[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits] [IPR]

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

                                                               J. Arkko
   Internet Draft                                              Ericsson
   Document: draft-arkko-pppext-eap-aka-01.txt             H. Haverinen
   Expires: December 2001                                         Nokia
                                                          November 2001


                          EAP AKA Authentication


Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.


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

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

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


Abstract

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for authentication and session key distribution using the
   UMTS AKA authentication mechanism. AKA is based on symmetric keys,
   and runs typically in a UMTS Subscriber Identity Module, a smart
   card like device. AKA provides also backward compatibility to GSM
   authentication, making it possible to use EAP AKA for authenticating
   both GSM and UMTS subscribers.


Table of Contents


   Status of this Memo................................................1
   Abstract...........................................................1
   1. Introduction and Motivation.....................................2
   2. Conventions used in this document...............................3
   3. Protocol Overview...............................................5
   4. IMSI Privacy Support...........................................10
   5. Message Format.................................................12


   Arkko and Haverinen     Expires in six months             [Page 1]


                        EAP AKA Authentication           November 2001


   6. Messages.......................................................13
   6.1. EAP-Response/Identity........................................13
   6.2. EAP-Request/USIM-Challenge...................................14
   6.3. EAP-Response/USIM-Challenge..................................18
   6.4. EAP-Response/USIM-Authentication-Reject......................20
   6.5. EAP-Response/USIM-Synchronization-Failure....................20
   6.6. EAP-Request/USIM-IMSI........................................21
   6.7. EAP-Response/USIM-IMSI.......................................22
   7. Interoperability with GSM......................................23
   8. IANA and Protocol Numbering Considerations.....................24
   9. Security Considerations........................................24
   10. Intellectual Property Right Notices...........................24
   Acknowledgements..................................................25
   Authors' Addresses................................................25

1. Introduction and Motivation

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for authentication and session key distribution using the
   UMTS AKA authentication mechanism [1]. The Universal Mobile
   Telecommunications System (UMTS) is a global third generation mobile
   network standard.

   AKA is based on challenge-response mechanisms and symmetric
   cryptography. AKA typically runs in a UMTS Subscriber Identity
   Module (USIM), a smart card like device. However, the applicability
   of AKA is not limited to client devices with smart cards, but the
   AKA mechanisms could also be implemented in host software, for
   example AKA also provides backward compatibility to the GSM
   authentication mechanism [2]. Compared to the GSM mechanism, AKA
   provides substantially longer key lengths and the authentication of
   the server side as well as the client side.

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

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

   - The use of the third generation mobile network authentication
     infrastructure in the context of wireless LANs and IEEE 801.1x
     technology through EAP over Wireless [3, 4].

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

   AKA works in the following manner:

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




   Arkko and Haverinen     Expires in six months             [Page 2]


                        EAP AKA Authentication           November 2001

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

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

   - The USIM verifies the AUTN, again based on the secret key and the
     sequence number. If this process is successful (the AUTN is valid
     and the sequence number used to generate AUTN is within the
     correct range), the USIM produces an authentication result, RES
     and sends this to the home environment.

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

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

   For a specification of the AKA mechanisms and how the cryptographic
   values AUTN, RES, IK, CK and AUTS are calculated, see reference [1].

   It is also possible that the home environment delegates the actual
   authentication task to an intermediate node. In this case the
   authentication vector or parts of it are delivered to the
   intermediate node, enabling it to perform the comparison between RES
   and XRES, and possibly also use CK and IK. In EAP AKA, the EAP
   server node is such an intermediate node.

   In the third generation mobile networks, AKA is used both for radio
   network authentication and IP multimedia service authentication
   purposes. Different user identities and formats are used for these;
   the radio network uses the International Mobile Subscriber
   Identifier (IMSI), whereas the IP multimedia service uses the
   Network Access Identifier (NAI) [5].


2. Conventions used in this document

   The following terms will be used through this document:


   AAA protocol

      Authentication, Authorization and Accounting protocol


   Arkko and Haverinen     Expires in six months             [Page 3]


                        EAP AKA Authentication           November 2001

   AAA server

      In this document, AAA server refers to the network element that
      resides on the border of Internet AAA network and GSM network.
      Cf. EAP server

   AKA

      Authentication and Key Agreement

   AuC

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

   EAP

      Extensible Authentication Protocol [6].

   EAP server

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

   GSM

      Global System for Mobile communications.

   NAI

      Network Access Identifier [5].

   AUTN

      Authentication value generated by the AuC which together with the
      RAND authenticates the server to the client, 128 bits [1].

