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rfc:rfc4186

Network Working Group H. Haverinen, Ed. Request for Comments: 4186 Nokia Category: Informational J. Salowey, Ed.

                                                         Cisco Systems
                                                          January 2006
           Extensible Authentication Protocol Method for
           Global System for Mobile Communications (GSM)
               Subscriber Identity Modules (EAP-SIM)

Status of This Memo

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

Copyright Notice

 Copyright (C) The Internet Society (2006).

IESG Note

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

Abstract

 This document specifies an Extensible Authentication Protocol (EAP)
 mechanism for authentication and session key distribution using the
 Global System for Mobile Communications (GSM) Subscriber Identity
 Module (SIM).  GSM is a second generation mobile network standard.
 The EAP-SIM mechanism specifies enhancements to GSM authentication
 and key agreement whereby multiple authentication triplets can be
 combined to create authentication responses and session keys of
 greater strength than the individual GSM triplets.  The mechanism
 also includes network authentication, user anonymity support, result
 indications, and a fast re-authentication procedure.

Haverinen & Salowey Informational [Page 1] RFC 4186 EAP-SIM Authentication January 2006

Table of Contents

 1. Introduction ....................................................4
 2. Terms ...........................................................5
 3. Overview ........................................................8
 4. Operation ......................................................10
    4.1. Version Negotiation .......................................10
    4.2. Identity Management .......................................11
         4.2.1. Format, Generation and Usage of Peer Identities ....11
         4.2.2. Communicating the Peer Identity to the Server ......17
         4.2.3. Choice of Identity for the EAP-Response/Identity ...19
         4.2.4. Server Operation in the Beginning of
                EAP-SIM Exchange ...................................19
         4.2.5. Processing of EAP-Request/SIM/Start by the Peer ....20
         4.2.6. Attacks Against Identity Privacy ...................21
         4.2.7. Processing of AT_IDENTITY by the Server ............22
    4.3. Message Sequence Examples (Informative) ...................23
         4.3.1. Full Authentication ................................24
         4.3.2. Fast Re-authentication .............................25
         4.3.3. Fall Back to Full Authentication ...................26
         4.3.4. Requesting the Permanent Identity 1 ................27
         4.3.5. Requesting the Permanent Identity 2 ................28
         4.3.6. Three EAP-SIM/Start Roundtrips .....................28
 5. Fast Re-Authentication .........................................30
    5.1. General ...................................................30
    5.2. Comparison to UMTS AKA ....................................31
    5.3. Fast Re-authentication Identity ...........................31
    5.4. Fast Re-authentication Procedure ..........................33
    5.5. Fast Re-authentication Procedure when Counter Is
         Too Small .................................................36
 6. EAP-SIM Notifications ..........................................37
    6.1. General ...................................................37
    6.2. Result Indications ........................................39
    6.3. Error Cases ...............................................40
         6.3.1. Peer Operation .....................................40
         6.3.2. Server Operation ...................................41
         6.3.3. EAP-Failure ........................................42
         6.3.4. EAP-Success ........................................42
 7. Key Generation .................................................43
 8. Message Format and Protocol Extensibility ......................45
    8.1. Message Format ............................................45
    8.2. Protocol Extensibility ....................................47
 9. Messages .......................................................48
    9.1. EAP-Request/SIM/Start .....................................48
    9.2. EAP-Response/SIM/Start ....................................49
    9.3. EAP-Request/SIM/Challenge .................................49
    9.4. EAP-Response/SIM/Challenge ................................50
    9.5. EAP-Request/SIM/Re-authentication .........................51

Haverinen & Salowey Informational [Page 2] RFC 4186 EAP-SIM Authentication January 2006

    9.6. EAP-Response/SIM/Re-authentication ........................51
    9.7. EAP-Response/SIM/Client-Error .............................52
    9.8. EAP-Request/SIM/Notification ..............................52
    9.9. EAP-Response/SIM/Notification .............................53
 10. Attributes ....................................................53
    10.1. Table of Attributes ......................................53
    10.2. AT_VERSION_LIST ..........................................54
    10.3. AT_SELECTED_VERSION ......................................55
    10.4. AT_NONCE_MT ..............................................55
    10.5. AT_PERMANENT_ID_REQ ......................................56
    10.6. AT_ANY_ID_REQ ............................................56
    10.7. AT_FULLAUTH_ID_REQ .......................................57
    10.8. AT_IDENTITY ..............................................57
    10.9. AT_RAND ..................................................58
    10.10. AT_NEXT_PSEUDONYM .......................................59
    10.11. AT_NEXT_REAUTH_ID .......................................59
    10.12. AT_IV, AT_ENCR_DATA, and AT_PADDING .....................60
    10.13. AT_RESULT_IND ...........................................62
    10.14. AT_MAC ..................................................62
    10.15. AT_COUNTER ..............................................63
    10.16. AT_COUNTER_TOO_SMALL ....................................63
    10.17. AT_NONCE_S ..............................................64
    10.18. AT_NOTIFICATION .........................................64
    10.19. AT_CLIENT_ERROR_CODE ....................................65
 11. IANA Considerations ...........................................66
 12. Security Considerations .......................................66
    12.1. A3 and A8 Algorithms .....................................66
    12.2. Identity Protection ......................................66
    12.3. Mutual Authentication and Triplet Exposure ...............67
    12.4. Flooding the Authentication Centre .......................69
    12.5. Key Derivation ...........................................69
    12.6. Cryptographic Separation of Keys and Session
          Independence .............................................70
    12.7. Dictionary Attacks .......................................71
    12.8. Credentials Re-use .......................................71
    12.9. Integrity and Replay Protection, and Confidentiality .....72
    12.10. Negotiation Attacks .....................................73
    12.11. Protected Result Indications ............................73
    12.12. Man-in-the-Middle Attacks ...............................74
    12.13. Generating Random Numbers ...............................74
 13. Security Claims ...............................................74
 14. Acknowledgements and Contributions ............................75
    14.1. Contributors .............................................75
    14.2. Acknowledgements .........................................75
         14.2.1. Contributors' Addresses ...........................77
 15. References ....................................................78
    15.1. Normative References .....................................78
    15.2. Informative References ...................................79

Haverinen & Salowey Informational [Page 3] RFC 4186 EAP-SIM Authentication January 2006

 Appendix A.  Test Vectors .........................................81
    A.1.  EAP-Request/Identity .....................................81
    A.2.  EAP-Response/Identity ....................................81
    A.3.  EAP-Request/SIM/Start ....................................82
    A.4.  EAP-Response/SIM/Start ...................................82
    A.5.  EAP-Request/SIM/Challenge ................................83
    A.6.  EAP-Response/SIM/Challenge ...............................86
    A.7.  EAP-Success ..............................................86
    A.8.  Fast Re-authentication ...................................86
    A.9.  EAP-Request/SIM/Re-authentication ........................87
    A.10.  EAP-Response/SIM/Re-authentication ......................89
 Appendix B.  Pseudo-Random Number Generator .......................90

1. Introduction

 This document specifies an Extensible Authentication Protocol (EAP)
 [RFC3748] mechanism for authentication and session key distribution
 using the Global System for Mobile Communications (GSM) Subscriber
 Identity Module (SIM).
 GSM is a second generation mobile network standard.  Second
 generation mobile networks and third generation mobile networks use
 different authentication and key agreement mechanisms.  EAP-AKA
 [EAP-AKA] specifies an EAP method that is based on the Authentication
 and Key Agreement (AKA) mechanism used in 3rd generation mobile
 networks.
 GSM authentication is based on a challenge-response mechanism.  The
 A3/A8 authentication and key derivation algorithms that run on the
 SIM can be given a 128-bit random number (RAND) as a challenge.  The
 SIM runs operator-specific algorithms, which take the RAND and a
 secret key Ki (stored on the SIM) as input, and produce a 32-bit
 response (SRES) and a 64-bit long key Kc as output.  The Kc key is
 originally intended to be used as an encryption key over the air
 interface, but in this protocol, it is used for deriving keying
 material and is not directly used.  Hence, the secrecy of Kc is
 critical to the security of this protocol.  For more information
 about GSM authentication, see [GSM-03.20].  See Section 12.1 for more
 discussion about the GSM algorithms used in EAP-SIM.
 The lack of mutual authentication is a weakness in GSM
 authentication.  The derived 64-bit cipher key (Kc) is not strong
 enough for data networks in which stronger and longer keys are
 required.  Hence, in EAP-SIM, several RAND challenges are used for
 generating several 64-bit Kc keys, which are combined to constitute
 stronger keying material.  In EAP-SIM, the client issues a random
 number NONCE_MT to the network in order to contribute to key
 derivation, and to prevent replays of EAP-SIM requests from previous

Haverinen & Salowey Informational [Page 4] RFC 4186 EAP-SIM Authentication January 2006

 exchanges.  The NONCE_MT can be conceived as the client's challenge
 to the network.  EAP-SIM also extends the combined RAND challenges
 and other messages with a message authentication code in order to
 provide message integrity protection along with mutual
 authentication.
 EAP-SIM specifies optional support for protecting the privacy of
 subscriber identity using the same concept as the GSM, which uses
 pseudonyms/temporary identifiers.  It also specifies an optional fast
 re-authentication procedure.
 The security of EAP-SIM builds on underlying GSM mechanisms.  The
 security properties of EAP-SIM are documented in Section 11 of this
 document.  Implementers and users of EAP-SIM are advised to carefully
 study the security considerations in Section 11 in order to determine
 whether the security properties are sufficient for the environment in
 question, especially as the secrecy of Kc keys is essential to the
 security of EAP-SIM.  In brief, EAP-SIM is in no sense weaker than
 the GSM mechanisms.  In some cases EAP-SIM provides better security
 properties than the underlying GSM mechanisms, particularly if the
 SIM credentials are only used for EAP-SIM and are not re-used from
 GSM/GPRS.  Many of the security features of EAP-SIM rely upon the
 secrecy of the Kc values in the SIM triplets, so protecting these
 values is key to the security of the EAP-SIM protocol.
 The 3rd Generation Partnership Project (3GPP) has specified an
 enhanced Authentication and Key Agreement (AKA) architecture for the
 Universal Mobile Telecommunications System (UMTS).  The 3rd
 generation AKA mechanism includes mutual authentication, replay
 protection, and derivation of longer session keys.  EAP-AKA [EAP-AKA]
 specifies an EAP method that is based on the 3rd generation AKA.
 EAP-AKA, which is a more secure protocol, may be used instead of
 EAP-SIM, if 3rd generation identity modules and 3G network
 infrastructures are available.

2. Terms

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
 The terms and abbreviations "authenticator", "backend authentication
 server", "EAP server", "peer", "Silently Discard", "Master Session
 Key (MSK)", and "Extended Master Session Key (EMSK)" in this document
 are to be interpreted as described in [RFC3748].

Haverinen & Salowey Informational [Page 5] RFC 4186 EAP-SIM Authentication January 2006

 This document frequently uses the following terms and abbreviations:
 AAA protocol
       Authentication, Authorization, and Accounting protocol
 AuC
       Authentication Centre.  The GSM network element that provides
       the authentication triplets for authenticating
       the subscriber.
 Authentication vector
       GSM triplets can be alternatively called authentication
       vectors.
 EAP
       Extensible Authentication Protocol
 Fast re-authentication
       An EAP-SIM authentication exchange that is based on keys
       derived upon a preceding full authentication exchange.
       The GSM authentication and key exchange algorithms are not
       used in the fast re-authentication procedure.
 Fast Re-authentication Identity
       A fast re-authentication identity of the peer, including an NAI
       realm portion in environments where a realm is used.  Used on
       fast re-authentication only.
 Fast Re-authentication Username
       The username portion of fast re-authentication identity,
       i.e., not including any realm portions.
 Full authentication
       An EAP-SIM authentication exchange based on the GSM
       authentication and key agreement algorithms.
 GSM
       Global System for Mobile communications.

Haverinen & Salowey Informational [Page 6] RFC 4186 EAP-SIM Authentication January 2006

 GSM Triplet
       The tuple formed by the three GSM authentication values RAND,
       Kc, and SRES.
 IMSI
       International Mobile Subscriber Identifier, used in GSM to
       identify subscribers.
 MAC
       Message Authentication Code
 NAI
       Network Access Identifier
 Nonce
       A value that is used at most once or that is never repeated
       within the same cryptographic context.  In general, a nonce can
       be predictable (e.g., a counter) or unpredictable (e.g., a
       random value).  Since some cryptographic properties may depend
       on the randomness of the nonce, attention should be paid to
       whether a nonce is required to be random or not.  In this
       document, the term nonce is only used to denote random nonces,
       and it is not used to denote counters.
 Permanent Identity
       The permanent identity of the peer, including an NAI realm
       portion in environments where a realm is used.  The permanent
       identity is usually based on the IMSI.  Used on full
       authentication only.
 Permanent Username
       The username portion of permanent identity, i.e., not including
       any realm portions.
 Pseudonym Identity
       A pseudonym identity of the peer, including an NAI realm
       portion in environments where a realm is used.  Used on
       full authentication only.

Haverinen & Salowey Informational [Page 7] RFC 4186 EAP-SIM Authentication January 2006

 Pseudonym Username
       The username portion of pseudonym identity, i.e., not including
       any realm portions.
 SIM
       Subscriber Identity Module.  The SIM is traditionally a smart
       card distributed by a GSM operator.

3. Overview

 Figure 1 shows an overview of the EAP-SIM full authentication
 procedure, wherein optional protected success indications are not
 used.  The authenticator typically communicates with an EAP server
 that is located on a backend authentication server using an AAA
 protocol.  The authenticator shown in the figure is often simply
 relaying EAP messages to and from the EAP server, but these backend
 AAA communications are not shown.
   Peer                                               Authenticator
     |                               EAP-Request/Identity       |
     |<---------------------------------------------------------|
     |                                                          |
     | EAP-Response/Identity                                    |
     |--------------------------------------------------------->|
     |                                                          |
     |                  EAP-Request/SIM/Start (AT_VERSION_LIST) |
     |<---------------------------------------------------------|
     |                                                          |
     | EAP-Response/SIM/Start (AT_NONCE_MT, AT_SELECTED_VERSION)|
     |--------------------------------------------------------->|
     |                                                          |
     |           EAP-Request/SIM/Challenge (AT_RAND, AT_MAC)    |
     |<---------------------------------------------------------|
 +-------------------------------------+                        |
 | Peer runs GSM algorithms, verifies  |                        |
 | AT_MAC and derives session keys     |                        |
 +-------------------------------------+                        |
     | EAP-Response/SIM/Challenge (AT_MAC)                      |
     |--------------------------------------------------------->|
     |                                                          |
     |                                             EAP-Success  |
     |<---------------------------------------------------------|
     |                                                          |
          Figure 1: EAP-SIM full authentication procedure

Haverinen & Salowey Informational [Page 8] RFC 4186 EAP-SIM Authentication January 2006

 The first EAP Request issued by the authenticator is
 EAP-Request/Identity.  On full authentication, the peer's response
 includes either the user's International Mobile Subscriber Identity
 (IMSI) or a temporary identity (pseudonym) if identity privacy is in
 effect, as specified in Section 4.2.
 Following the peer's EAP-Response/Identity packet, the peer receives
 EAP Requests of Type 18 (SIM) from the EAP server and sends the
 corresponding EAP Responses.  The EAP packets that are of the Type
 SIM also have a Subtype field.  On full authentication, the first
 EAP-Request/SIM packet is of the Subtype 10 (Start).  EAP-SIM packets
 encapsulate parameters in attributes, encoded in a Type, Length,
 Value format.  The packet format and the use of attributes are
 specified in Section 8.
 The EAP-Request/SIM/Start packet contains the list of EAP-SIM
 versions supported by the EAP server in the AT_VERSION_LIST
 attribute.  This packet may also include attributes for requesting
 the subscriber identity, as specified in Section 4.2.
 The peer responds to a EAP-Request/SIM/Start with the
 EAP-Response/SIM/Start packet, which includes the AT_NONCE_MT
 attribute that contains a random number NONCE_MT, chosen by the peer,
 and the AT_SELECTED_VERSION attribute that contains the version
 number selected by the peer.  The version negotiation is protected by
 including the version list and the selected version in the
 calculation of keying material (Section 7).
 After receiving the EAP Response/SIM/Start, the EAP server obtains n
 GSM triplets for use in authenticating the subscriber, where n = 2 or
 n = 3.  From the triplets, the EAP server derives the keying
 material, as specified in Section 7.  The triplets may be obtained by
 contacting an Authentication Centre (AuC) on the GSM network; per GSM
 specifications, between 1 and 5 triplets may be obtained at a time.
 Triplets may be stored in the EAP server for use at a later time, but
 triplets MUST NOT be re-used, except in some error cases that are
 specified in Section 10.9.
 The next EAP Request the EAP Server issues is of the type SIM and
 subtype Challenge (11).  It contains the RAND challenges and a
 message authentication code attribute AT_MAC to cover the challenges.
 The AT_MAC attribute is a general message authentication code
 attribute that is used in many EAP-SIM messages.
 On receipt of the EAP-Request/SIM/Challenge message, the peer runs
 the GSM authentication algorithm and calculates a copy of the message
 authentication code.  The peer then verifies that the calculated MAC
 equals the received MAC.  If the MAC's do not match, then the peer

Haverinen & Salowey Informational [Page 9] RFC 4186 EAP-SIM Authentication January 2006

 sends the EAP-Response/SIM/Client-Error packet and the authentication
 exchange terminates.
 Since the RANDs given to a peer are accompanied by the message
 authentication code AT_MAC, and since the peer's NONCE_MT value
 contributes to AT_MAC, the peer is able to verify that the EAP-SIM
 message is fresh (i.e., not a replay) and that the sender possesses
 valid GSM triplets for the subscriber.
 If all checks out, the peer responds with the
 EAP-Response/SIM/Challenge, containing the AT_MAC attribute that
 covers the peer's SRES response values (Section 9.4).  The EAP server
 verifies that the MAC is correct.  Because protected success
 indications are not used in this example, the EAP server sends the
 EAP-Success packet, indicating that the authentication was
 successful.  (Protected success indications are discussed in
 Section 6.2.)  The EAP server may also include derived keying
 material in the message it sends to the authenticator.  The peer has
 derived the same keying material, so the authenticator does not
 forward the keying material to the peer along with EAP-Success.
 EAP-SIM also includes a separate fast re-authentication procedure
 that does not make use of the A3/A8 algorithms or the GSM
 infrastructure.  Fast re-authentication is based on keys derived on
 full authentication.  If the peer has maintained state information
 for fast re-authentication and wants to use fast re-authentication,
 then the peer indicates this by using a specific fast
 re-authentication identity instead of the permanent identity or a
 pseudonym identity.  The fast re-authentication procedure is
 described in Section 5.

4. Operation

4.1. Version Negotiation

 EAP-SIM includes version negotiation so as to allow future
 developments in the protocol.  The version negotiation is performed
 on full authentication and it uses two attributes, AT_VERSION_LIST,
 which the server always includes in EAP-Request/SIM/Start, and
 AT_SELECTED_VERSION, which the peer includes in
 EAP-Response/SIM/Start on full authentication.
 AT_VERSION_LIST includes the EAP-SIM versions supported by the
 server.  If AT_VERSION_LIST does not include a version that is
 implemented by the peer and allowed in the peer's security policy,
 then the peer MUST send the EAP-Response/SIM/Client-Error packet
 (Section 9.7) to the server with the error code "unsupported
 version".  If a suitable version is included, then the peer includes

Haverinen & Salowey Informational [Page 10] RFC 4186 EAP-SIM Authentication January 2006

 the AT_SELECTED_VERSION attribute, containing the selected version in
 the EAP-Response/SIM/Start packet.  The peer MUST only indicate a
 version that is included in the AT_VERSION_LIST.  If several versions
 are acceptable, then the peer SHOULD choose the version that occurs
 first in the version list.
 The version number list of AT_VERSION_LIST and the selected version
 of AT_SELECTED_VERSION are included in the key derivation procedure
 (Section 7).  If an attacker modifies either one of these attributes,
 then the peer and the server derive different keying material.
 Because K_aut keys are different, the server and peer calculate
 different AT_MAC values.  Hence, the peer detects that AT_MAC,
 included in EAP-Request/SIM/Challenge, is incorrect and sends the
 EAP-Response/SIM/Client-Error packet.  The authentication procedure
 terminates.

