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

   Network Working Group                                       J. Arkko
   Internet Draft                                              Ericsson
   Document: draft-arkko-pppext-eap-aka-11.txt             H. Haverinen
   Expires: 27 April, 2004                                        Nokia
                                                       27 October, 2003


                          EAP AKA Authentication


Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

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

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

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

   Comments should be submitted to the eap@frascone.com mailing list.

Abstract

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for authentication and session key distribution using the
   Universal Mobile Telecommunications System (UMTS) Authentication and
   Key Agreement (AKA) mechanism. UMTS AKA is based on symmetric keys,
   and runs typically in a UMTS Subscriber Identity Module, a smart
   card like device.

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


Table of Contents

   Status of this Memo................................................1
   Abstract...........................................................1
   1. Introduction and Motivation.....................................3
   2. Terms and Conventions Used in This Document.....................4
   3. Protocol Overview...............................................6
   4. Operation......................................................11

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   4.1. Identity Management..........................................11
   4.2. Re-authentication............................................25
   4.3. EAP/AKA Notifications........................................31
   4.4. Error Cases..................................................32
   4.5. Key Generation...............................................34
   5. Message Format and Protocol Extensibility......................35
   5.1. Message Format...............................................35
   5.2. Protocol Extensibility.......................................37
   6. Messages.......................................................37
   6.1. EAP-Request/AKA-Identity.....................................37
   6.2. EAP-Response/AKA-Identity....................................38
   6.3. EAP-Request/AKA-Challenge....................................38
   6.4. EAP-Response/AKA-Challenge...................................39
   6.5. EAP-Response/AKA-Authentication-Reject.......................39
   6.6. EAP-Response/AKA-Synchronization-Failure.....................39
   6.7. EAP-Request/AKA-Reauthentication.............................39
   6.8. EAP-Response/AKA-Reauthentication............................40
   6.9. EAP-Response/AKA-Client-Error................................40
   6.10. EAP-Request/AKA-Notification................................40
   6.11. EAP-Response/AKA-Notification...............................41
   7. Attributes.....................................................41
   7.1. Table of Attributes..........................................41
   7.2. AT_MAC.......................................................42
   7.3. AT_IV, AT_ENCR_DATA and AT_PADDING...........................43
   7.4. AT_CHECKCODE.................................................45
   7.5. AT_PERMANENT_ID_REQ..........................................47
   7.6. AT_ANY_ID_REQ................................................47
   7.7. AT_FULLAUTH_ID_REQ...........................................47
   7.8. AT_IDENTITY..................................................48
   7.9. AT_RAND......................................................48
   7.10. AT_AUTN.....................................................49
   7.11. AT_RES......................................................49
   7.12. AT_AUTS.....................................................49
   7.13. AT_NEXT_PSEUDONYM...........................................50
   7.14. AT_NEXT_REAUTH_ID...........................................50
   7.15. AT_COUNTER..................................................51
   7.16. AT_COUNTER_TOO_SMALL........................................51
   7.17. AT_NONCE_S..................................................51
   7.18. AT_NOTIFICATION.............................................52
   7.19. AT_CLIENT_ERROR_CODE........................................53
   8. IANA and Protocol Numbering Considerations.....................53
   9. Security Considerations........................................54
   9.1. Identity Protection..........................................55
   9.2. Mutual Authentication........................................55
   9.3. Key Derivation...............................................55
   9.4. Brute-Force and Dictionary Attacks...........................55
   9.5. Integrity Protection, Replay Protection and Confidentiality..55


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   9.6. Negotiation Attacks..........................................56
   9.7. Fast Reconnect...............................................56
   9.8. Acknowledged Result Indications..............................56
   9.9. Man-in-the-middle Attacks....................................57
   9.10. Generating Random Numbers...................................57
   10. Security Claims...............................................57
   11. Intellectual Property Right Notices...........................58
   Acknowledgements and Contributions................................58
   Authors' Addresses................................................58
   Annex A. Pseudo-Random Number Generator...........................59

1. Introduction and Motivation

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for authentication and session key distribution using the
   UMTS AKA authentication mechanism [TS 33.102]. UMTS is a global
   third generation mobile network standard.

   AKA is based on challenge-response mechanisms and symmetric
   cryptography. AKA typically runs in a UMTS Subscriber Identity
   Module (USIM). Compared to the GSM mechanism, UMTS AKA provides
   substantially longer key lengths and mutual authentication.

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

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

   - The use of the third generation mobile network authentication
     infrastructure in the context of wireless LANs

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

   AKA works in the following manner:

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

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

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

   - The USIM verifies the AUTN, again based on the secret key and the
     sequence number. If this process is successful (the AUTN is valid

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     and the sequence number used to generate AUTN is within the
     correct range), the USIM produces an authentication result, RES
     and sends this to the home environment.

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

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

   For a specification of the AKA mechanisms and how the cryptographic
   values AUTN, RES, IK, CK and AUTS are calculated, see [TS 33.102].

   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 third generation mobile networks, AKA is used both for radio
   network authentication and IP multimedia service authentication
   purposes. Different user identities and formats are used for these;
   the radio network uses the International Mobile Subscriber
   Identifier (IMSI), whereas the IP multimedia service uses the
   Network Access Identifier (NAI) [RFC 2486].


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 [RFC 2119].

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

   This document frequently uses the following terms and abbreviations:



   AAA protocol

      Authentication, Authorization and Accounting protocol

   AKA

      Authentication and Key Agreement




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   AuC

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



   EAP

      Extensible Authentication Protocol [EAP].

   GSM

      Global System for Mobile communications.

   NAI

      Network Access Identifier [RFC 2486].

   AUTN

      Authentication value generated by the AuC which together with the
      RAND authenticates the server to the peer, 128 bits [TS 33.102].

