

Point-to-Point Extensions Working Group           H. Haverinen (editor) 
Internet Draft                                                    Nokia 
                                                    J. Salowey (editor) 
                                                                  Cisco 
                                                              June 2003 
                                                                        
 
 
                         EAP SIM Authentication 
                 draft-haverinen-pppext-eap-sim-11.txt 
 
 
Status of this Memo 

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

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

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

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

   The list of Internet-Draft Shadow Directories can be accessed at: 
        http://www.ietf.org/shadow.html. 

   This document is an individual submission for the Point-to-Point 
   Extensions Working Group of the Internet Engineering Task Force 
   (IETF).  Comments should be submitted to the ietf-ppp@merit.edu 
   mailing list. 

   Distribution of this memo is unlimited. 

Abstract 

   This document specifies an Extensible Authentication Protocol (EAP) 
   mechanism for authentication and session key distribution using the 
   GSM Subscriber Identity Module (SIM). The 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 and a re-authentication 
   procedure. 



  
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Table of Contents 

    
   Status of this Memo.........................................1 
   Abstract....................................................1 
   Table of Contents...........................................2 
   1. Introduction.............................................3 
   2. Terms....................................................4 
   3. Overview.................................................5 
   4. Version Negotiation......................................7 
   5. Identity Management......................................8 
   5.1. User identity in EAP-Response/Identity.................8 
   5.2. Obtaining Subscriber Identity via EAP/SIM Messages....10 
   5.3. Identity Privacy Support..............................13 
   6. Re-Authentication.......................................20 
   7. Message Format..........................................25 
   8. Message Authentication and Encryption...................26 
   8.1. AT_MAC Attribute......................................26 
   8.2. AT_CHECKCODE Attribute................................27 
   8.3. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes.........29 
   9. EAP-Request/SIM/Start...................................30 
   10. EAP-Response/SIM/Start.................................32 
   11. EAP-Request/SIM/Challenge..............................34 
   12. EAP-Response/SIM/Challenge.............................38 
   13. EAP-Request/SIM/Re-authentication......................39 
   14. EAP-Response/SIM/Re-authentication.....................43 
   15. Error Cases and the Usage of EAP-Failure and EAP-Success45 
   15.1. Processing Erroneous Packets.........................45 
   15.2. EAP-Failure..........................................46 
   15.3. EAP-Success..........................................46 
   16. EAP/SIM Notifications..................................46 
   17. Key Generation.........................................50 
   18. IANA Considerations....................................52 
   19. Security Considerations................................53 
   19.1. Identity Protection..................................53 
   19.2. Mutual Authentication and Triplet Exposure...........53 
   19.3. Key Derivation.......................................54 
   19.4. Dictionary Attacks...................................56 
   19.5. Credentials Reuse....................................56 
   19.6. Integrity Protection, Replay Protection and Confidentiality
       56 
   19.7. Negotiation Attacks..................................57 
   19.8. Fast Reconnect.......................................57 
   19.9. Acknowledged Result Indications......................58 
   19.10. Man-in-the-middle Attacks...........................58 
   19.11. Generating Random Numbers...........................58 
   20. Security Claims........................................58 
   21. Intellectual Property Right Notice.....................59 
   22. Acknowledgements and Contributions.....................59 
   References.................................................60 
   Editors' and Contributors' Contact Information.............62 
   Annex A. Test Vectors......................................63 
   Annex B. Pseudo-Random Number Generator....................64 
  
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1. Introduction 

   This document specifies an Extensible Authentication Protocol (EAP) 
   [1] mechanism for authentication and session key distribution using 
   the GSM Subscriber Identity Module (SIM). 

   GSM authentication is based on a challenge-response mechanism. The 
   A3/A8 authentication algorithms that run on the SIM can be given a 
   128-bit random number (RAND) as a challenge. The SIM runs an 
   operator-specific algorithm, which takes the RAND and a secret key 
   Ki stored on the SIM as input, and produces 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 not 
   directly used. Please find more information about GSM authentication 
   in [2]. 

   In EAP/SIM, several RAND challenges are used for generating several 
   64-bit Kc keys, which are combined to constitute stronger keying 
   material. EAP/SIM also enhances the basic GSM authentication 
   mechanism by accompanying the RAND challenges and other messages 
   with a message authentication code in order to provide mutual 
   authentication. 

   EAP/SIM specifies optional support for protecting the privacy of 
   subscriber identity and an optional re-authentication procedure. 

   The security of EAP/SIM builds on underlying GSM mechanisms. The 
   security properties of EAP/SIM are documented in Section 19 of this 
   document. Implementers and users of EAP/SIM are advised to carefully 
   study the security considerations in Section 19 in order to 
   determine whether the security properties are sufficient for the 
   environment in question. 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 not re-used 
   from GSM/GPRS. In any case, if the GSM authentication mechanisms are 
   considered to be sufficient for use on the cellular networks, then 
   EAP/SIM is expected to be sufficiently secure for other networks. 

   The 3rd Generation Partnership Project (3GPP) has specified an 
   enhanced Authentication and Key Exchange (AKA) architecture for the 
   Universal Mobile Telecommunications System (UMTS). The UMTS AKA 
   mechanism includes mutual authentication, replay protection and 
   derivation of longer session keys. EAP AKA [21] specifies an EAP 
   method that is based on UMTS AKA. EAP AKA may be used instead of 
   EAP/SIM if the security properties of EAP/SIM are not considered 
   sufficient. 




  
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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 RFC 2119 [3]. 

   This document frequently uses the following terms and abbreviations: 

   AAA protocol 

      Authentication, Authorization and Accounting protocol 

   AAA server 

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

   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. 

   Client 

      The entity that processes the EAP protocol on the supplicant. 
      Typically, it is the end that needs to be authenticated by the 
      authenticator. The Client includes a SIM that provides the 
      subscriber credentials and securely executes sensible 
      cryptographic calculations. 

   EAP 

      Extensible Authentication Protocol. 

   EAP Server 

      The network element that terminates the EAP protocol and performs 
      the authentication of the EAP/SIM client. In this document, we 
      assume that  the EAP server functionality is implemented in a AAA 
      server. 

   GSM 

      Global System for Mobile communications. 




  
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   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 

   SIM 

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

3. Overview 

   Figure 1 shows an overview of the EAP/SIM full authentication 
   procedure. This version of EAP/SIM exchange uses three roundtrips to 
   authenticate the user and generate keying material. In this 
   document, the term EAP Server refers to the network element that 
   terminates the EAP protocol. The Authenticator typically 
   communicates with the user's EAP server using an AAA protocol. The 
   AAA communications is not shown in the figure. 

   The first EAP Request issued by the Authenticator is EAP-
   Request/Identity. The client's response includes either the user's 
   International Mobile Subscriber Identity (IMSI) or a temporary 
   identity (pseudonym), as specified in Section 5.3. 

   Following the client's EAP-Response/Identity packet, the client 
   receives EAP Requests of type 18 (SIM) from the Authenticator 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 7. 

   The EAP-Request/SIM/Start packet contains the list of EAP/SIM 
   version supported by the Authenticator in the AT_VERSION_LIST 
   attribute. This packet may also include attributes for requesting 
   the subscriber identity, as specified in Section 5.3. 

  
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   The client responds to 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 client, and 
   the AT_SELECTED_VERSION attribute that contains the version number 
   selected by the client. The version negotiation is protected by 
   including the version list and the selected version in the 
   calculation of keying material (Section 17). The client MUST NOT 
   reuse the NONCE_MT value from previous sessions but the client MUST 
   choose it freshly for each EAP/SIM authentication exchange. The 
   client SHOULD use a good source of randomness to generate NONCE_MT. 

   In this document, we assume that the EAP server is implemented on 
   the AAA server and has an interface to the GSM network, so it 
   operates as a gateway between the Internet AAA network and the GSM 
   authentication infrastructure. After receiving the EAP 
   Response/SIM/Start, the EAP server obtains n GSM triplets from the 
   user's home operator's Authentication Centre (AuC) on the GSM 
   network, where n = 1, n = 2 or n = 3. From the triplets, the EAP 
   server derives the keying material, as specified in Section 17.  

   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 EAP server MUST NOT reuse the RAND values (triplets) from 
   previous successful sessions but the server MUST obtain fresh RANDs 
   for each EAP/SIM authentication exchange. However, if client 
   authentication fails, the server MAY reuse the RANDs in the next 
   authentication attempt. 

   On receipt of the EAP-Request/SIM/Challenge message, the client runs 
   the GSM authentication algorithm and calculates a copy of the 
   message authentication code. The client then verifies that the 
   calculated MAC equals the received MAC. If the MAC's do not match, 
   then the client silently ignores the EAP packet and does not send 
   any authentication values to the network. Eventually, if another 
   EAP-Request/SIM/Challenge packet with a valid AT_MAC is not 
   received, the connection establishment will time out. 

   Since the RAND's given to a client are accompanied with the message 
   authentication code AT_MAC, and since the client's NONCE_MT value 
   contributes to AT_MAC, the client is able to verify that the EAP SIM 
   message is fresh (not a replay) and that the sender possesses valid 
   GSM triplets for the subscriber.  

   If all checks out, the client responds with the EAP-
   Response/SIM/Challenge, containing the AT_MAC attribute that covers 
   the client's SRES response values (Section 12). The EAP server 
   verifies that the MAC is correct and sends the EAP-Success packet, 
   indicating that the authentication was successful. The EAP server 
   may also include derived keying material in the message it sends to 
   the Authenticator. 
  
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   The EAP-Request/SIM/Challenge, EAP-Response/SIM/Challenge, or the 
   packets used on re-authentication may optionally include the 
   AT_CHECKCODE attribute, which enables the protocol peers to ensure 
   the integrity of the EAP-Request/SIM/Start and EAP-
   Response/SIM/Start packets. AT_CHECKCODE is specified in Section 
   8.2. 