4.2. Identity Management

4.2.1. Format, Generation and Usage of Peer Identities

4.2.1.1. General

 In the beginning of EAP authentication, the Authenticator or the EAP
 server usually issues the EAP-Request/Identity packet to the peer.
 The peer responds with the EAP-Response/Identity, which contains the
 user's identity.  The formats of these packets are specified in
 [RFC3748].
 GSM subscribers are identified with the International Mobile
 Subscriber Identity (IMSI) [GSM-03.03].  The IMSI is a string of not
 more than 15 digits.  It is composed of a three digit Mobile Country
 Code (MCC), a two or three digit Mobile Network Code (MNC), and a
 Mobile Subscriber Identification Number (MSIN) of no more than 10
 digits.  MCC and MNC uniquely identify the GSM operator and help
 identify the AuC from which the authentication vectors need to be
 retrieved for this subscriber.
 Internet AAA protocols identify users with the Network Access
 Identifier (NAI) [RFC4282].  When used in a roaming environment, the
 NAI is composed of a username and a realm, separated with "@"
 (username@realm).  The username portion identifies the subscriber
 within the realm.
 This section specifies the peer identity format used in EAP-SIM.  In
 this document, the term "identity" or "peer identity" refers to the
 whole identity string that is used to identify the peer.  The peer

Haverinen & Salowey Informational [Page 11] RFC 4186 EAP-SIM Authentication January 2006

 identity may include a realm portion.  "Username" refers to the
 portion of the peer identity that identifies the user, i.e., the
 username does not include the realm portion.

4.2.1.2. Identity Privacy Support

 EAP-SIM includes optional identity privacy (anonymity) support that
 can be used to hide the cleartext permanent identity and thereby make
 the subscriber's EAP exchanges untraceable to eavesdroppers.  Because
 the permanent identity never changes, revealing it would help
 observers to track the user.  The permanent identity is usually based
 on the IMSI, which may further help the tracking, because the same
 identifier may be used in other contexts as well.  Identity privacy
 is based on temporary identities, or pseudonyms, which are equivalent
 to but separate from the Temporary Mobile Subscriber Identities
 (TMSI) that are used on cellular networks.  Please see Section 12.2
 for security considerations regarding identity privacy.

4.2.1.3. Username Types in EAP-SIM identities

 There are three types of usernames in EAP-SIM peer identities:
 (1) Permanent usernames.  For example,
 1123456789098765@myoperator.com might be a valid permanent identity.
 In this example, 1123456789098765 is the permanent username.
 (2) Pseudonym usernames.  For example, 3s7ah6n9q@myoperator.com might
 be a valid pseudonym identity.  In this example, 3s7ah6n9q is the
 pseudonym username.
 (3) Fast re-authentication usernames.  For example,
 53953754@myoperator.com might be a valid fast re-authentication
 identity.  In this case, 53953754 is the fast re-authentication
 username.  Unlike permanent usernames and pseudonym usernames, fast
 re-authentication usernames are one-time identifiers, which are not
 re-used across EAP exchanges.
 The first two types of identities are used only on full
 authentication and the last one only on fast re-authentication.  When
 the optional identity privacy support is not used, the non-pseudonym
 permanent identity is used on full authentication.  The fast
 re-authentication exchange is specified in Section 5.

4.2.1.4. Username Decoration

 In some environments, the peer may need to decorate the identity by
 prepending or appending the username with a string, in order to
 indicate supplementary AAA routing information in addition to the NAI

Haverinen & Salowey Informational [Page 12] RFC 4186 EAP-SIM Authentication January 2006

 realm.  (The usage of an NAI realm portion is not considered
 decoration.)  Username decoration is out of the scope of this
 document.  However, it should be noted that username decoration might
 prevent the server from recognizing a valid username.  Hence,
 although the peer MAY use username decoration in the identities that
 the peer includes in EAP-Response/Identity, and although the EAP
 server MAY accept a decorated peer username in this message, the peer
 or the EAP server MUST NOT decorate any other peer identities that
 are used in various EAP-SIM attributes.  Only the identity used in
 the EAP-Response/Identity may be decorated.

4.2.1.5. NAI Realm Portion

 The peer MAY include a realm portion in the peer identity, as per the
 NAI format.  The use of a realm portion is not mandatory.
 If a realm is used, the realm MAY be chosen by the subscriber's home
 operator and it MAY be a configurable parameter in the EAP-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-SIM users.  This convention makes it easy to
 recognize that the NAI identifies a GSM subscriber.  Such a reserved
 NAI realm may be a useful 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
 is specified in [3GPP-TS-23.003].
 Some old implementations derive the realm name from the IMSI by
 concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits
 of IMSI, and ".owlan.org".  For example, if the IMSI is
 123456789098765, and the MNC is three digits long, then the derived
 realm name is "mnc456.mcc123.owlan.org".  As there are no DNS servers
 running at owlan.org, these realm names can only be used with
 manually configured AAA routing.  New implementations SHOULD use the
 mechanism specified in [3GPP-TS-23.003] instead of owlan.org.
 The IMSI is a string of digits without any explicit structure, so the
 peer may not be able to determine the length of the MNC portion.  If
 the peer is not able to determine whether the MNC is two or three
 digits long, the peer MAY use a 3-digit MNC.  If the correct length
 of the MNC is two, then the MNC used in the realm name includes the
 first digit of the MSIN.  Hence, when configuring AAA networks for

Haverinen & Salowey Informational [Page 13] RFC 4186 EAP-SIM Authentication January 2006

 operators that have 2-digit MNCs, the network SHOULD also be prepared
 for realm names with incorrect, 3-digit MNCs.

4.2.1.6. Format of the Permanent Username

 The non-pseudonym permanent username SHOULD be derived from the IMSI.
 In this case, the permanent username MUST be of the format "1" |
 IMSI, where the character "|" denotes concatenation.  In other words,
 the first character of the username is the digit one (ASCII value 31
 hexadecimal), followed by the IMSI.  The IMSI is encoded as an ASCII
 string that consists of not more than 15 decimal digits (ASCII values
 between 30 and 39 hexadecimal), one character per IMSI digit, in the
 order specified in [GSM-03.03].  For example, a permanent username
 derived from the IMSI 295023820005424 would be encoded as the ASCII
 string "1295023820005424" (byte values in hexadecimal notation: 31 32
 39 35 30 32 33 38 32 30 30 30 35 34 32 34).
 The EAP server MAY use the leading "1" as a hint to try EAP-SIM as
 the first authentication method during method negotiation, rather
 than, for example EAP/AKA.  The EAP-SIM server MAY propose EAP-SIM,
 even if the leading character was not "1".
 Alternatively, an implementation MAY choose a permanent username that
 is not based on the IMSI.  In this case, the selection of the
 username, its format, and its processing is out of the scope of this
 document.  In this case, the peer implementation MUST NOT prepend any
 leading characters to the username.

4.2.1.7. Generating Pseudonyms and Fast Re-authentication Identities by

        the Server
 Pseudonym usernames and fast re-authentication identities are
 generated by the EAP server.  The EAP server produces pseudonym
 usernames and fast re-authentication identities in an
 implementation-dependent manner.  Only the EAP server needs to be
 able to map the pseudonym username to the permanent identity, or to
 recognize a fast re-authentication identity.
 EAP-SIM includes no provisions to ensure that the same EAP server
 that generated a pseudonym username will be used on the
 authentication exchange when the pseudonym username is used.  It is
 recommended that the EAP servers implement some centralized mechanism
 to allow all EAP servers of the home operator to map pseudonyms
 generated by other severs to the permanent identity.  If no such
 mechanism is available, then the EAP server failing to understand a
 pseudonym issued by another server can request the that peer send the
 permanent identity.

Haverinen & Salowey Informational [Page 14] RFC 4186 EAP-SIM Authentication January 2006

 When issuing a fast re-authentication identity, the EAP server may
 include a realm name in the identity to make the fast
 re-authentication request be forwarded to the same EAP server.
 When generating fast re-authentication identities, the server SHOULD
 choose a fresh, new fast re-authentication identity that is different
 from the previous ones that were used after the same full
 authentication exchange.  A full authentication exchange and the
 associated fast re-authentication exchanges are referred to here as
 the same "full authentication context".  The fast re-authentication
 identity SHOULD include a random component.  This random component
 works as a full authentication context identifier.  A
 context-specific fast re-authentication identity can help the server
 to detect whether its fast re-authentication state information
 matches that of its peer (in other words, whether the state
 information is from the same full authentication exchange).  The
 random component also makes the fast re-authentication identities
 unpredictable, so an attacker cannot initiate a fast
 re-authentication exchange to get the server's EAP-Request/SIM/
 Re-authentication packet.
 Transmitting pseudonyms and fast re-authentication identities from
 the server to the peer is discussed in Section 4.2.1.8.  The
 pseudonym is transmitted as a username, without an NAI realm, and the
 fast re-authentication identity is transmitted as a complete NAI,
 including a realm portion if a realm is required.  The realm is
 included in the fast re-authentication identity to allow the server
 to include a server-specific realm.
 Regardless of the construction method, the pseudonym username MUST
 conform to the grammar specified for the username portion of an NAI.
 The fast 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 fast re-authentication identities they generate are unique.
 In any case, it is necessary that permanent usernames, pseudonym
 usernames, and fast re-authentication usernames are separate and
 recognizable from each other.  It is also desirable that EAP-SIM and
 EAP-AKA [EAP-AKA] usernames be distinguishable from each other as an
 aid for the server on which method to offer.
 In general, it is the task of the EAP server and the policies of its
 administrator to ensure sufficient separation of the usernames.
 Pseudonym usernames and fast re-authentication usernames are both
 produced and used by the EAP server.  The EAP server MUST compose
 pseudonym usernames and fast re-authentication usernames so that it
 can determine if an NAI username is an EAP-SIM pseudonym username or

Haverinen & Salowey Informational [Page 15] RFC 4186 EAP-SIM Authentication January 2006

 an EAP-SIM fast re-authentication username.  For instance, when the
 usernames have been derived from the IMSI, the server could use
 different leading characters in the pseudonym usernames and fast
 re-authentication usernames (e.g., the pseudonym could begin with a
 leading "3" character).  When mapping a fast re-authentication
 identity to a permanent identity, the server SHOULD only examine the
 username portion of the fast re-authentication identity and ignore
 the realm portion of the identity.
 Because the peer may fail to save a pseudonym username sent in an
 EAP-Request/SIM/Challenge, for example due to malfunction, the EAP
 server SHOULD maintain at least the most recently used pseudonym
 username in addition to the most recently issued pseudonym username.
 If the authentication exchange is not completed successfully, then
 the server SHOULD NOT overwrite the pseudonym username that was
 issued during the most recent successful authentication exchange.

4.2.1.8. Transmitting Pseudonyms and Fast Re-authentication Identities

        to the Peer
 The server transmits pseudonym usernames and fast re-authentication
 identities to the peer in cipher, using the AT_ENCR_DATA attribute.
 The EAP-Request/SIM/Challenge message MAY include an encrypted
 pseudonym username and/or an encrypted fast re-authentication
 identity in the value field of the AT_ENCR_DATA attribute.  Because
 identity privacy support and fast re-authentication are optional
 implementations, the peer MAY ignore the AT_ENCR_DATA attribute and
 always use the permanent identity.  On fast re-authentication
 (discussed in Section 5), the server MAY include a new, encrypted
 fast re-authentication identity in the
 EAP-Request/SIM/Re-authentication message.
 On receipt of the EAP-Request/SIM/Challenge, the peer MAY decrypt the
 encrypted data in AT_ENCR_DATA.  If the authentication exchange is
 successful, and the encrypted data includes a pseudonym username,
 then the peer may use the obtained pseudonym username on the next
 full authentication.  If a fast re-authentication identity is
 included, then the peer MAY save it together with other fast
 re-authentication state information, as discussed in Section 5, for
 the next fast re-authentication.  If the authentication exchange does
 not complete successfully, the peer MUST ignore the received
 pseudonym username and the fast re-authentication identity.
 If the peer does not receive a new pseudonym username in the
 EAP-Request/SIM/Challenge message, the peer MAY use an old pseudonym
 username instead of the permanent username on the next full
 authentication.  The username portions of fast re-authentication

Haverinen & Salowey Informational [Page 16] RFC 4186 EAP-SIM Authentication January 2006

 identities are one-time usernames, which the peer MUST NOT re-use.
 When the peer uses a fast re-authentication identity in an EAP
 exchange, the peer MUST discard the fast re-authentication identity
 and not re-use it in another EAP authentication exchange, even if the
 authentication exchange was not completed.

4.2.1.9. Usage of the Pseudonym by the Peer

 When the optional identity privacy support is used on full
 authentication, the peer MAY use a pseudonym username received as
 part of a previous full authentication sequence as the username
 portion of the NAI.  The peer MUST NOT modify the pseudonym username
 received in AT_NEXT_PSEUDONYM.  However, as discussed above, the peer
 MAY need to decorate the username in some environments by appending
 or prepending the username with a string that indicates supplementary
 AAA routing information.
 When using a pseudonym username in an environment where a realm
 portion is used, the peer concatenates the received pseudonym
 username with the "@" character and an NAI realm portion.  The
 selection of the NAI realm is discussed above.  The peer can select
 the realm portion similarly, regardless of whether it uses the
 permanent username or a pseudonym username.

4.2.1.10. Usage of the Fast Re-authentication Identity by the Peer

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

4.2.2. Communicating the Peer Identity to the Server

4.2.2.1. General

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

Haverinen & Salowey Informational [Page 17] RFC 4186 EAP-SIM Authentication January 2006

 re-authentication identity maintained by the server, and if the
 server agrees to use fast re-authentication, then a fast
 re-authentication exchange is performed, as described in Section 5.
 The peer identity can also be transmitted from the peer to the server
 using EAP-SIM messages instead of the 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/SIM/Start message, and the peer includes the AT_IDENTITY
 attribute, which contains the peer's identity, in the
 EAP-Response/SIM/Start 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 that the
 peer send its permanent identity.
 The identity format in the AT_IDENTITY attribute is the same as in
 the EAP-Response/Identity packet (except that identity decoration is
 not allowed).  The AT_IDENTITY attribute contains a permanent
 identity, a pseudonym identity, or a fast re-authentication identity.
 Please note that the EAP-SIM peer and the EAP-SIM server only process
 the AT_IDENTITY attribute; entities that only pass through EAP
 packets do not process this attribute.  Hence, the authenticator and
 other intermediate AAA elements (such as possible AAA proxy servers)
 will continue to refer to the peer with the original identity from
 the EAP-Response/Identity packet unless the identity authenticated in
 the AT_IDENTITY attribute is communicated to them in another way
 within the AAA protocol.

4.2.2.2. Relying on EAP-Response/Identity Discouraged

 The EAP-Response/Identity packet is not method-specific, so in many
 implementations it may be handled by an EAP Framework.  This
 introduces an additional layer of processing between the EAP peer and
 EAP server.  The extra layer of processing may cache identity
 responses or add decorations to the identity.  A modification of the
 identity response will cause the EAP peer and EAP server to use
 different identities in the key derivation, which will cause the
 protocol to fail.
 For this reason, it is RECOMMENDED that the EAP peer and server use
 the method-specific identity attributes in EAP-SIM, and the server is
 strongly discouraged from relying upon the EAP-Response/Identity.

Haverinen & Salowey Informational [Page 18] RFC 4186 EAP-SIM Authentication January 2006

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

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

 If EAP-SIM peer is started upon receiving an EAP-Request/Identity
 message, then the peer MAY use an EAP-SIM identity in the EAP-
 Response/Identity packet.  In this case, the peer performs the
 following steps.
 If the peer has maintained fast re-authentication state information
 and wants to use fast re-authentication, then the peer transmits the
 fast re-authentication identity in EAP-Response/Identity.
 Else, if the peer has a pseudonym username available, then the peer
 transmits the pseudonym identity in EAP-Response/Identity.
 In other cases, the peer transmits the permanent identity in
 EAP-Response/Identity.

4.2.4. Server Operation in the Beginning of EAP-SIM Exchange

 As discussed in Section 4.2.2.2, the server SHOULD NOT rely on an
 identity string received in EAP-Response/Identity.  Therefore, the
 RECOMMENDED way to start an EAP-SIM exchange is to ignore any
 received identity strings.  The server SHOULD begin the EAP-SIM
 exchange by issuing the EAP-Request/SIM/Start packet with an
 identity-requesting attribute to indicate that the server wants the
 peer to include an identity in the AT_IDENTITY attribute of the EAP-
 Response/SIM/Start message.  Three methods to request an identity
 from the peer are discussed below.
 If the server chooses not to ignore the contents of EAP-
 Response/Identity, then the server may have already received an EAP-
 SIM identity in this packet.  However, if the EAP server has not
 received any EAP-SIM peer identity (permanent identity, pseudonym
 identity, or fast re-authentication identity) from the peer when
 sending the first EAP-SIM 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/SIM/Start message with the
 AT_PERMANENT_ID_REQ attribute to indicate that the server wants the
 peer to include the permanent identity in the AT_IDENTITY attribute

Haverinen & Salowey Informational [Page 19] RFC 4186 EAP-SIM Authentication January 2006

 of the EAP-Response/SIM/Start message.  This is done in the following
 cases:
 o  The server does not support fast re-authentication or identity
    privacy.
 o  The server decided to process a received identity, and the server
    recognizes the received identity as a pseudonym identity but the
    server is not able to map the pseudonym identity to a permanent
    identity.
 The server issues the EAP-Request/SIM/Start 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/SIM/Start message.  This is done in the following cases:
 o  The server does not support fast re-authentication and the server
    supports identity privacy.
 o  The server decided to process a received identity, and the server
    recognizes the received identity as a re-authentication identity
    but the server is not able to map the re-authentication identity
    to a permanent identity.
 The server issues the EAP-Request/SIM/Start 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 ignores a received EAP-Response/Identity, the
 server does not have any identity, or the server does not recognize
 the format of a received identity.

4.2.5. Processing of EAP-Request/SIM/Start by the Peer

 Upon receipt of an EAP-Request/SIM/Start message, the peer MUST
 perform the following steps.
 If the EAP-Request/SIM/Start does not include an identity request
 attribute, then the peer responds with EAP-Response/SIM/Start without
 AT_IDENTITY.  The peer includes the AT_SELECTED_VERSION and
 AT_NONCE_MT attributes, because the exchange is a full authentication
 exchange.
 If the EAP-Request/SIM/Start includes AT_PERMANENT_ID_REQ, and if the
 peer does not have a pseudonym available, then the peer MUST respond
 with EAP-Response/SIM/Start and include the permanent identity in

Haverinen & Salowey Informational [Page 20] RFC 4186 EAP-SIM Authentication January 2006

 AT_IDENTITY.  If the peer has a pseudonym available, then the peer
 MAY refuse to send the permanent identity; hence, in this case the
 peer MUST either respond with EAP-Response/SIM/Start and include the
 permanent identity in AT_IDENTITY or respond with EAP-Response/SIM/
 Client-Error packet with the code "unable to process packet".
 If the EAP-Request/SIM/Start includes AT_FULL_AUTH_ID_REQ, and if the
 peer has a pseudonym available, then the peer SHOULD respond with
 EAP-Response/SIM/Start and include the pseudonym identity in
 AT_IDENTITY.  If the peer does not have a pseudonym when it receives
 this message, then the peer MUST respond with EAP-Response/SIM/Start
 and include the permanent identity in AT_IDENTITY.  The Peer MUST NOT
 use a re-authentication identity in the AT_IDENTITY attribute.
 If the EAP-Request/SIM/Start includes AT_ANY_ID_REQ, and if the peer
 has maintained fast re-authentication state information and the peer
 wants to use fast re-authentication, then the peer responds with
 EAP-Response/SIM/Start and includes the fast re-authentication
 identity in AT_IDENTITY.  Else, if the peer has a pseudonym identity
 available, then the peer responds with EAP-Response/SIM/Start and
 includes the pseudonym identity in AT_IDENTITY.  Else, the peer
 responds with EAP-Response/SIM/Start and includes the permanent
 identity in AT_IDENTITY.
 An EAP-SIM exchange may include several EAP/SIM/Start rounds.  The
 server may issue a second EAP-Request/SIM/Start if it was not able to
 recognize the identity that the peer used in the previous AT_IDENTITY
 attribute.  At most, three EAP/SIM/Start rounds can be used, so the
 peer MUST NOT respond to more than three EAP-Request/SIM/Start
 messages within an EAP exchange.  The peer MUST verify that the
 sequence of EAP-Request/SIM/Start packets that the peer receives
 comply with the sequencing rules defined in this document.  That is,
 AT_ANY_ID_REQ can only be used in the first EAP-Request/SIM/Start; in
 other words, AT_ANY_ID_REQ MUST NOT be used in the second or third
 EAP-Request/SIM/Start.  AT_FULLAUTH_ID_REQ MUST NOT be used if the
 previous EAP-Request/SIM/Start included AT_PERMANENT_ID_REQ.  The
 peer operation, in cases when it receives an unexpected attribute or
 an unexpected message, is specified in Section 6.3.1.

