Network Working Group H. Haverinen (editor) Internet Draft Nokia J. Salowey (editor) Cisco Expires: 27 April, 2004 27 October, 2003 EAP SIM Authentication draft-haverinen-pppext-eap-sim-12.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 documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at: http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at: http://www.ietf.org/shadow.html. Comments should be submitted to the eap@frascone.com mailing list. 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. Haverinen and Salowey [Page 1] Internet Draft EAP SIM Authentication 27 October, 2003 Table of Contents Status of this Memo.........................................1 Abstract....................................................1 Table of Contents...........................................2 1. Introduction.............................................3 2. Terms....................................................4 3. Overview.................................................6 4. Operation................................................8 4.1. Version Negotiation....................................8 4.2. Identity Management....................................9 4.3. Re-Authentication.....................................25 4.4. EAP/SIM Notifications.................................30 4.5. Error Cases...........................................31 4.6. Key Generation........................................33 5. Message Format and Protocol Extensibility...............35 5.1. Message Format........................................35 5.2. Protocol Extensibility................................37 6. Messages................................................37 6.1. EAP-Request/SIM/Start.................................37 6.2. EAP-Response/SIM/Start................................38 6.3. EAP-Request/SIM/Challenge.............................38 6.4. EAP-Response/SIM/Challenge............................39 6.5. EAP-Request/SIM/Re-authentication.....................40 6.6. EAP-Response/SIM/Re-authentication....................40 6.7. EAP-Response/SIM/Client-Error.........................40 6.8. EAP-Request/SIM/Notification..........................40 6.9. EAP-Response/SIM/Notification.........................41 7. Attributes..............................................41 7.1. Table of Attributes...................................41 7.2. AT_MAC................................................42 7.3. AT_IV, AT_ENCR_DATA and AT_PADDING....................43 7.4. AT_VERSION_LIST.......................................45 7.5. AT_SELECTED_VERSION...................................46 7.6. AT_NONCE_MT...........................................46 7.7. AT_PERMANENT_ID_REQ...................................46 7.8. AT_ANY_ID_REQ.........................................47 7.9. AT_FULLAUTH_ID_REQ....................................47 7.10. AT_IDENTITY..........................................47 7.11. AT_RAND..............................................48 7.12. AT_NEXT_PSEUDONYM....................................49 7.13. AT_NEXT_REAUTH_ID....................................49 7.14. AT_COUNTER...........................................50 7.15. AT_COUNTER_TOO_SMALL.................................50 7.16. AT_NONCE_S...........................................50 7.17. AT_NOTIFICATION......................................51 7.18. AT_CLIENT_ERROR_CODE.................................52 8. IANA Considerations.....................................52 9. Security Considerations.................................54 9.1. Identity Protection...................................54 9.2. Mutual Authentication and Triplet Exposure............54 9.3. Key Derivation........................................55 Haverinen and Salowey Expires: 27 April, 2004 [Page 2] Internet Draft EAP SIM Authentication 27 October, 2003 9.4. Dictionary Attacks....................................56 9.5. Credentials Reuse.....................................56 9.6. Integrity and Replay Protection, and Confidentiality..57 9.7. Negotiation Attacks...................................57 9.8. Fast Reconnect........................................58 9.9. Acknowledged Result Indications.......................58 9.10. Man-in-the-middle Attacks............................58 9.11. Generating Random Numbers............................59 10. Security Claims........................................59 11. Intellectual Property Right Notice.....................59 12. Acknowledgements and Contributions.....................59 12.1. Contributors.........................................59 12.2. Acknowledgements.....................................60 Normative References.......................................60 Informative References.....................................61 Editors' and Contributors' Contact Information.............63 Annex A. Test Vectors......................................64 Annex B. Pseudo-Random Number Generator....................72 1. Introduction This document specifies an