





Network Working Group                                      M. Vanderveen
Request for Comments: 4763                                    H. Soliman
Category: Informational                    Qualcomm Flarion Technologies
                                                           November 2006


             Extensible Authentication Protocol Method for
     Shared-secret Authentication and Key Establishment (EAP-SAKE)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and in particular notes that the decision to publish is not
   based on IETF review for such things as security, congestion control,
   or inappropriate interaction with deployed protocols.  The RFC Editor
   has chosen to publish this document at its discretion.  Readers of
   this document should exercise caution in evaluating its value for
   implementation and deployment.  See RFC 3932 for more information.

Abstract

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for Shared-secret Authentication and Key Establishment
   (SAKE).  This RFC is published as documentation for the IANA
   assignment of an EAP Type for a vendor's EAP method per RFC 3748.
   The specification has passed Designated Expert review for this IANA
   assignment.













Vanderveen & Soliman         Informational                      [Page 1]

RFC 4763                        EAP-SAKE                   November 2006


Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Protocol Description ............................................4
      3.1. Overview and Motivation of EAP-SAKE ........................4
      3.2. Protocol Operation .........................................5
           3.2.1. Successful Exchange .................................5
           3.2.2. Authentication Failure ..............................7
           3.2.3. Identity Management ................................11
           3.2.4. Obtaining Peer Identity ............................11
           3.2.5. Key Hierarchy ......................................13
           3.2.6. Key Derivation .....................................15
           3.2.7. Ciphersuite Negotiation ............................17
           3.2.8. Message Integrity and Encryption ...................17
           3.2.9. Fragmentation ......................................21
           3.2.10. Error Cases .......................................21
      3.3. Message Formats ...........................................22
           3.3.1. Message Format Summary .............................22
           3.3.2. Attribute Format ...................................23
           3.3.3. Use of AT_ENCR_DATA Attribute ......................25
           3.3.4. EAP.Request/SAKE/Challenge Format ..................26
           3.3.5. EAP.Response/SAKE/Challenge Format .................28
           3.3.6. EAP.Request/SAKE/Confirm Format ....................30
           3.3.7. EAP.Response/SAKE/Confirm Format ...................32
           3.3.8. EAP.Response/SAKE/Auth-Reject Format ...............33
           3.3.9. EAP.Request/SAKE/Identity Format ...................34
           3.3.10. EAP.Response/SAKE/Identity Format .................36
           3.3.11. Other EAP Messages Formats ........................37
   4. IANA Considerations ............................................37
   5. Security Considerations ........................................38
      5.1. Denial-of-Service Attacks .................................38
      5.2. Root Secret Considerations ................................38
      5.3. Mutual Authentication .....................................39
      5.4. Integrity Protection ......................................39
      5.5. Replay Protection .........................................39
      5.6. Confidentiality ...........................................40
      5.7. Key Derivation, Strength ..................................40
      5.8. Dictionary Attacks ........................................41
      5.9. Man-in-the-Middle Attacks .................................41
      5.10. Result Indication Protection .............................41
      5.11. Cryptographic Separation of Keys .........................41
      5.12. Session Independence .....................................41
      5.13. Identity Protection ......................................42
      5.14. Channel Binding ..........................................42
      5.15. Ciphersuite Negotiation ..................................42
      5.16. Random Number Generation .................................43
   6. Security Claims ................................................43



Vanderveen & Soliman         Informational                      [Page 2]

RFC 4763                        EAP-SAKE                   November 2006


   7. Acknowledgements ...............................................44
   8. References .....................................................44
      8.1. Normative References ......................................44
      8.2. Informative References ....................................45

1.  Introduction

   The Extensible Authentication Protocol (EAP), described in [EAP],
   provides a standard mechanism for support of multiple authentication
   methods.  EAP is also used within IEEE 802 networks through the IEEE
   802.11i [IEEE802.11i] framework.

   EAP supports several authentication schemes, including smart cards,
   Kerberos, Public Key, One Time Passwords, TLS, and others.  This
   document defines an authentication scheme, called the EAP-SAKE.
   EAP-SAKE supports mutual authentication and session key derivation,
   based on a static pre-shared secret data.  This RFC is published as
   documentation for the IANA assignment of an EAP Type for a vendor's
   EAP method per RFC 3748.  The specification has passed Designated
   Expert review for this IANA assignment.

2.  Terminology

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  The key
   words  "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", "SHOULD",
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
   are to be interpreted as described in BCP 14 [KEYWORDS].

   In addition to the terms used in [EAP], this document frequently uses
   the following terms and abbreviations:

   MIC   Message Integrity Check

   SMS   SAKE Master Secret

   NAI   Network Access Identifier














Vanderveen & Soliman         Informational                      [Page 3]

RFC 4763                        EAP-SAKE                   November 2006


3.  Protocol Description

3.1.  Overview and Motivation of EAP-SAKE

   The EAP-SAKE authentication protocol is a native EAP authentication
   method.  That is, a stand-alone version of EAP-SAKE outside of EAP is
   not defined.  EAP-SAKE is designed to enable mutual authentication,
   based on pre-shared keys and well-known public cryptographic
   algorithms.  This method is ideal for subscription-based public
   access networks, e.g., cellular networks.

   EAP-SAKE assumes a long-term or pre-shared secret known only to the
   Peer and the Server.  This pre-shared secret is called the Root
   Secret.  The Root Secret consists of a 16-byte part Root-Secret-A,
   and 16-byte part Root-Secret-B.  The Root-Secret-A secret is reserved
   for use local to the EAP-SAKE method, i.e., to mutually authenticate
   the EAP Peer and EAP Server.  The Root-Secret-B secret is used to
   derive other quantities such as the Master Session Key (MSK) and
   Extended Master Session Key (EMSK).  Root-Secret-B and Root-Secret-A
   MUST be cryptographically separate.

   When a Backend Authentication Server is used, the Server typically
   communicates with the Authenticator using an AAA protocol.  The AAA
   communications are outside the scope of this document.

   Some of the advantages of EAP-SAKE are as follows:

   o It is based on well-established Bellare-Rogaway mutual
     authentication protocol.

   o It supports key derivation for local EAP method use and for export
     to other third parties to use them independently of EAP.

   o It employs only two request/response exchanges.

   o It relies on the (corrected) IEEE 802.11i function for key
     derivation and message integrity.  This way a device implementing a
     lower-layer secure association protocol compliant with IEEE 802.11i
     standard will not need additional cryptographic code.

   o Its encryption algorithm is securely negotiated (with no extra
     messages), yet encryption use is optional.

   o Keys used for authentication and those used for encryption are
     cryptographically separate.

   o User identity anonymity can be optionally supported.




Vanderveen & Soliman         Informational                      [Page 4]

RFC 4763                        EAP-SAKE                   November 2006


   o No synchronization (e.g., of counters) needed between server and
     peer.

   o There is no one-time key pre-processing step.

3.2.  Protocol Operation

   EAP-SAKE uses four messages consisting of two EAP request/response
   exchanges.  The EAP.Success and EAP.Failure messages shown in the
   figures are not part of the EAP-SAKE method.  As a convention, method
   attributes are named AT_XX, where XX is the name of the parameter the
   attribute value is set to.

3.2.1.  Successful Exchange

   A successful EAP-SAKE authentication exchange is depicted in Figure
   1.  The following steps take place:

   Peer                                                       Server
       |                                                          |
       |                        EAP.Request/SAKE/Challenge        |
       |                        (AT_RAND_S, AT_SERVERID)          |
   1   |<---------------------------------------------------------|
       |                                                          |
       | EAP.Response/SAKE/Challenge                              |
       | (AT_RAND_P, AT_PEERID, AT_SPI_P, AT_MIC_P)               |
   2   |--------------------------------------------------------->|
       |                                                          |
       |                        EAP.Request/SAKE/Confirm          |
       |                        (AT_SPI_S, AT_ENCR_DATA, AT_MIC_S)|
   3   |<---------------------------------------------------------|
       |                                                          |
       | EAP.Response/SAKE/Confirm                                |
       | (AT_MIC_P)                                               |
   4   |--------------------------------------------------------->|
       |                                                          |
       |                                                          |
       |                                         EAP-Success      |
   5   |<---------------------------------------------------------|
       |                                                          |

      Figure 1.  EAP-SAKE Authentication Procedure (with ciphersuite
                               negotiation)

   1.  The EAP server sends the first message of the EAP-SAKE exchange.
       This message is an EAP.Request message of type SAKE and subtype
       Challenge.  The EAP.Request/SAKE/Challenge message contains the
       attributes: AT_RAND_S, whose value is a nonce freshly generated



Vanderveen & Soliman         Informational                      [Page 5]

RFC 4763                        EAP-SAKE                   November 2006


       by the Server; and AT_SERVERID, which carries an identifier of
       the Server (a fully qualified domain name, such as the realm of
       the user NAI [NAI]).  The AT_SERVERID attribute is OPTIONAL but
       SHOULD be included in this message.  The purpose of the
       AT_SERVERID is to aid the Peer in selecting the correct security
       association (i.e., Root-Secret, PEERID, and SERVERID) to use
       during this EAP phase.