4.2.6. 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/SIM/
 Start 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

Haverinen & Salowey Informational [Page 21] RFC 4186 EAP-SIM Authentication January 2006

 EAP-Response/SIM/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/SIM/Client-Error to the EAP packet with
 AT_PERMANENT_ID_REQ, because the peer believes that the valid network
 is able to map the pseudonym identity to the peer's permanent
 identity.  (Alternatively, the conservative peer may accept
 AT_PERMANENT_ID_REQ in certain circumstances, for example, if the
 pseudonym was received a long time ago.)  The benefit of this policy
 is that it protects the peer against active attacks on anonymity.  On
 the other hand, a "liberal" peer always accepts the
 AT_PERMANENT_ID_REQ and responds with the permanent identity.  The
 benefit of this policy is that it works even if the valid network
 sometimes loses pseudonyms and is not able to map them to the
 permanent identity.

4.2.7. Processing of AT_IDENTITY by the Server

 When the server receives an EAP-Response/SIM/Start 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-Request/SIM/Notification with AT_NOTIFICATION code "General
 failure" (16384), 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/SIM/
 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/SIM/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/SIM/Start
 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/SIM/Challenge.

Haverinen & Salowey Informational [Page 22] RFC 4186 EAP-SIM Authentication January 2006

 If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid
 fast re-authentication identity and the server agrees on using
 re-authentication, then the server proceeds with fast
 re-authentication by sending EAP-Request/SIM/Re-authentication
 (Section 5).
 If the server used AT_ANY_ID_REQ, and if the peer sent an
 EAP-Response/SIM/Start with only AT_IDENTITY (indicating
 re-authentication), but the server is not able to map the identity to
 a permanent identity, then the server sends EAP-Request/SIM/Start
 with AT_FULLAUTH_ID_REQ.
 If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid
 fast re-authentication identity that the server is able to map to a
 permanent identity, and if the server does not want to use fast
 re-authentication, then the server sends EAP-Request/SIM/Start
 without any identity requesting attributes.
 If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an
 identity that the server recognizes as a pseudonym identity but the
 server is not able to map the pseudonym identity to a permanent
 identity, then the server sends EAP-Request/SIM/Start 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/SIM/Start with AT_FULLAUTH_ID_REQ.

4.3. Message Sequence Examples (Informative)

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

Haverinen & Salowey Informational [Page 23] RFC 4186 EAP-SIM Authentication January 2006

4.3.1. Full Authentication

 This case for full authentication is illustrated below in Figure 2.
 In this case, AT_IDENTITY contains either the permanent identity or a
 pseudonym identity.  The same sequence is also used in case the
 server uses the AT_FULLAUTH_ID_REQ in EAP-Request/SIM/Start.
    Peer                                             Authenticator
      |                                                       |
      |                            +------------------------------+
      |                            | Server does not have a       |
      |                            | Subscriber identity available|
      |                            | When starting EAP-SIM        |
      |                            +------------------------------+
      |                                                       |
      |          EAP-Request/SIM/Start                        |
      |          (AT_ANY_ID_REQ, AT_VERSION_LIST)             |
      |<------------------------------------------------------|
      |                                                       |
      |                                                       |
      | EAP-Response/SIM/Start                                |
      | (AT_IDENTITY, AT_NONCE_MT,                            |
      |  AT_SELECTED_VERSION)                                 |
      |------------------------------------------------------>|
      |                                                       |
       Figure 2: Requesting any identity, full authentication
 If the peer uses its full authentication identity and the AT_IDENTITY
 attribute contains a valid permanent identity or a valid pseudonym
 identity that the EAP server is able to map to the permanent
 identity, then the full authentication sequence proceeds as usual
 with the EAP Server issuing the EAP-Request/SIM/Challenge message.

Haverinen & Salowey Informational [Page 24] RFC 4186 EAP-SIM Authentication January 2006

4.3.2. Fast Re-authentication

 The case when the server uses the AT_ANY_ID_REQ and the peer wants to
 perform fast re-authentication is illustrated below in Figure 3.
    Peer                                             Authenticator
      |                                                       |
      |                            +------------------------------+
      |                            | Server does not have a       |
      |                            | Subscriber identity available|
      |                            | When starting EAP-SIM        |
      |                            +------------------------------+
      |                                                       |
      |        EAP-Request/SIM/Start                          |
      |        (AT_ANY_ID_REQ, AT_VERSION_LIST)               |
      |<------------------------------------------------------|
      |                                                       |
      |                                                       |
      | EAP-Response/SIM/Start                                |
      | (AT_IDENTITY containing a fast re-auth. identity)     |
      |------------------------------------------------------>|
      |                                                       |
     Figure 3: Requesting any identity, fast re-authentication
 On fast re-authentication, if the AT_IDENTITY attribute contains a
 valid fast re-authentication identity and the server agrees on using
 fast re-authentication, then the server proceeds with the fast
 re-authentication sequence and issues the EAP-Request/SIM/
 Re-authentication packet, as specified in Section 5.

Haverinen & Salowey Informational [Page 25] RFC 4186 EAP-SIM Authentication January 2006

4.3.3. Fall Back to Full Authentication

 Figure 4 illustrates cases in which the server does not recognize the
 fast re-authentication identity the peer used in AT_IDENTITY, and
 issues a second EAP-Request/SIM/Start message.
    Peer                                             Authenticator
      |                                                       |
      |                            +------------------------------+
      |                            | Server does not have a       |
      |                            | Subscriber identity available|
      |                            | When starting EAP-SIM        |
      |                            +------------------------------+
      |                                                       |
      |        EAP-Request/SIM/Start                          |
      |        (AT_ANY_ID_REQ, AT_VERSION_LIST)               |
      |<------------------------------------------------------|
      |                                                       |
      |                                                       |
      | EAP-Response/SIM/Start                                |
      | (AT_IDENTITY containing a fast re-auth. identity)     |
      |------------------------------------------------------>|
      |                                                       |
      |                            +------------------------------+
      |                            | Server does not recognize    |
      |                            | The fast re-auth.            |
      |                            | Identity                     |
      |                            +------------------------------+
      |                                                       |
      |     EAP-Request/SIM/Start                             |
      |     (AT_FULLAUTH_ID_REQ, AT_VERSION_LIST)             |
      |<------------------------------------------------------|
      |                                                       |
      |                                                       |
      | EAP-Response/SIM/Start                                |
      | (AT_IDENTITY with a full-auth. identity, AT_NONCE_MT, |
      |  AT_SELECTED_VERSION)                                 |
      |------------------------------------------------------>|
      |                                                       |
            Figure 4: Fall back to full authentication

Haverinen & Salowey Informational [Page 26] RFC 4186 EAP-SIM Authentication January 2006

4.3.4. Requesting the Permanent Identity 1

 Figure 5 illustrates the case in which the EAP server fails to map
 the pseudonym identity included in the EAP-Response/Identity packet
 to a valid permanent identity.
    Peer                                             Authenticator
       |                                                       |
       |                               EAP-Request/Identity    |
       |<------------------------------------------------------|
       |                                                       |
       | EAP-Response/Identity                                 |
       | (Includes a pseudonym)                                |
       |------------------------------------------------------>|
       |                                                       |
       |                            +------------------------------+
       |                            | Server fails to map the      |
       |                            | Pseudonym to a permanent id. |
       |                            +------------------------------+
       |  EAP-Request/SIM/Start                                |
       |  (AT_PERMANENT_ID_REQ, AT_VERSION_LIST)               |
       |<------------------------------------------------------|
       |                                                       |
       | EAP-Response/SIM/Start                                |
       | (AT_IDENTITY with permanent identity, AT_NONCE_MT,    |
       |  AT_SELECTED_VERSION)                                 |
       |------------------------------------------------------>|
       |                                                       |
            Figure 5: Requesting the permanent identity
 If the server recognizes the permanent identity, then the
 authentication sequence proceeds as usual with the EAP Server issuing
 the EAP-Request/SIM/Challenge message.

Haverinen & Salowey Informational [Page 27] RFC 4186 EAP-SIM Authentication January 2006

4.3.5. Requesting the Permanent Identity 2

 Figure 6 illustrates the case in which the EAP server fails to map
 the pseudonym included in the AT_IDENTITY attribute to a valid
 permanent identity.
    Peer                                             Authenticator
       |                                                       |
       |                            +------------------------------+
       |                            | Server does not have a       |
       |                            | Subscriber identity available|
       |                            | When starting EAP-SIM        |
       |                            +------------------------------+
       |        EAP-Request/SIM/Start                          |
       |        (AT_ANY_ID_REQ, AT_VERSION_LIST)               |
       |<------------------------------------------------------|
       |                                                       |
       |EAP-Response/SIM/Start                                 |
       |(AT_IDENTITY with a pseudonym identity, AT_NONCE_MT,   |
       | AT_SELECTED_VERSION)                                  |
       |------------------------------------------------------>|
       |                           +-------------------------------+
       |                           | Server fails to map the       |
       |                           | Pseudonym in AT_IDENTITY      |
       |                           | to a valid permanent identity |
       |                           +-------------------------------+
       |                                                       |
       |                EAP-Request/SIM/Start                  |
       |                (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) |
       |<------------------------------------------------------|
       |                                                       |
       | EAP-Response/SIM/Start                                |
       | (AT_IDENTITY with permanent identity,                 |
       |  AT_NONCE_MT, AT_SELECTED_VERSION)                    |
       |------------------------------------------------------>|
       |                                                       |
 Figure 6: Requesting a permanent identity (two EAP-SIM Start rounds)

4.3.6. Three EAP-SIM/Start Roundtrips

 In the worst case, there are three EAP/SIM/Start round trips before
 the server obtains an acceptable identity.  This case is illustrated
 in Figure 7.

Haverinen & Salowey Informational [Page 28] RFC 4186 EAP-SIM Authentication January 2006

    Peer                                             Authenticator
     |                                                       |
     |                            +------------------------------+
     |                            | Server does not have a       |
     |                            | Subscriber identity available|
     |                            | When starting EAP-SIM        |
     |                            +------------------------------+
     |        EAP-Request/SIM/Start                          |
     |        (Includes AT_ANY_ID_REQ, AT_VERSION_LIST)      |
     |<------------------------------------------------------|
     |                                                       |
     | EAP-Response/SIM/Start                                |
     | (AT_IDENTITY with fast re-auth. identity)             |
     |------------------------------------------------------>|
     |                                                       |
     |                            +------------------------------+
     |                            | Server does not accept       |
     |                            | The fast re-auth.            |
     |                            | Identity                     |
     |                            +------------------------------+
     |     EAP-Request/SIM/Start                             |
     |     (AT_FULLAUTH_ID_REQ, AT_VERSION_LIST)             |
     |<------------------------------------------------------|
     |                                                       |
     :                                                       :
     :                                                       :
     :                                                       :
     :                                                       :
     |EAP-Response/SIM/Start                                 |
     |(AT_IDENTITY with a pseudonym identity, AT_NONCE_MT,   |
     | AT_SELECTED_VERSION)                                  |
     |------------------------------------------------------>|
     |                                                       |
     |                           +-------------------------------+
     |                           | Server fails to map the       |
     |                           | Pseudonym in AT_IDENTITY      |
     |                           | to a valid permanent identity |
     |                           +-------------------------------+
     |           EAP-Request/SIM/Start                       |
     |           (AT_PERMANENT_ID_REQ, AT_VERSION_LIST)      |
     |<------------------------------------------------------|
     |                                                       |
     | EAP-Response/SIM/Start                                |
     | (AT_IDENTITY with permanent identity, AT_NONCE_MT,    |
     |  AT_SELECTED_VERSION)                                 |
     |------------------------------------------------------>|
     |                                                       |
              Figure 7: Three EAP-SIM Start rounds

Haverinen & Salowey Informational [Page 29] RFC 4186 EAP-SIM Authentication January 2006

 After the last EAP-Response/SIM/Start message, the full
 authentication sequence proceeds as usual.  If the EAP Server
 recognizes the permanent identity and is able to proceed, the server
 issues the EAP-Request/SIM/Challenge message.

5. Fast Re-Authentication

5.1. General

 In some environments, EAP authentication may be performed frequently.
 Because the EAP-SIM full authentication procedure makes use of the
 GSM SIM A3/A8 algorithms, and therefore requires 2 or 3 fresh
 triplets from the Authentication Centre, the full authentication
 procedure is not very well suited for frequent use.  Therefore,
 EAP-SIM includes a more inexpensive fast re-authentication procedure
 that does not make use of the SIM A3/A8 algorithms and does not need
 new triplets from the Authentication Centre.  Re-authentication can
 be performed in fewer roundtrips than the full authentication.
 Fast re-authentication is optional to implement for both the EAP-SIM
 server and peer.  On each EAP authentication, either one of the
 entities may also fall back on full authentication if it does not
 want to use fast re-authentication.
 Fast re-authentication is based on the keys derived on the preceding
 full authentication.  The same K_aut and K_encr keys that were used
 in full authentication are used to protect EAP-SIM packets and
 attributes, and the original Master Key from full authentication is
 used to generate a fresh Master Session Key, as specified in Section
 7.
 The fast re-authentication exchange makes use of an unsigned 16-bit
 counter, included in the AT_COUNTER attribute.  The counter has three
 goals: 1) it can be used to limit the number of successive
 reauthentication exchanges without full authentication 2) it
 contributes to the keying material, and 3) it protects the peer and
 the server from replays.  On full authentication, both the server and
 the peer initialize the counter to one.  The counter value of at
 least one is used on the first fast re-authentication.  On subsequent
 fast re-authentications, the counter MUST be greater than on any of
 the previous re-authentications.  For example, on the second fast
 re-authentication, the counter value is two or greater.  The
 AT_COUNTER attribute is encrypted.
 Both the peer and the EAP server maintain a copy of the counter.  The
 EAP server sends its counter value to the peer in the fast
 re-authentication request.  The peer MUST verify that its counter
 value is less than or equal to the value sent by the EAP server.

Haverinen & Salowey Informational [Page 30] RFC 4186 EAP-SIM Authentication January 2006

 The server includes an encrypted server random nonce (AT_NONCE_S) in
 the fast re-authentication request.  The AT_MAC attribute in the
 peer's response is calculated over NONCE_S to provide a
 challenge/response authentication scheme.  The NONCE_S also
 contributes to the new Master Session Key.
 Both the peer and the server SHOULD have an upper limit for the
 number of subsequent fast re-authentications allowed before a full
 authentication needs to be performed.  Because a 16-bit counter is
 used in fast re-authentication, the theoretical maximum number of
 re-authentications is reached when the counter value reaches FFFF
 hexadecimal.
 In order to use fast re-authentication, the peer and the EAP server
 need to store the following values: Master Key, latest counter value
 and the next fast re-authentication identity.  K_aut, K_encr may
 either be stored or derived again from MK.  The server may also need
 to store the permanent identity of the user.

5.2. Comparison to UMTS AKA

 When analyzing the fast re-authentication exchange, it may be helpful
 to compare it with the UMTS Authentication and Key Agreement (AKA)
 exchange, which it resembles closely.  The counter corresponds to the
 UMTS AKA sequence number, NONCE_S corresponds to RAND, AT_MAC in
 EAP-Request/SIM/Re-authentication corresponds to AUTN, the AT_MAC in
 EAP-Response/SIM/Re-authentication corresponds to RES,
 AT_COUNTER_TOO_SMALL corresponds to AUTS, and encrypting the counter
 corresponds to the usage of the Anonymity Key.  Also, the key
 generation on fast re-authentication, with regard to random or fresh
 material, is similar to UMTS AKA -- the server generates the NONCE_S
 and counter values, and the peer only verifies that the counter value
 is fresh.
 It should also be noted that encrypting the AT_NONCE_S, AT_COUNTER,
 or AT_COUNTER_TOO_SMALL attributes is not important to the security
 of the fast re-authentication exchange.

5.3. Fast Re-authentication Identity

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

Haverinen & Salowey Informational [Page 31] RFC 4186 EAP-SIM Authentication January 2006

 If the EAP server supports fast re-authentication, it MAY include the
 skippable AT_NEXT_REAUTH_ID attribute in the encrypted data of
 EAP-Request/SIM/Challenge message (Section 9.3).  This attribute
 contains a new fast re-authentication identity for the next fast
 re-authentication.  The attribute also works as a capability flag
 that, indicating that the server supports fast re-authentication, and
 that the server wants to continue using fast re-authentication within
 the current context.  The peer MAY ignore this attribute, in which
 case it MUST use full authentication next time.  If the peer wants to
 use re-authentication, it uses this fast re-authentication identity
 on next authentication.  Even if the peer has a fast
 re-authentication identity, the peer MAY discard the fast
 re-authentication identity and use a pseudonym or the permanent
 identity instead, in which case full authentication MUST be
 performed.  If the EAP server does not include the AT_NEXT_REAUTH_ID
 in the encrypted data of EAP-Request/SIM/Challenge or
 EAP-Request/SIM/ Re-authentication, then the peer MUST discard its
 current fast re-authentication state information and perform a full
 authentication next time.
 In environments where a realm portion is needed in the peer identity,
 the fast re-authentication identity received in AT_NEXT_REAUTH_ID
 MUST contain both a username portion and a realm portion, as per the
 NAI format.  The EAP Server can choose an appropriate realm part in
 order to have the AAA infrastructure route subsequent fast
 re-authentication related requests to the same AAA server.  For
 example, the realm part MAY include a portion that is specific to the
 AAA server.  Hence, it is sufficient to store the context required
 for fast re-authentication in the AAA server that performed the full
 authentication.
 The peer MAY use the fast re-authentication identity in the
 EAP-Response/Identity packet or, in response to the server's
 AT_ANY_ID_REQ attribute, the peer MAY use the fast re-authentication
 identity in the AT_IDENTITY attribute of the EAP-Response/SIM/Start
 packet.
 The peer MUST NOT modify the username portion of the fast
 re-authentication identity, but the peer MAY modify the realm portion
 or replace it with another realm portion.  The peer might need to
 modify the realm in order to influence the AAA routing, for example,
 to make sure that the correct server is reached.  It should be noted
 that sharing the same fast re-authentication key among several
 servers may have security risks, so changing the realm portion of the
 NAI in order to change the EAP server is not desirable.

Haverinen & Salowey Informational [Page 32] RFC 4186 EAP-SIM Authentication January 2006

 Even if the peer uses a fast re-authentication identity, the server
 may want to fall back on full authentication, for example because the
 server does not recognize the fast re-authentication identity or does
 not want to use fast re-authentication.  In this case, the server
 starts the full authentication procedure by issuing an
 EAP-Request/SIM/Start packet.  This packet always starts a full
 authentication sequence if it does not include the AT_ANY_ID_REQ
 attribute.  If the server was not able to recover the peer's identity
 from the fast re-authentication identity, the server includes either
 the AT_FULLAUTH_ID_REQ or the AT_PERMANENT_ID_REQ attribute in this
 EAP request.

