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Versions: 00 01 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 4187

                                                               J. Arkko
   Internet Draft                                              Ericsson
   Document: draft-arkko-pppext-eap-aka-00.txt             H. Haverinen
   Expires: December 2001                                         Nokia
                                                               May 2001


                          EAP AKA Authentication


Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.


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

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

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


Abstract

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for authentication and session key distribution using the
   UMTS AKA authentication mechanism. AKA is based on symmetric keys,
   and runs in a UMTS Subscriber Identity Module, a smart card like
   device. AKA provides also backward compatibility to GSM
   authentication, making it possible to use EAP AKA for authenticating
   both GSM and UMTS subscribers.


Table of Contents


   Status of this Memo................................................1
   Abstract...........................................................1
   1. Introduction and Motivation.....................................3
   2. Conventions used in this document...............................4
   3. Protocol Overview...............................................5
   4. Messages.......................................................11
   4.1. EAP-Response/Identity........................................11



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   4.2. EAP-Request/USIM-Challenge...................................12
   4.3. EAP-Response/USIM-Challenge..................................14
   4.4. EAP-Response/USIM-Authentication-Reject......................15
   4.5. EAP-Response/USIM-GSM-Authentication-Reject..................15
   4.6. EAP-Response/USIM-Synchronization-Failure....................16
   5. Interoperability with GSM......................................17
   6. IANA Considerations............................................18
   7. Security Considerations........................................18
   8. Intellectual Property Right Notices............................18
   Acknowledgements..................................................18
   Authors' Addresses................................................18











































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1. Introduction and Motivation

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for authentication and session key distribution using the
   UMTS AKA authentication mechanism [1]. The Universal Mobile
   Telecommunications System (UMTS) is a global third generation mobile
   network standard.

   AKA is based on challenge-response mechanisms and symmetric
   cryptography. AKA runs in a UMTS Subscriber Identity Module (USIM),
   a smart card like device. AKA provides also backwards compatibility
   to the GSM authentication mechanism [2]. Compared to the GSM
   mechanism, AKA provides substantially longer key lengths and the
   authentication of the server side as well as the client side.

   The introduction of AKA inside EAP allows several new applications.
   These include the following:

   - The use of the AKA also as a secure PPP authentication method in
     devices that already contain an USIM.

   - The use of the third generation mobile network authentication
     infrastructure in the context of wireless LANs and IEEE 801.1x
     technology through EAP over Wireless [3, 4].

   - Relying on AKA and the existing infrastructure in a seamless way
     with any other technology that can use EAP.

   AKA works in the following manner:

   - The USIM and the home environment have agreed on a secret key
     beforehand.

   - The actual authentication process starts by having the home
     environment produce an authentication vector, based on the secret
     key and a sequence number. The authentication vector contains a
     random part RAND, an authenticator part AUTN used for
     authenticating the network to the USIM, an expected result part
     XRES, a session key for integrity check IK, and a session key for
     encryption CK.

   - The RAND and the AUTN are delivered to the USIM.

   - The USIM verifies the AUTN, again based on the secret key and the
     sequence number. If this process is successful (the AUTN is valid
     and the sequence number used to generate AUTN is within the
     correct range), the USIM produces an authentication result, RES
     and sends this to the home environment.

   - The home environment verifies the correct result from the USIM. If
     the result is correct, IK and CK can be used to protect further
     communications between the USIM and the home environment.



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   When verifying AUTN, the USIM may detect that the sequence number
   the network uses is not within the correct range. In this case, the
   USIM calculates a sequence number synchronization parameter AUTS and
   sends it to the network. AKA authentication may then be retried with
   a new authentication vector generated using the synchronized
   sequence number.

   For a full specification of the AKA algorithms and how the
   cryptographic values AUTN, RES, IK, CK and AUTS are calculated, see
   reference [1].

   It is also possible that the home environment delegates the actual
   authentication task to an intermediate node. In this case the
   authentication vector or parts of it are delivered to the
   intermediate node, enabling it to perform the comparison between RES
   and XRES, and possibly also use CK and IK.