   AUTS

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

   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, ie. not including any
      realm portions.

   Pseudonym Identity

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

   Pseudonym Username

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




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   Re-authentication Identity

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

   Re-authentication Username

      The username portion of re-authentication identity, ie. not
      including any realm portions.

   RAND

      Random number generated by the AuC, 128 bits [TS 33.102].

   RES

      Authentication result from the peer, which together with the RAND
      authenticates the peer to the server, 128 bits [TS 33.102].

   SQN

      Sequence number used in the authentication process, 48 bits [TS
      33.102].

   SIM

      Subscriber Identity Module. The SIM is an application
      traditionally resident on smart cards distributed by GSM
      operators.

   SRES

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

   USIM

      UMTS Subscriber Identity Module. USIM is an application that is
      resident e.g. on smart cards distributed by UMTS operators.


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

3. Protocol Overview

   The message flow below shows the basic successful full
   authentication exchange in EAP AKA. At the minimum, EAP AKA uses two
   roundtrips to authorize the user 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

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   Identity (IMSI), or a temporary identity (pseudonym) if identity
   privacy is in effect, as specified in Section 4.1. (As specified in
   [EAP], 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 also
   Section 4.1.2 for specification how to obtain the identity via EAP
   AKA messages.)

   Next, the EAP server starts the actual AKA protocol by sending an
   EAP-Request/AKA-Challenge message. EAP AKA packets encapsulate
   parameters in attributes, encoded in a Type, Length, Value format.
   The packet format and the use of attributes are specified in Section
   5. The EAP-Request/AKA-Challenge message contains a random number
   (AT_RAND) and 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 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 a USIM) 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.




























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


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














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       Peer                                              Authenticator
          |                                                       |
          |                      EAP-Request/Identity             |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs UMTS algorithms, |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                                                       |
          |                      EAP-Request/AKA-Challenge        |
          |                      (AT_RAND, AT_AUTN, AT_MAC)       |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Peer runs UMTS algorithms on USIM,  |                     |
      | possibly verifies AUTN, and sends an|                     |
      | invalid response                    |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/AKA-Challenge                            |
          | (AT_RES, AT_MAC)                                      |
          |------------------------------------------------------>|
          |                                                       |
          |              +------------------------------------------+
          |              | Server checks the given RES and the MAC, |
          |              | and finds one of them incorrct.          |
          |              +------------------------------------------+
          |                                                       |
          |                                          EAP-Failure  |
          |<------------------------------------------------------|


   The next message flow shows the 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 as AKA in general produces in UMTS.











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


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



















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

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

   In addition to the full authentication scenarios described above,
   EAP AKA includes a re-authentication procedure, which is specified
   in Section 4.2. Re-authentication is based on keys derived on full
   authentication. If the peer has maintained state information for re-
   authentication and wants to use re-authentication, then the peer
   indicates this by using a specific re-authentication identity
   instead of the permanent identity or a pseudonym identity. The re-
   authentication procedure is described in Section 4.2.

4. Operation

4.1. Identity Management

4.1.1. Format, Generation and Usage of Peer Identities


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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
   [EAP].

   UMTS subscribers are identified with the International Mobile
   Subscriber Identity (IMSI) [TS 23.003]. The IMSI is composed of a
   three digit Mobile Country Code (MCC), a two or three digit Mobile
   Network Code (MNC) and a not more than 10 digit Mobile Subscriber
   Identification Number (MSIN). In other words, the IMSI is a string
   of not more than 15 digits. MCC and MNC uniquely identify the 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) [RFC 2486]. 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.

Identity Privacy Support

   EAP/AKA includes optional identity privacy (anonymity) support that
   can be used to hide the cleartext permanent identity and thereby to
   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 used in other contexts as well. Identity privacy
   is based on temporary identities, or pseudonyms, which are
   equivalent to but separate from the Temporary Mobile Subscriber
   Identities (TMSI) that are used on cellular networks. Please see
   Section 9.1 for security considerations regarding identity privacy.

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.




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

   (3) Re-authentication usernames. For example,
   43953754a@myoperator.com might be a valid re-authentication
   identity. In this case, 43953754 is the re-authentication username.

   The first two types of identities are only used on full
   authentication and the last one only on re-authentication. When the
   optional identity privacy support is not used, the non-pseudonym
   permanent identity is used on full authentication. The re-
   authentication exchange is specified in Section 4.2.

sername 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 a 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 the
   peer includes in EAP-Response/Identity, and 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.

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 operator and it
   MAY a configurable parameter in the EAP/SIM 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 a UMTS subscriber. Such reserved NAI realm may be
   useful as a hint as to 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 as it select 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 [Draft 3GPP TS 23.234].
   Alternatively, the realm name may be obtained by concatenating
   "mnc", the MNC digits of IMSI, ".mcc", the MCC digits of IMSI and
   ".owlan.org". For example, if the IMSI is 123456789098765, and the

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   MNC is three digits long, then the derived realm name is
   "mnc456.mcc123.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.

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 0x30), followed by the IMSI. The IMSI is an ASCII string that
   consists of not more than 15 decimal digits (ASCII values between
   0x30 and 0x39) as specified in [TS 23.003].

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

   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.

Generating Pseudonyms and Re-authentication Identities by the Server

   Pseudonym usernames and re-authentication identities are generated
   by the EAP server. The EAP server produces pseudonym usernames and
   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 re-authentication
   identity. Regardless of construction method, the pseudonym username
   MUST conform to the grammar specified for the username portion of an
   NAI. The re-authentication identity also 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 re-authentication identities they generate are unique.

