. Key Cache Synchronization .................................40
      3.7. Key Strength ..............................................40
      3.8. Key Wrap ..................................................41
   4. Handoff Vulnerabilities ........................................41
      4.1. EAP Pre-Authentication ....................................43
      4.2. Proactive Key Distribution ................................44
      4.3. AAA Bypass ................................................46
   5. Security Considerations ........................................50
      5.1. Peer and Authenticator Compromise .........................51
      5.2. Cryptographic Negotiation .................................53
      5.3. Confidentiality and Authentication ........................54
      5.4. Key Binding ...............................................59
      5.5. Authorization .............................................60
      5.6. Replay Protection .........................................63
      5.7. Key Freshness .............................................64
      5.8. Key Scope Limitation ......................................66
      5.9. Key Naming ................................................66
      5.10. Denial-of-Service Attacks ................................67
   6. References .....................................................68
      6.1. Normative References ......................................68
      6.2. Informative References ....................................68
   Acknowledgments ...................................................74
   Appendix A - Exported Parameters in Existing Methods ..............75

1.  Introduction

   The Extensible Authentication Protocol (EAP), defined in [RFC3748],
   was designed to enable extensible authentication for network access
   in situations in which the Internet Protocol (IP) protocol is not
   available.  Originally developed for use with Point-to-Point Protocol
   (PPP) [RFC1661], it has subsequently also been applied to IEEE 802
   wired networks [IEEE-802.1X], Internet Key Exchange Protocol version
   2 (IKEv2) [RFC4306], and wireless networks such as [IEEE-802.11] and
   [IEEE-802.16e].

   EAP is a two-party protocol spoken between the EAP peer and server.
   Within EAP, keying material is generated by EAP authentication
   algorithms, known as "methods".  Part of this keying material can be
   used by EAP methods themselves, and part of this material can be
   exported.  In addition to the export of keying material, EAP methods
   can also export associated parameters such as authenticated peer and
   server identities and a unique EAP conversation identifier, and can
   import and export lower-layer parameters known as "channel binding
   parameters", or simply "channel bindings".

   This document specifies the EAP key hierarchy and provides a
   framework for the transport and usage of keying material and
   parameters generated by EAP methods.  It also provides a detailed
   security analysis, describing the conditions under which the
   requirements described in "Guidance for Authentication,
   Authorization, and Accounting (AAA) Key Management" [RFC4962] can be
   satisfied.

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 [RFC2119].

1.2.  Terminology

   The terms "Cryptographic binding", "Cryptographic separation", "Key
   strength" and "Mutual authentication" are defined in [RFC3748] and
   are used with the same meaning in this document, which also
   frequently uses the following terms:

   4-Way Handshake
      A pairwise Authentication and Key Management Protocol (AKMP)
      defined in [IEEE-802.11], which confirms mutual possession of a
      Pairwise Master Key by two parties and distributes a Group Key.

   AAA  Authentication, Authorization, and Accounting
      AAA protocols with EAP support include "RADIUS Support for EAP"
      [RFC3579] and "Diameter EAP Application" [RFC4072].  In this
      document, the terms "AAA server" and "backend authentication
      server" are used interchangeably.

   AAA-Key
      The term AAA-Key is synonymous with Master Session Key (MSK).
      Since multiple keys can be transported by AAA, the term is
      potentially confusing and is not used in this document.

   Authenticator
      The entity initiating EAP authentication.

   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].

   Channel Binding
      A secure mechanism for ensuring that a subset of the parameters
      transmitted by the authenticator (such as authenticator
      identifiers and properties) are agreed upon by the EAP peer and
      server.  It is expected that the parameters are also securely
      agreed upon by the EAP peer and authenticator via the lower layer
      if the authenticator advertised the parameters.

   Derived Keying Material
      Keys derived from EAP keying material, such as Transient Session
      Keys (TSKs).

   EAP Keying Material
      Keys derived by an EAP method; this includes exported keying
      material (MSK, Extended MSK (EMSK), Initialization Vector (IV)) as
      well as local keying material such as Transient EAP Keys (TEKs).

   EAP Pre-Authentication
      The use of EAP to pre-establish EAP keying material on an
      authenticator prior to arrival of the peer at the access network
      managed by that authenticator.

   EAP Re-Authentication
      EAP authentication between an EAP peer and a server with whom the
      EAP peer shares valid unexpired EAP keying material.

   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.

   Exported Keying Material
      The EAP Master Session Key (MSK), Extended Master Session Key
      (EMSK), and Initialization Vector (IV).

   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.  The EMSK
      MUST be at least as long as the MSK in size.

   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 (Transport Layer Security) [RFC5216], 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 it is
      OPTIONAL for EAP methods to generate it.  However, when it is
      generated, it MUST be unpredictable.

   Keying Material
      Unless otherwise qualified, the term "keying material" refers to
      EAP keying material as well as derived keying material.

   Key Scope
      The parties to whom a key is available.

   Key Wrap
      The encryption of one symmetric cryptographic key in another.  The
      algorithm used for the encryption is called a key wrap algorithm
      or a key encryption algorithm.  The key used in the encryption
      process is called a key-encryption key (KEK).

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

   Lower Layer
      The lower layer is responsible for carrying EAP frames between the
      peer and authenticator.

   Lower-Layer Identity
      A name used to identify the EAP peer and authenticator within the
      lower layer.

   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.

   Network Access Server (NAS)
      A device that provides an access service for a user to a network.

   Pairwise Master Key (PMK)
      Lower layers use the MSK in a lower-layer dependent manner.  For
      instance, in IEEE 802.11 [IEEE-802.11], Octets 0-31 of the MSK are
      known as the Pairwise Master Key (PMK); the Temporal Key Integrity
      Protocol (TKIP) and Advanced Encryption Standard Counter Mode with
      CBC-MAC Protocol (AES CCMP) ciphersuites derive their Transient
      Session Keys (TSKs) solely from the PMK, whereas the Wired
      Equivalent Privacy (WEP) ciphersuite, as noted in "IEEE 802.1X
      RADIUS Usage Guidelines" [RFC3580], derives its TSKs from both
      halves of the MSK.  In [IEEE-802.16e], the MSK is truncated to 20
      octets for PMK and 20 octets for PMK2.

   Peer
      The entity that responds to the authenticator.  In [IEEE-802.1X],
      this entity is known as the Supplicant.

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

   Secure Association Protocol
      An exchange that occurs between the EAP peer and authenticator in
      order to manage security associations derived from EAP exchanges.
      The protocol establishes unicast and (optionally) multicast
      security associations, which include symmetric keys and a context
      for the use of the keys.  An example of a Secure Association
      Protocol is the 4-way handshake defined within [IEEE-802.11].

   Session-Id
      The EAP Session-Id uniquely identifies an EAP authentication
      exchange between an EAP peer (as identified by the Peer-Id(s)) and
      server (as identified by the Server-Id(s)).  For more information,
      see Section 1.4.

   Transient EAP Keys (TEKs)
      Session keys that 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.  The TEKs are stored locally by the EAP method
      and are not exported.  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.

   Transient Session Keys (TSKs)
      Keys used to protect data exchanged after EAP authentication has
      successfully completed using the ciphersuite negotiated between
      the EAP peer and authenticator.

1.3.  Overview

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

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

   Of these phases, phase 0, 1b, and 2 are handled external to EAP.
   phases 0 and 2 are handled by the lower-layer protocol, and phase 1b
   is typically handled by a AAA protocol.

   In the discovery phase (phase 0), peers locate authenticators and
   discover their capabilities.  A peer can locate an authenticator
   providing access to a particular network, or a peer can locate an
   authenticator behind a bridge with which it desires to establish a
   Secure Association.  Discovery can occur manually or automatically,
   depending on the lower layer over which EAP runs.

   The authentication phase (phase 1) can begin once the peer and
   authenticator discover each other.  This phase, if it occurs, always
   includes EAP authentication (phase 1a).  Where the chosen EAP method
   supports key derivation, in phase 1a, EAP keying material is derived
   on both the peer and the EAP server.

   An additional step (phase 1b) is needed in deployments that include a
   backend authentication server, in order to transport keying material
   from the backend authentication server to the authenticator.  In
   order to obey the principle of mode independence (see Section 1.6.1),
   where a backend authentication server is present, all keying material
   needed by the lower layer is transported from the EAP server to the
   authenticator.  Since existing TSK derivation and transport
   techniques depend solely on the MSK, in existing implementations,
   this is the only keying material replicated in the AAA key transport
   phase 1b.

   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).  The Secure Association
   Protocol exchange (phase 2) 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 between the parties is shown in
   Figure 1.

   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 1: Conversation Overview

1.3.1.  Examples

   Existing EAP lower layers implement phase 0, 2a, and 2b in different
   ways:

   PPP
      The Point-to-Point Protocol (PPP), defined in [RFC1661], does not
      support discovery, nor does it include a Secure Association
      Protocol.

   PPPoE
      PPP over Ethernet (PPPoE), defined in [RFC2516], includes support
      for a Discovery stage (phase 0).  In this step, the EAP peer sends
      a PPPoE Active Discovery Initiation (PADI) packet to the broadcast
      address, indicating the service it is requesting.  The Access
      Concentrator replies with a PPPoE Active Discovery Offer (PADO)
      packet containing its name, the service name, and an indication of
      the services offered by the concentrator.  The discovery phase is
      not secured.  PPPoE, like PPP, does not include a Secure
      Association Protocol.

   IKEv2
      Internet Key Exchange v2 (IKEv2), defined in [RFC4306], includes
      support for EAP and handles the establishment of unicast security
      associations (phase 2a).  However, the establishment of multicast
      security associations (phase 2b) typically does not involve EAP
      and needs to be handled by a group key management protocol such as
      Group Domain of Interpretation (GDOI) [RFC3547], Group Secure
      Association Key Management Protocol (GSAKMP) [RFC4535], Multimedia
      Internet KEYing  (MIKEY) [RFC3830], or Group Key Distribution
      Protocol (GKDP) [GKDP].  Several mechanisms have been proposed for
      the discovery of IPsec security gateways.  [RFC2230] discusses the
      use of Key eXchange (KX) Resource Records (RRs) for IPsec gateway
      discovery; while KX RRs are supported by many Domain Name Service
      (DNS) server implementations, they have not yet been widely
      deployed.  Alternatively, DNS SRV RRs [RFC2782] can be used for
      this purpose.  Where DNS is used for gateway location, DNS
      security mechanisms such as DNS Security (DNSSEC) ([RFC4033],
      [RFC4035]), TSIG [RFC2845], and Simple Secure Dynamic Update
      [RFC3007] are available.

   IEEE 802.11
      IEEE 802.11, defined in [IEEE-802.11], handles discovery via the
      Beacon and Probe Request/Response mechanisms.  IEEE 802.11 Access
      Points (APs) periodically announce their Service Set Identifiers
      (SSIDs) as well as capabilities using Beacon frames.  Stations can

      query for APs by sending a Probe Request.  Neither Beacon nor
      Probe Request/Response frames are secured.  The 4-way handshake
      defined in [IEEE-802.11] enables the derivation of unicast (phase
      2a) and multicast/broadcast (phase 2b) secure associations.  Since
      the group key exchange transports a group key from the AP to the
      station, two 4-way handshakes can be needed in order to support
      peer-to-peer communications.  A proof of the security of the IEEE
      802.11 4-way handshake, when used with EAP-TLS, is provided in
      [He].

   IEEE 802.1X
      IEEE 802.1X-2004, defined in [IEEE-802.1X], does not support
      discovery (phase 0), nor does it provide for derivation of unicast
      or multicast secure associations.

1.4.  EAP Key Hierarchy

   As illustrated in Figure 2, the EAP method key derivation 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 as well as additional information.  This
   information is typically used outside of the EAP method to determine
   whether to grant access to a service.  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 the peer stores information necessary to choose which
   certificate to use for which service.  Based on the long-term
   credential established between the peer and the server, methods
   derive two types of EAP keying material:

      (a) Keying material calculated locally by the EAP method but not
          exported, such as the Transient EAP Keys (TEKs).

      (b) Keying material exported by the EAP method: Master Session Key
          (MSK), Extended Master Session Key (EMSK), Initialization
          Vector (IV).

   As noted in [RFC3748] Section 7.10:

      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 also MAY export the IV; however, the use of the IV is
   deprecated.  The EMSK MUST NOT be provided to an entity outside the
   EAP server or peer, nor is it permitted to pass any quantity to an
   entity outside the EAP server or peer from which the EMSK could be
   computed without breaking some cryptographic assumption, such as
   inverting a one-way function.

   EAP methods supporting key derivation and mutual authentication
   SHOULD export a method-specific EAP conversation identifier known as
   the Session-Id, as well as one or more method-specific peer
   identifiers (Peer-Id(s)) and MAY export one or more method-specific
   server identifiers (Server-Id(s)).  EAP methods MAY also support the
   import and export of channel binding parameters.  EAP method
   specifications developed after the publication of this document MUST
   define the Peer-Id, Server-Id, and Session-Id.  The Peer-Id(s) and
   Server-Id(s), when provided, identify the entities involved in
   generating EAP keying material.  For existing EAP methods, the
   Peer-Id, Server-Id, and Session-Id are defined in Appendix A.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         ---+
|                                                         |            ^
|                EAP Method                               |            |
|                                                         |            |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+   |            |
| |                                 |   |             |   |            |
| |       EAP Method Key            |<->| Long-Term   |   |            |
| |         Derivation              |   | Credential  |   |            |
| |                                 |   |             |   |            |
| |                                 |   +-+-+-+-+-+-+-+   |  Local to  |
| |                                 |                     |       EAP  |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                     |     Method |
|   |             |               |                       |            |
|   |             |               |                       |            |
|   |             |               |                       |            |
|   |             |               |                       |            |
|   |         +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+-+ |            |
|   |         | TEK       | |MSK, EMSK  | |IV           | |            |
|   |         |Derivation | |Derivation | |Derivation   | |            |
|   |         |           | |           | |(Deprecated) | |            |
|   |         +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+-+ |            |
|   |               ^             |               |       |            |
|   |               |             |               |       |            V
+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+         ---+
    |               |             |               |                    ^
    |               |             |               |           Exported |
    | Peer-Id(s),   | channel     | MSK (64+B)    | IV (64B)      by   |
    | Server-Id(s), | bindings    | EMSK (64+B)   | (Optional)    EAP  |
    | Session-Id    | & Result    |               |             Method |
    V               V             V               V                    V

     Figure 2:  EAP Method Parameter Import/Export

   Peer-Id

      If an EAP method that generates keys authenticates one or more
      method-specific peer identities, those identities are exported by
      the method as the Peer-Id(s).  It is possible for more than one
      Peer-Id to be exported by an EAP method.  Not all EAP methods
      provide a method-specific peer identity; where this is not
      defined, the Peer-Id is the null string.  In EAP methods that do
      not support key generation, the Peer-Id MUST be the null string.
      Where an EAP method that derives keys does not provide a Peer-Id,
      the EAP server will not authenticate the identity of the EAP peer
      with which it derived keying material.

   Server-Id

      If an EAP method that generates keys authenticates one or more
      method-specific server identities, those identities are exported
      by the method as the Server-Id(s).  It is possible for more than
      one Server-Id to be exported by an EAP method.  Not all EAP
      methods provide a method-specific server identity; where this is
      not defined, the Server-Id is the null string.  If the EAP method
      does not generate keying material, the Server-Id MUST be the null
      string.  Where an EAP method that derives keys does not provide a
      Server-Id, the EAP peer will not authenticate the identity of the
      EAP server with which it derived EAP keying material.