Extensible Authentication Protocol (EAP) [EAP] 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. Hence the secrecy of Kc is critical to the security of this protocol. Please find more information about GSM authentication in [GSM 03.20]. The lack of mutual authentication is a weakness in GSM authentication. The 64 bit cipher key (Kc) that is derived is not strong enough for data networks where stronger and longer keys are required. Hence in EAP/SIM, several RAND challenges are used for generating several 64-bit Kc keys, which are combined to constitute stronger keying material. In EAP/SIM the client issues a random number NONCE_MT to the network, in order to contribute to key derivation, and to prevent replays of EAP/SIM requests from previous exchanges. The NONCE_MT can be conceived as the client's challenge to the network. EAP/SIM also extends the combined RAND challenges and other messages with a message authentication code in order to provide message integrity protection along with mutual authentication. EAP/SIM specifies optional support for protecting the privacy of subscriber identity using the same concept as GSM, which is using Haverinen and Salowey Expires: 27 April, 2004 [Page 3] Internet Draft EAP SIM Authentication 27 October, 2003 pseudonyms/temporary identifiers. It also specifies 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 9 of this document. Implementers and users of EAP/SIM are advised to carefully study the security considerations in Section 9 in order to determine whether the security properties are sufficient for the environment in question, especially as the secrecy of Kc keys is key to the security of EAP/SIM. In brief, EAP/SIM is in no sense weaker than the GSM mechanisms. In some cases EAP/SIM provides better security properties than the underlying GSM mechanisms, particularly if the SIM credentials are only used for EAP/SIM and not re-used from GSM/GPRS. Many of the security features of EAP_SIM rely upon the secrecy of the Kc values in the SIM triplets, so protecting these values is key to the security of the EAP-SIM protocol. 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 [EAP AKA] specifies an EAP method that is based on UMTS AKA. EAP AKA, which is a more secure protocol, may be used instead of EAP/SIM, if USIMs and 3G network infrastructure are available. 2. Terms The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119]. The terms and abbreviations "authenticator", "backend authentication server", "EAP server", "Silently Discard", "Master Session Key (MSK)", and "Extended Master Session Key (EMSK)" in this document are to be interpreted as described in [EAP]. This document frequently uses the following terms and abbreviations: AAA protocol Authentication, Authorization and Accounting protocol AuC Authentication Centre. The GSM network element that provides the authentication triplets for authenticating the subscriber. Haverinen and Salowey Expires: 27 April, 2004 [Page 4] Internet Draft EAP SIM Authentication 27 October, 2003 Authentication vector GSM triplets can be alternatively called authentication vectors. EAP Extensible Authentication Protocol. GSM Global System for Mobile communications. 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 Permanent Identity The permanent identity of the peer, including an NAI realm portion in environments where a realm is used. The permanent identity is usually based on the IMSI. Used on full authentication only. Permanent Username The username portion of permanent identity, ie. not including any realm portions. Pseudonym Identity A pseudonym identity of the peer, including an NAI realm portion in environments where a real is used. Used on full authentication only. Pseudonym Username The username portion of pseudonym identity, ie. not including any realm portions. Haverinen and Salowey Expires: 27 April, 2004 [Page 5] Internet Draft EAP SIM Authentication 27 October, 2003 Re-authentication Identity A re-authentication identity of the peer, including an NAI realm portion in environments where a real is used. Used on re- authentication only. Re-authentication Username The username portion of re-authentication identity, ie. not including any realm portions. 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. The authenticator typically communicates with an EAP server that is located on a backend authentication server using an AAA protocol. The AAA communications is not shown in the figure. The first EAP Request issued by the network is EAP-Request/Identity. On full authentication, the peer's response includes either the user's International Mobile Subscriber Identity (IMSI) or a temporary identity (pseudonym) if identity privacy is in effect, as specified in Section 4.2. Following the peer's EAP-Response/Identity packet, the peer receives EAP Requests of type 18 (SIM) from the EAP server and sends the corresponding EAP Responses. The EAP packets that are of the Type SIM also have a Subtype field. On full authentication, the