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Updated by: 5448 INFORMATIONAL
Errata Exist
Network Working Group                                           J. Arkko
Request for Comments: 4187                                      Ericsson
Category: Informational                                     H. Haverinen
                                                                   Nokia
                                                            January 2006


      Extensible Authentication Protocol Method for 3rd Generation
               Authentication and Key Agreement (EAP-AKA)

Status of This Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2006).

IESG Note

   The EAP-AKA protocol was developed by 3GPP.  The documentation of
   EAP-AKA is provided as information to the Internet community.  While
   the EAP WG has verified that EAP-AKA is compatible with EAP as
   defined in RFC 3748, no other review has been done, including
   validation of the security claims.  The IETF has also not reviewed
   the security of the underlying UMTS AKA algorithms.

Abstract

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for authentication and session key distribution that uses
   the Authentication and Key Agreement (AKA) mechanism.  AKA is used in
   the 3rd generation mobile networks Universal Mobile
   Telecommunications System (UMTS) and CDMA2000.  AKA is based on
   symmetric keys, and typically runs in a Subscriber Identity Module,
   which is a UMTS Subscriber Identity Module, USIM, or a (Removable)
   User Identity Module, (R)UIM, similar to a smart card.

   EAP-AKA includes optional identity privacy support, optional result
   indications, and an optional fast re-authentication procedure.









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Table of Contents

   1. Introduction and Motivation .....................................4
   2. Terms and Conventions Used in This Document .....................5
   3. Protocol Overview ...............................................9
   4. Operation ......................................................15
      4.1. Identity Management .......................................15
           4.1.1. Format, Generation, and Usage of Peer Identities ...15
           4.1.2. Communicating the Peer Identity to the Server ......21
           4.1.3. Choice of Identity for the EAP-Response/Identity ...23
           4.1.4. Server Operation in the Beginning of
                  EAP-AKA Exchange ...................................23
           4.1.5. Processing of EAP-Request/AKA-Identity by
                  the Peer ...........................................24
           4.1.6. Attacks against Identity Privacy ...................25
           4.1.7. Processing of AT_IDENTITY by the Server ............26
      4.2. Message Sequence Examples (Informative) ...................27
           4.2.1. Usage of AT_ANY_ID_REQ .............................27
           4.2.2. Fall Back on Full Authentication ...................28
           4.2.3. Requesting the Permanent Identity 1 ................29
           4.2.4. Requesting the Permanent Identity 2 ................30
           4.2.5. Three EAP/AKA-Identity Round Trips .................30
   5. Fast Re-Authentication .........................................32
      5.1. General ...................................................32
      5.2. Comparison to AKA .........................................33
      5.3. Fast Re-Authentication Identity ...........................33
      5.4. Fast Re-Authentication Procedure ..........................35
      5.5. Fast Re-Authentication Procedure when Counter is
           Too Small .................................................37
   6. EAP-AKA Notifications ..........................................38
      6.1. General ...................................................38
      6.2. Result Indications ........................................39
      6.3. Error Cases ...............................................40
           6.3.1. Peer Operation .....................................41
           6.3.2. Server Operation ...................................41
           6.3.3. EAP-Failure ........................................42
           6.3.4. EAP-Success ........................................42
   7. Key Generation .................................................43
   8. Message Format and Protocol Extensibility ......................45
      8.1. Message Format ............................................45
      8.2. Protocol Extensibility ....................................47
   9. Messages .......................................................48
      9.1. EAP-Request/AKA-Identity ..................................48
      9.2. EAP-Response/AKA-Identity .................................48
      9.3. EAP-Request/AKA-Challenge .................................49
      9.4. EAP-Response/AKA-Challenge ................................49
      9.5. EAP-Response/AKA-Authentication-Reject ....................50
      9.6. EAP-Response/AKA-Synchronization-Failure ..................50



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      9.7. EAP-Request/AKA-Reauthentication ..........................50
      9.8. EAP-Response/AKA-Reauthentication .........................51
      9.9. EAP-Response/AKA-Client-Error .............................52
      9.10. EAP-Request/AKA-Notification .............................52
      9.11. EAP-Response/AKA-Notification ............................52
   10. Attributes ....................................................53
      10.1. Table of Attributes ......................................53
      10.2. AT_PERMANENT_ID_REQ ......................................54
      10.3. AT_ANY_ID_REQ ............................................54
      10.4. AT_FULLAUTH_ID_REQ .......................................54
      10.5. AT_IDENTITY ..............................................55
      10.6. AT_RAND ..................................................55
      10.7. AT_AUTN ..................................................56
      10.8. AT_RES ...................................................56
      10.9. AT_AUTS ..................................................57
      10.10. AT_NEXT_PSEUDONYM .......................................57
      10.11. AT_NEXT_REAUTH_ID .......................................58
      10.12. AT_IV, AT_ENCR_DATA, and AT_PADDING .....................58
      10.13. AT_CHECKCODE ............................................60
      10.14. AT_RESULT_IND ...........................................62
      10.15. AT_MAC ..................................................63
      10.16. AT_COUNTER ..............................................64
      10.17. AT_COUNTER_TOO_SMALL ....................................64
      10.18. AT_NONCE_S ..............................................65
      10.19. AT_NOTIFICATION .........................................65
      10.20. AT_CLIENT_ERROR_CODE ....................................66
   11. IANA and Protocol Numbering Considerations ....................66
   12. Security Considerations .......................................68
      12.1. Identity Protection ......................................69
      12.2. Mutual Authentication ....................................69
      12.3. Flooding the Authentication Centre .......................69
      12.4. Key Derivation ...........................................70
      12.5. Brute-Force and Dictionary Attacks .......................70
      12.6. Protection, Replay Protection, and Confidentiality .......70
      12.7. Negotiation Attacks ......................................71
      12.8. Protected Result Indications .............................72
      12.9. Man-in-the-Middle Attacks ................................72
      12.10. Generating Random Numbers ...............................73
   13. Security Claims ...............................................73
   14. Acknowledgements and Contributions ............................74
   15. References ....................................................74
      15.1. Normative References .....................................74
      15.2. Informative References ...................................76
   Appendix A.  Pseudo-Random Number Generator .......................77







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1.  Introduction and Motivation

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for authentication and session key distribution that uses
   the 3rd generation Authentication and Key Agreement mechanism,
   specified for Universal Mobile Telecommunications System (UMTS) in
   [TS33.102] and for CDMA2000 in [S.S0055-A].  UMTS and CDMA2000 are
   global 3rd generation mobile network standards that use the same AKA
   mechanism.

   2nd generation mobile networks and 3rd generation mobile networks use
   different authentication and key agreement mechanisms.  The Global
   System for Mobile communications (GSM) is a 2nd generation mobile
   network standard, and EAP-SIM [EAP-SIM] specifies an EAP mechanism
   that is based on the GSM authentication and key agreement primitives.

   AKA is based on challenge-response mechanisms and symmetric
   cryptography.  AKA typically runs in a UMTS Subscriber Identity
   Module (USIM) or a CDMA2000 (Removable) User Identity Module
   ((R)UIM).  In this document, both modules are referred to as identity
   modules.  Compared to the 2nd generation mechanisms such as GSM AKA,
   the 3rd generation AKA provides substantially longer key lengths and
   mutual authentication.

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

   o  The use of the AKA also as a secure PPP authentication method in
      devices that already contain an identity module.
   o  The use of the 3rd generation mobile network authentication
      infrastructure in the context of wireless LANs
   o  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:

   o  The identity module and the home environment have agreed on a
      secret key beforehand.  (The "home environment" refers to the home
      operator's authentication network infrastructure.)
   o  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 identity module, an expected
      result part XRES, a 128-bit session key for integrity check IK,
      and a 128-bit session key for encryption CK.





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   o  The RAND and the AUTN are delivered to the identity module.
   o  The identity module 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 identity module produces an
      authentication result RES and sends it to the home environment.
   o  The home environment verifies the correct result from the identity
      module.  If the result is correct, IK and CK can be used to
      protect further communications between the identity module and the
      home environment.

   When verifying AUTN, the identity module may detect that the sequence
   number the network uses is not within the correct range.  In this
   case, the identity module 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 [TS33.102] for
   UMTS and [S.S0055-A] for CDMA2000.

   In EAP-AKA, the EAP server node obtains the authentication vectors,
   compares RES and XRES, and uses CK and IK in key derivation.

   In the 3rd generation mobile networks, AKA is used for both 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) [RFC4282].

2.  Terms and Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The terms and abbreviations "authenticator", "backend authentication
   server", "EAP server", "peer", "Silently Discard", "Master Session
   Key (MSK)", and "Extended Master Session Key (EMSK)" in this document
   are to be interpreted as described in [RFC3748].

   This document frequently uses the following terms and abbreviations.
   The AKA parameters are specified in detail in [TS33.102] for UMTS and
   [S.S0055-A] for CDMA2000.





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   AAA protocol

         Authentication, Authorization and Accounting protocol

   AKA

         Authentication and Key Agreement

   AuC

         Authentication Centre.  The mobile network element that can
         authenticate subscribers in the mobile networks.

   AUTN

         AKA parameter.  AUTN is an authentication value generated by
         the AuC, which, together with the RAND, authenticates the
         server to the peer, 128 bits.

   AUTS

         AKA parameter.  A value generated by the peer upon
         experiencing a synchronization failure, 112 bits.

   EAP

         Extensible Authentication Protocol [RFC3748]

   Fast Re-Authentication

         An EAP-AKA authentication exchange that is based on keys
         derived upon a preceding full authentication exchange.  The
         3rd Generation AKA is not used in the fast re-authentication
         procedure.

   Fast Re-Authentication Identity

         A fast re-authentication identity of the peer, including an
         NAI realm portion in environments where a realm is used.
         Used on re-authentication only.

   Fast Re-Authentication Username

         The username portion of fast re-authentication identity,
         i.e., not including any realm portions.






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   Full Authentication

         An EAP-AKA authentication exchange that is based on the
         3rd Generation AKA procedure.

   GSM

         Global System for Mobile communications.

   NAI

         Network Access Identifier [RFC4282]

   Identity Module

         Identity module is used in this document to refer to the
         part of the mobile device that contains authentication and
         key agreement primitives.  The identity module may be an
         integral part of the mobile device or it may be an application
         on a smart card distributed by a mobile operator.  USIM and
         (R)UIM are identity modules.

   Nonce

         A value that is used at most once or that is never repeated
         within the same cryptographic context.  In general, a nonce can
         be predictable (e.g., a counter) or unpredictable (e.g., a
         random value).  Because some cryptographic properties may
         depend on the randomness of the nonce, attention should be paid
         to whether a nonce is required to be random or not.  In this
         document, the term nonce is only used to denote random nonces,
         and it is not used to denote counters.

   Permanent Identity

         The permanent identity of the peer, including an NAI realm
         portion in environments where a realm is used.  The permanent
         identity is usually based on the IMSI.  Used on full
         authentication only.

   Permanent Username

         The username portion of permanent identity, i.e., not including
         any realm portions.







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   Pseudonym Identity

         A pseudonym identity of the peer, including an NAI realm
         portion in environments where a realm is used.  Used on full
         authentication only.

   Pseudonym Username

         The username portion of pseudonym identity, i.e., not including
         any realm portions.

   RAND

         An AKA parameter.  Random number generated by the AuC,
         128 bits.

   RES

         Authentication result from the peer, which, together with
         the RAND, authenticates the peer to the server,
         128 bits.

   (R)UIM

         CDMA2000 (Removable) User Identity Module.  (R)UIM is an
         application that is resident on devices such as smart cards,
         which may be fixed in the terminal or distributed by CDMA2000
         operators (when removable).

   SQN

         An AKA parameter.  Sequence number used in the authentication
         process, 48 bits.

   SIM

         Subscriber Identity Module.  The SIM is traditionally a smart
         card distributed by a GSM operator.

   SRES

         The authentication result parameter in GSM, corresponds to
         the RES parameter in 3G AKA, 32 bits.








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   UAK

         UIM Authentication Key, used in CDMA2000 AKA.  Both the
         identity module and the network can optionally generate the UAK
         during the AKA computation in CDMA2000.  UAK is not used in
         this version of EAP-AKA.

   UIM

         Please see (R)UIM.

   USIM

         UMTS Subscriber Identity Module.  USIM is an application that
         is resident on devices such as smart cards distributed by UMTS
         operators.

3.  Protocol Overview

   Figure 1 shows the basic, successful full authentication exchange in
   EAP-AKA, when optional result indications are not used.  The
   authenticator typically communicates with an EAP server that is
   located on a backend authentication server using an AAA protocol.
   The authenticator shown in the figure is often simply relaying EAP
   messages to and from the EAP server, but these backend AAA
   communications are not shown.  At the minimum, EAP-AKA uses two
   roundtrips to authenticate and authorize the peer and generate
   session keys.  As in other EAP schemes, an identity request/response
   message pair is usually exchanged first.  On full authentication, the
   peer's identity response includes either the user's International
   Mobile Subscriber Identity (IMSI), or a temporary identity
   (pseudonym) if identity privacy is in effect, as specified in
   Section 4.1.  (As specified in [RFC3748], the initial identity
   request is not required, and MAY be bypassed in cases where the
   network can presume the identity, such as when using leased lines,
   dedicated dial-ups, etc.  Please see Section 4.1.2 for specification
   of how to obtain the identity via EAP AKA messages.)

   After obtaining the subscriber identity, the EAP server obtains an
   authentication vector (RAND, AUTN, RES, CK, IK) for use in
   authenticating the subscriber.  From the vector, the EAP server
   derives the keying material, as specified in Section 6.4.  The vector
   may be obtained by contacting an Authentication Centre (AuC) on the
   mobile network; for example, per UMTS specifications, several vectors
   may be obtained at a time.  Vectors may be stored in the EAP server
   for use at a later time, but they may not be reused.





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   In CDMA2000, the vector may include a sixth value called the User
   Identity Module Authentication Key (UAK).  This key is not used in
   EAP-AKA.

   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 8.  The EAP-Request/AKA-Challenge message contains a RAND
   random number (AT_RAND), a network authentication token (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 and fast re-authentication support, as
   described in Section 4.1.  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 peer runs the AKA algorithm (typically using an identity module)
   and verifies the AUTN.  If this is successful, the peer 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 peer, and the AT_MAC
   attribute to integrity protect the EAP message.

   The EAP server verifies that the RES and the MAC in the EAP-Response/
   AKA-Challenge packet are correct.  Because protected success
   indications are not used in this example, the EAP server sends the
   EAP-Success packet, indicating that the authentication was
   successful.  (Protected success indications are discussed in
   Section 6.2.)  The EAP server may also include derived keying
   material in the message it sends to the authenticator.  The peer has
   derived the same keying material, so the authenticator does not
   forward the keying material to the peer along with EAP-Success.


















