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                                                               J. Arkko
   Internet Draft                                              Ericsson
   Document: draft-arkko-pppext-eap-aka-04.txt             H. Haverinen
   Expires: December 2002                                         Nokia
                                                              June 2002


                          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 typically 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.....................................2
   2. Conventions used in this document...............................3
   3. Protocol Overview...............................................5
   4. Obtaining Subscriber Identity via EAP AKA Messages.............10
   5. Identity Privacy Support.......................................11


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   6. Message Format.................................................14
   7. Message Integrity and Privacy Protection.......................16
   7.1. AT_MAC Attribute.............................................16
   7.2. AT_IV and AT_ENCR_DATA Attributes............................16
   8. Messages.......................................................17
   8.1. EAP-Response/Identity........................................18
   8.2. EAP-Request/AKA-Challenge....................................19
   8.3. EAP-Response/AKA-Challenge...................................22
   8.4. EAP-Response/AKA-Authentication-Reject.......................24
   8.5. EAP-Response/AKA-Synchronization-Failure.....................24
   8.6. EAP-Request/AKA-Identity.....................................25
   8.7. EAP-Response/AKA-Identity....................................26
   9. Key Derivation.................................................28
   10. Interoperability with GSM.....................................29
   11. IANA and Protocol Numbering Considerations....................30
   12. Security Considerations.......................................31
   13. Intellectual Property Right Notices...........................31
   Acknowledgements and Contributions................................31
   Authors' Addresses................................................31

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 typically runs in a UMTS Subscriber Identity
   Module (USIM), a smart card like device. However, the applicability
   of AKA is not limited to client devices with smart cards, but the
   AKA mechanisms could also be implemented in host software, for
   example. AKA also provides backward 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.


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   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.

   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 specification of the AKA mechanisms 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. Such delivery MUST be
   done in a secure manner. In EAP AKA, the EAP server node is such an
   intermediate node.

   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:

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      AAA protocol

      Authentication, Authorization and Accounting protocol

   AAA server

      The AAA server is responsible for storing shared secrets and
      other credential information necessary for the authentication of
      users. Cf. EAP server

   AKA

      Authentication and Key Agreement

   AuC

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

   Authenticator

      The entity that terminates the protocol carrying EAP used by the
      client, such as a Network Access Server (NAS) terminating the PPP
      link. The EAP server may be co-located in the Authenticator. In
      this case, the Authenticator may actually authenticate the user
      based on information received from the AAA server.

   EAP

      Extensible Authentication Protocol [6].

   EAP server

      The network element that terminates the EAP protocol. Typically,
      the EAP server functionality is implemented in a AAA server.

   GSM

      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].





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   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. The SIM is an application
      traditionally resident on 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. USIM is an application that is
      resident e.g. on smart cards 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 [7]

3. Protocol Overview

   In this document, the term EAP Server refers to the network element
   that terminates the EAP protocol. Usually the EAP server is separate
   from the authenticator device, which is the network element closest
   to the client, such as a Network Access Server (NAS) or an IEEE
   802.1X bridge. Alternatively, the EAP server functionality may be
   co-located in the authenticator although typically, the the EAP
   server functionality is implemented on a separate AAA server with
   whom the authenticator communicates using an AAA protocol. (The
   exact AAA communications are outside the scope of this document,
   however.)

   The below message flow shows the basic successful authentication
   case with the EAP AKA. The EAP AKA uses two roundtrips to authorize
   the user and generate session keys. As in other EAP schemes, first
   an identity request/response message pair is exchanged. (As

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   specified in [6], the initial identity request is not required, and
   MAY be bypassed in cases where the authenticator can presume the
   identity, such as when using leased lines, dedicated dial-ups, etc.
   Please see also Section 4 for specification how to obtain the
   identity via EAP AKA messages.)

   Next, the EAP server starts the actual AKA protocol by sending an
   EAP-Request/AKA-Challenge message. This message contains a random
   number (RAND) and an authorization vector (AUTN). The EAP-
   Request/AKA-Challenge message MAY optionally contain encrypted data,
   which is used for IMSI privacy support, as described in Section 5.
   The encrypted data is not shown in the figures of this section. 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 EAP server and proceeds to send the EAP-Response/AKA-
   Challenge. This message contains a result parameter that allows the
   EAP server in turn to verify that the client is a legitimate one.

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


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   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
   attribute is not included in EAP-Request/AKA-Challenge packet.


   The second message flow shows how the EAP server 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/AKA-Challenge packet.