     Client                                               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)                          | 
       |<---------------------------------------------------------| 
       |                                                          | 
   +-------------------------------------+                        | 
   | Client 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 

   EAP SIM also includes a separate re-authentication procedure, which 
   does not make use of the A3/A8 algorithms or the GSM infrastructure. 
   Re-authentication is based on keys derived on full authentication. 

4. 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 
  
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   (Section 9), which the server includes in EAP-Request/SIM/Start, and 
   AT_SELECTED_VERSION (Section 10), which the client includes in EAP-
   Response/SIM/Start. 

   AT_VERSION_LIST includes the EAP/SIM versions supported by the 
   server. The server MUST only include versions that it implements and 
   that are allowed in its security policy. The versions are listed in 
   the order of preference, most preferred versions first. At least one 
   version number MUST be included. The version number for the protocol 
   described in this document is one (0x0001). 

   If AT_VERSION_LIST does not include a version that is implemented by 
   the client and allowed in the client's security policy, then the 
   client MUST silently ignore the EAP-Request/SIM/Start packet. If a 
   suitable version is included, then the client includes the 
   AT_SELECTED_VERSION attribute, containing the selected version, in 
   the EAP-Response/SIM/Start packet. The client MUST only indicate a 
   version that is included in AT_VERSION_LIST. If several versions are 
   acceptable, then the client 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 17). If an attacker modifies either one of these 
   attributes, then the client and the server will derive different 
   keying material. Because K_aut keys are different, the server and 
   client will calculate different AT_MAC values. Hence, the client 
   will detect that AT_MAC is incorrect and discard the EAP-
   Request/SIM/Challenge packet. The authentication procedure will time 
   out. 

5. Identity Management 

5.1. User identity in EAP-Response/Identity 

   In the beginning of EAP authentication, the Authenticator issues the 
   EAP-Request/Identity packet to the client. The client responds with 
   EAP-Response/Identity, which contains the user's identity. The 
   formats of these packets are specified in [1]. 

   GSM subscribers are identified with the International Mobile 
   Subscriber Identity (IMSI) [4]. The IMSI is composed of a three 
   digit Mobile Country Code (MCC), a two or three digit Mobile Network 
   Code (MNC) and a not more than 10 digit Mobile Subscriber 
   Identification Number (MSIN). In other words, the IMSI is a string 
   of not more than 15 digits. MCC and MNC uniquely identify the GSM 
   operator. 

   Internet AAA protocols identify users with the Network Access 
   Identifier (NAI) [5]. 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. The AAA nodes use the realm portion of the NAI to 
  
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   route AAA requests to the correct AAA server. The realm name used in 
   this protocol MAY be chosen by the operator and it MAY a 
   configurable parameter in the EAP/SIM client implementation. In this 
   case, the client 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 reserved NAI realm may be 
   useful as a hint as to the first authentication method to use during 
   method negotiation. 

   There are three types of NAI username portions in EAP/SIM: non-
   pseudonym permanent usernames, pseudonym usernames and re-
   authentication usernames. The first two are only used on full 
   authentication and the last one only on re-authentication. When the 
   optional identity privacy support is not used, the non-pseudonym 
   permanent username is used.  

   The non-pseudonym permanent username MAY be derived from the IMSI. 
   In this case, the permanent username MUST be of the format "1imsi". 
   In other words, the first character of the username is the digit one 
   (ASCII value 0x31), followed by the IMSI. The IMSI is an ASCII 
   string that consists of not more than 15 decimal digits (ASCII 
   values between 0x30 and 0x39) as specified in [4]. 

   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 a local matter. In this 
   case, the client implementation MUST NOT prepend any leading 
   characters to the username. 

   When the optional identity privacy support is used on full 
   authentication, the client MAY use the pseudonym received as part of 
   the previous full authentication sequence as the username portion of 
   the NAI, as specified in Section 5.3. The client MUST NOT modify the 
   pseudonym received in AT_NEXT_PSEUDONYM. For example, the client 
   MUST NOT prepend any leading characters in the pseudonym. 

   On re-authentication, the client uses the re-authentication identity 
   received as part of the previous authentication sequence as the NAI. 
   A new re-authentication identity may be delivered as part of both 
   full authentication and re-authentication. The client MUST NOT 
   modify the re-authentication identity received in AT_NEXT_REAUTH_ID. 
   For example, the client MUST NOT prepend any leading characters in 
   the re-authentication identity. 

   If no configured realm name is available, the client MAY derive the 
   realm name from the MCC and MNC portions of the IMSI. A recommended 
   way to derive the realm from the IMSI will be specified in [6]. 
  
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   Alternatively, the realm name may be obtained by concatenating 
   "mnc", the MNC digits of IMSI, ".mcc", the MCC digits of IMSI and 
   ".owlan.org". For example, if the IMSI is 123456789098765, and the 
   MNC is three digits long, then the derived realm name is 
   "mnc456.mcc123.owlan.org". 

   If the client is not able to determine whether the MNC is two or 
   three digits long, the client MAY use a 3-digit MNC. If the correct 
   length of the MNC is two, then the MNC used in the realm name will 
   include the first digit of MSIN. Hence, when configuring AAA 
   networks for operators that have 2-digit MNC's, the network SHOULD 
   also be prepared for realm names with incorrect 3-digit MNC's. 

5.2. Obtaining Subscriber Identity via EAP/SIM Messages 

   It may be useful to obtain the identity of the subscriber through 
   means other than EAP Request/Identity. This can eliminate the need 
   for an identity request when using EAP method negotiation. If this 
   was not possible then it might not be possible to negotiate EAP/SIM 
   as the second method since it is not specified how to deal with a 
   new EAP Request/Identity.  

   If the EAP server has not received any identity (permanent identity, 
   pseudonym or re-authentication identity) from the client when 
   sending the first EAP/SIM request, then the EAP server SHOULD issue 
   the EAP-Request/SIM/Start packet and includes the AT_ANY_ID_REQ 
   attribute (specified in Section 9). This attribute does not contain 
   any data.  

   If the EAP server has received an EAP-Response/Identity packet but 
   the contents do not appear to be a valid permanent identity, 
   pseudonym or a re-authentication identity, the EAP server SHOULD 
   issue an EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ 
   attribute.  

   In some environments the intermediate entities or software layers in 
   the client may modify the identity string in the EAP-
   Response/Identity packet. For example, some EAP layer 
   implementations may cache the identity string from the first 
   authentication and do not obtain a new identity string from the EAP 
   method implementation on subsequent authentication exchanges. 
   Because the identity string is used in key derivation, such 
   modifications will result in failed authentication unless the EAP 
   server uses the AT_ANY_ID_REQ attribute to obtain an unmodified copy 
   of the identity string. Therefore, in cases when there is a 
   possibility that an intermediate element or software layer may 
   modify the EAP-Response/Identity packet, the EAP server SHOULD 
   always use the AT_ANY_ID_REQ attribute, even if the identity 
   received in EAP-Response/Identity was valid. 

   The AT_ANY_ID_REQ attribute requests the client to include the 
   AT_IDENTITY attribute (specified in Section 10) in the EAP-
   Response/SIM/Start packet. The identity format in the AT_IDENTITY 
  
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   attribute is the same as in the EAP-Response/Identity packet. The 
   AT_IDENTITY attribute contains a permanent identity, a pseudonym 
   identity or a re-authentication identity. If the server does not 
   support re-authentication, it uses the AT_FULLAUTH_ID_REQ attribute 
   instead of the AT_ANY_ID_REQ attribute to directly request for a 
   full authentication identity (either the permanent identity or a 
   pseudonym identity). If the server uses the AT_FULLAUTH_ID_REQ 
   attribute, the client MUST NOT use a re-authentication identity in 
   the AT_IDENTITY attribute.  

   The use of pseudonyms for anonymity is specified in Section 5.3. The 
   use of re-authentication identities is specified in Section 6. 

   This case for full authentication is illustrated in the figure 
   below. 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. 

   Client                                             Authenticator 
          |                                                       | 
          |                            +------------------------------+ 
          |                            | Server does not have any     | 
          |                            | 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)                                 | 
          |------------------------------------------------------>| 
          |                                                       | 
    

   If the client wants to perform full authentication, it includes the 
   permanent identity or a pseudonym identity in the AT_IDENTITY 
   attribute. The client may use these identities in response to either 
   AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ. In this case, the client MUST 
   include AT_NONCE_MT and AT_SELECTED_VERSION attributes in EAP-
   Response/SIM/Start message, as required on full authentication.  

   If the server uses the AT_ANY_ID_REQ and the client wants to perform 
   re-authentication, then the client includes a re-authentication 
   identity in the AT_IDENTITY attribute. On re-authentication, the 
   client MUST NOT include AT_NONCE_MT or AT_SELECTED_VERSION 
   attributes. This case is illustrated below. 


  
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   Client                                             Authenticator 
          |                                                       | 
          |                            +------------------------------+ 
          |                            | Server does not have any     | 
          |                            | 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 re-authentication identity) | 
          |------------------------------------------------------>| 
          |                                                       | 
    

   If the client 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 decode to the 
   permanent identity, then the full authentication sequence proceeds 
   as usual with the EAP Server issuing the EAP-Request/SIM/Challenge 
   message. 

   On re-authentication, if the AT_IDENTITY attribute contains a valid 
   re-authentication identity and the server agrees on using re-
   authentication, then the server proceeds with the re-authentication 
   sequence and issues the EAP-Request/SIM/Re-authentication packet, as 
   specified in Section 6. If the server does not recognize the re-
   authentication identity, then the server issues a second EAP-
   Request/SIM/Start message and includes the AT_FULLAUTH_ID_REQ 
   attribute. In this case, a second EAP/SIM/Start round trip is 
   required. The messages used on the first roundtrip are ignored. 
   (However, all EAP/SIM/Start messages are taken into account when 
   calculating the checkcode for AT_CHECKCODE. AT_CHECKCODE is 
   specified in Section 8.2.) This is illustrated below. 