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



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                        EAP AKA Authentication        27 October, 2003

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

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

Transmitting Pseudonyms and Re-authentication Identities to the Peer

   The server transmits pseudonym usernames and 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 re-authentication identity in
   the value field of the AT_ENCR_DATA attribute. Because identity
   privacy support and re-authentication are optional to implement, the
   peer MAY ignore the AT_ENCR_DATA attribute and always use the
   permanent identity. On re-authentication (discussed in Section 4.2),
   the server MAY include a new encrypted 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 re-authentication identity is
   included, then the peer MAY save it and other re-authentication
   state information, as discussed in Section 4.2, for the next 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 re-authentication
   identities are one-time usernames, which the peer MUST NOT re-use.

Usage of the Pseudonym by the Peer

   When the optional identity privacy support is used on full
   authentication, the peer MAY use the pseudonym username received as
   part of the previous full authentication sequence as the username
   portion of the NAI. The peer MUST NOT modify the pseudonym username

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                        EAP AKA Authentication        27 October, 2003

   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 a NAI realm portion. The
   selection of the NAI realm is discussed above.

Usage of the Re-authentication Identity by the Peer

   On re-authentication, the peer uses the re-authentication identity,
   received as part of the previous authentication sequence. A new re-
   authentication identity may be delivered as part of both full
   authentication and re-authentication. The peer MUST NOT modify the
   username part of the re-authentication identity received in
   AT_NEXT_REAUTH_ID, except in cases when username decoration is
   required. Even in these cases, the "root" 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

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 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 re-authentication identity, and the server
   recognized the identity and agrees on using re-authentication, then
   a re-authentication exchange is performed, as described in Section
   4.2.

   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
   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 the peer to
   send its permanent identity. The EAP-Request/AKA-Challenge, EAP-
   Response/AKA-Challenge, or the packets used on 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 0.

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                        EAP AKA Authentication        27 October, 2003

   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 re-authentication identity.

   Obtaining the subscriber identity via EAP/AKA messages is useful if
   the server does not have any EAP/AKA peer identity at the beginning
   of the EAP/AKA exchange or does not recognize the identity the peer
   used in EAP-Response/Identity.  This may happen if, for example, the
   EAP-Response/Identity has been issued by some EAP method other than
   EAP/AKA or if intermediate entities or software layers in the peer
   have modified the identity string in the EAP-Response/Identity
   packet. Also, some EAP layer implementations may cache the identity
   string from the first EAP authentication and do not obtain a new
   identity string from the EAP method implementation on subsequent
   authentication exchanges.

   As the identity string is used in key derivation, any of these cases
   will result in failed authentication unless the EAP server uses
   EAP/AKA attributes to obtain an unmodified copy of the identity
   string.  Therefore, unless the EAP server can be certain that no
   intermediate element or software layer has modified the EAP-
   Response/Identity packet, the EAP server SHOULD always use the
   EAP/AKA attributes to obtain the identity, even if the identity
   received in EAP-Response/Identity was valid.

   Please note that the EAP/AKA peer and the EAP/AKA server only
   process the AT_IDENTITY attribute and entities that only pass
   through EAP packets do not process this attribute. Hence, if the EAP
   server is not co-located in the authenticator, then the
   authenticator and other intermediate AAA elements (such as possible
   AAA proxy servers) will continue to refer to the peer with the
   original identity from the EAP-Response/Identity packet regardless
   of whether the AT_IDENTITY attribute is used in EAP/AKA to transmit
   another identity.

Choice of Identity for the EAP-Response/Identity

   If EAP/AKA peer is started upon receiving an EAP-Request/Identity
   message, then the peer performs the following steps.

   If the peer has maintained re-authentication state information and
   if the peer wants to use re-authentication, then the peer transmits
   the 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.





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                        EAP AKA Authentication        27 October, 2003

Server Operation in the Beginning of EAP/AKA Exchange

   If the EAP server has not received any identity (permanent identity,
   pseudonym identity or 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 message 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:

   - The server does not support re-authentication or identity privacy.

   - The server received an identity that it recognizes 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:

   - The server does not support re-authentication and the server
   supports identity privacy

   - The server received an identity that it recognizes 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/SIM/Start message, and the server does not indicate any
   preferred type for the identity. This is done in other cases, such
   as when the server does not have any identity, or the server does
   not recognize the format of a received identity.

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 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".


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                        EAP AKA Authentication        27 October, 2003

   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 includes the pseudonym identity
   in AT_IDENTITY. If the peer does not have a pseudonym when it
   receives this message, then 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." The Peer MUST NOT use a 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 re-authentication state information and the peer
   wants to use re-authentication, then the peer responds with EAP-
   Response/AKA-Identity and includes the 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. 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 is specified in Section 4.4.1.

Attacks against Identity Privacy

   The section above specifies two possible ways the peer can operate
   upon receipt of AT_PERMANENT_ID_REQ. This is because a received
   AT_PERMANENT_ID_REQ does not necessarily originate from the valid
   network, but an active attacker may transmit an EAP-Request/AKA-
   Identity packet with an AT_PERMANENT_ID_REQ attribute to the 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,
   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

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                        EAP AKA Authentication        27 October, 2003

   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.

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 EAP
   Failure and the EAP exchange terminates. 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 re-authentication identity and the server agrees on using re-
   authentication, then the server proceeds with re-authentication by
   sending EAP-Request/ AKA-Reauthentication (Section 4.2).

   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 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 re-authentication identity, which the server is able to map to
   a permanent identity, and if the server does not want to use 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


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                        EAP AKA Authentication        27 October, 2003

   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.


4.1.3. Message Sequence Examples (Informative)

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

sage of AT_ANY_ID_REQ

   Obtaining the peer identity with EAP/AKA attributes is illustrated
   in the figure 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)                                         |
          |------------------------------------------------------>|
          |                                                       |

all Back on Full Authentication

   The figure below illustrates the case when the server does not
   recognize the re-authentication identity the peer used in
   AT_IDENTITY.