   Session-Id

      The Session-Id uniquely identifies an EAP session between an EAP
      peer (as identified by the Peer-Id) and server (as identified by
      the Server-Id).  Where non-expanded EAP Type Codes are used (EAP
      Type Code not equal to 254), the EAP Session-Id is the
      concatenation of the single octet EAP Type Code and a temporally
      unique identifier obtained from the method (known as the
      Method-Id):

      Session-Id = Type-Code || Method-Id

      Where expanded EAP Type Codes are used, the EAP Session-Id
      consists of the Expanded Type Code (including the Type, Vendor-Id
      (in network byte order) and Vendor-Type fields (in network byte
      order) defined in [RFC3748] Section 5.7), concatenated with a
      temporally unique identifier obtained from the method (Method-Id):

      Session-Id = 0xFE || Vendor-Id || Vendor-Type || Method-Id

      The Method-Id is typically constructed from nonces or counters
      used within the EAP method exchange.  The inclusion of the Type
      Code or Expanded Type Code in the EAP Session-Id ensures that each
      EAP method has a distinct Session-Id space.  Since an EAP session
      is not bound to a particular authenticator or specific ports on
      the peer and authenticator, the authenticator port or identity are
      not included in the Session-Id.

   Channel Binding

      Channel binding is the process by which lower-layer parameters are
      verified for consistency between the EAP peer and server.  In
      order to avoid introducing media dependencies, EAP methods that
      transport channel binding parameters MUST treat this data as
      opaque octets.  See Section 5.3.3 for further discussion.

1.4.1.  Key Naming

   Each key created within the EAP key management framework has a name
   (a unique identifier), as well as a scope (the parties to whom the
   key is available).  The scope of exported keying material and TEKs is
   defined by the authenticated method-specific peer identities
   (Peer-Id(s)) and the authenticated server identities (Server-Id(s)),
   where available.

   MSK and EMSK Names
        The MSK and EMSK are exported by the EAP peer and EAP server,
        and MUST be named using the EAP Session-Id and a binary or
        textual indication of the EAP keying material being referred to.

   PMK Name
        This document does not specify a naming scheme for the Pairwise
        Master Key (PMK).  The PMK is only identified by the name of the
        key from which it is derived.

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

   TEK Name
        Transient EAP Keys (TEKs) MAY be named; their naming is
        specified in the EAP method specification.

   TSK Name
        Transient Session Keys (TSKs) are typically named.  Their naming
        is specified in the lower layer so that the correct set of TSKs
        can be identified for processing a given packet.

1.5.  Security Goals

   The goal of the EAP conversation is to derive fresh session keys
   between the EAP peer and authenticator that are known only to those
   parties, and for both the EAP peer and authenticator to demonstrate
   that they are authorized to perform their roles either by each other
   or by a trusted third party (the backend authentication server).

   Completion of an EAP method exchange (phase 1a) supporting key
   derivation results in the derivation of EAP keying material (MSK,
   EMSK, TEKs) known only to the EAP peer (identified by the Peer-Id(s))
   and EAP server (identified by the Server-Id(s)).  Both the EAP peer
   and EAP server know this keying material to be fresh.  The Peer-Id
   and Server-Id are discussed in Sections 1.4, 2.4, and 2.5 as well as
   in Appendix A.  Key freshness is discussed in Sections 3.4, 3.5, and
   5.7.

   Completion of the AAA exchange (phase 1b) results in the transport of
   keying material from the EAP server (identified by the Server-Id(s))
   to the EAP authenticator (identified by the NAS-Identifier) without
   disclosure to any other party.  Both the EAP server and EAP
   authenticator know this keying material to be fresh.  Disclosure
   issues are discussed in Sections 3.8 and 5.3; security properties of
   AAA protocols are discussed in Sections 5.1 - 5.9.

   The backend authentication server is trusted to transport keying
   material only to the authenticator that was established with the
   peer, and it is trusted to transport that keying material to no other
   parties.  In many systems, EAP keying material established by the EAP
   peer and EAP server are combined with publicly available data to
   derive other keys.  The backend authentication server is trusted to
   refrain from deriving these same keys or acting as a
   man-in-the-middle even though it has access to the keying material
   that is needed to do so.

   The authenticator is also a trusted party.  The authenticator is
   trusted not to distribute keying material provided by the backend
   authentication server to any other parties.  If the authenticator
   uses a key derivation function to derive additional keying material,
   the authenticator is trusted to distribute the derived keying
   material only to the appropriate party that is known to the peer, and
   no other party.  When this approach is used, care must be taken to
   ensure that the resulting key management system meets all of the
   principles in [RFC4962], confirming that keys used to protect data
   are to be known only by the peer and authenticator.

   Completion of the Secure Association Protocol (phase 2) results in
   the derivation or transport of Transient Session Keys (TSKs) known
   only to the EAP peer (identified by the Peer-Id(s)) and authenticator
   (identified by the NAS-Identifier).  Both the EAP peer and
   authenticator know the TSKs to be fresh.  Both the EAP peer and
   authenticator demonstrate that they are authorized to perform their
   roles.  Authorization issues are discussed in Sections 4.3.2 and 5.5;
   security properties of Secure Association Protocols are discussed in
   Section 3.1.

1.6.  EAP Invariants

   Certain basic characteristics, known as "EAP Invariants", hold true
   for EAP implementations:

      Mode independence
      Media independence
      Method independence
      Ciphersuite independence

1.6.1.  Mode Independence

   EAP is typically deployed to support extensible network access
   authentication in situations where a peer desires network access via
   one or more authenticators.  Where authenticators are deployed
   standalone, the EAP conversation occurs between the peer and
   authenticator, and the authenticator locally implements one or more
   EAP methods.  However, when utilized in "pass-through" mode, EAP
   enables the deployment of new authentication methods without
   requiring the development of new code on the authenticator.

   While the authenticator can implement some EAP methods locally and
   use those methods to authenticate local users, it can at the same
   time act as a pass-through for other users and methods, forwarding
   EAP packets back and forth between the backend authentication server
   and the peer.  This is accomplished by encapsulating EAP packets
   within the Authentication, Authorization, and Accounting (AAA)
   protocol spoken between the authenticator and backend authentication
   server.  AAA protocols supporting EAP include RADIUS [RFC3579] and
   Diameter [RFC4072].

   It is a fundamental property of EAP that at the EAP method layer, the
   conversation between the EAP peer and server is unaffected by whether
   the EAP authenticator is operating in "pass-through" mode.  EAP
   methods operate identically in all aspects, including key derivation
   and parameter import/export, regardless of whether or not the
   authenticator is operating as a pass-through.

   The successful completion of an EAP method that supports key
   derivation results in the export of EAP keying material and
   parameters on the EAP peer and server.  Even though the EAP peer or
   server can import channel binding parameters that can include the
   identity of the EAP authenticator, this information is treated as
   opaque octets.  As a result, within EAP, the only relevant identities
   are the Peer-Id(s) and Server-Id(s).  Channel binding parameters are
   only interpreted by the lower layer.

   Within EAP, the primary function of the AAA protocol is to maintain
   the principle of mode independence.  As far as the EAP peer is
   concerned, its conversation with the EAP authenticator, and all
   consequences of that conversation, are identical, regardless of the
   authenticator mode of operation.

1.6.2.  Media Independence

   One of the goals of EAP is to allow EAP methods to function on any
   lower layer meeting the criteria outlined in [RFC3748] Section 3.1.
   For example, as described in [RFC3748], EAP authentication can be run
   over PPP [RFC1661], IEEE 802 wired networks [IEEE-802.1X], and
   wireless networks such as 802.11 [IEEE-802.11] and 802.16
   [IEEE-802.16e].

   In order to maintain media independence, it is necessary for EAP to
   avoid consideration 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 be restricted to identifiers
   associated with a particular usage environment (e.g., Medium Access
   Control (MAC) addresses).

   Note that media independence can 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.
   Channel binding parameters are treated as opaque octets by EAP
   methods so that handling them does not require media-specific
   knowledge.

1.6.3.  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 having to implement
   the EAP methods configured for use by peers.  In fact, since a
   pass-through authenticator need not implement any EAP methods at all,
   it cannot be assumed to support any EAP method-specific code.  As
   noted in [RFC3748] Section 2.3:

      Compliant pass-through authenticator implementations MUST by
      default forward EAP packets of any Type.

   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 Local Area Network (WLAN)
   authentication [RFC4017].  However, despite this, it is possible for
   the peer and authenticator to interoperate as long as a suitable EAP
   method is supported both on the EAP peer and server.

1.6.4.  Ciphersuite Independence

   Ciphersuite Independence is a requirement for media independence.
   Since lower-layer ciphersuites vary between media, media independence
   requires that exported EAP keying material be large enough (with
   sufficient entropy) to handle any ciphersuite.

   While EAP methods can negotiate the ciphersuite used in protection of
   the EAP conversation, the ciphersuite used for the protection of the
   data exchanged after EAP authentication has completed is negotiated
   between the peer and authenticator within the lower layer, outside of
   EAP.

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

   Since the ciphersuites used to protect data depend on the lower
   layer, requiring that EAP methods have knowledge of lower-layer
   ciphersuites would compromise the principle of media independence.
   As a result, methods export EAP keying material that is ciphersuite
   independent.  Since ciphersuite negotiation occurs in the lower
   layer, there is no need for lower-layer ciphersuite negotiation
   within EAP.

   In order to allow a ciphersuite to be usable within the EAP keying
   framework, the ciphersuite specification needs to describe how TSKs
   suitable for use with the ciphersuite are derived from exported EAP
   keying material.  To maintain method independence, algorithms for
   deriving TSKs MUST NOT depend on the EAP method, although algorithms
   for TEK derivation MAY be specific to the EAP method.

   Advantages of ciphersuite-independence include:

   Reduced update requirements
        Ciphersuite independence enables EAP methods to be used with new
        ciphersuites without requiring the methods to be updated.  If
        EAP methods were to specify how to derive transient session keys
        for each ciphersuite, they would need to be updated each time a
        new ciphersuite is developed.  In addition, backend
        authentication servers might not be usable with all EAP-capable
        authenticators, since the backend authentication server would
        also need to be updated each time support for a new ciphersuite
        is added to the authenticator.

   Reduced EAP method complexity
        Ciphersuite independence enables EAP methods to avoid having to
        include ciphersuite-specific code.  Requiring each EAP method to
        include ciphersuite-specific code for transient session key
        derivation would increase method complexity and result in
        duplicated effort.

   Simplified configuration
        Ciphersuite independence enables EAP method implementations on
        the peer and server to avoid having to configure
        ciphersuite-specific parameters.  The ciphersuite is negotiated
        between the peer and authenticator outside of EAP.  Where the
        authenticator operates in "pass-through" mode, the EAP server is
        not a party to this negotiation, nor is it involved in the data
        flow between the EAP peer and authenticator.  As a result, the
        EAP server does not have knowledge of the ciphersuites and
        negotiation policies implemented by the peer and authenticator,
        nor is it aware of the ciphersuite negotiated between them.  For
        example, since Encryption Control Protocol (ECP) negotiation
        occurs after authentication, when run over PPP, the EAP peer and

        server cannot anticipate the negotiated ciphersuite, and
        therefore, this information cannot be provided to the EAP
        method.

2.  Lower-Layer Operation

   On completion of EAP authentication, EAP keying material and
   parameters exported by the EAP method are provided to the lower layer
   and AAA layer (if present).  These include the Master Session Key
   (MSK), Extended Master Session Key (EMSK), Peer-Id(s), Server-Id(s),
   and Session-Id.  The Initialization Vector (IV) is deprecated, but
   might be provided.

   In order to preserve the security of EAP keying material derived
   within methods, lower layers MUST NOT export keys passed down by EAP
   methods.  This implies that EAP keying material passed down to a
   lower layer is for the exclusive use of that lower layer and MUST NOT
   be used within another lower layer.  This prevents compromise of one
   lower layer from compromising other applications using EAP keying
   material.

   EAP keying material provided to a lower layer MUST NOT be transported
   to another entity.  For example, EAP keying material passed down to
   the EAP peer lower layer MUST NOT leave the peer;  EAP keying
   material passed down or transported to the EAP authenticator lower
   layer MUST NOT leave the authenticator.

   On the EAP server, keying material and parameters requested by and
   passed down to the AAA layer MAY be replicated to the AAA layer on
   the authenticator (with the exception of the EMSK).  On the
   authenticator, the AAA layer provides the replicated keying material
   and parameters to the lower layer over which the EAP authentication
   conversation took place.  This enables mode independence to be
   maintained.

   The EAP layer, as well as the peer and authenticator layers, MUST NOT
   modify or cache keying material or parameters (including channel
   bindings) passing in either direction between the EAP method layer
   and the lower layer or AAA layer.

2.1.  Transient Session Keys

   Where explicitly supported by the lower layer, lower layers MAY cache
   keying material, including exported EAP keying material and/or TSKs;
   the structure of this key cache is defined by the lower layer.  So as
   to enable interoperability, new lower-layer specifications MUST
   describe key caching behavior.  Unless explicitly specified by the
   lower layer, the EAP peer, server, and authenticator MUST assume that

   peers and authenticators do not cache keying material.  Existing EAP
   lower layers and AAA layers handle the generation of transient
   session keys and caching of EAP keying material in different ways:

   IEEE 802.1X-2004
        When used with wired networks, IEEE 802.1X-2004 [IEEE-802.1X]
        does not support link-layer ciphersuites, and as a result, it
        does not provide for the generation of TSKs or caching of EAP
        keying material and parameters.  Once EAP authentication
        completes, it is assumed that EAP keying material and parameters
        are discarded; on IEEE 802 wired networks, there is no
        subsequent Secure Association Protocol exchange.  Perfect
        Forward Secrecy (PFS) is only possible if the negotiated EAP
        method supports this.

   PPP
        PPP, defined in [RFC1661], does not include support for a Secure
        Association Protocol, nor does it support caching of EAP keying
        material or parameters.  PPP ciphersuites derive their TSKs
        directly from the MSK, as described in [RFC2716] Section 3.5.
        This is NOT RECOMMENDED, since if PPP were to support caching of
        EAP keying material, this could result in TSK reuse.  As a
        result, once the PPP session is terminated, EAP keying material
        and parameters MUST be discarded.  Since caching of EAP keying
        material is not permitted within PPP, there is no way to handle
        TSK re-key without EAP re-authentication.  Perfect Forward
        Secrecy (PFS) is only possible if the negotiated EAP method
        supports this.

   IKEv2
        IKEv2, defined in [RFC4306], only uses the MSK for
        authentication purposes and not key derivation.  The EMSK, IV,
        Peer-Id, Server-Id or Session-Id are not used.  As a result, the
        TSKs derived by IKEv2 are cryptographically independent of the
        EAP keying material and re-key of IPsec SAs can be handled
        without requiring EAP re-authentication.  Within IKEv2, it is
        possible to negotiate PFS, regardless of which EAP method is
        negotiated.  IKEv2 as specified in [RFC4306] does not cache EAP
        keying material or parameters; once IKEv2 authentication
        completes, it is assumed that EAP keying material and parameters
        are discarded.  The Session-Timeout Attribute is therefore
        interpreted as a limit on the VPN session time, rather than an
        indication of the MSK key lifetime.

   IEEE 802.11
        IEEE 802.11 enables caching of the MSK, but not the EMSK, IV,
        Peer-Id, Server-Id, or Session-Id.  More details about the
        structure of the cache are available in [IEEE-802.11].  In IEEE

        802.11, TSKs are derived from the MSK using a Secure Association
        Protocol known as the 4-way handshake, which includes a nonce
        exchange.  This guarantees TSK freshness even if the MSK is
        reused.  The 4-way handshake also enables TSK re-key without EAP
        re-authentication.  PFS is only possible within IEEE 802.11 if
        caching is not enabled and the negotiated EAP method supports
        PFS.

   IEEE 802.16e
        IEEE 802.16e, defined in [IEEE-802.16e], supports caching of the
        MSK, but not the EMSK, IV, Peer-Id, Server-Id, or Session-Id.
        IEEE 802.16e supports a Secure Association Protocol in which
        TSKs are chosen by the authenticator without any contribution by
        the peer.  The TSKs are encrypted, authenticated, and integrity
        protected using the MSK and are transported from the
        authenticator to the peer.  TSK re-key is possible without EAP
        re-authentication.  PFS is not possible even if the negotiated
        EAP method supports it.

   AAA
        Existing implementations and specifications for RADIUS/EAP
        [RFC3579] or Diameter EAP [RFC4072] do not support caching of
        keying material or parameters.  In existing AAA clients, proxy
        and server implementations, exported EAP keying material (MSK,
        EMSK, and IV), as well as parameters and derived keys are not
        cached and MUST be presumed lost after the AAA exchange
        completes.

