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                                                               J. Arkko
   Internet Draft                                              Ericsson
   Document: draft-arkko-pppext-eap-aka-07.txt             H. Haverinen
   Expires: March 2003                                            Nokia
                                                          December 2002


                          EAP AKA Authentication


Status of this Memo

   This document is an Internet-Draft and is subject to 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
   Universal Mobile Telecommunications System (UMTS) Authentication and
   Key Agreement (AKA) mechanism. UMTS AKA is based on symmetric keys,
   and runs typically in a UMTS Subscriber Identity Module, a smart
   card like device.

   EAP AKA includes optional identity privacy support and an optional
   re-authentication procedure.


Table of Contents

   Status of this Memo................................................1
   Abstract...........................................................1
   1. Introduction and Motivation.....................................2
   2. Terms and Conventions Used in This Document.....................4
   3. Protocol Overview...............................................6
   4. Identity Management............................................10
   4.1. User Identity in EAP-Response/Identity.......................10

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   4.2. Obtaining Subscriber Identity via EAP AKA Messages...........12
   4.3. Identity Privacy Support.....................................14
   5. Re-authentication..............................................19
   6. Message Format.................................................25
   7. Message Authentication and Encryption..........................26
   7.1. AT_MAC Attribute.............................................26
   7.2. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes................27
   8. Messages.......................................................28
   8.1. EAP-Request/AKA-Challenge....................................28
   8.2. EAP-Response/AKA-Challenge...................................32
   8.3. EAP-Response/AKA-Authentication-Reject.......................33
   8.4. EAP-Response/AKA-Synchronization-Failure.....................34
   8.5. EAP-Request/AKA-Identity.....................................35
   8.6. EAP-Response/AKA-Identity....................................36
   8.7. EAP-Request/AKA-Reauthentication.............................37
   8.8. EAP-Response/AKA-Reauthentication............................40
   8.9. EAP/AKA Notifications........................................42
   9. Error Cases....................................................45
   10. Key Derivation................................................45
   11. IANA and Protocol Numbering Considerations....................47
   12. Security Considerations.......................................48
   12.1. Identity Protection and Privacy.............................48
   12.2. Mutual Authentication.......................................49
   12.3. Key Derivation Considerations...............................49
   12.4. Brute-Force and Dictionary Attacks..........................49
   12.5. Packet Modification Attacks.................................49
   12.6. Negotiation Attacks.........................................49
   12.7. Fast Reconnect..............................................50
   12.8. Unreliable Media............................................50
   12.9. Man-in-the-middle Attacks...................................50
   12.10. Generating Random Numbers..................................50
   13. Intellectual Property Right Notices...........................51
   Acknowledgements and Contributions................................51
   Authors' Addresses................................................51
   Annex A. Key Derivation for IEEE 802.11...........................52
   Annex B. Pseudo-Random Number Generator...........................53

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].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). Compared to the GSM mechanism, UMTS AKA provides
   substantially longer key lengths and mutual authentication.


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   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 802.1x
     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.

   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


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   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. Terms and Conventions Used in This Document

   The following terms will be used through this document:


   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.




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   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].

   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]




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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 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 message flow below shows the basic successful full
   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 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 4.2 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. EAP AKA packets encapsulate
   parameters in attributes, encoded in a Type, Length, Value format.
   The packet format and the use of attributes are specified in Section
   6. The EAP-Request/AKA-Challenge message contains a random number
   (AT_RAND) and an authorization vector (AT_AUTN), and a message
   authentication code AT_MAC. The EAP-Request/AKA-Challenge message
   MAY optionally contain encrypted data, which is used for Identity
   privacy support, as described in Section 4.3. The AT_MAC attribute
   contains a message authentication code covering the EAP packet. 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 authenticate the client, and the AT_MAC
   attribute to integrity protect the EAP message.














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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     |
          |                         (AT_RAND, AT_AUTN, AT_MAC)    |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM,|                     |
      | verifies AUTN and MAC, derives RES  |                     |
      | and session key                     |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/AKA-Challenge                            |
          | (AT_RES, AT_MAC)                                      |
          |------------------------------------------------------>|
          |                                                       |
          |                          +--------------------------------+
          |                          | Server checks the given RES,   |
          |                          | and MAC and finds them correct.|
          |                          +--------------------------------+
          |                                                       |
          |                                          EAP-Success  |
          |<------------------------------------------------------|


   The second message flow shows how the EAP server rejects the Client
   due to a failed authentication. The same flow is also used in the
   GSM compatible mode, except that the AT_AUTN attribute and AT_MAC
   attribute are not used in the messages.














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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        |
          |                      (AT_RAND, AT_AUTN, AT_MAC)       |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM,|                     |
      | possibly verifies AUTN, and sends an|                     |
      | invalid response                    |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/AKA-Challenge                            |
          | (AT_RES, AT_MAC)                                      |
          |------------------------------------------------------>|
          |                                                       |
          |              +------------------------------------------+
          |              | Server checks the given RES and the MAC, |
          |              | and finds one of them incorrct.          |
          |              +------------------------------------------+
          |                                                       |
          |                                          EAP-Failure  |
          |<------------------------------------------------------|


   The next message flow shows the client rejecting the AUTN of the EAP
   server.

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








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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/AKA-Challenge     |
          |                         (AT_RAND, AT_AUTN, AT_MAC)    |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM |                     |
      | and discovers AUTN that can not be  |                     |
      | verified                            |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/AKA-Authentication-Reject                |
          |------------------------------------------------------>|
          |                                                       |
          |                                                       |
          |                                          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. 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/AKA-Challenge     |
          |                         (AT_RAND, AT_AUTN, AT_MAC)    |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM |                     |
      | and discovers AUTN that contains an |                     |
      | inappropriate sequence number       |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/AKA-Synchronization-Failure              |
          | (AT_AUTS)                                             |
          |------------------------------------------------------>|
          |                                                       |
          |                              +---------------------------+
          |                              | Perform resynchronization |
          |                              | Using AUTS and            |
          |                              | the sent RAND             |
          |                              +---------------------------+
          |                                                       |

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

   In addition to the full authentication scenarios described above,
   EAP AKA includes a re-authentication procedure, which is specified
   in Section 5.

4. Identity Management

   This section specifies user identity management and identity privacy
   support.

4.1. User Identity in EAP-Response/Identity

   In the beginning of an 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].

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   UMTS subscribers are identified with the International Mobile
   Subscriber Identity (IMSI) [8]. 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
   Identification Number (MSIN). In other words, the IMSI is a string
   of not more than 15 digits. MCC and MNC uniquely identify the
   operator.

   Internet AAA protocols identify users with the Network Access
   Identifier (NAI) [5]. 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. The AAA nodes use the realm portion of the NAI to
   route AAA requests to the correct AAA server. The realm name used in
   this protocol MAY be chosen 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. Operators MAY reserve a specific realm name for
   EAP/AKA users. This convention makes it easy to recognize that the
   NAI identifies a subscriber that uses EAP/AKA. Such a reserved NAI
   realm may be a useful hint to the first authentication method to use
   during method negotiation.