       Client                                             Authenticator
          |                                                       |
          |                      EAP-Request/Identity             |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs UMTS algorithms, |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/AKA-Challenge     |
          |                         (RAND, AUTN)                  |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM,|                     |
      | possibly verifies AUTN, and sends an|                     |
      | invalid response                    |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/AKA-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 EAP
   server. This flow is not used in the GSM compatible mode.

   The client sends an explicit error message (EAP-Response/AKA-
   Authentication-Reject) to the Authenticator, as usual in AKA when
   AUTN is incorrect. This allows the EAP server to produce the same
   error statistics as AKA in general produces in UMTS. Please note

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   that this behavior is different from other EAP/AKA error cases, such
   as when encountering an incorrect AT_MAC attribute, when the client
   silently discards the EAP/AKA message.



       Client                                             Authenticator
          |                                                       |
          |                      EAP-Request/Identity             |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs UMTS algorithms, |
          |                            | generates RAND and a bad AUTN|
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/AKA-Challenge     |
          |                         (RAND, AUTN)                  |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM |                     |
      | and discovers AUTN that can not be  |                     |
      | verified                            |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/AKA-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/AKA-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/Nak message [6], 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/AKA-Challenge     |
          |                         (RAND)                        |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client does not accept the GSM      |                     |
      | compatible version of this protocol.|                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/Nak                                      |
          |------------------------------------------------------>|
          |                                                       |
          |                                                       |
          |                                          EAP-Failure  |
          |<------------------------------------------------------|

   The AKA uses shared secrets between the Client and the Client's home
   operator 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. HereƆs 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/AKA-Challenge     |
          |                         (RAND, AUTN)                  |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM |                     |
      | and discovers AUTN that contains an |                     |
      | inappropriate sequence number       |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/AKA-Synchronization-Failure              |
          | (AUTS)                                                |
          |------------------------------------------------------>|
          |                                                       |
          |                              +---------------------------+
          |                              | Perform resynchronization |
          |                              | 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/AKA-Challenge message.

4. Obtaining Subscriber Identity via EAP AKA Messages

   It may be useful to obtain the identity of the subscriber through
   means other than EAP Request/Identity. This can eliminate the need
   for an identity request when using EAP method negotiation. If this
   was not possible then it might not be possible to negotiate EAP/AKA
   as the second method since it is not specified how to deal with a
   new EAP Request/Identity.

   If the EAP server does not have any identity (IMSI or pseudonym)
   available when sending the first EAP/AKA request (usually EAP-
   Request/AKA-Challenge), then the EAP server issues the EAP-
   Request/AKA-Identity as the first message and includes the
   AT_IDENTITY_REQ attribute (Section 8.6). This attribute does not
   contain any data. It requests the client to include the AT_IDENTITY

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   attribute (specified in Section 8.7) in the EAP-Response/AKA-
   Identity. The AT_IDENTITY attribute contains the current identity of
   the subscriber (IMSI or pseudonym). The use of pseudonyms for
   anonymity is specified in Section 5.

   This case is illustrated in the figure below.

   Client                                             Authenticator
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not have any     |
          |                            | Subscriber identity available|
          |                            | When starting EAP/AKA        |
          |                            +------------------------------+
          |                                                       |
          |                            EAP-Request/AKA-Identity   |
          |                            (Includes AT_IDENTITY_REQ) |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          | EAP-Response/AKA-Idenity                              |
          | (Includes AT_IDENTITY)                                |
          |------------------------------------------------------>|
          |                                                       |

   If the AT_IDENTITY attribute contains a valid cleartext identity or
   a pseudonym identity that the EAP server is able to decode to the
   cleartext identity, then the authentication sequence proceeds as
   usual with the EAP Server issuing the EAP-Request/AKA-Challenge
   message. The operation in the case when the AT_IDENTITY attribute
   contains a pseudonym that the EAP server fails to decode is
   specified in Section 5.

5. Identity Privacy Support

   In the very first connection to an EAP server, the client always
   transmits the cleartext identity (IMSI) in the EAP-Response/Identity
   packet or in the AT_IDENTITY attribute. In subsequent connections,
   the optional identity privacy support can be used to hide the
   identity and to make the connections unlinkable to a passive
   eavesdropper.

   The EAP-Request/AKA-Challenge message MAY include an encrypted
   pseudonym in the value field of the AT_ENCR_DATA attribute. The
   AT_IV and AT_MAC attributes are also used to transport the pseudonym
   to the client, as described in Section 8.2. Because the identity
   privacy support is optional to implement, the client MAY ignore the
   AT_IV, AT_ENCR_DATA, and AT_MAC attributes and always transmit the
   cleartext identity in the EAP-Response/Identity packet and in the
   AT_IDENTITY attribute.

