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rfc:rfc4187

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.

Arkko & Haverinen Informational [Page 1] RFC 4187 EAP-AKA Authentication January 2006

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

Arkko & Haverinen Informational [Page 2] RFC 4187 EAP-AKA Authentication January 2006

    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

Arkko & Haverinen Informational [Page 3] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 4] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 5] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 6] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 7] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 8] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 9] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 10] RFC 4187 EAP-AKA Authentication January 2006

     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

Arkko & Haverinen Informational [Page 11] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 12] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 13] RFC 4187 EAP-AKA Authentication January 2006

      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.

Arkko & Haverinen Informational [Page 14] RFC 4187 EAP-AKA Authentication January 2006

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

Arkko & Haverinen Informational [Page 15] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 16] RFC 4187 EAP-AKA Authentication January 2006

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

Arkko & Haverinen Informational [Page 17] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 18] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 19] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 20] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 21] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 22] RFC 4187 EAP-AKA Authentication January 2006

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:

Arkko & Haverinen Informational [Page 23] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 24] RFC 4187 EAP-AKA Authentication January 2006

 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,

Arkko & Haverinen Informational [Page 25] RFC 4187 EAP-AKA Authentication January 2006

 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).

Arkko & Haverinen Informational [Page 26] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 27] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 28] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 29] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 30] RFC 4187 EAP-AKA Authentication January 2006

     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.

Arkko & Haverinen Informational [Page 31] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 32] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 33] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 34] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 35] RFC 4187 EAP-AKA Authentication January 2006

      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

Arkko & Haverinen Informational [Page 36] RFC 4187 EAP-AKA Authentication January 2006

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

Arkko & Haverinen Informational [Page 37] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 38] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 39] RFC 4187 EAP-AKA Authentication January 2006

 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,

Arkko & Haverinen Informational [Page 40] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 41] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 42] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 43] RFC 4187 EAP-AKA Authentication January 2006

 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).

Arkko & Haverinen Informational [Page 44] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 45] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 46] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 47] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 48] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 49] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 50] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 51] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 52] RFC 4187 EAP-AKA Authentication January 2006

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

Arkko & Haverinen Informational [Page 53] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 54] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 55] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 56] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 57] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 58] RFC 4187 EAP-AKA Authentication January 2006

  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

Arkko & Haverinen Informational [Page 59] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 60] RFC 4187 EAP-AKA Authentication January 2006

  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.

Arkko & Haverinen Informational [Page 61] RFC 4187 EAP-AKA Authentication January 2006

 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            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Arkko & Haverinen Informational [Page 62] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 63] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 64] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 65] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 66] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 67] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 68] RFC 4187 EAP-AKA Authentication January 2006

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

Arkko & Haverinen Informational [Page 69] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 70] RFC 4187 EAP-AKA Authentication January 2006

 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

Arkko & Haverinen Informational [Page 71] RFC 4187 EAP-AKA Authentication January 2006

 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.

Arkko & Haverinen Informational [Page 72] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 73] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 74] RFC 4187 EAP-AKA Authentication January 2006

 [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.

Arkko & Haverinen Informational [Page 75] RFC 4187 EAP-AKA Authentication January 2006

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.

Arkko & Haverinen Informational [Page 76] 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

Arkko & Haverinen Informational [Page 77] RFC 4187 EAP-AKA Authentication January 2006

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

Arkko & Haverinen Informational [Page 78] RFC 4187 EAP-AKA Authentication January 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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Arkko & Haverinen Informational [Page 79]

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