   AUTS

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

   RAND

      Random number generated by the AuC, 128 bits [1].

   RES

      Authentication result from the client, which together with the
      RAND authenticates the client to the server, 128 bits [1].

   SQN

      Sequence number used in the authentication process, 48 bits [1].

   Arkko and Haverinen     Expires in six months             [Page 4]


                        EAP AKA Authentication           November 2001

   SIM

      Subscriber Identity Module. SIM cards are 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. These cards are smart cards
      Similar to SIMs and are distributed by UMTS operators.


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC 2119 [7]

3. Protocol Overview

   In this document, the term EAP Server refers to the network element
   that terminates the EAP protocol. Usually the EAP server is separate
   from the authenticator device, which is the network element closest
   to the client, such as a Network Access Server (NAS) or an IEEE
   802.1X bridge. Typically, the authenticator does not contain the EAP
   server functionality, but the EAP server functionality is
   implemented on a separate AAA server with whom the authenticator
   communicates using an AAA protocol. (The exact AAA communications is
   outside the scope of this document, however.)

   The below message flow shows the basic successful authentication
   case with the EAP AKA. The EAP AKA uses two roundtrips to authorize
   the user and generate session keys. As in other EAP schemes, first
   an identity request/response message pair is exchanged. (For this
   particular EAP protocol, the identity request is defined to be
   optional, to shorten the authentication process to a minimal one.)

   Next, the EAP server starts the actual AKA protocol by sending an
   EAP-Request/USIM-Challenge message. This message contains a random
   number (RAND) and an authorization vector (AUTN). The EAP-
   Request/USIM-Challenge message MAY optionally contain encrypted
   data, which is used for IMSI privacy support, as described in
   Section 4. The encrypted data is not shown in the figures of this
   section. The client runs the AKA algorithm (perhaps inside an USIM)
   and verifies the AUTN. If this is successful, the client is talking
   to a legitimate EAP server and proceeds to send the EAP-
   Response/USIM-Challenge. This message contains a result parameter
   that allows the EAP server in turn to verify that the client is a
   legitimate one.




   Arkko and Haverinen     Expires in six months             [Page 5]


                        EAP AKA Authentication           November 2001

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

   When EAP AKA is run in the GSM compatible mode, the message flow is
   otherwise identical to the message flow below except that the AUTN
   attribute is not included in EAP-Request/USIM-Challenge packet.


   The second message flow shows how the EAP server rejects the Client
   due to failed authentication. The same flow is also used in the GSM
   compatible mode, except that the AUTN parameter is not included in
   the EAP-Request/USIM-Challenge packet.










   Arkko and Haverinen     Expires in six months             [Page 6]


                        EAP AKA Authentication           November 2001

       Client                                             Authenticator
          |                                                       |
          |                      EAP-Request/Identity (optional)  |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs UMTS algorithms, |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/USIM-Challenge    |
          |                         (RAND, AUTN)                  |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM,|                     |
      | possibly verifies AUTN, and sends an|                     |
      | invalid response                    |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/USIM-Challenge                           |
          | (RES)                                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server checks the given RES, |
          |                            | and finds it incorrect.      |
          |                            +------------------------------+
          |                                                       |
          |                                          EAP-Failure  |
          |<------------------------------------------------------|


   The next message flow shows the client rejecting the AUTN of the EAP
   server. This flow is not used in the GSM compatible mode.
















   Arkko and Haverinen     Expires in six months             [Page 7]


                        EAP AKA Authentication           November 2001

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


   Networks that are not UMTS aware use the GSM compatible version of
   this protocol even for UMTS subscribers. In this case, the AUTN
   parameter is not included in the EAP-Request/USIM-Challenge packet.
   If a UMTS capable client does not want to accept the use of the GSM
   compatible mode, the client can reject the authentication with the
   EAP-Response/Nak message [6], as shown in the following figure:

















   Arkko and Haverinen     Expires in six months             [Page 8]


                        EAP AKA Authentication           November 2001

   Client                                             Authenticator
          |                                                       |
          |                      EAP-Request/Identity (optional)  |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs GSM algorithms,  |
          |                            | generates RAND               |
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/USIM-Challenge    |
          |                         (RAND)                        |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client does not accept the GSM      |                     |
      | compatible version of this protocol.|                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/Nak                                      |
          |------------------------------------------------------>|
          |                                                       |
          |                                                       |
          |                                          EAP-Failure  |
          |<------------------------------------------------------|