   On receipt of the EAP-Request/AKA-Challenge, the client verifies the
   AT_AUTN attribute before looking at the AT_ENCR_DATA or AT_MAC
   attributes. If the AUTN is invalid, then the client MUST ignore the

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   AT_IV, AT_ENCR_DATA and AT_MAC attributes. If AUTN is valid, then
   the client MAY derive the K_encr and K_int keys as described in
   Section 9 and verify the AT_MAC attribute. If the AT_MAC attribute
   is valid, then the client MAY decrypt the encrypted data and use the
   pseudonym in the next authentication. If the MAC is invalid, then
   the encrypted data MUST be ignored and the whole EAP packet MAY be
   silently ignored.

   The EAP server produces pseudonyms in an implementation-dependent
   manner. Please see [8] for examples on how to produce pseudonyms.
   Only the EAP server needs to be able to map the pseudonym to the
   cleartext identity. Regardless of construction method, the pseudonym
   MUST conform to the grammar specified for the username portion of an
   NAI. The EAP AKA server MAY produce pseudonyms that begin with a
   leading "0" character in order to be able to use the leading
   character as a hint in EAP method negotiation during next
   authentication.

   On the next connection to the EAP server, the client MAY transmit
   the received pseudonym in the first EAP-Response/Identity packet.
   The client concatenates the received pseudonym with the "@"
   character and the NAI realm portion. The client MUST use the same
   realm portion that it used in the connection when it received the
   pseudonym.

   If the EAP server issues the EAP-Request/AKA-Identity packet and
   requests the client to include the AT_IDENTITY attribute in the EAP-
   Response/AKA-Identity packet, as specified in Section 4, the client
   MAY transmit a pseudonym in the AT_IDENTITY packet. If the EAP
   server successfully decodes the pseudonym to a known identity, then
   the authentication proceeds with the EAP-Request/AKA-Challenge
   packet as usual.

   If the EAP server fails to decode the pseudonym to a known client
   name, then the EAP server requests the cleartext identity (non-
   pseudonym identity) by issuing the EAP-Request/AKA-Identity packet
   to the client. In this case, the EAP request packet includes
   AT_PERMANENT_IDENTITY_REQ to request the client to send its non-
   pseudonym identity. The client responds with the EAP-Response/AKA-
   Identity, which includes the client's identity in the clear in the
   AT_PERMANENT_IDENTITY attribute.

   The EAP server issues the EAP-Request/AKA-Identity message also in
   the case when it received the undecodable pseudonym in AT_IDENTITY
   included the EAP-Response/AKA-Identity. In this case, there are two
   EAP/AKA-Identity round trips. The authentication sequence proceeds
   similarly in both cases.

   Please note that the EAP/AKA client and the EAP/AKA server only
   process the AKA-Identity packets and entities that only pass through
   EAP packets do not process these packets. Hence, if the EAP server
   is not co-located in the authenticator, then the authenticator and
   other intermediate AAA elements (such as possible AAA proxy servers)
   will continue to refer to the client with the original pseudonym

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   identity from the EAP-Response/Identity packet regardless if the
   decoding fails in the EAP server.

   The figure below illustrates the case when an undecodable pseudonym
   is received in EAP-Response/Identity.

   Client                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes a pseudonym)                                |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server fails to decode the   |
          |                            | Pseudonym.                   |
          |                            +------------------------------+
          |                                                       |
          |                   EAP-Request/AKA-Identity            |
          |                  (Includes AT_PERMANENT_IDENTITY_REQ) |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (Includes cleartext identity in AT_PERMANENT_IDENTITY)|
          |------------------------------------------------------>|
          |                                                       |


   After receiving the EAP-Response/AKA-Identity packet, the EAP server
   issues the EAP-Request/AKA-Challenge and the authentication proceeds
   as usual.

   The figure below illustrates the case when the EAP server fails to
   decode the pseudonym included in the AT_IDENTITY attribute.


















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   Client                                             Authenticator
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not have any     |
          |                            | Subscriber identity available|
          |                            | When starting EAP/AKA        |
          |                            +------------------------------+
          |                                                       |
          |                            EAP-Request/AKA-Identity   |
          |                            (Includes AT_IDENTITY_REQ) |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          |EAP-Response/AKA-Identity                              |
          |(Includes a pseudonym AT_IDENTITY)                     |
          |------------------------------------------------------>|
          |                                                       |
          |                                                       |
          |                            +------------------------------+
          |                            | Server fails to decode the   |
          |                            | Pseudonym in AT_IDENTITY     |
          |                            +------------------------------+
          |                                                       |
          |                  EAP-Request/AKA-Identity             |
          |                  (Includes AT_PERMANENT_IDENTITY_REQ) |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (Includes AT_PERMANENT_IDENTITY)                      |
          |------------------------------------------------------>|
          |                                                       |

   After the latter EAP-Response/AKA-Identity message, the
   authentication sequence proceeds as usual with the EAP Server
   issuing the EAP-Request/AKA-Challenge message.