   In the third generation mobile networks, AKA is used both for radio
   network authentication and IP multimedia service authentication
   purposes. Different user identities and formats are used for these;
   the radio network uses the International Mobile Subscriber
   Identifier (IMSI), whereas the IP multimedia service uses the
   Network Access Identifier (NAI) [5].


2. Conventions used in this document

   The following terms will be used through this document:


      AAA protocol

         Authentication, Authorization and Accounting protocol

      AAA server

         In this document, AAA server refers to the network element
         that resides on the border of Internet AAA network and GSM
         network.

     AKA

        Authentication and Key Agreement

      AuC

         Authentication Centre. The mobile network element that can
         authorize subscribers either in GSM or in UMTS networks.

      EAP

         Extensible Authentication Protocol [6].

      GSM

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         Global System for Mobile communications.

      NAI

         Network Access Identifier [5].

      AUTN

         Authentication value generated by the AuC which together with
         the RAND authenticates the server to the client, 128 bits [1].

      AUTS

         A value generated by the client upon experiencing a
         synchronization failure, 112 bits.

      RAND

         Random number generated by the AuC, 128 bits [1].

      RES

         Authentication result from the client, which together with the
         RAND authenticates the client to the server, 128 bits [1].

      SQN

         Sequence number used in the authentication process, 48 bits
         [1].

      SIM

         Subscriber Identity Module. SIM cards are smart cards
         distributed by GSM operators.

      SRES

         The authentication result parameter in GSM, corresponds to the
         RES parameter in UMTS aka, 32 bits.

      USIM

         UMTS Subscriber Identity Module. These cards are smart cards
         Similar to SIMs and are distributed by UMTS operators.


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC 2119 [8]

3. Protocol Overview



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   The EAP AKA uses two roundtrips to authorize the user and generate
   session keys. The authenticator typically communicates with the
   user's AAA server using an AAA protocol. (The exact AAA
   communications outside the scope of this document, however.)

   The below message flow shows the basic successful authentication
   case with the EAP AKA. As in other EAP schemes, first an identity
   request/response message pair is exchanged. (For this particular EAP
   protocol, the identity request is defined to be optional, to shorten
   the authentication process to a minimal one.)

   Next, the authenticator starts the actual AKA protocol by sending an
   EAP-Request/USIM-Challenge message. This message contains a random
   number and an authorization vector. The client runs the AKA
   algorithm (perhaps inside an USIM) and verifies the AUTN. If this is
   successful, the client is talking to a legitimate authenticator and
   proceeds to send the EAP-Response/USIM-Challenge. This message
   contains a result parameter that allows the authenticator in turn to
   verify that the client is a legitimate one.




































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       Client                                             Authenticator
          |                                                       |
          |                      EAP-Request/Identity (optional)  |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs UMTS algorithms, |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/USIM-Challenge    |
          |                         (Lifetime, RAND, AUTN)        |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM,|                     |
      | verifies AUTN, derives RES          |                     |
      | and session key                     |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/USIM-Challenge                           |
          | (RES)                                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server checks the given RES, |
          |                            | and finds it correct.        |
          |                            +------------------------------+
          |                                                       |
          |                                          EAP-Success  |
          |<------------------------------------------------------|

   When EAP AKA is run in the GSM compatible mode, the message flow is
   otherwise identical to the message flow below except that the AUTN
   parameter is not included in EAP-Request/USIM-Challenge packet.


   An optional lifetime may be associated to the challenge message.
   This specifies the server side's limit on how long the ciphering and
   integrity keys generated as a part of the authentication process can
   be used. (The use of such keys is outside the scope of this
   document.)

   The second message flow shows how the Authenticator rejects the
   Client due to failed authentication. The same flow is also used in
   the GSM compatible mode, except that the AUTN parameter is not
   included in the EAP-Request/USIM-Challenge packet.




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       Client                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs UMTS algorithms, |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/USIM-Challenge    |
          |                         (Lifetime, RAND, AUTN)        |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM,|                     |
      | possibly verifies AUTN, and sends an|                     |
      | invalid response                    |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/USIM-Challenge                           |
          | (RES)                                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server checks the given RES, |
          |                            | and finds it incorrect.      |
          |                            +------------------------------+
          |                                                       |
          |                                          EAP-Failure  |
          |<------------------------------------------------------|


   The next message flow shows the client rejecting the AUTN of the
   Authenticator. This flow is not used in the GSM compatible mode.
