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





















   Arkko and Haverinen     Expires in six months             [Page 9]


                        EAP AKA Authentication           November 2001

       Client                                             Authenticator
          |                                                       |
          |                      EAP-Request/Identity (optional)  |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes user's NAI)                                 |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server runs UMTS algorithms, |
          |                            | generates RAND and AUTN.     |
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/USIM-Challenge    |
          |                         (RAND, AUTN)                  |
          |<------------------------------------------------------|
          |                                                       |
      +-------------------------------------+                     |
      | Client runs UMTS algorithms on USIM |                     |
      | and discovers AUTN that contains an |                     |
      | inappropriate sequence number       |                     |
      +-------------------------------------+                     |
          |                                                       |
          | EAP-Response/USIM-Synchronization-Failure             |
          | (AUTS)                                                |
          |------------------------------------------------------>|
          |                                                       |
          |                              +---------------------------+
          |                              | Perform resynchronization |
          |                              | towards the AAA using     |
          |                              | AUTS and the sent RAND    |
          |                              +---------------------------+
          |                                                       |

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

4. IMSI Privacy Support

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

   The EAP-Request/USIM-Challenge message MAY include an encrypted
   pseudonym in the value field of the AT_ENCR_DATA attribute. The
   AT_IV and AT_MAC attributes are also used to transport the pseudonym
   to the client, as described in Section 6.2. Because the IMSI privacy
   support is optional to implement, the client MAY ignore the AT_IV,
   AT_ENCR_DATA, and AT_MAC attributes and always transmit the IMSI in
   the EAP-Response/Identity packet.

   Arkko and Haverinen     Expires in six months            [Page 10]


                        EAP AKA Authentication           November 2001

   On receipt of the EAP-Request/USIM-Challenge, the client verifies
   the AT_AUTN attribute before looking at the AT_ENCR_DATA or AT_MAC
   attributes. If the AUTN is invalid, then the client MUST ignore the
   AT_IV, AT_ENCR_DATA and AT_MAC attributes. If AUTN is valid, then
   the client MAY derive the K_encr and K_int keys as described in
   Section 6.2 and verify the AT_MAC attribute. If the AT_MAC attribute
   is valid, then the client MAY decrypt the encrypted data and use the
   pseudonym in the next authentication. If the MAC is invalid, then
   the encrypted data MUST be ignored and the whole EAP packet MAY be
   silently ignored.

   The EAP server produces pseudonyms in an implementation-dependent
   manner. Please see [8] for examples on how to produce pseudonyms.
   The pseudonyms need to be reversible to the IMSI only on the EAP
   server. Regardless of construction method, the pseudonym MUST
   conform to the grammar specified for the username portion of an NAI.

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

   If the EAP server fails to decode the pseudonym to a known client
   name, then the EAP server requests the regular IMSI (non-pseudonym
   identity) by issuing the EAP-Request/USIM-IMSI packet to the client.
   This packet includes no attributes. The client responds with the
   EAP-Response/USIM-IMSI, which includes the client's IMSI in the
   clear. This case is illustrated in the figure below.

   Client                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes a pseudonym)                                |
          |------------------------------------------------------>|
          |                                                       |
          |                            +------------------------------+
          |                            | Server fails to decode the   |
          |                            | Pseudonym.                   |
          |                            +------------------------------+
          |                                                       |
          |                         EAP-Request/USIM-IMSI         |
          |<------------------------------------------------------|
          |                                                       |
          |                                                       |
          | EAP-Response/USIM-IMSI                                |
          | (IMSI)                                                |
          |------------------------------------------------------>|
          |                                                       |


   Arkko and Haverinen     Expires in six months            [Page 11]


                        EAP AKA Authentication           November 2001


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

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

5. Message Format

   The Type-Data of the EAP AKA packets begins with a 1-octet Subtype
   field, which is followed by a 2-octet reserved field. The rest of
   the Type-Data consists of attributes that are encoded in Type,
   Length, Value format. The figure below shows the generic format of
   an attribute.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Attribute Type |    Length     | Value...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Attribute Type

      Indicates the particular type of attribute. The attribute type
      values are listed in Section 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 may be two or more bytes in length. The
      type and length fields determine the format and length of the
      value field.

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

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

   Arkko and Haverinen     Expires in six months            [Page 12]


                        EAP AKA Authentication           November 2001

   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.

6. Messages

6.1. EAP-Response/Identity

   In the beginning of EAP authentication, the Authenticator issues the
   EAP-Request/Identity packet to the client. The client responds with
   EAP-Response/Identity, which contains the user's identity. The
   formats of these packets are specified in [6].