   If the client believes that the server should be able to decode the
   pseudonym identity, the client MAY refuse to send a clear text
   identity. In this case, the client silently ignores the EAP-
   Request/AKA-Identity packet that contains AT_PERMANENT_IDENTITY_REQ.
   This is necessary in some environments to prevent Man-in-the-Middle
   attackers from claiming to be servers that do not recognize the
   pseudonym, in an effort to find out the true identity of the user.

   Because the keys that are used to protect the pseudonym are derived
   from the AKA cipher key (CK) and the AKA integrity key (IK), the
   identity privacy support is not available when EAP AKA is used in
   the GSM compatible mode.

6. Message Format

   The Type-Data of the EAP AKA packets begins with a 1-octet Subtype
   field, which is followed by a 2-octet reserved field. The rest of

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   the Type-Data consists of attributes that are encoded in Type,
   Length, Value format. The figure below shows the generic format of
   an attribute.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Attribute Type |    Length     | Value...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Attribute Type

      Indicates the particular type of attribute. The attribute type
      values are listed in Section 11.

   Length

      Indicates the length of this attribute in multiples of 4 bytes.
      The maximum length of an attribute is 1024 bytes. The length
      includes the Attribute Type and Length bytes.

   Value

      The particular data associated with this attribute. This field is
      always included and it may be two or more bytes in length. The
      type and length fields determine the format and length of the
      value field.

   When an attribute numbered within the range 0 through 127 is
   encountered but not recognized, the EAP/AKA message containing that
   attribute MUST be silently discarded. These attributes are called
   non-skippable attributes.

   When an attribute numbered in the range 128 through 255 is
   encountered but not recognized that particular attribute is ignored,
   but the rest of the attributes and message data MUST still be
   processed. The Length field of the attribute is used to skip the
   attribute value in searching for the next attribute. These
   attributes are called skippable attributes.

   EAP/AKA packets do not include a version field. However, should
   there be reason to revise this protocol in the future, new non-
   skippable or skippable attributes could be specified in order to
   implement revised EAP/AKA versions in a backward-compatible manner.

   Unless otherwise specified, the order of the attributes in an EAP
   AKA message is insignificant, and an EAP AKA implementation should
   not assume a certain order to be used.

   Attributes can be encapsulated within other attributes. In other
   words, the value field of an attribute type can be specified to
   contain other attributes.


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7. Message Integrity and Privacy Protection

   This section specifies EAP/AKA attributes for attribute encryption
   and EAP/AKA message integrity protection.

   Encryption and integrity protection are based on the AKA session
   keys CK and IK. Because the CK and IK keys are derived from the RAND
   challenge, these attributes can only be used in the EAP-Request/AKA-
   Challenge message and any EAP/AKA messages sent after EAP-
   Requets/AKA-Challenge. For example, these attributes cannot be used
   in EAP-Request/AKA-Identity, because the RAND challenge has not yet
   been transmitted at that point. As there is no key derivation
   specification for the GSM mode, attribute encryption and message
   integrity protection are not available in the GSM mode.

7.1. AT_MAC Attribute

   The AT_MAC attribute can optionally be used for EAP/AKA message
   integrity protection. Whenever AT_ENCR_DATA (Section 7.2) is
   included in an EAP message, it MUST be followed (not necessarily
   immediately) by an AT_MAC attribute. Messages that do not meet this
   condition MUST be silently discarded.

   The value field of the AT_MAC attribute contains two reserved bytes
   followed by a message authentication code (MAC). The MAC is
   calculated over the whole EAP packet with the exception that the
   value field of the MAC attribute is set to zero when calculating the
   MAC. The reserved bytes are set to zero when sending and ignored on
   reception. The format of the AT_MAC attribute 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_MAC    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           MAC                                 |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The MAC algorithm is HMAC-SHA1-128 [9] keyed hash value. (The HMAC-
   SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by
   truncating the output to 16 bytes. Hence, the length of the MAC is
   16 bytes.) The integrity protection key (K_int) used in the
   calculation of the MAC is derived from the AKA integrity key (IK)
   and cipher key (CK), as specified in Section 9.

7.2. AT_IV and AT_ENCR_DATA Attributes

   AT_IV and AT_ENCR_DATA attributes can be optionally used to transmit
   encrypted information between the EAP/AKA client and server.