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       Peer                                             Authenticator
          |                      EAP-Request/Identity             |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server runs AKA algorithms,  |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                         EAP-Request/AKA-Challenge     |
          |                         (AT_RAND, AT_AUTN, AT_MAC)    |
          |<------------------------------------------------------|
      +-------------------------------------+                     |
      | Peer runs AKA algorithms,           |                     |
      | 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  |
          |<------------------------------------------------------|

              Figure 1: EAP-AKA full authentication procedure





















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   Figure 2 shows how the EAP server rejects the Peer due to a failed
   authentication.

       Peer                                              Authenticator
          |                      EAP-Request/Identity             |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server runs AKA algorithms,  |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                      EAP-Request/AKA-Challenge        |
          |                      (AT_RAND, AT_AUTN, AT_MAC)       |
          |<------------------------------------------------------|
      +-------------------------------------+                     |
      | Peer runs AKA algorithms,           |                     |
      | 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 incorrect.         |
          |              +------------------------------------------+
          |                      EAP-Request/AKA-Notification     |
          |<------------------------------------------------------|
          | EAP-Response/AKA-Notification                         |
          |------------------------------------------------------>|
          |                                          EAP-Failure  |
          |<------------------------------------------------------|

                    Figure 2: Peer authentication fails














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   Figure 3 shows the peer rejecting the AUTN of the EAP server.

   The peer sends an explicit error message (EAP-Response/
   AKA-Authentication-Reject) to the EAP server, as usual in AKA when
   AUTN is incorrect.  This allows the EAP server to produce the same
   error statistics that AKA generally produces in UMTS or CDMA2000.

        Peer                                             Authenticator
          |                      EAP-Request/Identity             |
          |<------------------------------------------------------|
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server runs AKA algorithms,  |
          |                            | generates RAND and a bad AUTN|
          |                            +------------------------------+
          |                         EAP-Request/AKA-Challenge     |
          |                         (AT_RAND, AT_AUTN, AT_MAC)    |
          |<------------------------------------------------------|
      +-------------------------------------+                     |
      | Peer runs AKA algorithms            |                     |
      | and discovers AUTN that can not be  |                     |
      | verified                            |                     |
      +-------------------------------------+                     |
          | EAP-Response/AKA-Authentication-Reject                |
          |------------------------------------------------------>|
          |                                          EAP-Failure  |
          |<------------------------------------------------------|

                  Figure 3: Network authentication fails

   The AKA uses shared secrets between the Peer and the Peer's home
   operator, together with a sequence number, to actually perform an
   authentication.  In certain circumstances, shown in Figure 4, it is
   possible for the sequence numbers to get out of sequence.















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        Peer                                             Authenticator
          |                      EAP-Request/Identity             |
          |<------------------------------------------------------|
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server runs AKA algorithms,  |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                         EAP-Request/AKA-Challenge     |
          |                         (AT_RAND, AT_AUTN, AT_MAC)    |
          |<------------------------------------------------------|
      +-------------------------------------+                     |
      | Peer runs AKA algorithms            |                     |
      | and discovers AUTN that contains an |                     |
      | inappropriate sequence number       |                     |
      +-------------------------------------+                     |
          | EAP-Response/AKA-Synchronization-Failure              |
          | (AT_AUTS)                                             |
          |------------------------------------------------------>|
          |                              +---------------------------+
          |                              | Perform resynchronization |
          |                              | Using AUTS and            |
          |                              | the sent RAND             |
          |                              +---------------------------+
          |                                                       |

                 Figure 4: Sequence number synchronization

   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 fast re-authentication procedure, which is
   specified in Section 5.  Fast re-authentication is based on keys
   derived on full authentication.  If the peer has maintained state
   information for re-authentication and wants to use fast
   re-authentication, then the peer indicates this by using a specific
   fast re-authentication identity instead of the permanent identity or
   a pseudonym identity.









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4.  Operation

4.1.  Identity Management

4.1.1.  Format, Generation, and Usage of Peer Identities

4.1.1.1.  General

   In the beginning of EAP authentication, the Authenticator or the EAP
   server usually issues the EAP-Request/Identity packet to the peer.
   The peer responds with EAP-Response/Identity, which contains the
   user's identity.  The formats of these packets are specified in
   [RFC3748].

   Subscribers of mobile networks are identified with the International
   Mobile Subscriber Identity (IMSI) [TS23.003].  The IMSI is a string
   of not more than 15 digits.  It is composed of a Mobile Country Code
   (MCC) of 3 digits, a Mobile Network Code (MNC) of 2 or 3 digits, and
   a Mobile Subscriber Identification Number (MSIN) of not more than 10
   digits.  MCC and MNC uniquely identify the GSM operator and help
   identify the AuC from which the authentication vectors need to be
   retrieved for this subscriber.

   Internet AAA protocols identify users with the Network Access
   Identifier (NAI) [RFC4282].  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.

   This section specifies the peer identity format used in EAP-AKA.  In
   this document, the term identity or peer identity refers to the whole
   identity string that is used to identify the peer.  The peer identity
   may include a realm portion.  "Username" refers to the portion of the
   peer identity that identifies the user, i.e., the username does not
   include the realm portion.

4.1.1.2.  Identity Privacy Support

   EAP-AKA includes optional identity privacy (anonymity) support that
   can be used to hide the cleartext permanent identity and thereby make
   the subscriber's EAP exchanges untraceable to eavesdroppers.  Because
   the permanent identity never changes, revealing it would help
   observers to track the user.  The permanent identity is usually based
   on the IMSI, which may further help the tracking, because the same
   identifier may be used in other contexts as well.  Identity privacy
   is based on temporary identities, or pseudonyms, which are equivalent





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   to but separate from the Temporary Mobile Subscriber Identities
   (TMSI) that are used on cellular networks.  Please see Section 12.1
   for security considerations regarding identity privacy.

4.1.1.3.  Username Types in EAP-AKA Identities

   There are three types of usernames in EAP-AKA peer identities:

   (1) Permanent usernames.  For example,
   0123456789098765@myoperator.com might be a valid permanent identity.
   In this example, 0123456789098765 is the permanent username.

   (2) Pseudonym usernames.  For example, 2s7ah6n9q@myoperator.com might
   be a valid pseudonym identity.  In this example, 2s7ah6n9q is the
   pseudonym username.

   (3) Fast re-authentication usernames.  For example,
   43953754@myoperator.com might be a valid fast re-authentication
   identity.  In this case, 43953754 is the fast re-authentication
   username.  Unlike permanent usernames and pseudonym usernames, fast
   re-authentication usernames are one-time identifiers, which are not
   re-used across EAP exchanges.

   The first two types of identities are used only on full
   authentication, and the last type only on fast re-authentication.
   When the optional identity privacy support is not used, the
   non-pseudonym permanent identity is used on full authentication.  The
   fast re-authentication exchange is specified in Section 5.

4.1.1.4.  Username Decoration

   In some environments, the peer may need to decorate the identity by
   prepending or appending the username with a string, in order to
   indicate supplementary AAA routing information in addition to the NAI
   realm.  (The usage of an NAI realm portion is not considered to be
   decoration.)  Username decoration is out of the scope of this
   document.  However, it should be noted that username decoration might
   prevent the server from recognizing a valid username.  Hence,
   although the peer MAY use username decoration in the identities that
   the peer includes in EAP-Response/Identity, and although the EAP
   server MAY accept a decorated peer username in this message, the peer
   or the EAP server MUST NOT decorate any other peer identities that
   are used in various EAP-AKA attributes.  Only the identity used in
   EAP-Response/Identity may be decorated.







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4.1.1.5.  NAI Realm Portion

   The peer MAY include a realm portion in the peer identity, as per the
   NAI format.  The use of a realm portion is not mandatory.

   If a realm is used, the realm MAY be chosen by the subscriber's home
   operator and it MAY be a configurable parameter in the EAP-AKA peer
   implementation.  In this case, the peer 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 an AKA subscriber.  Such a reserved
   NAI realm may be useful as a hint of the first authentication method
   to use during method negotiation.  When the peer is using a pseudonym
   username instead of the permanent username, the peer selects the
   realm name portion similarly to how it selects the realm portion when
   using the permanent username.

   If no configured realm name is available, the peer MAY derive the
   realm name from the MCC and MNC portions of the IMSI.  A RECOMMENDED
   way to derive the realm from the IMSI, using the realm
   3gppnetwork.org, will be specified in [TS23.003].

   Some old implementations derive the realm name from the IMSI by
   concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits
   of IMSI, and ".owlan.org".  For example, if the IMSI is
   123456789098765, and the MNC is three digits long, then the derived
   realm name is "mnc456.mcc123.owlan.org".  As there are no DNS servers
   running at owlan.org, these realm names can only be used with
   manually configured AAA routing.  New implementations SHOULD use the
   mechanism specified in [TS23.003] instead of owlan.org.

   The IMSI is a string of digits without any explicit structure, so the
   peer may not be able to determine the length of the MNC portion.  If
   the peer is not able to determine whether the MNC is two or three
   digits long, the peer MAY use a 3-digit MNC.  If the correct length
   of the MNC is two, then the MNC used in the realm name includes the
   first digit of MSIN.  Hence, when configuring AAA networks for
   operators that have 2-digit MNC's, the network SHOULD also be
   prepared for realm names with incorrect 3-digit MNC's.

4.1.1.6.  Format of the Permanent Username

   The non-pseudonym permanent username SHOULD be derived from the IMSI.
   In this case, the permanent username MUST be of the format "0" |
   IMSI, where the character "|" denotes concatenation.  In other words,
   the first character of the username is the digit zero (ASCII value 30
   hexadecimal), followed by the IMSI.  The IMSI is an ASCII string that
   consists of not more than 15 decimal digits (ASCII values between 30



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   and 39 hexadecimal), one character per IMSI digit, in the order as
   specified in [TS23.003].  For example, a permanent username derived
   from the IMSI 295023820005424 would be encoded as the ASCII string
   "0295023820005424" (byte values in hexadecimal notation: 30 32 39 35
   30 32 33 38 32 30 30 30 35 34 32 34)

   The EAP server MAY use the leading "0" as a hint to try EAP-AKA as
   the first authentication method during method negotiation, rather
   than using, for example, EAP-SIM.  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 out of the scope of this
   document.  In this case, the peer implementation MUST NOT prepend any
   leading characters to the username.

4.1.1.7.  Generating Pseudonyms and Fast Re-Authentication Identities by
          the Server

   Pseudonym usernames and fast re-authentication identities are
   generated by the EAP server.  The EAP server produces pseudonym
   usernames and fast re-authentication identities in an
   implementation-dependent manner.  Only the EAP server needs to be
   able to map the pseudonym username to the permanent identity, or to
   recognize a fast re-authentication identity.

   EAP-AKA includes no provisions to ensure that the same EAP server
   that generated a pseudonym username will be used on the
   authentication exchange when the pseudonym username is used.  It is
   recommended that the EAP servers implement some centralized mechanism
   to allow all EAP servers of the home operator to map pseudonyms
   generated by other severs to the permanent identity.  If no such
   mechanism is available, then the EAP server, failing to understand a
   pseudonym issued by another server, can request the peer to send the
   permanent identity.

   When issuing a fast re-authentication identity, the EAP server may
   include a realm name in the identity that will cause the fast
   re-authentication request to be forwarded to the same EAP server.

   When generating fast re-authentication identities, the server SHOULD
   choose a fresh, new fast re-authentication identity that is different
   from the previous ones that were used after the same full
   authentication exchange.  A full authentication exchange and the
   associated fast re-authentication exchanges are referred to here as
   the same "full authentication context".  The fast re-authentication
   identity SHOULD include a random component.  The random component



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   works as a full authentication context identifier.  A context-
   specific fast re-authentication identity can help the server to
   detect whether its fast re-authentication state information matches
   the peer's fast re-authentication state information (in other words,
   whether the state information is from the same full authentication
   exchange).  The random component also makes the fast re-
   authentication identities unpredictable, so an attacker cannot
   initiate a fast re-authentication exchange to get the server's
   EAP-Request/AKA-Reauthentication packet.

   Transmitting pseudonyms and fast re-authentication identities from
   the server to the peer is discussed in Section 4.1.1.8.  The
   pseudonym is transmitted as a username, without an NAI realm, and the
   fast re-authentication identity is transmitted as a complete NAI,
   including a realm portion if a realm is required.  The realm is
   included in the fast re-authentication identity in order to allow the
   server to include a server-specific realm.

   Regardless of construction method, the pseudonym username MUST
   conform to the grammar specified for the username portion of an NAI.
   Also, the fast re-authentication identity MUST conform to the NAI
   grammar.  The EAP servers that the subscribers of an operator can use
   MUST ensure that the pseudonym usernames and the username portions
   used in fast re-authentication identities that they generate are
   unique.

   In any case, it is necessary that permanent usernames, pseudonym
   usernames, and fast re-authentication usernames 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 to the
   server when deciding which method to offer.

   In general, it is the task of the EAP server and the policies of its
   administrator to ensure sufficient separation of the usernames.
   Pseudonym usernames and fast re-authentication usernames are both
   produced and used by the EAP server.  The EAP server MUST compose
   pseudonym usernames and fast re-authentication usernames so that it
   can recognize if an NAI username is an EAP-AKA pseudonym username or
   an EAP-AKA fast re-authentication username.  For instance, when the
   usernames have been derived from the IMSI, the server could use
   different leading characters in the pseudonym usernames and fast
   re-authentication usernames (e.g., the pseudonym could begin with a
   leading "2" character).  When mapping a fast re-authentication
   identity to a permanent identity, the server SHOULD only examine the
   username portion of the fast re-authentication identity and ignore
   the realm portion of the identity.





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   Because the peer may fail to save a pseudonym username that was sent
   in an EAP-Request/AKA-Challenge (for example, due to malfunction),
   the EAP server SHOULD maintain, at least, the most recently used
   pseudonym username in addition to the most recently issued pseudonym
   username.  If the authentication exchange is not completed
   successfully, then the server SHOULD NOT overwrite the pseudonym
   username that was issued during the most recent successful
   authentication exchange.

4.1.1.8.  Transmitting Pseudonyms and Fast Re-Authentication Identities
          to the Peer

   The server transmits pseudonym usernames and fast re-authentication
   identities to the peer in cipher, using the AT_ENCR_DATA attribute.

   The EAP-Request/AKA-Challenge message MAY include an encrypted
   pseudonym username and/or an encrypted fast re-authentication
   identity in the value field of the AT_ENCR_DATA attribute.  Because
   identity privacy support and fast re-authentication are optional to
   implement, the peer MAY ignore the AT_ENCR_DATA attribute and always
   use the permanent identity.  On fast re-authentication (discussed in
   Section 5), the server MAY include a new, encrypted fast re-
   authentication identity in the EAP-Request/AKA-Reauthentication
   message.

   On receipt of the EAP-Request/AKA-Challenge, the peer MAY decrypt the
   encrypted data in AT_ENCR_DATA; and if a pseudonym username is
   included, the peer may use the obtained pseudonym username on the
   next full authentication.  If a fast re-authentication identity is
   included, then the peer MAY save it together with other fast re-
   authentication state information, as discussed in Section 5, for the
   next fast re-authentication.