   2.  The Peer responds by sending an EAP.Response message of type SAKE
       and subtype Challenge.  The EAP.Response/SAKE/Challenge message
       contains the attributes: AT_RAND_P, which carries a nonce freshly
       generated by the Peer; AT_PEERID, which carries a Peer
       identifier; AT_SPI_P, which carries a list of one or more
       ciphersuites supported by the Peer;  and AT_MIC_P, containing a
       message integrity check.  The AT_SPI_P and AT_PEERID attributes
       are OPTIONAL.  The AT_PEERID attribute SHOULD be included, in
       order to cover the case of multi-homed hosts.  A supported
       ciphersuite is represented by a value local to the EAP-SAKE
       method, or "Security Parameter Index", see section 3.2.8.3.  The
       Peer uses both nonces, along with the Root-Secret-A key, to
       derive a SAKE Master Secret (SMS) and Temporary EAP Keys (TEKs),
       which also include the TEK-Auth and TEK-Cipher keys.  The MIC_P
       value is computed based on both nonces RAND_S and RAND_P, and the
       entire EAP packet, using the key TEK-Auth, see Section 3.2.6.

   3.  Upon receipt of the EAP.Response/SAKE/Challenge message, the
       Server uses both nonces RAND_S and RAND_P, along with the Root-
       Secret-A key, to compute the SMS and TEK in exactly the same way
       the Peer did.  Following that, the Server computes the Peer's
       MIC_P in exactly the same way the Peer did.  The Server then
       compares the computed MIC_P with the MIC_P it received from the
       Peer.  If they match, the Server considers the Peer
       authenticated.  If encryption is needed, the Server selects the
       strongest ciphersuite from the Peer's ciphersuite list SPI_P, or
       a suitable ciphersuite if the Peer did not include the AT_SPI_P
       attribute.  The Server sends an EAP.Request message of type SAKE
       and subtype Confirm, containing the attributes: AT_SPI_S,
       carrying the ciphersuite chosen by the Server; AT_ENCR_DATA,
       containing encrypted data; and AT_MIC_S, carrying a message
       integrity check.  The AT_SPI_S and AT_ENCR_DATA are OPTIONAL
       attributes, included if confidentiality and/or user identity
       anonymity is desired.  Other OPTIONAL attributes that MAY be
       included are AT_NEXT_TMPID, and AT_MSK_LIFE.  As before, the
       MIC_S value is computed using both nonces RAND_S and RAND_P, and
       the entire EAP packet, using the key TEK-Auth.






Vanderveen & Soliman         Informational                      [Page 6]

RFC 4763                        EAP-SAKE                   November 2006


   4.  If the Peer receives the EAP.Request/SAKE/Confirm message
       indicating successful authentication by the Server, the Peer
       computes the MIC_S in the same manner as the Server did.  The
       Peer then compares the received MIC_S with the MIC_S it computed.
       If they match, the Peer considers the Server authenticated, and
       it sends an EAP.Response message of type SAKE and subtype
       Confirm, with the attribute AT_MIC_P containing a message
       integrity check, computed in the same manner as before.

   5.  After a successful EAP-SAKE exchange, the Server concludes the
       EAP conversation by sending an EAP.Success message to the Peer.

   All EAP-SAKE messages contain a Session ID, which is chosen by the
   Server, sent in the first message, and replicated in subsequent
   messages until an EAP.Success or EAP.Failure is sent.  Upon receipt
   of an EAP-SAKE message, both Peer and Server MUST check the format of
   the message to ensure that it is well-formed and contains a valid
   Session ID.

   Note that the Session ID is introduced mainly for replay protection
   purposes, as it helps uniquely identify a session between a Peer and
   a Server.  In most cases, there is a one-to-one relationship between
   the Session ID and the Peer/Server pair.

   The parameters used by the EAP-SAKE method are summarized in the
   table below:

     Name     Length (bytes)  Description
   ---------+---------------+-------------
   RAND_P      16             Peer nonce
   RAND_S      16             Server nonce
   MIC_P       16             Peer MIC
   MIC_S       16             Server MIC
   SPI_P       variable       Peer ciphersuite preference(s)
   SPI_S       variable       Server chosen ciphersuite
   PEERID      variable       Peer identifier
   SERVERID    variable       Server identifier (FQDN)

3.2.2.  Authentication Failure

   If the Authenticator receives an EAP.Failure message from the Server,
   the Authenticator MUST terminate the connection with the Peer
   immediately.

   The Server considers the Peer to have failed authentication if either
   of the two received MIC_P values does not match the computed MIC_P.





Vanderveen & Soliman         Informational                      [Page 7]

RFC 4763                        EAP-SAKE                   November 2006


   The Server SHOULD deny authorization for a Peer that does not
   advertise any of the ciphersuites that are considered acceptable
   (e.g., by local system policy) and send an EAP.Failure message.

   In case of authentication failure, the Server MUST send an
   EAP.Failure message to the Peer as in Figure 2.  The following steps
   take place:

   Peer                                                       Server
       |                                                          |
       |                        EAP.Request/SAKE/Challenge        |
       |                        (AT_RAND_S, AT_SERVERID)          |
   1   |<---------------------------------------------------------|
       |                                                          |
       | EAP.Response/SAKE/Challenge                              |
       | (AT_RAND_P, AT_PEERID, AT_SPI_P, AT_MIC_P)               |
   2   |--------------------------------------------------------->|
       |                                                          |
       |               +-------------------------------------------+
       |               | Server finds MIC_P invalid.               |
       |               +-------------------------------------------+
       |                                                          |
       |                                             EAP-Failure  |
   3   |<---------------------------------------------------------|

         Figure 2.  EAP-SAKE Authentication Procedure, Peer Fails
                              Authentication

   1.  As in step 1 of Figure 1.

   2.  As in step 2 of Figure 1.

   3.  Upon receipt of the EAP.Response/SAKE/Challenge message, the
       Server uses both nonces RAND_S and RAND_P, along with the Root-
       Secret-A key, to compute the SMS and TEK in exactly the same way
       the Peer did.  Following that, the Server computes the Peer's MIC
       in exactly the same way the Peer did.  The Server then compares
       the computed MIC_P with the MIC_P it received from the Peer.
       Since they do not match, the Server considers the Peer to have
       failed authentication and sends an EAP.Failure message back to
       the Peer.










Vanderveen & Soliman         Informational                      [Page 8]

RFC 4763                        EAP-SAKE                   November 2006


   If the AT_SPI_S attribute does not contain one of the SPI values that
   the Peer listed in the previous message, or if the Peer did not
   include an AT_SPI_P attribute yet does not accept the ciphersuite the
   Server has chosen, then the Peer SHOULD silently discard this
   message.  Alternatively, the Peer MAY send a SAKE/Auth-Reject message
   back to the Server; in response to this message, the Server MUST send
   an EAP.Failure message to the Peer.

   The AT_SPI_S attribute MUST be included if encryption is to be used.
   The Server knows whether or not encryption is to be used, as it is
   usually the Server that needs to protect some data intended for the
   Peer (e.g., temporary ID, group keys, etc).  If the Peer receives an
   AT_SPI_S attribute yet there is no AT_ENCR_DATA attribute, the Peer
   SHOULD process the message and skip the AT_SPI_S attribute.

   The Peer considers the Server to have failed authentication if the
   received and the computed MIC_S values do not match.  In this case,
   the Peer MUST send to the Server an EAP.Response message of type SAKE
   and subtype Auth-Reject, indicating an authentication failure.  In
   this case, the Server MUST send an EAP.Failure message to the Peer as
   in Figure 3.  The following steps take place:






























Vanderveen & Soliman         Informational                      [Page 9]

RFC 4763                        EAP-SAKE                   November 2006


    Peer                                                       Server
        |                                                          |
        |                        EAP.Request/SAKE/Challenge        |
        |                        (AT_RAND_S, AT_SERVERID)          |
    1   |<---------------------------------------------------------|
        |                                                          |
        | EAP.Response/SAKE/Challenge                              |
        | (AT_RAND_P, AT_PEERID, AT_SPI_P, AT_MIC_P)               |
    2   |--------------------------------------------------------->|
        |                                                          |
        |                          EAP.Request/SAKE/Confirm        |
        |                        (AT_SPI_S, AT_ENCR_DATA, AT_MIC_S)|
    3   |<---------------------------------------------------------|
        |                                                          |
      +-----------------------------------------------+            |
      | Peer finds MIC_S invalid.                     |            |
      +-----------------------------------------------+            |
        |                                                          |
        | EAP.Response/SAKE/Auth-Reject                            |
     4  |--------------------------------------------------------->|
        |                                                          |
        |                                             EAP-Failure  |
     5  |<---------------------------------------------------------|
        |                                                          |

        Figure 3.  EAP-SAKE Authentication Procedure, Server Fails
                              Authentication

   1.  As in step 1 of Figure 1.

   2.  As in step 2 of Figure 1.

   3.  As in step 3 of Figure 1.

   4.  The Peer receives a EAP.Request/SAKE/Confirm message indicating
       successful authentication by the Server.  The Peer computes the
       MIC_S in the same manner as the Server did.  The Peer then
       compares the received MIC_S with the MIC_S it computed.  Since
       they do not match, the Peer considers the Server to have failed
       authentication.  In this case, the Peer responds with an
       EAP.Response message of type SAKE and subtype Auth-Reject,
       indicating authentication failure.