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                        EAP AKA Authentication        27 October, 2003

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

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

Requesting the Permanent Identity 1

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












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                        EAP AKA Authentication        27 October, 2003

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

   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.

Requesting the Permanent Identity 2


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






















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                        EAP AKA Authentication        27 October, 2003

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

Three EAP/AKA-Identity Round Trips

   The figure below illustrates the case with three EAP/AKA-Identity
   round trips.


















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                        EAP AKA Authentication        27 October, 2003

       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 re-authentication identity)         |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not accept       |
          |                            | The re-authentication        |
          |                            | Identity                     |
          |                            +------------------------------+
          |                                                       |
          |     EAP-Request/AKA-Identity                          |
          |     (AT_FULLAUTH_ID_REQ)                              |
          |<------------------------------------------------------|
          |                                                       |
          |EAP-Response/AKA-Identity                              |
          |(AT_IDENTITY with a pseudonym identity)                |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server fails to decode the   |
          |                            | Pseudonym in AT_IDENTITY     |
          |                            +------------------------------+
          |                                                       |
          |           EAP-Request/AKA-Identity                    |
          |           (AT_PERMANENT_ID_REQ)                       |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with permanent identity)                 |
          |------------------------------------------------------>|
          |                                                       |

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

4.2. Re-authentication

4.2.1. General

   In some environments, EAP authentication may be performed
   frequently. Because the EAP AKA full authentication procedure makes

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                        EAP AKA Authentication        27 October, 2003

   use of the UMTS AKA algorithms, and it 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
   re-authentication procedure that does not make use of the UMTS AKA
   algorithms and does not need new vectors from the Authentication
   Centre.

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

   Re-authentication is based on the keys derived on the preceding full
   authentication. The same K_aut and K_encr keys as in full
   authentication are used to protect EAP AKA packets and attributes,
   and the original Master Key from full authentication is used to
   generate a fresh Master Session Key, as specified in Section 4.5.

   On re-authentication, the peer protects against replays with an
   unsigned 16-bit counter, included in the AT_COUNTER attribute. 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 re-authentication. On subsequent re-authentications, the
   counter MUST be greater than on any of the previous re-
   authentications. For example, on the second re-authentication,
   counter value is two or greater etc. The AT_COUNTER attribute is
   encrypted.

   The server includes an encrypted server nonce (AT_NONCE_S) in the
   re-authentication request. The AT_MAC attribute in the 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.

   Both the peer and the server SHOULD have an upper limit for the
   number of subsequent re-authentications allowed before a full
   authentication needs to be performed. Because a 16-bit counter is
   used in re-authentication, the theoretical maximum number of re-
   authentications is reached when the counter value reaches 0xFFFF.
   In order to use re-authentication, the peer and the EAP server need
   to store the following values: Master Key, latest counter value and
   the next re-authentication identity. K_aut, K_encr may either be
   stored or derived again from MK. The server may also need to store
   the permanent identity of the user.

4.2.2. Re-authentication Identity

   The re-authentication procedure makes use of separate re-
   authentication user identities. Pseudonyms and the permanent
   identity are reserved for full authentication only. If a re-
   authentication identity is lost and the network does not recognize
   it, the EAP server can fall back on full authentication.


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                        EAP AKA Authentication        27 October, 2003

   If the EAP server supports re-authentication, it MAY include the
   skippable AT_NEXT_REAUTH_ID attribute in the encrypted data of EAP-
   Request/AKA-Challenge message. This attribute contains a new re-
   authentication identity for the next re-authentication. The peer MAY
   ignore this attribute, in which case it will use full authentication
   next time. If the peer wants to use re-authentication, it uses this
   re-authentication identity on next authentication. Even if the peer
   has a re-authentication 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.

   In environments where a real portion is needed in the peer identity,
   the 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 re-authentication
   related requests to the same AAA server. For example, the realm part
   MAY include a portion that is specific to the AAA server. Hence, it
   is sufficient to store the context required for re-authentication in
   the AAA server that performed the full authentication.

   The peer MAY use the re-authentication identity in the EAP-
   Response/Identity packet or, in response to server's AT_ANY_ID_REQ
   attribute, the peer MAY use the 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 re-authentication
   identity, but the peer MAY modify the realm portion or replace it
   with another realm portion.

   Even if the peer uses a re-authentication identity, the server may
   want to fall back on full authentication, for example because the
   server does not recognize the re-authentication identity or does not
   want to use re-authentication. If the server was able to decode the
   re-authentication identity to the permanent identity, the server
   issues the EAP-Request/AKA-Challenge packet to initiate full
   authentication. If the server was not able to recover the peer's
   identity from the re-authentication identity, the server starts the
   full authentication procedure by issuing an EAP-Request/AKA-Identity
   packet. This packet always starts a full authentication sequence if
   it does not include the AT_ANY_ID_REQ attribute.

4.2.3. Re-authentication Procedure

   The following figure illustrates the re-authentication procedure.
   Encrypted attributes are denoted with '*'. The peer uses its re-
   authentication identity in the EAP-Response/Identity packet. As
   discussed above, an alternative way to communicate the re-
   authentication identity to the server is for the 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 the peer to send its identity by
   including the AT_ANY_ID_REQ attribute in the EAP-Request/AKA-
   Identity packet.

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   If the server recognizes the re-authentication identity and agrees
   on using re-authentication, then the server sends the EAP-
   Request/AKA-Reauthentication packet to the 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 re-authentication identity.

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

   The peer verifies that the counter value is fresh (greater than any
   previously used value). The peer also verifies that AT_MAC is
   correct. The peer MAY save the next 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 re-
   authentication has succeeded and the server sends the EAP-Success
   packet to the peer.

