   If the peer does not receive a new pseudonym username in the
   EAP-Request/AKA-Challenge message, the peer MAY use an old pseudonym
   username instead of the permanent username on next full
   authentication.  The username portions of fast re-authentication
   identities are one-time usernames, which the peer MUST NOT re-use.
   When the peer uses a fast re-authentication identity in an EAP
   exchange, the peer MUST discard the fast re-authentication identity
   and not re-use it in another EAP authentication exchange, even if the
   authentication exchange was not completed.

4.1.1.9.  Usage of the Pseudonym by the Peer

   When the optional identity privacy support is used on full
   authentication, the peer MAY use a pseudonym username received as
   part of a previous full authentication sequence as the username



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   portion of the NAI.  The peer MUST NOT modify the pseudonym username
   received in AT_NEXT_PSEUDONYM.  However, as discussed above, the peer
   MAY need to decorate the username in some environments by appending
   or prepending the username with a string that indicates supplementary
   AAA routing information.

   When using a pseudonym username in an environment where a realm
   portion is used, the peer concatenates the received pseudonym
   username with the "@" character and an NAI realm portion.  The
   selection of the NAI realm is discussed above.  The peer can select
   the realm portion similarly, regardless of whether it uses the
   permanent username or a pseudonym username.

4.1.1.10.  Usage of the Fast Re-Authentication Identity by the Peer

   On fast re-authentication, the peer uses the fast re-authentication
   identity received as part of the previous authentication sequence.  A
   new fast re-authentication identity may be delivered as part of both
   full authentication and fast re-authentication.  The peer MUST NOT
   modify the username part of the fast re-authentication identity
   received in AT_NEXT_REAUTH_ID, except in cases when username
   decoration is required.  Even in these cases, the "root" fast
   re-authentication username must not be modified, but it may be
   appended or prepended with another string.

4.1.2.  Communicating the Peer Identity to the Server

4.1.2.1.  General

   The peer identity MAY be communicated to the server with the
   EAP-Response/Identity message.  This message MAY contain the
   permanent identity, a pseudonym identity, or a fast re-authentication
   identity.  If the peer uses the permanent identity or a pseudonym
   identity, which the server is able to map to the permanent identity,
   then the authentication proceeds as discussed in the overview of
   Section 3.  If the peer uses a fast re-authentication identity, and
   if the fast re-authentication identity matches with a valid fast
   re-authentication identity maintained by the server, then a fast
   re-authentication exchange is performed, as described in Section 5.

   The peer identity can also be transmitted from the peer to the server
   using EAP-AKA messages instead of EAP-Response/Identity.  In this
   case, the server includes an identity requesting attribute
   (AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ) in the
   EAP-Request/AKA-Identity message; and the peer includes the
   AT_IDENTITY attribute, which contains the peer's identity, in the
   EAP-Response/AKA-Identity message.  The AT_ANY_ID_REQ attribute is a
   general identity requesting attribute, which the server uses if it



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   does not specify which kind of an identity the peer should return in
   AT_IDENTITY.  The server uses the AT_FULLAUTH_ID_REQ attribute to
   request either the permanent identity or a pseudonym identity.  The
   server uses the AT_PERMANENT_ID_REQ attribute to request that the
   peer send its permanent identity.  The EAP-Request/AKA-Challenge,
   EAP-Response/AKA-Challenge, or the packets used on fast re-
   authentication may optionally include the AT_CHECKCODE attribute,
   which enables the protocol peers to ensure the integrity of the
   AKA-Identity packets.  AT_CHECKCODE is specified in Section 10.13.

   The identity format in the AT_IDENTITY attribute is the same as in
   the EAP-Response/Identity packet (except that identity decoration is
   not allowed).  The AT_IDENTITY attribute contains a permanent
   identity, a pseudonym identity, or a fast re-authentication identity.

   Please note that only the EAP-AKA peer and the EAP-AKA server process
   the AT_IDENTITY attribute and entities that pass through; EAP packets
   do not process this attribute.  Hence, the authenticator and other
   intermediate AAA elements (such as possible AAA proxy servers) will
   continue to refer to the peer with the original identity from the
   EAP-Response/Identity packet unless the identity authenticated in the
   AT_IDENTITY attribute is communicated to them in another way within
   the AAA protocol.

4.1.2.2.  Relying on EAP-Response/Identity Discouraged

   The EAP-Response/Identity packet is not method specific; therefore,
   in many implementations it may be handled by an EAP Framework.  This
   introduces an additional layer of processing between the EAP peer and
   EAP server.  The extra layer of processing may cache identity
   responses or add decorations to the identity.  A modification of the
   identity response will cause the EAP peer and EAP server to use
   different identities in the key derivation, which will cause the
   protocol to fail.

   For this reason, it is RECOMMENDED that the EAP peer and server use
   the method-specific identity attributes in EAP-AKA, and the server is
   strongly discouraged from relying upon the EAP-Response/Identity.

   In particular, if the EAP server receives a decorated identity in
   EAP-Response/Identity, then the EAP server MUST use the
   identity-requesting attributes to request the peer to send an
   unmodified and undecorated copy of the identity in AT_IDENTITY.








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4.1.3.  Choice of Identity for the EAP-Response/Identity

   If EAP-AKA peer is started upon receiving an EAP-Request/Identity
   message, then the peer MAY use an EAP-AKA identity in the EAP-
   Response/Identity packet.  In this case, the peer performs the
   following steps.

   If the peer has maintained fast re-authentication state information
   and if the peer wants to use fast re-authentication, then the peer
   transmits the fast re-authentication identity in
   EAP-Response/Identity.

   Else, if the peer has a pseudonym username available, then the peer
   transmits the pseudonym identity in EAP-Response/Identity.

   In other cases, the peer transmits the permanent identity in
   EAP-Response/Identity.

4.1.4.  Server Operation in the Beginning of EAP-AKA Exchange

   As discussed in Section 4.1.2.2, the server SHOULD NOT rely on an
   identity string received in EAP-Response/Identity.  Therefore, the
   RECOMMENDED way to start an EAP-AKA exchange is to ignore any
   received identity strings.  The server SHOULD begin the EAP-AKA
   exchange by issuing the EAP-Request/AKA-Identity packet with an
   identity-requesting attribute to indicate that the server wants the
   peer to include an identity in the AT_IDENTITY attribute of the EAP-
   Response/AKA-Identity message.  Three methods to request an identity
   from the peer are discussed below.

   If the server chooses to not ignore the contents of
   EAP-Response/Identity, then the server may already receive an EAP-AKA
   identity in this packet.  However, if the EAP server has not received
   any EAP-AKA peer identity (permanent identity, pseudonym identity, or
   fast re-authentication identity) from the peer when sending the first
   EAP-AKA request, or if the EAP server has received an
   EAP-Response/Identity packet but the contents do not appear to be a
   valid permanent identity, pseudonym identity, or a re-authentication
   identity, then the server MUST request an identity from the peer
   using one of the methods below.

   The server sends the EAP-Request/AKA-Identity message with the
   AT_PERMANENT_ID_REQ attribute to indicate that the server wants the
   peer to include the permanent identity in the AT_IDENTITY attribute
   of the EAP-Response/AKA-Identity message.  This is done in the
   following cases:





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   o  The server does not support fast re-authentication or identity
      privacy.
   o  The server decided to process a received identity, and the server
      recognizes the received identity as a pseudonym identity, but the
      server is not able to map the pseudonym identity to a permanent
      identity.

   The server issues the EAP-Request/AKA-Identity packet with the
   AT_FULLAUTH_ID_REQ attribute to indicate that the server wants the
   peer to include a full authentication identity (pseudonym identity or
   permanent identity) in the AT_IDENTITY attribute of the
   EAP-Response/AKA-Identity message.  This is done in the following
   cases:

   o  The server does not support fast re-authentication and the server
      supports identity privacy
   o  The server decided to process a received identity, and the server
      recognizes the received identity as a re-authentication identity
      but the server is not able to map the re-authentication identity
      to a permanent identity

   The server issues the EAP-Request/AKA-Identity packet with the
   AT_ANY_ID_REQ attribute to indicate that the server wants the peer to
   include an identity in the AT_IDENTITY attribute of the
   EAP-Response/AKA-Identity message, and the server does not indicate
   any preferred type for the identity.  This is done in other cases,
   such as when the server ignores a received EAP-Response/Identity,
   when the server does not have any identity, or when the server does
   not recognize the format of a received identity.

4.1.5.  Processing of EAP-Request/AKA-Identity by the Peer

   Upon receipt of an EAP-Request/AKA-Identity message, the peer MUST
   perform the following steps.

   If the EAP-Request/AKA-Identity includes AT_PERMANENT_ID_REQ, and if
   the peer does not have a pseudonym available, then the peer MUST
   respond with EAP-Response/AKA-Identity and include the permanent
   identity in AT_IDENTITY.  If the peer has a pseudonym available, then
   the peer MAY refuse to send the permanent identity; hence, in this
   case the peer MUST either respond with EAP-Response/AKA-Identity and
   include the permanent identity in AT_IDENTITY or respond with
   EAP-Response/AKA-Client-Error packet with code "unable to process
   packet".

   If the EAP-Request/AKA-Identity includes AT_FULL_AUTH_ID_REQ, and if
   the peer has a pseudonym available, then the peer SHOULD respond with
   EAP-Response/AKA-Identity and include the pseudonym identity in



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   AT_IDENTITY.  If the peer does not have a pseudonym when it receives
   this message, then the peer MUST respond with EAP-Response/
   AKA-Identity and include the permanent identity in AT_IDENTITY.  The
   Peer MUST NOT use a fast re-authentication identity in the
   AT_IDENTITY attribute.

   If the EAP-Request/AKA-Identity includes AT_ANY_ID_REQ, and if the
   peer has maintained fast re-authentication state information and
   wants to use fast re-authentication, then the peer responds with
   EAP-Response/AKA-Identity and includes the fast re-authentication
   identity in AT_IDENTITY.  Else, if the peer has a pseudonym identity
   available, then the peer responds with EAP-Response/AKA-Identity and
   includes the pseudonym identity in AT_IDENTITY.  Else, the peer
   responds with EAP-Response/AKA-Identity and includes the permanent
   identity in AT_IDENTITY.

   An EAP-AKA exchange may include several EAP/AKA-Identity rounds.  The
   server may issue a second EAP-Request/AKA-Identity, if it was not
   able to recognize the identity the peer used in the previous
   AT_IDENTITY attribute.  At most three EAP/AKA-Identity rounds can be
   used, so the peer MUST NOT respond to more than three
   EAP-Request/AKA-Identity messages within an EAP exchange.  The peer
   MUST verify that the sequence of EAP-Request/AKA-Identity packets the
   peer receives comply with the sequencing rules defined in this
   document.  That is, AT_ANY_ID_REQ can only be used in the first
   EAP-Request/AKA-Identity; in other words, AT_ANY_ID_REQ MUST NOT be
   used in the second or third EAP-Request/AKA-Identity.
   AT_FULLAUTH_ID_REQ MUST NOT be used if the previous
   EAP-Request/AKA-Identity included AT_PERMANENT_ID_REQ.  The peer
   operation, in cases when it receives an unexpected attribute or an
   unexpected message, is specified in Section 6.3.1.

4.1.6.  Attacks against Identity Privacy

   The section above specifies two possible ways the peer can operate
   upon receipt of AT_PERMANENT_ID_REQ because a received
   AT_PERMANENT_ID_REQ does not necessarily originate from the valid
   network.  However, an active attacker may transmit an
   EAP-Request/AKA-Identity packet with an AT_PERMANENT_ID_REQ attribute
   to the peer, in an effort to find out the true identity of the user.
   If the peer does not want to reveal its permanent identity, then the
   peer sends the EAP-Response/AKA-Client-Error packet with the error
   code "unable to process packet", and the authentication exchange
   terminates.

   Basically, there are two different policies that the peer can employ
   with regard to AT_PERMANENT_ID_REQ.  A "conservative" peer assumes
   that the network is able to maintain pseudonyms robustly.  Therefore,



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   if a conservative peer has a pseudonym username, the peer responds
   with EAP-Response/AKA-Client-Error to the EAP packet with
   AT_PERMANENT_ID_REQ, because the peer believes that the valid network
   is able to map the pseudonym identity to the peer's permanent
   identity.  (Alternatively, the conservative peer may accept
   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 peer against active attacks on anonymity.  On
   the other hand, a "liberal" peer 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 map them to the
   permanent identity.

4.1.7.  Processing of AT_IDENTITY by the Server

   When the server receives an EAP-Response/AKA-Identity message with
   the AT_IDENTITY (in response to the server's identity requesting
   attribute), the server MUST operate as follows.

   If the server used AT_PERMANENT_ID_REQ, and if the AT_IDENTITY does
   not contain a valid permanent identity, then the server sends an
   EAP-Request/AKA-Notification packet with AT_NOTIFICATION code
   "General failure" (16384) to terminate the EAP exchange.  If the
   server recognizes the permanent identity and is able to continue,
   then the server proceeds with full authentication by sending
   EAP-Request/AKA-Challenge.

   If the server used AT_FULLAUTH_ID_REQ, and if AT_IDENTITY contains a
   valid permanent identity or a pseudonym identity that the server can
   map to a valid permanent identity, then the server proceeds with full
   authentication by sending EAP-Request/AKA-Challenge.  If AT_IDENTITY
   contains a pseudonym identity that the server is not able to map to a
   valid permanent identity, or an identity that the server is not able
   to recognize or classify, then the server sends EAP-Request/
   AKA-Identity with AT_PERMANENT_ID_REQ.

   If the server used AT_ANY_ID_REQ, and if the AT_IDENTITY contains a
   valid permanent identity or a pseudonym identity that the server can
   map to a valid permanent identity, then the server proceeds with full
   authentication by sending EAP-Request/ AKA-Challenge.

   If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid
   fast re-authentication identity and the server agrees on using
   re-authentication, then the server proceeds with fast
   re-authentication by sending EAP-Request/AKA-Reauthentication
   (Section 5).




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   If the server used AT_ANY_ID_REQ, and if the peer sent an EAP-
   Response/AKA-Identity with AT_IDENTITY that contains an identity that
   the server recognizes as a fast re-authentication identity, but the
   server is not able to map the identity to a permanent identity, then
   the server sends EAP-Request/AKA-Identity with AT_FULLAUTH_ID_REQ.

   If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid
   fast re-authentication identity, which the server is able to map to a
   permanent identity, and if the server does not want to use fast
   re-authentication, then the server proceeds with full authentication
   by sending EAP-Request/AKA-Challenge.

   If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an
   identity that the server recognizes as a pseudonym identity but the
   server is not able to map the pseudonym identity to a permanent
   identity, then the server sends EAP-Request/AKA-Identity with
   AT_PERMANENT_ID_REQ.