   The EAP AKA mechanism uses the NAI format [5] as the identity.
   In order to facilitate the use of the existing cellular roaming
   infrastructure, the subscriber's IMSI is used as the client
   identifier. When IMSI privacy is not used, the EAP AKA client
   transmits the user's IMSI within the NAI in the EAP
   Response/Identity packet. The NAI is of the format "0imsi@realm". In
   other words, the first character is the digit zero (ASCII value
   0x30), followed by the IMSI, followed by the @ character and the
   realm. The IMSI is an ASCII string that consists of not more than 15
   decimal digits (ASCII values between 0x30 and 0x39) as specified in
   [9].

   When the optional IMSI privacy support is used, the client MAY use
   the pseudonym received as part of the previous authentication
   sequence as the user name portion of the NAI, as specified in
   Section 4.

   The AAA network routes AAA requests to the correct AAA server using
   the realm part of the NAI. Because cellular roaming can be used with
   EAP AKA, the AAA request can be routed to an AAA server in the
   visited network instead of the server indicated in the NAI realm.
   The operators need to agree on this special AAA routing in advance.
   It is recommended that operators should reserve the realm portion of
   NAI for EAP AKA users exclusively, so that exactly the same realm is
   not used with other authentication methods. This convention makes it
   easy to recognize that the NAI identifies a UMTS or GSM subscriber
   of this operator, which may be useful when configuring the routing
   rules in the visited AAA networks.

   In the EAP AKA protocol, the EAP-Request/Identity message is
   optional when applicable. If the client can positively determine
   that it has to authenticate, it MAY send an unsolicited EAP-
   Response/Identity to the authenticator with an EAP Identifier value
   it has picked up itself. The client MUST NOT send an unsolicited
   EAP-Response/Identity if it has already received an EAP-
   Request/Identity packet. The client MUST send an EAP-
   Response/Identity to all received EAP-Request/Identity packets,

   Arkko and Haverinen     Expires in six months            [Page 13]


                        EAP AKA Authentication           November 2001

   using the Identifier value in the EAP-Request/Identity. If the
   authenticator receives an unsolicited EAP-Response/Identity, it
   SHOULD process the packet as if it had requested it. If the
   authenticator receives an EAP-Response/Identity with an incorrect
   Identifier value in response to the first EAP-Request/Identity it
   has sent to the client, then the authenticator SHOULD still accept
   the EAP-Response/Identity packet.

6.2. EAP-Request/USIM-Challenge

   The format of the EAP-Request/USIM-Challenge packet is shown below.












































   Arkko and Haverinen     Expires in six months            [Page 14]


                        EAP AKA Authentication           November 2001

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Subtype    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    AT_RAND    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             RAND                              |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    AT_AUTN    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                        AUTN (optional)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_IV     | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                 Initialization Vector (optional)              |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_ENCR_DATA  | Length        |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                    Encrypted Data (optional)                  |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_MAC    | Length = 6    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           MAC (optional)                      |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      1 for Request

   Identifier

      See [6]



   Arkko and Haverinen     Expires in six months            [Page 15]


                        EAP AKA Authentication           November 2001

   Length

      The length of the EAP Request packet.

   Type

      TBD

   Subtype

      1 for USIM-Challenge

   Reserved

      Set to zero when sending, ignored on reception.

   AT_RAND

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

   AT_AUTN

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

   AT_IV

      The value field 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. This attribute MUST be included if and only if the
      AT_ENCR_DATA is included. Messages that do not meet this
      condition MUST be silently discarded.

   AT_ENCR_DATA

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

      The encryption key (K_encr) is derived from the AKA Cipher Key
      (CK) with the following formula. The result of the SHA-1 hash
      value [12] is truncated to 128 bits by ignoring the 32 rightmost

   Arkko and Haverinen     Expires in six months            [Page 16]


                        EAP AKA Authentication           November 2001

      bits. The notation A|0 denotes A concatenated with the byte zero
      0x00.

         K_encr = 128 leftmost bits of SHA1(CK|0)

      The plaintext consists of nested attributes as described below.

   AT_MAC

      This attribute is optional, but it MUST be included whenever the
      AT_ENCR_DATA attribute is included. Messages that do not meet
      this condition MUST be silently discarded.

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

      The MAC algorithm is HMAC-SHA1 [13] keyed hash value, so the
      length of the MAC is 20 bytes.