   5.  Upon receipt of a EAP.Response/SAKE/Auth-Reject message, the
       Server sends an EAP.Failure message back to the Peer.






Vanderveen & Soliman         Informational                     [Page 10]

RFC 4763                        EAP-SAKE                   November 2006


3.2.3.  Identity Management

   It may be advisable to assign a temporary identifier (TempID) to a
   Peer when user anonymity is desired.  The TempID is delivered to the
   Peer in the EAP.Request/SAKE/Confirm message.  The TempID follows the
   format of any NAI [NAI] and is generated by the EAP Server that
   engages in the EAP-SAKE authentication task with the Peer.  EAP
   servers SHOULD be configurable with TempID spaces that can be
   distinguished from the permanent identity space.  For instance, a
   specific realm could be assigned for TempIDs (e.g., tmp.example.biz).

   A TempID is not assigned an explicit lifetime.  The TempID is valid
   until the Server requests the permanent identifier, or the Peer
   triggers the start of a new EAP session by sending in its permanent
   identifier.  A Peer MUST be able to trigger an EAP session at any
   time using its permanent identifier.  A new TempID assigned during a
   successful EAP session MUST replace the existing TempID for future
   transactions between the Peer and the Server.

   Note that the delivery of a TempID does not imply that the Server
   considers the Peer authenticated; the Server still has to check the
   MIC in the EAP.Response/SAKE/Confirm message.  In case the EAP phase
   ends with an EAP.Failure message, then the Server and the Peer MUST
   consider the TempID that was just delivered as invalid.  Hence, the
   Peer MUST NOT use this TempID the next time it tries to authenticate
   with the Server.

3.2.4.  Obtaining Peer Identity

   The types of identities that a Peer may possess are permanent and
   temporary identities, referred to as PermID and TempID, respectively.
   A PermID can be an NAI associated with the Root Secret, and is long-
   lived.  A TempID is an identifier generated through the EAP method
   and that the Peer can use to identify itself during subsequent EAP
   sessions with the Server.  The purpose of the TempID is to allow for
   user anonymity support.  The use of TempIDs is optional in the EAP-
   SAKE method.

   The Server MAY request the Peer ID via the EAP.Request/SAKE/Identity
   message, as shown in Figure 4.  This case may happen if, for example,
   the Server wishes to initiate an EAP-SAKE session for a Peer it does
   not have a subscriber identifier for.  The following steps take
   place:








Vanderveen & Soliman         Informational                     [Page 11]

RFC 4763                        EAP-SAKE                   November 2006


   Peer                                                          Server
          |                                                          |
          |                         +---------------------------------+
          |                         | Server wishes to initiate       |
          |                         | an EAP-SAKE session             |
          |                         |                                 |
          |                         +---------------------------------+
          |                                                          |
          |                        EAP.Request/SAKE/Identity         |
          |                        (AT_ANY_ID_REQ, AT_SERVERID)      |
     1    |<---------------------------------------------------------|
          |                                                          |
          | EAP.Response/SAKE/Identity                               |
          | (AT_PEERID)                                              |
     2    |--------------------------------------------------------->|
          |                                                          |
        +--------------------------------------------------------------+
        | If identity found, normal EAP-SAKE authentication follows.   |
        +--------------------------------------------------------------+

                 Figure 4.  Server Requests Peer Identity

   1.  The Server sends an EAP.Request message of type SAKE and subtype
       Identity, with the attribute AT_ANY_ID_REQ, indicating a request
       for any Peer identifier.

   2.  The Peer constructs an EAP.Response message of type SAKE and
       subtype Identity, with the attribute AT_PEER_ID containing any
       Peer identifier (PermID or TempID).

   If the Server cannot find the Peer identity reported in the
   EAP.Response/SAKE/Identity message, or if it does not recognize the
   format of the Peer identifier, the Server MAY send an EAP.Failure
   message to the Peer.

   If the Server is unable to look up a Peer by its TempID, or if policy
   dictates that the Peer should now use its permanent id, then the
   Server may specifically ask for the permanent identifier, as in
   Figure 5.  The following steps occur:












Vanderveen & Soliman         Informational                     [Page 12]

RFC 4763                        EAP-SAKE                   November 2006


   Peer                                                       Server
       |                                                          |
       |                         +---------------------------------+
    1  |                         | Server obtains TempID but       |
       |                         | requires PermID                 |
       |                         +---------------------------------+
       |                                                          |
       |                        EAP.Request/SAKE/Identity         |
       |                        (AT_PERM_ID_REQ, AT_SERVERID)     |
    2  |<---------------------------------------------------------|
       |                                                          |
       | EAP.Response/SAKE/Identity                               |
       | (AT_PEERID)                                              |
    3  |--------------------------------------------------------->|
       |                                                          |
       |                         +---------------------------------+
       |                         | Server finds and uses           |
       |                         | Peer PermID to start a          |
       |                         | EAP-SAKE authentication phase   |
       |                         +---------------------------------+
       |
    +---------------------------------------------------------------+
    |  Normal EAP-SAKE authentication follows.                      |
    +---------------------------------------------------------------+

       Figure 5.  Server Is Given a TempID as Peer Identity; Server
                             Requires a PermID

   1.  The Peer (or the Authenticator on behalf of the Peer) sends an
       EAP.Request message of type Identity and includes the TempID.

   2.  The Server requires a PermID instead, so it sends an EAP.Request
       message of type SAKE and subtype Identity with attributes
       AT_PERM_ID_REQ and AT_SERVERID.

   3.  The Peer sends an EAP.Response message of type SAKE and subtype
       Identity containing the attribute AT_PEERID carrying the Peer
       PermID.

3.2.5.  Key Hierarchy

   EAP-SAKE uses a three-level key hierarchy.

   Level 1 contains the pre-shared secret called Root Secret.  This is a
   32-byte high-entropy string partitioned into the Root-Secret-A part
   (16 bytes) and the Root-Secret-B part (16 bytes).





Vanderveen & Soliman         Informational                     [Page 13]

RFC 4763                        EAP-SAKE                   November 2006


   Level 2 contains the key derivation key called the SAKE Master Secret
   (SMS).  SMS-A is derived from the Root-Secret-A key and the Peer and
   Server nonces using the EAP-SAKE Key-Derivation Function (KDF), and
   similarly for SMS-B.  The SMS is known only to the Peer and Server
   and is not made known to other parties.

   Level 3 contains session keys, such as Transient EAP Keys (TEK),
   Master Session Key (MSK), and Extended MSK (EMSK).  TEKs are keys for
   use local to the EAP method only.  They are derived from the SMS-A
   and the nonces using the EAP-SAKE KDF.  They are partitioned into a
   16-byte TEK-Auth, used to compute the MICs, and a 16-byte TEK-Cipher,
   used to encrypt selected attributes.  Since the SMS is fresh, so are
   the derived TEKs.

   +--------------------+                       +--------------------+
   |  Root-Secret-A     |                       |  Root-Secret-B     |
   | (pre-shared secret)|                       | (pre-shared secret)|
   +--------------------+                       +--------------------+
          |                                       |
          V                                       V
   +-------------------+                        +--------------------+
   | SAKE Master Secret|<---RAND_S------------->| SAKE Master Secret |
   |    (SMS-A)        |        |               |   (SMS-B)          |
   |                   |<-------]---RAND_P----->|                    |
   +-------------------+        |     |         +--------------------+
          |                     |     |                |
          V                     |     |                V
   +--------------------+       |     |         +--------------------+
   | Transient EAP Keys |<------+-----+-------->|  Session Keys:     |
   | TEK-Auth,TEK-Cipher|<------------+-------->|  MSK, EMSK         |
   +--------------------+                       +--------------------+

             Figure 6.  Key Hierarchy for the EAP-SAKE Method

   On another branch of level 3 of the key hierarchy are the MSK and
   EMSK, each mandated to be 64 bytes long.  They are derived from the
   SMS-B and the nonces using the EAP-SAKE KDF.  Again, since the SMS is
   fresh, so are the derived MSK/EMSK.  The MSK is meant to be exported
   and relayed to other parties.  The EMSK is reserved for future use,
   such as derivation of application-specific keys, and is not shared
   with other parties.










Vanderveen & Soliman         Informational                     [Page 14]

RFC 4763                        EAP-SAKE                   November 2006


   The EAP-SAKE key material is summarized in the table below.