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   The value field of AT_IV contains two reserved bytes followed by a
   16-byte initialization vector required by the AT_ENCR_DATA
   attribute. The reserved bytes are set to zero when sending and
   ignored on reception. The AT_IV attribute MUST be included if and
   only if the AT_ENCR_DATA is included. Messages that do not meet this
   condition MUST be silently discarded.

   The sender of the AT_IV attribute chooses the initialization vector
   by random. The sender MUST NOT reuse the initialization vector value
   from previous EAP AKA packets but the sender MUST choose it freshly
   for each AT_IV attribute. The sends SHOULD use a good source of
   randomness to generate the initialization vector. The format of
   AT_IV 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 = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                 Initialization Vector                         |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of the AT_ENCR_DATA attribute consists of two
   reserved bytes followed by bytes encrypted using the Advanced
   Encryption Standard (AES) [10] in the Cipher Block Chaining (CBC)
   mode of operation, using the initialization vector from the AT_IV
   attribute. The reserved bytes are set to zero when sending and
   ignored on reception. Please see [11] for a description of the CBC
   mode. The format of the AT_ENCR_DATA attribute 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_ENCR_DATA  | Length        |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                    Encrypted Data                             .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The encryption key (K_encr) is derived is derived from the AKA
   integrity key (IK) and cipher key (CK), as specified in Section 9.
   The plaintext consists of nested EAP/AKA attributes.


8. Messages




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8.1. 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 subscriber's IMSI is used as the client
   identifier. When used in a roaming environment, the NAI is composed
   of a username and a realm, separated with "@" (username@realm). The
   username portion identifies the subscriber within the realm.

   There are two types of NAI username portions in EAP AKA: non-
   pseudonym permanent usernames and pseudonym usernames. When identity
   privacy is not used, the non-pseudonym permanent username is used.
   The non-pseudonym permanent username is of the format "0imsi". In
   other words, the first character of the username is the digit zero
   (ASCII value 0x30), followed by the IMSI. The IMSI is an ASCII
   string that consists of not more than 15 decimal digits (ASCII
   values between 0x30 and 0x39) as specified in [13].

   The EAP server MAY use the leading "0" as a hint to try EAP/AKA as
   the first authentication method during method negotiation, rather
   than for example EAP/SIM. The EAP/AKA server MAY propose EAP/AKA
   even if the leading character was not "0".

   When the optional identity privacy support is used, the client MAY
   use the pseudonym received as part of the previous authentication
   sequence as the username portion of the NAI, as specified in Section
   5. The client MUST NOT modify the pseudonym received in
   AT_PSEUDONYM. For example, the client MUST NOT append any leading
   characters in the pseudonym.

   The AAA network routes AAA requests to the correct AAA server using
   the realm part of the NAI. The realm part MAY be decided by the
   operator and it MAY be a configurable parameter in the EAP/AKA
   client implementation. In this case, the client is typically
   configured with the NAI realm of the home operator.
   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. Network operators that wish to
   apply this approach must make the necessary arrangements before this
   special routing can be enabled. Operators MAY reserve a specific
   realm portion of NAI for EAP AKA users. This convention makes it
   easy to recognize that the NAI identifies a UMTS or GSM subscriber.
   Such reserved NAI realm may be useful as a hint as to the first
   authentication method to use during method negotiation.

   If no configured realm name is available in the client, the client
   MAY derive the realm name from the IMSI. The IMSI is composed of a
   three digit Mobile Country Code (MCC), a two or three digit Mobile
   Network Code (MNC) and a not more than 10 digit Mobile Subscriber

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   Identification Number (MSIN). In other words, the IMSI is a string
   of not more than 15 digits. MCC and MNC uniquely identify the
   operator. A NAI realm name can be derived from the IMSI by
   concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits
   of IMSI and ".owlan.org". For example, if the IMSI is
   123456789098765, and the MNC is three digits long, then the derived
   realm name is "mnc456.mcc123.owlan.org".

   If the client is not able to determine whether the MNC is two or
   three digits long, the client MAY use a 3-digit MNC. If the correct
   length of the MNC is two, then the MNC used in the realm name will
   include the first digit of MSIN. Hence, when configuring AAA
   networks for operators that have 2-digit MNC's, the network SHOULD
   also be prepared for realm names with incorrect 3-digit MNC's.

8.2. EAP-Request/AKA-Challenge

   The format of the EAP-Request/AKA-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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    AT_RAND    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             RAND                              |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    AT_AUTN    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                        AUTN (optional)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_IV     | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                 Initialization Vector (optional)              |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_ENCR_DATA  | Length        |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                    Encrypted Data (optional)                  |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_MAC    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           MAC (optional)                      |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      1 for Request

   Identifier

      See [6]



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   Length

      The length of the EAP Request packet.