      The integrity protection key (K_int) used in the calculation of
      the MAC is derived from the AKA integrity key (IK) with the
      following formula. The notation A|0 denotes A concatenated with
      the byte zero 0x00.

         K_int = SHA1(IK|0)

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

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




   Arkko and Haverinen     Expires in six months            [Page 17]


                        EAP AKA Authentication           November 2001

   AT_PSEUDONYM

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

   AT_PADDING

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

6.3. EAP-Response/USIM-Challenge

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

   EAP-Response/USIM-Challenge MAY include the AT_MAC attribute to
   integrity protect the EAP packet. Later versions of this protocol
   MAY make use of the AT_ENCR_DATA and AT_IV attributes in this
   message to include encrypted (skippable) attributes. AT_MAC,
   AT_ENCR_DATA and AT_IV attributes are not shown in the figure below.
   If present, they are processed as in EAP-Request/USIM-Challenge
   packet. The EAP server MUST process EAP-Response/USIM-Challenge
   messages that include these attributes even if the server did not
   implement these optional attributes.













   Arkko and Haverinen     Expires in six months            [Page 18]


                        EAP AKA Authentication           November 2001

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Subtype     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AT_RES    |    Length     |          RES Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                                                               |
   |                             RES                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      See [6]

   Length

      The length of the EAP Response packet.

   Type

      TBD

   Subtype

      1 for USIM-Challenge

   Reserved

      Set to zero when sending, ignored on reception.

   AT_RES

      This attribute MUST be included in EAP-Response/USIM-Challenge.
      The value field of this attribute begins with the 2-byte RES
      Length, which is identifies the exact length of the RES (or SRES)
      in bits. The RES length is followed by the UMTS AKA RES or GSM
      SRES parameter. According to the specification [14] the length of
      the AKA RES can vary between 32 and 128 bits. The GSM SRES
      parameter is always 32 bits long. Because the length of the
      AT_RES attribute must be a multiple of 4 bytes, the sender pads
      the RES with zero bits where necessary.



   Arkko and Haverinen     Expires in six months            [Page 19]


                        EAP AKA Authentication           November 2001

6.4. EAP-Response/USIM-Authentication-Reject

   The format of the EAP-Response/USIM-Authentication-Reject packet is
   shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Subtype     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      See [6]

   Length

      The length of the EAP Response packet.

   Type

      TBD

   Subtype

      2 for USIM-Authentication-Reject

   Reserved

      Set to zero on sending, ignored on reception.


6.5. EAP-Response/USIM-Synchronization-Failure

   The format of the EAP-Response/USIM-Synchronization-Failure packet
   is shown below.











   Arkko and Haverinen     Expires in six months            [Page 20]


                        EAP AKA Authentication           November 2001

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Subtype     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
   |    AT_AUTS    | Length = 4    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                             AUTS                              |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The semantics of the fields is described below:

   Code

      2 for Response

   Identifier

      See [6]

   Length

      The length of the EAP Response packet, 20.

   Type

      TBD

   Subtype

      4 for USIM-Synchronization-Failure

   AT_AUTS

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

6.6. EAP-Request/USIM-IMSI

   The format of the EAP-Request/USIM-IMSI packet is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Subtype     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Arkko and Haverinen     Expires in six months            [Page 21]


                        EAP AKA Authentication           November 2001

   The semantics of the fields is described below:

   Code

      1 for Request

   Identifier

      See [6]

   Length

      The length of the EAP Request packet.

   Type

      TBD

   Subtype

      5 for USIM-IMSI

   Reserved

      Set to zero on sending, ignored on reception.

6.7. EAP-Response/USIM-IMSI

   The format of the EAP-Response/USIM-IMSI packet is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Subtype     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    AT_IMSI    | Length = 5    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             IMSI                              |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The semantics of the fields is described below:

   Code

      2 for Response




   Arkko and Haverinen     Expires in six months            [Page 22]


                        EAP AKA Authentication           November 2001

   Identifier

      See [6]

   Length

      The length of the EAP Response packet.

   Type

      TBD

   Subtype

      5 for USIM-IMSI

   Reserved

      Set to zero on sending, ignored on reception.

   AT_IMSI

      This attribute MUST be included in EAP-Response/USIM-IMSI. The
      value field of this attribute contains two reserved bytes
      followed by the IMSI, represented as an ASCII string that
      consists of not more than 15 decimal digits (ASCII values between
      0x30 and 0x39) [9]. The reserved bytes are set to zero on sending
      and ignored on reception. The IMSI characters are followed by one
      or more "F" characters (ASCII value 0x46). They are included to
      make the length of the value field 16 bytes.