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       Client                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs UMTS algorithms, |
          |                            | generates RAND and a bad AUTN|
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/USIM-Challenge    |
          |                         (Lifetime, RAND, AUTN)        |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM |                     |
      | and discovers AUTN that can not be  |                     |
      | verified                            |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/USIM-Authentication-Reject               |
          |------------------------------------------------------>|
          |                                                       |
          |                                                       |
          |                                          EAP-Failure  |
          |<------------------------------------------------------|


   Networks that are not UMTS aware use the GSM compatible version of
   this protocol even for UMTS subscribers. In this case, the AUTN
   parameter is not included in the EAP-Request/USIM-Challenge packet.
   If a UMTS capable client does not want to accept the use of the GSM
   compatible mode, the client can reject the authentication with the
   EAP-Response/USIM-GSM-Authentication-Reject message, as shown in the
   following figure:
















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   Client                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs GSM algorithms,  |
          |                            | generates RAND               |
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/USIM-Challenge    |
          |                         (Lifetime, RAND)              |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client does not accept the GSM      |                     |
      | compatible version of this protocol.|                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/USIM-GSM-Authentication-Reject           |
          |------------------------------------------------------>|
          |                                                       |
          |                                                       |
          |                                          EAP-Failure  |
          |<------------------------------------------------------|

   The AKA uses shared secrets between the Client and the Authenticator
   together with a sequence number to actually perform an
   authentication. In certain circumstances it is possible for the
   sequence numbers to get out of sequence. Here's what happens then:





















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       Client                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs UMTS algorithms, |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/USIM-Challenge    |
          |                         (Lifetime, RAND, AUTN)        |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM |                     |
      | and discovers AUTN that contains an |                     |
      | inappropriate sequence number       |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/USIM-Synchronization-Failure             |
          | (AUTS)                                                |
          |------------------------------------------------------>|
          |                                                       |
          |                              +---------------------------+
          |                              | Perform resynchronization |
          |                              | towards the AAA using     |
          |                              | AUTS and the sent RAND    |
          |                              +---------------------------+
          |                                                       |

   After the resynchronization process takes place in the server and
   AAA side, the process continues by the server side sending a new
   EAP-Request/USIM-Challenge message.

4. Messages

4.1. EAP-Response/Identity

   In the beginning of EAP authentication, the Authenticator issues the
   EAP-Request/Identity packet to the client. The client responds with
   EAP-Response/Identity, which contains the user's identity. The
   formats of these packets are specified in [6].

   The EAP AKA mechanism uses the NAI format [5] as the identity.
   In order to facilitate the use of the existing cellular roaming
   infrastructure, the EAP AKA client transmits the user's IMSI within
   the NAI in the EAP Response/Identity packet. The NAI is of the
   format "0imsi@realm". In other words, the first character is the
   digit zero (ASCII 0x30), followed by the IMSI, followed by the @

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   character and the realm. The IMSI is an ASCII string that consists
   of not more than 15 decimal digits (ASCII values between 0x30 and
   0x39) as specified in [9].

   The AAA network routes AAA requests to the correct AAA server using
   the realm part of the NAI. Because cellular roaming can be used with
   EAP AKA, the AAA request can be routed to an AAA server in the
   visited network instead of the server indicated in the NAI realm.
   The operators need to agree on this special AAA routing in advance.
   It is recommended that operators should reserve the realm portion of
   NAI used with EAP AKA to UMTS and GSM subscribers only, so that
   exactly the same realm is not used with other authentication
   methods. This convention makes it easy to recognize that the NAI
   identifies a UMTS or GSM subscriber of this operator, which may be
   useful when configuring the routing rules in the visited AAA
   networks.