   Type

      TBD

   Subtype

      1 for AKA-Challenge

   Reserved

      Set to zero when sending, ignored on reception.

   AT_RAND

      The value field of this attribute contains two reserved bytes
      followed by the AKA RAND parameter, 16 bytes (128 bits). The
      reserved bytes are set to zero when sending and ignored on
      reception. The AT_RAND attribute MUST be present in EAP-
      Request/AKA-Challenge.

   AT_AUTN

      The value field of this attribute contains two reserved bytes
      followed by the AKA AUTN parameter, 16 bytes (128 bits). The
      reserved bytes are set to zero when sending and ignored on
      reception. The AT_AUTN attribute MUST NOT be included in the GSM
      compatible mode of this protocol; otherwise it MUST be included.

   AT_IV

      See Section 7.2.

   AT_ENCR_DATA

      See Section 7.2. The nested attributes that are included in the
      plaintext of AT_ENCR_DATA are described below.

   AT_MAC

      See Section 7.1.

   In the EAP-Request/AKA-Challege message, the AT_IV, AT_ENCR_DATA and
   AT_MAC attributes are used for IMSI privacy. The plaintext of the
   AT_ENCR_DATA value field consists of nested attributes, which are
   shown below. Later versions of this protocol MAY specify additional
   attributes to be included within the encrypted data.





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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_PSEUDONYM  | Length        | Actual Pseudonym Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                           Pseudonym                           .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AT_PADDING   | Length        | Padding...                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   AT_PSEUDONYM

      This attribute is optional. The value field of this attribute
      begins with 2-byte actual pseudonym length, which specifies the
      length of the pseudonym in bytes. This field is followed by a
      pseudonym username, of the indicated actual length, that the
      client can use in the next authentication, as described in
      Section 5. The username does not include any terminating null
      characters. Because the length of the attribute must be a
      multiple of 4 bytes, the sender pads the pseudonym with zero
      bytes when necessary.

   AT_PADDING

      The encryption algorithm 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 AT_ENCR_DATA.
      The AT_PADDING attribute is not included if the total length of
      other nested attributes within the AT_ENCR_DATA attribute is a
      multiple of 16 bytes. As usual, the Length of the Padding
      attribute includes the Attribute Type and Attribute Length
      fields. The Length of the Padding attribute is 4, 8 or 12 bytes.
      It is chosen so that the length of the value field of the
      AT_ENCR_DATA attribute becomes a multiple of 16 bytes. 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
      set to zero, and silently drop the message if this verification
      fails.

8.3. EAP-Response/AKA-Challenge

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

   As specified in Section 7, EAP-Response/AKA-Challenge MAY include
   the AT_MAC attribute to integrity protect the EAP packet. Later
   versions of this protocol MAY make use of the AT_ENCR_DATA and AT_IV
   attributes in this message to include encrypted (skippable)
   attributes. AT_MAC, AT_ENCR_DATA and AT_IV attributes are not shown

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   in the figure below. If present, they are processed as in EAP-
   Request/AKA-Challenge packet. The EAP server MUST process EAP-
   Response/AKA-Challenge messages that include these attributes even
   if the server did not implement these optional attributes.

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_RES    |    Length     |          RES Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                                                               |
   |                             RES                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      See [6]

   Length

      The length of the EAP Response packet.

   Type

      TBD

   Subtype

      1 for AKA-Challenge

   Reserved

      Set to zero when sending, ignored on reception.

   AT_RES

      This attribute MUST be included in EAP-Response/AKA-Challenge.
      The value field of this attribute begins with the 2-byte RES
      Length, which is identifies the exact length of the RES (or SRES)
      in bits. The RES length is followed by the UMTS AKA RES or GSM
      SRES parameter. According to the specification [14] the length of
      the AKA RES can vary between 32 and 128 bits. The GSM SRES

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      parameter is always 32 bits long. Because the length of the
      AT_RES attribute must be a multiple of 4 bytes, the sender pads
      the RES with zero bits where necessary.

8.4. EAP-Response/AKA-Authentication-Reject

   The format of the EAP-Response/AKA-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     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      See [6]

   Length

      The length of the EAP Response packet.

   Type

      TBD

   Subtype

      2 for AKA-Authentication-Reject

   Reserved

      Set to zero on sending, ignored on reception.


8.5. EAP-Response/AKA-Synchronization-Failure

   The format of the EAP-Response/AKA-Synchronization-Failure 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
   |    AT_AUTS    | Length = 4    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                             AUTS                              |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      See [6]

   Length

      The length of the EAP Response packet, 20.