   If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an
   identity that the server is not able to recognize or classify, then
   the server sends EAP-Request/AKA-Identity with AT_FULLAUTH_ID_REQ.

4.2.  Message Sequence Examples (Informative)

   This section contains non-normative message sequence examples to
   illustrate how the peer identity can be communicated to the server.

4.2.1.  Usage of AT_ANY_ID_REQ

   Obtaining the peer identity with EAP-AKA attributes is illustrated in
   Figure 5 below.

       Peer                                             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)                                         |
          |------------------------------------------------------>|
          |                                                       |
                     Figure 5: Usage of AT_ANY_ID_REQ



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4.2.2.  Fall Back on Full Authentication

   Figure 6 illustrates the case when the server does not recognize the
   fast re-authentication identity the peer used in AT_IDENTITY.

       Peer                                             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 fast re-auth. identity)     |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server does not recognize    |
          |                            | The fast re-auth.            |
          |                            | Identity                     |
          |                            +------------------------------+
          |     EAP-Request/AKA-Identity                          |
          |     (AT_FULLAUTH_ID_REQ)                              |
          |<------------------------------------------------------|
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with a full-auth. Identity)              |
          |------------------------------------------------------>|
          |                                                       |

                Figure 6: Fall back on full authentication

   If the server recognizes the fast 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.













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4.2.3.  Requesting the Permanent Identity 1

   Figure 7 illustrates the case when the EAP server fails to decode a
   pseudonym identity included in the EAP-Response/Identity packet.

       Peer                                             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)                 |
          |------------------------------------------------------>|
          |                                                       |

               Figure 7: Requesting the permanent identity 1

   If the server recognizes the permanent identity, then the
   authentication sequence proceeds as usual with the EAP Server issuing
   the EAP-Request/AKA-Challenge message.






















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4.2.4.  Requesting the Permanent Identity 2

   Figure 8 illustrates the case when the EAP server fails to decode the
   pseudonym included in the AT_IDENTITY attribute.

       Peer                                             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)                 |
          |------------------------------------------------------>|
          |                                                       |

               Figure 8: Requesting the permanent identity 2

4.2.5.  Three EAP/AKA-Identity Round Trips

   Figure 9 illustrates the case with three EAP/AKA-Identity round
   trips.














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       Peer                                             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 fast re-auth. identity)             |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server does not accept       |
          |                            | The fast 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)                 |
          |------------------------------------------------------>|
          |                                                       |

                   Figure 9: Three EAP-AKA Start rounds

   After the last EAP-Response/AKA-Identity message, the full
   authentication sequence proceeds as usual.



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5.  Fast Re-Authentication

5.1.  General

   In some environments, EAP authentication may be performed frequently.
   Because the EAP-AKA full authentication procedure uses the AKA
   algorithms, and therefore requires fresh 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 fast re-authentication
   procedure that does not make use of the AKA algorithms and does not
   need new vectors from the Authentication Centre.

   Fast re-authentication is optional to implement for both the EAP-AKA
   server and peer.  On each EAP authentication, either one of the
   entities may fall back on full authentication if is does not want to
   use fast re-authentication.

   Fast re-authentication is based on the keys derived on the preceding
   full authentication.  The same K_aut and K_encr keys used in full
   authentication are used to protect EAP-AKA packets and attributes,
   and the original Master Key from full authentication is used to
   generate a fresh Master Session Key, as specified in Section 7.

   The fast re-authentication exchange makes use of an unsigned 16-bit
   counter, included in the AT_COUNTER attribute.  The counter has three
   goals: 1) it can be used to limit the number of successive
   reauthentication exchanges without full-authentication 2) it
   contributes to the keying material, and 3) it protects the peer and
   the server from replays.  On full authentication, both the server and
   the peer initialize the counter to one.  The counter value of at
   least one is used on the first fast re-authentication.  On subsequent
   fast re-authentications, the counter MUST be greater than on any of
   the previous fast re-authentications.  For example, on the second
   fast re-authentication, counter value is two or greater, etc.  The
   AT_COUNTER attribute is encrypted.

   Both the peer and the EAP server maintain a copy of the counter.  The
   EAP server sends its counter value to the peer in the fast
   re-authentication request.  The peer MUST verify that its counter
   value is less than or equal to the value sent by the EAP server.

   The server includes an encrypted server random nonce (AT_NONCE_S) in
   the fast re-authentication request.  The AT_MAC attribute in the
   peer's response is calculated over NONCE_S to provide a
   challenge/response authentication scheme.  The NONCE_S also
   contributes to the new Master Session Key.




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   Both the peer and the server SHOULD have an upper limit for the
   number of subsequent fast re-authentications allowed before a full
   authentication needs to be performed.  Because a 16-bit counter is
   used in fast re-authentication, the theoretical maximum number of
   re-authentications is reached when the counter value reaches FFFF
   hexadecimal.  In order to use fast re-authentication, the peer and
   the EAP server need to store the following values: Master Key, latest
   counter value and the next fast re-authentication identity.  K_aut
   and K_encr may either be stored or derived again from MK.  The server
   may also need to store the permanent identity of the user.

5.2.  Comparison to AKA

   When analyzing the fast re-authentication exchange, it may be helpful
   to compare it with the 3rd generation Authentication and Key
   Agreement (AKA) exchange used on full authentication.  The counter
   corresponds to the AKA sequence number, NONCE_S corresponds to RAND,
   the AT_MAC in EAP-Request/AKA-Reauthentication corresponds to AUTN,
   the AT_MAC in EAP-Response/AKA-Reauthentication corresponds to RES,
   AT_COUNTER_TOO_SMALL corresponds to AUTS, and encrypting the counter
   corresponds to the usage of the Anonymity Key.  Also, the key
   generation on fast re-authentication, with regard to random or fresh
   material, is similar to AKA -- the server generates the NONCE_S and
   counter values, and the peer only verifies that the counter value is
   fresh.

   It should also be noted that encrypting the AT_NONCE_S, AT_COUNTER,
   or AT_COUNTER_TOO_SMALL attributes is not important to the security
   of the fast re-authentication exchange.

5.3.  Fast Re-Authentication Identity

   The fast re-authentication procedure makes use of separate
   re-authentication user identities.  Pseudonyms and the permanent
   identity are reserved for full authentication only.  If a fast
   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 fast 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 fast re-authentication.  The
   attribute also works as a capability flag that indicates that the
   server supports fast re-authentication and that the server wants to
   continue using fast re-authentication within the current context.
   The peer MAY ignore this attribute, in which case it will use full
   authentication next time.  If the peer wants to use fast
   re-authentication, it uses this fast re-authentication identity on
   next authentication.  Even if the peer has a fast re-authentication



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   identity, the peer MAY discard the re-authentication identity and use
   a pseudonym or the permanent identity instead, in which case full
   authentication MUST be performed.  If the EAP server does not include
   the AT_NEXT_REAUTH_ID in the encrypted data of
   EAP-Request/AKA-Challenge or EAP-Request/AKA-Reauthentication, then
   the peer MUST discard its current fast re-authentication state
   information and perform a full authentication next time.

   In environments where a realm portion is needed in the peer identity,
   the fast re-authentication identity received in AT_NEXT_REAUTH_ID
   MUST contain both a username portion and a realm portion, as per the
   NAI format.  The EAP Server can choose an appropriate realm part in
   order to have the AAA infrastructure route subsequent fast
   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 fast re-authentication in the AAA server that performed the full
   authentication.

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

   The peer MUST NOT modify the username portion of the fast
   re-authentication identity, but the peer MAY modify the realm portion
   or replace it with another realm portion.  The peer might need to
   modify the realm in order to influence the AAA routing, for example,
   to make sure that the correct server is reached.  It should be noted
   that sharing the same fast re-authentication key among several
   servers may have security risks, so changing the realm portion of the
   NAI in order to change the EAP server is not desirable.

   Even if the peer uses a fast re-authentication identity, the server
   may want to fall back on full authentication, for example, because
   the server does not recognize the fast re-authentication identity or
   does not want to use fast re-authentication.  If the server was able
   to decode the fast 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 peer's identity from the fast 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.





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5.4.  Fast Re-Authentication Procedure

   Figure 10 illustrates the fast re-authentication procedure.  In this
   example, the optional protected success indication is not used.
   Encrypted attributes are denoted with '*'.  The peer uses its fast
   re-authentication identity in the EAP-Response/Identity packet.  As
   discussed above, an alternative way to communicate the fast
   re-authentication identity to the server is for the peer to use the
   AT_IDENTITY attribute in the EAP-Response/AKA-Identity message.  This
   latter case is not illustrated in the figure below, and it is only
   possible when the server requests that the peer send its identity by
   including the AT_ANY_ID_REQ attribute in the EAP-Request/AKA-Identity
   packet.

   If the server recognizes the identity as a valid fast
   re-authentication identity, and if the server agrees to use fast
   re-authentication, then the server sends the EAP- Request/AKA-
   Reauthentication packet to the peer.  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 fast
   re-authentication identity.

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

   The peer verifies that AT_MAC is correct and that the counter value
   is fresh (greater than any previously used value).  The peer MAY save
   the next fast re-authentication identity from the encrypted
   AT_NEXT_REAUTH_ID for next time.  If all checks are successful, the
   peer 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 fast re-authentication has succeeded and the server
   sends the EAP-Success packet to the peer.

   If protected success indications (Section 6.2) were used, the
   EAP-Success packet would be preceded by an EAP-AKA notification
   round.



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        Peer                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes a fast 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)            |
          |<------------------------------------------------------|
          |                                                       |
          :                                                       :
          :                                                       :


          :                                                       :
          :                                                       :
          |                                                       |
     +-----------------------------------------------+            |
     | Peer verifies AT_MAC and the freshness of     |            |
     | the counter. Peer 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  |
          |<------------------------------------------------------|
          |                                                       |

                        Figure 10: Reauthentication







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5.5.  Fast Re-Authentication Procedure when Counter is Too Small

   If the peer 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 Figure 11.  Encrypted attributes
   are denoted with '*'.

        Peer                                             Authenticator
          |          EAP-Request/AKA-Identity                     |
          |          (AT_ANY_ID_REQ)                              |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY)                                         |
          | (Includes a fast 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 peer has included AT_COUNTER_TOO_SMALL|
          |            +----------------------------------------------+
          |                        EAP-Request/AKA-Challenge      |
          |<------------------------------------------------------|
     +---------------------------------------------------------------+
     |                Normal full authentication follows.            |
     +---------------------------------------------------------------+
          |                                                       |

            Figure 11: Fast re-authentication counter too small

   In the figure above, the first three messages are similar to the
   basic fast re-authentication case.  When the peer detects that the
   counter value is not fresh, it includes the AT_COUNTER_TOO_SMALL
   attribute in EAP-Response/AKA-Reauthentication.  This attribute



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   doesn't contain any data but it is a request for the server to
   initiate full authentication.  In this case, the peer 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 counter value as in the
   EAP-Request/AKA-Reauthentication packet.  If not, the server
   terminates the authentication exchange by sending the
   EAP-Request/AKA-Notification packet with AT_NOTIFICATION code
   "General failure" (16384).  If all checks on the packet are
   successful, the server transmits an EAP-Request/AKA-Challenge packet
   and the full authentication procedure is performed as usual.  Because
   the server already knows the subscriber identity, it MUST NOT use the
   EAP-Request/AKA-Identity packet to request the identity.

   It should be noted that in this case, peer identity is only
   transmitted in the AT_IDENTITY attribute at the beginning of the
   whole EAP exchange.  The fast re-authentication identity used in this
   AT_IDENTITY attribute will be used in key derivation (see Section 7).

6.  EAP-AKA Notifications

6.1.  General

   EAP-AKA does not prohibit the use of the EAP Notifications as
   specified in [RFC3748].  EAP Notifications can be used at any time in
   the EAP-AKA exchange.  It should be noted that EAP-AKA does not
   protect EAP Notifications.  EAP-AKA also specifies method-specific
   EAP-AKA notifications, which are protected in some cases.

   The EAP server can use EAP-AKA notifications to convey notifications
   and result indications (Section 6.2) to the peer.

   The server MUST use notifications in cases discussed in
   Section 6.3.2.  When the EAP server issues an
   EAP-Request/AKA-Notification packet to the peer, the peer MUST
   process the notification packet.  The peer MAY show a notification
   message to the user and the peer MUST respond to the EAP server with
   an EAP-Response/AKA-Notification packet, even if the peer did not
   recognize the notification code.

   An EAP-AKA full authentication exchange or a fast re-authentication
   exchange MUST NOT include more than one EAP-AKA notification round.

   The notification code is a 16-bit number.  The most significant bit
   is called the Success bit (S bit).  The S bit specifies whether the
   notification implies failure.  The code values with the S bit set to
   zero (code values 0...32767) are used on unsuccessful cases.  The



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   receipt of a notification code from this range implies failed EAP
   exchange, so the peer can use the notification as a failure
   indication.  After receiving the EAP-Response/AKA-Notification for
   these notification codes, the server MUST send the EAP-Failure
   packet.

   The receipt of a notification code with the S bit set to one (values
   32768...65536) does not imply failure.  Notification code "Success"
   (32768) has been reserved as a general notification code to indicate
   successful authentication.

   The second most significant bit of the notification code is called
   the Phase bit (P bit).  It specifies at which phase of the EAP-AKA
   exchange the notification can be used.  If the P bit is set to zero,
   the notification can only be used after a successful EAP/AKA-
   Challenge round in full authentication or a successful EAP/AKA-
   Reauthentication round in re-authentication.  A re-authentication
   round is considered successful only if the peer has successfully
   verified AT_MAC and AT_COUNTER attributes, and does not include the
   AT_COUNTER_TOO_SMALL attribute in EAP-Response/AKA-Reauthentication.

   If the P bit is set to one, the notification can only by used before
   the EAP/AKA-Challenge round in full authentication or before the
   EAP/AKA-Reauthentication round in reauthentication.  These
   notifications can only be used to indicate various failure cases.  In
   other words, if the P bit is set to one, then the S bit MUST be set
   to zero.

   Section 9.10 and Section 9.11 specify what other attributes must be
   included in the notification packets.

   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.

6.2.  Result Indications

   As discussed in Section 6.3, the server and the peer use explicit
   error messages in all error cases.  If the server detects an error
   after successful authentication, the server uses an EAP-AKA
   notification to indicate failure to the peer.  In this case, the
   result indication is integrity and replay protected.





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   By sending an EAP-Response/AKA-Challenge packet or an
   EAP-Response/AKA-Reauthentication packet (without
   AT_COUNTER_TOO_SMALL), the peer indicates that it has successfully
   authenticated the server and that the peer's local policy accepts the
   EAP exchange.  In other words, these packets are implicit success
   indications from the peer to the server.