        In order to avoid key reuse, the AAA layer MUST delete
        transported keys once they are sent.  The AAA layer MUST NOT
        retain keys that it has previously sent.  For example, a AAA
        layer that has transported the MSK MUST delete it, and keys MUST
        NOT be derived from the MSK from that point forward.

2.2.  Authenticator and Peer Architecture

   This specification does not impose constraints on the architecture of
   the EAP authenticator or peer.  For example, any of the authenticator
   architectures described in [RFC4118] can be used.  As a result, lower
   layers need to identify EAP peers and authenticators unambiguously,
   without incorporating implicit assumptions about peer and
   authenticator architectures.

   For example, it is possible for multiple base stations and a
   "controller" (e.g., WLAN switch) to comprise a single EAP
   authenticator.  In such a situation, the "base station identity" is
   irrelevant to the EAP method conversation, except perhaps as an
   opaque blob to be used in channel binding.  Many base stations can
   share the same authenticator identity.  An EAP authenticator or peer:

      (a) can contain one or more physical or logical ports;
      (b) can advertise itself as one or more "virtual" authenticators
          or peers;
      (c) can utilize multiple CPUs;
      (d) can support clustering services for load balancing or
          failover.

   Both the EAP peer and authenticator can have more than one physical
   or logical port.  A peer can simultaneously access the network via
   multiple authenticators, or via multiple physical or logical ports on
   a given authenticator.  Similarly, an authenticator can offer network
   access to multiple peers, each via a separate physical or logical
   port.  When a single physical authenticator advertises itself as
   multiple virtual authenticators, it is possible for a single physical
   port to belong to multiple virtual authenticators.

   An authenticator can be configured to communicate with more than one
   EAP server, each of which is configured to communicate with a subset
   of the authenticators.  The situation is illustrated in Figure 3.

2.3.  Authenticator Identification

   The EAP method conversation is between the EAP peer and server.  The
   authenticator identity, if considered at all by the EAP method, is
   treated as an opaque blob for the purpose of channel binding (see
   Section 5.3.3).  However, the authenticator identity is important in
   two other exchanges - the AAA protocol exchange and the Secure
   Association Protocol conversation.

   The AAA conversation is between the EAP authenticator and the backend
   authentication server.  From the point of view of the backend
   authentication server, keying material and parameters are transported
   to the EAP authenticator identified by the NAS-Identifier Attribute.
   Since an EAP authenticator MUST NOT share EAP keying material or
   parameters with another party, if the EAP peer or backend
   authentication server detects use of EAP keying material and
   parameters outside the scope defined by the NAS-Identifier, the
   keying material MUST be considered compromised.

   The Secure Association Protocol conversation is between the peer and
   the authenticator.  For lower layers that support key caching, it is
   particularly important for the EAP peer, authenticator, and backend
   server to have a consistent view of the usage scope of the
   transported keying material.  In order to enable this, it is
   RECOMMENDED that the Secure Association Protocol explicitly
   communicate the usage scope of the EAP keying material passed down to
   the lower layer, rather than implicitly assuming that this is defined
   by the authenticator and peer endpoint addresses.

                     +-+-+-+-+
                     | EAP   |
                     | Peer  |
                     +-+-+-+-+
                       | | |  Peer Ports
                      /  |  \
                     /   |   \
                    /    |    \
                   /     |     \
                  /      |      \
                 /       |       \
                /        |        \
               /         |         \     Authenticator
            | | |      | | |      | | |   Ports
          +-+-+-+-+  +-+-+-+-+  +-+-+-+-+
          |       |  |       |  |       |
          | Auth1 |  | Auth2 |  | Auth3 |
          |       |  |       |  |       |
          +-+-+-+-+  +-+-+-+-+  +-+-+-+-+
               \        | \         |
                \       |  \        |
                 \      |   \       |
   EAP over AAA   \     |    \      |
     (optional)    \    |     \     |
                    \   |      \    |
                     \  |       \   |
                      \ |        \  |
                   +-+-+-+-+-+  +-+-+-+-+-+  Backend
                   |  EAP    |  |  EAP    |  Authentication
                   | Server1 |  | Server2 |  Servers
                   +-+-+-+-+-+  +-+-+-+-+-+

   Figure 3: Relationship between EAP Peer, Authenticator, and Server

   Since an authenticator can have multiple ports, the scope of the
   authenticator key cache cannot be described by a single endpoint
   address.  Similarly, where a peer can have multiple ports and sharing
   of EAP keying material and parameters between peer ports of the same

   link type is allowed, the extent of the peer key cache cannot be
   communicated by using a single endpoint address.  Instead, it is
   RECOMMENDED that the EAP peer and authenticator consistently identify
   themselves utilizing explicit identifiers, rather than endpoint
   addresses or port identifiers.

   AAA protocols such as RADIUS [RFC3579] and Diameter [RFC4072] provide
   a mechanism for the identification of AAA clients; since the EAP
   authenticator and AAA client MUST be co-resident, this mechanism is
   applicable to the identification of EAP authenticators.

   RADIUS [RFC2865] requires that an Access-Request packet contain one
   or more of the NAS-Identifier, NAS-IP-Address, and NAS-IPv6-Address
   attributes.  Since a NAS can have more than one IP address, the
   NAS-Identifier Attribute is RECOMMENDED for explicit identification
   of the authenticator, both within the AAA protocol exchange and the
   Secure Association Protocol conversation.

   Problems that can arise where the peer and authenticator implicitly
   identify themselves using endpoint addresses include the following:

   (a)  It is possible that the peer will not be able to determine which
        authenticator ports are associated with which authenticators.
        As a result, the EAP peer will be unable to utilize the
        authenticator key cache in an efficient way, and will also be
        unable to determine whether EAP keying material has been shared
        outside its authorized scope, and therefore needs to be
        considered compromised.

   (b)  It is possible that the authenticator will not be able to
        determine which peer ports are associated with which peers,
        preventing the peer from communicating with it utilizing
        multiple peer ports.

   (c)  It is possible that the peer will not be able to determine with
        which virtual authenticator it is communicating.  For example,
        multiple virtual authenticators can share a MAC address, but
        utilize different NAS-Identifiers.

   (d)  It is possible that the authenticator will not be able to
        determine with which virtual peer it is communicating.  Multiple
        virtual peers can share a MAC address, but utilize different
        Peer-Ids.

   (e)  It is possible that the EAP peer and server will not be able to
        verify the authenticator identity via channel binding.

   For example, problems (a), (c), and (e) occur in [IEEE-802.11], which
   utilizes peer and authenticator MAC addresses within the 4-way
   handshake.  Problems (b) and (d) do not occur since [IEEE-802.11]
   only allows a virtual peer to utilize a single port.

   The following steps enable lower-layer identities to be securely
   verified by all parties:

   (f)  Specify the lower-layer parameters used to identify the
        authenticator and peer.  As noted earlier, endpoint or port
        identifiers are not recommended for identification of the
        authenticator or peer when it is possible for them to have
        multiple ports.

   (g)  Communicate the lower-layer identities between the peer and
        authenticator within phase 0.  This allows the peer and
        authenticator to determine the key scope if a key cache is
        utilized.

   (h)  Communicate the lower-layer authenticator identity between the
        authenticator and backend authentication server within the NAS-
        Identifier Attribute.

   (i)  Include the lower-layer identities within channel bindings (if
        supported) in phase 1a, ensuring that they are communicated
        between the EAP peer and server.

   (j)  Support the integrity-protected exchange of identities within
        phase 2a.

   (k)  Utilize the advertised lower-layer identities to enable the peer
        and authenticator to verify that keys are maintained within the
        advertised scope.

2.3.1.  Virtual Authenticators

   When a single physical authenticator advertises itself as multiple
   virtual authenticators, if the virtual authenticators do not maintain
   logically separate key caches, then by authenticating to one virtual
   authenticator, the peer can gain access to the other virtual
   authenticators sharing a key cache.

   For example, where a physical authenticator implements "Guest" and
   "Corporate Intranet" virtual authenticators, an attacker acting as a
   peer could authenticate with the "Guest" virtual authenticator and
   derive EAP keying material.  If the "Guest" and "Corporate Intranet"
   virtual authenticators share a key cache, then the peer can utilize
   the EAP keying material derived for the "Guest" network to obtain
   access to the "Corporate Intranet" network.

   The following steps can be taken to mitigate this vulnerability:

   (a)  Authenticators are REQUIRED to cache associated authorizations
        along with EAP keying material and parameters and to apply
        authorizations to the peer on each network access, regardless of
        which virtual authenticator is being accessed.  This ensures
        that an attacker cannot obtain elevated privileges even where
        the key cache is shared between virtual authenticators, and a
        peer obtains access to one virtual authenticator utilizing a key
        cache entry created for use with another virtual authenticator.

   (b)  It is RECOMMENDED that physical authenticators maintain separate
        key caches for each virtual authenticator.  This ensures that a
        cache entry created for use with one virtual authenticator
        cannot be used for access to another virtual authenticator.
        Since a key cache entry can no longer be shared between virtual
        authentications, this step provides protection beyond that
        offered in (a).  This is valuable in situations where
        authorizations are not used to enforce access limitations.  For
        example, where access is limited using a filter installed on a
        router rather than using authorizations provided to the
        authenticator, a peer can gain unauthorized access to resources
        by exploiting a shared key cache entry.

   (c)  It is RECOMMENDED that each virtual authenticator identify
        itself consistently to the peer and to the backend
        authentication server, so as to enable the peer to verify the
        authenticator identity via channel binding (see Section 5.3.3).

   (d)  It is RECOMMENDED that each virtual authenticator identify
        itself distinctly, in order to enable the peer and backend
        authentication server to tell them apart.  For example, this can
        be accomplished by utilizing a distinct value of the NAS-
        Identifier Attribute.

2.4.  Peer Identification

   As described in [RFC3748] Section 7.3, the peer identity provided in
   the EAP-Response/Identity can be different from the peer identities
   authenticated by the EAP method.  For example, the identity provided

   in the EAP-Response/Identity can be a privacy identifier as described
   in "The Network Access Identifier" [RFC4282] Section 2.  As noted in
   [RFC4284], it is also possible to utilize a Network Access Identifier
   (NAI) for the purposes of source routing; an NAI utilized for source
   routing is said to be "decorated" as described in [RFC4282] Section
   2.7.

   When the EAP peer provides the Network Access Identity (NAI) within
   the EAP-Response/Identity, as described in [RFC3579], the
   authenticator copies the NAI included in the EAP-Response/Identity
   into the User-Name Attribute included within the Access-Request.  As
   the Access-Request is forwarded toward the backend authentication
   server, AAA proxies remove decoration from the NAI included in the
   User-Name Attribute; the NAI included within the
   EAP-Response/Identity encapsulated in the Access-Request remains
   unchanged.  As a result, when the Access-Request arrives at the
   backend authentication server, the EAP-Response/Identity can differ
   from the User-Name Attribute (which can have some or all of the
   decoration removed).  In the absence of a Peer-Id, the backend
   authentication server SHOULD use the contents of the User-Name
   Attribute, rather than the EAP-Response/Identity, as the peer
   identity.

   It is possible for more than one Peer-Id to be exported by an EAP
   method.  For example, a peer certificate can contain more than one
   peer identity; in a tunnel method, peer identities can be
   authenticated within both an outer and inner exchange, and these
   identities could be different in type and contents.  For example, an
   outer exchange could provide a Peer-Id in the form of a Relative
   Distinguished Name (RDN), whereas an inner exchange could identify
   the peer via its NAI or MAC address.  Where EAP keying material is
   determined solely from the outer exchange, only the outer Peer-Id(s)
   are exported; where the EAP keying material is determined from both
   the inner and outer exchanges, then both the inner and outer
   Peer-Id(s) are exported by the tunnel method.

2.5.  Server Identification

   It is possible for more than one Server-Id to be exported by an EAP
   method.  For example, a server certificate can contain more than one
   server identity; in a tunnel method, server identities could be
   authenticated within both an outer and inner exchange, and these
   identities could be different in type and contents.  For example, an
   outer exchange could provide a Server-Id in the form of an IP
   address, whereas an inner exchange could identify the server via its
   Fully-Qualified Domain Name (FQDN) or hostname.  Where EAP keying
   material is determined solely from the outer exchange, only the outer
   Server-Id(s) are exported by the EAP method; where the EAP keying
   material is determined from both the inner and outer exchanges, then
   both the inner and outer Server-Id(s) are exported by the EAP method.

   As shown in Figure 3, an authenticator can be configured to
   communicate with multiple EAP servers; the EAP server that an
   authenticator communicates with can vary according to configuration
   and network and server availability.  While the EAP peer can assume
   that all EAP servers within a realm have access to the credentials
   necessary to validate an authentication attempt, it cannot assume
   that all EAP servers share persistent state.

   Authenticators can be configured with different primary or secondary
   EAP servers, in order to balance the load.  Also, the authenticator
   can dynamically determine the EAP server to which requests will be
   sent; in the event of a communication failure, the authenticator can
   fail over to another EAP server.  For example, in Figure 3,
   Authenticator2 can be initially configured with EAP server1 as its
   primary backend authentication server, and EAP server2 as the backup,
   but if EAP server1 becomes unavailable, EAP server2 can become the
   primary server.

   In general, the EAP peer cannot direct an authentication attempt to a
   particular EAP server within a realm, this decision is made by AAA
   clients, nor can the peer determine with which EAP server it will be
   communicating, prior to the start of the EAP method conversation.
   The Server-Id is not included in the EAP-Request/Identity, and since
   the EAP server may be determined dynamically, it typically is not
   possible for the authenticator to advertise the Server-Id during the
   discovery phase.  Some EAP methods do not export the Server-Id so
   that it is possible that the EAP peer will not learn with which
   server it was conversing after the EAP conversation completes
   successfully.

   As a result, an EAP peer, on connecting to a new authenticator or
   reconnecting to the same authenticator, can find itself communicating
   with a different EAP server.  Fast reconnect, defined in [RFC3748]

   Section 7.2, can fail if the EAP server with which the peer
   communicates is not the same one with which it initially established
   a security association.  For example, an EAP peer attempting an
   EAP-TLS session resume can find that the new EAP-TLS server will not
   have access to the TLS Master Key identified by the TLS Session-Id,
   and therefore the session resumption attempt will fail, requiring
   completion of a full EAP-TLS exchange.

   EAP methods that export the Server-Id MUST authenticate the server.
   However, not all EAP methods supporting mutual authentication provide
   a non-null Server-Id; some methods only enable the EAP peer to verify
   that the EAP server possesses a long-term secret, but do not provide
   the identity of the EAP server.  In this case, the EAP peer will know
   that an authenticator has been authorized by an EAP server, but will
   not confirm the identity of the EAP server.  Where the EAP method
   does not provide a Server-Id, the peer cannot identify the EAP server
   with which it generated keying material.  This can make it difficult
   for the EAP peer to identify the location of a key possessed by that
   EAP server.

   As noted in [RFC5216] Section 5.2, EAP methods supporting
   authentication using server certificates can determine the Server-Id
   from the subject or subjectAltName fields in the server certificate.
   Validating the EAP server identity can help the EAP peer to decide
   whether a specific EAP server is authorized.  In some cases, such as
   where the certificate extensions defined in [RFC4334] are included in
   the server certificate, it can even be possible for the peer to
   verify some channel binding parameters from the server certificate.

   It is possible for problems to arise in situations where the EAP
   server identifies itself differently to the EAP peer and
   authenticator.  For example, it is possible that the Server-Id
   exported by EAP methods will not be identical to the Fully Qualified
   Domain Name (FQDN) of the backend authentication server.  Where
   certificate-based authentication is used within RADIUS or Diameter,
   it is possible that the subjectAltName used in the backend
   authentication server certificate will not be identical to the
   Server-Id or backend authentication server FQDN.  This is not
   normally an issue in EAP, as the authenticator will be unaware of the
   identities used between the EAP peer and server.  However, this can
   be an issue for key caching, if the authenticator is expected to
   locate a backend authentication server corresponding to a Server-Id
   provided by an EAP peer.