  
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   Client                                             Authenticator 
          |                                                       | 
          |                            +------------------------------+ 
          |                            | Server does not have any     | 
          |                            | 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 re-authentication identity) | 
          |------------------------------------------------------>| 
          |                                                       | 
          |                            +------------------------------+ 
          |                            | Server does not recognize    | 
          |                            | The re-authentication        | 
          |                            | 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)                                 | 
          |------------------------------------------------------>| 
          |                                                       | 
    

   If the server recognizes the re-authentication identity, but still 
   wants to fall back on full authentication, the server may issue the 
   EAP-Request/SIM/Start packet without any identity request attributes 
   (AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ). In this case, the 
   server only includes the AT_VERSION_LIST attribute, and full 
   authentication proceeds as usual. The client does not include any 
   identity attributes in the EAP-Response/SIM/Start packet.  

   An extra EAP/SIM/Start round trip is also required in cases when the 
   AT_IDENTITY attribute contains a pseudonym identity that the EAP 
   server fails to decode. The operation in this case is specified in 
   Section 5.3. 

5.3. Identity Privacy Support 

   EAP/SIM includes optional identity privacy (anonymity) support that 
   can be used to hide the cleartext permanent identity and to make the 
   subscriber's connections unlinkable to eavesdroppers. Identity 
  
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   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 19.1 for security considerations regarding identity privacy. 

   If identity privacy is not used or if the client does not have any 
   pseudonyms or re-authentication identities are available, the client 
   transmits the permanent identity in the EAP-Response/Identity packet 
   or in the AT_IDENTITY attribute. 

   The EAP-Request/SIM/Challenge message MAY include an encrypted 
   pseudonym in the value field of the AT_ENCR_DATA attribute. The 
   AT_IV and AT_MAC attributes are also used to transport the pseudonym 
   to the client, as described in Section 11. Because the identity 
   privacy support is optional to implement, the client MAY ignore the 
   AT_IV and AT_ENCR_DATA attributes and always transmit the permanent 
   identity in the EAP-Response/Identity packet and in the AT_IDENTITY 
   attribute. 

   On receipt of the EAP-Request/SIM/Challenge, the client verifies the 
   AT_MAC attribute before looking at the AT_ENCR_DATA attribute. If 
   the AT_MAC is invalid, then the client MUST silently discard the EAP 
   packet. If the AT_MAC attribute is valid, then the client MAY 
   decrypt the encrypted data in AT_ENCR_DATA and use the obtained 
   pseudonym on the next full authentication.  

   If the client does not receive a new pseudonym in the EAP-
   Request/SIM/Challenge message, the client MAY use an old pseudonym 
   instead of the permanent identity on next full authentication. 

   The EAP server produces pseudonyms in an implementation-dependent 
   manner. Please see [7] for examples on how to produce pseudonyms. 
   Only the EAP server needs to be able to map the pseudonym to the 
   permanent identity. Regardless of construction method, the pseudonym 
   MUST conform to the grammar specified for the username portion of an 
   NAI. 

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

   In general, it is the task of the EAP server and the policies of its 
   administrator to ensure sufficient separation in the usernames. 
   Pseudonyms, for instance, are both produced and used by the EAP 
   server. The EAP server MUST compose pseudonyms so that it can 
   recognize if a NAI username is an EAP SIM pseudonym. For instance, 
   when the usernames have been derived from the IMSI, the pseudonym 
   could begin with a leading "3" character. 

   On the next full authentication with the EAP server, the client MAY 
   transmit the received pseudonym in the first EAP-Response/Identity 
   packet. The client concatenates the received pseudonym with the "@" 
  
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   character and the NAI realm portion. The client selects the realm 
   name portion similarly as it select the realm name portion when 
   using the permanent identity. If the EAP server successfully decodes 
   the pseudonym received in the EAP-Response/Identity packet to a 
   known client permanent identity, the authentication proceeds with 
   the EAP-Request/SIM/Start message as usual. 

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

   If the EAP server requests the client to include its identity in the 
   EAP-Response/SIM/Start packet, as specified in Section 5.2, the 
   client MAY transmit the received pseudonym in the AT_IDENTITY 
   attribute. If the EAP server successfully decodes the pseudonym to a 
   known identity, then the authentication proceeds with the EAP-
   Request/SIM/Challenge packet as usual. 

   If the EAP server fails to decode the pseudonym to a known identity, 
   then the EAP server requests the permanent identity (non-pseudonym 
   identity) by including the AT_PERMANENT_ID_REQ attribute (Section 9) 
   in the EAP-Request/SIM/Start message. Because another EAP server may 
   have generated the pseudonym using a different coding scheme, the 
   EAP server SHOULD use AT_PERMANENT_ID_REQ also in cases when it does 
   not recognize the format of the client identity. 

   The EAP server issues the EAP-Request/SIM/Start message also in the 
   case when it received the undecodable pseudonym in AT_IDENTITY 
   included the EAP-Response/SIM/Start packet. In this case, an extra 
   EAP/SIM/Start round trip is required. 

   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 client, in an effort to find out the true identity of the user. 
   The client MAY silently discard any EAP-Request/SIM/Start messages 
   that include AT_PERMANENT_ID_REQ for a while in order to wait for an 
   EAP-Request/SIM/Start packet without AT_PERMANENT_ID_REQ. If the 
   valid network sent the message, the message will be retransmitted, 
   so the client can reconsider replying to the message when it 
   receives a retransmission. 

   Basically, there are two different policies that the client can 
   employ with regard to AT_PERMANENT_ID_REQ. A "conservative" client 
   assumes that the network is able to maintain pseudonyms robustly. 
   Therefore, if a conservative client has a pseudonym, the client 
   silently ignores the EAP packet with AT_PERMANENT_ID_REQ, because 
   the client believes that the valid network is able to decode the 
   pseudonym. (Alternatively, the conservative client may respond to 
   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 client against active attacks on anonymity. 
  
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   On the other hand, a "liberal" client 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 decode them to the 
   permanent identity. 

   Regardless how the identity is communicated to the server, the full 
   authentication message sequence and the attributes are the same in 
   all cases. For example, AT_NONCE_MT and AT_SELECTED_VERSION are 
   always included in the EAP-Response/SIM/Start packet on full 
   authentication, even if they were already transmitted in the 
   previous EAP-Response/SIM/Start. AT_VERSION_LIST is also included in 
   every EAP-Request/SIM/Start message. The values used on the last 
   EAP/SIM/Start round trip are used and the previous EAP/SIM/Start 
   round trips is ignored. (However, all EAP/SIM/Start rounds are taken 
   into account when calculating the checkcode for AT_CHECKCODE. 
   AT_CHECKCODE is specified in Section 8.2). The NONCE_MT value and 
   the version negotiation attributes included in the last EAP-
   Response/SIM/Start packet are used in all calculations. The EAP/SIM 
   client MAY use the same NONCE_MT value in both EAP-
   Response/SIM/Start packets. 

   The value field of the AT_PERMANENT_ID_REQ does not contain any data 
   but the attribute is included to request the client to include the 
   AT_IDENTITY attribute (Section 10) with the permanent authentication 
   identity in the EAP-Response/SIM/Start message. In this case, the 
   AT_IDENTITY attribute contains the client's permanent identity in 
   the clear.  

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

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













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

   If the server does not recognize the permanent identity, or if the 
   server is not able to continue the authentication exchange with the 
   client after receiving the permanent identity, then the server 
   issues the EAP Failure packet and the authentication exchange 
   terminates. 

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
















  
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   Client                                             Authenticator 
          |                                                       | 
          |                            +------------------------------+ 
          |                            | Server does not have any     | 
          |                            | 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 decode the   | 
          |                            | Pseudonym in AT_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)                    | 
          |------------------------------------------------------>| 
          |                                                       | 
    
   In the worst case, there are three EAP/SIM/Start round trips before 
   the server has obtained an acceptable identity: on the first round, 
   the client sends its re-authentication identity in AT_IDENTITY. The 
   server fails to accept it and request a full authentication identity 
   with a second EAP-Request/SIM/Start. The client responds with a 
   pseudonym identity in AT_IDENTITY. The server fails to decode the 
   pseudonym and has to issue a third EAP-Request/SIM/Start, including 
   AT_PERMANENT_ID_REQ. Finally, the server accepts the client's EAP-
   Response/SIM/Start with the AT_IDENTITY attribute and proceeds with 
   full authentication. This is illustrated in the figure below. 








  
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       Client                                             Authenticator 
          |                                                       | 
          |                            +------------------------------+ 
          |                            | Server does not have any     | 
          |                            | 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 re-authentication identity)         | 
          |------------------------------------------------------>| 
          |                                                       | 
          |                            +------------------------------+ 
          |                            | Server does not accept       | 
          |                            | The re-authentication        | 
          |                            | 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 decode the   | 
          |                            | Pseudonym in AT_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)                                 | 
          |------------------------------------------------------>| 
          |                                                       | 
    
   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. If the server does not 
   recognize the permanent identity, or if the server is not able to 
   continue the authentication exchange with the client after receiving 
  
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   the permanent identity, then the server issues the EAP Failure 
   packet and the authentication exchange terminates. 

6. Re-Authentication 

   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 it therefore requires 2 or 
   3 fresh triplets from the Authentication Centre, the full 
   authentication procedure is not very well suitable for frequent use. 
   Therefore, EAP SIM includes a more inexpensive 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.  

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

   Re-authentication is based on the keys derived on the preceding full 
   authentication. The same K_aut and K_encr keys as in full 
   authentication are used to protect EAP 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 17.  

   On re-authentication, the client protects against replays with an 
   unsigned 16-bit counter, included in the AT_COUNTER attribute. On 
   full authentication, both the server and the client initialize the 
   counter to one. The counter value of at least one is used on the 
   first re-authentication. On subsequent re-authentications, the 
   counter MUST be greater than on any of the previous re-
   authentications. For example, on the second re-authentication, 
   counter value is two or greater etc. The AT_COUNTER attribute is 
   encrypted. 