5.4. Fast Re-authentication Procedure

 Figure 8 illustrates the fast re-authentication procedure.  In this
 example, the optional protected success indication is not used.
 Encrypted attributes are denoted with '*'.  The peer uses its
 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/SIM/Start message.  This
 latter case is not illustrated in the figure below, and it is only
 possible when the server requests that the peer send its identity by
 including the AT_ANY_ID_REQ attribute in the EAP-Request/SIM/Start
 packet.
 If the server recognizes the identity as a valid fast
 re-authentication identity, and if the server agrees to use fast
 re-authentication, then the server sends the EAP-Request/SIM/
 Re-authentication packet to the peer.  This packet MUST include the
 encrypted AT_COUNTER attribute, with a fresh counter value, the
 encrypted AT_NONCE_S attribute that contains a random number chosen
 by the server, the AT_ENCR_DATA and the AT_IV attributes used for
 encryption, and the AT_MAC attribute that contains a message
 authentication code over the packet.  The packet MAY also include an
 encrypted AT_NEXT_REAUTH_ID attribute that contains the next fast
 re-authentication identity.
 Fast re-authentication identities are one-time identities.  If the
 peer does not receive a new fast re-authentication identity, it MUST
 use either the permanent identity or a pseudonym identity on the next
 authentication to initiate full authentication.
 The peer verifies that AT_MAC is correct, and that the counter value
 is fresh (greater than any previously used value).  The peer MAY save
 the next fast re-authentication identity from the encrypted
 AT_NEXT_REAUTH_ID for next time.  If all checks are successful, the
 peer responds with the EAP-Response/SIM/Re-authentication packet,

Haverinen & Salowey Informational [Page 33] RFC 4186 EAP-SIM Authentication January 2006

 including the AT_COUNTER attribute with the same counter value and
 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/SIM/
 Re-authentication packet.  If these checks are successful, the
 re-authentication has succeeded and the server sends the EAP-Success
 packet to the peer.
 If protected success indications (Section 6.2) were used, the
 EAP-Success packet would be preceded by an EAP-SIM notification
 round.

Haverinen & Salowey Informational [Page 34] RFC 4186 EAP-SIM Authentication January 2006

     Peer                                             Authenticator
        |                                                       |
        |                               EAP-Request/Identity    |
        |<------------------------------------------------------|
        |                                                       |
        | EAP-Response/Identity                                 |
        | (Includes a fast re-authentication identity)          |
        |------------------------------------------------------>|
        |                                                       |
        |                          +--------------------------------+
        |                          | Server recognizes the identity |
        |                          | and agrees to use fast         |
        |                          | re-authentication              |
        |                          +--------------------------------+
        |                                                       |
        :                                                       :
        :                                                       :
        :                                                       :
        :                                                       :
        |  EAP-Request/SIM/Re-authentication                    |
        |  (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 fast re-  |            |
   | authentication identity for next re-auth.     |            |
   +-----------------------------------------------+            |
        |                                                       |
        | EAP-Response/SIM/Re-authentication                    |
        | (AT_IV, AT_ENCR_DATA, *AT_COUNTER with same value,    |
        |  AT_MAC)                                              |
        |------------------------------------------------------>|
        |                          +--------------------------------+
        |                          | Server verifies AT_MAC and     |
        |                          | the counter                    |
        |                          +--------------------------------+
        |                                                       |
        |                                          EAP-Success  |
        |<------------------------------------------------------|
        |                                                       |
                  Figure 8: Fast Re-authentication

Haverinen & Salowey Informational [Page 35] RFC 4186 EAP-SIM Authentication January 2006

5.5. Fast Re-authentication Procedure when Counter Is Too Small

 If the peer does not accept the counter value of EAP-Request/SIM/
 Re-authentication, it indicates the counter synchronization problem
 by including the encrypted AT_COUNTER_TOO_SMALL in EAP-Response/SIM/
 Re-authentication.  The server responds with EAP-Request/SIM/Start to
 initiate a normal full authentication procedure.  This is illustrated
 in Figure 9.  Encrypted attributes are denoted with '*'.
     Peer                                             Authenticator
        |          EAP-Request/SIM/Start                        |
        |          (AT_ANY_ID_REQ, AT_VERSION_LIST)             |
        |<------------------------------------------------------|
        |                                                       |
        | EAP-Response/SIM/Start                                |
        | (AT_IDENTITY)                                         |
        | (Includes a fast re-authentication identity)          |
        |------------------------------------------------------>|
        |                                                       |
        |  EAP-Request/SIM/Re-authentication                    |
        |  (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/SIM/Re-authentication                    |
        | (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/SIM/Start          |
        |                        (AT_VERSION_LIST)              |
        |<------------------------------------------------------|
   +---------------------------------------------------------------+
   |                Normal full authentication follows.            |
   +---------------------------------------------------------------+
        |                                                       |
        Figure 9: Fast Re-authentication, counter is not fresh

Haverinen & Salowey Informational [Page 36] RFC 4186 EAP-SIM Authentication January 2006

 In the figure above, the first three messages are similar to the
 basic fast re-authentication case.  When the peer detects that the
 counter value is not fresh, it includes the AT_COUNTER_TOO_SMALL
 attribute in EAP-Response/SIM/Re-authentication.  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 counter value as in the
 EAP-Request/SIM/Re-authentication packet.  If not, the server
 terminates the authentication exchange by sending the
 EAP-Request/SIM/Notification with AT_NOTIFICATION code "General
 failure" (16384).  If all checks on the packet are successful, the
 server transmits a new EAP-Request/SIM/Start packet and the full
 authentication procedure is performed as usual.  Since the server
 already knows the subscriber identity, it MUST NOT include
 AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ, or AT_PERMANENT_ID_REQ in the
 EAP-Request/SIM/Start.
 It should be noted that in this case, peer identity is only
 transmitted in the AT_IDENTITY attribute at the beginning of the
 whole EAP exchange.  The fast re-authentication identity used in this
 AT_IDENTITY attribute will be used in key derivation (see Section 7).

6. EAP-SIM Notifications

6.1. General

 EAP-SIM does not prohibit the use of the EAP Notifications as
 specified in [RFC3748].  EAP Notifications can be used at any time in
 the EAP-SIM exchange.  It should be noted that EAP-SIM does not
 protect EAP Notifications.  EAP-SIM also specifies method-specific
 EAP-SIM notifications that are protected in some cases.
 The EAP server can use EAP-SIM notifications to convey notifications
 and result indications (Section 6.2) to the peer.
 The server MUST use notifications in cases discussed in
 Section 6.3.2.  When the EAP server issues an
 EAP-Request/SIM/Notification packet to the peer, the peer MUST
 process the notification packet.  The peer MAY show a notification
 message to the user and the peer MUST respond to the EAP server with
 an EAP-Response/SIM/Notification packet, even if the peer did not
 recognize the notification code.

Haverinen & Salowey Informational [Page 37] RFC 4186 EAP-SIM Authentication January 2006

 An EAP-SIM full authentication exchange or a fast re-authentication
 exchange MUST NOT include more than one EAP-SIM notification round.
 The notification code is a 16-bit number.  The most significant bit
 is called the Success bit (S bit).  The S bit specifies whether the
 notification implies failure.  The code values with the S bit set to
 zero (code values 0...32767) are used on unsuccessful cases.  The
 receipt of a notification code from this range implies a failed EAP
 exchange, so the peer can use the notification as a failure
 indication.  After receiving the EAP-Response/SIM/Notification for
 these notification codes, the server MUST send the EAP-Failure
 packet.
 The receipt of a notification code with the S bit set to one (values
 32768...65536) does not imply failure.  Notification code "Success"
 (32768) has been reserved as a general notification code to indicate
 successful authentication.
 The second most significant bit of the notification code is called
 the Phase bit (P bit).  It specifies at which phase of the EAP-SIM
 exchange the notification can be used.  If the P bit is set to zero,
 the notification can only be used after a successful
 EAP/SIM/Challenge round in full authentication or a successful
 EAP/SIM/Re-authentication round in reauthentication.  A
 re-authentication round is considered successful only if the peer has
 successfully verified AT_MAC and AT_COUNTER attributes, and does not
 include the AT_COUNTER_TOO_SMALL attribute in
 EAP-Response/SIM/Re-authentication.
 If the P bit is set to one, the notification can only by used before
 the EAP/SIM/Challenge round in full authentication, or before the
 EAP/SIM/Re-authentication round in reauthentication.  These
 notifications can only be used to indicate various failure cases.  In
 other words, if the P bit is set to one, then the S bit MUST be set
 to zero.
 Section 9.8 and Section 9.9 specify what other attributes must be
 included in the notification packets.
 Some of the notification codes are authorization related and, hence,
 are not usually considered part of the responsibility of an EAP
 method.  However, they are included as part of EAP-SIM because there
 are currently no other ways to convey this information to the user in
 a localizable way, and the information is potentially useful for the
 user.  An EAP-SIM server implementation may decide never to send
 these EAP-SIM notifications.

Haverinen & Salowey Informational [Page 38] RFC 4186 EAP-SIM Authentication January 2006

6.2. Result Indications

 As discussed in Section 6.3, the server and the peer use explicit
 error messages in all error cases.  If the server detects an error
 after successful authentication, the server uses an EAP-SIM
 notification to indicate failure to the peer.  In this case, the
 result indication is integrity and replay protected.
 By sending an EAP-Response/SIM/Challenge packet or an
 EAP-Response/SIM/Re-authentication packet (without
 AT_COUNTER_TOO_SMALL), the peer indicates that it has successfully
 authenticated the server and that the peer's local policy accepts the
 EAP exchange.  In other words, these packets are implicit success
 indications from the peer to the server.
 EAP-SIM also supports optional protected success indications from the
 server to the peer.  If the EAP server wants to use protected success
 indications, it includes the AT_RESULT_IND attribute in the
 EAP-Request/SIM/Challenge or the EAP-Request/SIM/Re-authentication
 packet.  This attribute indicates that the EAP server would like to
 use result indications in both successful and unsuccessful cases.  If
 the peer also wants this, the peer includes AT_RESULT_IND in
 EAP-Response/SIM/Challenge or EAP-Response/SIM/Re-authentication.
 The peer MUST NOT include AT_RESULT_IND if it did not receive
 AT_RESULT_IND from the server.  If both the peer and the server used
 AT_RESULT_IND, then the EAP exchange is not complete yet, but an
 EAP-SIM notification round will follow.  The following EAP-SIM
 notification may indicate either failure or success.
 Success indications with the AT_NOTIFICATION code "Success" (32768)
 can only be used if both the server and the peer indicate they want
 to use them with AT_RESULT_IND.  If the server did not include
 AT_RESULT_IND in the EAP-Request/SIM/Challenge or
 EAP-Request/SIM/Re-authentication packet, or if the peer did not
 include AT_RESULT_IND in the corresponding response packet, then the
 server MUST NOT use protected success indications.
 Because the server uses the AT_NOTIFICATION code "Success" (32768) to
 indicate that the EAP exchange has completed successfully, the EAP
 exchange cannot fail when the server processes the EAP-SIM response
 to this notification.  Hence, the server MUST ignore the contents of
 the EAP-SIM response it receives from the
 EAP-Request/SIM/Notification with this code.  Regardless of the
 contents of the EAP-SIM response, the server MUST send EAP-Success as
 the next packet.

Haverinen & Salowey Informational [Page 39] RFC 4186 EAP-SIM Authentication January 2006

6.3. Error Cases

 This section specifies the operation of the peer and the server in
 error cases.  The subsections below require the EAP-SIM peer and
 server to send an error packet (EAP-Response/SIM/Client-Error from
 the peer or EAP-Request/SIM/Notification from the server) 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.

6.3.1. Peer Operation

 In general, if an EAP-SIM peer detects an error in a received EAP-SIM
 packet, the EAP-SIM implementation responds with the
 EAP-Response/SIM/Client-Error packet.  In response to the
 EAP-Response/SIM/Client-Error, the EAP server MUST issue the
 EAP-Failure packet and the authentication exchange terminates.
 By default, the peer uses the client error code 0, "unable to process
 packet".  This error code is used in the following cases:
 o  EAP exchange is not acceptable according to the peer's local
    policy.
 o  the peer is not able to parse the EAP request, i.e., the EAP
    request is malformed.
 o  the peer encountered a malformed attribute.
 o  wrong attribute types or duplicate attributes have been included
    in the EAP request.
 o  a mandatory attribute is missing.
 o  unrecognized, non-skippable attribute.
 o  unrecognized or unexpected EAP-SIM Subtype in the EAP request.
 o  A RAND challenge repeated in AT_RAND.
 o  invalid AT_MAC.  The peer SHOULD log this event.
 o  invalid pad bytes in AT_PADDING.

Haverinen & Salowey Informational [Page 40] RFC 4186 EAP-SIM Authentication January 2006

 o  the peer does not want to process AT_PERMANENT_ID_REQ.
 Separate error codes have been defined for the following error cases
 in Section 10.19:
 As specified in Section 4.1, when processing the AT_VERSION_LIST
 attribute, which lists the EAP-SIM versions supported by the server,
 if the attribute does not include a version that is implemented by
 the peer and allowed in the peer's security policy, then the peer
 MUST send the EAP-Response/SIM/Client-Error packet with the error
 code "unsupported version".
 If the number of RAND challenges is smaller than what is required by
 peer's local policy when processing the AT_RAND attribute, the peer
 MUST send the EAP-Response/SIM/Client-Error packet with the error
 code "insufficient number of challenges".
 If the peer believes that the RAND challenges included in AT_RAND are
 not fresh e.g., because it is capable of remembering some previously
 used RANDs, the peer MUST send the EAP-Response/SIM/Client-Error
 packet with the error code "RANDs are not fresh".

6.3.2. Server Operation

 If an EAP-SIM server detects an error in a received EAP-SIM response,
 the server MUST issue the EAP-Request/SIM/Notification packet with an
 AT_NOTIFICATION code that implies failure.  By default, the server
 uses one of the general failure codes ("General failure after
 authentication" (0), or "General failure" (16384)).  The choice
 between these two codes depends on the phase of the EAP-SIM exchange,
 see Section 6.  When the server issues an EAP-
 Request/SIM/Notification that implies failure, the error cases
 include the following:
 o  the server is not able to parse the peer's EAP response
 o  the server encounters a malformed attribute, a non-recognized
    non-skippable attribute, or a duplicate attribute
 o  a mandatory attribute is missing or an invalid attribute was
    included
 o  unrecognized or unexpected EAP-SIM Subtype in the EAP Response
 o  invalid AT_MAC.  The server SHOULD log this event.
 o  invalid AT_COUNTER

Haverinen & Salowey Informational [Page 41] RFC 4186 EAP-SIM Authentication January 2006

6.3.3. EAP-Failure

 The EAP-SIM server sends EAP-Failure in two cases:
 1) In response to an EAP-Response/SIM/Client-Error packet the server
    has received from the peer, or
 2) Following an EAP-SIM notification round, when the AT_NOTIFICATION
    code implies failure.
 The EAP-SIM server MUST NOT send EAP-Failure in cases other than
 these two.  However, it should be noted that even though the EAP-SIM
 server would not send an EAP-Failure, an authorization decision that
 happens outside EAP-SIM, such as in the AAA server or in an
 intermediate AAA proxy, may result in a failed exchange.
 The peer MUST accept the EAP-Failure packet in case 1) and case 2),
 above.  The peer SHOULD silently discard the EAP-Failure packet in
 other cases.

6.3.4. EAP-Success

 On full authentication, the server can only send EAP-Success after
 the EAP/SIM/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/SIM/Challenge packet.
 If the peer did not indicate that it wants to use protected success
 indications with AT_RESULT_IND (as discussed in Section 6.2) on full
 authentication, then the peer MUST accept EAP-Success after a
 successful EAP/SIM/Challenge round.
 If the peer indicated that it wants to use protected success
 indications with AT_RESULT_IND (as discussed in Section 6.2), then
 the peer MUST NOT accept EAP-Success after a successful
 EAP/SIM/Challenge round.  In this case, the peer MUST only accept
 EAP-Success after receiving an EAP-SIM Notification with the
 AT_NOTIFICATION code "Success" (32768).
 On fast re-authentication, EAP-Success can only be sent after the
 EAP/SIM/Re-authentication 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/SIM/Re-authentication packet.
 If the peer did not indicate that it wants to use protected success
 indications with AT_RESULT_IND (as discussed in Section 6.2) on fast

Haverinen & Salowey Informational [Page 42] RFC 4186 EAP-SIM Authentication January 2006

 re-authentication, then the peer MUST accept EAP-Success after a
 successful EAP/SIM/Re-authentication round.
 If the peer indicated that it wants to use protected success
 indications with AT_RESULT_IND (as discussed in Section 6.2), then
 the peer MUST NOT accept EAP-Success after a successful EAP/SIM/Re-
 authentication round.  In this case, the peer MUST only accept
 EAP-Success after receiving an EAP-SIM Notification with the
 AT_NOTIFICATION code "Success" (32768).
 If the peer receives an EAP-SIM notification (Section 6) that
 indicates failure, then the peer MUST no longer accept the
 EAP-Success packet, even if the server authentication was
 successfully completed.

7. Key Generation

 This section specifies how keying material is generated.
 On EAP-SIM full authentication, a Master Key (MK) is derived from the
 underlying GSM authentication values (Kc keys), the NONCE_MT, and
 other relevant context as follows.
 MK = SHA1(Identity|n*Kc| NONCE_MT| Version List| Selected Version)
 In the formula above, the "|" character denotes concatenation.
 "Identity" denotes the peer identity string without any terminating
 null characters.  It is the identity from the last AT_IDENTITY
 attribute sent by the peer in this exchange, or, if AT_IDENTITY was
 not used, it is the identity from the EAP-Response/Identity packet.
 The identity string is included as-is, without any changes.  As
 discussed in Section 4.2.2.2, relying on EAP-Response/Identity for
 conveying the EAP-SIM peer identity is discouraged, and the server
 SHOULD use the EAP-SIM method-specific identity attributes.
 The notation n*Kc in the formula above denotes the n Kc values
 concatenated.  The Kc keys are used in the same order as the RAND
 challenges in AT_RAND attribute.  NONCE_MT denotes the NONCE_MT value
 (not the AT_NONCE_MT attribute, but only the nonce value).  The
 Version List includes the 2-byte-supported version numbers from
 AT_VERSION_LIST, in the same order as in the attribute.  The Selected
 Version is the 2-byte selected version from AT_SELECTED_VERSION.
 Network byte order is used, just as in the attributes.  The hash
 function SHA-1 is specified in [SHA-1].  If several EAP/SIM/Start
 roundtrips are used in an EAP-SIM exchange, then the NONCE_MT,
 Version List and Selected version from the last EAP/SIM/Start round
 are used, and the previous EAP/SIM/Start rounds are ignored.