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        Peer                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes a re-authentication identity)               |
          |------------------------------------------------------>|
          |                                                       |
          |                          +--------------------------------+
          |                          | Server recognizes the identity |
          |                          | and agrees on using fast       |
          |                          | re-authentication              |
          |                          +--------------------------------+
          |                                                       |
          |  EAP-Request/AKA-Reauthentication                     |
          |  (AT_IV, AT_ENCR_DATA, *AT_COUNTER,                   |
          |   *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC)            |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
     +-----------------------------------------------+            |
     | 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  |
          |<------------------------------------------------------|
          |                                                       |

4.2.4. 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 the following figure. Encrypted attributes are
   denoted with '*'.





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

   In the figure above, the first three messages are similar to the
   basic re-authentication case. When the peer detects that the counter
   value is not fresh, it includes the AT_COUNTER_TOO_SMALL attribute
   in EAP-Response/AKA-Reauthentication. This attribute doesn't contain
   any data but it is a request for the server to initiate full
   authentication. In this case, the 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 as in the EAP-
   Request/AKA-Reauthentication packet. If not, the server silently
   discards the EAP-Response/AKA-Reauthentication packet. If all checks
   on the packet are successful, the server transmits a EAP-
   Request/AKA-Challenge packet and the full authentication procedure
   is performed as usual. Since the server already knows the subscriber
   identity, it MUST NOT use the EAP-Request/AKA-Identity packet to
   request the identity.


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4.3. EAP/AKA Notifications

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

   The EAP server MAY issue an EAP-Request/AKA-Notification packet to
   the peer. 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.

   The notification code is a 16-bit number. The most significant bit
   is called the Failure bit (F bit). The F bit specifies whether the
   notification implies failure. The code values with the F bit set to
   zero (code values 0...32767) are used on unsuccessful cases. The
   receipt of a notification code from this range implies failed
   authentication, 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 F bit set to one (values
   32768...65536) does not imply failure, so the peer MUST NOT change
   its state when it receives such a notification. (This version of the
   protocol does not specify any notification codes with the F bit set
   to one.)

   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 the EAP/AKA-Challenge round
   in full authentication or the EAP/AKA-Reauthentication round in
   reautentication. For these notifications, the AT_MAC attribute MUST
   be included in both EAP-Request/AKA-Notification and EAP-
   Response/AKA-Notification.

   If the P bit is set to one, the notification can only by used before
   the EAP/AKA-Challenge round in full authentication or the EAP/AKA-
   Reauthentication round in reauthentication. For these notifications,
   the AT_MAC attribute MUST NOT be included in either EAP-Request/AKA-
   Notification or EAP-Response/AKA-Notification. (This version of the
   protocol does not specify any notification codes with the P bit set
   to one.)

   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

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   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.

4.4. 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 or EAP
   Failure) in error cases. However, implementations SHOULD NOT rely
   upon the correct error reporting behavior of the peer,
   authenticator, or the server.  It is possible for error and other
   messages to be lost in transit 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.

4.4.1. Peer Operation

   Two special error messages have been specified for error cases that
   are related to the processing of the UMTS 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
   6.5), 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 6.6), 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:

   - the peer is not able to parse the EAP request, i.e. the EAP
   request is malformed

   - the peer encountered a malformed attribute

   - wrong attribute types or duplicate attributes have been included
   in the EAP request

   - a mandatory attribute is missing

   - unrecognized non-skippable attribute

   - unrecognized or unexpected EAP/AKA Subtype in the EAP request

   - invalid AT_MAC


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   - invalid AT_CHECKCODE

   - invalid pad bytes in AT_PADDING

   - the peer does not want to process AT_PERMANENT_ID_REQ

4.4.2. Server Operation

   If an EAP/AKA server detects an error in a received EAP/AKA
   response, the server MUST issue the EAP Failure packet and the
   authentication exchange terminates. The errors cases when the server
   issues an EAP Failure include the following:

   - the server is not able to parse the peer's EAP response

   - the server encounters a malformed attribute, a non-recognized non-
   skippable attribute, or a duplicate attribute

   - a mandatory attribute is missing or an invalid attribute was
   included

   - unrecognized or unexpected EAP/AKA Subtype in the EAP Response

   - invalid AT_MAC

   - invalid AT_CHECKCODE

   - invalid AT_COUNTER

4.4.3. Failure

   As normally in EAP, the EAP server sends the EAP-Failure packet to
   the peer when the authentication procedure fails on the EAP Server.
   In EAP/AKA, this may occur for example if the EAP server does not
   recognize the peer identity, or if the EAP server is not able to
   obtain the authentication vectors for the subscriber or the
   authentication exchange times out. The server may also send EAP
   Failure if there is an error in the received EAP/AKA response, as
   discussed in Section 4.4.2.

   The server can send EAP-Failure at any time in the EAP exchange. The
   peer MUST process EAP-Failure.

4.4.4. 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.

   On 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

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   successfully authenticated the server and sent the EAP-Response/AKA-
   Reauthentication packet.

   If the peer receives an EAP/AKA notification (section 4.3) that
   indicates failure, then the peer MUST no longer accept the EAP-
   Success packet even if the server authentication was successfully
   completed.

4.5. Key Generation

   This section specifies how keying material is generated.

   On EAP AKA full authentication, a Master Key (MK) is derived from
   the underlying UMTS 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 AT_IDENTITY attribute
   from the last EAP-Response/AKA-Identity packet, or, if AT_IDENTITY
   was not used, the identity from the EAP-Response/Identity packet.
   The identity string is included as-is, without any changes and
   including the possible identity decoration. 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 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 new values exchanged in the 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
   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.