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

   As discussed in Section 5.3, in some environments the client may 
   assume that the network can reliably store pseudonyms and therefore 
   the client may fail to respond to the AT_PERMANENT_ID_REQ attribute. 
   The network SHOULD store pseudonyms on a reliable database. Because 
   one of the objectives of the re-authentication procedure is to 
   reduce load on the network, the re-authentication procedure does not 
   require the EAP server to contact a reliable database. Therefore, 
   the re-authentication procedure makes use of separate re-
   authentication user identities. Pseudonyms and the permanent 
   identity are reserved for full authentication only. The network does 
  
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   not need to store re-authentication identities as carefully as 
   pseudonyms. If a re-authentication identity is lost and the network 
   does not recognize it, the EAP server can fall back on full 
   authentication. 

   If the EAP server supports re-authentication, it MAY include the 
   skippable AT_NEXT_REAUTH_ID attribute in the encrypted data of EAP-
   Request/SIM/Challenge message (Section 11). This attribute contains 
   a new re-authentication identity for the next re-authentication. The 
   client MAY ignore this attribute, in which case it will use full 
   authentication next time. If the client wants to use re-
   authentication, it uses this re-authentication identity on next 
   authentication. Even if the client has a re-authentication identity, 
   the client MAY discard the re-authentication identity and use a 
   pseudonym or the permanent identity instead, in which case full 
   authentication will be performed. 

   The re-authentication identity received in AT_NEXT_REAUTH_ID 
   contains both the username portion and the realm portion of the 
   Network Access Identifier. The EAP Server can choose an appropriate 
   realm part in order to have the AAA infrastructure route subsequent 
   re-authentication related requests to the same AAA server. For 
   example, the realm part MAY include a portion that is specific to 
   the AAA server. Hence, it is sufficient to store the context 
   required for re-authentication in the AAA server that performed the 
   full authentication. 

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

   Even if the client uses a re-authentication identity, the server may 
   want to fall back on full authentication, for example because the 
   server does not recognize the re-authentication identity or does not 
   want to use re-authentication. 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 client's identity from the re-authentication 
   identity, the server includes either the AT_FULLAUTH_ID_REQ or the 
   AT_PERMANENT_ID_REQ attribute in this EAP request. (As specified in 
   Sections 5.2 and 5.3, the server MAY use AT_ANY_ID_REQ, 
   AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ attributes if it does not 
   know the client's identity.) 

   Both the client and the server SHOULD have an upper limit for the 
   number of subsequent re-authentications allowed before a full 
   authentication needs to be performed. Because a 16-bit counter is 
   used in re-authentication, the theoretical maximum number of re-
   authentications is reached when the counter value reaches 0xFFFF. 


  
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   In order to use re-authentication, the client and the server need to 
   store the following values: Master Key, K_aut, K_encr, latest 
   counter value and the next re-authentication identity. 

   The following figure illustrates the re-authentication procedure. 
   Encrypted attributes are denoted with '*'. The client 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 client 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 the client to send its identity by 
   including the AT_ANY_ID_REQ attribute in the EAP-Request/SIM/Start 
   packet. 

   If the server recognizes the re-authentication identity and agrees 
   on using re-authentication, then the server sends the EAP-
   Request/SIM/Re-authentication packet to the client. This packet MUST 
   include the encrypted AT_COUNTER attribute, with a fresh counter 
   value, the encrypted AT_NONCE_S attribute that contains a random 
   number chosen by the server, the AT_ENCR_DATA and the AT_IV 
   attributes used for encryption, and the AT_MAC attribute that 
   contains a message authentication code over the packet. The packet 
   MAY also include an encrypted AT_NEXT_REAUTH_ID attribute that 
   contains the next re-authentication identity.  

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

   The client verifies that the counter value is fresh (greater than 
   any previously used value). The client also verifies that AT_MAC is 
   correct. The client MAY save the next re-authentication identity 
   from the encrypted AT_NEXT_REAUTH_ID for next time. If all checks 
   are successful, the client responds with the EAP-Response/SIM/Re-
   authentication packet, including the AT_COUNTER attribute with the 
   same counter value and the AT_MAC attribute. 

   The server verifies the AT_MAC attribute and also verifies that the 
   counter value is the same that it used in the EAP-Request/SIM/Re-
   authentication packet. If these checks are successful, the re-
   authentication has succeeded and the server sends the EAP-Success 
   packet to the client. 









  
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   Client                                             Authenticator 
          |                                                       | 
          |                               EAP-Request/Identity    | 
          |<------------------------------------------------------| 
          |                                                       | 
          | EAP-Response/Identity                                 | 
          | (Includes a re-authentication identity)               | 
          |------------------------------------------------------>| 
          |                                                       | 
          |                          +--------------------------------+ 
          |                          | Server recognizes the identity | 
          |                          | and agrees on using fast       | 
          |                          | re-authentication              | 
          |                          +--------------------------------+ 
          |                                                       | 
          |  EAP-Request/SIM/Re-authentication                    | 
          |  (AT_IV, AT_ENCR_DATA, *AT_COUNTER,                   | 
          |   *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC)            | 
          |<------------------------------------------------------| 
          |                                                       | 
          |                                                       | 
     +-----------------------------------------------+            | 
     | Client verifies AT_MAC and the freshness of   |            | 
     | the counter. Client MAY store the new 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  | 
          |<------------------------------------------------------| 
          |                                                       | 
    
   If the client 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 the following 
   figure. Encrypted attributes are denoted with '*'. 





  
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   Client                                             Authenticator 
          |                                                       | 
          |                               EAP-Request/Identity    | 
          |<------------------------------------------------------| 
          |                                                       | 
          | EAP-Response/Identity                                 | 
          | (Includes a 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 client has included AT_COUNTER_TOO_SMALL| 
          |            +----------------------------------------------+ 
          |                                                       | 
          |                        EAP-Request/SIM/Start          | 
          |                        (AT_VERSION_LIST)              | 
          |<------------------------------------------------------| 
          |                                                       | 
     +---------------------------------------------------------------+ 
     |                Normal full authentication follows.            | 
     +---------------------------------------------------------------+ 
          |                                                       | 
    

   In the figure above, the first three messages are similar to the 
   basic re-authentication case. When the client 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 client MUST ignore 
   the contents of the server's AT_NEXT_REAUTH_ID attribute. 

   On receipt of AT_COUNTER_TOO_SMALL, the server verifies AT_MAC and 
   verifies that AT_COUNTER contains the same as in the EAP-
   Request/SIM/Re-authentication packet. If not, the server silently 
   discards the EAP-Response/SIM/Re-authentication packet. 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 
  
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   identity, it MUST NOT include AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or 
   AT_PERMANENT_ID_REQ in the EAP-Request/SIM/Start. 

7. Message Format 

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

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

   Attribute Type 

      Indicates the particular type of attribute. The attribute type 
      values are listed in Section 18. 

   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. 

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

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

   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. 

  
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   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. Message Authentication and Encryption 

   This section specifies EAP/SIM attributes for attribute encryption 
   and EAP/SIM message authentication. 

   Because the K_encr and K_aut keys derived from the RAND challenges 
   (as specified in Section 17) are required to process the integrity 
   protection and encryption attributes, these attributes can only be 
   used in the EAP-Request/SIM/Challenge message and any EAP/SIM 
   messages sent after EAP-Requets/SIM/Challenge. For example, these 
   attributes cannot be used in EAP-Request/SIM/Start. 

8.1. AT_MAC Attribute 

   The AT_MAC attribute is used for EAP/SIM message authentication. The 
   AT_MAC attribute MUST be included in all EAP/SIM packets whenever 
   key material (K_aut) has been derived. However, AT_MAC MUST NOT be 
   included in packets with the EAP/SIM message Subtype Start, even if 
   key material from the previous authentication was available. 
   Messages that do not meet these conditions MUST be silently 
   discarded. 

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

   The contents of the message-specific data, if present, are specified 
   separately for each EAP/SIM message. The message-specific data is 
   included in order to protect data that is not transmitted with the 
   EAP packet. 

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

  
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   The MAC algorithm is HMAC-SHA1-128 [8] keyed hash value. (The HMAC-
   SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by 
   truncating the output to 16 bytes. Hence, the length of the MAC is 
   16 bytes.) The derivation of the authentication key (K_aut) used in 
   the calculation of the MAC is specified in Section 17.  

8.2. AT_CHECKCODE Attribute 

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

   The AT_CHECKCODE attribute MAY be used to protect the EAP/SIM Start 
   messages. AT_CHECKCODE is included in EAP-Request/SIM/Challenge 
   and/or EAP-Response/SIM/Challenge upon full authentication. In re-
   authentication, AT_CHECKCODE can be included in EAP-Request/SIM/Re-
   authentication and/or EAP-Response/SIM/Re-authentication. Because 
   the AT_MAC attribute is used in these messages, AT_CHECKCODE will be 
   integrity protected with AT_MAC. 

   The format of the AT_CHECKCODE attribute is shown below. 

    0                   1                   2                   3 
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   | AT_CHECKCODE  | Length        |           Reserved            | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                                                               | 
   |                     Checkcode (0 or 20 bytes)                 | 
   |                                                               | 
   |                                                               | 
   |                                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    

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

   The checkcode is a hash value, calculated with SHA1 [11], over the 
   EAP-Request/SIM/Start and EAP-Response/SIM/Start packets exchanged 
   in this authentication exchange. The packets are included in the 
   order that they were transmitted, that is, starting with the first 
   EAP-Request/SIM/Start message, followed by the corresponding EAP-
   Response/SIM/Start, followed by the second EAP-Request/SIM/Start (if 
   used) etc. EAP packets are included in the hash calculation "as-is", 
  
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   as they were transmitted or received. All reserved bytes, padding 
   bytes etc. that are specified for various attributes are included as 
   such, and the receiver must not reset them to zero. No delimiter 
   bytes, padding or any other framing are included between the EAP 
   packets when calculating the checkcode. 