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 The Master Key is fed into a Pseudo-Random number Function (PRF)
 which generates separate Transient EAP Keys (TEKs) for protecting
 EAP-SIM packets, as well as a Master Session Key (MSK) for link layer
 security, and an Extended Master Session Key (EMSK) for other
 purposes.  On fast re-authentication, the same TEKs MUST be used for
 protecting EAP packets, but a new MSK and a new EMSK MUST be derived
 from the original MK and from new values exchanged in the fast
 re-authentication.
 EAP-SIM requires two TEKs for its own purposes; the authentication
 key K_aut is to be used with the AT_MAC attribute, and the encryption
 key K_encr is to be used with the AT_ENCR_DATA attribute.  The same
 K_aut and K_encr keys are used in full authentication and subsequent
 fast re-authentications.
 Key derivation is based on the random number generation specified in
 NIST Federal Information Processing Standards (FIPS) Publication
 186-2 [PRF].  The pseudo-random number generator is specified in the
 change notice 1 (2001 October 5) of [PRF] (Algorithm 1).  As
 specified in the change notice (page 74), when Algorithm 1 is used as
 a general-purpose pseudo-random number generator, the "mod q" term in
 step 3.3 is omitted.  The function G used in the algorithm is
 constructed via the 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 Appendix B.
 160-bit XKEY and XVAL values are used, so b = 160.  On each full
 authentication, the Master Key is used as the initial secret seed-key
 XKEY.  The optional user input values (XSEED_j) in step 3.1 are set
 to zero.
 On full authentication, the resulting 320-bit random numbers (x_0,
 x_1, ..., x_m-1) are concatenated and partitioned into suitable-sized
 chunks and used as keys in the following order: K_encr (128 bits),
 K_aut (128 bits), Master Session Key (64 bytes), Extended Master
 Session Key (64 bytes).
 On fast re-authentication, the same pseudo-random number generator
 can be used to generate a new Master Session Key and a new Extended
 Master Session Key.  The seed value XKEY' is calculated as follows:
 XKEY' = SHA1(Identity|counter|NONCE_S| MK)
 In the formula above, the Identity denotes the fast re-authentication
 identity, without any terminating null characters, from the
 AT_IDENTITY attribute of the EAP-Response/SIM/Start packet, or, if

Haverinen & Salowey Informational [Page 44] RFC 4186 EAP-SIM Authentication January 2006

 EAP-Response/SIM/Start was not used on fast re-authentication, it
 denotes the identity string from the EAP-Response/Identity packet.
 The counter denotes the counter value from the AT_COUNTER attribute
 used in the EAP-Response/SIM/Re-authentication 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/SIM/Re-authentication packet.  The MK is the Master Key
 derived on the preceding full authentication.
 On fast re-authentication, the pseudo-random number generator is run
 with the new seed value XKEY', and the resulting 320-bit random
 numbers (x_0, x_1, ..., x_m-1) are concatenated and partitioned into
 two 64-byte chunks and used as the new 64-byte Master Session Key and
 the new 64-byte Extended Master Session Key.  Note that because
 K_encr and K_aut are not derived on fast re-authentication, the
 Master Session Key and the Extended Master Session key are obtained
 from the beginning of the key stream (x_0, x_1, ...).
 The first 32 bytes of the MSK can be used as the Pairwise Master Key
 (PMK) for IEEE 802.11i.
 When the RADIUS attributes specified in [RFC2548] are used to
 transport keying material, then the first 32 bytes of the MSK
 correspond to MS-MPPE-RECV-KEY and the second 32 bytes to
 MS-MPPE-SEND-KEY.  In this case, only 64 bytes of keying material
 (the MSK) are used.
 When generating the initial Master Key, the hash function is used as
 a mixing function to combine several session keys (Kc's) generated by
 the GSM authentication procedure and the random number NONCE_MT into
 a single session key.  There are several reasons for this.  The
 current GSM session keys are, at most, 64 bits, so two or more of
 them are needed to generate a longer key.  By using a one-way
 function to combine the keys, we are assured that, even if an
 attacker managed to learn one of the EAP-SIM session keys, it
 wouldn't help him in learning the original GSM Kc's.  In addition,
 since we include the random number NONCE_MT in the calculation, the
 peer is able to verify that the EAP-SIM packets it receives from the
 network are fresh and not replays (also see Section 11).

8. Message Format and Protocol Extensibility

8.1. Message Format

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

Haverinen & Salowey Informational [Page 45] RFC 4186 EAP-SIM Authentication January 2006

 Length and Identifier fields in the EAP header are also specified in
 [RFC3748].  In EAP-SIM, the Type field is set to 18.
 In EAP-SIM, the Type-Data begins with an EAP-SIM header that consists
 of a 1-octet Subtype field and a 2-octet reserved field.  The Subtype
 values used in EAP-SIM are defined in the IANA considerations section
 of the EAP-AKA specification [EAP-AKA].  The formats of the EAP
 header and the EAP-SIM 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 that immediately follows the EAP-SIM 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |      Type     |    Length     |  Value...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute Type
       Indicates the particular type of attribute.  The attribute type
       values are listed in the IANA considerations section of the
       EAP-AKA specification [EAP-AKA].
 Length
       Indicates the length of this attribute in multiples of four
       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.

Haverinen & Salowey Informational [Page 46] RFC 4186 EAP-SIM Authentication January 2006

 Attributes numbered within the range 0 through 127 are called
 non-skippable attributes.  When an EAP-SIM peer encounters a
 non-skippable attribute that the peer does not recognize, the peer
 MUST send the EAP-Response/SIM/Client-Error packet, which terminates
 the authentication exchange.  If an EAP-SIM server encounters a
 non-skippable attribute that the server does not recognize, then the
 server sends the EAP-Request/SIM/Notification packet with an
 AT_NOTIFICATION code, which implies general failure ("General failure
 after authentication" (0), or "General failure" (16384), depending on
 the phase of the exchange), which terminates the authentication
 exchange.
 Attributes within the range of 128 through 255 are called skippable
 attributes.  When a skippable attribute is encountered and is not
 recognized, it is ignored.  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.
 Unless otherwise specified, the order of the attributes in an EAP-SIM
 message is insignificant and an EAP-SIM 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.

8.2. Protocol Extensibility

 EAP-SIM can be extended by specifying new attribute types.  If
 skippable attributes are used, it is possible to extend the protocol
 without breaking old implementations.
 However, any new attributes added to the EAP-Request/SIM/Start or
 EAP-Response/SIM/Start packets would not be integrity-protected.
 Therefore, these messages MUST NOT be extended in the current version
 of EAP-SIM.  If the list of supported EAP-SIM versions in the
 AT_VERSION_LIST does not include versions other than 1, then the
 server MUST NOT include attributes other than those specified in this
 document in the EAP-Request/SIM/Start message.  Note that future
 versions of this protocol might specify new attributes for
 EAP-Request/SIM/Start and still support version 1 of the protocol.
 In this case, the server might send an EAP-Request/SIM/Start message
 that includes new attributes and indicates support for protocol
 version 1 and other versions in the AT_VERSION_LIST attribute.  If
 the peer selects version 1, then the peer MUST ignore any other
 attributes included in EAP-Request/SIM/Start, other than those
 specified in this document.  If the selected EAP-SIM version in
 peer's AT_SELECTED_VERSION is 1, then the peer MUST NOT include other

Haverinen & Salowey Informational [Page 47] RFC 4186 EAP-SIM Authentication January 2006

 attributes aside from those specified in this document in the
 EAP-Response/SIM/Start message.
 When specifying new attributes, it should be noted that EAP-SIM does
 not support message fragmentation.  Hence, the sizes of the new
 extensions MUST be limited so that the maximum transfer unit (MTU) of
 the underlying lower layer is not exceeded.  According to [RFC3748],
 lower layers must provide an EAP MTU of 1020 bytes or greater, so any
 extensions to EAP-SIM SHOULD NOT exceed the EAP MTU of 1020 bytes.
 Because EAP-SIM supports version negotiation, new versions of the
 protocol can also be specified by using a new version number.

9. Messages

 This section specifies the messages used in EAP-SIM.  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.  The general message format is
 specified in Section 8.1.

9.1. EAP-Request/SIM/Start

 In full authentication the first SIM-specific EAP Request is
 EAP-Request/SIM/Start.  The EAP/SIM/Start roundtrip is used for two
 purposes.  In full authentication this packet is used to request the
 peer to send the AT_NONCE_MT attribute to the server.  In addition,
 as specified in Section 4.2, the Start round trip may be used by the
 server for obtaining the peer identity.  As discussed in Section 4.2,
 several Start rounds may be required to obtain a valid peer identity.
 The server MUST always include the AT_VERSION_LIST attribute.
 The server MAY include one of the following identity-requesting
 attributes: AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, or
 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 received a response from the peer, it MUST NOT
 issue a new EAP-Request/SIM/Start packet if it has previously issued
 an EAP-Request/SIM/Start message either without any identity
 requesting attributes or with the AT_PERMANENT_ID_REQ attribute.
 If the server has received a response from the peer, it MUST NOT
 issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ or
 AT_FULLAUTH_ID_REQ attributes if it has previously issued an
 EAP-Request/SIM/Start message with the AT_FULLAUTH_ID_REQ attribute.

Haverinen & Salowey Informational [Page 48] RFC 4186 EAP-SIM Authentication January 2006

 If the server has received a response from the peer, it MUST NOT
 issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ
 attribute if the server has previously issued an
 EAP-Request/SIM/Start message with the AT_ANY_ID_REQ attribute.
 This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.

9.2. EAP-Response/SIM/Start

 The peer sends EAP-Response/SIM/Start in response to a valid
 EAP-Request/SIM/Start from the server.
 If and only if the server's EAP-Request/SIM/Start includes one of the
 identity-requesting attributes, then the peer MUST include the
 AT_IDENTITY attribute.  The usage of AT_IDENTITY is defined in
 Section 4.2.
 The AT_NONCE_MT attribute MUST NOT be included if the AT_IDENTITY
 with a fast re-authentication identity is present for fast
 re-authentication.  AT_NONCE_MT MUST be included in all other cases
 (full authentication).
 The AT_SELECTED_VERSION attribute MUST NOT be included if the
 AT_IDENTITY attribute with a fast re-authentication identity is
 present for fast re-authentication.  In all other cases,
 AT_SELECTED_VERSION MUST be included (full authentication).  This
 attribute is used in version negotiation, as specified in
 Section 4.1.
 This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.

9.3. EAP-Request/SIM/Challenge

 The server sends the EAP-Request/SIM/Challenge after receiving a
 valid EAP-Response/SIM/Start that contains AT_NONCE_MT and
 AT_SELECTED_VERSION, and after successfully obtaining the subscriber
 identity.
 The AT_RAND attribute MUST be included.
 The AT_RESULT_IND attribute MAY be included.  The usage of this
 attribute is discussed in Section 6.2.
 The AT_MAC attribute MUST be included.  For
 EAP-Request/SIM/Challenge, the MAC code is calculated over the
 following data:
 EAP packet| NONCE_MT

Haverinen & Salowey Informational [Page 49] RFC 4186 EAP-SIM Authentication January 2006

 The EAP packet is represented as specified in Section 8.1.  It is
 followed by the 16-byte NONCE_MT value from the peer's AT_NONCE_MT
 attribute.
 The EAP-Request/SIM/Challenge packet MAY include encrypted attributes
 for identity privacy and for communicating the next fast
 re-authentication identity.  In this case, the AT_IV and AT_ENCR_DATA
 attributes are included (Section 10.12).
 The plaintext of the AT_ENCR_DATA value field consists of nested
 attributes.  The nested attributes MAY include AT_PADDING (as
 specified in Section 10.12).  If the server supports identity privacy
 and wants to communicate a pseudonym to the peer for the next full
 authentication, then the nested encrypted attributes include the
 AT_NEXT_PSEUDONYM attribute.  If the server supports
 re-authentication and wants to communicate a fast re-authentication
 identity to the peer, then the nested encrypted attributes include
 the AT_NEXT_REAUTH_ID attribute.
 When processing this message, the peer MUST process AT_RAND before
 processing other attributes.  Only if AT_RAND is verified to be
 valid, the peer derives keys and verifies AT_MAC.  The operation in
 case an error occurs is specified in Section 6.3.1.

9.4. EAP-Response/SIM/Challenge

 The peer sends EAP-Response/SIM/Challenge in response to a valid
 EAP-Request/SIM/Challenge.
 Sending this packet indicates that the peer has successfully
 authenticated the server and that the EAP exchange will be accepted
 by the peer's local policy.  Hence, if these conditions are not met,
 then the peer MUST NOT send EAP-Response/SIM/Challenge, but the peer
 MUST send EAP-Response/SIM/Client-Error.
 The AT_MAC attribute MUST be included.  For EAP-
 Response/SIM/Challenge, the MAC code is calculated over the following
 data:
 EAP packet| n*SRES
 The EAP packet is represented as specified in Section 8.1.  The EAP
 packet bytes are immediately followed by the two or three SRES values
 concatenated, denoted above with the notation n*SRES.  The SRES
 values are used in the same order as the corresponding RAND
 challenges in the server's AT_RAND attribute.

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 The AT_RESULT_IND attribute MAY be included if it was included in
 EAP-Request/SIM/Challenge.  The usage of this attribute is discussed
 in Section 6.2.
 Later versions of this protocol MAY make use of the AT_ENCR_DATA and
 AT_IV attributes in this message to include encrypted (skippable)
 attributes.  The EAP server MUST process EAP-Response/SIM/Challenge
 messages that include these attributes even if the server did not
 implement these optional attributes.

9.5. EAP-Request/SIM/Re-authentication

 The server sends the EAP-Request/SIM/Re-authentication message if it
 wants to use fast re-authentication, and if it has received a valid
 fast re-authentication identity in EAP-Response/Identity or
 EAP-Response/SIM/Start.
 AT_MAC MUST be included.  No message-specific data is included in the
 MAC calculation.  See Section 10.14.
 The AT_RESULT_IND attribute MAY be included.  The usage of this
 attribute is discussed in Section 6.2.
 The AT_IV and AT_ENCR_DATA attributes MUST be included.  The
 plaintext consists of the following nested encrypted attributes,
 which MUST be included: AT_COUNTER and AT_NONCE_S.  In addition, the
 nested encrypted attributes MAY include the following attributes:
 AT_NEXT_REAUTH_ID and AT_PADDING.

9.6. EAP-Response/SIM/Re-authentication

 The client sends the EAP-Response/SIM/Re-authentication packet in
 response to a valid EAP-Request/SIM/Re-authentication.
 The AT_MAC attribute MUST be included.  For
 EAP-Response/SIM/Re-authentication, the MAC code is calculated over
 the following data:
 EAP packet| NONCE_S
 The EAP packet is represented as specified in Section 8.1.  It is
 followed by the 16-byte NONCE_S value from the server's AT_NONCE_S
 attribute.
 The AT_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

Haverinen & Salowey Informational [Page 51] RFC 4186 EAP-SIM Authentication January 2006

 encrypted attributes, and it is included in cases specified in
 Section 5.  The AT_PADDING attribute MAY be included.
 The AT_RESULT_IND attribute MAY be included if it was included in
 EAP-Request/SIM/Re-authentication.  The usage of this attribute is
 discussed in Section 6.2.
 Sending this packet without AT_COUNTER_TOO_SMALL indicates that the
 peer has successfully authenticated the server and that the EAP
 exchange will be accepted by the peer's local policy.  Hence, if
 these conditions are not met, then the peer MUST NOT send
 EAP-Response/SIM/Re-authentication, but the peer MUST send
 EAP-Response/SIM/Client-Error.

9.7. EAP-Response/SIM/Client-Error

 The peer sends EAP-Response/SIM/Client-Error in error cases, as
 specified in Section 6.3.1.
 The AT_CLIENT_ERROR_CODE attribute MUST be included.
 The AT_MAC, AT_IV, or AT_ENCR_DATA attributes MUST NOT be used with
 this packet.

9.8. EAP-Request/SIM/Notification

 The usage of this message is specified in Section 6.  The
 AT_NOTIFICATION attribute MUST be included.
 The AT_MAC attribute MUST be included if the P bit of the
 notification code in AT_NOTIFICATION is set to zero, and MUST NOT be
 included in cases when the P bit is set to one.  The P bit is
 discussed in Section 6.
 No message-specific data is included in the MAC calculation.  See
 Section 10.14.
 If EAP-Request/SIM/Notification is used on a fast re-authentication
 exchange, and if the P bit in AT_NOTIFICATION is set to zero, then
 AT_COUNTER is used for replay protection.  In this case, the
 AT_ENCR_DATA and AT_IV attributes MUST be included, and the
 encapsulated plaintext attributes MUST include the AT_COUNTER
 attribute.  The counter value included in AT_COUNTER MUST be the same
 as in the EAP-Request/SIM/Re-authentication packet on the same fast
 re-authentication exchange.

Haverinen & Salowey Informational [Page 52] RFC 4186 EAP-SIM Authentication January 2006

9.9. EAP-Response/SIM/Notification

 The usage of this message is specified in Section 6.  This packet is
 an acknowledgement of EAP-Request/SIM/Notification.
 The AT_MAC attribute MUST be included in cases when the P bit of the
 notification code in AT_NOTIFICATION of EAP-Request/SIM/Notification
 is set to zero, and MUST NOT be included in cases when the P bit is
 set to one.  The P bit is discussed in Section 6.
 No message-specific data is included in the MAC calculation, see
 Section 10.14.
 If EAP-Request/SIM/Notification is used on a fast re-authentication
 exchange, and if the P bit in AT_NOTIFICATION is set to zero, then
 AT_COUNTER is used for replay protection.  In this case, the
 AT_ENCR_DATA and AT_IV attributes MUST be included, and the
 encapsulated plaintext attributes MUST include the AT_COUNTER
 attribute.  The counter value included in AT_COUNTER MUST be the same
 as in the EAP-Request/SIM/Re-authentication packet on the same fast
 re-authentication exchange.

10. Attributes

 This section specifies the format of message attributes.  The
 attribute type numbers are specified in the IANA considerations
 section of the EAP-AKA specification [EAP-AKA].

10.1. Table of Attributes

 The following table provides a guide to which attributes may be found
 in which kinds of messages, and in what quantity.  Messages are
 denoted with numbers in parentheses as follows: (1)
 EAP-Request/SIM/Start, (2) EAP-Response/SIM/Start, (3)
 EAP-Request/SIM/Challenge, (4) EAP-Response/SIM/Challenge, (5)
 EAP-Request/SIM/Notification, (6) EAP-Response/SIM/Notification, (7)
 EAP-Response/SIM/Client-Error, (8) EAP-Request/SIM/Re-authentication,
 and (9) EAP-Response/SIM/Re-authentication.  The column denoted with
 "Encr" indicates whether the attribute is a nested attribute that
 MUST be included within AT_ENCR_DATA, and the column denoted with
 "Skip" indicates whether the attribute is a skippable attribute.
 "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
 future versions of the protocol.

Haverinen & Salowey Informational [Page 53] RFC 4186 EAP-SIM Authentication January 2006

            Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9)  Encr Skip
      AT_VERSION_LIST  1   0   0   0   0   0   0   0   0   N     N
  AT_SELECTED_VERSION  0  0-1  0   0   0   0   0   0   0   N     N
          AT_NONCE_MT  0  0-1  0   0   0   0   0   0   0   N     N
  AT_PERMANENT_ID_REQ 0-1  0   0   0   0   0   0   0   0   N     N
        AT_ANY_ID_REQ 0-1  0   0   0   0   0   0   0   0   N     N
   AT_FULLAUTH_ID_REQ 0-1  0   0   0   0   0   0   0   0   N     N
          AT_IDENTITY  0  0-1  0   0   0   0   0   0   0   N     N
              AT_RAND  0   0   1   0   0   0   0   0   0   N     N
    AT_NEXT_PSEUDONYM  0   0  0-1  0   0   0   0   0   0   Y     Y
    AT_NEXT_REAUTH_ID  0   0  0-1  0   0   0   0  0-1  0   Y     Y
                AT_IV  0   0  0-1  0* 0-1 0-1  0   1   1   N     Y
         AT_ENCR_DATA  0   0  0-1  0* 0-1 0-1  0   1   1   N     Y
           AT_PADDING  0   0  0-1  0* 0-1 0-1  0  0-1 0-1  Y     N
        AT_RESULT_IND  0   0  0-1 0-1  0   0   0  0-1 0-1  N     Y
               AT_MAC  0   0   1   1  0-1 0-1  0   1   1   N     N
           AT_COUNTER  0   0   0   0  0-1 0-1  0   1   1   Y     N
 AT_COUNTER_TOO_SMALL  0   0   0   0   0   0   0   0  0-1  Y     N
           AT_NONCE_S  0   0   0   0   0   0   0   1   0   Y     N
      AT_NOTIFICATION  0   0   0   0   1   0   0   0   0   N     N
 AT_CLIENT_ERROR_CODE  0   0   0   0   0   0   1   0   0   N     N
 It should be noted that attributes AT_PERMANENT_ID_REQ,
 AT_ANY_ID_REQ, and AT_FULLAUTH_ID_REQ are mutually exclusive; 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.

10.2. AT_VERSION_LIST

 The format of the AT_VERSION_LIST 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_VERSION_L..| Length        | Actual Version List Length    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Supported Version 1          |  Supported Version 2          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  .                                                               .
  .                                                               .
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Supported Version N           |     Padding                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This attribute is used in version negotiation, as specified in
 Section 4.1.  The attribute contains the version numbers supported by
 the EAP-SIM server.  The server MUST only include versions that it

Haverinen & Salowey Informational [Page 54] RFC 4186 EAP-SIM Authentication January 2006

 implements and that are allowed in its security policy.  The server
 SHOULD list the versions in the order of preference, with the most
 preferred versions listed first.  At least one version number MUST be
 included.  The version number for the protocol described in this
 document is one (0001 hexadecimal).
 The value field of this attribute begins with 2-byte Actual Version
 List Length, which specifies the length of the Version List in bytes,
 not including the Actual Version List Length attribute length.  This
 field is followed by the list of the versions supported by the
 server, which each have a length of 2 bytes.  For example, if there
 is only one supported version, then the Actual Version List Length is
 2.  Because the length of the attribute must be a multiple of 4
 bytes, the sender pads the value field with zero bytes when
 necessary.