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   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.

   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).

   On re-authentication, the same pseudo-random number generator can be
   used to generate a new Master Session Key and new Initialization
   Vectors. The seed value XKEY' is calculated as follows:
   XKEY' = SHA1(Identity|counter|NONCE_S| MK)

   In the formula above, the Identity denotes the 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 re-authentication, the
   identity string from the EAP-Response/Identity packet. The counter
   denotes the counter value from AT_COUNTER attribute used in the EAP-
   Response/AKA-Reauthentication packet. The counter is used in network
   byte order. NONCE_S denotes the 16-byte NONCE_S value from the
   AT_NONCE_S attribute used in the EAP-Request/AKA-Reauthentication
   packet. The MK is the Master Key derived on the preceding full
   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.

   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 [RFC 2548] 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.

5. Message Format and Protocol Extensibility

5.1. Message Format

   As specified in [EAP], 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 [EAP]. In
   EAP/AKA, the Type field is set to 23.



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   In EAP/AKA, the Type-Data begins with an EAP/AKA header that
   consists of a 1-octet Subtype field, and a 2-octet reserved field.
   The Subtype values used in EAP/AKA are defined in Section 8. 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 8.

   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 the EAP Failure packet 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,

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   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 to be used.

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


5.2. 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 7.4,
   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 [EAP],
   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.

6. 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 5.1.

6.1. EAP-Request/AKA-Identity

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


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   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.

6.2. 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_IDENITY is defined in Section 4.1.

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

6.3. 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 7.2.

   The AT_CHECKCODE attribute MAY be included, and in certain cases
   specified in Section 7.4, 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 7.3).

   The plaintext of the AT_ENCR_DATA value field consist of nested
   attributes. The nested attributes MAY include AT_PADDING (as
   specified in Section 7.3). If the server supports identity privacy

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   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 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.

6.4. EAP-Response/AKA-Challenge

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

   The AT_MAC attribute MUST be included. In EAP-Response/AKA-
   Challenge, there is no message-specific data covered by the MAC, see
   Section 7.2.

   The AT_RES attribute MUST be included.

   The AT_CHECKCODE attribute MAY be included, and in certain cases
   specified in Section 7.4, it MUST be included.

   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.


6.5. 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.

6.6. 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.


6.7. EAP-Request/AKA-Reauthentication


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

   The AT_MAC attribute MUST be included. No message-specific data is
   included in the MAC calculation, see Section 7.2.

   The AT_CHECKCODE attribute MAY be included, and in certain cases
   specified in Section 7.4, 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.


6.8. 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 5.1. 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 7.4, 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 4.2. The AT_PADDING attribute MAY be included.

6.9. EAP-Response/AKA-Client-Error

   The peer sends EAP-Response/AKA-Client-Error in error cases, as
   specified in Section 4.4.1.

   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.

6.10. EAP-Request/AKA-Notification

   The usage of this message is specified in Section 4.3.

   The AT_NOTIFICATION attribute MUST be included.



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   The AT_MAC attribute is included in cases discussed in Section 4.3.
   No message-specific data is included in the MAC calculation. See
   Section 7.2.

   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. These attributes MAY be included only if the P bit of
   the notification code in AT_NOTIFICATION is set to zero.

6.11. EAP-Response/AKA-Notification

   The usage of this message is specified in Section 4.3. Because this
   packet is only an acknowledgement of EAP-Request/AKA-Notification,
   it does not contain any mandatory attributes.

   The AT_MAC attribute is included in cases described in Section 4.3.
   No message-specific data is included in the MAC calculation. See
   Section 7.2.

   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. These attributes MAY be included only if the P bit of
   the notification code in the AT_NOTIFICATION attribute of the
   server's EAP-Request/AKA-Notification packet is set to zero.

7. Attributes

   This section specifies the format of message attributes. The
   attribute type numbers are specified in Section 8.

7.1. 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-Re-authentication, (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.





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              Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)(11) E
                 AT_MAC  0   0   1   1  0-1 0-1  0   1   1   0   0   N
                  AT_IV  0   0  0-1  0*  0*  0*  0   1   1   0   0   N
           AT_ENCR_DATA  0   0  0-1  0*  0*  0*  0   1   1   0   0   N
             AT_PADDING  0   0  0-1  0*  0*  0*  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_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_COUNTER  0   0   0   0   0   0   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


   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 and AT_ENCR_DATA is included, then both of the
   attributes MUST be included.

7.2. AT_MAC

   The AT_MAC attribute is used for EAP/AKA message authentication.
   Section 6 specifies 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, 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 5.1. 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 6.

   The format of the AT_MAC attribute is shown below.







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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_MAC    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           MAC                                 |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The MAC algorithm is HMAC-SHA1-128 [RFC 2104] 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 4.5.

   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. If the message authentication code is invalid,
   then the recipient MUST ignore all other attributes in the message
   and operate as specified in Section 4.4.

7.3. 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/SIM 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 4.4 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
   by random. The sender MUST NOT reuse the initialization vector value
   from previous EAP AKA packets and the sender MUST choose it freshly
   for each AT_IV attribute. The sender SHOULD use a good source of
   randomness to generate the initialization vector. Please see [RFC
   1750] for more information about generating random numbers for
   security applications. The format of AT_IV is shown below.












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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_IV     | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                 Initialization Vector                         |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of the AT_ENCR_DATA attribute consists of two
   reserved bytes followed by cipher text bytes encrypted using the
   Advanced Encryption Standard (AES) [AES] in the Cipher Block
   Chaining (CBC) mode of operation using the initialization vector
   from the AT_IV attribute. The reserved bytes are set to zero when
   sending and ignored on reception. Please see [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 4.5.