   Messages are included in request/response pairs; in other words only 
   full "round trips" are included. Packets that are silently discarded 
   are not included. The EAP server must only include an EAP-
   Request/SIM/Start in the calculation once it has received a 
   corresponding response, with the same Identifier value. 
   Retransmissions or requests to which the server does not receive 
   response are not included. 

   The client must include the EAP-Request/SIM/Start and the 
   corresponding response in the calculation only if the client 
   receives a subsequent EAP-Request/SIM/Challenge, or a follow-up EAP-
   Request/SIM/Start with different attributes (attribute types) than 
   in the first EAP-Request/SIM/Start. After sending EAP-
   Response/SIM/Start, if the client receives another EAP-
   Request/SIM/Start with the same attributes as in the previous 
   request, then the client's response to the first request must have 
   been lost. In this case the client must not include the first 
   request and its response in the calculation of the checkcode. 

   The AT_CHECKCODE attribute is optional to implement. It is specified 
   in order to allow protecting the EAP/SIM/Start messages and any 
   future extensions to them. The implementation of AT_CHECKCODE is 
   recommended.  

   If the receiver of AT_CHECKCODE implements this attribute, then the 
   receiver MUST check that the checkcode is correct. If the checkcode 
   is invalid, the receiver must terminate the authentication exchange. 

   If the EAP_/SIM/Start messages are extended with new attributes then 
   AT_CHECKCODE must be implemented and used. More specifically, if the 
   server includes any other attributes than AT_PERMANENT_ID_REQ, 
   AT_FULLAUTH_ID_REQ, AT_ANY_ID_REQ or AT_VERSION_LIST in the EAP-
   Request/SIM/Start packet, then the server MUST include AT_CHECKCODE 
   in EAP-Request/SIM/Challenge or EAP-Request/SIM/Re-authentication. 
   If the client includes any other attributes than AT_NONCE_MT, 
   AT_IDENTITY or AT_SELECTED_VERSION in the EAP-Response/SIM/Start 
   message, then the client MUST include AT_CHECKCODE in EAP-
   Response/SIM/Challenge or EAP-Response/SIM/Re-authentication.  

   If the server implements the processing of any other attribute than 
   AT_NONCE_MT, AT_IDENTITY or AT_SELECTED_VERSION in the EAP-
   Response/SIM/Start message, then the server MUST implement 
   AT_CHECKCODE. In this case, if the server receives any other 
   attribute than AT_NONCE_MT, AT_IDENTITY or AT_SELECTED_VERSION in 
   the EAP-Response/SIM/Start message, then the server MUST check that 
   AT_CHECKCODE is present in EAP-Response/SIM/Challenge or EAP-

  
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   Response/SIM/Re-authentication. If AT_CHECKCODE is not included, the 
   server must terminate the authentication exchange. 

   Similarly, if the client implements the processing of any other 
   attribute than AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, 
   AT_ANY_ID_REQ or AT_VERSION_LIST in the EAP-Request/SIM/Start 
   packet, then the client MUST implement AT_CHECKCODE. In this case, 
   if the client receives any other attribute than AT_PERMANENT_ID_REQ, 
   AT_FULLAUTH_ID_REQ, AT_ANY_ID_REQ or AT_VERSION_LIST in the EAP-
   Request/SIM/Start packet, then the client MUST check that 
   AT_CHECKCODE is present in EAP-Request/SIM/Challenge or EAP-
   Request/SIM/Re-authentication. If the attribute was not included, 
   the client must terminate the authentication exchange. 

8.3. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes 

   AT_IV and AT_ENCR_DATA attributes can be used to transmit encrypted 
   information between the EAP/SIM client 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. Messages that do not meet this 
   condition MUST be silently discarded.  

   The sender of the AT_IV attribute chooses the initialization vector 
   by random. The sender MUST NOT reuse the initialization vector value 
   from previous EAP SIM packets but the sender MUST choose it freshly 
   for each AT_IV attribute. The sends SHOULD use a good source of 
   randomness to generate the initialization vector. 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 bytes encrypted using the Advanced 
   Encryption Standard (AES) [9] 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 [10] for a description of the CBC 
   mode. The format of the AT_ENCR_DATA attribute is shown below. 

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

   The derivation of the encryption key (K_encr) is specified in 
   Section 17. 

   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 (0x00) on sending. The recipient of the message MUST 
   verify that the pad bytes are set to zero, and silently drop the 
   message if this verification fails. 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...                    | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               | 
   |                                                               | 
   |                                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    
9. EAP-Request/SIM/Start 

   The first SIM specific EAP Request is of subtype Start. The 
   EAP/SIM/Start roundtrip is used for two purposes. On full 
   authentication, this packet is used to request the client to send 
   the AT_NONCE_MT attribute to the server. In addition, as specified 
   in Section 5.2, the Start round trip may be used for obtaining the 
   client identity to the server. The format of the EAP 
   Request/SIM/Start packet is shown below. 

    

  
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       0                   1                   2                   3 
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |     Code      |  Identifier   |            Length             | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |     Type      |    Subtype    |           Reserved            | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |AT_PERM..._REQ | Length = 1    |           Reserved            | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |AT_FULL..._REQ | Length = 1    |           Reserved            | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |AT_ANY_ID_REQ  | Length = 1    |           Reserved            | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      | AT_VERSION_L..| Length        | Actual Version List Length    | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |  Supported Version 1          |  Supported Version 2          | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      .                                                               . 
      .                                                               . 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      | Supported Version N           |     Padding                   | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    
   Code 

      1 for Request  

   Identifier 

      See [1]. 

   Length 

      The length of the EAP packet. 

   Type 

      18 

   Subtype 

      10 

   Reserved 

      Set to zero on sending, ignored on reception 

   AT_PERMANENT_ID_REQ 

      The AT_PERMANENT_ID_REQ attribute is optional to include and it 
      is included in the cases defined in Section 5.3. It MUST NOT be 
      included if AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ is included. The 

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

   AT_FULLAUTH_ID_REQ 

      The AT_FULLAUTH_ID_REQ attribute is optional to include and it is 
      included in the cases defined in Section 5.3. It MUST NOT be 
      included if AT_ANY_ID_REQ or AT_PERMANENT_ID_REQ is included. The 
      value field only contains two reserved bytes, which are set to 
      zero on sending and ignored on reception. 

   AT_ANY_ID_REQ 

      The AT_ANY_ID_REQ attribute is optional and it is included in the 
      cases defined in Section 5.2. It MUST NOT be included if 
      AT_PERMANENT_ID_REQ or AT_FULLAUTH_ID_REQ is included. The value 
      field only contains two reserved bytes, which are set to zero on 
      sending and ignored on reception. 

   AT_VERSION_LIST 

      The AT_VERSION_LIST attribute MUST be included. This attribute is 
      used in version negotiation, as specified in Section 4. 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 bytes. This field is 
      followed by the list of supported version, each 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. EAP-Response/SIM/Start 

   The format of the EAP Response/SIM/Start packet is shown below. 


















  
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       0                   1                   2                   3 
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |     Code      |  Identifier   |            Length             | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |     Type      |    Subtype    |           Reserved            | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |AT_NONCE_MT    | Length = 5    |           Reserved            | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |                                                               | 
      |                           NONCE_MT                            | 
      |                                                               | 
      |                                                               | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      | AT_IDENTITY   | Length        | Actual Identity Length        | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      |                                                               | 
      .                       Identity (optional)                     . 
      .                                                               . 
      |                                                               | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
      | AT_SELECTED...| Length = 1    |    Selected Version           | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    

   Code 

      2 for Response 

   Identifier 

      See [1]. 

   Length 

      The length of the EAP packet. 

   Type 

      18 

   Subtype 

      10 

   Reserved 

      Set to zero when sending, ignored on reception. 

   AT_NONCE_MT 

      The AT_NONCE_MT attribute MUST NOT be included on re-
      authentication, that is, if the AT_IDENTITY with a re-
  
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      authentication identity is included. AT_NONCE_MT MUST be included 
      in all other cases (full authentication). The value field 
      contains two reserved bytes followed by a random number generated 
      by the client (16 bytes) freshly for this EAP/SIM authentication. 
      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. 

   AT_IDENTITY 

      The AT_IDENTITY attribute is optional to include and it is 
      included in cases defined in Section 5.2 and 5.3. The value field 
      of this attribute begins with 2-byte actual identity length, 
      which specifies the length of the identity in bytes. This field 
      is followed by the subscriber identity of the indicated actual 
      length, in the same Network Access Identifier format that is used 
      in EAP-Response/Identity, i.e. including the NAI realm portion. 
      The identity is the permanent identity, a pseudonym identity or a 
      re-authentication identity. The identity format is specified in 
      Section 5.1. 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. 

   AT_SELECTED_VERSION 

      The AT_SELECTED_VERSION attribute MUST NOT be included on re-
      authentication, that is, if the AT_IDENTITY attribute with a re-
      authentication identity is included. In all other cases, 
      AT_SELECTED_VERSION MUST be included (full authentication). This 
      attribute is used in version negotiation, as specified in Section 
      4. The value field of this attribute contains a two-byte version 
      number, which indicates the EAP/SIM version that the client wants 
      to use. 

11. EAP-Request/SIM/Challenge 

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















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

   Code 

      1 for Request  

   Identifier 

      See [1] 


  
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   Length 

      The length of the EAP packet. 

   Type 

      18 

   Subtype 

      11 

   Reserved 

      Set to zero when sending, ignored on reception. 