10.3. AT_SELECTED_VERSION

 The format of the AT_SELECTED_VERSION 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_SELECTED...| Length = 1    |    Selected Version           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This attribute is used in version negotiation, as specified in
 Section 4.1.  The value field of this attribute contains a two-byte
 version number, which indicates the EAP-SIM version that the peer
 wants to use.

10.4. AT_NONCE_MT

 The format of the AT_NONCE_MT attribute is shown below.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |AT_NONCE_MT    | Length = 5    |           Reserved            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  |                           NONCE_MT                            |
  |                                                               |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Haverinen & Salowey Informational [Page 55] RFC 4186 EAP-SIM Authentication January 2006

 The value field of the NONCE_MT attribute contains two reserved bytes
 followed by a random number freshly generated by the peer (16 bytes
 long) for this EAP-SIM authentication exchange.  The random number is
 used as a seed value for the new keying material.  The reserved bytes
 are set to zero upon sending and ignored upon reception.
 The peer MUST NOT re-use the NONCE_MT value from a previous EAP-SIM
 authentication exchange.  If an EAP-SIM exchange includes several
 EAP/SIM/Start rounds, then the peer SHOULD use the same NONCE_MT
 value in all EAP-Response/SIM/Start packets.  The peer SHOULD use a
 good source of randomness to generate NONCE_MT.  Please see [RFC4086]
 for more information about generating random numbers for security
 applications.

10.5. 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 4.2.  The
 value field contains only two reserved bytes, which are set to zero
 on sending and ignored on reception.

10.6. 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 4.2.  The value
 field contains only two reserved bytes, which are set to zero on
 sending and ignored on reception.

Haverinen & Salowey Informational [Page 56] RFC 4186 EAP-SIM Authentication January 2006

10.7. 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_FULLAUTH_...| Length = 1    |           Reserved            |
  +---------------+---------------+-------------------------------+
 The use of the AT_FULLAUTH_ID_REQ is defined in Section 4.2.  The
 value field contains only two reserved bytes, which are set to zero
 on sending and ignored on reception.

10.8. 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 (optional)                     .
  .                                                               .
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The use of the AT_IDENTITY is defined in Section 4.2.  The value
 field of this attribute begins with a 2-byte actual identity length,
 which specifies the length of the identity in bytes.  This field is
 followed by the subscriber identity of the indicated actual length.
 The identity is the permanent identity, a pseudonym identity, or a
 fast re-authentication identity.  The identity format is specified in
 Section 4.2.1.  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.

Haverinen & Salowey Informational [Page 57] RFC 4186 EAP-SIM Authentication January 2006

10.9. 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        |           Reserved            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  .                            n*RAND                             .
  .                                                               .
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The value field of this attribute contains two reserved bytes
 followed by n GSM RANDs, each 16 bytes long.  The value of n can be
 determined by the attribute length.  The reserved bytes are set to
 zero upon sending and ignored upon reception.
 The number of RAND challenges (n) MUST be two or three.  The peer
 MUST verify that the number of RAND challenges is sufficient
 according to the peer's policy.  The server MUST use different RAND
 values.  In other words, a RAND value can only be included once in
 AT_RAND.  When processing the AT_RAND attribute, the peer MUST check
 that the RANDs are different.
 The EAP server MUST obtain fresh RANDs for each EAP-SIM full
 authentication exchange.  More specifically, the server MUST consider
 RANDs it included in AT_RAND to be consumed if the server receives an
 EAP-Response/SIM/Challenge packet with a valid AT_MAC, or an
 EAP-Response/SIM/Client-Error with the code "insufficient number of
 challenges" or "RANDs are not fresh".  However, in other cases (if
 the server does not receive a response to its
 EAP-Request/SIM/Challenge packet, or if the server receives a
 response other than the cases listed above), the server does not need
 to consider the RANDs to be consumed, and the server MAY re-use the
 RANDs in the AT_RAND attribute of the next full authentication
 attempt.

Haverinen & Salowey Informational [Page 58] RFC 4186 EAP-SIM Authentication January 2006

10.10. AT_NEXT_PSEUDONYM

 The format of the AT_NEXT_PSEUDONYM attribute is shown below.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | AT_NEXT_PSEU..| Length        | Actual Pseudonym Length       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  .                          Next Pseudonym                       .
  .                                                               .
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The value field of this attribute begins with the 2-byte actual
 pseudonym length, which specifies the length of the following
 pseudonym in bytes.  This field is followed by a pseudonym username
 that the peer can use in the next authentication.  The username MUST
 NOT include any realm portion.  The username does not include any
 terminating null characters.  Because the length of the attribute
 must be a multiple of 4 bytes, the sender pads the pseudonym with
 zero bytes when necessary.  The username encoding MUST follow the
 UTF-8 transformation format [RFC3629].  This attribute MUST always be
 encrypted by encapsulating it within the AT_ENCR_DATA attribute.

10.11. AT_NEXT_REAUTH_ID

 The format of the AT_NEXT_REAUTH_ID attribute is shown below.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | AT_NEXT_REAU..| Length        | Actual Re-Auth Identity Length|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  .               Next Fast Re-authentication Username            .
  .                                                               .
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The value field of this attribute begins with the 2-byte actual
 re-authentication identity length which specifies the length of the
 following fast re-authentication identity in bytes.  This field is
 followed by a fast re-authentication identity that the peer can use
 in the next fast re-authentication, as described in Section 5.  In
 environments where a realm portion is required, the fast
 re-authentication identity includes both a username portion and a

Haverinen & Salowey Informational [Page 59] RFC 4186 EAP-SIM Authentication January 2006

 realm name portion.  The fast re-authentication identity does not
 include any terminating null characters.  Because the length of the
 attribute must be a multiple of 4 bytes, the sender pads the fast
 re-authentication identity with zero bytes when necessary.  The
 identity encoding MUST follow the UTF-8 transformation format
 [RFC3629].  This attribute MUST always be encrypted by encapsulating
 it within the AT_ENCR_DATA attribute.

10.12. AT_IV, AT_ENCR_DATA, and AT_PADDING

 AT_IV and AT_ENCR_DATA attributes can be used to transmit encrypted
 information between the EAP-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 6.3 specifies the operation if a
 packet that does not meet this condition is encountered.
 The sender of the AT_IV attribute chooses the initialization vector
 at random.  The sender MUST NOT re-use the initialization vector
 value from previous EAP-SIM packets.  The sender SHOULD use a good
 source of randomness to generate the initialization vector.  Please
 see [RFC4086] for more information about generating random numbers
 for security applications.  The format of AT_IV is shown below.
   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] with a 128-bit key 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.

Haverinen & Salowey Informational [Page 60] RFC 4186 EAP-SIM Authentication January 2006

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | AT_ENCR_DATA  | Length        |           Reserved            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  .                    Encrypted Data                             .
  .                                                               .
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The derivation of the encryption key (K_encr) is specified in Section
 7.
 The plaintext consists of nested EAP-SIM attributes.
 The encryption algorithm requires the length of the plaintext to be a
 multiple of 16 bytes.  The sender may need to include the AT_PADDING
 attribute as the last attribute within AT_ENCR_DATA.  The AT_PADDING
 attribute is not included if the total length of other nested
 attributes within the AT_ENCR_DATA attribute is a multiple of 16
 bytes.  As usual, the Length of the Padding attribute includes the
 Attribute Type and Attribute Length fields.  The length of the
 Padding attribute is 4, 8, or 12 bytes.  It is chosen so that the
 length of the value field of the AT_ENCR_DATA attribute becomes a
 multiple of 16 bytes.  The actual pad bytes in the value field are
 set to zero (00 hexadecimal) on sending.  The recipient of the
 message MUST verify that the pad bytes are set to zero.  If this
 verification fails on the peer, then it MUST send the
 EAP-Response/SIM/Client-Error packet with the error code "unable to
 process packet" to terminate the authentication exchange.  If this
 verification fails on the server, then the server sends the peer the
 EAP-Request/SIM/Notification packet with an AT_NOTIFICATION code that
 implies failure to terminate the authentication exchange.  The format
 of the AT_PADDING attribute is shown below.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  AT_PADDING   | Length        | Padding...                    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
  |                                                               |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Haverinen & Salowey Informational [Page 61] RFC 4186 EAP-SIM Authentication January 2006

10.13. AT_RESULT_IND

 The format of the AT_RESULT_IND attribute is shown below.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  AT_RESULT_...| Length = 1    |           Reserved            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The value field of this attribute consists of two reserved bytes,
 which are set to zero upon sending and ignored upon reception.  This
 attribute is always sent unencrypted, so it MUST NOT be encapsulated
 within the AT_ENCR_DATA attribute.

10.14. AT_MAC

 The AT_MAC attribute is used for EAP-SIM message authentication.
 Section 8 specifies in which messages AT_MAC MUST be included.
 The value field of the AT_MAC attribute contains two reserved bytes
 followed by a keyed message authentication code (MAC).  The MAC is
 calculated over the whole EAP packet and concatenated with optional
 message-specific data, with the exception that the value field of the
 MAC attribute is set to zero when calculating the MAC.  The EAP
 packet includes the EAP header that begins with the Code field, the
 EAP-SIM header that begins with the Subtype field, and all the
 attributes, as specified in Section 8.1.  The reserved bytes in
 AT_MAC are set to zero when sending and ignored on reception.  The
 contents of the message-specific data that may be included in the MAC
 calculation are specified separately for each EAP-SIM message in
 Section 9.
 The format of the AT_MAC attribute is shown below.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     AT_MAC    | Length = 5    |           Reserved            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  |                           MAC                                 |
  |                                                               |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Haverinen & Salowey Informational [Page 62] RFC 4186 EAP-SIM Authentication January 2006

 The MAC algorithm is an HMAC-SHA1-128 [RFC2104] keyed hash value.
 (The HMAC-SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value
 by truncating the output to the first 16 bytes.  Hence, the length of
 the MAC is 16 bytes.  The derivation of the authentication key
 (K_aut) used in the calculation of the MAC is specified in Section 7.
 When the AT_MAC attribute is included in an EAP-SIM message, the
 recipient MUST process the AT_MAC attribute before looking at any
 other attributes, except when processing EAP-Request/SIM/Challenge.
 The processing of EAP-Request/SIM/Challenge is specified in Section
 9.3.  If the message authentication code is invalid, then the
 recipient MUST ignore all other attributes in the message and operate
 as specified in Section 6.3.

10.15. 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.
 This attribute MUST always be encrypted by encapsulating it within
 the AT_ENCR_DATA attribute.

10.16. 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.  This
 attribute MUST always be encrypted by encapsulating it within the
 AT_ENCR_DATA attribute.

Haverinen & Salowey Informational [Page 63] RFC 4186 EAP-SIM Authentication January 2006

10.17. AT_NONCE_S

 The format of the AT_NONCE_S attribute is shown below.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | AT_NONCE_S    | Length = 5    |           Reserved            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  |                                                               |
  |                            NONCE_S                            |
  |                                                               |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The value field of the AT_NONCE_S attribute contains two reserved
 bytes followed by a random number freshly generated by the server (16
 bytes) for this EAP-SIM fast re-authentication.  The random number is
 used as a challenge for the peer and also as a seed value for the new
 keying material.  The reserved bytes are set to zero upon sending and
 ignored upon reception.  This attribute MUST always be encrypted by
 encapsulating it within the AT_ENCR_DATA attribute.
 The server MUST NOT re-use the NONCE_S value from any previous
 EAP-SIM fast re-authentication exchange.  The server SHOULD use a
 good source of randomness to generate NONCE_S.  Please see [RFC4086]
 for more information about generating random numbers for security
 applications.

10.18. 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    |S|P|  Notification Code        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The value field of this attribute contains a two-byte notification
 code.  The first and second bit (S and P) of the notification code
 are interpreted as described in Section 6.
 The notification code values listed below have been reserved.  The
 descriptions below illustrate the semantics of the notifications.

Haverinen & Salowey Informational [Page 64] RFC 4186 EAP-SIM Authentication January 2006

 The peer implementation MAY use different wordings when presenting
 the notifications to the user.  The "requested service" depends on
 the environment where EAP-SIM is applied.
 0 - General failure after authentication.  (Implies failure, used
 after successful authentication.)
 16384 - General failure.  (Implies failure, used before
 authentication.)
 32768 - Success.  User has been successfully authenticated.  (Does
 not imply failure, used after successful authentication).  The usage
 of this code is discussed in Section 6.2.
 1026 - User has been temporarily denied access to the requested
 service.  (Implies failure, used after successful authentication.)
 1031 - User has not subscribed to the requested service.  (Implies
 failure, used after successful authentication.)

10.19. 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
  1    "unsupported version": the peer does not support any of
       the versions listed in AT_VERSION_LIST
  2    "insufficient number of challenges": the peer's policy
       requires more triplets than the server included in AT_RAND
  3    "RANDs are not fresh": the peer believes that the RAND
       challenges included in AT_RAND were not fresh

Haverinen & Salowey Informational [Page 65] RFC 4186 EAP-SIM Authentication January 2006

11. IANA Considerations

 IANA has assigned the EAP type number 18 for this protocol.
 EAP-SIM shares most of the protocol design, such as attributes and
 message Subtypes, with EAP-AKA [EAP-AKA].  EAP-SIM protocol numbers
 should be administered in the same IANA registry as EAP-AKA.  The
 initial values are listed in [EAP-AKA] for both protocols, so this
 document does not require any new registries or parameter allocation.
 As a common registry is used for EAP-SIM and EAP-AKA, the protocol
 number allocation policy for both protocols is specified in
 [EAP-AKA].

12. Security Considerations

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

12.1. A3 and A8 Algorithms

 The GSM A3 and A8 algorithms are used in EAP-SIM.  [GSM-03.20]
 specifies the general GSM authentication procedure and the external
 interface (inputs and outputs) of the A3 and A8 algorithms.  The
 operation of these functions falls completely within the domain of an
 individual operator, and therefore, the functions are specified by
 each operator rather than being fully standardised.  The GSM-MILENAGE
 algorithm, specified publicly in [3GPP-TS-55.205], is an example
 algorithm set for A3 and A8 algorithms.
 The security of the A3 and A8 algorithms is important to the security
 of EAP-SIM.  Some A3/A8 algorithms have been compromised; see [GSM-
 Cloning] for discussion about the security of COMP-128 version 1.
 Note that several revised versions of the COMP-128 A3/A8 algorithm
 have been devised after the publication of these weaknesses and that
 the publicly specified GSM-MILENAGE algorithm is not vulnerable to
 any known attacks.

12.2. Identity Protection

 EAP-SIM includes optional identity privacy support that protects the
 privacy of the subscriber identity against passive eavesdropping.
 This document only specifies a mechanism to deliver pseudonyms from
 the server to the peer as part of an EAP-SIM exchange.  Hence, a peer
 that has not yet performed any EAP-SIM exchanges does not typically
 have a pseudonym available.  If the peer does not have a pseudonym
 available, then the privacy mechanism cannot be used, but the

Haverinen & Salowey Informational [Page 66] RFC 4186 EAP-SIM Authentication January 2006

 permanent identity 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 to attempt to learn
 the subscriber's permanent identity.  However, as discussed in
 Section 4.2.2, the terminal can refuse to send the cleartext
 permanent identity 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.  If an external security mechanism is in use, the identity
 privacy features of EAP-SIM may not be useful.  The security
 considerations of using an external security mechanism with EAP-SIM
 are beyond the scope of this document.

12.3. Mutual Authentication and Triplet Exposure

 EAP-SIM provides mutual authentication.  The peer believes that the
 network is authentic because the network can calculate a correct
 AT_MAC value in the EAP-Request/SIM/Challenge packet.  To calculate
 AT_MAC it is sufficient to know the RAND and Kc values from the GSM
 triplets (RAND, SRES, Kc) used in the authentication.  Because the
 network selects the RAND challenges and the triplets, an attacker
 that knows n (2 or 3) GSM triplets for the subscriber is able to
 impersonate a valid network to the peer.  (Some peers MAY employ an
 implementation-specific counter-measure against impersonating a valid
 network by re-using a previously used RAND; see below.)  In other
 words, the security of EAP-SIM is based on the secrecy of Kc keys,
 which are considered secret intermediate results in the EAP-SIM
 cryptographic calculations.
 Given physical access to the SIM card, it is easy to obtain any
 number of GSM triplets.
 Another way to obtain triplets is to mount an attack on the peer
 platform via a virus or other malicious piece of software.  The peer
 SHOULD be protected against triplet querying attacks by malicious
 software.  Care should be taken not to expose Kc keys to attackers
 when they are stored or handled by the peer, or transmitted between
 subsystems of the peer.  Steps should be taken to limit the
 transport, storage, and handling of these values outside a protected
 environment within the peer.  However, the virus protection of the
 peer and the security capabilities of the peer's operating system are
 outside the scope of this document.

Haverinen & Salowey Informational [Page 67] RFC 4186 EAP-SIM Authentication January 2006

 The EAP-SIM server typically obtains the triplets from the Home
 Location Register (HLR).  An attacker might try to obtain triplets by
 attacking against the network used between the EAP-SIM server and the
 HLR.  Care should be taken not to expose Kc keys to attackers when
 they are stored or handled by the EAP-SIM server, or transmitted
 between the EAP server and the HLR.  Steps should be taken to limit
 the transport, storage, and handling of these values outside a
 protected environment.  However, the protection of the communications
 between the EAP-SIM server and the HLR is outside the scope of this
 document.
 If the same SIM credentials are also used for GSM traffic, the
 triplets could be revealed in the GSM network; see Section 12.8.
 In GSM, the network is allowed to re-use the RAND challenge in
 consecutive authentication exchanges.  This is not allowed in
 EAP-SIM.  The EAP-SIM server is mandated to use fresh triplets (RAND
 challenges) in consecutive authentication exchanges, as specified in
 Section 3.  EAP-SIM does not mandate any means for the peer to check
 if the RANDs are fresh, so the security of the scheme leans on the
 secrecy of the triplets.  However, the peer MAY employ
 implementation-specific mechanisms to remember some of the previously
 used RANDs, and the peer MAY check the freshness of the server's
 RANDs.  The operation in cases when the peer detects that the RANDs
 are not fresh is specified in Section 6.3.1.
 Preventing the re-use of authentication vectors has been taken into
 account in the design of the UMTS Authentication and Key Agreement
 (AKA), which is used in EAP-AKA [EAP-AKA].  In cases when the triplet
 re-use properties of EAP-SIM are not considered sufficient, it is
 advised to use EAP-AKA.
 Note that EAP-SIM mutual authentication is done with the EAP server.
 In general, EAP methods do not authenticate the identity or services
 provided by the EAP authenticator (if distinct from the EAP server)
 unless they provide the so-called channel bindings property.  The
 vulnerabilities related to this have been discussed in [RFC3748],
 [EAP-Keying], [Service-Identity].
 EAP-SIM does not provide the channel bindings property, so it only
 authenticates the EAP server.  However, ongoing work such as
 [Service-Identity] may provide such support as an extension to
 popular EAP methods such as EAP-TLS, EAP-SIM, or EAP-AKA.

Haverinen & Salowey Informational [Page 68] RFC 4186 EAP-SIM Authentication January 2006

12.4. Flooding the Authentication Centre

 The EAP-SIM server typically obtains authentication vectors from the
 Authentication Centre (AuC).  EAP-SIM introduces a new usage for the
 AuC.  The protocols between the EAP-SIM server and the AuC are out of
 the scope of this document.  However, it should be noted that a
 malicious EAP-SIM peer may generate a lot of protocol requests to
 mount a denial of service attack.  The EAP-SIM server implementation
 SHOULD take this into account and SHOULD take steps to limit the
 traffic that it generates towards the AuC, preventing the attacker
 from flooding the AuC and from extending the denial of service attack
 from EAP-SIM to other users of the AuC.