   Where the backend authentication server FQDN differs from the
   subjectAltName in the backend authentication server certificate, it
   is possible that the AAA client will not be able to determine whether
   it is talking to the correct backend authentication server.  Where

   the Server-Id and backend authentication server FQDN differ, it is
   possible that the combination of the key scope (Peer-Id(s), Server-
   Id(s)) and EAP conversation identifier (Session-Id) will not be
   sufficient to determine where the key resides.  For example, the
   authenticator can identify backend authentication servers by their IP
   address (as occurs in RADIUS), or using a Fully Qualified Domain Name
   (as in Diameter).  If the Server-Id does not correspond to the IP
   address or FQDN of a known backend authentication server, then it may
   not be possible to locate which backend authentication server
   possesses the key.

3.  Security Association Management

   EAP, as defined in [RFC3748], supports key derivation, but does not
   provide for the management of lower-layer security associations.
   Missing functionality includes:

   (a)  Security Association negotiation.  EAP does not negotiate
        lower-layer unicast or multicast security associations,
        including cryptographic algorithms or traffic profiles.  EAP
        methods only negotiate cryptographic algorithms for their own
        use, not for the underlying lower layers.  EAP also does not
        negotiate the traffic profiles to be protected with the
        negotiated ciphersuites; in some cases the traffic to be
        protected can have lower-layer source and destination addresses
        different from the lower-layer peer or authenticator addresses.

   (b)  Re-key.  EAP does not support the re-keying of exported EAP
        keying material without EAP re-authentication, although EAP
        methods can support "fast reconnect" as defined in [RFC3748]
        Section 7.2.1.

   (c)  Key delete/install semantics.  EAP does not synchronize
        installation or deletion of keying material on the EAP peer and
        authenticator.

   (d)  Lifetime negotiation.  EAP does not support lifetime negotiation
        for exported EAP keying material, and existing EAP methods also
        do not support key lifetime negotiation.

   (e)  Guaranteed TSK freshness.  Without a post-EAP handshake, TSKs
        can be reused if EAP keying material is cached.

   These deficiencies are typically addressed via a post-EAP handshake
   known as the Secure Association Protocol.

3.1.  Secure Association Protocol

   Since neither EAP nor EAP methods provide for establishment of
   lower-layer security associations, it is RECOMMENDED that these
   facilities be provided within the Secure Association Protocol,
   including:

   (a)  Entity Naming.  A basic feature of a Secure Association Protocol
        is the explicit naming of the parties engaged in the exchange.
        Without explicit identification, the parties engaged in the
        exchange are not identified and the scope of the EAP keying
        parameters negotiated during the EAP exchange is undefined.

   (b)  Mutual proof of possession of EAP keying material.  During the
        Secure Association Protocol, the EAP peer and authenticator MUST
        demonstrate possession of the keying material transported
        between the backend authentication server and authenticator
        (e.g., MSK), in order to demonstrate that the peer and
        authenticator have been authorized.  Since mutual proof of
        possession is not the same as mutual authentication, the peer
        cannot verify authenticator assertions (including the
        authenticator identity) as a result of this exchange.
        Authenticator identity verification is discussed in Section 2.3.

   (c)  Secure capabilities negotiation.  In order to protect against
        spoofing during the discovery phase, ensure selection of the
        "best" ciphersuite, and protect against forging of negotiated
        security parameters, the Secure Association Protocol MUST
        support secure capabilities negotiation.  This includes the
        secure negotiation of usage modes, session parameters (such as
        security association identifiers (SAIDs) and key lifetimes),
        ciphersuites and required filters, including confirmation of
        security-relevant capabilities discovered during phase 0.  The
        Secure Association Protocol MUST support integrity and replay
        protection of all capability negotiation messages.

   (d)  Key naming and selection.  Where key caching is supported, it is
        possible for the EAP peer and authenticator to share more than
        one key of a given type.  As a result, 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.  Use of the key naming mechanism
        described in Section 1.4.1 is RECOMMENDED.

        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.

   (e)  Generation of fresh transient session keys (TSKs).  Where the
        lower layer supports caching of keying material, the EAP peer
        lower layer can initiate a new session using keying material
        that was derived in a previous session.  Were the TSKs to be
        derived solely from a portion of the exported EAP keying
        material, this would result in reuse of the session keys that
        could expose the underlying ciphersuite to attack.

        In lower layers where caching of keying material is supported,
        the Secure Association Protocol phase is REQUIRED, and MUST
        support the derivation of fresh unicast and multicast TSKs, even
        when the transported keying material provided by the backend
        authentication server is not fresh.  This is typically supported
        via the exchange of nonces or counters, which are then mixed
        with the keying material in order to generate fresh unicast
        (phase 2a) and possibly multicast (phase 2b) session keys.  By
        not using exported EAP keying material directly to protect data,
        the Secure Association Protocol protects it against compromise.

   (f)  Key lifetime management.  This includes explicit key lifetime
        negotiation or seamless re-key.  EAP does not support the
        re-keying of EAP keying material without re-authentication, and
        existing EAP methods do not support key lifetime negotiation.
        As a result, the Secure Association Protocol MAY handle the
        re-key and determination of the key lifetime.  Where key caching
        is supported, secure negotiation of key lifetimes is
        RECOMMENDED.  Lower layers that support re-key, but not key
        caching, may not require key lifetime negotiation.  For example,
        a difference between IKEv1 [RFC2409] and IKEv2 [RFC4306] is that
        in IKEv1 SA lifetimes were negotiated; in IKEv2, each end of the
        SA is responsible for enforcing its own lifetime policy on the
        SA and re-keying the SA when necessary.

   (g)  Key state resynchronization.  It is possible for the peer or
        authenticator to reboot or reclaim resources, clearing portions
        or all of the key cache.  Therefore, key lifetime negotiation
        cannot guarantee that the key cache will remain synchronized,
        and it may not be possible for the peer to determine before
        attempting to use a key whether it exists within the
        authenticator cache.  It is therefore RECOMMENDED for the EAP
        lower layer to provide a mechanism for key state

        resynchronization, either via the SAP or using a lower layer
        indication (see [RFC3748] Section 3.4).  Where the peer and
        authenticator do not jointly possess a key with which to protect
        the resynchronization exchange, secure resynchronization is not
        possible, and alternatives (such as an initiation of EAP
        re-authentication after expiration of a timer) are needed to
        ensure timely resynchronization.

   (h)  Key scope synchronization.  To support key scope determination,
        the Secure Association Protocol SHOULD provide a mechanism by
        which the peer can determine the scope of the key cache on each
        authenticator and by which the authenticator can determine the
        scope of the key cache on a peer.  This includes negotiation of
        restrictions on key usage.

   (i)  Traffic profile negotiation.  The traffic to be protected by a
        lower-layer security association will not necessarily have the
        same lower-layer source or destination address as the EAP peer
        and authenticator, and it is possible for the peer and
        authenticator to negotiate multiple security associations, each
        with a different traffic profile.  Where this is the case, the
        profile of protected traffic SHOULD be explicitly negotiated.
        For example, in IKEv2 it is possible for an Initiator and
        Responder to utilize EAP for authentication, then negotiate a
        Tunnel Mode Security Association (SA), which permits passing of
        traffic originating from hosts other than the Initiator and
        Responder.  Similarly, in IEEE 802.16e, a Subscriber Station
        (SS) can forward traffic to the Base Station (BS), which
        originates from the Local Area Network (LAN) to which it is
        attached.  To enable this, Security Associations within IEEE
        802.16e are identified by the Connection Identifier (CID), not
        by the EAP peer and authenticator MAC addresses.  In both IKEv2
        and IEEE 802.16e, multiple security associations can exist
        between the EAP peer and authenticator, each with their own
        traffic profile and quality of service parameters.

   (j)  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.

   (k)  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.  See [RFC3748]
        Section 2.4 for more discussion.

3.2.  Key Scope

   Absent explicit specification within the lower layer, after the
   completion of phase 1b, transported keying material, and parameters
   are bound to the EAP peer and authenticator, but are not bound to a
   specific peer or authenticator port.

   While EAP keying material passed down to the lower layer is not
   intrinsically bound to particular authenticator and peer ports, TSKs
   MAY be bound to particular authenticator and peer ports by the Secure
   Association Protocol.  However, a lower layer MAY also permit TSKs to
   be used on multiple peer and/or authenticator ports, provided that
   TSK freshness is guaranteed (such as by keeping replay counter state
   within the authenticator).

   In order to further limit the key scope, the following measures are
   suggested:

   (a)  The lower layer MAY specify additional restrictions on key
        usage, such as limiting the use of EAP keying material and
        parameters on the EAP peer to the port over which the EAP
        conversation was conducted.

   (b)  The backend authentication server and authenticator MAY
        implement additional attributes in order to further restrict the
        scope of keying material.  For example, in IEEE 802.11, the
        backend authentication server can provide the authenticator with
        a list of authorized Called or Calling-Station-Ids and/or SSIDs
        for which keying material is valid.

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

3.3.  Parent-Child Relationships

   When an EAP re-authentication takes place, new EAP keying material is
   exported by the EAP method.  In EAP lower layers where EAP
   re-authentication eventually results in TSK replacement, the maximum

   lifetime of derived keying material (including TSKs) can be less than
   or equal to that of EAP keying material (MSK/EMSK), but it cannot be
   greater.

   Where TSKs are derived from or are wrapped by exported EAP keying
   material, compromise of that exported EAP keying material implies
   compromise of TSKs.  Therefore, if EAP keying material is considered
   stale, not only SHOULD EAP re-authentication be initiated, but also
   replacement of child keys, including TSKs.

   Where EAP keying material is used only for entity authentication but
   not for TSK derivation (as in IKEv2), compromise of exported EAP
   keying material does not imply compromise of the TSKs.  Nevertheless,
   the compromise of EAP keying material could enable an attacker to
   impersonate an authenticator, so that EAP re-authentication and TSK
   re-key are RECOMMENDED.

   With respect to IKEv2, Section 5.2 of [RFC4718], "IKEv2
   Clarifications and Implementation Guidelines", states:

      Rekeying the IKE_SA and reauthentication are different concepts in
      IKEv2.  Rekeying the IKE_SA establishes new keys for the IKE_SA
      and resets the Message ID counters, but it does not authenticate
      the parties again (no AUTH or EAP payloads are involved)...  This
      means that reauthentication also establishes new keys for the
      IKE_SA and CHILD_SAs.  Therefore while rekeying can be performed
      more often than reauthentication, the situation where
      "authentication lifetime" is shorter than "key lifetime" does not
      make sense.

   Child keys that are used frequently (such as TSKs that are used for
   traffic protection) can expire sooner than the exported EAP keying
   material on which they are dependent, so that it is advantageous to
   support re-key of child keys prior to EAP re-authentication.  Note
   that deletion of the MSK/EMSK does not necessarily imply deletion of
   TSKs or child keys.

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

3.4.  Local Key Lifetimes

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

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

   EAP methods MAY cache local EAP keying material (TEKs) that can
   persist for multiple EAP conversations when fast reconnect is used
   [RFC3748].  For example, EAP methods based on TLS (such as EAP-TLS
   [RFC5216]) derive and cache the TLS Master Secret, typically for
   substantial time periods.  The lifetime of other local EAP keying
   material calculated within the EAP method is defined by the method.
   Note that in general, when using fast reconnect, there is no
   guarantee that the original long-term credentials are still in the
   possession of the peer.  For instance, a smart-card holding the
   private key for EAP-TLS may have been removed.  EAP servers SHOULD
   also verify that the long-term credentials are still valid, such as
   by checking that certificate used in the original authentication has
   not yet expired.

3.5.  Exported and Calculated Key Lifetimes

   The following mechanisms are available for communicating the lifetime
   of keying material between the EAP peer, server, and authenticator:

      AAA protocols  (backend authentication server and authenticator)
      Lower-layer mechanisms (authenticator and peer)
      EAP method-specific negotiation (peer and server)

   Where the EAP method does not support the negotiation of the lifetime
   of exported EAP keying material, and a key lifetime negotiation
   mechanism is not provided by the lower layer, it is possible that
   there will not be a way for the peer to learn the lifetime of keying
   material.  This can leave the peer uncertain of how long the
   authenticator will maintain keying material within the key cache.  In
   this case the lifetime of keying material can be managed as a system
   parameter on the peer and authenticator; a default lifetime of 8
   hours is RECOMMENDED.

3.5.1.  AAA Protocols

   AAA protocols such as RADIUS [RFC2865] and Diameter [RFC4072] can be
   used to communicate the maximum key lifetime from the backend
   authentication server to the authenticator.

   The Session-Timeout Attribute is defined for RADIUS in [RFC2865] and
   for Diameter in [RFC4005].  Where EAP is used for authentication,
   [RFC3580] Section 3.17, indicates that a Session-Timeout Attribute
   sent in an Access-Accept along with a Termination-Action value of
   RADIUS-Request specifies the maximum number of seconds of service
   provided prior to EAP re-authentication.

   However, there is also a need to be able to specify the maximum
   lifetime of cached keying material.  Where EAP pre-authentication is
   supported, cached keying material can be pre-established on the
   authenticator prior to session start and will remain there until
   expiration.  EAP lower layers supporting caching of keying material
   MAY also persist that material after the end of a session, enabling
   the peer to subsequently resume communication utilizing the cached
   keying material.  In these situations it can be desirable for the
   backend authentication server to specify the maximum lifetime of
   cached keying material.

   To accomplish this, [IEEE-802.11] overloads the Session-Timeout
   Attribute, assuming that it represents the maximum time after which
   transported keying material will expire on the authenticator,
   regardless of whether transported keying material is cached.

   An IEEE 802.11 authenticator receiving transported keying material is
   expected to initialize a timer to the Session-Timeout value, and once
   the timer expires, the transported keying material expires.  Whether
   this results in session termination or EAP re-authentication is
   controlled by the value of the Termination-Action Attribute.  Where
   EAP re-authentication occurs, the transported keying material is
   replaced, and with it, new calculated keys are put in place.  Where
   session termination occurs, transported and derived keying material
   is deleted.

   Overloading the Session-Timeout Attribute is problematic in
   situations where it is necessary to control the maximum session time
   and key lifetime independently.  For example, it might be desirable
   to limit the lifetime of cached keying material to 5 minutes while
   permitting a user once authenticated to remain connected for up to an
   hour without re-authenticating.  As a result, in the future,
   additional attributes can be specified to control the lifetime of
   cached keys; these attributes MAY modify the meaning of the
   Session-Timeout Attribute in specific circumstances.

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

3.5.2.  Lower-Layer Mechanisms

   Lower-layer mechanisms can be used to enable the lifetime of keying
   material to be negotiated between the peer and authenticator.  This
   can be accomplished either using the Secure Association Protocol or
   within the lower-layer transport.

   Where TSKs are established as the result of a Secure Association
   Protocol exchange, it is RECOMMENDED that the Secure Association
   Protocol include support for TSK re-key.  Where the TSK is taken
   directly from the MSK, there is no need to manage the TSK lifetime as
   a separate parameter, since the TSK lifetime and MSK lifetime are
   identical.

3.5.3.  EAP Method-Specific Negotiation

   As noted in [RFC3748] Section 7.10:

      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.

   However, EAP does not itself support the negotiation of lifetimes for
   exported EAP keying material such as the MSK, EMSK, and IV.

   While EAP itself does not support lifetime negotiation, it would be
   possible to specify methods that do.  However, systems that rely on
   key lifetime negotiation within EAP methods would only function with
   these methods.  Also, there is no guarantee that the key lifetime
   negotiated within the EAP method would be compatible with backend
   authentication server policy.  In the interest of method independence
   and compatibility with backend authentication server implementations,
   management of the lifetime of keying material SHOULD NOT be provided
   within EAP methods.

3.6.  Key Cache Synchronization

   Key lifetime negotiation alone cannot guarantee key cache
   synchronization.  Even where a lower-layer exchange is run
   immediately after EAP in order to determine the lifetime of keying
   material, it is still possible for the authenticator to purge all or
   part of the key cache prematurely (e.g., due to reboot or need to
   reclaim memory).

   The lower layer can utilize the Discovery phase 0 to improve key
   cache synchronization.  For example, if the authenticator manages the
   key cache by deleting the oldest key first, the relative creation
   time of the last key to be deleted could be advertised within the
   Discovery phase, enabling the peer to determine whether keying
   material had been prematurely expired from the authenticator key
   cache.