   ===================================================================
   Key              Size      Scope          Lifetime  Use
                  (bytes)
   ===================================================================
   Root-Secret-A    16        Peer, Server   Device    Derive TEK
   --------------------------------------------------------------------
   Root-Secret-B    16        Peer, Server   Device    Derive MSK, EMSK
   --------------------------------------------------------------------
   TEK-Auth         16        Peer, Server   MSK Life  Compute MICs
   --------------------------------------------------------------------
   TEK-Cipher       16        Peer, Server   MSK Life  Encrypt attribute
   --------------------------------------------------------------------
   MSK              64        Peer, Server,  MSK Life  Derive keys for
                              Authenticator            lower-layer use
   --------------------------------------------------------------------
   EMSK             64        Peer, Server   MSK Life  Reserved
   --------------------------------------------------------------------

   A key name format is not provided in this version.

   Since this version of EAP-SAKE does not support fast re-
   authentication, the lifetime of the TEKs is to extend only until the
   next EAP mutual authentication.  The MSK lifetime dictates when the
   next mutual authentication is to take place.  The Server MAY convey
   the MSK lifetime to the Peer in the AT_MSK_LIFE attribute.  Note that
   EAP-SAKE does not support key lifetime negotiation.

   The EAP-SAKE Method-Id is 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.

3.2.6.  Key Derivation

3.2.6.1.  Key-Derivation Function

   For the rest of this document, KDF-X denotes the EAP-SAKE Key-
   Derivation Function whose output is X bytes.  This function is the
   pseudo-random function of [IEEE802.11i].

   The function takes three strings of bytes of arbitrary lengths: a
   Key, a Label, and a Msg, and outputs a string Out of length X bytes
   as follows:

   Out = KDF-X (Key, Label, Msg)





Vanderveen & Soliman         Informational                     [Page 15]

RFC 4763                        EAP-SAKE                   November 2006


   The KDF is a keyed hash function with key "Key" and operating on
   input "Label | Msg".  The convention followed herein is that
   concatenation is denoted by |, FLOOR denotes the floor function, and
   [x...y] denotes bytes x through y.

   The label is an ASCII character string.  It is included in the exact
   form it is given without a length byte or trailing null character.

   Below, "Length" denotes a single unsigned octet with values between 0
   and 255 (bytes).  The following functions are defined (see [HMAC],
   [SHA1]):

   H-SHA1(Key, Label, Msg, Length) := HMAC-SHA1(Key, Label|Y|Msg|Length)
   where Y := 0x00
   KDF-16(Key, Label, Msg) := KDF(Key, Label, Msg, 16)
   KDF-32(Key, Label, Msg) := KDF(Key, Label, Msg, 32)
   KDF-128(Key, Label, Msg) := KDF(Key, Label, Msg, 128)

   KDF(Key, Label, Msg, Length)
   R := []  ;; null string
   for i := 0 to FLOOR(Length/20)-1 do
   R := R | H-SHA1(Key, Label, Msg, i)
   return R[0...(Length-1)]

3.2.6.2.  Transient EAP Keys Derivation

   The Peer and Server derive the SMS and then the TEK as follows:

   SMS-A = KDF-16 (Root-Secret-A, "SAKE Master Secret A", RAND_P|RAND_S)
   TEK = KDF-32 (SMS-A, "Transient EAP Key", RAND_S | RAND_P)
   TEK-Auth = TEK[0...15]
   TEK-Cipher = TEK[16...31]

3.2.6.3.  Extended/Master Session Key Derivation

   The Peer and the Server derive the MSK/EMSK, as follows:

   SMS-B = KDF-16 (Root-Secret-B, "SAKE Master Secret B", RAND_P|RAND_S)
   Session-Key-Block=KDF-128(SMS-B, "Master Session Key", RAND_S|RAND_P)
   MSK = Session-Key-Block[0...63]
   EMSK = Session-Key-Block[64...127]

   The derivation above affords the required cryptographic separation
   between the MSK and EMSK.  That is, knowledge of the EMSK does not
   immediately lead to knowledge of the MSK, nor does knowledge of the
   MSK immediately lead to knowledge of the EMSK.





Vanderveen & Soliman         Informational                     [Page 16]

RFC 4763                        EAP-SAKE                   November 2006


3.2.7.  Ciphersuite Negotiation

   A ciphersuite is identified by a numeric value called the Security
   Parameter Index (SPI).  The SPI is used here in the EAP-SAKE method
   in order to negotiate a ciphersuite between the Peer and the Server
   for EAP data protection only.  Obviously, this ciphersuite can only
   be used late in the EAP conversation, after it has been agreed upon
   by both the Peer and the Server.

   The EAP method may or may not need to use this ciphersuite, since
   attribute encryption is optional.  For example, if the temporary
   identifier needs to be delivered to the Peer and needs to be
   encrypted, then the negotiated ciphersuite will be used for this
   task.  If there are no attributes that need encryption as they are
   passed to the Peer, then this ciphersuite is never used.

   As with most other methods employing ciphersuite negotiation, the
   following exchange is employed: the Peer sends an ordered list of one
   or more supported ciphersuites, starting with the most preferred one,
   in a field SPI_P.  The Server then sends back the one ciphersuite
   chosen in a field SPI_S.  The Server SHOULD choose the strongest
   ciphersuite supported by both of them.

   Ciphersuite negotiation for data protection is achieved via SAKE
   Signaling as follows.  In the EAP.Response/SAKE/Challenge, the Peer
   sends a list of supported ciphersuites, SPI_P, and protects that with
   a MIC.  In the EAP.Request/SAKE/Confirm, the Server sends one
   selected ciphersuite, SPI_S, and signs that with a MIC.  Finally, the
   Peer confirms the selected ciphersuite and readiness to use it in a
   signed EAP.Response/SAKE/Confirm message.  The negotiation is secure
   because of the Message Integrity Checks that cover the entire EAP
   message.

3.2.8.  Message Integrity and Encryption

   This section specifies the EAP/SAKE attributes used for message
   integrity and attribute encryption: AT_MIC_S, AT_MIC_P, AT_IV,
   AT_ENCR_DATA, and AT_PADDING.  Only the AT_MIC_S and AT_MIC_P are
   mandatory to use during the EAP-SAKE exchange.

   Because the TEKs necessary for protection of the attributes and for
   message authentication are derived using the nonces RAND_S and
   RAND_P, the AT_MIC_S and AT_MIC_P attributes can only be used in the
   EAP.Response/SAKE/Challenge message and any messages sent thereafter.







Vanderveen & Soliman         Informational                     [Page 17]

RFC 4763                        EAP-SAKE                   November 2006


3.2.8.1.  The AT_MIC_S and AT_MIC_P Attributes

   The AT_MIC_S and AT_MIC_P attributes are used for EAP-SAKE message
   integrity.  The AT_MIC_S attribute MUST be included in all EAP-SAKE
   packets that the Server sends whenever key material (TEK) has been
   derived.  That is, the AT_MIC_S attribute MUST be included in the
   EAP.Request/SAKE/Confirm message.  The AT_MIC_S MUST NOT be included
   in EAP.Request/SAKE/Challenge messages, or EAP.Request/SAKE/Identity
   messages.

   The AT_MIC_P attribute MUST be included in all EAP-SAKE packets the
   Peer sends whenever key material (TEK) has been derived.  That is,
   the AT_MIC_P attribute MUST be included in the
   EAP.Response/SAKE/Challenge and EAP.Response/SAKE/Confirm messages.

   The AT_MIC_P attribute MUST NOT be included in the
   EAP.Response/SAKE/Auth-Reject message since the Peer has not
   confirmed that it has the same TEK as the Server.

   Messages that do not meet the conditions specified above MUST be
   silently discarded upon reception.

   The value field of the AT_MIC_S and AT_MIC_P attributes contain a
   message integrity check (MIC).  The MIC is calculated over the entire
   EAP packet, prepended with the Server nonce and identifier and the
   Peer nonce and identifier.  The value field of the MIC attribute is
   set to zero when calculating the MIC.  Including the Server and Peer
   nonces and identifiers aids in detecting replay and man-in-the-middle
   attacks.

   The Peer computes its MIC as follows:

   MIC_P = KDF-16 (TEK-Auth, "Peer MIC", RAND_S | RAND_P |
   PEERID | 0x00 | SERVERID | 0x00 | <EAP-packet>),

   while the Server computes its MIC as

   MIC_S = KDF-16 (TEK-Auth, "Server MIC", RAND_P |RAND_S |
   SERVERID | 0x00 | PEERID | 0x00 | <EAP-packet>).

   Here, <EAP-packet> denotes the entire EAP packet, used as a string of
   bytes, the MIC value field set to zero.  0x00 denotes a single octet
   value used to delimit SERVERID and PEERID.  The PEERID and SERVERID,
   respectively, are the ones carried in the corresponding attributes in
   the SAKE/Challenge messages.






Vanderveen & Soliman         Informational                     [Page 18]

RFC 4763                        EAP-SAKE                   November 2006


   In case the SAKE/Challenge exchange was preceded by an
   EAP.Request/SAKE/Identity message containing the AT_SERVERID
   Attribute, then the SERVERID value in the MIC_P and MIC_S computation
   MUST be set to the value of this attribute.