   The plaintext consists of nested EAP/AKA attributes.

   The encryption algorithm requires the length of the plaintext to be
   a multiple of 16 bytes. The sender may need to include the
   AT_PADDING attribute as the last attribute within AT_ENCR_DATA. The
   AT_PADDING attribute is not included if the total length of other
   nested attributes within the AT_ENCR_DATA attribute is a multiple of
   16 bytes. As usual, the Length of the Padding attribute includes the
   Attribute Type and Attribute Length fields. The length of the
   Padding attribute is 4, 8 or 12 bytes. It is chosen so that the
   length of the value field of the AT_ENCR_DATA attribute becomes a
   multiple of 16 bytes. The actual pad bytes in the value field are
   set to zero (0x00) on sending. The recipient of the message MUST
   verify that the pad bytes are set to zero. If this 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 EAP Failure, and the
   authentication exchange terminates. The format of the AT_PADDING
   attribute is shown below.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AT_PADDING   | Length        | Padding...                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


7.4. 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 re-authentication.

   The AT_CHECKCODE attribute MAY be used to protect the EAP/AKA-
   Identity messages. AT_CHECKCODE is included in EAP-Request/AKA-
   Challenge and/or EAP-Response/AKA-Challenge upon full
   authentication. In re-authentication, AT_CHECKCODE MAY be included
   in EAP-Request/AKA-Reauthentication and/or EAP-Response/AKA-
   Reauthentication. 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.

    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 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


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   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. The EAP server must only include an EAP-
   Request/AKA-Identity in the calculation once it has received a
   corresponding response, with the same Identifier value.
   Retransmissions or requests to which the server does not receive
   response are not included.

   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 different attributes (attribute types)
   than in the first EAP-Request/AKA-Identity. After sending EAP-
   Response/AKA-Identity, if the peer receives another EAP-Request/AKA-
   Identity with the same attributes as in the previous request, then
   the peer's response to the first request must have been lost. In
   this case the peer must not include the first request and its
   response in the calculation of the checkcode.

   The AT_CHECKCODE attribute is optional to implement. It is specified
   in order to allow protecting 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 4.4.

   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 other attributes 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 other attributes 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 other attribute than AT_IDENTITY in the EAP-
   Response/AKA-Identity message, then the server MUST check that

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   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 4.4.

   Similarly, if the peer implements the processing of any other
   attribute 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
   other attribute 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 4.4.

7.5. 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.

7.6. 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.

7.7. 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_ANY_ID_REQ  | Length = 1    |           Reserved            |
      +---------------+---------------+-------------------------------+




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   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.

7.8. 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
   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.

7.9. AT_RAND

   The format of the AT_RAND attribute is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    AT_RAND    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             RAND                              |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute contains two reserved bytes
   followed by the AKA RAND parameter, 16 bytes (128 bits). The
   reserved bytes are set to zero when sending and ignored on
   reception.



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7.10. 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.


7.11. 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 is identifies the exact length of the RES in bits. The RES
   length is followed by the UMTS AKA RES parameter. According to [TS
   33.105] 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.

7.12. AT_AUTS

   The format of the AT_AUTS attribute is shown below.









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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
   |    AT_AUTS    | Length = 4    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                             AUTS                              |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

7.13. 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 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 [RFC2279].

7.14. 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 Re-authentication Username             .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   The value field of this attribute begins with 2-byte actual re-
   authentication identity length which specifies the length of the
   following re-authentication identity in bytes. This field is
   followed by a re-authentication identity that the peer can use in
   the next re-authentication, as described in Section 4.2. In
   environments where a realm portion is required, the re-
   authentication identity includes both a username portion and a realm
   name portion. The re-authentication identity does not include any
   terminating null characters. Because the length of the attribute
   must be a multiple of 4 bytes, the sender pads the re-authentication
   identity with zero bytes when necessary. The identity encoding MUST
   follow the UTF-8 transformation format [RFC2279].

7.15. 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.

7.16. 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.

7.17. AT_NONCE_S

   The format of the AT_NONCE_S attribute is shown below.













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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AT_COUNTER   | Length = 1    |           Counter             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 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 generated by the server (16 bytes)
   freshly for this EAP/AKA re-authentication. The random number is
   used as challenge for the peer and also a seed value for the new
   keying material. The reserved bytes are set to zero upon sending and
   ignored upon reception.

   The server MUST choose the NONCE_S value freshly for each EAP/AKA
   re-authentication exchange. The server SHOULD use a good source of
   randomness to generate NONCE_S. Please see [RFC 1750] for more
   information about generating random numbers for security
   applications.

7.18. 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    |F|P|  Notification Code        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute contains a two-byte notification
   code. The first and second bit (F and P) of the notification code
   are interpreted as described in Section 4.3.

   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.

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

   1031 - User has not subscribed to the requested service (implies
   failure, used after the challenge round)



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7.19. 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


8. IANA and Protocol Numbering Considerations

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

   IANA has assigned the number 23 for EAP AKA authentication.

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

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






















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

        AT_RAND.........................................1
        AT_AUTN.........................................2
        AT_RES..........................................3
        AT_AUTS.........................................4
        AT_PADDING......................................6
        AT_PERMANENT_ID_REQ............................10
        AT_MAC.........................................11
        AT_NOTIFICATION................................12
        AT_ANY_ID_REQ..................................13
        AT_IDENTITY....................................14
        AT_FULLAUTH_ID_REQ.............................17
        AT_COUNTER.....................................19
        AT_COUNTER_TOO_SMALL...........................20
        AT_NONCE_S.....................................21
        AT_CLIENT_ERROR_CODE...........................22

        AT_IV.........................................129
        AT_ENCR_DATA..................................130
        AT_NEXT_PSEUDONYM.............................132
        AT_NEXT_REAUTH_ID.............................133
        AT_CHECKCODE..................................134

   The AT_NOTIFICATION attribute contains a notification code value.
   Values 1024, 1026 and 1031 have been specified in Section 7.18 of
   this document.