3.7.  Key Strength

   As noted in Section 2.1, EAP lower layers determine TSKs in different
   ways.  Where exported EAP keying material is utilized in the
   derivation, encryption or authentication of TSKs, it is possible for
   EAP key generation to represent the weakest link.

   In order to ensure that methods produce EAP keying material of an
   appropriate symmetric key strength, it is RECOMMENDED that EAP
   methods utilizing public key cryptography choose a public key that
   has a cryptographic strength providing the required level of attack
   resistance.  This is typically provided by configuring EAP methods,
   since there is no coordination between the lower layer and EAP method
   with respect to minimum required symmetric key strength.

   Section 5 of BCP 86 [RFC3766] offers advice on the required RSA or DH
   module and DSA subgroup size in bits, for a given level of attack
   resistance in bits.  The National Institute for Standards and
   Technology (NIST) also offers advice on appropriate key sizes in
   [SP800-57].

3.8.  Key Wrap

   The key wrap specified in [RFC2548], which is based on an MD5-based
   stream cipher, has known problems, as described in [RFC3579] Section
   4.3.  RADIUS uses the shared secret for multiple purposes, including
   per-packet authentication and 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 concerns exist relating to the
   security of this hash [MD5Collision].

   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, [RFC3579] Section 4.2
   recommends running RADIUS over IPsec.  The same approach is taken in
   Diameter EAP [RFC4072], which in Section 4.1.3 defines the
   EAP-Master-Session-Key Attribute-Value Pair (AVP) in clear-text, to
   be protected by IPsec or TLS.

4.  Handoff Vulnerabilities

   A handoff occurs when an EAP peer moves to a new authenticator.
   Several mechanisms have been proposed for reducing handoff latency
   within networks utilizing EAP.  These include:

   EAP pre-authentication
      In EAP pre-authentication, an EAP peer pre-establishes EAP keying
      material with an authenticator prior to arrival.  EAP
      pre-authentication only affects the timing of EAP authentication,
      but does not shorten or eliminate EAP (phase 1a) or AAA (phase 1b)
      exchanges;  Discovery (phase 0) and Secure Association Protocol
      (phase 2) exchanges occur as described in Section 1.3.  As a
      result, the primary benefit is to enable EAP authentication to be
      removed from the handoff critical path, thereby reducing latency.
      Use of EAP pre-authentication within IEEE 802.11 is described in
      [IEEE-802.11] and [8021XPreAuth].

   Proactive key distribution
      In proactive key distribution, keying material and authorizations
      are transported from the backend authentication server to a
      candidate authenticator in advance of a handoff.  As a result, EAP
      (phase 1a) is not needed, but the Discovery (phase 0), and Secure
      Association Protocol exchanges (phase 2) are still necessary.
      Within the AAA exchange (phase 1b), authorization and key
      distribution functions are typically supported, but not
      authentication.  Proactive key distribution is described in
      [MishraPro], [IEEE-03-084], and [HANDOFF].

   Key caching
      Caching of EAP keying material enables an EAP peer to re-attach to
      an authenticator without requiring EAP (phase 1a) or AAA (phase
      1b) exchanges.  However, Discovery (phase 0) and Secure
      Association Protocol (phase 2) exchanges are still needed.  Use of
      key caching within IEEE 802.11 is described in [IEEE-802.11].

   Context transfer
      In context transfer schemes, keying material and authorizations
      are transferred between a previous authenticator and a new
      authenticator.  This can occur in response to a handoff request by
      the EAP peer, or in advance, as in proactive key distribution.  As
      a result, EAP (phase 1a) is eliminated, but not the Discovery
      (phase 0) or Secure Association Protocol exchanges (phase 2).  If
      a secure channel can be established between the new and previous
      authenticator without assistance from the backend authentication
      server, then the AAA exchange (phase 1b) can be eliminated;
      otherwise, it is still needed, although it can be shortened.
      Context transfer protocols are described in [IEEE-802.11F] (now
      deprecated) and "Context Transfer Protocol (CXTP)" [RFC4067].
      "Fast Authentication Methods for Handovers between IEEE 802.11
      Wireless LANs" [Bargh] analyzes fast handoff techniques, including
      context transfer mechanisms.

   Token distribution
      In token distribution schemes, the EAP peer is provided with a
      credential, subsequently enabling it to authenticate with one or
      more additional authenticators.  During the subsequent
      authentications, EAP (phase 1a) is eliminated or shortened; the
      Discovery (phase 0) and Secure Association Protocol exchanges
      (phase 2) still occur, although the latter can be shortened.  If
      the token includes authorizations and can be validated by an
      authenticator without assistance from the backend authentication
      server, then the AAA exchange (phase 1b) can be eliminated;
      otherwise, it is still needed, although it can be shortened.
      Token-based schemes, initially proposed in early versions of IEEE
      802.11i [IEEE-802.11i], are described in [Token], [Tokenk], and
      [SHORT-TERM].

   The sections that follow discuss the security vulnerabilities
   introduced by the above schemes.

4.1.  EAP Pre-Authentication

   EAP pre-authentication differs from a normal EAP conversation
   primarily with respect to the lower-layer encapsulation.  For
   example, in [IEEE-802.11], EAP pre-authentication frames utilize a
   distinct Ethertype, but otherwise conforms to the encapsulation
   described in [IEEE-802.1X].  As a result, an EAP pre-authentication
   conversation differs little from the model described in Section 1.3,
   other than the introduction of a delay between phase 1 and phase 2.

   EAP pre-authentication relies on lower-layer mechanisms for discovery
   of candidate authenticators.  Where discovery can provide information
   on candidate authenticators outside the immediate listening range,
   and the peer can determine whether it already possesses valid EAP
   keying material with candidate authenticators, the peer can avoid
   unnecessary EAP pre-authentications and can establish EAP keying
   material well in advance, regardless of the coverage overlap between
   authenticators.  However, if the peer can only discover candidate
   authenticators within listening range and cannot determine whether it
   can reuse existing EAP keying material, then it is possible that the
   peer will not be able to complete EAP pre-authentication prior to
   connectivity loss or that it can pre-authenticate multiple times with
   the same authenticator, increasing backend authentication server
   load.

   Since a peer can complete EAP pre-authentication with an
   authenticator without eventually attaching to it, it is possible that
   phase 2 will not occur.  In this case, an Accounting-Request
   signifying the start of service will not be sent, or will only be
   sent with a substantial delay after the completion of authentication.

   As noted in "IEEE 802.1X RADIUS Usage Guidelines" [RFC3580], the AAA
   exchange resulting from EAP pre-authentication differs little from an
   ordinary exchange described in "RADIUS Support for EAP" [RFC3579].
   For example, since in IEEE 802.11 [IEEE-802.11] an Association
   exchange does not occur prior to EAP pre-authentication, the SSID is
   not known by the authenticator at authentication time, so that an
   Access-Request cannot include the SSID within the Called-Station-Id
   attribute as described in [RFC3580] Section 3.20.

   Since only the absence of an SSID in the Called-Station-Id attribute
   distinguishes an EAP pre-authentication attempt, if the authenticator
   does not always include the SSID for a normal EAP authentication
   attempt, it is possible that the backend authentication server will
   not be able to determine whether a session constitutes an EAP
   pre-authentication attempt, potentially resulting in authorization or
   accounting problems.  Where the number of simultaneous sessions is
   limited, the backend authentication server can refuse to authorize a
   valid EAP pre-authentication attempt or can enable the peer to engage
   in more simultaneous sessions than they are authorized for.  Where
   EAP pre-authentication occurs with an authenticator which the peer
   never attaches to, it is possible that the backend accounting server
   will not be able to determine whether the absence of an
   Accounting-Request was due to packet loss or a session that never
   started.

   In order to enable pre-authentication requests to be handled more
   reliably, it is RECOMMENDED that AAA protocols explicitly identify
   EAP pre-authentication.  In order to suppress unnecessary EAP
   pre-authentication exchanges, it is RECOMMENDED that authenticators
   unambiguously identify themselves as described in Section 2.3.

4.2.  Proactive Key Distribution

   In proactive key distribution schemes, the backend authentication
   server transports keying material and authorizations to an
   authenticator in advance of the arrival of the peer.  The
   authenticators selected to receive the transported key material are
   selected based on past patterns of peer movement between
   authenticators known as the "neighbor graph".  In order to reduce
   handoff latency, proactive key distribution schemes typically only
   demonstrate proof of possession of transported keying material
   between the EAP peer and authenticator.  During a handoff, the
   backend authentication server is not provided with proof that the
   peer successfully authenticated to an authenticator; instead, the
   authenticator generates a stream of accounting messages without a
   corresponding set of authentication exchanges.  As described in
   [MishraPro], knowledge of the neighbor graph can be established via
   static configuration or analysis of authentication exchanges.  In

   order to prevent corruption of the neighbor graph, new neighbor graph
   entries can only be created as the result of a successful EAP
   exchange, and accounting packets with no corresponding authentication
   exchange need to be verified to correspond to neighbor graph entries
   (e.g., corresponding to handoffs between neighbors).

   In order to prevent compromise of one authenticator from resulting in
   compromise of other authenticators, cryptographic separation needs to
   be maintained between the keying material transported to each
   authenticator.  However, even where cryptographic separation is
   maintained, an attacker compromising an authenticator can still
   disrupt the operation of other authenticators.  As noted in [RFC3579]
   Section 4.3.7, in the absence of spoofing detection within the AAA
   infrastructure, it is possible for EAP authenticators to impersonate
   each other.  By forging NAS identification attributes within
   authentication messages, an attacker compromising one authenticator
   could corrupt the neighbor graph, tricking the backend authentication
   server into transporting keying material to arbitrary authenticators.
   While this would not enable recovery of EAP keying material without
   breaking fundamental cryptographic assumptions, it could enable
   subsequent fraudulent accounting messages, or allow an attacker to
   disrupt service by increasing load on the backend authentication
   server or thrashing the authenticator key cache.

   Since proactive key distribution requires the distribution of derived
   keying material to candidate authenticators, the effectiveness of
   this scheme depends on the ability of backend authentication server
   to anticipate the movement of the EAP peer.  Since proactive key
   distribution relies on backend authentication server knowledge of the
   neighbor graph, it is most applicable to intra-domain handoff
   scenarios.  However, in inter-domain handoff, where there can be many
   authenticators, peers can frequently connect to authenticators that
   have not been previously encountered, making it difficult for the
   backend authentication server to derive a complete neighbor graph.

   Since proactive key distribution schemes typically require
   introduction of server-initiated messages as described in [RFC5176]
   and [HANDOFF], security issues described in [RFC5176] Section 6 are
   applicable, including authorization (Section 6.1) and replay
   detection (Section 6.3) problems.

4.3.  AAA Bypass

   Fast handoff techniques that enable elimination of the AAA exchange
   (phase 1b) differ fundamentally from typical network access scenarios
   (dial-up, wired LAN, etc.) that include user authentication as well
   as authorization for the offered service.  Where the AAA exchange
   (phase 1b) is omitted, authorizations and keying material are not
   provided by the backend authentication server, and as a result, they
   need to be supplied by other means.  This section describes some of
   the implications.

4.3.1.  Key Transport

   Where transported keying material is not supplied by the backend
   authentication server, it needs to be provided by another party
   authorized to access that keying material.  As noted in Section 1.5,
   only the EAP peer, authenticator, and server are authorized to
   possess transported keying material.  Since EAP peers do not trust
   each other, if the backend authentication server does not supply
   transported keying material to a new authenticator, it can only be
   provided by a previous authenticator.

   As noted in Section 1.5, the goal of the EAP conversation is to
   derive session keys known only to the peer and the authenticator.  If
   keying material is replicated between a previous authenticator and a
   new authenticator, then the previous authenticator can possess
   session keys used between the peer and new authenticator.  Also, the
   new authenticator can possess session keys used between the peer and
   the previous authenticator.

   If a one-way function is used to derive the keying material to be
   transported to the new authenticator, then the new authenticator
   cannot possess previous session keys without breaking a fundamental
   cryptographic assumption.

4.3.2.  Authorization

   As a part of the authentication process, the backend authentication
   server determines the user's authorization profile and transmits the
   authorizations to the authenticator along with the transported keying
   material.  Typically, the profile is determined based on the user
   identity, but a certificate presented by the user can also provide
   authorization information.

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

   (a)  Is this a legitimate user of this network?

   (b)  Is the user allowed to access this network?

   (c)  Is the user permitted to access this network on this day and at
        this time?

   (d)  Is the user within the concurrent session limit?

   (e)  Are there any fraud, credit limit, or other concerns that could
        lead to access denial?

   (f)  If access is to be granted, what are the service parameters
        (mandatory tunneling, bandwidth, filters, and so on) to be
        provisioned for the user?

   While the authorization decision is, in principle, simple, the
   distributed decision making process can add complexity.  Where
   brokers or proxies are involved, all of the AAA entities in the chain
   from the authenticator to the home backend authentication server are
   involved in the decision.  For example, a broker can deny access even
   if the home backend authentication 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 concurrent session
   limits).  In addition to the Accept/Reject decisions made by AAA
   entities, service 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 is.  As a result, the
   authenticator has no way to know on what the decision was based.  Was
   a set of authorization parameters sent because this service is always
   provided to the user, or was the decision based on the time of day
   and the capabilities of the authenticator?

4.3.3.  Correctness

   When the AAA exchange (phase 1b) is bypassed, several challenges
   arise in ensuring correct authorization:

   Theft of service
      Bypassing the AAA exchange (phase 1b) SHOULD NOT enable a user to
      extend their network access or gain access to services they are
      not entitled to.

   Consideration of network-wide state
      Handoff techniques SHOULD NOT render the backend authentication
      server incapable of keeping track of network-wide state.  For
      example, a backend authentication server can need to keep track of
      simultaneous user sessions.

   Elevation of privilege
      Backend authentication servers often perform conditional
      evaluation, in which the authorizations returned in an
      Access-Accept message are contingent on the authenticator or on
      dynamic state such as the time of day.  In this situation,
      bypassing the AAA exchange could enable unauthorized access unless
      the restrictions are explicitly encoded within the authorizations
      provided by the backend authentication server.

   A handoff mechanism that provides proper authorization is said to be
   "correct".  One condition for correctness is as follows:

      For a handoff to be "correct" it MUST establish on the new
      authenticator the same authorizations as would have been created
      had the new authenticator completed a AAA conversation with the
      backend authentication server.

   A properly designed handoff scheme will only succeed if it is
   "correct" in this way.  If a successful handoff would establish
   "incorrect" authorizations, it is preferable for it to fail.  Where
   the supported services differ between authenticators, a handoff that
   bypasses the backend authentication server is likely to fail.
   Section 1.1 of [RFC2865] 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.

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

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

   In order to perform a correct handoff, if a new authenticator is
   provided with RADIUS authorizations for a known but unavailable
   service, then it MUST process these authorizations the same way it
   would handle a RADIUS Access-Accept requesting an unavailable

   service;  this MUST cause the handoff to fail.  However, if a new
   authenticator is provided with authorizations including unknown
   attributes, then these attributes MAY be ignored.  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].
   Although it can 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 an authenticator provide a service that it
   does not implement.  This implies that if the new authenticator were
   to complete a AAA conversation, it would be likely to receive
   different service instructions.  Failure of the handoff is the
   desired result since it will cause the new authenticator to go back
   to the backend server in order to receive the appropriate service
   definition.

   Handoff mechanisms that bypass the backend authentication server are
   most likely to be successful when employed in a homogeneous
   deployment within a single administrative domain.  In a heterogeneous
   deployment, the backend authentication server can 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.  Where a backend authentication server
   would send different authorizations to the new authenticator than
   were sent to a previous authenticator, transferring authorizations
   between the previous authenticator and the new authenticator will
   result in incorrect authorization.