   If the AT_SERVERID was not included in either the SAKE/Challenge
   message or the SAKE/Identity message, then the value of the SERVERID
   used in the computation of MIC_P and MIC_S MUST be empty.  If the
   AT_PEERID was not included in the SAKE/Challenge message, and there
   was no EAP.Response/SAKE/Identity message preceding the
   SAKE/Challenge messages, then the value of the PEERID used in the
   computation of MIC_P and MIC_S MUST be empty.

   The Server and Peer identity are included in the computation of the
   signed responses so that the Peer and Server can verify each other's
   identities, and the possession of a common secret, the TEK-Auth.
   However, since the AT_SERVERID is not explicitly signed with a MIC by
   the Server, EAP-SAKE does not claim channel binding.

3.2.8.2.  The AT_IV, AT_ENCR_DATA, and AT_PADDING Attributes

   The AT_IV and AT_ENCR_DATA attributes can be used to transmit
   encrypted information between the Server and the Peer.  The value
   field of AT_IV contains an initialization vector (IV) if one is
   required by the encryption algorithm used.  It is not mandatory that
   the AT_IV attribute be included whenever the AT_ENCR_DATA attribute
   is.

   However, the AT_IV attribute MUST NOT be included unless the
   AT_ENCR_DATA is included.  Messages that do not meet this condition
   MUST be silently discarded.

   Attributes can be encrypted only after a ciphersuite has been agreed
   on, i.e., in any message starting with the Server's
   EAP.Request/SAKE/Confirm message.  The attributes MUST be encrypted
   using algorithms corresponding to the SPI value specified by the
   AT_SPI_S attribute.  The attributes MUST be encrypted using the TEK-
   Cipher key, whose derivation is specified in Section 3.2.6.2.

   If an IV is required by the encryption algorithm, then the sender of
   the AT_IV attribute MUST NOT reuse the IV value from previous EAP-
   SAKE packets.  The sender MUST choose a new value for each AT_IV
   attribute.  The sender SHOULD use a good random number generator to
   generate the initialization vector (see [RFC4086] for guidelines).







Vanderveen & Soliman         Informational                     [Page 19]

RFC 4763                        EAP-SAKE                   November 2006


   The value field of the AT_ENCR_DATA attribute consists of bytes
   encrypted using the ciphersuite specified in the AT_SPI_S attribute.
   The encryption key is the TEK-Cipher, and the plaintext consists of
   one or more concatenated EAP-SAKE attributes.

   The default encryption algorithm, as described in Section 3.2.8.3,
   requires the length of the plaintext to be a multiple of 16 bytes.
   The sender MAY need to include the AT_PADDING attribute as the last
   attribute within the value field of the AT_ENCR_DATA attribute.  The
   length of the padding is chosen so that the length of the value field
   of the AT_ENCR_DATA attribute becomes a multiple of 16 bytes.  The
   AT_PADDING attribute SHOULD NOT be included if the total length of
   other attributes present within the AT_ENCR_DATA attribute is a
   multiple of 16 bytes.  The length of the AT_PADDING attribute
   includes the Attribute Type and Attribute Length fields.  The actual
   pad bytes in the value field are set to zero (0x00) on sending.  The
   recipient of the message MUST verify that the pad bytes are zero
   after decryption and MUST silently discard the message otherwise.

   The MIC computed on the entire EAP message can be used to obviate the
   need for special integrity protection or message authentication of
   the encrypted attributes.

   An example of the AT_ENCR_DATA attribute is shown in Section 3.3.3.

3.2.8.3.  Security Parameter Index (SPI) Considerations

   An SPI value is a variable-length string identifying at least an
   encryption algorithm and possibly a "security association".  EAP-SAKE
   does not mandate the format of the SPI; it only mandates that the
   default encryption algorithm be supported if encryption is supported.
   That is, if an implementation compliant with this document supports
   encryption, then it MUST support the AES-CBC cipher.

   The default encryption algorithm AES-CBC involves the AES block
   cipher [AES] in the Cipher-Block-Chaining (CBC) mode of operation
   [CBC].

   The Peer in the EAP-SAKE method can send up to four SPI values in its
   SPI_P field.  Because the length of the AT_SPI_P and AT_SPI_S
   attributes must each be a multiple of 2 bytes, the sender pads the
   value field with zero bytes when necessary (the AT_PADDING attribute
   is not used here).  For example, the value field of the AT_SPI_S
   contains one byte with the chosen SPI, followed by one byte of zeros.







Vanderveen & Soliman         Informational                     [Page 20]

RFC 4763                        EAP-SAKE                   November 2006


3.2.9.  Fragmentation

   The EAP-SAKE method does not require fragmentation, as the messages
   do not get excessively long.  That is, EAP packets are well within
   the limit of the maximum transmission unit of other layers (link,
   network).  The only variable fields are those carrying NAIs, which
   are limited by their length field to 256 bytes.

3.2.10.  Error Cases

   Malformed messages: As a general rule, if a Peer or Server receives
   an EAP-SAKE packet that contains an error, the implementation SHOULD
   silently discard this packet, not change state, and not send any EAP
   messages to the other party.  Examples of such errors include a
   missing mandatory attribute, an attribute that is not allowed in this
   type of message, and unrecognized subtypes or attributes.

   Non-matching Session Id: If a Peer or Server receives an EAP-SAKE
   packet with a Session Id field not matching the Session Id from the
   previous packet in this session, that entity SHOULD silently discard
   this packet (not applicable for the first message of an EAP-SAKE
   session).

   Peer Authorization Failure: It may be possible that a Peer is not
   authorized for services, such as when the terminal device is reported
   stolen.  In that case, the Server SHOULD send an EAP.Failure message.

   Unexpected EAP.Success: A Server MUST NOT send an EAP-Success message
   before the SAKE/Challenge and SAKE/Confirm rounds.  The Peer MUST
   silently discard any EAP.Success packets before the Peer has
   successfully authenticated the Server via the
   EAP.Response/SAKE/Confirm packet.

   The Peer and Server SHOULD log all error cases.

















Vanderveen & Soliman         Informational                     [Page 21]

RFC 4763                        EAP-SAKE                   November 2006


3.3.  Message Formats

3.3.1.  Message Format Summary

   A summary of the EAP packet format [EAP] is shown below for
   convenience.  The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |   Identifier  |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=EAP-SAKE  |    Version=2  | Session ID    |   Subtype     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code

   1 - Request

   2 - Response

   Identifier

      The identifier field is one octet and aids in matching responses
      with requests.

   Length

      The Length field is two octets and indicates the number of octets
      in an EAP message including the Code, Identifier, Length, Type,
      and Data fields.

   Type

      To be assigned.

   Version

      The EAP-SAKE method as described herein is version 2.

   Session ID

      The Session ID is a 1-byte random number that MUST be freshly
      generated by the Server that must match all EAP messages in a
      particular EAP conversation.






Vanderveen & Soliman         Informational                     [Page 22]

RFC 4763                        EAP-SAKE                   November 2006


   Subtype

      EAP-SAKE subtype: SAKE/Challenge, SAKE/Confirm, SAKE/Auth-Reject,
      and SAKE/Identity.  All messages of type "EAP-SAKE" that are not
      of these subtypes MUST silently discarded.

      Message Name          Subtype Value (decimal)
      =============================================
      SAKE/Challenge        1
      SAKE/Confirm          2
      SAKE/Auth-Reject      3
      SAKE/Identity         4

3.3.2.  Attribute Format

   The EAP-SAKE attributes that are part of the EAP-SAKE packet follow
   the Type-Length-Value format with 1-byte Type, 1-byte Length, and
   variable-length Value (up to 255 bytes).  The Length field is in
   octets and includes the length of the Type and Length fields.  The
   EAP-SAKE attribute format is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |  Value...                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      1-byte unsigned integer; see Table below.

   Length

      The total number of octets in the attribute, including Type and
      Length.

   Value

      Attribute-specific.












Vanderveen & Soliman         Informational                     [Page 23]

RFC 4763                        EAP-SAKE                   November 2006


   The following attribute types are allocated.