   Type

      TBD

   Subtype

      4 for AKA-Synchronization-Failure

   AT_AUTS

      This attribute MUST be included in EAP-Response/AKA-
      Synchronization-Failure. The value field of this attribute
      contains the AKA AUTS parameter, 112 bits (14 bytes).

8.6. EAP-Request/AKA-Identity

   The format of the EAP-Request/AKA-Identity 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     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AT_ID..._REQ   | Length = 1    |           Reserved            |

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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AT_PERM..._REQ | Length = 1    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The semantics of the fields is described below:

   Code

      1 for Request

   Identifier

      See [6]

   Length

      The length of the EAP Request packet.

   Type

      TBD

   Subtype

      5 for AKA-Identity

   Reserved

      Set to zero on sending, ignored on reception.

   AT_PERMANENT_IDENTITY_REQ

      The AT_PERMANENT_IDENTITY_REQ attribute is optional and it is
      included in the cases defined in Section 5. It MUST NOT be
      included if AT_IDENTITY_REQ is included. The value field only
      contains two reserved bytes, which are set to zero on sending and
      ignored on reception.

   AT_IDENTITY_REQ

      The AT_IDENTITY_REQ attribute is optional and it is included in
      the cases defined in Section 4. It MUST NOT be included if
      AT_PERMANENT_IDENTITY_REQ is included. The value field only
      contains two reserved bytes, which are set to zero on sending and
      ignored on reception.

8.7. EAP-Response/AKA-Identity

   The format of the EAP-Response/AKA-Identity 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_PERM...    | Length        | Actual Identity Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .               Cleartext Identity (optional)                   .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_IDENTITY   | Length        | Actual Identity Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                 Current Identity (optional)                   .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      See [6]

   Length

      The length of the EAP Response packet.

   Type

      TBD

   Subtype

      5 for AKA-Identity

   Reserved

      Set to zero on sending, ignored on reception.

   AT_PERMANENT_IDENTITY

      This attribute is optional and it is included in EAP-
      Response/AKA-Identity in cases specified in Section 5. It MUST
      NOT be included if AT_IDENTITY is included. The value field of
      this attribute begins with 2-byte actual identity length, which

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      specifies the length of the identity in bytes. This field is
      followed by the non-pseudonym permanent Network Access Identifier
      username portion of the indicated actual length. The EAP/AKA
      username format is specified in Section 8.1. The username does
      not include any terminating null characters. Because the length
      of the attribute must be a multiple of 4 bytes, the sender pads
      the identity with zero bytes when necessary.

   AT_IDENTITY

      The AT_IDENTITY attribute is optional and it is included in cases
      defined in Section 4. It MUST NOT be included if
      AT_PERMANENT_IDENTITY is included. The value field of this
      attribute begins with 2-byte actual identity length, which
      specifies the length of the identity in bytes. This field is
      followed by the Network Access Identifier username portion of the
      indicated actual length. The username format is specified in
      Section 8.1. The username is either the non-pseudonym permanent
      username or a pseudonym username. The username does not include
      any terminating null characters. Because the length of the
      attribute must be a multiple of 4 bytes, the sender pads the
      identity with zero bytes when necessary.

9. Key Derivation

   This section specifies how EAP AKA keying material is derived from
   the IK and CK keys. Because IK and CK are not available in the GSM
   mode, this key derivation specification can only be applied in the
   UMTS AKA mode.

   EAP AKA requires two keys for its own purposes, an integrity
   protection key K_int and an encryption key K_encr, to be used with
   the AT_MAC and AT_ENCR_DATA attributes. In addition, it is possible
   to derive additional key material, such as a master key to be used
   with IEEE 802.11i.

   Key derivation is based on the random number generation specified in
   NIST Federal Information Processing Standards Publication 186-2
   [15]. The random number generator is specified in the change notice
   1 (2001 October 5)of [15] (Algorithm 1). As specified in the change
   notice (page 74), when Algorithm 1 is used as a general-purpose
   random number generator, the "mod q" term in step 3.3 is omitted.
   The function G used in the algorithm is constructed via Secure Hash
   Standard as specified in Appendix 3.3 of the standard.

   160-bit XKEY and XVAL values are used, so b = 160. The initial
   secret seed value XKEY is computed from the AKA integrity key IK and
   cipher key CK with the following formula:

      XKEY = SHA1(IK|CK)

   The notation IK|CK denotes IK concatenated with CK.

   The optional user input values (XSEED_j) are set to zero.

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   The resulting 160-bit random numbers x_0, x_1, ..., x_m-1 are
   concatenated and partitioned into suitable-sized chunks and used as
   keys in the following order: K_encr (128 bits), K_int (128 bits),
   EAP application specific keys. The number of random number generator
   iterations (m) depends on the amount of required keying material.