   Virtual LAN (VLAN) support is defined in [IEEE-802.1Q]; RADIUS
   support for dynamic VLANs is described in [RFC3580] and [RFC4675].
   If some authenticators support dynamic VLANs while others do not,
   then attributes present in the Access-Request (such as the
   NAS-Port-Type, NAS-IP-Address, NAS-IPv6-Address, and NAS-Identifier)
   could be examined by the backend authentication server to determine
   when VLAN attributes will be returned, and if so, which ones.
   However, if the backend authenticator is bypassed, then a handoff
   occurring between authenticators supporting different VLAN
   capabilities could result in a user obtaining access to an
   unauthorized VLAN (e.g., a user with access to a guest VLAN being
   given unrestricted access to the network).

   Similarly, it is preferable for a handoff between an authenticator
   providing confidentiality and another that does not to fail, since if
   the handoff were successful, the user would be moved from a secure to
   an insecure channel without permission from the backend
   authentication server.

5.  Security Considerations

   The EAP threat model is described in [RFC3748] Section 7.1.  The
   security properties of EAP methods (known as "security claims") are
   described in [RFC3748] Section 7.2.1.  EAP method requirements for
   applications such as Wireless LAN authentication are described in
   [RFC4017].  The RADIUS threat model is described in [RFC3579] Section
   4.1, and responses to these threats are described in [RFC3579],
   Sections 4.2 and 4.3.

   However, in addition to threats against EAP and AAA, there are other
   system level threats.  In developing the threat model, it is assumed
   that:

      All traffic is visible to the attacker.
      The attacker can alter, forge, or replay messages.
      The attacker can reroute messages to another principal.
      The attacker can be a principal or an outsider.
      The attacker can compromise any key that is sufficiently old.

   Threats arising from these assumptions include:

   (a)  An attacker can compromise or steal an EAP peer or
        authenticator, in an attempt to gain access to other EAP peers
        or authenticators or to obtain long-term secrets.

   (b)  An attacker can attempt a downgrade attack in order to exploit
        known weaknesses in an authentication method or cryptographic
        algorithm.

   (c)  An attacker can try to modify or spoof packets, including
        Discovery or Secure Association Protocol frames, EAP or AAA
        packets.

   (d)  An attacker can attempt to induce an EAP peer, authenticator, or
        server to disclose keying material to an unauthorized party, or
        utilize keying material outside the context that it was intended
        for.

   (e)  An attacker can alter, forge, or replay packets.

   (f)  An attacker can cause an EAP peer, authenticator, or server to
        reuse a stale key.  Use of stale keys can also occur
        unintentionally.  For example, a poorly implemented backend
        authentication server can provide stale keying material to an
        authenticator, or a poorly implemented authenticator can reuse
        nonces.

   (g)  An authenticated attacker can attempt to obtain elevated
        privilege in order to access information that it does not have
        rights to.

   (h)  An attacker can attempt a man-in-the-middle attack in order to
        gain access to the network.

   (i)  An attacker can compromise an EAP authenticator in an effort to
        commit fraud.  For example, a compromised authenticator can
        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.

   (j)  An attacker can launch a denial-of-service attack against the
        EAP peer, authenticator, or backend authentication server.

   In order to address these threats, [RFC4962] Section 3 describes
   required and recommended security properties.  The sections that
   follow analyze the compliance of EAP methods, AAA protocols, and
   Secure Association Protocols with those guidelines.

5.1.  Peer and Authenticator Compromise

   Requirement: In the event that an authenticator is compromised or
   stolen, an attacker can gain access to the network through that
   authenticator, or can obtain the credentials needed for the
   authenticator/AAA client to communicate with one or more backend
   authentication servers.  Similarly, if a peer is compromised or
   stolen, an attacker can obtain credentials needed to communicate with
   one or more authenticators.  A mandatory requirement from [RFC4962]
   Section 3:

      Prevent the Domino effect

      Compromise of a single peer MUST NOT compromise keying material
      held by any other peer within the system, including session keys
      and long-term keys.  Likewise, compromise of a single
      authenticator MUST NOT compromise keying material held by any
      other authenticator within the system.  In the context of a key

      hierarchy, this means that the compromise of one node in the key
      hierarchy must not disclose the information necessary to
      compromise other branches in the key hierarchy.  Obviously, the
      compromise of the root of the key hierarchy will compromise all of
      the keys; however, a compromise in one branch MUST NOT result in
      the compromise of other branches.  There are many implications of
      this requirement; however, two implications deserve highlighting.
      First, the scope of the keying material must be defined and
      understood by all parties that communicate with a party that holds
      that keying material.  Second, a party that holds keying material
      in a key hierarchy must not share that keying material with
      parties that are associated with other branches in the key
      hierarchy.

      Group keys are an obvious exception.  Since all members of the
      group have a copy of the same key, compromise of any one of the
      group members will result in the disclosure of the group key.

   Some of the implications of the requirement are as follows:

   Key Sharing
        In order to be able to determine whether keying material has
        been shared, it is necessary for the identity of the EAP
        authenticator (NAS-Identifier) to be defined and understood by
        all parties that communicate with it.  EAP lower-layer
        specifications such as [IEEE-802.11], [IEEE-802.16e],
        [IEEE-802.1X], IKEv2 [RFC4306], and PPP [RFC1661] do not involve
        key sharing.

   AAA Credential Sharing
        AAA credentials (such as RADIUS shared secrets, IPsec pre-shared
        keys or certificates) MUST NOT be shared between AAA clients,
        since if one AAA client were compromised, this would enable an
        attacker to impersonate other AAA clients to the backend
        authentication server, or even to impersonate a backend
        authentication server to other AAA clients.

   Compromise of Long-Term Credentials
        An attacker obtaining keying material (such as TSKs, TEKs, or
        the MSK) MUST NOT be able to obtain long-term user credentials
        such as pre-shared keys, passwords, or private-keys without
        breaking a fundamental cryptographic assumption.  The mandatory
        requirements of [RFC4017] Section 2.2 include generation of EAP
        keying material, capability to generate EAP keying material with
        128 bits of effective strength, resistance to dictionary
        attacks, shared state equivalence, and protection against
        man-in-the-middle attacks.

5.2.  Cryptographic Negotiation

   Mandatory requirements from [RFC4962] Section 3:

      Cryptographic algorithm independent

      The AAA key management protocol MUST be cryptographic algorithm
      independent.  However, an EAP method MAY depend on a specific
      cryptographic algorithm.  The ability to negotiate the use of a
      particular cryptographic algorithm provides resilience against
      compromise of a particular cryptographic algorithm.  Algorithm
      independence is also REQUIRED with a Secure Association Protocol
      if one is defined.  This is usually accomplished by including an
      algorithm identifier and parameters in the protocol, and by
      specifying the algorithm requirements in the protocol
      specification.  While highly desirable, the ability to negotiate
      key derivation functions (KDFs) is not required.  For
      interoperability, at least one suite of mandatory-to-implement
      algorithms MUST be selected.  Note that without protection by
      IPsec as described in [RFC3579] Section 4.2, RADIUS [RFC2865] does
      not meet this requirement, since the integrity protection
      algorithm cannot be negotiated.

      This requirement does not mean that a protocol must support both
      public-key and symmetric-key cryptographic algorithms.  It means
      that the protocol needs to be structured in such a way that
      multiple public-key algorithms can be used whenever a public-key
      algorithm is employed.  Likewise, it means that the protocol needs
      to be structured in such a way that multiple symmetric-key
      algorithms can be used whenever a symmetric-key algorithm is
      employed.

      Confirm ciphersuite selection

      The selection of the "best" ciphersuite SHOULD be securely
      confirmed.  The mechanism SHOULD detect attempted roll-back
      attacks.

   EAP methods satisfying [RFC4017] Section 2.2 mandatory requirements
   and AAA protocols utilizing transmission-layer security are capable
   of addressing downgrade attacks.  [RFC3748] Section 7.2.1 describes
   the "protected ciphersuite negotiation" security claim that refers to
   the ability of an EAP method to negotiate the ciphersuite used to
   protect the EAP method conversation, as well as to integrity protect
   the ciphersuite negotiation.  [RFC4017] Section 2.2 requires EAP
   methods satisfying this security claim.  Since TLS v1.2 [RFC5246] and
   IKEv2 [RFC4306] support negotiation of Key Derivation Functions
   (KDFs), EAP methods based on TLS or IKEv2 will, if properly designed,

   inherit this capability.  However, negotiation of KDFs is not
   required by RFC 4962 [RFC4962], and EAP methods based on neither TLS
   nor IKEv2 typically do not support KDF negotiation.

   When AAA protocols utilize TLS [RFC5246] or IPsec [RFC4301] for
   transmission layer security, they can leverage the cryptographic
   algorithm negotiation support provided by IKEv2 [RFC4306] or TLS
   [RFC5246].  RADIUS [RFC3579] by itself does not support cryptographic
   algorithm negotiation and relies on MD5 for integrity protection,
   authentication, and confidentiality.  Given the known weaknesses in
   MD5 [MD5Collision], this is undesirable, and can be addressed via use
   of RADIUS over IPsec, as described in [RFC3579] Section 4.2.

   To ensure against downgrade attacks within lower-layer protocols,
   algorithm independence is REQUIRED with lower layers using EAP for
   key derivation.  For interoperability, at least one suite of
   mandatory-to-implement algorithms MUST be selected.  Lower-layer
   protocols supporting EAP for key derivation SHOULD also support
   secure ciphersuite negotiation as well as KDF negotiation.

   As described in [RFC1968], PPP ECP does not support secure
   ciphersuite negotiation.  While [IEEE-802.16e] and [IEEE-802.11]
   support ciphersuite negotiation for protection of data, they do not
   support negotiation of the cryptographic primitives used within the
   Secure Association Protocol, such as message integrity checks (MICs)
   and KDFs.

5.3.  Confidentiality and Authentication

   Mandatory requirements from [RFC4962] Section 3:

      Authenticate all parties

      Each party in the AAA key management protocol MUST be
      authenticated to the other parties with whom they communicate.
      Authentication mechanisms MUST maintain the confidentiality of any
      secret values used in the authentication process.  When a secure
      association protocol is used to establish session keys, the
      parties involved in the secure association protocol MUST identify
      themselves using identities that are meaningful in the lower-layer
      protocol environment that will employ the session keys.  In this
      situation, the authenticator and peer may be known by different
      identifiers in the AAA protocol environment and the lower-layer
      protocol environment, making authorization decisions difficult
      without a clear key scope.  If the lower-layer identifier of the

      peer will be used to make authorization decisions, then the pair
      of identifiers associated with the peer MUST be authorized by the
      authenticator and/or the AAA server.

      AAA protocols, such as RADIUS [RFC2865] and Diameter [RFC3588],
      provide a mechanism for the identification of AAA clients; since
      the EAP authenticator and AAA client are always co-resident, this
      mechanism is applicable to the identification of EAP
      authenticators.

      When multiple base stations and a "controller" (such as a WLAN
      switch) comprise a single EAP authenticator, the "base station
      identity" is not relevant; the EAP method conversation takes place
      between the EAP peer and the EAP server.  Also, many base stations
      can share the same authenticator identity.  The authenticator
      identity is important in the AAA protocol exchange and the secure
      association protocol conversation.

      Authentication mechanisms MUST NOT employ plaintext passwords.
      Passwords may be used provided that they are not sent to another
      party without confidentiality protection.

      Keying material confidentiality and integrity

      While preserving algorithm independence, confidentiality and
      integrity of all keying material MUST be maintained.

   Conformance to these requirements is analyzed in the sections that
   follow.

5.3.1.  Spoofing

   Per-packet authentication and integrity protection provides
   protection against spoofing attacks.

   Diameter [RFC3588] provides support for per-packet authentication and
   integrity protection via use of IPsec or TLS.  RADIUS/EAP [RFC3579]
   provides for per-packet authentication and integrity protection via
   use of the Message-Authenticator Attribute.

   [RFC3748] Section 7.2.1 describes the "integrity protection" security
   claim and [RFC4017] Section 2.2 requires EAP methods supporting this
   claim.

   In order to prevent forgery of Secure Association Protocol frames,
   per-frame authentication and integrity protection is RECOMMENDED on
   all messages.  IKEv2 [RFC4306] supports per-frame integrity

   protection and authentication, as does the Secure Association
   Protocol defined in [IEEE-802.16e].  [IEEE-802.11] supports per-frame
   integrity protection and authentication on all messages within the
   4-way handshake except the first message.  An attack leveraging this
   omission is described in [Analysis].

5.3.2.  Impersonation

   Both RADIUS [RFC2865] and Diameter [RFC3588] implementations are
   potentially vulnerable to a rogue authenticator impersonating another
   authenticator.  While both protocols support mutual authentication
   between the AAA client/authenticator and the backend authentication
   server, the security mechanisms vary.

   In RADIUS, the shared secret used for authentication is determined by
   the source address of the RADIUS packet.  However, when RADIUS
   Access-Requests are forwarded by a proxy, the NAS-IP-Address,
   NAS-Identifier, or NAS-IPv6-Address attributes received by the RADIUS
   server will not correspond to the source address.  As noted in
   [RFC3579] Section 4.3.7, if the first-hop proxy does not check the
   NAS identification attributes against the source address in the
   Access-Request packet, it is possible for a rogue authenticator to
   forge NAS-IP-Address [RFC2865], NAS-IPv6-Address [RFC3162], or
   NAS-Identifier [RFC2865] attributes in order to impersonate another
   authenticator; attributes such as the Called-Station-Id [RFC2865] and
   Calling-Station-Id [RFC2865] can be forged as well.  Among other
   things, this can result in messages (and transported keying material)
   being sent to the wrong authenticator.

   While [RFC3588] requires use of the Route-Record AVP, this utilizes
   Fully Qualified Domain Names (FQDNs), so that impersonation detection
   requires DNS A, AAAA, and PTR Resource Records (RRs) to be properly
   configured.  As a result, Diameter is as vulnerable to this attack as
   RADIUS, if not more so.  [RFC3579] Section 4.3.7 recommends
   mechanisms for impersonation detection; to prevent access to keying
   material by proxies without a "need to know", it is necessary to
   allow the backend authentication server to communicate with the
   authenticator directly, such as via the redirect functionality
   supported in [RFC3588].

5.3.3.  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 can enable an authenticator to impersonate
   another authenticator or communicate incorrect information via
   out-of-band mechanisms (such as via AAA or the lower layer).

   Where EAP is used in pass-through mode, the EAP peer does not verify
   the identity of the pass-through authenticator within the EAP
   conversation.  Within the Secure Association Protocol, the EAP peer
   and authenticator only demonstrate mutual possession of the
   transported keying material; they do not mutually authenticate.  This
   creates a potential security vulnerability, described in [RFC3748]
   Section 7.15.

   As described in [RFC3579] Section 4.3.7, it is possible for a
   first-hop AAA proxy to detect a AAA client attempting to impersonate
   another authenticator.  However, it is possible for a pass-through
   authenticator acting as a AAA client to provide correct information
   to the backend authentication server while communicating misleading
   information to the EAP peer via the lower layer.

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

   As noted in [RFC3748] Section 7.15, this vulnerability can be
   addressed by 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.  Typically, the EAP
   method imports channel binding parameters from the lower layer on the
   peer, and transmits them securely to the EAP server, which exports
   them to the lower layer or AAA layer.  However, transport can occur
   from EAP server to peer, or can be bi-directional.  On the side of
   the exchange (peer or server) where channel binding is verified, the
   lower layer or AAA layer passes the result of the verification (TRUE
   or FALSE) up to the EAP method.  While the verification can be done
   either by the peer or the server, typically only the server has the
   knowledge to determine the correctness of the values, as opposed to
   merely verifying their equality.  For further discussion, see
   [EAP-SERVICE].

   It is also possible to perform channel binding without transporting
   data over EAP, as described in [EAP-CHANNEL].  In this approach the
   EAP method includes channel binding parameters in the calculation of
   exported EAP keying material, making it impossible for the peer and

   authenticator to complete the Secure Association Protocol if there is
   a mismatch in the channel binding parameters.  However, this approach
   can only be applied where methods generating EAP keying material are
   used along with lower layers that utilize EAP keying material.  For
   example, this mechanism would not enable verification of channel
   binding on wired IEEE 802 networks using [IEEE-802.1X].