   In the EAP AKA protocol, the EAP-Request/Identity message is
   optional when applicable. If the client can positively determine
   that it has to authenticate, it MAY send an unsolicited EAP-
   Response/Identity to the authenticator with an Identifier value it
   has picked up itself. The client MUST NOT send an unsolicited EAP-
   Response/Identity if it has already received an EAP-Request/Identity
   packet. The client MUST send an EAP-Response/Identity to all
   received EAP-Request/Identity packets, using the Identifier value in
   the EAP-Request/Identity. If the authenticator receives an
   unsolicited EAP-Response/Identity, it SHOULD process the packet as
   if it had requested it. If the authenticator receives an EAP-
   Response/Identity with an incorrect Identifier value in response to
   the first EAP-Request/Identity it has sent to the client, then the
   authenticator SHOULD still accept the EAP-Response/Identity packet.

4.2. EAP-Request/USIM-Challenge

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




















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    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      |    Subtype    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Key Lifetime                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             RAND                              |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                        AUTN (optional)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

      Code

         1 for Request

      Identifier

         See [6]

      Length

         The length of the EAP Request packet.
         44, if AUTN is included (UMTS AKA).
         28, if AUTN is excluded (GSM compatible mode).

      Type

         TBD

      Subtype

         1 for USIM-Challenge

      Reserved

         Set to zero when sending, ignored on reception.

      Key lifetime

         This expresses how long the cipher and integrity keys may be
         used. This value is expressed in seconds, and the value of
         zero means they may be used indefinitely.


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      RAND

         The AKA RAND parameter, 16 bytes (128 bits).

      AUTN

         The AKA AUTN parameter, 16 bytes (128 bits).

4.3. EAP-Response/USIM-Challenge

   The format of the EAP-Response/USIM-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      |   Subtype     |  ResLength    |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             RES                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

      Code

         2 for Response

      Identifier

         See [6]

      Length

         The length of the EAP Response packet, 12..40.

      Type

         TBD

      Subtype

         1 for USIM-Challenge

      ResLength

         This is the length of the RES parameter in bits. According to
         the specification [10] this parameter can vary between 32 and
         128 bits. In the GSM compatible mode, the RES field contains
         the GSM SRES parameter which is always 32 bits long.


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      Reserved

         Set to zero when sending, ignored on reception.

      RES

         The AKA RES parameter, 32..128 bits. The Length parameter
         specifies the total length of the payload and identifies the
         at the same time indirectly also the size of the RES in bytes.
         The ResLength field identifies the exact length in bits. The
         sender may pad the RES with zero bits and bytes where
         necessary. In the GSM compatible mode, the RES field contains
         the GSM SRES parameter.


4.4. EAP-Response/USIM-Authentication-Reject

   The format of the EAP-Response/USIM-Authentication-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      |   Subtype     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

      Code

        2 for Response

      Identifier

        See [6]

      Length

        The length of the EAP Response packet, 12.

      Type

        TBD

      Subtype

        2 for USIM-Authentication-Reject


4.5. EAP-Response/USIM-GSM-Authentication-Reject



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   The format of the EAP-Response/USIM-GSM-Authentication-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      |   Subtype     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The semantics of the fields is described below:

      Code

         2 for Response

      Identifier

         See [6]

      Length

         The length of the EAP Response packet, 6.

      Type

         TBD

      Subtype

         3 for USIM-GSM-Authentication-Reject


4.6. EAP-Response/USIM-Synchronization-Failure

   The format of the EAP-Response/USIM-Synchronization-Failure 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      |   Subtype     |             AUTS              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:


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      Code

         2 for Response

      Identifier

         See [6]

      Length

         The length of the EAP Response packet, 20.

      Type

         TBD

      Subtype

         4 for USIM-Synchronization-Failure

      AUTS

         The AKA AUTS parameter, 112 bits (14 bytes).


5. Interoperability with GSM

   The EAP AKA protocol is able to authenticate both UMTS and GSM
   users, if the subscriber's operator's network is UMTS aware. This is
   because the home network will be able to determine from the
   subscriber records whether the subscriber is equipped with a UMTS
   USIM or a GSM SIM. A UMTS aware home network will hence always use
   UMTS AKA with UMTS subscribers and GSM authentication with GSM
   subscribers. With GSM subscribers, the EAP AKA protocol is always
   used in the GSM compatible mode.

   It is not possible to use a GSM AuC to authenticate UMTS
   subscribers. (Note that if the home network doesn't support an
   authentication method it should not distribute SIMs for that
   method.)