   There are three types of NAI username portions in EAP/AKA: non-
   pseudonym permanent usernames, pseudonym usernames and re-
   authentication usernames. The first two are only used on full
   authentication and the last one only on re-authentication. When the
   optional identity privacy support is not used, the non-pseudonym
   permanent username is used.

   The non-pseudonym permanent username MAY be derived from the IMSI.
   In this case, the permanent username MUST be 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 [8]

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

   Alternatively, an  implementation may choose a permanent username
   that is not based on the IMSI. In this case the selection of the
   username, its format, and its processing is a local matter. In this
   case, the client implementation MUST NOT append any leading
   characters to the username.

   When the optional identity privacy support is used on full
   authentication, the client MAY use the pseudonym received upon the
   previous full authentication sequence as the username portion of the
   NAI, as specified in Section 4.3. The client MUST NOT modify the


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   pseudonym received in AT_NEXT_PSEUDONYM. For example, the client
   MUST NOT append any leading characters in the pseudonym.

   On re-authentication, the client uses the re-authentication identity
   received upon the previous authentication sequence as the NAI. A new
   re-authentication identity may be delivered as part of both full
   authentication and re-authentication. The client MUST NOT modify the
   re-authentication identity received in AT_NEXT_REAUTH_ID but the
   client must use the re-authentication identity as it is. For
   example, the client MUST NOT append any leading characters in the
   re-authentication identity.

   If no configured realm name is available, the client MAY derive the
   realm name from the MCC and MNC portions of the IMSI. A recommended
   way to derive the realm from the IMSI will be specified in [9].
   Alternatively, the realm name may be obtained 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 MNCs, the network SHOULD
   also be prepared for realm names with incorrect 3-digit MNCs.

4.2. 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 not all EAP implementations support
   multiple EAP Identity requests..

   If the EAP server has not received any identity (permanent identity,
   pseudonym or re-authentication identity) from the client when
   sending the first EAP/AKA request, then the EAP server may issue the
   EAP-Request/AKA-Identity packet and includes the AT_ANY_ID_REQ
   attribute (specified in Section 8.5). This attribute does not
   contain any data.

   The AT_ANY_ID_REQ attribute requests the client to include the
   AT_IDENTITY attribute (specified in Section 8.6) in the EAP-
   Response/AKA-Identity packet. The identity format in the AT_IDENTITY
   attribute is the same as in the Type-Data field of the EAP-
   Response/Identity packet. The AT_IDENTITY attribute contains a
   permanent identity, a pseudonym identity or a re-authentication
   identity. If the server does not support re-authentication, it uses
   the AT_FULLAUTH_ID_REQ attribute instead of the AT_ANY_ID_REQ
   attribute to directly request for a full authentication identity
   (either the permanent identity or a pseudonym identity). If the

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   server uses the AT_FULLAUTH_ID_REQ attribute, the client MUST NOT
   use a re-authentication identity in the AT_IDENTITY attribute.

   The use of pseudonyms for anonymity is specified in Section 4.3. The
   use of re-authentication identities is specified in Section 5.

   The full authentication case is illustrated in the figure below. In
   this case, AT_IDENTITY contains either the permanent identity or a
   pseudonym identity. The same sequence is also used in case the
   server uses the AT_FULLAUTH_ID_REQ in EAP-Request/AKA-Identity

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

   If the client wants to perform full authentication, it includes the
   permanent identity or a pseudonym identity in the AT_IDENTITY
   attribute. The client may use these identities in response to either
   AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ. If the server uses the
   AT_ANY_ID_REQ and the client wants to perform re-authentication,
   then the client includes a re-authentication identity in the
   AT_IDENTITY attribute.

   If the client uses its full authentication identity and the
   AT_IDENTITY attribute contains a valid permanent identity or a valid
   pseudonym identity that the EAP server is able to decode to the
   permanent identity, then the full authentication sequence proceeds
   as usual with the EAP Server issuing the EAP-Request/AKA-Challenge
   message.

   On re-authentication, if the AT_IDENTITY attribute contains a valid
   re-authentication identity and the server agrees on using re-
   authentication, then the server proceeds with the re-authentication
   sequence and issues the EAP-Request/AKA-Reauthentication packet, as
   specified in Section 5. If the server does not recognize the re-
   authentication identity, then it issues a second EAP-Request/AKA-
   Identity message and includes the AT_FULLAUTH_ID_REQ attribute. In
   this case, a second EAP/AKA-Identity round trip is required. The
   messages used on the first roundtrip are ignored. This is
   illustrated below.

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       Client                                             Authenticator
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not have any     |
          |                            | Subscriber identity available|
          |                            | When starting EAP/AKA        |
          |                            +------------------------------+
          |                                                       |
          |        EAP-Request/AKA-Identity                       |
          |        (AT_ANY_ID_REQ)                                |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY containing a re-authentication identity) |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not recognize    |
          |                            | The re-authentication        |
          |                            | Identity                     |
          |                            +------------------------------+
          |                                                       |
          |     EAP-Request/AKA-Identity                          |
          |     (AT_FULLAUTH_ID_REQ)                              |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with a full-auth. Identity)              |
          |------------------------------------------------------>|
          |                                                       |

   If the server recognizes the re-authentication identity, but still
   wants to fall back on full authentication, the server may issue the
   EAP-Request/AKA-Challenge packet. In this case, the full
   authentication procedure proceeds as usual.

   An extra EAP/AKA-Identity round trip is also required in cases when
   the AT_IDENTITY attribute contains a pseudonym identity that the EAP
   server fails to decode. The operation in this case is specified in
   Section 4.3.

4.3. Identity Privacy Support

   EAP/AKA includes optional identity privacy (anonymity) support that
   can be used to hide the cleartext permanent identity and to make the
   subscriber's connections unlinkable to eavesdroppers. Identity
   privacy is based on temporary identities, or pseudonyms, which are
   equivalent to but separate from the Temporary Mobile Subscriber
   Identities (TMSI) that are used on cellular networks. Please see
   Section 12.1 for security considerations concerning identity
   privacy.

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                        EAP AKA Authentication           December 2002


   If identity privacy is not used or if the client does not have any
   pseudonyms or re-authentication identities available, the client
   transmits the permanent identity in the EAP-Response/Identity packet
   or in the AT_IDENTITY attribute.

   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 8.1. Because the identity
   privacy support is optional to implement, the client MAY ignore the
   AT_IV and AT_ENCR_DATA attributes and always transmit the permanent
   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_MAC attribute before looking at the AT_ENCR_DATA attribute. If
   the AT_MAC is invalid, then the client MUST silently discard the EAP
   packet. If the AT_MAC attribute is valid, then the client MAY
   decrypt the encrypted data in AT_ENCR_DATA and use the obtained
   pseudonym on the next full authentication.

   If the client does not receive a new pseudonym in the EAP-
   Request/AKA-Challenge message, the client MAY use an old pseudonym
   instead of the permanent identity on next full authentication.

   The EAP server produces pseudonyms in an implementation-dependent
   manner. Only the EAP server needs to be able to map the pseudonym to
   the permanent identity. Regardless of construction method, the
   pseudonym MUST conform to the grammar specified for the username
   portion of an NAI.