   AT_RAND 

      The AT_RAND attribute MUST be included. The value field of this 
      attribute contains two reserved bytes followed by n GSM RANDs 
      (each 16 bytes long). The reserved bytes are set to zero upon 
      sending and ignored upon reception.  

      The number of RAND challenges MUST be one, two or three. The 
      client MUST silently ignore the EAP-Request/SIM/Challenge 
      message, if the number of RAND challenges is smaller than what is 
      required by client's local policy. 

      If two or three RAND challenges are used, the server MUST use 
      different RAND values. In other words, a RAND value can only be 
      included once in AT_RAND. The client MUST check that the RANDs 
      are different. If the client detects that the server has repeated 
      a RAND value, the client MUST terminate the EAP exchange. 

   AT_IV 

      The AT_IV attribute is optional to include. See section 8.3. 

   AT_ENCR_DATA 

      The AT_ENCR_DATA attribute is optional to include. See section 
      8.3. The plaintext consists of nested attributes as described 
      below. 

   AT_CHECKCODE 

      The AT_CHECKCODE attribute is optional to include. See section 
      8.2. 

   AT_MAC 

      AT_MAC MUST be included. For EAP-Request/SIM/Challenge, the MAC 
      code is calculated over the following data: 
  
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          EAP packet| NONCE_MT 
      The EAP packet is represented as specified in Section 8.1. It is 
      followed by the 16-byte NONCE_MT value from the client's 
      AT_NONCE_MT attribute. 

   The AT_IV, AT_ENCR_DATA and AT_MAC attributes are used for identity 
   privacy and for communicating the next re-authentication identity. 
   The plaintext of the AT_ENCR_DATA value field consists of nested 
   attributes, which 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 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   | AT_NEXT_PSEU..| Length        | Actual Pseudonym Length       | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                                                               | 
   .                          Next Pseudonym                       . 
   .                                                               . 
   |                                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   | AT_NEXT_REAU..| Length        | Actual Re-Auth Identity Length| 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                                                               | 
   .                   Next Re-authentication Username             . 
   .                                                               . 
   |                                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |  AT_PADDING   | Length        | Padding...                    | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               | 
   |                                                               | 
   |                                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    

   AT_NEXT_PSEUDONYM 

      The AT_NEXT_PSEUDONYM attribute is optional to include. The value 
      field of this attribute begins with 2-byte actual pseudonym 
      length, which specifies the length of the pseudonym in bytes. 
      This field is followed by a pseudonym username, of the indicated 
      actual length, that the client can use in the next 
      authentication, as described in Section 5.3. 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.  

   AT_NEXT_REAUTH_ID 

      The AT_NEXT_REAUTH_ID attribute is optional to include. The value 
      field of this attribute begins with 2-byte actual re-
      authentication identity length, which specifies the length of the 
      re-authentication identity in bytes. This field is followed by a 
  
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      re-authentication identity, of the indicated actual length, that 
      the client can use in the next re-authentication, as described in 
      Section 6. The re-authentication identity includes both a 
      username portion and a realm name portion. The re-authentication 
      identity does not include any terminating null characters. 
      Because the length of the attribute must be a multiple of 4 
      bytes, the sender pads the re-authentication identity with zero 
      bytes when necessary.  

   AT_PADDING 

      The AT_PADDING attribute is optional. See section 8.3 

12. EAP-Response/SIM/Challenge 

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

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

     0                   1                   2                   3 
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |     Code      |  Identifier   |            Length             | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |     Type      |    Subtype    |           Reserved            | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    | AT_CHECKCODE  | Length = 6    |           Reserved            | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |                                                               | 
    |                   Checkcode (optional)                        | 
    |                                                               | 
    |                                                               | 
    |                                                               | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |  AT_MAC       | Length = 5    |           Reserved            | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |                                                               | 
    |                                                               | 
    |                           MAC                                 | 
    |                                                               | 
    |                                                               | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    

   Code 

      2 for Response 
  
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   Identifier 

      See [1]. 

   Length 

      The length of the EAP packet. 

   Type 

      18 

   Subtype 

      11 

   Reserved 

      Set to zero when sending, ignored on reception. 

   AT_CHECKCODE 

      The AT_CHECKCODE attribute is optional to include. See section 
      8.2. 

   AT_MAC 

      AT_MAC 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 one, 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 AT_RAND attribute. 

13. EAP-Request/SIM/Re-authentication 

   The format of the EAP-Request/SIM/Re-authentication packet is shown 
   below.  













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

   Code 

      1 for Request 

   Identifier 

      See [1]. 

   Length 

      The length of the EAP packet. 



  
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   Type 

      18 

   Subtype 

      13 

   Reserved 

      Set to zero when sending, ignored on reception. 

   AT_IV 

      The AT_IV attribute is MUST be included. See section 8.3. 

   AT_ENCR_DATA 

      The AT_ENCR_DATA attribute MUST be included. See section 8.3. The 
      plaintext consists of nested attributes as described below. 

   AT_CHECKCODE 

      The AT_CHECKCODE attribute is optional to include. See section 
      8.2. 

   AT_MAC 

      AT_MAC MUST be included. No message-specific data is included in 
      the MAC calculation. See Section 8.1. 

   The AT_IV and AT_ENCR_DATA attributes are used for communicating 
   encrypted attributes. The plaintext of the AT_ENCR_DATA value field 
   consists of nested attributes, which are shown below. 



















  
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    0                   1                   2                   3 
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |  AT_COUNTER   | Length = 1    |           Counter             | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   | AT_NONCE_S    | Length = 5    |           Reserved            | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                                                               | 
   |                                                               | 
   |                            NONCE_S                            | 
   |                                                               | 
   |                                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   | AT_NEXT_REAU..| Length        | Actual Re-Auth Identity Length| 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                                                               | 
   .                   Next Re-authentication Username             . 
   .                                                               . 
   |                                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |  AT_PADDING   | Length        | Padding...                    | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               | 
   |                                                               | 
   |                                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    

   AT_COUNTER 

      The AT_COUNTER attribute MUST be included. The value field 
      consists of a 16-bit unsigned integer counter value, represented 
      in network byte order. 

   AT_NONCE_S 

      The AT_NONCE_S attribute MUST be included. The value field 
      contains two reserved bytes followed by a random number generated 
      by the server (16 bytes) freshly for this EAP/SIM re-
      authentication. The random number is used as challenge for the 
      client and also a seed value for the new keying material. The 
      reserved bytes are set to zero upon sending and ignored upon 
      reception. 

   AT_NEXT_REAUTH_ID 

      The AT_NEXT_REAUTH_ID attribute is optional to include. The 
      attribute is described in Section 11. 

   AT_PADDING 

      The AT_PADDING attribute is optional to include. See section 8.3 

  
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14. EAP-Response/SIM/Re-authentication 

   The format of the EAP-Response/SIM/Re-authentication packet is shown 
   below.  

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

   Code 

      2 for Response 

   Identifier 

      See [1]. 


  
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   Length 

      The length of the EAP packet. 

   Type 

      18 

   Subtype 

      13 

   Reserved 

      Set to zero when sending, ignored on reception. 

   AT_IV 

      The AT_IV attribute is MUST be included. See section 8.3. 

   AT_ENCR_DATA 

      The AT_ENCR_DATA attribute MUST be included. See section 8.3. The 
      plaintext consists of nested attributes as described below. 

   AT_CHECKCODE 

      The AT_CHECKCODE attribute is optional to include. See section 
      8.2. 

   AT_MAC 

      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 are used for communicating 
   encrypted attributes. The plaintext of the AT_ENCR_DATA value field 
   consists of nested attributes, which are shown below. 









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

   AT_COUNTER 

      The AT_COUNTER attribute MUST be included. The format of this 
      attribute is specified in Section 13. 

   AT_COUNTER_TOO_SMALL 

      The AT_COUNTER_TOO_SMALL attribute is optional to include, and it 
      is included in cases specified in Section 6. 

   AT_PADDING 

      The AT_PADDING attribute is optional to include. See section 8.3 

15. Error Cases and the Usage of EAP-Failure and EAP-Success 

15.1. Processing Erroneous Packets  

   In general, if an EAP/SIM client or server implementation detects an 
   error in a received EAP/SIM packet, the EAP/SIM implementation 
   silently ignores the EAP packet, does not change its state and does 
   not send any EAP messages to its peer. Examples of such errors, 
   specified in detail elsewhere in this document, are an invalid 
   AT_MAC value, insufficient number of RAND challenges included in 
   AT_RAND, no acceptable version included in AT_VERSION_LIST, a 
   mandatory attribute is missing, illegal attributes included and an 
   unrecognized non-skippable attribute.  

   If the EAP/SIM client receives an EAP/SIM Request of an unrecognized 
   subtype, the EAP/SIM client MUST silently discard the EAP request. 

   The rationale for this error case behavior is that an active 
   attacker may have sent the erroneous packet. As the EAP/SIM client 
   or server does not process the packet and does not change its state, 
   it is possible to successfully process a valid packet if such packet 
   is received later. If no valid packets are received, the 
   authentication exchange will eventually time out.  

  
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15.2. EAP-Failure 

   As normally in EAP, the EAP server sends the EAP-Failure packet to 
   the client when the authentication procedure fails on the EAP 
   Server. In EAP/SIM, this may occur for example if the EAP server 
   does not recognize the user identity, or if the EAP server is not 
   able to obtain the GSM triplets for the subscriber or the 
   authentication exchange times out. 

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

15.3. EAP-Success 

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

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

   If the client receives an EAP/SIM notification (section 16) that 
   indicates failure, then the client MUST no longer accept the EAP-
   Success packet even if the server authentication was successfully 
   completed. 

16. EAP/SIM Notifications 

   The EAP-Request/Notification, specified in [1], can be used to 
   convey a displayable message from the authenticator to the client. 
   Because these messages are textual messages, it may be hard for the 
   client to present them in the user's preferred language. Therefore, 
   EAP/SIM uses a separate EAP/SIM message subtype to transmit 
   localizable notification codes instead of the EAP-
   Request/Notification packet. 