   -----------------------------------------------------------------
   Attr.  Name    Length
                (bytes)       Skippable      Description
   -----------------------------------------------------------------
   AT_RAND_S     18           No        Server Nonce RAND_S
   AT_RAND_P     18           No        Peer Nonce RAND_P
   AT_MIC_S      10           No        Server MIC
   AT_MIC_P      10           No        Peer MIC
   AT_SERVERID   variable     No        Server FQDN
   AT_PEERID     variable     No        Peer NAI (tmp, perm)
   AT_SPI_S      variable     No        Server chosen SPI SPI_S
   AT_SPI_P      variable     No        Peer SPI list SPI_P
   AT_ANY_ID_REQ    4         No        Requires any Peer Id (tmp, perm)
   AT_PERM_ID_REQ   4         No        Requires Peer's permanent Id/NAI
   AT_ENCR_DATA  Variable     Yes       Contains encrypted attributes
   AT_IV         Variable     Yes       IV for encrypted attributes
   AT_PADDING    2 to 18      Yes       Padding for encrypted attributes
   AT_NEXT_TMPID variable     Yes       TempID for next EAP-SAKE phase

   AT_MSK_LIFE      6         Yes       MSK Lifetime in seconds
   -----------------------------------------------------------------




























Vanderveen & Soliman         Informational                     [Page 24]

RFC 4763                        EAP-SAKE                   November 2006


3.3.3.  Use of AT_ENCR_DATA Attribute

   An example of the AT_ENCR_DATA attribute, as used in the
   EAP.Request/SAKE/Confirm message, is shown below:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_IV         | Length = 18   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                 Initialization Vector                         |
   |                                                               |
   |                               |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |AT_ENCR_DATA   | Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+}e
   | AT_NEXT_TMPID | Length        |                               |}n
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |}c
   |                                                               |}r
   .                    Peer TempID                                |}y
   .                                                               |}p
   .                                                               |}t
   |                                                               |}e
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+}d
   |   AT_MIC_S     | Length = 10  |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                       MIC_S                                   |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                               |AT_PADDING     | Length=2      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





















Vanderveen & Soliman         Informational                     [Page 25]

RFC 4763                        EAP-SAKE                   November 2006


3.3.4.  EAP.Request/SAKE/Challenge Format

   The format of the EAP.Request/SAKE/Challenge packet is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=EAP-SAKE  |    Version=2  | Session ID    |   Subtype=1   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   AT_RAND_S    | Length = 18  |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                     RAND_S                                    |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                               | AT_SERVERID   | Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   :                                                               :
   |                 Server ID                                     |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      1 for Request

   Identifier

      A random number.  See [EAP].

   Length

      The length of the entire EAP packet in octets.

   Type

      EAP-SAKE

   Version

      2





Vanderveen & Soliman         Informational                     [Page 26]

RFC 4763                        EAP-SAKE                   November 2006


   Session ID

      A random number chosen by the server to identify this EAP-Session.

   Subtype

      1 for SAKE/Challenge

   AT_RAND_S

      The value field of this attribute contains the Server nonce RAND_S
      parameter.  The RAND_S attribute MUST be present in
      EAP.Request/SAKE/Challenge.

   AT_SERVERID

      The value field of this attribute contains the Server identifier
      (e.g., a non-null terminated string).  The AT_SERVERID attribute
      SHOULD be present in EAP.Request/SAKE Challenge.
































Vanderveen & Soliman         Informational                     [Page 27]

RFC 4763                        EAP-SAKE                   November 2006


3.3.5.  EAP.Response/SAKE/Challenge Format

   The format of the EAP.Response/SAKE/Challenge packet is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=EAP-SAKE  |    Version=2  | Session ID    |   Subtype=1   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   AT_RAND_P    | Length = 18  |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                     RAND_P                                    |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                               | AT_PEERID     | Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   :                     Peer NAI                                  :
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                               | AT_SPI_P      |  Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   SPIP                        | AT_MIC_P      |  Length = 18  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             MIC_P                             |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      A number that MUST match the Identifier field from the
      corresponding Request.

   Length

      The length of the entire EAP packet in octets.




Vanderveen & Soliman         Informational                     [Page 28]

RFC 4763                        EAP-SAKE                   November 2006


   Type

      EAP-SAKE

   Version

      2

   Session ID

      A number matching all other EAP messages in this EAP session.

   Subtype

      1 for SAKE/Challenge

   AT_RAND_P

      The value field of this attribute contains the Peer nonce RAND_P
      parameter.  The AT_RAND_P attribute MUST be present in the
      EAP.Response/SAKE/Challenge.

   AT_PEERID

      The value field of this attribute contains the NAI of the Peer.
      The Peer identity follows the same Network Access Identifier
      format that is used in EAP.Response/Identity, i.e., including the
      NAI realm portion.  The identity is the permanent identity, or a
      temporary identity.  The identity does not include any terminating
      null characters.  The AT_PEERID attribute is optional in the
      EAP.Response/SAKE/Challenge.

   AT_SPI_P

      The value field of this attribute contains the Peer's ciphersuite
      list SPI_P parameter.  The AT_SPI_P attribute is optional in the
      EAP.Response/SAKE/Challenge.

   AT_MIC_P

      The value field of this attribute contains the Peer MIC_P
      parameter.  The AT_MIC_P attribute MUST be present in the
      EAP.Response/SAKE/Challenge.








Vanderveen & Soliman         Informational                     [Page 29]

RFC 4763                        EAP-SAKE                   November 2006


3.3.6.  EAP.Request/SAKE/Confirm Format

   The format of the EAP.Request/SAKE/Confirm packet is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=EAP-SAKE  |    Version=2  | Session ID    |   Subtype=2   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   AT_SPI_S    | Length        |        SPI_S                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   AT_IV       | Length        |   Initialization Vector ...   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               :
   |                                                               |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               | AT_ENCR_DATA  | Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Encrypted Data...                       |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   AT_MSK_LIFE | Length=6      |    MSK Lifetime...            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |  AT_MIC_S     | Length=18     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             MIC_S                             |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      1 for Request

   Identifier

      A random number.  See [EAP].

   Length

      The length of the entire EAP packet in octets.




Vanderveen & Soliman         Informational                     [Page 30]

RFC 4763                        EAP-SAKE                   November 2006


   Type

      EAP-SAKE

   Version

      2

   Session ID

      A number matching all other EAP messages in this EAP session.

   Subtype

      2 for SAKE Confirm

   AT_SPI_S

      The value field of this attribute contains the Server chosen
      ciphersuite SPI_S parameter.  The AT_SPI_S attribute is optional
      in the EAP.Request/SAKE/Confirm.

   AT_IV

      This attribute is optional to use in this message.  The value
      field of this attribute contains the Initialization Vector that is
      used with the encrypted data following.

   AT_ENCR_DATA

      This attribute is optional to use in this message.  The encrypted
      data, if present, may contain an attribute AT_NEXT_TMPID,
      containing the NAI the Peer should use in the next EAP
      authentication.

   AT_MSK_LIFE

      This attribute is optional to use in this message.  The value
      field of this attribute contains the MSK Lifetime in seconds.

   AT_MIC_S

      The value field of this attribute contains the Server MIC_S
      parameter.  The AT_MIC_S attribute MUST be present in the
      EAP.Request/SAKE/Confirm.






Vanderveen & Soliman         Informational                     [Page 31]

RFC 4763                        EAP-SAKE                   November 2006


3.3.7.  EAP.Response/SAKE/Confirm Format

   The format of the EAP.Response/SAKE/Confirm packet is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=EAP-SAKE  |    Version=2  | Session ID    |   Subtype=2   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   AT_MIC_P     | Length = 18  |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                       MIC_P                                   |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |  AT_PADDING   | Length = 2    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      A number that MUST match the Identifier field from the
      corresponding Request.

   Length

      The length of the entire EAP packet in octets.

   Type

      EAP-SAKE

   Version

      2

   Session ID

      A number matching all other EAP messages in this EAP session.






Vanderveen & Soliman         Informational                     [Page 32]

RFC 4763                        EAP-SAKE                   November 2006


   Subtype

      2 for SAKE Confirm

   AT_MIC_P

      The value field of this attribute contains the Peer's MIC_P
      parameter.  The AT_MIC_P attribute MUST be present in the
      EAP.Response/SAKE/Confirm.

   AT_PADDING

      The value field is set to zero.  Added to achieve 32-bit alignment
      of the EAP-SAKE packet.

3.3.8.  EAP.Response/SAKE/Auth-Reject Format

   The format of the EAP.Response/SAKE/Auth-Reject packet is shown
   below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=EAP-SAKE  |    Version=2  | Session ID    |   Subtype=3   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      A number that MUST match the Identifier field from the
      corresponding Request.

   Length

      The length of the entire EAP packet in octets.

   Type

      EAP-SAKE





Vanderveen & Soliman         Informational                     [Page 33]

RFC 4763                        EAP-SAKE                   November 2006


   Version

      2

   Session ID

      A number matching all other EAP messages in this EAP session.

   Subtype

      3 for SAKE/Auth-Reject

3.3.9.  EAP.Request/SAKE/Identity Format

   The format of the EAP.Request/SAKE/Identity is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=EAP-SAKE  |    Version=2  | Session ID    |   Subtype=4   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AT_PERM_ID_REQ | Length = 4    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AT_ANY_ID_REQ  | Length = 4    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AT_SERVERID    | Length        |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               :
   |                       Server ID                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      1 for Request

   Identifier

      A random number.  See [EAP].

   Length

      The length of the entire EAP packet in octets.






Vanderveen & Soliman         Informational                     [Page 34]

RFC 4763                        EAP-SAKE                   November 2006


   Type

      EAP-SAKE

   Version

      2

   Session ID

      A number matching all other EAP messages in this EAP session.

   Subtype

      4 for SAKE/Identity

   AT_PERM_ID_REQ

      The AT_PERM_ID_REQ attribute is optional, to be included in cases
      where the Server requires the Peer to give its permanent
      identifier (i.e., PermID).  The AT_PERM_ID_REQ MUST NOT be
      included if the AT_ANY_ID_REQ attribute is included.  The value
      field only contains two reserved bytes, which are set to zero on
      sending and ignored on reception.