   In any case, it is necessary that permanent usernames and pseudonyms
   are separate and recognizable from each other. It is also desirable
   that EAP SIM and EAP AKA usernames be recognizable from each other
   as an aid for the server to which method to offer.

   In general, it is the task of the EAP server and the policies of its
   administrator to ensure sufficient separation in the usernames.
   Pseudonyms, for instance, are both produced and used by the EAP
   server. The EAP server MUST compose pseudonyms so that it can
   recognize if a NAI username is an EAP AKA pseudonym. For instance,
   when the usernames have been derived from the IMSI, the pseudonym
   could begin with a leading "2" character.

   The client MAY transmit the received pseudonym in the first EAP-
   Response/Identity packet of the next full authentication with the
   EAP server. The client concatenates the received pseudonym with the
   "@" character and the NAI realm portion. The client selects the
   realm name portion similarly as it select the realm name portion
   when using the permanent identity. If the EAP server successfully
   decodes the pseudonym received in the EAP-Response/Identity packet
   to a known client permanent identity, the authentication proceeds
   with the EAP-Request/AKA-Challenge message as usual.

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   Because the client may fail to save a pseudonym sent to in an EAP-
   Request/AKA-Challenge, for example due to malfunction, the EAP
   server SHOULD maintain at least one old pseudonym in addition to the
   most recent pseudonym.

   If the EAP server requests the client to include its identity in the
   EAP-Response/AKA-Identity packet, as specified in Section 4.2, the
   client MAY transmit the received pseudonym in the AT_IDENTITY
   attribute. 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 identity,
   then the EAP server requests the permanent identity (non-pseudonym
   identity) by including the AT_PERMANENT_ID_REQ attribute (Section
   8.5) in the EAP-Request/AKA-Challenge message.

   The EAP server issues the EAP-Request/AKA-Identity message also in
   the case when it received the undecodable pseudonym in AT_IDENTITY
   included in the EAP-Response/AKA-Identity packet. In this case, a
   second EAP/AKA-Identity round trip is required.

   A received AT_PERMANENT_ID_REQ does not necessarily originate from
   the valid network, but an active attacker may transmit an EAP-
   Request/AKA-Identity packet with an AT_PERMANENT_ID_REQ attribute to
   the client, in an effort to find out the true identity of the user.
   On receipt of EAP-Request/AKA-Identity that includes
   AT_PERMANENT_ID_REQ, the client MAY delay the processing of the
   message for a while in order to wait for another EAP AKA message
   that does not include the AT_PERMANENT_ID_REQ attribute.

   Basically, there are two different policies that the client can
   employ with regard to AT_PERMANENT_ID_REQ. A "conservative" client
   assumes that the network is able to maintain pseudonyms robustly.
   Therefore, if a conservative client has a pseudonym, the client
   silently ignores the EAP packet with AT_PERMANENT_ID_REQ, because
   the client believes that the valid network is able to decode the
   pseudonym. (Alternatively, the conservative client may respond to
   AT_PERMANENT_ID_REQ in certain circumstances, for example if the
   pseudonym was received a long time ago.) The benefit of this policy
   is that it protects the client against active attacks on anonymity.
   On the other hand, a "liberal" client always accepts the
   AT_PERMANENT_ID_REQ and responds with the permanent identity. The
   benefit of this policy is that it works even if the valid network
   sometimes loses pseudonyms and is not able to decode them to the
   permanent identity.

   The value field of the AT_PERMANENT_ID_REQ does not contain any data
   but the attribute is included to request the client to include the
   AT_IDENTITY attribute (Section 8.6) with the permanent
   authentication identity in the EAP-Response/AKA-Identity message. In
   this case, the AT_IDENTITY attribute contains the client's permanent
   identity in the clear.


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                        EAP AKA Authentication           December 2002

   Please note that the EAP/AKA client and the EAP/AKA server only
   process the AT_IDENTITY attribute. Entities that only pass EAP
   packets through do not process this attribute. 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 identity from the EAP-Response/Identity packet regardless
   if the decoding fails in the EAP server.

   The figure below illustrates the case when the EAP server fails to
   decode the pseudonym included in the EAP-Response/Identity packet.

       Client                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes a pseudonym)                                |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server fails to decode the   |
          |                            | Pseudonym.                   |
          |                            +------------------------------+
          |                                                       |
          |  EAP-Request/AKA-Identity                             |
          |  (AT_PERMANENT_ID_REQ)                                |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with permanent identity)                 |
          |------------------------------------------------------>|
          |                                                       |

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

   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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                        EAP AKA Authentication           December 2002

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

   In the worst case, there are three EAP/AKA-Identity round trips
   before the server has obtained an acceptable identity: on the first
   round, the client sends its re-authentication identity in
   AT_IDENTITY. The server fails to accept it and request a full
   authentication identity with a second EAP-Request/AKA-Identity. The
   client responds with a pseudonym identity in AT_IDENTITY. The server
   fails to decode the pseudonym and has to issue a third EAP-
   Request/AKA-Identity, including AT_PERMANENT_ID_REQ. Finally, the
   server accepts the client's EAP-Response/AKA-Identity with the
   AT_IDENTITY attribute and proceeds with full authentication. This is
   illustrated in the figure below.











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                        EAP AKA Authentication           December 2002

       Client                                             Authenticator
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not have any     |
          |                            | Subscriber identity available|
          |                            | When starting EAP/AKA        |
          |                            +------------------------------+
          |                                                       |
          |        EAP-Request/AKA-Identity                       |
          |        (AT_ANY_ID_REQ)                                |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with re-authentication identity)         |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not accept       |
          |                            | The re-authentication        |
          |                            | Identity                     |
          |                            +------------------------------+
          |                                                       |
          |     EAP-Request/AKA-Identity                          |
          |     (AT_FULLAUTH_ID_REQ)                              |
          |<------------------------------------------------------|
          |                                                       |
          |EAP-Response/AKA-Identity                              |
          |(AT_IDENTITY with a pseudonym identity)                |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server fails to decode the   |
          |                            | Pseudonym in AT_IDENTITY     |
          |                            +------------------------------+
          |                                                       |
          |           EAP-Request/AKA-Identity                    |
          |           (AT_PERMANENT_ID_REQ)                       |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with permanent identity)                 |
          |------------------------------------------------------>|
          |                                                       |

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

5. Re-authentication

   In some environments, EAP authentication may be performed
   frequently. Because the EAP AKA full authentication procedure makes
   use of the UMTS AKA algorithms, and it therefore requires fresh

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                        EAP AKA Authentication           December 2002

   authentication vectors from the Authentication Centre, the full
   authentication procedure may result in many network operations when
   used very frequently. Therefore, EAP AKA includes a more inexpensive
   re-authentication procedure that does not make use of the UMTS AKA
   algorithms and does not need new vectors from the Authentication
   Centre.

   Re-authentication is optional to implement for both the EAP AKA
   server and client. On each EAP authentication, either one of the
   entities may also fall back on full authentication if they do not
   want to use re-authentication.

   Re-authentication is based on the keys derived on the preceding full
   authentication. The same K_aut and K_encr keys as in full
   authentication are used to protect EAP AKA packets and attributes,
   and the original XKEY seed value from full authentication is used to
   generate fresh application specific keys, as specified in Section
   10.