   The EAP server MAY issue an EAP-Request/SIM/Notification packet to 
   the client. The client MAY show a notification message to the user 
   and the client MUST respond to the EAP server with an EAP-
   Response/SIM/Notification packet, even if the client did not 
   recognize the notification code. 

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

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

   If the P bit is set to one, the notification can only by used before 
   the EAP/SIM/Challenge round in full authentication or the 
   EAP/SIM/Reauthentication round in reauthentication. For these 
   notifications, the AT_MAC attribute MUST NOT be included in either 
   EAP-Request/SIM/Notification or EAP-Response/SIM/Notification. 

    

   Some of the notification codes are authorization related and hence 
   not usually considered as part of the responsibility of an EAP 
   method. However, they are included as part of EAP/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. 

   The format of the EAP-Request/SIM/Notification packet is shown 
   below. 


















  
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     0                   1                   2                   3 
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |     Code      |  Identifier   |            Length             | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |     Type      |    Subtype    |           Reserved            | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |AT_NOTIFICATION| Length = 1    |F|P|  Notification Code        | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |  AT_MAC       | Length = 5    |           Reserved            | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    |                                                               | 
    |                                                               | 
    |                              MAC                              | 
    |                                                               | 
    |                                                               | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    

   Code 

      1 for Request 

   Identifier 

      See [1]. 

   Length 

      The length of the EAP packet. 

   Type 

      18 

   Subtype 

      12 

   Reserved 

      Set to zero when sending, ignored on reception. 

   AT_NOTIFICATION 

      The AT_NOTIFICATION attribute MUST be included. The value field 
      of this attribute contains a two-byte notification code. The 
      first and second bit (F and P) of the notification code are 
      interpreted as described above. 

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

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

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

   AT_MAC 

      AT_MAC is included in cases described above. No message-specific 
      data is included in the MAC calculation. See Section 8.1. 

   The format of the EAP-Response/SIM/Notification packet is shown 
   below. Because this packet is only an acknowledgement of EAP-
   Request/SIM/Notification, it does not contain any mandatory 
   attributes. 

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

   Code 

      2 for Response 

   Identifier 

      See [1]. 

   Length 

      The length of the EAP packet. 

   Type 

      18 


  
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   Subtype 

      12 

   Reserved 

      Set to zero when sending, ignored on reception. 

   AT_MAC 

      AT_MAC is included in cases described above. No message-specific 
      data is included in the MAC calculation. See Section 8.1. 

17. 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 user identity string without any terminating 
   null characters. It is the identity from the AT_IDENTITY attribute 
   from the last EAP-Response/SIM/Start packet, or, if AT_IDENTITY was 
   not used, the identity from the EAP-Response/Identity packet. The 
   notation n*Kc 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 just 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 SHA1 is specified in [11]. 

   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 re-authentication, the same TEKs will be used for 
   protecting EAP packets, but a new MSK and a new EMSK will be derived 
   from the original MK and new values exchanged in the re-
   authentication. 

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

  
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   Key derivation is based on the random number generation specified in 
   NIST Federal Information Processing Standards (FIPS) Publication 
   186-2 [12]. The pseudo-random number generator is specified in the 
   change notice 1 (2001 October 5) of [12] (Algorithm 1). As specified 
   in the change notice (page 74), when Algorithm 1 is used as a 
   general-purpose pseudo-random number generator, the "mod q" term in 
   step 3.3 is omitted. The function G used in the algorithm is 
   constructed via Secure Hash Standard as specified in Appendix 3.3 of 
   the standard. For convenience, the random number algorithm with the 
   correct modification is cited in Annex 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.  
    
   The resulting 320-bit random numbers x_0, x_1, ..., x_m-1 are 
   concatenated and partitioned into suitable-sized chunks and used as 
   keys in the following order: K_encr (128 bits), K_aut (128 bits), 
   Master Session Key (64 bytes), Extended Master Session Key (64 
   bytes). 
    
   On re-authentication, the same pseudo-random number generator can be 
   used to generate a new Master Session Key and new Initialization 
   Vectors. The seed value XKEY' is calculated as follows: 

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

   In the formula above, the Identity denotes the re-authentication 
   user identity, without any terminating null characters, from the 
   AT_IDENTITY attribute of the EAP-Response/SIM/Start packet, or, if 
   EAP-Response/SIM/Start was not used on re-authentication, the 
   identity string from the EAP-Response/Identity packet. The counter 
   denotes the counter value from AT_COUNTER attribute used in the EAP-
   Response/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. The pseudo-random number generator is 
   run with the new seed value XKEY', and the resulting 320-bit random 
   numbers x_0, x_1, ..., x_m-1 are concatenated and partitioned into 
   64-byte chunks and used as the new 64-byte Master Session Key and 
   the new 64-byte Extended Master Session Key.  
    
   The first 32 bytes of the MSK can be used as the Pairwise Master Key 
   (PMK) for IEEE 802.11i. 

   When the RADIUS attributes specified in [14] 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. 


  
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   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 client is able to 
   verify that the EAP SIM packets it receives from the network are 
   fresh and not a replay. (Please see also Section 19.) 

18. IANA Considerations 

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

   IANA has assigned the EAP type number 18 for this protocol. 

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

        Start..........................................10 
        Challenge......................................11 
        Notification...................................12 
        Re-authentication..............................13 
    
   The Subtype-specific data is composed of attributes, which have 
   attribute type numbers. The following attribute types are specified: 

        AT_RAND.........................................1 
        AT_PADDING......................................6 
        AT_NONCE_MT.....................................7 
        AT_PERMANENT_ID_REQ............................10 
        AT_MAC.........................................11 
        AT_NOTIFICATION................................12 
        AT_ANY_ID_REQ..................................13 
        AT_IDENTITY....................................14 
        AT_VERSION_LIST................................15 
        AT_SELECTED_VERSION............................16 
        AT_FULLAUTH_ID_REQ.............................17 
        AT_COUNTER.....................................19 
        AT_COUNTER_TOO_SMALL...........................20 
        AT_NONCE_S.....................................21 
    
        AT_IV.........................................129 
        AT_ENCR_DATA..................................130 
        AT_NEXT_PSEUDONYM.............................132 
        AT_NEXT_REAUTH_ID.............................133 
        AT_CHECKCODE..................................134 
    
    
  
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   The AT_NOTIFICATION attribute contains a notification code value. 
   Values 1024, 1026 and 1031 have been specified in Section 16 of this 
   document. 

   The AT_VERSION_LIST and AT_SELECTED_VERSION attributes contain 
   version numbers. Version 1 has been specified in Section 4 of this 
   document. 

   All requests for value assignment from the various number spaces 
   described in this document require proper documentation, according 
   to the "Specification Required" policy described in [15]. Requests 
   must be specified in sufficient detail so that interoperability 
   between independent implementations is possible. Possible forms of 
   documentation include, but are not limited to, RFCs, the products of 
   another standards body (e.g. 3GPP), or permanently and readily 
   available vendor design notes. 

19. Security Considerations 

   The revised EAP base protocol [16] highlights several attacks that 
   are possible against the EAP protocol as there is no inherent 
   security mechanisms provided. This section discusses the claimed 
   security properties of EAP SIM as well as vulnerabilities and 
   security recommendations. 

19.1. Identity Protection 

   EAP/SIM includes optional identity privacy support that protects the 
   privacy of the subscriber identity against passive eavesdropping. 
   The mechanism cannot be used on the first connection with a given 
   server, when the 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 (Section 5.3) to learn the 
   subscriber's permanent identity. However, as discussed in Section 
   5.3, 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 client 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) [17] may be used. If an 
   external security mechanism is in use 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. 

19.2. Mutual Authentication and Triplet Exposure 

   EAP/SIM provides mutual authentication. The client believes that the 
   network is authentic because the network can calculate a correct 
  
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   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 hereby the triplets, an 
   attacker that knows n (1, 2 or 3) GSM triplets for the subscriber is 
   able to impersonate a valid network to the client. 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 client platform via a virus or other malicious piece of 
   software. The client SHOULD be protected against triplet querying 
   attacks by malicious software. 

   If the same SIM credentials are also used for GSM traffic, the 
   triplets could be revealed in the GSM network; see Section 19.5. 

   Since the security of EAP/SIM is based on the secrecy of Kc keys 
   care should be taken not to expose these values to attackers when 
   they are transmitted between entities, stored or handled. Steps 
   should be taken to limit the transport, storage and handling of 
   these values outside a protected environment. These considerations 
   are important at both the client and authenticator implementations. 

   In GSM, the network is allowed to reuse the RAND challenge in 
   consecutive authentication exchanges. This is not allowed in EAP/SIM 
   but the EAP/SIM server is mandated to use fresh triplets (RAND 
   challenges) in consecutive authentication exchanges, as specified in 
   Section 3. However, EAP SIM does not include means for the client to 
   check if the RANDs are fresh, so the security of the scheme leans on 
   the secrecy of the triplets.  

   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 [21]. In cases when the triplet re-
   use considerations of EAP SIM are not considered sufficient, it is 
   advised to use EAP AKA. 

19.3. Key Derivation 

   EAP/SIM supports key derivation. The key hierarchy is specified in 
   Section 17. 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 the operator-specific authentication algorithms, the strength of 
   the Ki key, and the quality of the RAND challenges, which is also 
   operator specific. 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. 

   EAP SIM client is recommended to require at least two triplets to be 
   used in each EAP SIM exchange (n = 2 or n = 3). The server is 

  
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   recommended to always include at least two triplets in EAP-
   Request/SIM/Challenge. 