   EAP-AKA also supports optional protected success indications from the
   server to the peer.  If the EAP server wants to use protected success
   indications, it includes the AT_RESULT_IND attribute in the
   EAP-Request/AKA-Challenge or the EAP-Request/AKA-Reauthentication
   packet.  This attribute indicates that the EAP server would like to
   use result indications in both successful and unsuccessful cases.  If
   the peer also wants this, the peer includes AT_RESULT_IND in
   EAP-Response/AKA-Challenge or EAP-Response/AKA-Reauthentication.  The
   peer MUST NOT include AT_RESULT_IND if it did not receive
   AT_RESULT_IND from the server.  If both the peer and the server used
   AT_RESULT_IND, then the EAP exchange is not complete yet, but an
   EAP-AKA notification round will follow.  The following EAP-AKA
   notification may indicate either failure or success.

   Success indications with the AT_NOTIFICATION code "Success" (32768)
   can only be used if both the server and the peer indicate they want
   to use them with AT_RESULT_IND.  If the server did not include
   AT_RESULT_IND in the EAP-Request/AKA-Challenge or
   EAP-Request/AKA-Reauthentication packet, or if the peer did not
   include AT_RESULT_IND in the corresponding response packet, then the
   server MUST NOT use protected success indications.

   Because the server uses the AT_NOTIFICATION code "Success" (32768) to
   indicate that the EAP exchange has completed successfully, the EAP
   exchange cannot fail when the server processes the EAP-AKA response
   to this notification.  Hence, the server MUST ignore the contents of
   the EAP-AKA response it receives to the EAP-Request/AKA-Notification
   with this code.  Regardless of the contents of the EAP-AKA response,
   the server MUST send EAP-Success as the next packet.

6.3.  Error Cases

   This section specifies the operation of the peer and the server in
   error cases.  The subsections below require the EAP-AKA peer and
   server to send an error packet (EAP-Response/AKA-Client-Error,
   EAP-Response/AKA-Authentication-Reject or
   EAP-Response/AKA-Synchronization-Failure from the peer and
   EAP-Request/AKA-Notification from the server) in error cases.
   However, implementations SHOULD NOT rely upon the correct error
   reporting behavior of the peer, authenticator, or server.  It is
   possible for error messages and other messages to be lost in transit,



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   or for a malicious participant to attempt to consume resources by not
   issuing error messages.  Both the peer and the EAP server SHOULD have
   a mechanism to clean up state even if an error message or EAP-Success
   is not received after a timeout period.

6.3.1.  Peer Operation

   Two special error messages have been specified for error cases that
   are related to the processing of the AKA AUTN parameter, as described
   in Section 3: (1) if the peer does not accept AUTN, the peer responds
   with EAP-Response/AKA-Authentication-Reject (Section 9.5), and the
   server issues EAP-Failure, and (2) if the peer detects that the
   sequence number in AUTN is not correct, the peer responds with
   EAP-Response/AKA-Synchronization-Failure (Section 9.6), and the
   server proceeds with a new EAP-Request/AKA-Challenge.

   In other error cases, when an EAP-AKA peer detects an error in a
   received EAP-AKA packet, the EAP-AKA peer responds with the
   EAP-Response/AKA-Client-Error packet.  In response to the
   EAP-Response/AKA-Client-Error, the EAP server MUST issue the
   EAP-Failure packet, and the authentication exchange terminates.

   By default, the peer uses the client error code 0, "unable to process
   packet".  This error code is used in the following cases:

   o  EAP exchange is not acceptable according to the peer's local
      policy.
   o  The peer is not able to parse the EAP request, i.e., the EAP
      request is malformed.
   o  The peer encountered a malformed attribute.
   o  Wrong attribute types or duplicate attributes have been included
      in the EAP request.
   o  A mandatory attribute is missing.
   o  Unrecognized non-skippable attribute.
   o  Unrecognized or unexpected EAP-AKA Subtype in the EAP request.
   o  Invalid AT_MAC.  The peer SHOULD log this event.
   o  Invalid AT_CHECKCODE.  The peer SHOULD log this event.
   o  Invalid pad bytes in AT_PADDING.
   o  The peer does not want to process AT_PERMANENT_ID_REQ.

6.3.2.  Server Operation

   If an EAP-AKA server detects an error in a received EAP-AKA response,
   the server MUST issue the EAP-Request/AKA-Notification packet with an
   AT_NOTIFICATION code that implies failure.  By default, the server
   uses one of the general failure codes ("General failure after
   authentication" (0) or "General failure" (16384)).  The choice




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   between these two codes depends on the phase of the EAP-AKA exchange,
   see Section 6.  The error cases when the server issues an
   EAP-Request/AKA-Notification that implies failure include the
   following:

   o  The server is not able to parse the peer's EAP response.
   o  The server encounters a malformed attribute, a non-recognized
      non-skippable attribute, or a duplicate attribute.
   o  A mandatory attribute is missing or an invalid attribute was
      included.
   o  Unrecognized or unexpected EAP-AKA Subtype in the EAP Response.
   o  Invalid AT_MAC.  The server SHOULD log this event.
   o  Invalid AT_CHECKCODE.  The server SHOULD log this event.
   o  Invalid AT_COUNTER.

6.3.3.  EAP-Failure

   The EAP-AKA server sends EAP-Failure in three cases:

   1.  In response to an EAP-Response/AKA-Client-Error packet the server
       has received from the peer, or

   2.  In response to an EAP-Response/AKA-Authentication-Reject packet
       the server has received from the peer, or

   3.  Following an EAP-AKA notification round, when the AT_NOTIFICATION
       code implies failure.

   The EAP-AKA server MUST NOT send EAP-Failure in other cases than
   these three.  However, it should be noted that even though the
   EAP-AKA server would not send an EAP-Failure, an authorization
   decision that happens outside EAP-AKA, such as in the AAA server or
   in an intermediate AAA proxy, may result in a failed exchange.

   The peer MUST accept the EAP-Failure packet in case 1), case 2), and
   case 3) above.  The peer SHOULD silently discard the EAP-Failure
   packet in other cases.

6.3.4.  EAP-Success

   On full authentication, the server can only send EAP-Success after
   the EAP/AKA-Challenge round.  The peer MUST silently discard any
   EAP-Success packets if they are received before the peer has
   successfully authenticated the server and sent the
   EAP-Response/AKA-Challenge packet.






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   If the peer did not indicate that it wants to use protected success
   indications with AT_RESULT_IND (as discussed in Section 6.2) on full
   authentication, then the peer MUST accept EAP-Success after a
   successful EAP/AKA-Challenge round.

   If the peer indicated that it wants to use protected success
   indications with AT_RESULT_IND (as discussed in Section 6.2), then
   the peer MUST NOT accept EAP-Success after a successful EAP/
   AKA-Challenge round.  In this case, the peer MUST only accept
   EAP-Success after receiving an EAP-AKA Notification with the
   AT_NOTIFICATION code "Success" (32768).

   On fast re-authentication, EAP-Success can only be sent after the
   EAP/AKA-Reauthentication round.  The peer MUST silently discard any
   EAP-Success packets if they are received before the peer has
   successfully authenticated the server and sent the
   EAP-Response/AKA-Reauthentication packet.

   If the peer did not indicate that it wants to use protected success
   indications with AT_RESULT_IND (as discussed in Section 6.2) on fast
   re-authentication, then the peer MUST accept EAP-Success after a
   successful EAP/AKA-Reauthentication round.

   If the peer indicated that it wants to use protected success
   indications with AT_RESULT_IND (as discussed in Section 6.2), then
   the peer MUST NOT accept EAP-Success after a successful EAP/AKA-
   Reauthentication round.  In this case, the peer MUST only accept
   EAP-Success after receiving an EAP-AKA Notification with the
   AT_NOTIFICATION code "Success" (32768).

   If the peer receives an EAP-AKA notification (Section 6) that
   indicates failure, then the peer MUST no longer accept the
   EAP-Success packet, even if the server authentication was
   successfully completed.

7.  Key Generation

   This section specifies how keying material is generated.

   On EAP-AKA full authentication, a Master Key (MK) is derived from the
   underlying AKA values (CK and IK keys), and the identity, as follows.

   MK = SHA1(Identity|IK|CK)

   In the formula above, the "|" character denotes concatenation.
   Identity denotes the peer identity string without any terminating
   null characters.  It is the identity from the last AT_IDENTITY
   attribute sent by the peer in this exchange, or, if AT_IDENTITY was



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   not used, the identity from the EAP-Response/Identity packet.  The
   identity string is included as-is, without any changes.  As discussed
   in Section 4.1.2.2, relying on EAP-Response/Identity for conveying
   the EAP-AKA peer identity is discouraged, and the server SHOULD use
   the EAP-AKA method-specific identity attributes.  The hash function
   SHA-1 is specified in [SHA-1].

   The Master Key is fed into a Pseudo-Random number Function (PRF),
   which generates separate Transient EAP Keys (TEKs) for protecting
   EAP-AKA packets, as well as a Master Session Key (MSK) for link layer
   security and an Extended Master Session Key (EMSK) for other
   purposes.  On fast re-authentication, the same TEKs MUST be used for
   protecting EAP packets, but a new MSK and a new EMSK MUST be derived
   from the original MK and from new values exchanged in the fast
   re-authentication.

   EAP-AKA requires two TEKs for its own purposes: the authentication
   key K_aut, to be used with the AT_MAC attribute, and the encryption
   key K_encr, to be used with the AT_ENCR_DATA attribute.  The same
   K_aut and K_encr keys are used in full authentication and subsequent
   fast re-authentications.

   Key derivation is based on the random number generation specified in
   NIST Federal Information Processing Standards (FIPS) Publication
   186-2 [PRF].  The pseudo-random number generator is specified in the
   change notice 1 (2001 October 5) of [PRF] (Algorithm 1).  As
   specified in the change notice (page 74), when Algorithm 1 is used as
   a general-purpose pseudo-random number generator, the "mod q" term in
   step 3.3 is omitted.  The function G used in the algorithm is
   constructed via Secure Hash Standard as specified in Appendix 3.3 of
   the standard.  It should be noted that the function G is very similar
   to SHA-1, but the message padding is different.  Please refer to
   [PRF] for full details.  For convenience, the random number algorithm
   with the correct modification is cited in Annex A.

   160-bit XKEY and XVAL values are used, so b = 160.  On each full
   authentication, the Master Key is used as the initial secret seed-key
   XKEY.  The optional user input values (XSEED_j) in step 3.1 are set
   to zero.

   On full authentication, 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), Master Session Key (64 bytes), Extended Master
   Session Key (64 bytes).






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   On fast re-authentication, the same pseudo-random number generator
   can be used to generate a new Master Session Key and a new Extended
   Master Session Key.  The seed value XKEY' is calculated as follows:

   XKEY' = SHA1(Identity|counter|NONCE_S| MK)

   In the formula above, the Identity denotes the fast re-authentication
   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 fast re-authentication, it
   denotes the identity string from the EAP-Response/Identity packet.
   The counter denotes the counter value from the 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 random
   NONCE_S value from the AT_NONCE_S attribute used in the
   EAP-Request/AKA-Reauthentication packet.  The MK is the Master Key
   derived on the preceding full authentication.

   On fast re-authentication, the pseudo-random number generator is run
   with the new seed value XKEY', and the resulting 320-bit random
   numbers x_0, x_1, ..., x_m-1 are concatenated and partitioned into
   64-byte chunks and used as the new 64-byte Master Session Key and the
   new 64-byte Extended Master Session Key.  Note that because K_encr
   and K_aut are not derived on fast re-authentication, the Master
   Session Key and the Extended Master Session key are obtained from the
   beginning of the key stream x_0, x_1, ....

   The first 32 bytes of the MSK can be used as the Pairwise Master Key
   (PMK) for IEEE 802.11i.

   When the RADIUS attributes specified in [RFC2548] are used to
   transport keying material, then the first 32 bytes of the MSK
   correspond to MS-MPPE-RECV-KEY and the second 32 bytes to
   MS-MPPE-SEND-KEY.  In this case, only 64 bytes of keying material
   (the MSK) are used.

8.  Message Format and Protocol Extensibility

8.1.  Message Format

   As specified in [RFC3748], EAP packets begin with the Code,
   Identifiers, Length, and Type fields, which are followed by
   EAP-method-specific Type-Data.  The Code field in the EAP header is
   set to 1 for EAP requests, and to 2 for EAP Responses.  The usage of
   the Length and Identifier fields in the EAP header is also specified
   in [RFC3748].  In EAP-AKA, the Type field is set to 23.





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   In EAP-AKA, the Type-Data begins with an EAP-AKA header that consists
   of a 1-octet Subtype field, and a 2-octet reserved field.  The
   Subtype values used in EAP-AKA are defined in Section 11.  The
   formats of the EAP header and the EAP-AKA header 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Subtype    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The rest of the Type-Data, immediately following the EAP-AKA header,
   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.

   Attributes numbered within the range 0 through 127 are called
   non-skippable attributes.  When an EAP-AKA peer encounters a
   non-skippable attribute type that the peer does not recognize, the
   peer MUST send the EAP-Response/AKA-Client-Error packet, and the
   authentication exchange terminates.  If an EAP-AKA server encounters
   a non-skippable attribute that the server does not recognize, then
   the server sends EAP-Request/AKA-Notification packet with an



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   AT_NOTIFICATION code that implies general failure ("General failure
   after authentication" (0), or "General failure" (16384), depending on
   the phase of the exchange), and the authentication exchange
   terminates.

   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.

   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 will be used.

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

8.2.  Protocol Extensibility

   EAP-AKA can be extended by specifying new attribute types.  If
   skippable attributes are used, it is possible to extend the protocol
   without breaking old implementations.  As specified in Section 10.13,
   if new attributes are specified for EAP-Request/AKA-Identity or
   EAP-Response/AKA-Identity, then the AT_CHECKCODE MUST be used to
   integrity protect the new attributes.

   When specifying new attributes, it should be noted that EAP-AKA does
   not support message fragmentation.  Hence, the sizes of the new
   extensions MUST be limited so that the maximum transfer unit (MTU) of
   the underlying lower layer is not exceeded.  According to [RFC3748],
   lower layers must provide an EAP MTU of 1020 bytes or greater, so any
   extensions to EAP-AKA SHOULD NOT exceed the EAP MTU of 1020 bytes.

   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.
   It is possible to introduce version negotiation in the
   EAP-Request/AKA-Identity and EAP-Response/AKA-Identity messages by
   specifying new skippable attributes.








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9.  Messages

   This section specifies the messages used in EAP-AKA.  It specifies
   when a message may be transmitted or accepted, which attributes are
   allowed in a message, which attributes are required in a message, and
   other message-specific details.  Message format is specified in
   Section 8.1.

9.1.  EAP-Request/AKA-Identity

   The EAP/AKA-Identity roundtrip MAY be used for obtaining the peer
   identity from the server.  As discussed in Section 4.1, several
   AKA-Identity rounds may be required in order to obtain a valid peer
   identity.

   The server MUST include one of the following identity requesting
   attributes: AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, AT_ANY_ID_REQ.
   These three attributes are mutually exclusive, so the server MUST NOT
   include more than one of the attributes.