   Even if K_encr or K_int were not used in the particular
   authentication sequence, they are derived and the EAP application
   specific material begins after K_int.

   For example, the EAP application specific material can be used for
   packet security between the client and the authenticator. Because
   the required keying material depends on the EAP application and the
   EAP key derivation standardization has not been finalized yet, exact
   rules of key derivation cannot be given here. As a guideline, the
   EAP application specific keys resulting from the key expansion
   scheme is used in the following order:

      any master session keys required,
      any encryption keys required,
      any integrity protection keys required,
      any initialization vectors required

   If separate keys or IV's are required for each direction, then the
   downlink material (to protect traffic to user) is taken before the
   uplink material (to protect traffic from user).

10. 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

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   reject the authentication with the EAP-Response/AKA-GSM-
   Authentication-Reject packet.

   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


11. IANA and Protocol Numbering Considerations

   The realm name "owlan.org" has been reserved for NAI realm names
   generated from the IMSI.

   IANA has assigned the number 23 for EAP AKA authentication.

   EAP AKA messages include a Subtype field. The following Subtypes are
   specified:

        AKA-Challenge...................................1
        AKA-Authentication-Reject.......................2
        AKA-Synchronization-Failure.....................4
        AKA-Identity....................................5

   The Subtype-specific data is composed of attributes, which have
   attribute type numbers. The following attribute types are specified:

        AT_RAND.........................................1
        AT_AUTN.........................................2
        AT_RES..........................................3
        AT_AUTS.........................................4
        AT_PERMANENT_IDENTITY...........................5
        AT_PADDING......................................6
        AT_PERMANENT_IDENTITY_REQ.......................7
        AT_IDENTITY_REQ.................................8
        AT_IDENTITY.....................................9

        AT_IV.........................................129
        AT_ENCR_DATA..................................130
        AT_MAC........................................131
        AT_PSEUDONYM..................................132

   All requests for value assignment from the various number spaces
   described in this document require proper documentation, according
   to the "Specification Required" policy described in [16]. Requests
   must be specified in sufficient detail so that interoperability
   between independent implementations is possible. Possible forms of
   documentation include, but are not limited to, RFCs, the products of


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   another standards body (e.g. 3GPP), or permanently and readily
   available vendor design notes.

12. 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.

13. 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 and Contributions

   The authors wish to thank Rolf Blom of Ericsson, Bernard Aboba of
   Microsoft, Arne Norefors of Ericsson, N.Asokan of Nokia, Valtteri
   Niemi of Nokia, Kaisa Nyberg of Nokia, Jukka-Pekka Honkanen of Nokia
   and Olivier Paridaens of Alcatel for interesting discussions in this
   problem space.

   The identiy privacy support is based on the identity privacy support
   of [8]. The attribute format is based on the extension format of
   Mobile IPv4 [17].

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. (NORMATIVE)

   [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. (NORMATIVE)


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

   [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.
         (INFORMATIVE)

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

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

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

   [8]   J. Carlson, B. Aboba, H. Haverinen, "EAP SRP-SHA1
         Authentication Protocol", draft-ietf-pppext-eap-srp-03.txt,
         July 2001 (work-in-progress). (INFORMATIVE)

   [9]   H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for
         Message Authentication", RFC2104, February 1997. (NORMATIVE)

   [10]  Federal Information Processing Standard (FIPS) draft standard,
         "Advanced Encryption Standard (AES)",
         http://csrc.nist.gov/publications/drafts/dfips-AES.pdf,
         September 2001. (NORMATIVE)

   [11]  US National Bureau of Standards, "DES Modes of Operation",
         Federal Information Processing Standard (FIPS) Publication 81,
         December 1980. (NORMATIVE)

   [12]  Federal Information Processing Standard (FIPS) Publication
         180-1, "Secure Hash Standard," National Institute of Standards
         and Technology, U.S. Department of Commerce, April 17, 1995.
         (NORMATIVE)

   [13]  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. (NORMATIVE)

   [14]  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 (NORMATIVE)



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   [15]  Federal Information Processing Standards (FIPS) Publication
         186-2 (with change notice), "Digital Signature Standard
         (DSS)", National Institute of Standards and Technology,
         January 27, 2000, (NORMATIVE)
         Available on-line at:
         http://csrc.nist.gov/publications/fips/fips186-2/
         fips186-2-change1.pdf

   [16]  T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", RFC 2434, October 1998.
         (NORMATIVE)

   [17]  C. Perkins (editor), "IP Mobility Support", RFC 2002, October
         1996. (INFORMATIVE)








































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