   AT_ANY_ID_REQ

      The AT_ANY_ID_REQ attribute is optional, to be included in cases
      where the Server requires the Peer to send any identifier (e.g.,
      PermID, TempID).  The AT_ANY_ID_REQ MUST NOT be included if
      AT_PERM_ID_REQ is included.  The value field only contains two
      reserved bytes, which are set to zero on sending and ignored on
      reception.  One of the AT_PERM_ID_REQ and AT_ANY_ID_REQ MUST be
      included.

   AT_SERVERID

      The value field of this attribute contains the identifier/realm of
      the Server.  The AT_SERVERID attribute is optional but RECOMMENDED
      to include in the EAP.Request/SAKE/Identity.











Vanderveen & Soliman         Informational                     [Page 35]

RFC 4763                        EAP-SAKE                   November 2006


3.3.10.  EAP.Response/SAKE/Identity Format

   The format of the EAP.Response/SAKE/Identity is shown below:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=EAP-SAKE  |    Version=2  | Session ID    |   Subtype=4   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   AT_PEERID   | Length        |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               :
   |                       Peer NAI                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      A number that MUST match the Identifier field from the
      corresponding Request.

   Length

      The length of the entire EAP packet.

   Type

      EAP-SAKE

   Version

      2

   Session ID

      A number matching all other EAP messages in this EAP session.

   Subtype

      4 for SAKE/Identity





Vanderveen & Soliman         Informational                     [Page 36]

RFC 4763                        EAP-SAKE                   November 2006


   AT_PEERID

      The value field of this attribute contains the NAI of the Peer.
      The AT_PEERID attribute MUST be present in
      EAP.Response/SAKE/Identity.

3.3.11.  Other EAP Messages Formats

   The format of the EAP.Request/Identity and EAP.Response/Identity
   packets is described in [EAP].  The user ID (e.g., NAI) SHOULD be
   present in this packet.

   The format of the EAP-Success and EAP-Failure packet is also shown in
   [EAP].

4.  IANA Considerations

   IANA allocated a new EAP Type for EAP-SAKE.

   EAP-SAKE messages include an 8-bit Subtype field.  The Subtype is a
   new numbering space for which IANA administration is required.  The
   following subtypes are specified in this memo:

   SAKE/Challenge.................1
   SAKE/Confirm...................2
   SAKE/Auth-Reject...............3
   SAKE/Identity..................4

   The Subtype-specific data is composed of attributes, which have an
   8-bit type number.  Attributes numbered within the range 0 through
   127 are called non-skippable attributes, and attributes within the
   range of 128 through 255 are called skippable attributes.  The EAP-
   SAKE attribute type number is a new numbering space for which IANA
   administration is required.  The following attribute types are
   specified:

   AT_RAND_S.......................................1
   AT_RAND_P.......................................2
   AT_MIC_S........................................3
   AT_MIC_P........................................4
   AT_SERVERID.....................................5
   AT_PEERID.......................................6
   AT_SPI_S........................................7
   AT_SPI_P........................................8
   AT_ANY_ID_REQ...................................9
   AT_PERM_ID_REQ.................................10





Vanderveen & Soliman         Informational                     [Page 37]

RFC 4763                        EAP-SAKE                   November 2006


   AT_ENCR_DATA..................................128
   AT_IV.........................................129
   AT_PADDING....................................130
   AT_NEXT_TMPID.................................131
   AT_MSK_LIFE...................................132

   All requests for value assignment from the two number spaces
   described in this memo require proper documentation, according to the
   "Specification Required" policy described in [IANA].

   All assignments of values from the two number spaces described in
   this memo require IETF consensus.

5.  Security Considerations

   The EAP specification [EAP] describes the security vulnerabilities of
   EAP, which does not include its method-specific security mechanisms.
   This section discusses the claimed security properties of the EAP-
   SAKE method, along with vulnerabilities and security recommendations.

5.1.  Denial-of-Service Attacks

   Since EAP-SAKE is not a tunneling method, the
   EAP.Response/SAKE/Auth-Reject, EAP.Success, and EAP.Failure packets
   are not integrity or replay protected.  This makes it possible for an
   attacker to spoof such messages.  Note that EAP.Response/SAKE/Auth-
   Reject cannot be protected with a MIC since an authentication failure
   indicates that the Server and Peer do not agree on a common key.

   Most importantly, an attacker cannot cause a Peer to accept an
   EAP.Success packet as indication that the Server considers the mutual
   authentication to have been achieved.  This is because a Peer does
   not accept EAP.Success packets before it has authenticated the Server
   or after it has considered the Server to have failed authentication.

5.2.  Root Secret Considerations

   If the Root Secret is known to any party other than the Server and
   Peer, then the mutual authentication and key establishment using
   EAP-SAKE is compromised.

   EAP-SAKE does not address how the Root Secret is generated or
   distributed to the Server and Peer.  It is RECOMMENDED that the
   entropy of the Root Secret be maximized.  The Root Secret SHOULD be
   machine-generated.






Vanderveen & Soliman         Informational                     [Page 38]

RFC 4763                        EAP-SAKE                   November 2006


   If the Root Secret is derived from a low-entropy, guessable quantity
   such as a human-selected password, then the EAP-SAKE key derivation
   is subject to on-line and off-line dictionary attacks.  To help
   identify whether such a password has been compromised,
   implementations SHOULD keep a log of the number of EAP-SAKE messages
   received with invalid MIC fields.  In these cases, a procedure for
   updating the Root Secret securely SHOULD be in place.

5.3.  Mutual Authentication

   Mutual authentication is accomplished via the SAKE/Challenge and
   SAKE/Confirm messages.  The EAP.Request/SAKE/Challenge contains the
   Server nonce RAND_S; the EAP.Response/SAKE/Challenge contains the
   Peer nonce RAND_P, along with the Peer MIC (MIC_P); and the
   EAP.Request/SAKE/Confirm contains the Server MIC (MIC_S).  Both MICs
   (MIC_S and MIC_P) are computed using both nonces RAND_S and RAND_P
   and are keyed by the TEK, a shared secret derived from the Root
   Secret.  The Server considers the Peer authenticated if the MIC_P it
   computes matches the one that the Peer sends.  Similarly, the Peer
   considers the Server authenticated if the MIC_S it computes matches
   the one that the Server sends.  The way the MICs are computed
   involves a keyed one-way hash function, which makes it
   computationally hard for an attacker to produce the correct MIC
   without knowledge of the shared secret.

5.4.  Integrity Protection

   Integrity protection of EAP-SAKE messages is accomplished through the
   use of the Message Integrity Checks (MIC), which are present in every
   message as soon as a common shared secret (TEK) is available, i.e.,
   any message after the EAP.Request/SAKE/Challenge.  An adversary
   cannot modify any of the MIC-protected messages without causing the
   recipient to encounter a MIC failure.  The extent of the integrity
   protection is commensurate with the security of the KDF used to
   derive the MIC, the length and entropy of the shared secret used by
   the KDF, and the length of the MIC.

5.5.  Replay Protection

   The first message of most session establishment protocols, such as
   EAP-SAKE, is subject to replay.  A replayed
   EAP.Request/SAKE/Challenge message results in the Peer sending an
   EAP.Response/SAKE/Challenge message back, which contains a MIC that
   was computed using the attacker's chosen nonce.  This poses a minimal
   risk to the compromise of the TEK-Auth key, and this EAP Session
   cannot proceed successfully as the Peer will find the Server's MIC
   invalid.




Vanderveen & Soliman         Informational                     [Page 39]

RFC 4763                        EAP-SAKE                   November 2006


   Replay protection is achieved via the RAND_S and RAND_P values,
   together with the Session ID field, which are included in the
   calculation of the MIC present in each packet subsequent to the EAP-
   SAKE/Challenge request packet.  The Session ID MUST be generated anew
   by the Server for each EAP session.  Session Ids also aid in
   identification of possible multiple EAP sessions between a Peer and a
   Server.  Within the same session, messages can be replayed by an
   attacker, but the state machine SHOULD be able to handle these cases.
   Note that a replay within a session is indistinguishable to a
   recipient from a network malfunction (e.g., message was first lost
   and then re-transmitted, so the recipient thinks it is a duplicate
   message).

   Replay protection between EAP sessions and within an EAP session is
   also accomplished via the MIC, which covers not only the entire EAP
   packet (including the Session ID) but also the nonces RAND_S and
   RAND_P.  Thus, the recipient of an EAP message can be assured that
   the message it just received is the one just sent by the other Peer
   and not a replay, since it contains a valid MIC of the recipient's
   nonce and the other Peer nonce.  As before, the extent of replay
   protection is commensurate with the security of the KDF used to
   derive the MIC, the length and entropy of the shared secret used by
   the KDF, and the length of the MIC.

5.6.  Confidentiality

   Confidentiality of EAP-SAKE attributes is supported through the use
   of the AT_ENCR_DATA and AT_IV attributes.  A ciphersuite is
   negotiated securely (see Section 3.2.7) and can be used to encrypt
   any attributes as needed.  The default ciphersuite contains a strong
   cipher based on AES.