first EAP-Request/SIM packet is of the Subtype 10 (Start). EAP SIM packets encapsulate parameters in attributes, encoded in a Type, Length, Value format. The packet format and the use of attributes are specified in Section 5. The EAP-Request/SIM/Start packet contains the list of EAP/SIM version supported by the EAP server in the AT_VERSION_LIST attribute. This packet may also include attributes for requesting the subscriber identity, as specified in Section 4.2. The peer responds to EAP-Request/SIM/Start with the EAP- Response/SIM/Start packet, which includes the AT_NONCE_MT attribute that contains a random number NONCE_MT, chosen by the peer, and the AT_SELECTED_VERSION attribute that contains the version number selected by the peer. The version negotiation is protected by including the version list and the selected version in the calculation of keying material (Section 4.6). Haverinen and Salowey Expires: 27 April, 2004 [Page 6] Internet Draft EAP SIM Authentication 27 October, 2003 After receiving the EAP Response/SIM/Start, the EAP server obtains n GSM triplets for use in authenticating the subscriber, where n = 2 or n = 3. From the triplets, the EAP server derives the keying material, as specified in Section 4.6. The triplets may be obtained by contacting an Authentication Centre (AuC) on the GSM network; per GSM specifications, between 1 and 5 triplets may be obtained at a time. 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. On receipt of the EAP-Request/SIM/Challenge message, the peer runs the GSM authentication algorithm and calculates a copy of the message authentication code. The peer then verifies that the calculated MAC equals the received MAC. If the MAC's do not match, then the peer sends the EAP-Response/SIM/Client-Error packet and the authentication exchange terminates. Since the RAND's given to a peer are accompanied with the message authentication code AT_MAC, and since the peer's NONCE_MT value contributes to AT_MAC, the peer is able to verify that the EAP SIM message is fresh (not a replay) and that the sender possesses valid GSM triplets for the subscriber. If all checks out, the peer responds with the EAP- Response/SIM/Challenge, containing the AT_MAC attribute that covers the peer's SRES response values (Section 6.4). 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. Haverinen and Salowey Expires: 27 April, 2004 [Page 7] Internet Draft EAP SIM Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<---------------------------------------------------------| | | | EAP-Response/Identity | |--------------------------------------------------------->| | | | EAP-Request/SIM/Start | | (AT_VERSION_LIST) | |<---------------------------------------------------------| | | | EAP-Response/SIM/Start | | (AT_NONCE_MT, AT_SELECTED_VERSION) | |--------------------------------------------------------->| | | | EAP-Request/SIM/Challenge | | (AT_RAND, AT_MAC) | |<---------------------------------------------------------| | | +-------------------------------------+ | | Peer runs GSM algorithms, | | | verifies AT_MAC and derives | | | session keys | | +-------------------------------------+ | | | | EAP-Response/SIM/Challenge | | (AT_MAC) | |--------------------------------------------------------->| | | | | | EAP-Success | |<---------------------------------------------------------| | | Figure 1 EAP/SIM full authentication procedure 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. If the peer has maintained state information for re-authentication and wants to use re-authentication, then the peer indicates this by using a specific re-authentication identity instead of the permanent identity or a pseudonym identity. The re-authentication procedure is described in Section 4.3. 4. Operation 4.1. Version Negotiation EAP/SIM includes version negotiation so as to allow future developments in the protocol. The version negotiation is performed on full authentication and it uses two attributes, AT_VERSION_LIST, Haverinen and Salowey Expires: 27 April, 2004 [Page 8] Internet Draft EAP SIM Authentication 27 October, 2003 which the server always includes in EAP-Request/SIM/Start, and AT_SELECTED_VERSION, which the peer includes in EAP- Response/SIM/Start on full authentication. AT_VERSION_LIST includes the EAP/SIM versions supported by the server. If AT_VERSION_LIST does not include a version that is implemented by the peer and allowed in the peer's security policy, then the peer MUST send the EAP-Response/SIM/Client-Error packet (Section 6.7) to the server with the error code "unsupported version". If a suitable version is included, then the peer includes the AT_SELECTED_VERSION attribute, containing the selected version, in the EAP-Response/SIM/Start packet. The peer MUST only indicate a version that is included in AT_VERSION_LIST. If several versions are acceptable, then the peer SHOULD choose the version that occurs first in the version list. The version number list of AT_VERSION_LIST and the selected version of AT_SELECTED_VERSION are included in the key derivation procedure (Section 4.6). If an attacker modifies either one of these attributes, then the peer and the server derive different keying material. Because K_aut keys are different, the server and peer calculate different AT_MAC values. Hence, the peer detects that AT_MAC included in EAP-Request/SIM/Challenge is incorrect and sends the EAP-Response/SIM/Client-Error packet. The authentication procedure terminates. 