   On re-authentication, the client protects against replays with an
   unsigned 16-bit counter, included in the AT_COUNTER attribute. On
   full authentication, both the server and the client initialize the
   counter to one. The counter value of at least one is used on the
   first re-authentication. On subsequent re-authentications, the
   counter MUST be greater than on any of the previous re-
   authentications. For example, on the second re-authentication,
   counter value is two or greater etc. The AT_COUNTER attribute is
   encrypted.

   The server includes an encrypted server nonce (AT_NONCE_S) in the
   re-authentication request. The AT_MAC attribute in the client's
   response is calculated over NONCE_S to provide a challenge/response
   authentication scheme. The NONCE_S also contributes to the new
   application specific keys.

   As discussed in Section 4.3, in some environments the client may
   assume that the network can reliably store pseudonyms and therefore
   the client may fail to respond to the AT_PERMANENT_ID_REQ attribute.
   The network SHOULD store pseudonyms on a reliable database. Because
   one of the objectives of the re-authentication procedure is to
   reduce load on the network, the re-authentication procedure does not
   require the EAP server to contact a reliable database. Therefore,
   the re-authentication procedure makes use of separate re-
   authentication user identities. Pseudonyms and the permanent
   identity are reserved for full authentication only. The network does
   not need to store re-authentication identities as carefully as
   pseudonyms. If a re-authentication identity is lost and the network
   does not recognize it, the EAP server can fall back on full
   authentication.

   If the EAP server supports re-authentication, it MAY include the
   skippable AT_NEXT_REAUTH_ID attribute in the encrypted data of EAP-
   Request/AKA-Challenge message. This attribute contains a new re-
   authentication identity for the next re-authentication. The client

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                        EAP AKA Authentication           December 2002

   MAY ignore this attribute, in which case it will use full
   authentication next time. If the client wants to use re-
   authentication, it uses this re-authentication identity on next
   authentication. Even if the client has a re-authentication identity,
   the client MAY discard the re-authentication identity and use a
   pseudonym or the permanent identity instead, in which case full
   authentication will be performed.

   The re-authentication identity received in AT_NEXT_REAUTH_ID
   contains both the username portion and the realm portion of the
   Network Access Identifier. The EAP Server can choose an appropriate
   realm part in order to have the AAA infrastructure route subsequent
   re-authentication related requests to the same AAA server. For
   example, the realm part MAY include a portion that is specific to
   the AAA server. Hence, it is sufficient to store the context
   required for re-authentication in the AAA server that performed the
   full authentication.

   The client MAY use the re-authentication identity in the EAP-
   Response/Identity packet or, in response to server's AT_ANY_ID_REQ
   attribute, the client MAY use the re-authentication identity in the
   AT_IDENTITY attribute of the EAP-Response/AKA-Identity packet.

   Even if the client uses a re-authentication identity, the server may
   want to fall back on full authentication, for example because the
   server does not recognize the re-authentication identity or does not
   want to use re-authentication. If the server was able to decode the
   re-authentication identity to the permanent identity, the server
   issues the EAP-Request/AKA-Challenge packet to initiate full
   authentication. If the server was not able to recover the client's
   identity from the re-authentication identity, the server starts the
   full authentication procedure by issuing an EAP-Request/AKA-Identity
   packet. This packet always starts a full authentication sequence if
   it does not include the AT_ANY_ID_REQ attribute. (As specified in
   Sections 4.2 and 4.3, the server MAY use AT_ANY_ID_REQ,
   AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ attributes if it does not
   know the client's identity.)

   Both the client and the server SHOULD have an upper limit for the
   number of subsequent re-authentications allowed before a full
   authentication needs to be performed. Because a 16-bit counter is
   used in re-authentication, the theoretical maximum number of re-
   authentications is reached when the counter value reaches 0xFFFF.

   In order to use re-authentication, the client and the server need to
   store the following values: original XKEY, K_aut, K_encr, latest
   counter value and the next re-authentication identity.

   The following figure illustrates the re-authentication procedure.
   Encrypted attributes are denoted with '*'. The client uses its re-
   authentication identity in the EAP-Response/Identity packet. As
   discussed above, an alternative way to communicate the re-
   authentication identity to the server is for the client to use the
   AT_IDENTITY attribute in the EAP-Response/AKA-Identity message. This

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                        EAP AKA Authentication           December 2002

   latter case is not illustrated in the figure below, and it is only
   possible when the server requests the client to send its identity by
   including the AT_ANY_ID_REQ attribute in the EAP-Request/AKA-
   Identity packet.

   If the server recognizes the re-authentication identity and agrees
   on using re-authentication, then the server sends the EAP-
   Request/AKA-Reauthentication packet to the client. This packet MUST
   include the encrypted AT_COUNTER attribute, with a fresh counter
   value, the encrypted AT_NONCE_S attribute that contains a random
   number chosen by the server, the AT_ENCR_DATA and the AT_IV
   attributes used for encryption, and the AT_MAC attribute that
   contains a message authentication code over the packet. The packet
   MAY also include an encrypted AT_NEXT_REAUTH_ID attribute that
   contains the next re-authentication identity.

   Re-authentication identities are one-time identities. If the client
   does not receive a new re-authentication identity, it MUST use
   either the permanent identity or a pseudonym identity on the next
   authentication to initiate full authentication.

   The client verifies that the counter value is fresh (greater than
   any previously used value). The client also verifies that AT_MAC is
   correct. The client MAY save the next re-authentication identity
   from the encrypted AT_NEXT_REAUTH_ID for next time. If all checks
   are successful, the client responds with the EAP-Response/AKA-
   Reauthentication packet, including the AT_COUNTER attribute with the
   same counter value and the AT_MAC attribute.

   The server verifies the AT_MAC attribute and also verifies that the
   counter value is the same that it used in the EAP-Request/AKA-
   Reauthentication packet. If these checks are successful, the re-
   authentication has succeeded and the server sends the EAP-Success
   packet to the client.





















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                        EAP AKA Authentication           December 2002

       Client                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes a re-authentication identity)               |
          |------------------------------------------------------>|
          |                                                       |
          |                          +--------------------------------+
          |                          | Server recognizes the identity |
          |                          | and agrees on using fast       |
          |                          | re-authentication              |
          |                          +--------------------------------+
          |                                                       |
          |  EAP-Request/AKA-Reauthentication                     |
          |  (AT_IV, AT_ENCR_DATA, *AT_COUNTER,                   |
          |   *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC)            |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
     +-----------------------------------------------+            |
     | Client verifies AT_MAC and the freshness of   |            |
     | the counter. Client MAY store the new re-     |            |
     | authentication identity for next re-auth.     |            |
     +-----------------------------------------------+            |
          |                                                       |
          | EAP-Response/AKA-Reauthentication                     |
          | (AT_IV, AT_ENCR_DATA, *AT_COUNTER with same value,    |
          |  AT_MAC)                                              |
          |------------------------------------------------------>|
          |                                                       |
          |                          +--------------------------------+
          |                          | Server verifies AT_MAC and     |
          |                          | the counter                    |
          |                          +--------------------------------+
          |                                                       |
          |                                          EAP-Success  |
          |<------------------------------------------------------|
          |                                                       |

   If the client does not accept the counter value of EAP-Request/AKA-
   Reauthentication, it indicates the counter synchronization problem
   by including the encrypted AT_COUNTER_TOO_SMALL in EAP-Response/AKA-
   Reauthentication. The server responds with EAP-Request/AKA-Challenge
   to initiate a normal full authentication procedure. This is
   illustrated in the following figure. Encrypted attributes are
   denoted with '*'.