   If the server has previously issued an EAP-Request/AKA-Identity
   message with the AT_PERMANENT_ID_REQ attribute, and if the server has
   received a response from the peer, then the server MUST NOT issue a
   new EAP-Request/AKA-Identity packet.

   If the server has previously issued an EAP-Request/AKA-Identity
   message with the AT_FULLAUTH_ID_REQ attribute, and if the server has
   received a response from the peer, then the server MUST NOT issue a
   new EAP-Request/AKA-Identity packet with the AT_ANY_ID_REQ or
   AT_FULLAUTH_ID_REQ attributes.

   If the server has previously issued an EAP-Request/AKA-Identity
   message with the AT_ANY_ID_REQ attribute, and if the server has
   received a response from the peer, then the server MUST NOT issue a
   new EAP-Request/AKA-Identity packet with the AT_ANY_ID_REQ.

   This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.

9.2.  EAP-Response/AKA-Identity

   The peer sends EAP-Response/AKA-Identity in response to a valid
   EAP-Request/AKA-Identity from the server.

   The peer MUST include the AT_IDENTITY attribute.  The usage of
   AT_IDENTITY is defined in Section 4.1.

   This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.




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9.3.  EAP-Request/AKA-Challenge

   The server sends the EAP-Request/AKA-Challenge on full authentication
   after successfully obtaining the subscriber identity.

   The AT_RAND attribute MUST be included.

   AT_MAC MUST be included.  In EAP-Request/AKA-Challenge, there is no
   message-specific data covered by the MAC, see Section 10.15.

   The AT_RESULT_IND attribute MAY be included.  The usage of this
   attribute is discussed in Section 6.2.

   The AT_CHECKCODE attribute MAY be included, and in certain cases
   specified in Section 10.13, it MUST be included.

   The EAP-Request/AKA-Challenge packet MAY include encrypted attributes
   for identity privacy and for communicating the next re-authentication
   identity.  In this case, the AT_IV and AT_ENCR_DATA attributes are
   included (Section 10.12).

   The plaintext of the AT_ENCR_DATA value field consists of nested
   attributes.  The nested attributes MAY include AT_PADDING (as
   specified in Section 10.12).  If the server supports identity privacy
   and wants to communicate a pseudonym to the peer for the next full
   authentication, then the nested encrypted attributes include the
   AT_NEXT_PSEUDONYM attribute.  If the server supports
   re-authentication and wants to communicate a fast re-authentication
   identity to the peer, then the nested encrypted attributes include
   the AT_NEXT_REAUTH_ID attribute.  Later versions of this protocol MAY
   specify additional attributes to be included within the encrypted
   data.

   When processing this message, the peer MUST process AT_RAND and
   AT_AUTN before processing other attributes.  Only if these attributes
   are verified to be valid, the peer derives keys and verifies AT_MAC.
   The operation in case an error occurs is specified in Section 6.3.1.

9.4.  EAP-Response/AKA-Challenge

   The peer sends EAP-Response/AKA-Challenge in response to a valid
   EAP-Request/AKA-Challenge.

   Sending this packet indicates that the peer has successfully
   authenticated the server and that the EAP exchange will be accepted
   by the peer's local policy.  Hence, if these conditions are not met,
   then the peer MUST NOT send EAP-Response/AKA-Challenge, but the peer
   MUST send EAP-Response/AKA-Client-Error.



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   The AT_MAC attribute MUST be included.  In
   EAP-Response/AKA-Challenge, there is no message-specific data covered
   by the MAC, see Section 10.15.

   The AT_RES attribute MUST be included.

   The AT_CHECKCODE attribute MAY be included, and in certain cases
   specified in Section 10.13, it MUST be included.

   The AT_RESULT_IND attribute MAY be included, if it was included in
   EAP-Request/AKA-Challenge.  The usage of this attribute is discussed
   in Section 6.2.

   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.  The EAP server MUST process EAP-Response/AKA-Challenge
   messages that include these attributes even if the server did not
   implement these optional attributes.

9.5.  EAP-Response/AKA-Authentication-Reject

   The peer sends the EAP-Response/AKA-Authentication-Reject packet if
   it does not accept the AUTN parameter.  This version of the protocol
   does not specify any attributes for this message.  Future versions of
   the protocol MAY specify attributes for this message.

   The AT_MAC, AT_ENCR_DATA, or AT_IV attributes MUST NOT be used in
   this message.

9.6.  EAP-Response/AKA-Synchronization-Failure

   The peer sends the EAP-Response/AKA-Synchronization-Failure, when the
   sequence number in the AUTN parameter is incorrect.

   The peer MUST include the AT_AUTS attribute.  Future versions of the
   protocol MAY specify other additional attributes for this message.

   The AT_MAC, AT_ENCR_DATA, or AT_IV attributes MUST NOT be used in
   this message.

9.7.  EAP-Request/AKA-Reauthentication

   The server sends the EAP-Request/AKA-Reauthentication message if it
   wants to use fast re-authentication, and if it has received a valid
   fast re-authentication identity in EAP-Response/Identity or
   EAP-Response/AKA-Identity.





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   The AT_MAC attribute MUST be included.  No message-specific data is
   included in the MAC calculation, see Section 10.15.

   The AT_RESULT_IND attribute MAY be included.  The usage of this
   attribute is discussed in Section 6.2.

   The AT_CHECKCODE attribute MAY be included, and in certain cases
   specified in Section 10.13, it MUST be included.

   The AT_IV and AT_ENCR_DATA attributes MUST be included.  The
   plaintext consists of the following nested encrypted attributes,
   which MUST be included: AT_COUNTER and AT_NONCE_S.  In addition, the
   nested encrypted attributes MAY include the following attributes:
   AT_NEXT_REAUTH_ID and AT_PADDING.

9.8.  EAP-Response/AKA-Reauthentication

   The client sends the EAP-Response/AKA-Reauthentication packet in
   response to a valid EAP-Request/AKA-Reauthentication.

   The AT_MAC attribute MUST be included.  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 8.1.  It is followed by the
   16-byte NONCE_S value from the server's AT_NONCE_S attribute.

   The AT_CHECKCODE attribute MAY be included, and in certain cases
   specified in Section 10.13, it MUST be included.

   The AT_IV and AT_ENCR_DATA attributes MUST be included.  The nested
   encrypted attributes MUST include the AT_COUNTER attribute.  The
   AT_COUNTER_TOO_SMALL attribute MAY be included in the nested
   encrypted attributes, and it is included in cases specified in
   Section 5.  The AT_PADDING attribute MAY be included.

   The AT_RESULT_IND attribute MAY be included, if it was included in
   EAP-Request/AKA-Reauthentication.  The usage of this attribute is
   discussed in Section 6.2.

   Sending this packet without AT_COUNTER_TOO_SMALL indicates that the
   peer has successfully authenticated the server and that the EAP
   exchange will be accepted by the peer's local policy.  Hence, if
   these conditions are not met, then the peer MUST NOT send
   EAP-Response/AKA-Reauthentication, but the peer MUST send
   EAP-Response/ AKA-Client-Error.






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9.9.  EAP-Response/AKA-Client-Error

   The peer sends EAP-Response/AKA-Client-Error in error cases, as
   specified in Section 6.3.1.

   The AT_CLIENT_ERROR_CODE attribute MUST be included.  The AT_MAC,
   AT_IV, or AT_ENCR_DATA attributes MUST NOT be used with this packet.

9.10.  EAP-Request/AKA-Notification

   The usage of this message is specified in Section 6.

   The AT_NOTIFICATION attribute MUST be included.

   The AT_MAC attribute MUST be included if the P bit of the
   AT_NOTIFICATION code is set to zero, and MUST NOT be included if the
   P bit is set to one.  The P bit is discussed in Section 6.

   No message-specific data is included in the MAC calculation.  See
   Section 10.15.

   If EAP-Request/AKA-Notification is used on a fast re-authentication
   exchange, and if the P bit in AT_NOTIFICATION is set to zero, then
   AT_COUNTER is used for replay protection.  In this case, the
   AT_ENCR_DATA and AT_IV attributes MUST be included, and the
   encapsulated plaintext attributes MUST include the AT_COUNTER
   attribute.  The counter value included in AT_COUNTER MUST be the same
   as in the EAP-Request/AKA-Reauthentication packet on the same fast
   re-authentication exchange.

9.11.  EAP-Response/AKA-Notification

   The usage of this message is specified in Section 6.  This packet is
   an acknowledgement of EAP-Request/AKA-Notification.

   The AT_MAC attribute MUST be included in cases when the P bit of the
   notification code in AT_NOTIFICATION of EAP-Request/AKA-Notification
   is set to zero, and MUST NOT be included in cases when the P bit is
   set to one.  The P bit is discussed in Section 6.

   If EAP-Request/AKA-Notification is used on a fast re-authentication
   exchange, and if the P bit in AT_NOTIFICATION is set to zero, then
   AT_COUNTER is used for replay protection.  In this case, the
   AT_ENCR_DATA and AT_IV attributes MUST be included, and the
   encapsulated plaintext attributes MUST include the AT_COUNTER
   attribute.  The counter value included in AT_COUNTER MUST be the same
   as in the EAP-Request/AKA-Reauthentication packet on the same fast
   re-authentication exchange.



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10.  Attributes

   This section specifies the format of message attributes.  The
   attribute type numbers are specified in Section 11.

10.1.  Table of Attributes

   The following table provides a guide to which attributes may be found
   in which kinds of messages, and in what quantity.  Messages are
   denoted with numbers in parentheses as follows: (1) EAP-Request/
   AKA-Identity, (2) EAP-Response/AKA-Identity, (3) EAP-Request/
   AKA-Challenge, (4) EAP-Response/AKA-Challenge, (5) EAP-Request/
   AKA-Notification, (6) EAP-Response/AKA-Notification, (7) EAP-
   Response/AKA-Client-Error (8) EAP-Request/AKA-Reauthentication, (9)
   EAP-Response/AKA-Reauthentication, (10) EAP-Response/AKA-
   Authentication-Reject, and (11) EAP-Response/AKA-Synchronization-
   Failure.  The column denoted with "E" indicates whether the attribute
   is a nested attribute that MUST be included within AT_ENCR_DATA.

   "0" indicates that the attribute MUST NOT be included in the message,
   "1" indicates that the attribute MUST be included in the message,
   "0-1" indicates that the attribute is sometimes included in the
   message, and "0*" indicates that the attribute is not included in the
   message in cases specified in this document, but MAY be included in
   the future versions of the protocol.

              Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)(11) E
    AT_PERMANENT_ID_REQ 0-1  0   0   0   0   0   0   0   0   0   0   N
          AT_ANY_ID_REQ 0-1  0   0   0   0   0   0   0   0   0   0   N
     AT_FULLAUTH_ID_REQ 0-1  0   0   0   0   0   0   0   0   0   0   N
            AT_IDENTITY  0  0-1  0   0   0   0   0   0   0   0   0   N
                AT_RAND  0   0   1   0   0   0   0   0   0   0   0   N
                AT_AUTN  0   0   1   0   0   0   0   0   0   0   0   N
                 AT_RES  0   0   0   1   0   0   0   0   0   0   0   N
                AT_AUTS  0   0   0   0   0   0   0   0   0   0   1   N
      AT_NEXT_PSEUDONYM  0   0  0-1  0   0   0   0   0   0   0   0   Y
      AT_NEXT_REAUTH_ID  0   0  0-1  0   0   0   0  0-1  0   0   0   Y
                  AT_IV  0   0  0-1  0* 0-1 0-1  0   1   1   0   0   N
           AT_ENCR_DATA  0   0  0-1  0* 0-1 0-1  0   1   1   0   0   N
             AT_PADDING  0   0  0-1  0* 0-1 0-1  0  0-1 0-1  0   0   Y
           AT_CHECKCODE  0   0  0-1 0-1  0   0   0  0-1 0-1  0   0   N
          AT_RESULT_IND  0   0  0-1 0-1  0   0   0  0-1 0-1  0   0   N
                 AT_MAC  0   0   1   1  0-1 0-1  0   1   1   0   0   N
             AT_COUNTER  0   0   0   0  0-1 0-1  0   1   1   0   0   Y
   AT_COUNTER_TOO_SMALL  0   0   0   0   0   0   0   0  0-1  0   0   Y
             AT_NONCE_S  0   0   0   0   0   0   0   1   0   0   0   Y
        AT_NOTIFICATION  0   0   0   0   1   0   0   0   0   0   0   N
   AT_CLIENT_ERROR_CODE  0   0   0   0   0   0   1   0   0   0   0   N



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   It should be noted that attributes AT_PERMANENT_ID_REQ,
   AT_ANY_ID_REQ, and AT_FULLAUTH_ID_REQ are mutually exclusive, so that
   only one of them can be included at the same time.  If one of the
   attributes AT_IV or AT_ENCR_DATA is included, then both of the
   attributes MUST be included.

10.2.  AT_PERMANENT_ID_REQ

   The format of the AT_PERMANENT_ID_REQ 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_PERM..._REQ | Length = 1    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The use of the AT_PERMANENT_ID_REQ is defined in Section 4.1.  The
   value field only contains two reserved bytes, which are set to zero
   on sending and ignored on reception.

10.3.  AT_ANY_ID_REQ

   The format of the AT_ANY_ID_REQ 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_ANY_ID_REQ  | Length = 1    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The use of the AT_ANY_ID_REQ is defined in Section 4.1.  The value
   field only contains two reserved bytes, which are set to zero on
   sending and ignored on reception.

10.4.  AT_FULLAUTH_ID_REQ

   The format of the AT_FULLAUTH_ID_REQ 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_FULLAUTH_...| Length = 1    |           Reserved            |
   +---------------+---------------+-------------------------------+

   The use of the AT_FULLAUTH_ID_REQ is defined in Section 4.1.  The
   value field only contains two reserved bytes, which are set to zero
   on sending and ignored on reception.




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10.5.  AT_IDENTITY

   The format of the AT_IDENTITY 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_IDENTITY   | Length        | Actual Identity Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                       Identity                                .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The use of the AT_IDENTITY is defined in Section 4.1.  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.
   The identity is the permanent identity, a pseudonym identity or a
   fast re-authentication identity.  The identity format is specified in
   Section 4.1.1.  The same identity format is used in the AT_IDENTITY
   attribute and the EAP-Response/Identity packet, with the exception
   that the peer MUST NOT decorate the identity it includes in
   AT_IDENTITY.  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.

10.6.  AT_RAND

   The format of the AT_RAND 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_RAND    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             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.




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10.7.  AT_AUTN

   The format of the AT_AUTN 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_AUTN    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                        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.

10.8.  AT_RES

   The format of the AT_RES 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_RES    |    Length     |          RES Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                                                               |
   |                             RES                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute begins with the 2-byte RES Length,
   which identifies the exact length of the RES in bits.  The RES length
   is followed by the AKA RES parameter.  According to [TS33.105], 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.