5.7.  Key Derivation, Strength

   EAP-SAKE derives a Master Key (for EAP use) and Master Session Key,
   as well as other lower-level keys, such as TEKs.  Some of the lower-
   level keys may or may not be used.  The strength (entropy) of all
   these keys is at most the strength of the Root Secret.

   The entropy of the MSK and of the EMSK, assuming that the Server and
   Peer 128-bit nonces are generated using good random number
   generators, is at most 256-bits.









Vanderveen & Soliman         Informational                     [Page 40]

RFC 4763                        EAP-SAKE                   November 2006


5.8.  Dictionary Attacks

   Dictionary attacks are not feasible to mount on the EAP-SAKE method
   because passwords are not used.  Instead, the Root Secret is
   machine-generated.  This does not necessarily pose provisioning
   problems.

5.9.  Man-in-the-Middle Attacks

   Resistance to man-in-the-middle attacks is provided through the
   integrity protection that each EAP message carries (i.e., Message
   Integrity Check field) as soon as a common key for this EAP session
   has been derived through mutual authentication.  As before, the
   extent of this resistance is commensurate with the strength of the
   MIC itself.  Man-in-the-middle attacks associated with the use of any
   EAP method within a tunneling or sequencing protocol are beyond the
   scope of this document.

5.10.  Result Indication Protection

   EAP-SAKE provides result indication protection in that it provides
   result indications, integrity protection, and replay protection.  The
   Server indicates that it has successfully authenticated the Peer by
   sending the EAP.Request/SAKE/Confirm message, which is integrity and
   replay protected.  The Peer indicates that it has successfully
   authenticated the Server by sending the EAP.Response/SAKE/Confirm
   message, which is also integrity and replay protected.

5.11.  Cryptographic Separation of Keys

   The TEKs used to protect EAP-SAKE packets (TEK-Auth, TEK-Cipher), the
   Master Session Key, and the Extended Master Session Key are
   cryptographically separate.  Information about any of these keys does
   not lead to information about any other keys.  We also note that it
   is infeasible to calculate the Root Secret from any or all of the
   TEKs, the MSK, or the EMSK.

5.12.  Session Independence

   Within each EAP-SAKE session, fresh keying material is generated.
   The keying material exported by this method from two independent
   EAP-SAKE sessions is cryptographically separate, as explained below.

   Both the Server and the Peer SHOULD generate fresh random numbers
   (i.e., nonces) for the EAP-SAKE exchange.  If either entity re-uses a
   random number from a previous session, then the fact that the other
   does use a freshly generated random number implies that the TEKs,
   MSK, and EMSK derived within this session are cryptographically



Vanderveen & Soliman         Informational                     [Page 41]

RFC 4763                        EAP-SAKE                   November 2006


   separate from the corresponding keys derived in the previous
   exchange.

   Therefore, compromise of MSK or EMSK on one exchange does not
   compromise the MSK and EMSK of previous or subsequent exchanges
   between a Peer and a Server.

5.13.  Identity Protection

   As seen from Section 3.2.3., the Server may assign a temporary NAI to
   a Peer in order to achieve user anonymity.  This identifier may be
   used by the Peer the next time it engages in an EAP-SAKE
   authentication phase with the Server.  The TempID is protected by
   sending it encrypted, within an AT_ENCR_DATA attribute, and signed by
   the Server with a MIC.  Thus, an eavesdropper cannot link the
   original PermID that the Peer first sends (e.g., on power-up) to any
   subsequent TempID values sent in the clear to the Server.

   The Server and Peer MAY be configured such that only TempID
   identities are exchanged after one initial EAP-SAKE phase that uses
   the Peer permanent identity.  In this case, in order to achieve
   maximum identity protection,  the TempID SHOULD be stored in non-
   volatile memory in the Peer and Server.  Thus, compliance with this
   document does not preclude or mandate Peer identity protection across
   the lifetime of the Peer.

5.14.  Channel Binding

   The Server identifier and Peer identifier MAY be sent in the
   SAKE/Challenge messages.  However, since there is no established
   authentication key at the time of the first message, the Server
   identifier is not integrity-protected here.

   All subsequent EAP-SAKE messages exchanged during a successful EAP-
   SAKE phase are integrity-protected, as they contain a Message
   Integrity Check (MIC).  The MIC is computed over the EAP message and
   also over the Server and Peer identities.  In that, both EAP
   endpoints can verify the identity of the other party.

5.15.  Ciphersuite Negotiation

   EAP-SAKE does not support negotiation of the ciphersuite used to
   integrity-protect the EAP conversation.  However, negotiation of a
   ciphersuite for data protection is supported.  This ciphersuite
   negotiation is protected in order to minimize the risk of down-
   negotiation or man-in-the-middle attacks.





Vanderveen & Soliman         Informational                     [Page 42]

RFC 4763                        EAP-SAKE                   November 2006


   This negotiation is secure because of the Message Integrity Checks
   (MICs) that cover the entire EAP messages used for ciphersuite
   negotiation (see Section 3.2.7.).  The extent of the security of the
   negotiation is commensurate with the security of the KDF used to
   derive the MICs, the length and entropy of the shared secret used by
   the KDF, and the length of the MICs.

5.16.  Random Number Generation

   EAP-SAKE supports key derivation from a 32-byte Root Secret.  The
   entropy of all other keys derived from it is reduced somewhat through
   the use of keyed hash functions (e.g.  KDF).  Thus, assuming
   optimistically that the effective key strength of the Root Secret is
   32 bytes, the effective key strengths of the derived keys is at most
   the effective key strength of the Root Secret quantities they are
   derived from: EMSK, at most 16 bytes; MSK, at most 16 bytes.

6.  Security Claims

   This section provides the security claims as required by [EAP].

      [a] Mechanism: EAP-SAKE is a challenge/response authentication and
          key establishment mechanism based on a symmetric pre-shared
          secret.

      [b] Security claims.  EAP-SAKE provides:

          Mutual authentication (Section 5.3)

          Integrity protection (Section 5.4)

          Replay protection (Section 5.5)

          Confidentiality (optional, Section 5.6 and Section 5.13)

          Key derivation (Section 5.7)

          Dictionary attack protection (Section 5.8)

          Protected result indication of successful authentication from
          Server and from Peer (Section 5.10)

          Session independence (Section 5.12)

      [c] Key strength.  EAP-SAKE supports key derivation with 256-bit
          effective key strength (Section 5.7)

      [d] Description of key hierarchy: see Section 3.2.5.



Vanderveen & Soliman         Informational                     [Page 43]

RFC 4763                        EAP-SAKE                   November 2006


      [e] Indication of vulnerabilities: EAP-Make does not provide:

          Fast reconnect

          Fragmentation

          Channel binding

          Cryptographic binding

7.  Acknowledgements

   Thanks to R. Dynarski for his helpful comments.

8.  References

8.1.  Normative References

   [AES]          National Institute of  Standards and Technology,
                  "Federal Information Processing Standards (FIPS)
                  Publication 197, Advanced Encryption Standard (AES)",
                  November 2001.  http://csrc.nist.gov/publications/
                  fips/fips197/fips-197.pdf

   [CBC]          National Institute of Standards and Technology, NIST
                  Special Publication 800-38A, "Recommendation for Block
                  Cipher Modes of Operation - Methods and Techniques",
                  December 2001.  http://csrc.nist.gov/publications/
                  drafts/Draft-NIST_SP800-38D_Public_Comment.pdf

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

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

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

   [IEEE802.11i]  "IEEE Standard for Information Technology-
                  Telecommunications and Information Exchange between
                  Systems - LAN/MAN Specific Requirements - Part 11:
                  Wireless Medium Access Control (MAC) and physical
                  layer (PHY) specifications: Amendment 6: Medium Access
                  Control (MAC) Security Enhancements", June 2004.



Vanderveen & Soliman         Informational                     [Page 44]

RFC 4763                        EAP-SAKE                   November 2006


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

   [SHA1]         National Institute of Standards and Technology, U.S.
                  Department of Commerce, Federal Information Processing
                  Standard (FIPS) Publication 180-1, "Secure Hash
                  Standard", April 1995.

8.2.  Informative References

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

   [RFC4086]      Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                  "Randomness Requirements for Security", BCP 106, RFC
                  4086, June 2005.

Authors' Addresses

   Michaela Vanderveen
   Qualcomm Flarion Technologies
   135 Rte. 202/206 South
   Bedminster, NJ 07921
   USA

   EMail: mvandervn@yahoo.com


   Hesham Soliman
   Qualcomm Flarion Technologies
   135 Rte. 202/206 South
   Bedminster, NJ 07921
   USA

   EMail: solimanhs@gmail.com
















Vanderveen & Soliman         Informational                     [Page 45]

RFC 4763                        EAP-SAKE                   November 2006


Full Copyright Statement

   Copyright (C) The IETF Trust (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78 and at www.rfc-editor.org/copyright.html, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST,
   AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
   THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY
   IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
   PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.






Vanderveen & Soliman         Informational                     [Page 46]