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                        EAP AKA Authentication           December 2002

       Client                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes a re-authentication identity)               |
          |------------------------------------------------------>|
          |                                                       |
          |  EAP-Request/AKA-Reauthentication                     |
          |  (AT_IV, AT_ENCR_DATA, *AT_COUNTER,                   |
          |   *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC)            |
          |<------------------------------------------------------|
          |                                                       |
     +-----------------------------------------------+            |
     | AT_MAC is valid but the counter is not fresh. |            |
     +-----------------------------------------------+            |
          |                                                       |
          | EAP-Response/AKA-Reauthentication                     |
          | (AT_IV, AT_ENCR_DATA, *AT_COUNTER_TOO_SMALL,          |
          |  *AT_COUNTER, AT_MAC)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |            +----------------------------------------------+
          |            | Server verifies AT_MAC but detects           |
          |            | That client has included AT_COUNTER_TOO_SMALL|
          |            +----------------------------------------------+
          |                                                       |
          |                        EAP-Request/AKA-Challenge      |
          |<------------------------------------------------------|
          |                                                       |
     +---------------------------------------------------------------+
     |                Normal full authentication follows.            |
     +---------------------------------------------------------------+
          |                                                       |

   In the figure above, the first three messages are similar to the
   basic re-authentication case. When the client detects that the
   counter value is not fresh, it includes the AT_COUNTER_TOO_SMALL
   attribute in EAP-Response/AKA-Reauthentication. This attribute
   doesn't contain any data but it is a request for the server to
   initiate full authentication. In this case, the client MUST ignore
   the contents of the server's AT_NEXT_REAUTH_ID attribute.

   On receipt of AT_COUNTER_TOO_SMALL, the server verifies AT_MAC and
   verifies that AT_COUNTER contains the same as in the EAP-
   Request/AKA-Reauthentication packet. If not, the server silently
   discards the EAP-Response/AKA-Reauthentication packet. If all checks
   on the packet are successful, the server transmits a EAP-
   Request/AKA-Challenge packet and the full authentication procedure
   is performed as usual. Since the server already knows the subscriber
   identity, it MUST NOT use the EAP-Request/AKA-Identity packet to
   request the identity.


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                        EAP AKA Authentication           December 2002

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
   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 is 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 when searching for the next attribute. These
   attributes are called skippable attributes.

   EAP/AKA packets do not include a version field. However, should
   there be a 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.


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                        EAP AKA Authentication           December 2002

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

7. Message Authentication and Encryption

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

   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 it. For
   example, these attributes cannot be used in EAP-Request/AKA-
   Identity, because the RAND challenge has not yet been transmitted at
   that point. Integrity protection with AT_MAC MUST be used in all
   messages when keys have been derived.

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, concatenated with optional
   message-specific data, 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 contents of the message-specific data, if present, are specified
   separately for each EAP/AKA message. The message-specific data is
   included in order to protect data that is not transmitted with the
   EAP packet.

   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 [10] keyed hash value. (The
      HMAC-SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value

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                        EAP AKA Authentication           December 2002

      by truncating the output to 16 bytes. Hence, the length of the
      MAC is 16 bytes.) The message authentication key (K_aut) used in
      the calculation of the MAC is derived from the AKA integrity key
      (IK) and cipher key (CK), as specified in Section 10.

7.2. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes

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

   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) [11] 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 [12] 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                             .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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   The encryption key (K_encr) is derived is derived from the AKA
   integrity key (IK) and cipher key (CK), as specified in Section10.
   The plaintext consists of nested EAP/AKA attributes.

   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. The format of the AT_PADDING
   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_PADDING   | Length        | Padding...                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

8. Messages

8.1. 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                                   |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_IV     | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                 Initialization Vector (optional)              |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_ENCR_DATA  | Length        |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                    Encrypted Data (optional)                  |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_MAC    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           MAC                                 |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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

      23

   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 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

      AT_MAC MUST be included. In EAP-Request/AKA-Challenge, there is
      no message-specific data covered by the 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 Identity privacy and for
   communicating the next re-authentication identity. 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_NEXT_PS... | Length        | Actual Pseudonym Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                        Next Pseudonym                         .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_NEXT_REAU..| Length        | Actual Re-Auth Identity Length|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                   Next Re-authentication Username             .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AT_PADDING   | Length        | Padding...                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   AT_NEXT_PSEUDONYM

      This attribute is optional. The value field of this attribute
      begins with a 2-byte actual pseudonym length, which specifies the
      length of the pseudonym in bytes. This field is followed by a
      pseudonym user name, of the indicated actual length, that the
      client can use in the next authentication, as described in
      Section 4.3. The user name 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_NEXT_REAUTH_ID

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

   AT_PADDING

      AT_PADDING is optional to include. See Section 7.2.


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8.2. EAP-Response/AKA-Challenge

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

   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
   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                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_MAC    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           MAC                                 |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      See [6]

   Length

      The length of the EAP Response packet.

   Type

      23




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   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 in bits.
      The RES length is followed by the UMTS AKA RES parameter.
      According to the specification [13] the length of the AKA RES can
      vary between 32 and 128 bits. 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.

   AT_MAC

      AT_MAC MUST be included. In EAP-Response/AKA-Challenge, there is
      no message-specific data covered by the MAC. See Section 7.1.

8.3. 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

      23

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   Subtype

      2 for AKA-Authentication-Reject

   Reserved

      Set to zero on sending, ignored on reception.


8.4. EAP-Response/AKA-Synchronization-Failure

   The format of the EAP-Response/AKA-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     |           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

      23

   Subtype

      4 for AKA-Synchronization-Failure






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   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).

8.5. 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_PERM..._REQ | Length = 1    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AT_FULL..._REQ | Length = 1    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AT_ANY_ID_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

      23

   Subtype

      5 for AKA-Identity

   Reserved

      Set to zero on sending, ignored on reception.

   AT_PERMANENT_ID_REQ

      The AT_PERMANENT_ID_REQ attribute is optional to include and it
      is included in the cases defined in Section 4.3. It MUST NOT be

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                        EAP AKA Authentication           December 2002

      included if AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ is included. The
      value field only contains two reserved bytes, which are set to
      zero on sending and ignored on reception.

   AT_FULLAUTH_ID_REQ

      The AT_FULLAUTH_ID_REQ attribute is optional to include and it is
      included in the cases defined in Section 4.2. It MUST NOT be
      included if AT_ANY_ID_REQ or AT_PERMANENT_ID_REQ is included. The
      value field only contains two reserved bytes, which are set to
      zero on sending and ignored on reception.