12.5. Key Derivation

 EAP-SIM supports key derivation.  The key hierarchy is specified in
 Section 7.  EAP-SIM combines several GSM triplets in order to
 generate stronger keying material and stronger AT_MAC values.  The
 actual strength of the resulting keys depends, among other things, on
 operator-specific parameters including authentication algorithms, the
 strength of the Ki key, and the quality of the RAND challenges.  For
 example, some SIM cards generate Kc keys with 10 bits set to zero.
 Such restrictions may prevent the concatenation technique from
 yielding strong session keys.  Because the strength of the Ki key is
 128 bits, the ultimate strength of any derived secret key material is
 never more than 128 bits.
 It should also be noted that a security policy that allows n=2 to be
 used may compromise the security of a future policy that requires
 three triplets, because adversaries may be able to exploit the
 messages exchanged when the weaker policy is applied.
 There is no known way to obtain complete GSM triplets by mounting an
 attack against EAP-SIM.  A passive eavesdropper can learn n*RAND and
 AT_MAC and may be able to link this information to the subscriber
 identity.  An active attacker that impersonates a GSM subscriber can
 easily obtain n*RAND and AT_MAC values from the EAP server for any
 given subscriber identity.  However, calculating the Kc and SRES
 values from AT_MAC would require the attacker to reverse the keyed
 message authentication code function HMAC-SHA1-128.
 As EAP-SIM does not expose any values calculated from an individual
 GSM Kc keys, it is not possible to mount a brute force attack on only
 one of the Kc keys in EAP-SIM.  Therefore, when considering brute
 force attacks on the values exposed in EAP-SIM, the effective length
 of EAP-SIM session keys is not compromised by the fact that they are

Haverinen & Salowey Informational [Page 69] RFC 4186 EAP-SIM Authentication January 2006

 combined from several shorter keys, i.e., the effective length of 128
 bits may be achieved.  For additional considerations, see Section
 12.8.

12.6. Cryptographic Separation of Keys and Session Independence

 The EAP Transient Keys used to protect EAP-SIM packets (K_encr,
 K_aut), the Master Session Key, and the Extended Master Session Key
 are cryptographically separate in EAP-SIM.  An attacker cannot derive
 any non-trivial information about any of these keys based on the
 other keys.  An attacker also cannot calculate the pre-shared secret
 (Ki) from the GSM Kc keys, from EAP-SIM K_encr, from EAP-SIM K_aut,
 from the Master Session Key, or from the Extended Master Session Key.
 Each EAP-SIM exchange generates fresh keying material, and the keying
 material exported from the method upon separate EAP-SIM exchanges is
 cryptographically separate.  The EAP-SIM peer contributes to the
 keying material with the NONCE_MT parameter, which must be chosen
 freshly for each full authentication exchange.  The EAP server is
 mandated to choose the RAND challenges freshly for each full
 authentication exchange.  If either the server or the peer chooses
 its random value (NONCE_MT or RAND challenges) freshly, even if the
 other entity re-used its value from a previous exchange, then the EAP
 Transient Keys, the Master Session Key, and the Extended Master
 Session Key will be different and cryptographically separate from the
 corresponding values derived upon the previous full authentication
 exchange.
 On fast re-authentication, freshness of the Master Session Key and
 the Extended Master Session Key is provided with a counter
 (AT_COUNTER).  The same EAP Transient Keys (K_encr, K_aut) that were
 used in the full authentication exchange are used to protect the EAP
 negotiation.  However, replay and integrity protection across all the
 fast re-authentication exchanges that use the same EAP Transient Keys
 is provided with AT_COUNTER.
 [RFC3748] defines session independence as the "demonstration that
 passive attacks (such as capture of the EAP conversation) or active
 attacks (including compromise of the MSK or EMSK) do not enable
 compromise of subsequent or prior MSKs or EMSKs".  Because the MSKs
 and EMSKs are separate between EAP exchanges, EAP-SIM supports this
 security claim.
 It should be noted that [Patel-2003], which predates [RFC3748], uses
 a slightly different meaning for session independence.  The EAP-SIM
 protocol does not allow the peer to ensure that different Kc key
 values would be used in different exchanges.  Only the server is able
 to ensure that fresh RANDs, and therefore, fresh Kc keys are used.

Haverinen & Salowey Informational [Page 70] RFC 4186 EAP-SIM Authentication January 2006

 Hence, the peer cannot guarantee EAP-SIM sessions to be independent
 with regard to the internal Kc values.  However, in EAP-SIM, the Kc
 keys are considered to be secret intermediate results, which are not
 exported outside the method.  See Section 12.3 for more information
 about RAND re-use.

12.7. Dictionary Attacks

 Because EAP-SIM is not a password protocol, it is not vulnerable to
 dictionary attacks.  (The pre-shared symmetric secret stored on the
 SIM card is not a passphrase, nor is it derived from a passphrase.)

12.8. Credentials Re-use

 EAP-SIM cannot prevent attacks over the GSM or GPRS radio networks.
 If the same SIM credentials are also used in GSM or GPRS, it is
 possible to mount attacks over the cellular interface.
 A passive attacker can eavesdrop GSM or GPRS traffic and obtain RAND,
 SRES pairs.  He can then use a brute force attack or other
 cryptanalysis techniques to obtain the 64-bit Kc keys used to encrypt
 the GSM or GPRS data.  This makes it possible to attack each 64-bit
 key separately.
 An active attacker can mount a "rogue GSM/GPRS base station attack",
 replaying previously seen RAND challenges to obtain SRES values.  He
 can then use a brute force attack to obtain the Kc keys.  If
 successful, the attacker can impersonate a valid network or decrypt
 previously seen traffic, because EAP-SIM does not provide perfect
 forward secrecy (PFS).
 Due to several weaknesses in the GSM encryption algorithms, the
 effective key strength of the Kc keys is much less than the expected
 64 bits (no more than 40 bits if the A5/1 GSM encryption algorithm is
 used; as documented in [Barkan-2003], an active attacker can force
 the peer to use the weaker A5/2 algorithm that can be broken in less
 than a second).
 Because the A5 encryption algorithm is not used in EAP-SIM, and
 because EAP-SIM does not expose any values calculated from individual
 Kc keys, it should be noted that these attacks are not possible if
 the SIM credentials used in EAP-SIM are not shared in GSM/GPRS.
 At the time this document was written, the 3rd Generation Partnership
 Project (3GPP) has started to work on fixes to these A5
 vulnerabilities.  One of the solution proposals discussed in 3GPP is
 integrity-protected A5 version negotiation, which would require the
 base station to prove knowledge of the Kc key before the terminal

Haverinen & Salowey Informational [Page 71] RFC 4186 EAP-SIM Authentication January 2006

 sends any values calculated from the Kc to the network.  Another
 proposal is so-called special RANDs, where some bits of the RAND
 challenge would be used for cryptographic separation by indicating
 the allowed use of the triplet, such as the allowed A5 algorithm in
 GSM or the fact that the triplet is intended for EAP-SIM.  This is
 currently a work in progress, and the mechanisms have not been
 selected yet.

12.9. Integrity and Replay Protection, and Confidentiality

 AT_MAC, AT_IV, AT_ENCR_DATA, and AT_COUNTER attributes are used to
 provide integrity, replay and confidentiality protection for EAP-SIM
 requests and responses.  Integrity protection with AT_MAC includes
 the EAP header.  These attributes cannot be used during the
 EAP/SIM/Start roundtrip.  However, the protocol values (user identity
 string, NONCE_MT, and version negotiation parameters) are
 (implicitly) protected by later EAP-SIM messages by including them in
 key derivation.
 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.
 Confidentiality protection is applied only to a part of the protocol
 fields.  The table of attributes in Section 10.1 summarizes which
 fields are confidentiality-protected.  It should be noted that the
 error and notification code attributes AT_CLIENT_ERROR_CODE and
 AT_NOTIFICATION are not confidential, but they are transmitted in the
 clear.  Identity protection is discussed in Section 12.2.
 On full authentication, replay protection of the EAP exchange is
 provided by the RAND values from the underlying GSM authentication
 scheme and the use of the NONCE_MT value.  Protection against replays
 of EAP-SIM messages is also based on the fact that messages that can
 include AT_MAC can only be sent once with a certain EAP-SIM Subtype,
 and on the fact that a different K_aut key will be used for
 calculating AT_MAC in each full authentication exchange.
 On fast re-authentication, a counter included in AT_COUNTER and a
 server random nonce is used to provide replay protection.  The
 AT_COUNTER attribute is also included in EAP-SIM notifications if it
 is used after successful authentication in order to provide replay
 protection between re-authentication exchanges.
 Because EAP-SIM is not a tunneling method, EAP-Request/Notification,
 EAP-Response/Notification, EAP-Success, or EAP-Failure packets are
 not confidential, integrity-protected, or replay-protected in
 EAP-SIM.  On physically insecure networks, this may enable an

Haverinen & Salowey Informational [Page 72] RFC 4186 EAP-SIM Authentication January 2006

 attacker to send false notifications to the peer and to mount denial
 of service attacks by spoofing these packets.  As discussed in
 Section 6.3, the peer will only accept EAP-Success after the peer
 successfully authenticates the server.  Hence, the attacker cannot
 force the peer to believe successful mutual authentication has
 occurred until the peer successfully authenticates the server or
 after the peer fails to authenticate the server.
 The security considerations of EAP-SIM result indications are covered
 in Section 12.11
 An eavesdropper will see the EAP-Request/Notification,
 EAP-Response/Notification, EAP-Success, and EAP-Failure packets sent
 in the clear.  With EAP-SIM, confidential information MUST NOT be
 transmitted in EAP Notification packets.

12.10. Negotiation Attacks

 EAP-SIM does not protect the EAP-Response/Nak packet.  Because
 EAP-SIM does not protect the EAP method negotiation, EAP method
 downgrading attacks may be possible, especially if the user uses the
 same identity with EAP-SIM and other EAP methods.
 EAP-SIM includes a version negotiation procedure.  In EAP-SIM the
 keying material derivation includes the version list and selected
 version to ensure that the protocol cannot be downgraded and that the
 peer and server use the same version of EAP-SIM.
 EAP-SIM does not support ciphersuite negotiation.

12.11. Protected Result Indications

 EAP-SIM supports optional protected success indications and
 acknowledged failure indications.  If a failure occurs after
 successful authentication, then the EAP-SIM failure indication is
 integrity- and replay-protected.
 Even if an EAP-Failure packet is lost when using EAP-SIM over an
 unreliable medium, then the EAP-SIM failure indications will help
 ensure that the peer and EAP server will know the other party's
 authentication decision.  If protected success indications are used,
 then the loss of Success packet will also be addressed by the
 acknowledged, integrity- and replay-protected EAP-SIM success
 indication.  If the optional success indications are not used, then
 the peer may end up believing that the server succeeded
 authentication, when it actually failed.  Since access will not be

Haverinen & Salowey Informational [Page 73] RFC 4186 EAP-SIM Authentication January 2006

 granted in this case, protected result indications are not needed
 unless the client is not able to realize it does not have access for
 an extended period of time.

12.12. 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-SIM Master Session Key, or keys derived from it,
 MAY be used as the integrity protection keys, or, if an external
 security mechanism such as PEAP is used, then the link integrity
 protection keys MAY be derived by the external security mechanism.
 There are man-in-the-middle attacks associated with the use of any
 EAP method within a tunneled protocol.  For instance, an early
 version of PEAP [PEAP-02] was vulnerable to this attack.  This
 specification does not address these attacks.  If EAP-SIM is used
 with a tunneling protocol, there should be cryptographic binding
 provided between the protocol and EAP-SIM to prevent
 man-in-the-middle attacks through rogue authenticators being able to
 setup one-way authenticated tunnels.  For example, newer versions of
 PEAP include such cryptographic binding.  The EAP-SIM Master Session
 Key MAY be used to provide the cryptographic binding.  However, the
 mechanism by which the binding is provided depends on the tunneling
 protocol and is beyond the scope of this document.

12.13. Generating Random Numbers

 An EAP-SIM implementation SHOULD use a good source of randomness to
 generate the random numbers required in the protocol.  Please see
 [RFC4086] for more information on generating random numbers for
 security applications.

13. Security Claims

 This section provides the security claims required by [RFC3748].
 Auth. mechanism: EAP-SIM is based on the GSM SIM mechanism, which is
 a challenge/response authentication and key agreement mechanism based
 on a symmetric 128-bit pre-shared secret.  EAP-SIM also makes use of
 a peer challenge to provide mutual authentication.
 Ciphersuite negotiation: No
 Mutual authentication: Yes (Section 12.3)
 Integrity protection: Yes (Section 12.9)

Haverinen & Salowey Informational [Page 74] RFC 4186 EAP-SIM Authentication January 2006

 Replay protection: Yes (Section 12.9)
 Confidentiality: Yes, except method-specific success and failure
 indications (Section 12.2, Section 12.9)
 Key derivation: Yes
 Key strength: EAP-SIM supports key derivation with 128-bit effective
 key strength (Section 12.5).  However, as discussed in Section 11, if
 the same credentials are used in GSM/GPRS and in EAP-SIM, then the
 key strength may be reduced considerably, basically to the same level
 as in GSM, by mounting attacks over GSM/GPRS.  For example an active
 attack using a false GSM/GPRS base station reduces the effective key
 strength to almost zero.
 Description of key hierarchy: Please see Section 7.
 Dictionary attack protection: N/A (Section 12.7)
 Fast reconnect: Yes
 Cryptographic binding: N/A
 Session independence: Yes (Section 12.6)
 Fragmentation: No
 Channel binding: No
 Indication of vulnerabilities: Vulnerabilities are discussed in
 Section 12.

14. Acknowledgements and Contributions

14.1. Contributors

 In addition to the editors, Nora Dabbous, Jose Puthenkulam, and
 Prasanna Satarasinghe were significant contributors to this document.
 Pasi Eronen and Jukka-Pekka Honkanen contributed Appendix A.

14.2. Acknowledgements

 Juha Ala-Laurila, N. Asokan, Jan-Erik Ekberg, Patrik Flykt,
 Jukka-Pekka Honkanen, Antti Kuikka, Jukka Latva, Lassi Lehtinen, Jyri
 Rinnemaa, Timo Takamaki, and Raimo Vuonnala contributed many original
 ideas and concepts to this protocol.

Haverinen & Salowey Informational [Page 75] RFC 4186 EAP-SIM Authentication January 2006

 N. Asokan, Pasi Eronen, and Jukka-Pekka Honkanen contributed and
 helped in innumerable ways during the development of the protocol.
 Valtteri Niemi and Kaisa Nyberg contributed substantially to the
 design of the key derivation and the fast re-authentication
 procedure, and have also provided their cryptographic expertise in
 many discussions related to this protocol.
 Simon Blake-Wilson provided very helpful comments on key derivation
 and version negotiation.
 Thanks to Greg Rose for his very valuable comments to an early
 version of this specification [S3-020125], and for reviewing and
 providing very useful comments on version 12.
 Thanks to Bernard Aboba, Vladimir Alperovich, Florent Bersani,
 Jacques Caron, Gopal Dommety, Augustin Farrugia, Mark Grayson, Max de
 Groot, Prakash Iyer, Nishi Kant, Victor Lortz, Jouni Malinen, Sarvar
 Patel, Tom Porcher, Michael Richardson, Stefan Schroeder, Uma
 Shankar, Jesse Walker, and Thomas Wieland for their contributions and
 critiques.  Special thanks to Max for proposing improvements to the
 MAC calculation.
 Thanks to Glen Zorn for reviewing this document and for providing
 very useful comments on the protocol.
 Thanks to Sarvar Patel for his review of the protocol [Patel-2003].
 Thanks to Bernard Aboba for reviewing this document for RFC 3748
 compliance.
 The identity privacy support is based on the identity privacy support
 of [EAP-SRP].  The attribute format is based on the extension format
 of Mobile IPv4 [RFC3344].
 This protocol has been partly developed in parallel with EAP-AKA
 [EAP-AKA], and hence this specification incorporates many ideas from
 Jari Arkko.

Haverinen & Salowey Informational [Page 76] RFC 4186 EAP-SIM Authentication January 2006

14.2.1. Contributors' Addresses

 Nora Dabbous
 Gemplus
 34 rue Guynemer
 92447 Issy les Moulineaux
 France
 Phone: +33 1 4648 2000
 EMail: nora.dabbous@gemplus.com
 Jose Puthenkulam
 Intel Corporation
 2111 NE 25th Avenue, JF2-58
 Hillsboro, OR 97124
 USA
 Phone: +1 503 264 6121
 EMail: jose.p.puthenkulam@intel.com
 Prasanna Satarasinghe
 Transat Technologies
 180 State Street, Suite 240
 Southlake, TX 76092
 USA
 Phone: + 1 817 4814412
 EMail: prasannas@transat-tech.com

Haverinen & Salowey Informational [Page 77] RFC 4186 EAP-SIM Authentication January 2006

15. References

15.1. Normative References

 [GSM-03.20]        European Telecommunications Standards  Institute,
                    "GSM Technical Specification GSM 03.20 (ETS 300
                    534):  "Digital cellular telecommunication system
                    (Phase 2); Security related network functions"",
                    August 1997.
 [RFC2119]          Bradner, S., "Key words for use in RFCs to
                    Indicate Requirement Levels", BCP 14, RFC 2119,
                    March 1997.
 [GSM-03.03]        European Telecommunications Standards Institute,
                    "GSM Technical Specification GSM 03.03 (ETS 300
                    523): "Digital cellular telecommunication system
                    (Phase 2); Numbering, addressing and
                    identification"", April 1997.
 [RFC2104]          Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                    Keyed-Hashing for Message Authentication", RFC
                    2104, February 1997.
 [RFC4282]          Aboba, B., Beadles, M., Arkko, J., and P. Eronen,
                    "The Network Access Identifier", RFC 4282,
                    December 2005.
 [AES]              National Institute of  Standards and Technology,
                    "Federal Information Processing Standards (FIPS)
                    Publication 197, "Advanced Encryption Standard
                    (AES)"", November 2001.
                    http://csrc.nist.gov/publications/fips/fips197/
                    fips-197.pdf
 [CBC]              National Institute of Standards and Technology,
                    "NIST Special Publication 800-38A, "Recommendation
                    for Block Cipher Modes of Operation - Methods and
                    Techniques"", December 2001.
                    http://csrc.nist.gov/publications/nistpubs/
                    800-38a/sp800-38a.pdf
 [SHA-1]            National Institute of Standards and Technology,
                    U.S.  Department of Commerce, "Federal Information
                    Processing Standard (FIPS) Publication 180-1,
                    "Secure Hash Standard"", April 1995.

Haverinen & Salowey Informational [Page 78] RFC 4186 EAP-SIM Authentication January 2006

 [PRF]              National Institute of Standards and Technology,
                    "Federal Information Processing Standards (FIPS)
                    Publication  186-2 (with change notice); Digital
                    Signature Standard (DSS)", January 2000.
                    Available on-line at:
                    http://csrc.nist.gov/publications/
                    fips/fips186-2/fips186-2-change1.pdf
 [RFC3629]          Yergeau, F., "UTF-8, a transformation format of
                    ISO 10646", STD 63, RFC 3629, November 2003.
 [RFC3748]          Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J.,
                    and H. Levkowetz, "Extensible Authentication
                    Protocol (EAP)", RFC 3748, June 2004.
 [EAP-AKA]          Arkko, J. and H. Haverinen, "Extensible
                    Authentication Protocol Method for 3rd Generation
                    Authentication and Key Agreement (EAP-AKA)", RFC
                    4187, January 2006.

15.2. Informative References

 [3GPP-TS-23.003]   3rd Generation Partnership Project, "3GPP
                    Technical Specification 3GPP TS 23.003 V6.8.0:
                    "3rd Generation Parnership Project; Technical
                    Specification Group Core Network; Numbering,
                    addressing and identification (Release 6)"",
                    December 2005.
 [3GPP-TS-55.205]   3rd Generation Partnership Project, "3GPP
                    Technical Specification 3GPP TS 55.205 V 6.0.0:
                    "3rd Generation Partnership Project; Technical
                    Specification Group Services and System Aspects;
                    Specification of the GSM-MILENAGE Algorithms: An
                    example algorithm set for the GSM Authentication
                    and Key Generation functions A3 and A8 (Release
                    6)"", December 2002.
 [PEAP]             Palekar, A., Simon, D., Zorn, G., Salowey, J.,
                    Zhou, H., and S. Josefsson, "Protected EAP
                    Protocol (PEAP) Version 2", Work in Progress,
                    October 2004.
 [PEAP-02]          Anderson, H., Josefsson, S., Zorn, G., Simon, D.,
                    and A. Palekar, "Protected EAP Protocol (PEAP)",
                    Work in Progress, February 2002.