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10.9.  AT_AUTS

   The format of the AT_AUTS 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_AUTS    | Length = 4    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                             AUTS                              |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute contains the AKA AUTS parameter,
   112 bits (14 bytes).

10.10.  AT_NEXT_PSEUDONYM

   The format of the AT_NEXT_PSEUDONYM 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_NEXT_PSEU..| Length        | Actual Pseudonym Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                          Next Pseudonym                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute begins with a 2-byte actual
   pseudonym length, which specifies the length of the following
   pseudonym in bytes.  This field is followed by a pseudonym username
   that the peer can use in the next authentication.  The username MUST
   NOT include any realm portion.  The username does not include any
   terminating null characters.  Because the length of the attribute
   must be a multiple of 4 bytes, the sender pads the pseudonym with
   zero bytes when necessary.  The username encoding MUST follow the
   UTF-8 transformation format [RFC3629].  This attribute MUST always be
   encrypted by encapsulating it within the AT_ENCR_DATA attribute.









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10.11.  AT_NEXT_REAUTH_ID

   The format of the AT_NEXT_REAUTH_ID 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_NEXT_REAU..| Length        | Actual Re-Auth Identity Length|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .              Next Fast Re-Authentication Username             .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute begins with a 2-byte actual
   re-authentication identity length which specifies the length of the
   following fast re-authentication identity in bytes.  This field is
   followed by a fast re-authentication identity that the peer can use
   in the next fast re-authentication, as described in Section 5.  In
   environments where a realm portion is required, the fast
   re-authentication identity includes both a username portion and a
   realm name portion.  The fast 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 fast
   re-authentication identity with zero bytes when necessary.  The
   identity encoding MUST follow the UTF-8 transformation format
   [RFC3629].  This attribute MUST always be encrypted by encapsulating
   it within the AT_ENCR_DATA attribute.

10.12.  AT_IV, AT_ENCR_DATA, and AT_PADDING

   AT_IV and AT_ENCR_DATA attributes can be used to transmit encrypted
   information between the EAP-AKA peer 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.  Section 6.3 specifies the operation if a
   packet that does not meet this condition is encountered.

   The sender of the AT_IV attribute chooses the initialization vector
   at random.  The sender MUST NOT reuse the initialization vector value
   from previous EAP-AKA packets.  The sender SHOULD use a good source
   of randomness to generate the initialization vector.  Please see
   [RFC4086] for more information about generating random numbers for
   security applications.  The format of AT_IV 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_IV     | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                 Initialization Vector                         |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of the AT_ENCR_DATA attribute consists of two
   reserved bytes followed by cipher text bytes.  The cipher text bytes
   are encrypted using the Advanced Encryption Standard (AES) [AES] with
   a 128-bit key in the Cipher Block Chaining (CBC) mode of operation,
   which uses the initialization vector from the AT_IV attribute.  The
   reserved bytes are set to zero when sending and ignored on reception.
   Please see [CBC] for a description of the CBC mode.  The format of
   the AT_ENCR_DATA attribute is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_ENCR_DATA  | Length        |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                    Encrypted Data                             .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The derivation of the encryption key (K_encr) is specified in
   Section 7.

   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 (00 hexadecimal) on sending.  The recipient of the
   message MUST verify that the pad bytes are set to zero.  If this



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   verification fails on the peer, then it MUST send the
   EAP-Response/AKA-Client-Error packet with the error code "unable to
   process packet" to terminate the authentication exchange.  If this
   verification fails on the server, then the server sends the
   EAP-Response/AKA-Notification packet with an AT_NOTIFICATION code
   that implies failure to terminate the authentication exchange.  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...                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

10.13.  AT_CHECKCODE

   The AT_MAC attribute is not used in the very first EAP-AKA messages
   during the AKA-Identity round, because keying material has not been
   derived yet.  The peer and the server may exchange one or more pairs
   of EAP-AKA messages of the Subtype AKA-Identity before keys are
   derived and before the AT_MAC attribute can be applied.  The EAP/-
   AKA-Identity messages may also be used upon fast re-authentication.

   The AT_CHECKCODE attribute MAY be used to protect the EAP/
   AKA-Identity messages.  In full authentication, the server MAY
   include the AT_CHECKCODE in EAP-Request/AKA-Challenge, and the peer
   MAY include AT_CHECKCODE in EAP-Response/AKA-Challenge.  In fast
   re-authentication, the server MAY include AT_CHECKCODE in
   EAP-Request/ AKA-Reauthentication, and the peer MAY include
   AT_CHECKCODE in EAP-Response/AKA-Reauthentication.  The fact that the
   peer receives an EAP-Request with AT_CHECKCODE does not imply that
   the peer would have to include AT_CHECKCODE in the corresponding
   response.  The peer MAY include AT_CHECKCODE even if the server did
   not include AT_CHECKCODE in the EAP request.  Because the AT_MAC
   attribute is used in these messages, AT_CHECKCODE will be integrity
   protected with AT_MAC.  The format of the AT_CHECKCODE attribute 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_CHECKCODE  | Length        |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Checkcode (0 or 20 bytes)                 |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of AT_CHECKCODE begins with two reserved bytes, which
   may be followed by a 20-byte checkcode.  If the checkcode is not
   included in AT_CHECKCODE, then the attribute indicates that no EAP/-
   AKA-Identity messages were exchanged.  This may occur in both full
   authentication and fast re-authentication.  The reserved bytes are
   set to zero when sending and ignored on reception.

   The checkcode is a hash value, calculated with SHA1 [SHA-1], over all
   EAP-Request/AKA-Identity and EAP-Response/AKA-Identity packets
   exchanged in this authentication exchange.  The packets are included
   in the order that they were transmitted, that is, starting with the
   first EAP-Request/AKA-Identity message, followed by the corresponding
   EAP-Response/AKA-Identity, followed by the second
   EAP-Request/AKA-Identity (if used), etc.

   EAP packets are included in the hash calculation "as-is" (as they
   were transmitted or received).  All reserved bytes, padding bytes,
   etc., that are specified for various attributes are included as such,
   and the receiver must not reset them to zero.  No delimiter bytes,
   padding, or any other framing are included between the EAP packets
   when calculating the checkcode.

   Messages are included in request/response pairs; in other words, only
   full "round trips" are included.  Packets that are silently discarded
   are not included, and retransmitted packets (that have the same
   Identifier value) are only included once.  (The base EAP protocol
   [RFC3748] ensures that requests and responses "match".)  The EAP
   server must only include an EAP-Request/AKA-Identity in the
   calculation after it has received a corresponding response with the
   same Identifier value.

   The peer must include the EAP-Request/AKA-Identity and the
   corresponding response in the calculation only if the peer receives a
   subsequent EAP-Request/AKA-Challenge or a follow-up EAP-Request/
   AKA-Identity with a different Identifier value than in the first
   EAP-Request/AKA-Identity.



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   The AT_CHECKCODE attribute is optional to implement.  It is specified
   in order to allow protection of the EAP/AKA-Identity messages and any
   future extensions to them.  The implementation of AT_CHECKCODE is
   RECOMMENDED.

   If the receiver of AT_CHECKCODE implements this attribute, then the
   receiver MUST check that the checkcode is correct.  If the checkcode
   is invalid, the receiver must operate as specified in Section 6.3.

   If the EAP/AKA-Identity messages are extended with new attributes,
   then AT_CHECKCODE MUST be implemented and used.  More specifically,
   if the server includes any attributes other than AT_PERMANENT_ID_REQ,
   AT_FULLAUTH_ID_REQ, or AT_ANY_ID_REQ in the EAP-Request/AKA-Identity
   packet, then the server MUST include AT_CHECKCODE in EAP-Request/
   AKA-Challenge or EAP-Request/AKA-Reauthentication.  If the peer
   includes any attributes other than AT_IDENTITY in the EAP-Response/
   AKA-Identity message, then the peer MUST include AT_CHECKCODE in
   EAP-Response/AKA-Challenge or EAP-Response/AKA-Reauthentication.

   If the server implements the processing of any other attribute than
   AT_IDENTITY for the EAP-Response/AKA-Identity message, then the
   server MUST implement AT_CHECKCODE.  In this case, if the server
   receives any attribute other than AT_IDENTITY in the
   EAP-Response/AKA-Identity message, then the server MUST check that
   AT_CHECKCODE is present in EAP-Response/AKA-Challenge or
   EAP-Response/ AKA-Reauthentication.  The operation when a mandatory
   attribute is missing is specified in Section 6.3.

   Similarly, if the peer implements the processing of any attribute
   other than AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, or AT_ANY_ID_REQ
   for the EAP-Request/AKA-Identity packet, then the peer MUST implement
   AT_CHECKCODE.  In this case, if the peer receives any attribute other
   than AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, or AT_ANY_ID_REQ in the
   EAP-Request/AKA-Identity packet, then the peer MUST check that
   AT_CHECKCODE is present in EAP-Request/AKA-Challenge or
   EAP-Request/AKA-Reauthentication.  The operation when a mandatory
   attribute is missing is specified in Section 6.3.

10.14.  AT_RESULT_IND

   The format of the AT_RESULT_IND 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_RESULT_...| Length = 1    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   The value field of this attribute consists of two reserved bytes,
   which are set to zero upon sending and ignored upon reception.  This
   attribute is always sent unencrypted, so it MUST NOT be encapsulated
   within the AT_ENCR_DATA attribute.

10.15.  AT_MAC

   The AT_MAC attribute is used for EAP-AKA message authentication.
   Section 9 specifies in which messages AT_MAC MUST be included.

   The value field of the AT_MAC attribute contains two reserved bytes
   followed by a keyed message authentication code (MAC).  The MAC is
   calculated over the whole EAP packet and 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 EAP
   packet includes the EAP header that begins with the Code field, the
   EAP-AKA header that begins with the Subtype field, and all the
   attributes, as specified in Section 8.1.  The reserved bytes in
   AT_MAC are set to zero when sending and ignored on reception.  The
   contents of the message-specific data that may be included in the MAC
   calculation are specified separately for each EAP-AKA message in
   Section 9.

   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 [RFC2104] keyed hash value.  (The
   HMAC-SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by
   truncating the output to 16 bytes.  Hence, the length of the MAC is
   16 bytes.)  The derivation of the authentication key (K_aut) used in
   the calculation of the MAC is specified in Section 7.

   When the AT_MAC attribute is included in an EAP-AKA message, the
   recipient MUST process the AT_MAC attribute before looking at any
   other attributes, except when processing EAP-Request/AKA-Challenge.
   The processing of EAP-Request/AKA-Challenge is specified in





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   Section 9.3.  If the message authentication code is invalid, then the
   recipient MUST ignore all other attributes in the message and operate
   as specified in Section 6.3.

10.16.  AT_COUNTER

   The format of the AT_COUNTER 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_COUNTER   | Length = 1    |           Counter             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of the AT_COUNTER attribute consists of a 16-bit
   unsigned integer counter value, represented in network byte order.
   This attribute MUST always be encrypted by encapsulating it within
   the AT_ENCR_DATA attribute.

10.17.  AT_COUNTER_TOO_SMALL

   The format of the AT_COUNTER_TOO_SMALL 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_COUNTER...| Length = 1    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute consists of two reserved bytes,
   which are set to zero upon sending and ignored upon reception.  This
   attribute MUST always be encrypted by encapsulating it within the
   AT_ENCR_DATA attribute.


















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10.18.  AT_NONCE_S

   The format of the AT_NONCE_S 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_NONCE_S    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                                                               |
   |                            NONCE_S                            |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of the AT_NONCE_S attribute contains two reserved
   bytes followed by a random number (16 bytes) that is freshly
   generated by the server for this EAP-AKA fast re-authentication.  The
   random number is used as challenge for the peer and also as a seed
   value for the new keying material.  The reserved bytes are set to
   zero upon sending and ignored upon reception.  This attribute MUST
   always be encrypted by encapsulating it within the AT_ENCR_DATA
   attribute.

   The server MUST NOT reuse the NONCE_S value from a previous EAP-AKA
   fast re-authentication exchange.  The server SHOULD use a good source
   of randomness to generate NONCE_S.  Please see [RFC4086] for more
   information about generating random numbers for security
   applications.

10.19.  AT_NOTIFICATION

   The format of the AT_NOTIFICATION 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_NOTIFICATION| Length = 1    |S|P|  Notification Code        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute contains a two-byte notification
   code.  The first and second bit (S and P) of the notification code
   are interpreted as described in Section 6.







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   The notification code values listed below have been reserved.  The
   descriptions below illustrate the semantics of the notifications.
   The peer implementation MAY use different wordings when presenting
   the notifications to the user.  The "requested service" depends on
   the environment where EAP-AKA is applied.

   0 - General failure after authentication.  (Implies failure, used
   after successful authentication.)

   16384 - General failure.  (Implies failure, used before
   authentication.)

   32768 - Success.  User has been successfully authenticated.  (Does
   not imply failure, used after successful authentication.)  The usage
   of this code is discussed in Section 6.2.

   1026 - User has been temporarily denied access to the requested
   service.  (Implies failure, used after successful authentication.)

   1031 - User has not subscribed to the requested service.  (Implies
   failure, used after successful authentication.)

10.20.  AT_CLIENT_ERROR_CODE

   The format of the AT_CLIENT_ERROR_CODE 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_CLIENT_ERR..| Length = 1    |     Client Error Code         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute contains a two-byte client error
   code.  The following error code values have been reserved.

   0 "unable to process packet": a general error code

11.  IANA and Protocol Numbering Considerations

   IANA has assigned the EAP type number 23 for EAP-AKA authentication.

   EAP-AKA shares most of the protocol design, such as attributes and
   message Subtypes, with EAP-SIM [EAP-SIM].  EAP-AKA protocol numbers
   should be administered in the same IANA registry with EAP-SIM.  This
   document establishes the registries and lists the initial protocol
   numbers for both protocols.