5.3.4.  Mutual Authentication

   [RFC3748] Section 7.2.1 describes the "mutual authentication" and
   "dictionary attack resistance" claims, and [RFC4017] requires EAP
   methods satisfying these claims.  EAP methods complying with
   [RFC4017] therefore provide for mutual authentication between the EAP
   peer and server.

   [RFC3748] Section 7.2.1 also describes the "Cryptographic binding"
   security claim, and [RFC4017] Section 2.2 requires support for this
   claim.  As described in [EAP-BINDING], EAP method sequences and
   compound authentication mechanisms can 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, [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 EAP keying material, it
   MAY be desirable to derive the compound key from a portion of the
   EMSK.  Where this is done, 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.

   Diameter [RFC3588] provides for per-packet authentication and
   integrity protection via IPsec or TLS, and RADIUS/EAP [RFC3579] also
   provides for per-packet authentication and integrity protection.
   Where the authenticator/AAA client and backend authentication server
   communicate directly and credible key wrap is used (see Section 3.8),
   this ensures that the AAA Key Transport (phase 1b) achieves its
   security objectives: mutually authenticating the AAA
   client/authenticator and backend authentication server and providing
   transported keying material to the EAP authenticator and to no other
   party.

   [RFC2607] Section 7 describes the security issues occurring when the
   authenticator/AAA client and backend authentication server do not
   communicate directly.  Where a AAA intermediary is present (such as a
   RADIUS proxy or a Diameter agent), and data object security is not
   used, transported keying material can be recovered by an attacker in
   control of the intermediary.  As discussed in Section 2.1, unless the
   TSKs are derived independently from EAP keying material (as in
   IKEv2), possession of transported keying material enables decryption
   of data traffic sent between the peer and the authenticator to whom
   the keying material was transported.  It also allows the AAA
   intermediary to impersonate the authenticator or the peer.  Since the
   peer does not authenticate to a AAA intermediary, it has no ability
   to determine whether it is authentic or authorized to obtain keying
   material.

   However, as long as transported keying material or keys derived from
   it are only utilized by a single authenticator, compromise of the
   transported keying material does not enable an attacker to
   impersonate the peer to another authenticator.  Vulnerability to
   compromise of a AAA intermediary can be mitigated by implementation
   of redirect functionality, as described in [RFC3588] and [RFC4072].

   The Secure Association Protocol does not provide for mutual
   authentication between the EAP peer and authenticator, only mutual
   proof of possession of transported keying material.  In order for the
   peer to verify the identity of the authenticator, mutual proof of
   possession needs to be combined with impersonation prevention and
   channel binding.  Impersonation prevention (described in Section
   5.3.2) enables the backend authentication server to determine that
   the transported keying material has been provided to the correct
   authenticator.  When utilized along with impersonation prevention,
   channel binding (described in Section 5.3.3) enables the EAP peer to
   verify that the EAP server has authorized the authenticator to
   possess the transported keying material.  Completion of the Secure
   Association Protocol exchange demonstrates that the EAP peer and the
   authenticator possess the transported keying material.

5.4.  Key Binding

   Mandatory requirement from [RFC4962] Section 3:

      Bind key to its context

      Keying material MUST be bound to the appropriate context.  The
      context includes the following:

      o  The manner in which the keying material is expected to be used.

      o  The other parties that are expected to have access to the
         keying material.

      o  The expected lifetime of the keying material.  Lifetime of a
         child key SHOULD NOT be greater than the lifetime of its parent
         in the key hierarchy.

      Any party with legitimate access to keying material can determine
      its context.  In addition, the protocol MUST ensure that all
      parties with legitimate access to keying material have the same
      context for the keying material.  This requires that the parties
      are properly identified and authenticated, so that all of the
      parties that have access to the keying material can be determined.

      The context will include the peer and NAS identities in more than
      one form.  One (or more) name form is needed to identify these
      parties in the authentication exchange and the AAA protocol.
      Another name form may be needed to identify these parties within
      the lower layer that will employ the session key.

   Within EAP, exported keying material (MSK, EMSK,IV) is bound to the
   Peer-Id(s) and Server-Id(s), which are exported along with the keying
   material.  However, not all EAP methods support authenticated server
   identities (see Appendix A).

   Within the AAA protocol, transported keying material is destined for
   the EAP authenticator identified by the NAS-Identifier Attribute
   within the request, and is for use by the EAP peer identified by the
   Peer-Id(s), User-Name [RFC2865], or Chargeable User Identity (CUI)
   [RFC4372] attributes.  The maximum lifetime of the transported keying
   material can be provided, as discussed in Section 3.5.1.  Key usage
   restrictions can also be included as described in Section 3.2.  Key
   lifetime issues are discussed in Sections 3.3, 3.4, and 3.5.

5.5.  Authorization

   Requirement: The Secure Association Protocol (phase 2) conversation
   may utilize different identifiers from the EAP conversation (phase
   1a), so that binding between the EAP and Secure Association Protocol
   identities is REQUIRED.

   Mandatory requirement from [RFC4962] Section 3:

      Peer and authenticator authorization

      Peer and authenticator authorization MUST be performed.  These
      entities MUST demonstrate possession of the appropriate keying
      material, without disclosing it.  Authorization is REQUIRED

      whenever a peer associates with a new authenticator.  The
      authorization checking prevents an elevation of privilege attack,
      and it ensures that an unauthorized authenticator is detected.

      Authorizations SHOULD be synchronized between the peer, NAS, and
      backend authentication server.  Once the AAA key management
      protocol exchanges are complete, all of these parties should hold
      a common view of the authorizations associated with the other
      parties.

      In addition to authenticating all parties, key management
      protocols need to demonstrate that the parties are authorized to
      possess keying material.  Note that proof of possession of keying
      material does not necessarily prove authorization to hold that
      keying material.  For example, within an IEEE 802.11, the 4-way
      handshake demonstrates that both the peer and authenticator
      possess the same EAP keying material.  However, by itself, this
      possession proof does not demonstrate that the authenticator was
      authorized by the backend authentication server to possess that
      keying material.  As noted in [RFC3579] in Section 4.3.7, where
      AAA proxies are present, it is possible for one authenticator to
      impersonate another, unless each link in the AAA chain implements
      checks against impersonation.  Even with these checks in place, an
      authenticator may still claim different identities to the peer and
      the backend authentication server.  As described in [RFC3748]
      Section 7.15, channel binding is required to enable the peer to
      verify that the authenticator claim of identity is both consistent
      and correct.

   Recommendation from [RFC4962] Section 3:

      Authorization restriction

      If peer authorization is restricted, then the peer SHOULD be made
      aware of the restriction.  Otherwise, the peer may inadvertently
      attempt to circumvent the restriction.  For example, authorization
      restrictions in an IEEE 802.11 environment include:

      o  Key lifetimes, where the keying material can only be used for a
         certain period of time;

      o  SSID restrictions, where the keying material can only be used
         with a specific IEEE 802.11 SSID;

      o  Called-Station-ID restrictions, where the keying material can
         only be used with a single IEEE 802.11 BSSID; and

      o  Calling-Station-ID restrictions, where the keying material can
         only be used with a single peer IEEE 802 MAC address.

   As described in Section 2.3, consistent identification of the EAP
   authenticator enables the EAP peer to determine the scope of keying
   material provided to an authenticator, as well as to confirm with the
   backend authentication server that an EAP authenticator proving
   possession of EAP keying material during the Secure Association
   Protocol was authorized to obtain it.

   Within the AAA protocol, the authorization attributes are bound to
   the transported keying material.  While the AAA exchange provides the
   AAA client/authenticator with authorizations relating to the EAP
   peer, neither the EAP nor AAA exchanges provide authorizations to the
   EAP peer.  In order to ensure that all parties hold the same view of
   the authorizations, it is RECOMMENDED that the Secure Association
   Protocol enable communication of authorizations between the EAP
   authenticator and peer.

   In lower layers where the authenticator consistently identifies
   itself to the peer and backend authentication server and the EAP peer
   completes the Secure Association Protocol exchange with the same
   authenticator through which it completed the EAP conversation,
   authorization of the authenticator is demonstrated to the peer by
   mutual authentication between the peer and authenticator as discussed
   in the previous section.  Identification issues are discussed in
   Sections 2.3, 2.4, and 2.5 and key scope issues are discussed in
   Section 3.2.

   Where the EAP peer utilizes different identifiers within the EAP
   method and Secure Association Protocol conversations, peer
   authorization can be difficult to demonstrate to the authenticator
   without additional restrictions.  This problem does not exist in
   IKEv2 where the Identity Payload is used for peer identification both
   within IKEv2 and EAP, and where the EAP conversation is
   cryptographically protected within IKEv2 binding the EAP and IKEv2
   exchanges.  However, within [IEEE-802.11], the EAP peer identity is
   not used within the 4-way handshake, so that it is necessary for the
   authenticator to require that the EAP peer utilize the same MAC
   address for EAP authentication as for the 4-way handshake.

5.6.  Replay Protection

   Mandatory requirement from [RFC4962] Section 3:

      Replay detection mechanism

      The AAA key management protocol exchanges MUST be replay
      protected, including AAA, EAP and Secure Association Protocol
      exchanges.  Replay protection allows a protocol message recipient
      to discard any message that was recorded during a previous
      legitimate dialogue and presented as though it belonged to the
      current dialogue.

   [RFC3748] Section 7.2.1 describes the "replay protection" security
   claim, and [RFC4017] Section 2.2 requires use of EAP methods
   supporting this claim.

   Diameter [RFC3588] provides support for replay protection via use of
   IPsec or TLS.  "RADIUS Support for EAP" [RFC3579] protects against
   replay of keying material via the Request Authenticator.  According
   to [RFC2865] Section 3:

      In Access-Request Packets, the Authenticator value is a 16 octet
      random number, called the Request Authenticator.

   However, some RADIUS packets are not replay protected.  In
   Accounting, Disconnect, and Care-of Address (CoA)-Request packets,
   the Request Authenticator contains a keyed Message Integrity Code
   (MIC) rather than a nonce.  The Response Authenticator in Accounting,
   Disconnect, and CoA-Response packets also contains a keyed MIC whose
   calculation does not depend on a nonce in either the Request or
   Response packets.  Therefore, unless an Event-Timestamp attribute is
   included or IPsec is used, it is possible that the recipient will not
   be able to determine whether these packets have been replayed.  This
   issue is discussed further in [RFC5176] Section 6.3.

   In order to prevent replay of Secure Association Protocol frames,
   replay protection is REQUIRED on all messages.  [IEEE-802.11]
   supports replay protection on all messages within the 4-way
   handshake; IKEv2 [RFC4306] also supports this.

5.7.  Key Freshness

   Requirement: A session key SHOULD be considered compromised if it
   remains in use beyond its authorized lifetime.  Mandatory requirement
   from [RFC4962] Section 3:

      Strong, fresh session keys

      While preserving algorithm independence, session keys MUST be
      strong and fresh.  Each session deserves an independent session
      key.  Fresh keys are required even when a long replay counter
      (that is, one that "will never wrap") is used to ensure that loss
      of state does not cause the same counter value to be used more
      than once with the same session key.

      Some EAP methods are capable of deriving keys of varying strength,
      and these EAP methods MUST permit the generation of keys meeting a
      minimum equivalent key strength.  BCP 86 [RFC3766] offers advice
      on appropriate key sizes.  The National Institute for Standards
      and Technology (NIST) also offers advice on appropriate key sizes
      in [SP800-57].

      A fresh cryptographic key is one that is generated specifically
      for the intended use.  In this situation, a secure association
      protocol is used to establish session keys.  The AAA protocol and
      EAP method MUST ensure that the keying material supplied as an
      input to session key derivation is fresh, and the secure
      association protocol MUST generate a separate session key for each
      session, even if the keying material provided by EAP is cached.  A
      cached key persists after the authentication exchange has
      completed.  For the AAA/EAP server, key caching can happen when
      state is kept on the server.  For the NAS or client, key caching
      can happen when the NAS or client does not destroy keying material
      immediately following the derivation of session keys.

      Session keys MUST NOT be dependent on one another.  Multiple
      session keys may be derived from a higher-level shared secret as
      long as a one-time value, usually called a nonce, is used to
      ensure that each session key is fresh.  The mechanism used to
      generate session keys MUST ensure that the disclosure of one
      session key does not aid the attacker in discovering any other
      session keys.

   EAP, AAA, and the lower layer each bear responsibility for ensuring
   the use of fresh, strong session keys.  EAP methods need to ensure
   the freshness and strength of EAP keying material provided as an
   input to session key derivation.  [RFC3748] Section 7.10 states:

      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.

   The contribution of nonces enables the EAP peer and server to ensure
   that exported EAP keying material is fresh.

   [RFC3748] Section 7.2.1 describes the "key strength" and "session
   independence" security claims, and [RFC4017] requires EAP methods
   supporting these claims as well as methods capable of providing
   equivalent key strength of 128 bits or greater.  See Section 3.7 for
   more information on key strength.

   The AAA protocol needs to ensure that transported keying material is
   fresh and is not utilized outside its recommended lifetime.  Replay
   protection is necessary for key freshness, but an attacker can
   deliver a stale (and therefore potentially compromised) key in a
   replay-protected message, so replay protection is not sufficient.  As
   discussed in Section 3.5, the Session-Timeout Attribute enables the
   backend authentication server to limit the exposure of transported
   keying material.

   The EAP Session-Id, described in Section 1.4, enables the EAP peer,
   authenticator, and server to distinguish EAP conversations.  However,
   unless the authenticator keeps track of EAP Session-Ids, the
   authenticator cannot use the Session-Id to guarantee the freshness of
   keying material.

   The Secure Association Protocol, described in Section 3.1, MUST
   generate a fresh session key for each session, even if the EAP keying
   material and parameters provided by methods are cached, or either the
   peer or authenticator lack a high entropy random number generator.  A
   RECOMMENDED method is for the peer and authenticator to each provide
   a nonce or counter used in session key derivation.  If a nonce is
   used, it is RECOMMENDED that it be at least 128 bits.  While
   [IEEE-802.11] and IKEv2 [RFC4306] satisfy this requirement,
   [IEEE-802.16e] does not, since randomness is only contributed from
   the Base Station.

5.8.  Key Scope Limitation

   Mandatory requirement from [RFC4962] Section 3:

      Limit key scope

      Following the principle of least privilege, parties MUST NOT have
      access to keying material that is not needed to perform their
      role.  A party has access to a particular key if it has access to
      all of the secret information needed to derive it.

      Any protocol that is used to establish session keys MUST specify
      the scope for session keys, clearly identifying the parties to
      whom the session key is available.

   Transported keying material is permitted to be accessed by the EAP
   peer, authenticator and server.  The EAP peer and server derive EAP
   keying material during the process of mutually authenticating each
   other using the selected EAP method.  During the Secure Association
   Protocol exchange, the EAP peer utilizes keying material to
   demonstrate to the authenticator that it is the same party that
   authenticated to the EAP server and was authorized by it.  The EAP
   authenticator utilizes the transported keying material to prove to
   the peer not only that the EAP conversation was transported through
   it (this could be demonstrated by a man-in-the-middle), but that it
   was uniquely authorized by the EAP server to provide the peer with
   access to the network.  Unique authorization can only be demonstrated
   if the EAP authenticator does not share the transported keying
   material with a party other than the EAP peer and server.  TSKs are
   permitted to be accessed only by the EAP peer and authenticator (see
   Section 1.5); TSK derivation is discussed in Section 2.1.  Since
   demonstration of authorization within the Secure Association Protocol
   exchange depends on possession of transported keying material, the
   backend authentication server can obtain TSKs unless it deletes the
   transported keying material after sending it.

5.9.  Key Naming

   Mandatory requirement from [RFC4962] Section 3:

      Uniquely named keys

      AAA key management proposals require a robust key naming scheme,
      particularly where key caching is supported.  The key name
      provides a way to refer to a key in a protocol so that it is clear
      to all parties which key is being referenced.  Objects that cannot
      be named cannot be managed.  All keys MUST be uniquely named, and
      the key name MUST NOT directly or indirectly disclose the keying

      material.  If the key name is not based on the keying material,
      then one can be sure that it cannot be used to assist in a search
      for the key value.