Haverinen & Salowey Informational [Page 79] RFC 4186 EAP-SIM Authentication January 2006

 [EAP-Keying]       Aboba, B., Simon, D., Arkko, J., Eronen, P., and
                    H.  Levkowetz, "Extensible Authentication Protocol
                    (EAP) Key Management Framework", Work in Progress,
                    October 2005.
 [Service-Identity] Arkko, J. and P. Eronen, "Authenticated Service
                    Information for the Extensible Authentication
                    Protocol (EAP)", Work in Progress, October 2004.
 [RFC4086]          Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                    "Randomness Requirements for Security", BCP 106,
                    RFC 4086, June 2005.
 [S3-020125]        Qualcomm, "Comments on draft EAP/SIM, 3rd
                    Generation Partnership Project document 3GPP TSG
                    SA WG3 Security S3#22, S3-020125", February 2002.
 [RFC3344]          Perkins, C., "IP Mobility Support for IPv4", RFC
                    3344, August 2002.
 [RFC2548]          Zorn, G., "Microsoft Vendor-specific RADIUS
                    Attributes ", RFC 2548, March 1999.
 [EAP-SRP]          Carlson, J., Aboba, B., and H. Haverinen, "EAP
                    SRP-SHA1 Authentication Protocol", Work in
                    Progress, July 2001.
 [GSM-Cloning]      Wagner, D., "GSM Cloning".  Web page about
                    COMP-128 version 1 vulnerabilities, available at
                    http://www.isaac.cs.berkeley.edu/isaac/gsm.html
 [Barkan-2003]      Barkan, E., Biham, E., and N. Keller, "Instant
                    Ciphertext-Only Cryptanalysis of GSM Encrypted
                    Communications".  available on-line at
                    http://cryptome.org/gsm-crack-bbk.pdf
 [Patel-2003]       Patel, S., "Analysis of EAP-SIM Session Key
                    Agreement".  Posted to the EAP mailing list 29
                    May,2003. http://
                    mail.frascone.com/pipermail/public/eap/2003-May/
                    001267.html

Haverinen & Salowey Informational [Page 80] RFC 4186 EAP-SIM Authentication January 2006

Appendix A. Test Vectors

 Test vectors for the NIST FIPS 186-2 pseudo-random number generator
 [PRF] are available at the following URL:
 http://csrc.nist.gov/encryption/dss/Examples-1024bit.pdf
 The following examples show the contents of EAP-SIM packets on full
 authentication and fast re-authentication.

A.1. EAP-Request/Identity

 The first packet is a plain Identity Request:
    01                   ; Code: Request
    00                   ; Identifier: 0
    00 05                ; Length: 5 octets
    01                   ; Type: Identity

A.2. EAP-Response/Identity

 The client's identity is "1244070100000001@eapsim.foo", so it
 responds with the following packet:
    02                   ; Code: Response
    00                   ; Identifier: 0
    00 20                ; Length: 32 octets
    01                   ; Type: Identity
       31 32 34 34       ; "1244070100000001@eapsim.foo"
       30 37 30 31
       30 30 30 30
       30 30 30 31
       40 65 61 70
       73 69 6d 2e
       66 6f 6f

Haverinen & Salowey Informational [Page 81] RFC 4186 EAP-SIM Authentication January 2006

A.3. EAP-Request/SIM/Start

 The server's first packet looks like this:
    01                   ; Code: Request
    01                   ; Identifier: 1
    00 10                ; Length: 16 octets
    12                   ; Type: EAP-SIM
       0a                ; EAP-SIM subtype: Start
       00 00             ; (reserved)
       0f                ; Attribute type: AT_VERSION_LIST
          02             ; Attribute length: 8 octets (2*4)
          00 02          ; Actual version list length: 2 octets
          00 01          ; Version: 1
          00 00          ; (attribute padding)

A.4. EAP-Response/SIM/Start

 The client selects a nonce and responds with the following packet:
    02                   ; Code: Response
    01                   ; Identifier: 1
    00 20                ; Length: 32 octets
    12                   ; Type: EAP-SIM
       0a                ; EAP-SIM subtype: Start
       00 00             ; (reserved)
       07                ; Attribute type: AT_NONCE_MT
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          01 23 45 67    ; NONCE_MT value
          89 ab cd ef
          fe dc ba 98
          76 54 32 10
       10                ; Attribute type: AT_SELECTED_VERSION
          01             ; Attribute length: 4 octets (1*4)
          00 01          ; Version: 1

Haverinen & Salowey Informational [Page 82] RFC 4186 EAP-SIM Authentication January 2006

A.5. EAP-Request/SIM/Challenge

 Next, the server selects three authentication triplets
       (RAND1,SRES1,Kc1) = (10111213 14151617 18191a1b 1c1d1e1f,
                            d1d2d3d4,
                            a0a1a2a3 a4a5a6a7)
       (RAND2,SRES2,Kc2) = (20212223 24252627 28292a2b 2c2d2e2f,
                            e1e2e3e4,
                            b0b1b2b3 b4b5b6b7)
       (RAND3,SRES3,Kc3) = (30313233 34353637 38393a3b 3c3d3e3f,
                            f1f2f3f4,
                            c0c1c2c3 c4c5c6c7)
 Next, the MK is calculated as specified in Section 7*.
 MK = e576d5ca 332e9930 018bf1ba ee2763c7 95b3c712
 And the other keys are derived using the PRNG:
       K_encr = 536e5ebc 4465582a a6a8ec99 86ebb620
       K_aut =  25af1942 efcbf4bc 72b39434 21f2a974
       MSK =    39d45aea f4e30601 983e972b 6cfd46d1
                c3637733 65690d09 cd44976b 525f47d3
                a60a985e 955c53b0 90b2e4b7 3719196a
                40254296 8fd14a88 8f46b9a7 886e4488
       EMSK =   5949eab0 fff69d52 315c6c63 4fd14a7f
                0d52023d 56f79698 fa6596ab eed4f93f
                bb48eb53 4d985414 ceed0d9a 8ed33c38
                7c9dfdab 92ffbdf2 40fcecf6 5a2c93b9
 Next, the server selects a pseudonym and a fast re-authentication
 identity (in this case, "w8w49PexCazWJ&xCIARmxuMKht5S1sxR
 DqXSEFBEg3DcZP9cIxTe5J4OyIwNGVzxeJOU1G" and
 "Y24fNSrz8BP274jOJaF17WfxI8YO7QX0
 0pMXk9XMMVOw7broaNhTczuFq53aEpOkk3L0dm@eapsim.foo", respectively).

Haverinen & Salowey Informational [Page 83] RFC 4186 EAP-SIM Authentication January 2006

 The following plaintext will be encrypted and stored in the
 AT_ENCR_DATA attribute:
       84               ; Attribute type: AT_NEXT_PSEUDONYM
          13            ; Attribute length: 76 octets (19*4)
          00 46         ; Actual pseudonym length: 70 octets
          77 38 77 34 39 50 65 78 43 61 7a 57 4a 26 78 43
          49 41 52 6d 78 75 4d 4b 68 74 35 53 31 73 78 52
          44 71 58 53 45 46 42 45 67 33 44 63 5a 50 39 63
          49 78 54 65 35 4a 34 4f 79 49 77 4e 47 56 7a 78
          65 4a 4f 55 31 47
          00 00          ; (attribute padding)
       85                ; Attribute type: AT_NEXT_REAUTH_ID
          16             ; Attribute length: 88 octets (22*4)
          00 51          ; Actual re-auth identity length: 81 octets
          59 32 34 66 4e 53 72 7a 38 42 50 32 37 34 6a 4f
          4a 61 46 31 37 57 66 78 49 38 59 4f 37 51 58 30
          30 70 4d 58 6b 39 58 4d 4d 56 4f 77 37 62 72 6f
          61 4e 68 54 63 7a 75 46 71 35 33 61 45 70 4f 6b
          6b 33 4c 30 64 6d 40 65 61 70 73 69 6d 2e 66 6f
          6f
          00 00 00       ; (attribute padding)
       06                ; Attribute type: AT_PADDING
          03             ; Attribute length: 12 octets (3*4)
          00 00 00 00
          00 00 00 00
          00 00
 The EAP packet looks like this:
    01                   ; Code: Request
    02                   ; Identifier: 2
    01 18                ; Length: 280 octets
    12                   ; Type: EAP-SIM
       0b                ; EAP-SIM subtype: Challenge
       00 00             ; (reserved)
       01                ; Attribute type: AT_RAND
          0d             ; Attribute length: 52 octets (13*4)
          00 00          ; (reserved)
          10 11 12 13    ; first RAND
          14 15 16 17
          18 19 1a 1b
          1c 1d 1e 1f
          20 21 22 23    ; second RAND
          24 25 26 27
          28 29 2a 2b
          2c 2d 2e 2f

Haverinen & Salowey Informational [Page 84] RFC 4186 EAP-SIM Authentication January 2006

          30 31 32 33    ; third RAND
          34 35 36 37
          38 39 3a 3b
          3c 3d 3e 3f
       81                ; Attribute type: AT_IV
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          9e 18 b0 c2    ; IV value
          9a 65 22 63
          c0 6e fb 54
          dd 00 a8 95
       82               ; Attribute type: AT_ENCR_DATA
          2d            ; Attribute length: 180 octets (45*4)
          00 00         ; (reserved)
          55 f2 93 9b bd b1 b1 9e a1 b4 7f c0 b3 e0 be 4c
          ab 2c f7 37 2d 98 e3 02 3c 6b b9 24 15 72 3d 58
          ba d6 6c e0 84 e1 01 b6 0f 53 58 35 4b d4 21 82
          78 ae a7 bf 2c ba ce 33 10 6a ed dc 62 5b 0c 1d
          5a a6 7a 41 73 9a e5 b5 79 50 97 3f c7 ff 83 01
          07 3c 6f 95 31 50 fc 30 3e a1 52 d1 e1 0a 2d 1f
          4f 52 26 da a1 ee 90 05 47 22 52 bd b3 b7 1d 6f
          0c 3a 34 90 31 6c 46 92 98 71 bd 45 cd fd bc a6
          11 2f 07 f8 be 71 79 90 d2 5f 6d d7 f2 b7 b3 20
          bf 4d 5a 99 2e 88 03 31 d7 29 94 5a ec 75 ae 5d
          43 c8 ed a5 fe 62 33 fc ac 49 4e e6 7a 0d 50 4d
       0b                ; Attribute type: AT_MAC
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          fe f3 24 ac    ; MAC value
          39 62 b5 9f
          3b d7 82 53
          ae 4d cb 6a
 The MAC is calculated over the EAP packet above (with MAC value set
 to zero), followed by the NONCE_MT value (a total of 296 bytes).

Haverinen & Salowey Informational [Page 85] RFC 4186 EAP-SIM Authentication January 2006

A.6. EAP-Response/SIM/Challenge

 The client's response looks like this:
    02                   ; Code: Response
    02                   ; Identifier: 2
    00 1c                ; Length: 28 octets
    12                   ; Type: EAP-SIM
       0b                ; EAP-SIM subtype: Challenge
       00 00             ; (reserved)
       0b                ; Attribute type: AT_MAC
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          f5 6d 64 33    ; MAC value
          e6 8e d2 97
          6a c1 19 37
          fc 3d 11 54
 The MAC is calculated over the EAP packet above (with MAC value set
 to zero), followed by the SRES values (a total of 40 bytes).

A.7. EAP-Success

 The last packet is an EAP-Success:
    03                   ; Code: Success
    02                   ; Identifier: 2
    00 04                ; Length: 4 octets

A.8. Fast Re-authentication

 When performing fast re-authentication, the EAP-Request/Identity
 packet is the same as usual.  The EAP-Response/Identity contains the
 fast re-authentication identity (from AT_ENCR_DATA attribute above):
    02                   ; Code: Response
    00                   ; Identifier: 0
    00 56                ; Length: 86 octets
    01                   ; Type: Identity
       59 32 34 66 4e 53 72 7a 38 42 50 32 37 34 6a 4f
       4a 61 46 31 37 57 66 78 49 38 59 4f 37 51 58 30
       30 70 4d 58 6b 39 58 4d 4d 56 4f 77 37 62 72 6f
       61 4e 68 54 63 7a 75 46 71 35 33 61 45 70 4f 6b
       6b 33 4c 30 64 6d 40 65 61 70 73 69 6d 2e 66 6f
       6f

Haverinen & Salowey Informational [Page 86] RFC 4186 EAP-SIM Authentication January 2006

A.9. EAP-Request/SIM/Re-authentication

 The server recognizes the reauthentication identity, so it will
 respond with EAP-Request/SIM/Re-authentication.  It retrieves the
 associated counter value, generates a nonce, and picks a new
 reauthentication identity (in this case,
 "uta0M0iyIsMwWp5TTdSdnOLvg2XDVf21OYt1vnfiMcs5dnIDHOIFVavIRzMR
 yzW6vFzdHW@eapsim.foo").
 The following plaintext will be encrypted and stored in the
 AT_ENCR_DATA attribute.  Note that AT_PADDING is not used because the
 length of the plaintext is a multiple of 16 bytes.
       13                ; Attribute type: AT_COUNTER
          01             ; Attribute length: 4 octets (1*4)
          00 01          ; Counter value
       15                ; Attribute type: AT_NONCE_S
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          01 23 45 67    ; NONCE_S value
          89 ab cd ef
          fe dc ba 98
          76 54 32 10
       85                ; Attribute type: AT_NEXT_REAUTH_ID
          16             ; Attribute length: 88 octets (22*4)
          00 51          ; Actual re-auth identity length: 81 octets
          75 74 61 30 4d 30 69 79 49 73 4d 77 57 70 35 54
          54 64 53 64 6e 4f 4c 76 67 32 58 44 56 66 32 31
          4f 59 74 31 76 6e 66 69 4d 63 73 35 64 6e 49 44
          48 4f 49 46 56 61 76 49 52 7a 4d 52 79 7a 57 36
          76 46 7a 64 48 57 40 65 61 70 73 69 6d 2e 66 6f
          6f
          00 00 00       ; (attribute padding)

Haverinen & Salowey Informational [Page 87] RFC 4186 EAP-SIM Authentication January 2006

 The EAP packet looks like this:
    01                   ; Code: Request
    01                   ; Identifier: 1
    00 a4                ; Length: 164 octets
    12                   ; Type: EAP-SIM
       0d                ; EAP-SIM subtype: Re-authentication
       00 00             ; (reserved)
       81                ; Attribute type: AT_IV
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          d5 85 ac 77    ; IV value
          86 b9 03 36
          65 7c 77 b4
          65 75 b9 c4
       82                ; Attribute type: AT_ENCR_DATA
          1d             ; Attribute length: 116 octets (29*4)
          00 00          ; (reserved)
          68 62 91 a9 d2 ab c5 8c aa 32 94 b6 e8 5b 44 84
          6c 44 e5 dc b2 de 8b 9e 80 d6 9d 49 85 8a 5d b8
          4c dc 1c 9b c9 5c 01 b9 6b 6e ca 31 34 74 ae a6
          d3 14 16 e1 9d aa 9d f7 0f 05 00 88 41 ca 80 14
          96 4d 3b 30 a4 9b cf 43 e4 d3 f1 8e 86 29 5a 4a
          2b 38 d9 6c 97 05 c2 bb b0 5c 4a ac e9 7d 5e af
          f5 64 04 6c 8b d3 0b c3 9b e5 e1 7a ce 2b 10 a6
       0b                ; Attribute type: AT_MAC
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          48 3a 17 99    ; MAC value
          b8 3d 7c d3
          d0 a1 e4 01
          d9 ee 47 70
 The MAC is calculated over the EAP packet above (with MAC value set
 to zero; a total of 164 bytes).
 Finally, the server derives new keys.  The XKEY' is calculated as
 described in Section 7*:
 XKEY' = 863dc120 32e08343 c1a2308d b48377f6 801f58d4

Haverinen & Salowey Informational [Page 88] RFC 4186 EAP-SIM Authentication January 2006

 The new MSK and EMSK are derived using the PRNG (note that K_encr and
 K_aut stay the same).
       MSK   =  6263f614 973895e1 335f7e30 cff028ee
                2176f519 002c9abe 732fe0ef 00cf167c
                756d9e4c ed6d5ed6 40eb3fe3 8565ca07
                6e7fb8a8 17cfe8d9 adbce441 d47c4f5e
       EMSK  =  3d8ff786 3a630b2b 06e2cf20 9684c13f
                6b82f992 f2b06f1b 54bf51ef 237f2a40
                1ef5e0d7 e098a34c 533eaebf 34578854
                b7721526 20a777f0 e0340884 a294fb73

A.10. EAP-Response/SIM/Re-authentication

 The client's response includes the counter as well.  The following
 plaintext will be encrypted and stored in the AT_ENCR_DATA attribute:
       13                ; Attribute type: AT_COUNTER
          01             ; Attribute length: 4 octets (1*4)
          00 01          ; Counter value
       06                ; Attribute type: AT_PADDING
          03             ; Attribute length: 12 octets (3*4)
          00 00 00 00
          00 00 00 00
          00 00
 The EAP packet looks like this:
    02                   ; Code: Response
    01                   ; Identifier: 1
    00 44                ; Length: 68 octets
    12                   ; Type: EAP-SIM
       0d                ; EAP-SIM subtype: Re-authentication
       00 00             ; (reserved)
       81                ; Attribute type: AT_IV
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          cd f7 ff a6    ; IV value
          5d e0 4c 02
          6b 56 c8 6b
          76 b1 02 ea
       82                ; Attribute type: AT_ENCR_DATA
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          b6 ed d3 82
          79 e2 a1 42
          3c 1a fc 5c
          45 5c 7d 56

Haverinen & Salowey Informational [Page 89] RFC 4186 EAP-SIM Authentication January 2006

       0b                ; Attribute type: AT_MAC
          05             ; Attribute length: 20 octets (5*4)
          00 00          ; (reserved)
          fa f7 6b 71    ; MAC value
          fb e2 d2 55
          b9 6a 35 66
          c9 15 c6 17
 The MAC is calculated over the EAP packet above (with MAC value set
 to zero), followed by the NONCE_S value (a total of 84 bytes).
 The next packet will be EAP-Success:
    03                   ; Code: Success
    01                   ; Identifier: 1
    00 04                ; Length: 4 octets

Appendix B. Pseudo-Random Number Generator

 The "|" character denotes concatenation, and "^" denotes
 exponentiation.
 Step 1: Choose a new, secret value for the seed-key, XKEY
 Step 2: In hexadecimal notation let
     t = 67452301 EFCDAB89 98BADCFE 10325476 C3D2E1F0
     This is the initial value for H0|H1|H2|H3|H4
     in the FIPS SHS [SHA-1]
 Step 3: For j = 0 to m - 1 do
       3.1 XSEED_j = 0 /* no optional user input */
       3.2 For i = 0 to 1 do
           a. XVAL = (XKEY + XSEED_j) mod 2^b
           b. w_i = G(t, XVAL)
           c. XKEY = (1 + XKEY + w_i) mod 2^b
       3.3 x_j = w_0|w_1

Haverinen & Salowey Informational [Page 90] RFC 4186 EAP-SIM Authentication January 2006

Authors' Addresses

 Henry Haverinen (editor)
 Nokia Enterprise Solutions
 P.O. Box 12
 FIN-40101 Jyvaskyla
 Finland
 EMail: henry.haverinen@nokia.com
 Joseph Salowey (editor)
 Cisco Systems
 2901 Third Avenue
 Seattle, WA  98121
 USA
 Phone: +1 206 256 3380
 EMail: jsalowey@cisco.com

Haverinen & Salowey Informational [Page 91] RFC 4186 EAP-SIM Authentication January 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Acknowledgement

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Haverinen & Salowey Informational [Page 92]

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