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   EAP-AKA and EAP-SIM messages include a Subtype field.  The Subtype is
   a new numbering space for which IANA administration is required.  The
   Subtype is an 8-bit integer.  The following Subtypes are specified in
   this document and in [EAP-SIM]:

        AKA-Challenge...................................1
        AKA-Authentication-Reject.......................2
        AKA-Synchronization-Failure.....................4
        AKA-Identity....................................5
        SIM-Start......................................10
        SIM-Challenge..................................11
        AKA-Notification and SIM-Notification..........12
        AKA-Reauthentication and SIM-Reauthentication..13
        AKA-Client-Error and SIM-Client-Error..........14

   The messages are composed of attributes, which have 8-bit attribute
   type numbers.  Attributes numbered within the range 0 through 127 are
   called non-skippable attributes, and attributes within the range of
   128 through 255 are called skippable attributes.  The EAP-AKA and
   EAP-SIM attribute type number is a new numbering space for which IANA
   administration is required.  The following attribute types are
   specified in this document in [EAP-SIM]:

        AT_RAND.........................................1
        AT_AUTN.........................................2
        AT_RES..........................................3
        AT_AUTS.........................................4
        AT_PADDING......................................6
        AT_NONCE_MT.....................................7
        AT_PERMANENT_ID_REQ............................10
        AT_MAC.........................................11
        AT_NOTIFICATION................................12
        AT_ANY_ID_REQ..................................13
        AT_IDENTITY....................................14
        AT_VERSION_LIST................................15
        AT_SELECTED_VERSION............................16
        AT_FULLAUTH_ID_REQ.............................17
        AT_COUNTER.....................................19
        AT_COUNTER_TOO_SMALL...........................20
        AT_NONCE_S.....................................21
        AT_CLIENT_ERROR_CODE...........................22
        AT_IV.........................................129
        AT_ENCR_DATA..................................130
        AT_NEXT_PSEUDONYM.............................132
        AT_NEXT_REAUTH_ID.............................133
        AT_CHECKCODE..................................134
        AT_RESULT_IND.................................135




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   The AT_NOTIFICATION attribute contains a 16-bit notification code
   value.  The most significant bit of the notification code is called
   the S bit (success) and the second most significant bit is called the
   P bit (phase).  If the S bit is set to zero, then the notification
   code indicates failure; notification codes with the S bit set to one
   do not indicate failure.  If the P bit is set to zero, then the
   notification code can only be used before authentication has
   occurred.  If the P bit is set to one, then the notification code can
   only be used after authentication.  The notification code is a new
   numbering space for which IANA administration is required.  The
   following values have been specified in this document and in
   [EAP-SIM].

        General failure after authentication......................0
        User has been temporarily denied access................1026
        User has not subscribed to the requested service.......1031
        General failure.......................................16384
        Success...............................................32768

   The AT_VERSION_LIST and AT_SELECTED_VERSION attributes, specified in
   [EAP-SIM], contain 16-bit EAP method version numbers.  The EAP method
   version number is a new numbering space for which IANA administration
   is required.  Value 1 for "EAP-SIM Version 1" has been specified in
   [EAP-SIM].  Version numbers are not currently used in EAP-AKA.

   The AT_CLIENT_ERROR_CODE attribute contains a 16-bit client error
   code.  The client error code is a new numbering space for which IANA
   administration is required.  Values 0, 1, 2, and 3 have been
   specified in this document and in [EAP-SIM].

   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 [RFC2434].  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 EAP specification [RFC3748] describes the security
   vulnerabilities of EAP, which does not include its own security
   mechanisms.  This section discusses the claimed security properties
   of EAP-AKA as well as vulnerabilities and security recommendations.






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12.1.  Identity Protection

   EAP-AKA includes optional Identity privacy support that protects the
   privacy of the subscriber identity against passive eavesdropping.
   This document only specifies a mechanism to deliver pseudonyms from
   the server to the peer as part of an EAP-AKA exchange.  Hence, a peer
   that has not yet performed any EAP-AKA exchanges does not typically
   have a pseudonym available.  If the peer does not have a pseudonym
   available, then the privacy mechanism cannot be used, and the
   permanent identity will have to be sent in the clear.  The terminal
   SHOULD store the pseudonym in 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 4.1.2) to
   learn the subscriber's IMSI.  However, as discussed in Section 4.1.2,
   the terminal can refuse to send the cleartext IMSI if it believes
   that the network should be able to recognize the pseudonym.

   If the peer 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) [PEAP]) 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 3rd generation AKA
   mechanisms [TS33.102] and [S.S0055-A].

   Note that this mutual authentication is with the EAP server.  In
   general, EAP methods do not authenticate the identity or services
   provided by the EAP authenticator (if distinct from the EAP server)
   unless they provide the so-called channel bindings property.  The
   vulnerabilities related to this have been discussed in [RFC3748],
   [EAPKeying], [ServiceIdentity].

   EAP-AKA does not provide the channel bindings property, so it only
   authenticates the EAP server.  However, ongoing work such as
   [ServiceIdentity] may provide such support as an extension to popular
   EAP methods such as EAP-TLS, EAP-SIM, or EAP-AKA.

12.3.  Flooding the Authentication Centre

   The EAP-AKA server typically obtains authentication vectors from the
   Authentication Centre (AuC).  EAP-AKA introduces a new usage for the
   AuC.  The protocols between the EAP-AKA server and the AuC are out of
   the scope of this document.  However, it should be noted that a



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   malicious EAP-AKA peer may generate a lot of protocol requests to
   mount a denial-of-service attack.  The EAP-AKA server implementation
   SHOULD take this into account and SHOULD take steps to limit the
   traffic that it generates towards the AuC, preventing the attacker
   from flooding the AuC and from extending the denial-of-service attack
   from EAP-AKA to other users of the AuC.

12.4.  Key Derivation

   EAP-AKA supports key derivation with 128-bit effective key strength.
   The key hierarchy is specified in Section 7.

   The Transient EAP Keys used to protect EAP-AKA packets (K_encr,
   K_aut), the Master Session Keys, and the Extended Master Session Keys
   are cryptographically separate.  An attacker cannot derive any
   non-trivial information about any of these keys based on the other
   keys.  An attacker also cannot calculate the pre-shared secret from
   AKA IK, AKA CK, EAP-AKA K_encr, EAP-AKA K_aut, the Master Session
   Key, or the Extended Master Session Key.

12.5.  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 AKA
   is not a password protocol (the pre-shared secret is not a
   passphrase, or derived from a passphrase), EAP-AKA is not vulnerable
   to dictionary attacks.

12.6.  Protection, Replay Protection, and Confidentiality

   AT_MAC, AT_IV, AT_ENCR_DATA, and AT_COUNTER attributes are used to
   provide integrity, replay, and confidentiality protection for EAP-AKA
   Requests and Responses.  Integrity protection with AT_MAC includes
   the EAP header.  Integrity protection (AT_MAC) is based on a keyed
   message authentication code.  Confidentiality (AT_ENCR_DATA and
   AT_IV) is based on a block cipher.

   Because keys are not available in the beginning of the EAP methods,
   the AT_MAC attribute cannot be used for protecting EAP/AKA-Identity
   messages.  However, the AT_CHECKCODE attribute can optionally be used
   to protect the integrity of the EAP/AKA-Identity roundtrip.

   Confidentiality protection is applied only to a part of the protocol
   fields.  The table of attributes in Section 10.1 summarizes which
   fields are confidentiality protected.  It should be noted that the
   error and notification code attributes AT_CLIENT_ERROR_CODE and
   AT_NOTIFICATION are not confidential, but they are transmitted in the
   clear.  Identity protection is discussed in Section 12.1.



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   On full authentication, replay protection of the EAP exchange is
   provided by RAND and AUTN values from the underlying AKA scheme.
   Protection against replays of EAP-AKA messages is also based on the
   fact that messages that can include AT_MAC can only be sent once with
   a certain EAP-AKA Subtype, and on the fact that a different K_aut key
   will be used for calculating AT_MAC in each full authentication
   exchange.

   On fast re-authentication, a counter included in AT_COUNTER and a
   server random nonce is used to provide replay protection.  The
   AT_COUNTER attribute is also included in EAP-AKA notifications, if
   they are used after successful authentication in order to provide
   replay protection between re-authentication exchanges.

   The contents of the user identity string are implicitly integrity
   protected by including them in key derivation.

   Because EAP-AKA is not a tunneling method, EAP-Request/Notification,
   EAP-Response/Notification, EAP-Success, or EAP-Failure packets are
   not confidential, integrity protected, or replay protected.  On
   physically insecure networks, this may enable an attacker to mount
   denial-of-service attacks by spoofing these packets.  As discussed in
   Section 6.3, the peer will only accept EAP-Success after the peer
   successfully authenticates the server.  Hence, the attacker cannot
   force the peer to believe successful mutual authentication has
   occurred before the peer successfully authenticates the server or
   after the peer failed to authenticate the server.

   The security considerations of EAP-AKA result indications are covered
   in Section 12.8

   An eavesdropper will see the EAP Notification, EAP_Success and
   EAP-Failure packets sent in the clear.  With EAP-AKA, confidential
   information MUST NOT be transmitted in EAP Notification packets.

12.7.  Negotiation Attacks

   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.

   As described in Section 8, EAP-AKA allows the protocol to be extended
   by defining new attribute types.  When defining such attributes, it
   should be noted that any extra attributes included in
   EAP-Request/AKA-Identity or EAP-Response/AKA-Identity packets are not





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   included in the MACs later on, and thus some other precautions must
   be taken to avoid modifications to them.

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

12.8.  Protected Result Indications

   EAP-AKA supports optional protected success indications, and
   acknowledged failure indications.  If a failure occurs after
   successful authentication, then the EAP-AKA failure indication is
   integrity and replay protected.

   Even if an EAP-Failure packet is lost when using EAP-AKA over an
   unreliable medium, then the EAP-AKA failure indications will help
   ensure that the peer and EAP server will know the other party's
   authentication decision.  If protected success indications are used,
   then the loss of Success packet will also be addressed by the
   acknowledged, integrity, and replay protected EAP-AKA success
   indication.  If the optional success indications are not used, then
   the peer may end up believing the server completed successful
   authentication, when actually it failed.  Because access will not be
   granted in this case, protected result indications are not needed
   unless the client is not able to realize it does not have access for
   an extended period of time.

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 Master Session Key or keys derived from it 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.  For instance, an early
   version of PEAP [PEAP-02] was vulnerable to this attack.  This
   specification does not address these attacks.  If EAP-AKA is used
   with a tunneling protocol, there should be cryptographic binding
   provided between the protocol and EAP-AKA to prevent
   man-in-the-middle attacks through rogue authenticators being able to
   setup one-way authenticated tunnels.  For example, newer versions of
   PEAP include such cryptographic binding.  The EAP-AKA Master Session
   Key MAY be used to provide the cryptographic binding.  However, the
   mechanism that provides the binding depends on the tunneling protocol
   and is beyond the scope of this document.




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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
   [RFC4086] for more information on generating random numbers for
   security applications.

13.  Security Claims

   This section provides the security claims required by [RFC3748].

   Auth.  Mechanism: EAP-AKA is based on the AKA mechanism, which is an
   authentication and key agreement mechanism based on a symmetric
   128-bit pre-shared secret.

   Ciphersuite negotiation: No

   Mutual authentication: Yes (Section 12.2)

   Integrity protection: Yes (Section 12.6)

   Replay protection: Yes (Section 12.6)

   Confidentiality: Yes, except method-specific success and failure
   indications (Section 12.1, Section 12.6)

   Key derivation: Yes

   Key strength: EAP-AKA supports key derivation with 128-bit effective
   key strength.

   Description of key hierarchy: Please see Section 7.

   Dictionary attack protection: N/A (Section 12.5)

   Fast reconnect: Yes

   Cryptographic binding: N/A

   Session independence: Yes (Section 12.4)

   Fragmentation: No

   Channel binding: No

   Indication of vulnerabilities.  Vulnerabilities are discussed in
   Section 12.




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14.  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,
   Pasi Eronen of Nokia, Olivier Paridaens of Alcatel, and Ilkka
   Uusitalo of Ericsson for interesting discussions in this problem
   space.

   Many thanks to Yoshihiro Ohba for reviewing the document.

   This protocol has been partly developed in parallel with EAP-SIM
   [EAP-SIM], and hence this specification incorporates many ideas from
   EAP-SIM, and many contributions from the reviewer's of EAP-SIM.

   The attribute format is based on the extension format of Mobile IPv4
   [RFC3344].

15.  References

15.1.  Normative References

   [TS33.102]        3rd Generation Partnership Project, "3GPP Technical
                     Specification 3GPP TS 33.102 V5.1.0: "Technical
                     Specification Group Services and System Aspects; 3G
                     Security; Security Architecture (Release 5)"",
                     December 2002.

   [S.S0055-A]       3rd Generation Partnership Project 2, "3GPP2
                     Enhanced Cryptographic Algorithms", September 2003.

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

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

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

   [TS23.003]        3rd Generation Partnership Project, "3GPP Technical
                     Specification 3GPP TS 23.003 V6.8.0: "3rd
                     Generation Parnership Project; Technical
                     Specification Group Core Network; Numbering,
                     addressing and identification (Release 6)"",
                     December 2005.



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   [RFC2104]         Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
                     Keyed-Hashing for Message Authentication",
                     RFC 2104, February 1997.

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

   [CBC]             National Institute of Standards and Technology,
                     "NIST Special Publication 800-38A, "Recommendation
                     for Block Cipher Modes of Operation - Methods and
                     Techniques"", December 2001,
                     http://csrc.nist.gov/publications/
                     nistpubs/800-38a/sp800-38a.pdf.

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

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

   [TS33.105]        3rd Generation Partnership Project, "3GPP Technical
                     Specification 3GPP TS 33.105 4.1.0: "Technical
                     Specification Group Services and System Aspects; 3G
                     Security; Cryptographic Algorithm Requirements
                     (Release 4)"", June 2001.

   [RFC3629]         Yergeau, F., "UTF-8, a transformation format of ISO
                     10646", STD 63, RFC 3629, November 2003.

   [RFC2434]         Narten, T. and H. Alvestrand, "Guidelines for
                     Writing an IANA Considerations Section in RFCs",
                     BCP 26, RFC 2434, October 1998.









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15.2.  Informative References

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

   [PEAP]            Palekar, A., Simon, D., Zorn, G., Salowey, J.,
                     Zhou, H., and S. Josefsson, "Protected EAP Protocol
                     (PEAP) Version 2", work in progress, October 2004.

   [PEAP-02]         Anderson, H., Josefsson, S., Zorn, G., Simon, D.,
                     and A.  Palekar, "Protected EAP Protocol (PEAP)",
                     work in progress, February 2002.

   [EAPKeying]       Aboba, B., Simon, D., Arkko, J., Eronen, P., and H.
                     Levkowetz, "Extensible Authentication Protocol
                     (EAP) Key Management Framework", work in progress,
                     October 2005.

   [ServiceIdentity] Arkko, J. and P. Eronen, "Authenticated Service
                     Information for the Extensible Authentication
                     Protocol (EAP)", Work in Progress, October 2004.

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

   [RFC3344]         Perkins, C., "IP Mobility Support for IPv4",
                     RFC 3344, August 2002.

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


















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RFC 4187                 EAP-AKA Authentication             January 2006


Appendix A.  Pseudo-Random Number Generator

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

   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 [SHA-1]

   Step 3: For j = 0 to m - 1 do
         3.1.  XSEED_j = 0 /* no 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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Authors' Addresses

   Jari Arkko
   Ericsson
   FIN-02420 Jorvas
   Finland

   EMail: jari.Arkko@ericsson.com


   Henry Haverinen
   Nokia Enterprise Solutions
   P.O. Box 12
   FIN-40101 Jyvaskyla
   Finland

   EMail: henry.haverinen@nokia.com


































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RFC 4187                 EAP-AKA Authentication             January 2006


Full Copyright Statement

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