   AT_ANY_ID_REQ

      The AT_ANY_ID_REQ attribute is optional and it is included in the
      cases defined in Section 4.2. It MUST NOT be included if
      AT_PERMANENT_ID_REQ or AT_FULLAUTH_ID_REQ is included. The value
      field only contains two reserved bytes, which are set to zero on
      sending and ignored on reception.

8.6. EAP-Response/AKA-Identity

   The format of the EAP-Response/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_IDENTITY   | Length        | Actual Identity Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                 Current Identity                              .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      See [6]

   Length

      The length of the EAP Response packet.



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   Type

      23

   Subtype

      5 for AKA-Identity

   Reserved

      Set to zero on sending, ignored on reception.

   AT_IDENTITY

      The AT_IDENTITY attribute is optional to include and it is
      included in cases defined in Section 4.2 and 4.3. 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 subscriber identity of the indicated actual
      length, in the same Network Access Identifier format that is used
      in EAP-Response/Identity, i.e. including the NAI realm portion.
      The identity is the permanent identity, a pseudonym identity or a
      re-authentication identity. The identity format is specified in
      Section 4.1. The identity 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.


8.7. EAP-Request/AKA-Reauthentication

   The format of the EAP-Request/AKA-Reauthentication 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_IV         | Length = 5    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                 Initialization Vector                         |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | AT_ENCR_DATA  | Length        |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                    Encrypted Data                             .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  AT_MAC       | Length = 5    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                              MAC                              |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code

      1 for Request

   Identifier

      See [6].

   Length

      The length of the EAP packet.

   Type

      23

   Subtype

      13

   Reserved

      Set to zero when sending, ignored on reception.


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   AT_IV

      The AT_IV attribute is MUST be included. See Section 7.2.

   AT_ENCR_DATA

      The AT_ENCR_DATA attribute MUST be included. See Section 7.2. The
      plaintext consists of nested attributes as described below.

   AT_MAC

      AT_MAC MUST be included. No message-specific data is included in
      the MAC calculation. See Section 7.1.

   The AT_IV and AT_ENCR_DATA attributes are used for communicating
   encrypted attributes. The plaintext of the AT_ENCR_DATA value field
   consists of nested attributes, which are 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_COUNTER   | Length = 1    |           Counter             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_NONCE_S    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                                                               |
   |                            NONCE_S                            |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_NEXT_REAU..| Length        | Actual Re-Auth Identity Length|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                   Next Re-authentication Username             .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AT_PADDING   | Length        | Padding...                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   AT_COUNTER

      The AT_COUNTER attribute MUST be included. The value field
      consists of a 16-bit unsigned integer counter value, represented
      in network byte order.

   AT_NONCE_S

      The AT_NONCE_S attribute MUST be included. The value field
      contains two reserved bytes followed by a random number generated

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      by the server (16 bytes) freshly for this EAP/AKA re-
      authentication. The random number is used as challenge for the
      client and also a seed value for the new keying material. The
      reserved bytes are set to zero upon sending and ignored upon
      reception.

   AT_NEXT_REAUTH_ID

      The AT_NEXT_REAUTH_ID attribute is optional to include. The
      attribute is described in Section 8.1.

   AT_PADDING

      The AT_PADDING attribute is optional to include. See section 7.2

8.8. EAP-Response/AKA-Reauthentication

   The format of the EAP-Response/AKA-Reauthentication 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_IV         | Length = 5    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                 Initialization Vector                         |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | AT_ENCR_DATA  | Length        |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                    Encrypted Data                             .
    .                                                               .
    |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  AT_MAC       | Length = 5    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                              MAC                              |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code

      2 for Response


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                        EAP AKA Authentication           December 2002

   Identifier

      See [6].

   Length

      The length of the EAP packet.

   Type

      23

   Subtype

      13

   Reserved

      Set to zero when sending, ignored on reception.

   AT_IV

      The AT_IV attribute is MUST be included. See Section 7.2.

   AT_ENCR_DATA

      The AT_ENCR_DATA attribute MUST be included. See Section 7.2. The
      plaintext consists of nested attributes as described below.

   AT_MAC

      For EAP-Response/AKA-Reauthentication, the MAC code is calculated
      over the following data:

          EAP packet| NONCE_S

      The EAP packet is represented as specified in Section 7.1. It is
      followed by the 16-byte NONCE_S value from the client's
      AT_NONCE_S attribute.

   The AT_IV and AT_ENCR_DATA attributes are used for communicating
   encrypted attributes. The plaintext of the AT_ENCR_DATA value field
   consists of nested attributes, which are 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AT_COUNTER   | Length = 1    |           Counter             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AT_COUNTER...| Length = 1    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AT_PADDING   | Length        | Padding...                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   AT_COUNTER

      The AT_COUNTER attribute MUST be included. The format of this
      attribute is specified in Section 8.7.

   AT_COUNTER_TOO_SMALL

      The AT_COUNTER_TOO_SMALL attribute is optional to include, and it
      is included in cases specified in Section 5.

   AT_PADDING

      The AT_PADDING attribute is optional to include. See section 7.2


8.9. EAP/AKA Notifications

   The EAP-Request/Notification, specified in [6], can be used to
   convey a displayable message from the authenticator to the client.
   Because these messages are textual messages, it may be hard for the
   client to present them in the userÆs preferred language. Therefore,
   EAP/AKA uses a separate EAP/AKA message subtype to transmit
   localizable notification codes instead of the EAP-
   Request/Notification packet.

   The EAP server MAY issue an EAP-Request/AKA-Notification packet to
   the client. The client MAY delay the processing of EAP-Request/AKA-
   Notification and wait for other EAP/AKA requests. If a valid EAP/AKA
   request of another subtype is received, the client MAY silently
   ignore the EAP-Request/AKA-Notification and process the other
   EAP/AKA request instead. If the client decides to process the EAP-
   Request/AKA-Notification, then the client MAY show a notification
   message to the user and the client MUST respond to the EAP server
   with an EAP-Response/AKA-Notification packet, even if the client did
   not recognize the notification code. The processing of EAP-
   Request/AKA-Notification MUST NOT result in any change in the client
   state. For example, the client MUST NOT assume that the receipt of
   an EAP-Request/AKA-Notification indicates failed authentication.




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   On full authentication, AT_MAC MUST be included in EAP/AKA
   Notification packets if a new K_aut key is available and MUST NOT be
   included if K_aut has not been derived yet. In other words, AT_MAC
   MUST be included on full authentication if and only if the EAP/AKA-
   Challenge roundtrip has been performed. On re-authentication,
   EAP/AKA-Notification packets MUST be protected with AT_MAC because
   K_aut is always available.

   Some of the notification codes are authorization related and hence
   not usually considered as part of the responsibility of an EAP
   method. However, they are included as part of EAP/AKA because there
   are currently no other ways to convey this information to the user
   in a localizable way, and the information is potentially useful for
   the user. An EAP/AKA server implementation may decide never to send
   these EAP/AKA notifications.

   The format of the EAP-Request/AKA-Notification 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_NOTIFICATION| Length = 1    |      Notification Code        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  AT_MAC       | Length = 5    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                              MAC                              |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code

      1 for Request

   Identifier

      See [6].

   Length

      The length of the EAP packet.