7. Interoperability with GSM

   The EAP AKA protocol is able to authenticate both UMTS and GSM
   users, if the subscriber's operator's network is UMTS aware. This is
   because the home network will be able to determine from the
   subscriber records whether the subscriber is equipped with a UMTS
   USIM or a GSM SIM. A UMTS aware home network will hence always use
   UMTS AKA with UMTS subscribers and GSM authentication with GSM
   subscribers. With GSM subscribers, the EAP AKA protocol is always
   used in the GSM compatible mode.

   It is not possible to use a GSM AuC to authenticate UMTS
   subscribers. (Note that if the home network doesn't support an
   authentication method it should not distribute SIMs for that
   method.)

   However, it is possible that the node actually terminating EAP and
   the node that stores the authentication keys (AuC) are separate, and
   support different authentication types. If the node terminating EAP
   is GSM-only but AuC is UMTS-aware, then authentication can still be
   achieved using the GSM compatible version of EAP AKA. This
   authentication will be weaker, since the GSM compatible mode does

   Arkko and Haverinen     Expires in six months            [Page 23]


                        EAP AKA Authentication           November 2001

   not provide for mutual authentication. Section 6.8.1.1 in [1]
   specifies how the GSM SRES parameter and the Kc key can be
   calculated on the USIM and the AuC. If a UMTS terminal does not want
   to accept the GSM compatible version of this protocol, then it can
   reject the authentication with the EAP-Response/USIM-GSM-
   Authentication-Reject packet.

   In conclusion, the following table shows which variant of the EAP
   AKA protocol should be run under different conditions:

   SIM    EAP node             AuC         EAP AKA mode
   ----------------------------------------------------
   GSM    (any)              (any)                  GSM
   UMTS   (any)                GSM            (illegal)
   UMTS   GSM             GSM+UMTS                  GSM
   UMTS   GSM+UMTS        GSM+UMTS                 UMTS


8. IANA and Protocol Numbering Considerations

   IANA has assigned the number TBD for EAP AKA authentication.

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

        USIM-Challenge..................................1
        USIM-Authentication-Reject......................2
        USIM-Synchronization-Failure....................4
        USIM-IMSI.......................................5


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

        AT_RAND.........................................1
        AT_AUTN.........................................2
        AT_RES..........................................3
        AT_AUTS.........................................4
        AT_IMSI.........................................5
        AT_PADDING......................................6

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

9. Security Considerations

   Implementations running the EAP AKA protocol will rely on the
   security of the AKA scheme, and the secrecy of the symmetric keys
   stored in the USIM and the AuC.

10. Intellectual Property Right Notices


   Arkko and Haverinen     Expires in six months            [Page 24]


                        EAP AKA Authentication           November 2001

   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

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

   The IMSI privacy support is based on the identity privacy support of
   [8]. The attribute format is based on the extension format of Mobile
   IPv4 [15].

Authors' Addresses

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

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

References

   [1]   3GPP Technical Specification 3GPP TS 33.102 V3.6.0: "Technical
         Specification Group Services and System Aspects; 3G Security;
         Security Architecture (Release 1999)", 3rd Generation
         Partnership Project, November 2000.

   [2]   GSM Technical Specification GSM 03.20 (ETS 300 534): "Digital
         cellular telecommunication system (Phase 2); Security related
         network functions", European Telecommunications Standards,
         Institute, August 1997.

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

   [4]   IEEE Draft 802.11eS/D1, "Draft Supplement to STANDARD FOR
         Telecommunications and Information Exchange between Systems -
         LAN/MAN Specific Requirements - Part 11: Wireless Medium
         Access Control (MAC) and physical layer (PHY) specifications:
         Specification for Enhanced Security", March 2001

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


   Arkko and Haverinen     Expires in six months            [Page 25]


                        EAP AKA Authentication           November 2001



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

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

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

   [9]   GSM Technical Specification GSM 03.03 (ETS 300 523): "Digital
         cellular telecommunication system (Phase 2); Numbering,
         addressing and identification", European Telecommunications
         Standards Institute, April 1997.

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

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

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

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

   [14]  3GPP Technical Specification 3GPP TS 33.105 V3.5.0: "Technical
         Specification Group Services and System Aspects; 3G Security;
         Cryptographic Algorithm Requirements (Release 1999)",
         3rdGeneration Partnership Project, October 2000

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















   Arkko and Haverinen     Expires in six months            [Page 26]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/