   However, it is possible that the node actually terminating EAP and
   the node that stores the authentication keys (AuC) are separate, and
   support different authentication types. If the node terminating EAP
   is GSM-only but AuC is UMTS-aware, then authentication can still be
   achieved using the GSM compatible version of EAP AKA. This
   authentication will be weaker, since the GSM compatible mode does
   not provide for mutual authentication. Section 6.8.1.1 in [1]
   specifies how the GSM SRES parameter and the Kc key can be
   calculated on the USIM and the AuC. If a UMTS terminal does not want
   to accept the GSM compatible version of this protocol, then it can
   reject the authentication with the EAP-Response/USIM-GSM-
   Authentication-Reject packet.


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   In conclusion, the following table shows which variant of the EAP
   AKA protocol should be run under different conditions:

   SIM    EAP node             AuC         EAP AKA mode
   ----------------------------------------------------
   GSM    (any)              (any)                  GSM
   UMTS   (any)                GSM            (illegal)
   UMTS   GSM             GSM+UMTS                  GSM
   UMTS   GSM+UMTS        GSM+UMTS                 UMTS


6. IANA Considerations

   IANA has assigned the number TBD for EAP AKA authentication.

7. Security Considerations

   Implementations running the EAP AKA protocol will rely on the
   security of the AKA scheme, and the secrecy of the symmetric keys
   stored in the USIM and the AuC.

8. Intellectual Property Right Notices

   On IPR related issues, Nokia and Ericsson refer to the their
   respective statements on patent licensing. Please see
   http://www.ietf.org/ietf/IPR/NOKIA and
   http://www.ietf.org/ietf/IPR/ERICSSON-General

Acknowledgements

   The authors wish to thank Rolf Blom of Ericsson, Bernard Aboba of
   Microsoft and Arne Norefors of Ericsson for interesting discussions
   in this problem space.

Authors' Addresses

   Jari Arkko
   Ericsson
   02420 Jorvas                 Phone:  +358 40 5079256
   Finland                      Email:  jari.arkko@ericsson.com

   Henry Haverinen
   Nokia Mobile Phones
   P.O. Box 88
   33721 Tampere                Phone: +358 50 594 4899
   Finland                      E-mail: henry.haverinen@nokia.com

References

   [1]   3GPP Technical Specification 3GPP TS 33.102 V3.6.0: "Technical
         Specification Group Services and System Aspects; 3G Security;
         Security Architecture (Release 1999)", 3rd Generation
         Partnership Project, November 2000.


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                           EAP AKA Authentication            May 2001



   [2]   GSM Technical Specification GSM 03.20 (ETS 300 534): "Digital
         cellular telecommunication system (Phase 2); Security related
         network functions", European Telecommunications Standards,
         Institute, August 1997.

   [3]   IEEE Draft P802.1X/D11, "Standards for Local Area and
         Metropolitan Area Networks: Standard for Port Based Network
         Access Control", March 2001

   [4]   IEEE Draft 802.11eS/D1, "Draft Supplement to STANDARD FOR
         Telecommunications and Information Exchange between Systems -
         LAN/MAN Specific Requirements - Part 11: Wireless Medium
         Access Control (MAC) and physical layer (PHY) specifications:
         Specification for Enhanced Security", March 2001

   [5]   Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
         2486, January 1999.

   [6]   L. Blunk, J. Vollbrecht, "PPP Extensible Authentication
         Protocol (EAP)", RFC 2284, March 1998.

   [8]   S. Bradner, "Key words for use in RFCs to indicate Requirement
         Levels", RFC 2119, March 1997.

   [9]   GSM Technical Specification GSM 03.03 (ETS 300 523): "Digital
         cellular telecommunication system (Phase 2); Numbering,
         addressing and identification", European Telecommunications
         Standards Institute, April 1997.

   [10]  3GPP Technical Specification 3GPP TS 33.105 V3.5.0: "Technical
         Specification Group Services and System Aspects; 3G Security;
         Cryptographic Algorithm Requirements (Release 1999)",
         3rdGeneration Partnership Project, October 2000




















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