   Type

      23



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   Subtype

      12

   Reserved

      Set to zero when sending, ignored on reception.

   AT_NOTIFICATION

      The AT_NOTIFICATION attribute MUST be included. The value field
      of this attribute contains a two-byte notification code. The
      following code values have been reserved. The descriptions below
      illustrate the semantics of the notifications. The client
      implementation MAY use different wordings when presenting the
      notifications to the user. The "requested service" depends on the
      environment where EAP/AKA is applied.

      1026 -           - User has been temporarily denied access to the requested
      service

      1031 - User has not subscribed to the requested service

   AT_MAC

      AT_MAC is included in cases described above. No message-specific
      data is included in the MAC calculation. See Section 7.1.


   The format of the EAP-Response/AKA-Notification packet is shown
   below. Because this packet is only an acknowledgement of EAP-
   Request/AKA-Notification, it does not contain any mandatory
   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_MAC       | Length = 5    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                              MAC                              |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code

      2 for Response


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   Identifier

      See [6].

   Length

      The length of the EAP packet.

   Type

      23

   Subtype

      12

   Reserved

      Set to zero when sending, ignored on reception.

   AT_MAC

      AT_MAC is included in cases described above. No message-specific
      data is included in the MAC calculation. See Section 7.1.



9. Error Cases

   In general, if an EAP/AKA client or server implementation detects an
   error in a received EAP/AKA packet, the EAP/AKA implementation
   silently ignores the EAP packet, does not change its state and does
   not send any EAP messages to its peer. Examples of such errors,
   specified in detail elsewhere in this document, are an invalid
   AT_MAC value, a mandatory attribute is missing, illegal attributes
   included and an unrecognized non-skippable attribute. If no valid
   packets are received, the authentication exchange will eventually
   time out.
   If the EAP/AKA client receives an EAP/AKA Request of an unrecognized
   subtype, the EAP/AKA client MUST silently discard the EAP request.

   As normally in EAP, the EAP server sends the EAP-Failure packet to
   the client when the authentication procedure fails on the EAP
   Server. In EAP/AKA, this may occur for example if the EAP server is
   not able to obtain authentication vectors for the subscriber or the
   authentication exchange times out.

10. Key Derivation

   This section specifies how EAP AKA keying material is derived from
   the IK and CK keys.

   EAP AKA requires two keys for its own purposes, a message
   authentication key K_aut and an encryption key K_encr, to be used

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   with the AT_MAC and AT_ENCR_DATA attributes. The same K_aut and
   K_encr keys are used in full authentication and subsequent re-
   authentications. In addition, it is possible to derive additional
   application specific key material, such as a master key to be used
   with IEEE 802.11i.

   Key derivation is based on the pseudo-random number generator
   specified in NIST Federal Information Processing Standards
   Publication 186-2 [14]. The pseudo-random number generator is
   specified in the change notice 1 (2001 October 5)of [14] (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 3.2 is omitted. The function G used in the algorithm is
   constructed via Secure Hash Standard as specified in Appendix 3.3 of
   the standard. For convenience, the pseudo-random number algorithm
   with the correct modification is cited in Annex B.

   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(Identity|IK|CK)

   In the formula above, the "|" character denotes concatenation.
   Identity denotes the user identity string without any terminating
   null characters. It is the identity from the AT_IDENTITY attribute
   from the last EAP-Response/AKA-Identity packet, or, if AT_IDENTITY
   was not used, the identity from the EAP-Response/Identity packet.

   The optional user input values (XSEED_j) in Step 3.1 are set to
   zero.

   The resulting 320-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_aut (128 bits),
   EAP application specific keys. The number of pseudo-random number
   generator iterations (m) depends on the amount of required keying
   material. The EAP application specific material immediately follows
   K_aut.

   On re-authentication, the same pseudo-random number generator can be
   used to generate new application specific keys. The seed value XKEYÆ
   is calculated as follows:

      XKEYÆ = SHA1(Identity|counter|NONCE_S|original XKEY)

   In the formula above, the Identity denotes the re-authentication
   user identity, without any terminating null characters, from the
   AT_IDENTITY attribute of the EAP-Response/AKA-Identity packet, or,
   if EAP-Response/AKA-Identity was not used on re-authentication, the
   identity string from the EAP-Response/Identity packet. The counter
   denotes the counter value from AT_COUNTER attribute used in the EAP-
   Response/AKA-Reauthentication packet. The counter is used in network
   byte order. NONCE_S denotes the 16-byte NONCE_S value from the

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   AT_NONCE_S attribute used in the EAP-Request/AKA-Reauthentication
   packet. The original XKEY is the XKEY value from the preceding full
   authentication. The pseudo-random number generator is run with the
   new seed value XKEYÆ, and the resulting 320-bit random numbers x_0,
   x_1, ..., x_m-1 are concatenated and partitioned into suitable-sized
   chunks and used as new application specific keys.

   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, rules
   of key derivation cannot be given here. ). However, please see Annex
   A for a specification of how keys for IEEE 802.11 are derived.

   If a master session key is required, then the master session key is
   the first EAP application specific key. On full authentication, the
   master session key immediately follows K_aut in the key stream
   resulting from the key expansion scheme. On re-authentication, the
   master session key is the first new application specific key that is
   derived.

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
        AKA-Notification...............................12
        AKA-Reauthentication...........................13


















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   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_PADDING......................................6
        AT_PERMANENT_ID_REQ............................10
        AT_MAC.........................................11
        AT_ANY_ID_REQ..................................13
        AT_IDENTITY....................................14
        AT_FULLAUTH_ID_REQ.............................17
        AT_COUNTER.....................................19
        AT_COUNTER_TOO_SMALL...........................20
        AT_NONCE_S.....................................21

        AT_IV.........................................129
        AT_ENCR_DATA..................................130
        AT_NEXT_PSEUDONYM.............................132
        AT_NEXT_REAUTH_ID.............................133

   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 [15]. 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
   another standards body (e.g. 3GPP), or permanently and readily
   available vendor design notes.

12. Security Considerations

   The protocol in this document is intended to provide the appropriate
   level of security to operate Extensible Authentication Protocol
   using the UMTS AKA procedure in both physically insecure
   environments and physically or otherwise secure environments.
   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. This section highlights any known
   vulnerabilities and the ways to address them.

   The revised EAP base protocol [16] highlights several attacks that
   are possible against the EAP protocol. Therefore it specifies
   security requirements needed for standardizing EAP methods. This
   section discusses how the security requirements are addressed in
   EAP/AKA.

12.1. Identity Protection and Privacy

   EAP/AKA includes optional Identity privacy support that protects the
   privacy of the subscriber identity against passive eavesdropping.
   The mechanism cannot be used on the first connection with a given

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   server, when the IMSI will have to be sent in the clear. The
   terminal SHOULD store the pseudonym in a non-volatile memory so that
   it can be maintained across reboots. An active attacker that
   impersonates the network may use the AT_PERMANENT_ID_REQ attribute
   (Section 4.3) to learn the subscriber's IMSI. However, as discussed
   in Section 4.3, the terminal can refuse to send the cleartext IMSI
   if it believes that the network should be able to recognize the
   pseudonym.