   EAP key names (defined in Section 1.4.1), along with the Peer-Id(s)
   and Server-Id(s), uniquely identify EAP keying material, and do not
   directly or indirectly expose EAP keying material.

   Existing AAA server implementations do not distribute key names along
   with the transported keying material.  However, Diameter EAP
   [RFC4072] Section 4.1.4 defines the EAP-Key-Name AVP for the purpose
   of transporting the EAP Session-Id.  Since the EAP-Key-Name AVP is
   defined within the RADIUS attribute space, it can be used either with
   RADIUS or Diameter.

   Since the authenticator is not provided with the name of the
   transported keying material by existing backend authentication server
   implementations, existing Secure Association Protocols do not utilize
   EAP key names.  For example, [IEEE-802.11] supports PMK caching; to
   enable the peer and authenticator to determine the cached PMK to
   utilize within the 4-way handshake, the PMK needs to be named.  For
   this purpose, [IEEE-802.11] utilizes a PMK naming scheme that is
   based on the key.  Since IKEv2 [RFC4306] does not cache transported
   keying material, it does not need to refer to transported keying
   material.

5.10.  Denial-of-Service Attacks

   Key caching can result in vulnerability to denial-of-service attacks.
   For example, EAP methods that create persistent state can be
   vulnerable to denial-of-service attacks on the EAP server by a rogue
   EAP peer.

   To address this vulnerability, EAP methods creating persistent state
   can limit the persistent state created by an EAP peer.  For example,
   for each peer an EAP server can choose to limit persistent state to a
   few EAP conversations, distinguished by the EAP Session-Id.  This
   prevents a rogue peer from denying access to other peers.

   Similarly, to conserve resources an authenticator can choose to limit
   the persistent state corresponding to each peer.  This can be
   accomplished by limiting each peer to persistent state corresponding
   to a few EAP conversations, distinguished by the EAP Session-Id.

   Whether creation of new TSKs implies deletion of previously derived
   TSKs depends on the EAP lower layer.  Where there is no implied
   deletion, the authenticator can choose to limit the number of TSKs
   and associated state that can be stored for each peer.

6.  References

6.1.  Normative References

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

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

   [RFC4962]      Housley, R. and B. Aboba, "Guidance for
                  Authentication, Authorization, and Accounting (AAA)
                  Key Management", BCP 132, RFC 4962, July 2007.

6.2.  Informative References

   [8021XPreAuth] Pack, S. and Y. Choi, "Pre-Authenticated Fast Handoff
                  in a Public Wireless LAN Based on IEEE 802.1x Model",
                  Proceedings of the IFIP TC6/WG6.8 Working Conference
                  on Personal Wireless Communications, p.175-182,
                  October 23-25, 2002.

   [Analysis]     He, C. and J. Mitchell, "Analysis of the 802.11i 4-Way
                  Handshake", Proceedings of the 2004 ACM Workshop on
                  Wireless Security, pp. 43-50, ISBN: 1-58113-925-X.

   [Bargh]        Bargh, M., Hulsebosch, R., Eertink, E., Prasad, A.,
                  Wang, H. and P. Schoo, "Fast Authentication Methods
                  for Handovers between IEEE 802.11 Wireless LANs",
                  Proceedings of the 2nd ACM international workshop on
                  Wireless mobile applications and services on WLAN
                  hotspots, October, 2004.

   [GKDP]         Dondeti, L., Xiang, J., and S. Rowles, "GKDP: Group
                  Key Distribution Protocol", Work in Progress, March
                  2006.

   [He]           He, C., Sundararajan, M., Datta, A. Derek, A. and J.
                  C.  Mitchell, "A Modular Correctness Proof of TLS and
                  IEEE 802.11i", ACM Conference on Computer and
                  Communications Security (CCS '05), November, 2005.

   [IEEE-802.11]  Institute of Electrical and Electronics Engineers,
                  "Information technology - Telecommunications and
                  information exchange between systems - Local and
                  metropolitan area networks - Specific Requirements
                  Part 11:  Wireless LAN Medium Access Control (MAC) and
                  Physical Layer (PHY) Specifications", IEEE Standard
                  802.11-2007, 2007.

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

   [IEEE-802.1Q]  IEEE Standards for Local and Metropolitan Area
                  Networks:  Draft Standard for Virtual Bridged Local
                  Area Networks, P802.1Q-2003, January 2003.

   [IEEE-802.11i] Institute of Electrical and Electronics Engineers,
                  "Supplement to Standard for Telecommunications and
                  Information Exchange Between Systems - LAN/MAN
                  Specific Requirements - Part 11: Wireless LAN Medium
                  Access Control (MAC) and Physical Layer (PHY)
                  Specifications:  Specification for Enhanced Security",
                  IEEE 802.11i/D1, 2001.

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

   [IEEE-802.16e] Institute of Electrical and Electronics Engineers,
                  "IEEE Standard for Local and Metropolitan Area
                  Networks: Part 16: Air Interface for Fixed and Mobile
                  Broadband Wireless Access Systems: Amendment for
                  Physical and Medium Access Control Layers for Combined
                  Fixed and Mobile Operations in Licensed Bands" IEEE
                  802.16e, August 2005.

   [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.

   [EAP-SERVICE]  Arkko, J. and P. Eronen, "Authenticated Service
                  Information for the Extensible Authentication Protocol
                  (EAP)", Work in Progress, October 2005.

   [SHORT-TERM]   Friedman, A., Sheffer, Y., and A. Shaqed, "Short-Term
                  Certificates", Work in Progress, June 2007.

   [HANDOFF]      Arbaugh, W. and B. Aboba, "Handoff Extension to
                  RADIUS", Work in Progress, October 2003.

   [EAP-CHANNEL]  Ohba, Y., Parthasrathy, M., and M. Yanagiya, "Channel
                  Binding Mechanism Based on Parameter Binding in Key
                  Derivation", Work in Progress, June 2007.

   [EAP-BINDING]  Puthenkulam, J., Lortz, V., Palekar, A., and D. Simon,
                  "The Compound Authentication Binding Problem", Work in
                  Progress, October 2003.

   [MD5Collision] Klima, V., "Tunnels in Hash Functions: MD5 Collisions
                  Within a Minute", Cryptology ePrint Archive, March
                  2006, http://eprint.iacr.org/2006/105.pdf

   [MishraPro]    Mishra, A., Shin, M. and W. Arbaugh, "Pro-active Key
                  Distribution using Neighbor Graphs", IEEE Wireless
                  Communications, vol. 11, February 2004.

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

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

   [RFC2230]      Atkinson, R., "Key Exchange Delegation Record for the
                  DNS", RFC 2230, November 1997.

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

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

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

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

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

   [RFC2782]      Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR
                  for specifying the location of services (DNS SRV)",
                  RFC 2782, February 2000.

   [RFC2845]      Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
                  Wellington, "Secret Key Transaction Authentication for
                  DNS (TSIG)", RFC 2845, May 2000.

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

   [RFC3007]      Wellington, B., "Secure Domain Name System (DNS)
                  Dynamic Update", RFC 3007, November 2000.

   [RFC3162]      Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",
                  RFC 3162, August 2001.

   [RFC3547]      Baugher, M., Weis, B., Hardjono, T., and H. Harney,
                  "The Group Domain of Interpretation", RFC 3547, July
                  2003.

   [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", BCP
                  86, RFC 3766, April 2004.

   [RFC3830]      Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and
                  K. Norrman, "MIKEY: Multimedia Internet KEYing", RFC
                  3830, August 2004.

   [RFC4005]      Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
                  "Diameter Network Access Server Application", RFC
                  4005, August 2005.

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

   [RFC4033]      Arends, R., Austein, R., Larson, M., Massey, D., and
                  S. Rose, "DNS Security Introduction and Requirements",
                  RFC 4033, March 2005.

   [RFC4035]      Arends, R., Austein, R., Larson, M., Massey, D., and
                  S. Rose, "Protocol Modifications for the DNS Security
                  Extensions", RFC 4035, March 2005.

   [RFC4067]      Loughney, J., Ed., Nakhjiri, M., Perkins, C., and R.
                  Koodli, "Context Transfer Protocol (CXTP)", RFC 4067,
                  July 2005.

   [RFC4072]      Eronen, P., Ed., Hiller, T., and G. Zorn, "Diameter
                  Extensible Authentication Protocol (EAP) Application",
                  RFC 4072, August 2005.

   [RFC4118]      Yang, L., Zerfos, P., and E. Sadot, "Architecture
                  Taxonomy for Control and Provisioning of Wireless
                  Access Points (CAPWAP)", RFC 4118, June 2005.

   [RFC4186]      Haverinen, H., Ed., and J. Salowey, Ed., "Extensible
                  Authentication Protocol Method for Global System for
                  Mobile Communications (GSM) Subscriber Identity
                  Modules (EAP-SIM)", RFC 4186, January 2006.

   [RFC4187]      Arkko, J. and H. Haverinen, "Extensible Authentication
                  Protocol Method for 3rd Generation Authentication and
                  Key Agreement (EAP-AKA)", RFC 4187, January 2006.

   [RFC4282]      Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
                  Network Access Identifier", RFC 4282, December 2005.

   [RFC4284]      Adrangi, F., Lortz, V., Bari, F., and P. Eronen,
                  "Identity Selection Hints for the Extensible
                  Authentication Protocol (EAP)", RFC 4284, January
                  2006.

   [RFC4301]      Kent, S. and K. Seo, "Security Architecture for the
                  Internet Protocol", RFC 4301, December 2005.

   [RFC4306]      Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
                  Protocol", RFC 4306, December 2005.

   [RFC4372]      Adrangi, F., Lior, A., Korhonen, J., and J. Loughney,
                  "Chargeable User Identity", RFC 4372, January 2006.

   [RFC4334]      Housley, R. and T. Moore, "Certificate Extensions and
                  Attributes Supporting Authentication in Point-to-Point
                  Protocol (PPP) and Wireless Local Area Networks
                  (WLAN)", RFC 4334, February 2006.

   [RFC4535]      Harney, H., Meth, U., Colegrove, A., and G. Gross,
                  "GSAKMP: Group Secure Association Key Management
                  Protocol", RFC 4535, June 2006.

   [RFC4763]      Vanderveen, M. and H. Soliman, "Extensible
                  Authentication Protocol Method for Shared-secret
                  Authentication and Key Establishment (EAP-SAKE)", RFC
                  4763, November 2006.

   [RFC4675]      Congdon, P., Sanchez, M., and B. Aboba, "RADIUS
                  Attributes for Virtual LAN and Priority Support", RFC
                  4675, September 2006.

   [RFC4718]      Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
                  Implementation Guidelines", RFC 4718, October 2006.

   [RFC4764]      Bersani, F. and H. Tschofenig, "The EAP-PSK Protocol:
                  A Pre-Shared Key Extensible Authentication Protocol
                  (EAP) Method", RFC 4764, January 2007.

   [RFC5176]      Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
                  Aboba, "Dynamic Authorization Extensions to Remote
                  Authentication Dial In User Service (RADIUS)", RFC
                  5176, January 2008.

   [RFC5216]      Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
                  Authentication Protocol", RFC 5216, March 2008.

   [RFC5246]      Dierks, T. and E. Rescorla, "The Transport Layer
                  Security (TLS) Protocol Version 1.2", RFC 5246, August
                  2008.

   [SP800-57]     National Institute of Standards and Technology,
                  "Recommendation for Key Management", Special
                  Publication 800-57, May 2006.

   [Token]        Fantacci, R., Maccari, L., Pecorella, T., and F.
                  Frosali, "A secure and performant token-based
                  authentication for infrastructure and mesh 802.1X
                  networks", IEEE Conference on Computer Communications,
                  June 2006.

   [Tokenk]       Ohba, Y., Das, S., and A. Duttak, "Kerberized Handover
                  Keying: A Media-Independent Handover Key Management
                  Architecture", Mobiarch 2007.

Acknowledgments

   Thanks to Ashwin Palekar, Charlie Kaufman, and Tim Moore of
   Microsoft, Jari Arkko of Ericsson, Dorothy Stanley of Aruba Networks,
   Bob Moskowitz of TruSecure, Jesse Walker of Intel, Joe Salowey of
   Cisco, and Russ Housley of Vigil Security for useful feedback.

Appendix A - Exported Parameters in Existing Methods

   This Appendix specifies Session-Id, Peer-Id, Server-Id and
   Key-Lifetime for EAP methods that have been published prior to this
   specification.  Future EAP method specifications MUST include a
   definition of the Session-Id, Peer-Id and Server-Id (could be the
   null string).  In the descriptions that follow, all fields comprising
   the Session-Id are assumed to be in network byte order.

   EAP-Identity

      The EAP-Identity method is defined in [RFC3748].  It does not
      derive keys, and therefore does not define the Session-Id.  The
      Peer-Id and Server-Id are the null string (zero length).

   EAP-Notification

      The EAP-Notification method is defined in [RFC3748].  It does not
      derive keys and therefore does not define the Session-Id.  The
      Peer-Id and Server-Id are the null string (zero length).

   EAP-MD5-Challenge

      The EAP-MD5-Challenge method is defined in [RFC3748].  It does not
      derive keys and therefore does not define the Session-Id.  The
      Peer-Id and Server-Id are the null string (zero length).

   EAP-GTC

      The EAP-GTC method is defined in [RFC3748].  It does not derive
      keys and therefore does not define the Session-Id.  The Peer-Id
      and Server-Id are the null string (zero length).

   EAP-OTP

      The EAP-OTP method is defined in [RFC3748].  It does not derive
      keys and therefore does not define the Session-Id.  The Peer-Id
      and Server-Id are the null string (zero length).

   EAP-AKA

      EAP-AKA is defined in [RFC4187].  The EAP-AKA Session-Id is the
      concatenation of the EAP Type Code (0x17) with the contents of the
      RAND field from the AT_RAND attribute, followed by the contents of
      the AUTN field in the AT_AUTN attribute:

      Session-Id = 0x17 || RAND || AUTN

      The Peer-Id is the contents of the Identity field from the
      AT_IDENTITY attribute, using only the Actual Identity Length
      octets from the beginning, however.  Note that the contents are
      used as they are transmitted, regardless of whether the
      transmitted identity was a permanent, pseudonym, or fast EAP
      re-authentication identity.  The Server-Id is the null string
      (zero length).

   EAP-SIM

      EAP-SIM is defined in [RFC4186].  The EAP-SIM Session-Id is the
      concatenation of the EAP Type Code (0x12) with the contents of the
      RAND field from the AT_RAND attribute, followed by the contents of
      the NONCE_MT field in the AT_NONCE_MT attribute:

      Session-Id = 0x12 || RAND || NONCE_MT

      The Peer-Id is the contents of the Identity field from the
      AT_IDENTITY attribute, using only the Actual Identity Length
      octets from the beginning, however.  Note that the contents are
      used as they are transmitted, regardless of whether the
      transmitted identity was a permanent, pseudonym, or fast EAP
      re-authentication identity.  The Server-Id is the null string
      (zero length).

   EAP-PSK

      EAP-PSK is defined in [RFC4764].  The EAP-PSK Session-Id is the
      concatenation of the EAP Type Code (0x2F) with the peer (RAND_P)
      and server (RAND_S) nonces:

      Session-Id = 0x2F || RAND_P || RAND_S

      The Peer-Id is the contents of the ID_P field and the Server-Id is
      the contents of the ID_S field.

   EAP-SAKE

      EAP-SAKE is defined in [RFC4763].  The EAP-SAKE Session-Id is the
      concatenation of the EAP Type Code (0x30) with the contents of the
      RAND_S field from the AT_RAND_S attribute, followed by the
      contents of the RAND_P field in the AT_RAND_P attribute:

      Session-Id = 0x30 || RAND_S || RAND_P

      Note that the EAP-SAKE Session-Id is not the same as the "Session
      ID" parameter chosen by the Server, which is sent in the first
      message, and replicated in subsequent messages.  The Peer-Id is
      contained within the value field of the AT_PEERID attribute and
      the Server-Id, if available, is contained in the value field of
      the AT_SERVERID attribute.

   EAP-TLS

      For EAP-TLS, the Peer-Id, Server-Id and Session-Id are defined in
      [RFC5216].

Authors' 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

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

    EMail: pasi.eronen@nokia.com

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