   The AT_CLIENT_ERROR_CODE attribute contains a client error code.
   Value 0 has been specified in Section 7.19 of this document.

   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 [RFC 2434].
   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.

   EAP AKA and EAP SIM [EAP SIM] are "sister" protocols with similar
   message structure and protocol numbering spaces. Many attributes and
   message Subtypes have the same protocol numbers in these two
   protocols. Hence, it is recommended that the same protocol number
   value SHOULD NOT be allocated for two different purposes in EAP AKA
   and EAP SIM.

9. Security Considerations

   The EAP base protocol specification [EAP] highlights several attacks
   that are possible against the EAP protocol. This section discusses


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   the claimed security properties of EAP AKA as well as
   vulnerabilities and security recommendations.

9.1. Identity Protection

   EAP/AKA includes optional Identity privacy support that protects the
   privacy of the subscriber identity against passive eavesdropping.
   The mechanism cannot be used on the first exchange with a given
   server, when the IMSI will have to be sent in the clear. The
   terminal SHOULD store the pseudonym in a non-volatile memory so that
   it can be maintained across reboots. An active attacker that
   impersonates the network may use the AT_PERMANENT_ID_REQ attribute
   (Section 1.1) to learn the subscriber's IMSI. However, as discussed
   in Section 1.1, 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.

9.2. Mutual Authentication

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

9.3. Key Derivation

   EAP/AKA supports key derivation with 128-bit effective key strength.
   The key hierarchy is specified in Section 0.

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

9.4. Brute-Force and Dictionary Attacks

   The effective strength of EAP/AKA values is 128 bits, and there are
   no known computationally feasible brute-force attacks. Because UMTS
   AKA is not a password protocol (the pre-shared secret must not be a
   weak password), EAP/AKA is not vulnerable to dictionary attacks.

9.5. Integrity Protection, Replay Protection and Confidentiality

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

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   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.

   On full authentication, replay protection is provided by RAND and
   AUTN values from the underlying UMTS AKA scheme. On re-
   authentication, a counter and a server nonce is used to provide
   replay protection.
   The contents of the EAP-Response/Identity packet are implicitly
   integrity protected by including them in key derivation.

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

   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.

9.6. 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 5, 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 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.

9.7. Fast Reconnect

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

9.8. Acknowledged Result Indications


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   EAP/AKA does not provide acknowledged or integrity protected Success
   or Failure indications.

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

   On physically insecure networks, an attacker may mount denial of
   service attacks by sending false EAP Success or EAP Failure
   indications. However, the attacker cannot force the peer or the EAP
   server to believe successful authentication has occurred when mutual
   authentication failed or has not happened yet.

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

9.10. 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
   [RFC 1750] for more information on generating random numbers for
   security applications.

10. Security Claims

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

   [a] Intended use. EAP AKA is intended for use over both physically
   insecure networks and physically or otherwise secure networks.
   Applicable media include but are not limited to PPP, IEEE 802 wired
   networks and IEEE 802.11.



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

   [c] Security claims. The security properties of the method are
   discussed in Section 9.

   [d] Key strength. EAP/AKA supports key derivation with 128-bit
   effective key strength.

   [e] Description of key hierarchy. Please see Section 0.

   [f] Indication of vulnerabilities. Vulnerabilities are discussed in
   Section 9.

11. Intellectual Property Right Notices

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

Acknowledgements and Contributions

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

   The attribute format is based on the extension format of Mobile IPv4
   [RFC 3344].

Authors' Addresses

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

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









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Annex A. Pseudo-Random Number Generator

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

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

   Step 2: In hexadecimal notation let
       t = 67452301 EFCDAB89 98BADCFE 10325476 C3D2E1F0
       This is the initial value for H0|H1|H2|H3|H4
       in the FIPS SHS [SHA-1]

   Step 3: For j = 0 to m - 1 do
         3.1 XSEED_j = 0 /* no optional user input */
         3.2 For i = 0 to 1 do
             a. XVAL = (XKEY + XSEED_j) mod 2^b
             b. w_i = G(t, XVAL)
             c. XKEY = (1 + XKEY + w_i) mod 2^b
         3.3 x_j = w_0|w_1





































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Normative References

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

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

   [EAP] L. Blunk et al., "Extensible Authentication Protocol (EAP)",
   draft-ietf-eap-rfc2284bis-05.txt, work-in-progress, September 2003.

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

   [TS 23.003] 3GPP Technical Specification 3GPP TS 23.003 V5.5.1: "3rd
   Generation Parnership Project; Technical Specification Group Core
   Network; Numbering, addressing and identification (Release 5)", 3rd
   Generation Partnership Project, January 2003

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

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

   [AES] Federal Information Processing Standards (FIPS) Publication
   197, "Advanced Encryption Standard (AES)", National Institute of
   Standards and Technology, November 26, 2001.
   http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf

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

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

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



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

Informative References

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

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

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

   [RFC 3344] C. Perkins (editor), "IP Mobility Support", RFC 3344,
   August 2002.

   [EAP SIM] H. Haverinen, J. Salowey, "EAP SIM Authentication", draft-
   haverinen-pppext-eap-sim-12.txt, October 2003, work in progress

   [TS 23.234] Draft 3GPP Technical Specification 3GPP TS 23.234 V
   1.4.0: "Technical Specification Group Services and System Aspects;
   3GPP system to Wireless Local Area Network (WLAN) Interworking;
   System Description", 3rd Generation Partnership Project, work in
   progress, January 2003.



























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