   In the case where one triplet is used (n = 1), if the client is the 
   attacker, he needs to guess 96 bits (64 in k_int and 32 in SRES) if 
   the server is the attacker he needs to guess 64 bits, but given the 
   128 bit nonce he cannot make any precomputations.  This provides a 
   reasonable level of security for the MAC exchange, but only provides 
   64 bits for the Master Session Key.  Therefore a single triplet can 
   only be used in the case where a Master Session Key is not needed or 
   is provided by an external security mechanism such as PEAP. However, 
   the use of a single triplet is strongly discouraged in this case 
   too, because it may enable an adversary to attack on individual 
   triplets. It should also be noted that a security policy that allows 
   n=1 to be used may compromise the security of a future policy that 
   requires more than one triplet, because adversaries will be able to 
   exploit the triplets they have learnt when the weaker policy was 
   applied. Security considerations for the use of an external security 
   mechanism with EAP/SIM are beyond the scope of this document. 

   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 when more than one triplet is used, it is not possible 
   to mount a brute force attack on just 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 combined from several 
   shorter keys, i.e the effective length of 128 bits may be achieved. 
   However, see Section 19.5.The EAP Transient Keys used to protect EAP 
   SIM packets (K_encr, K_aut) and the Master Session Key are 
   cryptographically separate. An attacker cannot derive any non-
   trivial information from K_encr or K_aut based on the Master Session 
   Key or vice versa. An attacker also cannot calculate the pre-shared 
   secret from the GSM Kc keys used, EAP SIM K_encr, EAP SIM K_aut or 
   from the Master Session Key. 

   Each EAP/SIM exchange generates fresh keying material. The EAP SIM 
   client contributes to the keying material with the NONCE_MT 
   parameter, which must be chosen freshly for each exchange. Hence, 
   even if the RAND challenges were reused from a previous session, the 
   session keys will be different. Please see section 19.2 for more 
   information about RAND reuse. 



  
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19.4. Dictionary Attacks 

   Because EAP/SIM is not a password protocol, it is not vulnerable to 
   dictionary attacks. (The pre-shared symmetric secret shall not be a 
   weak password.) 

19.5. Credentials Reuse 

   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 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. If the attacker can 
   obtain 1-3 Kc keys, he can then impersonate a valid network to an 
   EAP-SIM client. 

   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). 

   Because this attack requires the attacker to build a rogue GSM base 
   station (or at least eavesdrop the GSM traffic), the cost of the 
   attack is not negligible; it is the same cost as usually in GSM. 
   However, 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; an active attacker can force the client 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. 

19.6. Integrity Protection, Replay Protection and Confidentiality 

   AT_MAC, AT_IV and AT_ENCR_DATA attributes are used to provide 
   integrity, replay and confidentiality protection for EAP/SIM 
   Requests and Responses. Integrity protection includes the EAP 
   header. These attributes cannot be used during the EAP/SIM/Start 
   roundtrip. However, the protocol values (identity, NONCE_MT and 
   version negotiation parameters) are protected by later EAP/SIM 
   messages. The AT_CHECKCODE attribute can optionally be used to 
   protect the integrity of the EAP/SIM/Start roundtrip. 


  
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   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.  

   On full authentication, replay protection is provided by the 
   underlying GSM authentication scheme, which makes use of the RAND 
   value, and the use of the NONCE_MT value. On re-authentication, a 
   counter and a server nonce is used to provide replay protection. 

   Contents of the EAP-Response/Identity packet are implicitly 
   integrity protected by including them in key derivation. 

   Because EAP/SIM is not a tunneling method, EAP 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 attacker to send false notifications to 
   the peer and to mount denial of service attacks by spoofing these 
   packets. 

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

19.7. 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, ensuring the protocol cannot be downgraded and that the 
   client and server use the same version of EAP/SIM. 

   As described in Section 7, EAP/SIM allows the protocol to be 
   extended by defining new attribute types. When defining such 
   attributes, it should noted that any extra attributes included in 
   EAP-Request/SIM/Start or EAP-Response/SIM/Start packets are not 
   included in the MACs later on, and thus some other precautions must 
   be taken to avoid modifications to them. 

   EAP/SIM does not support ciphersuite negotiation. 

19.8. Fast Reconnect 

   EAP/SIM includes an optional re-authentication ("fast reconnect") 
   procedure, as recommended in [16] for EAP types that are intended 
   for physically insecure networks. 



  
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19.9. Acknowledged Result Indications 

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

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

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

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

19.11. 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 
   [18] for more information on generating random numbers for security 
   applications. 

20. Security Claims 

   This section provides the security claims required by [16]. 

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

   [b] 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 client challenge to provide mutual authentication. 

   [c] Security claims. The security properties of the method are 
   discussed in Section 19. 

   [d] Key strength. EAP SIM supports key derivation with 128-bit 
   effective key strength if several triplets are used per 
   authentication exchange, and 64-bit effective key strength if only 
   one triplet is used. (The policies in the client and the server can 
   prevent authentication exchanges with just one triplet.) However, as 
   discussed in Section 19, 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. 

   [e] Description of key hierarchy. Please see Section 17. 

   [f] Indication of vulnerabilities. Vulnerabilities are discussed in 
   Section 19. 

21. Intellectual Property Right Notice 

   On IPR related issues, Nokia refers to the Nokia Statement on Patent 
   licensing, see http://www.ietf.org/ietf/IPR/NOKIA. 

22. Acknowledgements and Contributions 

   In addition to the editors, Nora Dabbous, Jose Puthenkulam, and 
   Prasanna Satarasinghe are significant contributors of this document. 

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

   N. Asokan and Jukka-Pekka Honkanen have contributed and helped in 
   innumerable ways during the whole development of the protocol. 

   Valtteri Niemi and Kaisa Nyberg contributed substantially to the 
   design of the key derivation and the re-authentication procedure, 
   and have also provided their cryptographic expertise in many 
   discussions related to this protocol. 

   Simon Blake-Wilson provided most helpful comments on key derivation 
   and version negotiation. 
  
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   Thanks to Greg Rose for his most valuable comments [19]. 

   Thanks to Bernard Aboba, Vladimir Alperovich, Jacques Caron, Gopal 
   Dommety, Pasi Eronen, Augustin Farrugia, Mark Grayson, Max de Groot, 
   Prakash Iyer, Nishi Kant, Victor Lortz, Sarvar Patel, Stefan 
   Schr÷der, Jesse Walker and Thomas Wieland for their contributions 
   and critiques. Special thanks to Max for proposing improvements to 
   the MAC calculation. 

   The identity privacy support is based on the identity privacy 
   support of [7]. The attribute format is based on the extension 
   format of Mobile IPv4 [20]. 

   This protocol has been partly developed in parallel with EAP AKA 
   [21], and hence this specification incorporates many ideas from Jari 
   Arkko. 

References 

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

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

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

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

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

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

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

   [8]   H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for 
         Message Authentication", RFC 2104, February 1997. (NORMATIVE) 
 
  
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   [9]   Federal Information Processing Standard (FIPS) draft standard, 
         "Advanced Encryption Standard (AES)", 
         http://csrc.nist.gov/publications/drafts/dfips-AES.pdf, 
         September 2001. (NORMATIVE) 

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

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

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

   [13]  B. Aboba, D. Simon, "PPP EAP TLS Authentication Protocol", RFC 
         2716, October 1999 (INFORMATIVE) 

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

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

   [16]  L. Blunk, J. Vollbrecht, B. Aboba, "Extensible Authentication 
         Protocol (EAP)", draft-ietf-pppext-rfc2284bis-07.txt, work-in-
         progress, October 2002. (NORMATIVE) 

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

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

   [19]  Qualcomm, "Comments on draft EAP/SIM", 3rd Generation 
         Partnership Project document 3GPP TSG SA WG3 Security S3#22, 
         S3-020125, February 2002. (INFORMATIVE) 

 

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

   [21]  J. Arkko, H. Haverinen, "EAP AKA Authentication", draft-arkko-
         pppext-eap-aka-10.txt, June 2003 (work in progress). 
         (INFORMATIVE) 

Editors' and Contributors' Contact Information 

   Henry Haverinen 
   Nokia Mobile Phones 
   P.O. Box 88 
   FIN-33721 Tampere 
   Finland 
   E-mail: henry.haverinen@nokia.com 
   Phone: +358 50 594 4899 
    
   Joseph Salowey 
   Cisco Systems 
   2901 Third Avenue 
   Seattle, WA 98121 
   US 
   E-mail: jsalowey@cisco.com 
   Phone: +1 206 256 3380 
    
   Nora Dabbous  
   Gemplus  
   34 rue Guynemer 
   92447 Issy les Moulineaux   France  
   E-mail: nora.dabbous@gemplus.com  
   Phone: +33 1 4648 2000 
    
   Jose Puthenkulam 
   Intel Corporation 
   2111 NE 25th Avenue, JF2-58 
   Hillsboro, OR 97124 
   US 
   E-mail: jose.p.puthenkulam@intel.com 
   Phone: +1 503 264 6121 
    
   Prasanna Satarasinghe 
   Transat Technologies 
   180 State Street, Suite 240 
   Southlake, TX 76092 
   US 
   E-mail: prasannas@transat-tech.com 
   Phone: + 1 817 4814412 
    




  
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Annex A. Test Vectors 

   Test vectors for the NIST FIPS 186-2 pseudo-random number generator 
   [12] are available at the following URL: 
   http://csrc.nist.gov/encryption/dss/Examples-1024bit.pdf 
    
   TBD: Test vectors for EAP SIM values 














































  
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Annex B. Pseudo-Random Number Generator 

   The "|" character denotes concatenation, and "^" denotes involution. 
    
   Step 1: Choose a new, secret value for the seed-key, XKEY 
    
   Step 2: In hexadecimal notation let 
       t = 67452301 EFCDAB89 98BADCFE 10325476 C3D2E1F0 
       This is the initial value for H0|H1|H2|H3|H4  
       in the FIPS SHS [11] 
    
   Step 3: For j = 0 to m - 1 do 
         3.1 XSEED_j = 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 and Salowey   Expires in six months               [Page 64] 