   If the client and server cannot guarantee that the pseudonym will be
   maintained reliably and Identity privacy is required then additional
   protection from an external security mechanism such as Protected
   Extensible Authentication Protocol (PEAP) [17] may be used. The
   benefits and the security considerations of using an external
   security mechanism with EAP/AKA are beyond the scope of this
   document.

12.2. Mutual Authentication

   EAP/AKA provides mutual authentication via the UMTS AKA mechanisms.

12.3. Key Derivation Considerations

   EAP/AKA supports key derivation with 128-bit entropy. The key
   hierarchy is specified in Section 10.

12.4. Brute-Force and Dictionary Attacks

   The effective strength of EAP/AKA values is 128 bits, and there are
   no known computationally feasible brute-force attacks. Because UMTS
   AKA is not a password protocol, EAP/AKA is not vulnerable to
   dictionary attacks.

12.5. Packet Modification Attacks

   AT_MAC, AT_IV and AT_ENCR_DATA attributes are used to provide
   integrity, replay and confidentiality protection for EAP/AKA
   Requests and Responses. Integrity protection includes the EAP
   header.

   The contents of the EAP-Response/Identity packet are implicitly
   integrity protected by including them in key derivation.

   Because EAP/AKA is not a tunneling method, EAP Notification, EAP
   Success or EAP Failure packets are not protected. On physically
   insecure networks, this may enable an attacker to mount denial of
   service attacks by sending false EAP Notification, EAP Success or
   EAP Failure packets. However, the attacker cannot force the peers to
   believe successful authentication has occurred when mutual
   authentication failed or has not happened yet.

12.6. Negotiation Attacks



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   EAP/AKA does not protect the EAP-Response/Nak packet. Because
   EAP/AKA does not protect the EAP method negotiation, EAP method
   downgrading attacks may be possible, especially if the user uses the
   same identity with EAP/AKA and other EAP methods.

   EAP/AKA does not support ciphersuite negotiation or EAP/AKA protocol
   version negotiation.

12.7. Fast Reconnect

   EAP/AKA includes an optional re-authentication ("fast reconnect")
   procedure, as recommended in [16] for EAP types that are intended
   for physically insecure networks.

12.8. Unreliable Media

   EAP/AKA does not provide acknowledged Success or Failure
   indications. If a Success or Failure packet is lost when using
   EAP/AKA over an unreliable medium, and if the protocol over which
   EAP/AKA is transported does not address the possible loss of Success
   or Failure, then the peer and authenticator may end up having a
   different interpretation of the state of the authentication
   conversation.

12.9. Man-in-the-middle Attacks

   In order to avoid man-in-the-middle attacks and session hijacking,
   user data SHOULD be integrity protected on physically insecure
   networks. The EAP/AKA application specific keys or keys derived from
   them MAY be used as the integrity protection keys, or, if an
   external security mechanism such as PEAP is used, then the link
   integrity protection keys MAY be derived by the external security
   mechanism.

   There are man-in-the-middle attacks associated with the use of any
   EAP method within a tunneled protocol such as PEAP, or within a
   sequence of EAP methods followed by each other. This specification
   does not address these attacks. If EAP/AKA is used with a tunneling
   protocol or as part of a sequence of methods, there should be
   cryptographic binding provided between the protocols and EAP/AKA to
   prevent man-in-the-middle attacks through rogue authenticators being
   able to setup one-way authenticated tunnels. EAP/AKA application-
   specific keys MAY be used to provide the cryptographic binding.
   However the mechanism how the binding is provided depends on the
   tunneling or sequencing protocol, and it is beyond the scope of this
   document.

12.10. Generating Random Numbers

   An EAP/AKA implementation SHOULD use a good source of randomness to
   generate the random numbers required in the protocol. Please see
   [18] for more information on generating random numbers for security
   applications.


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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, Olivier Paridaens of Alcatel and Ilkka Uusitalo of Ericsson
   for interesting discussions in this problem space.

   The attribute format is based on the extension format of Mobile IPv4
   [19].

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

























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Annex A. Key Derivation for IEEE 802.11

   As specified in Section 12, application specific keying material can
   be derived with the pseudo-random function.

   The key hierarchy in IEEE 802.11i currently assumes that EAP methods
   produce a 256-bit long Pairwise Master Key (PMK). When a Pairwise
   Master Key is required, it is the first EAP application specific key
   that is derived. On full authentication, the PMK immediately follows
   K_aut in the key stream resulting from the key expansion scheme. On
   re-authentication, the PMK is the first new application specific key
   that is derived.

   For pre 802.11i networks, the signature key used to authenticate
   broadcast keys in IEEE 802.1x is selected as the first 256 bits of
   the EAP application specific keys immediately after K_aut. (On re-
   authentication, the first 256 application specific key bits are used
   as the signature key.)  The next 256 bits are used as the WEP
   session key.  The full 256-bit key is not usually used during WEP
   encryption, unused bits at then end should be ignored by the
   implementation. When the keys are transmitted from the AAA server to
   the access point using the RADIUS protocol the session key is placed
   in an MS-MPPE-RECV-KEY attribute and the signature key is placed in
   an MS-MPPE-SEND-KEY attribute. These attributes are defined in RFC
   2548.






























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Annex B. Pseudo-Random Number Generator

   The "|" character denotes concatenation, and "^" denotes involution.

   Step 1: Choose a new, secret value for the seed-key, XKEY

   Step 2: In hexadecimal notation let
       t = 67452301 EFCDAB89 98BADCFE 10325476 C3D2E1F0
       This is the initial value for H0|H1|H2|H3|H4
       in the FIPS SHS [12]

   Step 3: For j = 0 to m -                          - 1 do
         3.1 XSEED_j = optional user input
         3.2 For i = 0 to 1 do
             a. XVAL = (XKEY + XSEED_j) mod 2^b
             b. w_i = G(t, XVAL)
             c. XKEY = (1 + XKEY + w_i) mod 2^b
         3.3 x_j = w_0|w_1





































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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)

   [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]   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)

   [9]   Draft 3GPP Technical Specification 3GPP TS 23.234 V 1.2.0:
         "Technical Specification Group Services and System Aspects;
         3GPP system to Wireless Local Area Network (WLAN)
         Interworking; System Description", 3rd Generation Partnership
         Project, work in progress, November 2002. (INFORMATIVE)

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

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


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

   [13]  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)

   [14]  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

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

   [16]  L. Blunk, J. Vollbrecht, B. Aboba, "Extensible Authentication
         Protocol (EAP)", draft-ietf-pppext-rfc2284bis-07.txt, work-in-
         progress, October 2002. (NORMATIVE)

   [17]  H. Andersson, S. Josefsson, G. Zorn, D. Simon, A. Palekar,
         "Protected EAP Protocol (PEAP)", draft-josefsson-pppext-eap-
         tls-eap-05.txt, work-in-progress, September 2002.
         (IMFORMATIVE)

   [18]  D. Eastlake, 3rd, S. Crocker, J. Schiller, "Randomness
         Recommendations for Security",  RFC 1750 (Informational),
         December 1994. (INFORMATIVE)

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

















   Arkko and Haverinen     Expires in six months            [Page 55]


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