4.2. Identity Management 4.2.1 Format, Generation and Usage of Peer Identities General In the beginning of EAP authentication, the Authenticator or the EAP server usually issues the EAP-Request/Identity packet to the peer. The peer responds with EAP-Response/Identity, which contains the user's identity. The formats of these packets are specified in [EAP]. GSM subscribers are identified with the International Mobile Subscriber Identity (IMSI) [GSM 03.03]. The IMSI is composed of a three digit Mobile Country Code (MCC), a two or three digit Mobile Network Code (MNC) and a not more than 10 digit Mobile Subscriber Identification Number (MSIN). In other words, the IMSI is a string of not more than 15 digits. MCC and MNC uniquely identify the GSM operator and help identify the AuC from which the authentication vectors need to be retrieved for this subscriber. Internet AAA protocols identify users with the Network Access Identifier (NAI) [RFC 2486]. When used in a roaming environment, the NAI is composed of a username and a realm, separated with "@" (username@realm). The username portion identifies the subscriber within the realm. Haverinen and Salowey Expires: 27 April, 2004 [Page 9] Internet Draft EAP SIM Authentication 27 October, 2003 This section specifies the peer identity format used in EAP/SIM. In this document, the term identity or peer identity refers to the whole identity string that is used to identify the peer. The peer identity may include a realm portion. "Username" refers to the portion of the peer identity that identifies the user, i.e. the username does not include the realm portion. Identity Privacy Support EAP/SIM includes optional identity privacy (anonymity) support that can be used to hide the cleartext permanent identity and thereby to make the subscriber's EAP exchanges untraceable to eavesdroppers. Because the permanent identity never changes, revealing it would help observers to track the user. The permanent identity is usually based on the IMSI, which may further help the tracking, because the same identifier may used in other contexts as well. Identity privacy is based on temporary identities, or pseudonyms, which are equivalent to but separate from the Temporary Mobile Subscriber Identities (TMSI) that are used on cellular networks. Please see Section 9.1 for security considerations regarding identity privacy. Username Types in EAP/SIM identities There are three types of usernames in EAP/SIM peer identities: (1) Permanent usernames. For example, 1123456789098765@myoperator.com might be a valid permanent identity. In this example, 1123456789098765 is the permanent username. (2) Pseudonym usernames. For example, 3s7ah6n9q@myoperator.com might be a valid pseudonym identity. In this example, 3s7ah6n9q is the pseudonym username. (3) Re-authentication usernames. For example, 53953754a@myoperator.com might be a valid re-authentication identity. In this case, 53953754 is the re-authentication username. The first two types of identities are only used on full authentication and the last one only on re-authentication. When the optional identity privacy support is not used, the non-pseudonym permanent identity is used on full authentication. The re- authentication exchange is specified in Section 4.3. Username Decoration In some environments, the peer may need to decorate the identity by prepending or appending the username with a string, in order to indicate supplementary AAA routing information in addition to the NAI realm. (The usage of a NAI realm portion is not considered to be decoration.) Username decoration is out of the scope of this document. However, it should be noted that username decoration might prevent the server from recognizing a valid username. Hence, although the peer MAY use username decoration in the identities the Haverinen and Salowey Expires: 27 April, 2004 [Page 10] Internet Draft EAP SIM Authentication 27 October, 2003 peer includes in EAP-Response/Identity, and the EAP server MAY accept a decorated peer username in this message, the peer or the EAP server MUST NOT decorate any other peer identities that are used in various EAP/SIM attributes. Only the identity used in EAP- Response/Identity may be decorated. NAI Realm Portion The peer MAY include a realm portion in the peer identity, as per the NAI format. The use of a realm portion is not mandatory. If a realm is used, the realm MAY be chosen by the operator and it MAY a configurable parameter in the EAP/SIM peer implementation. In this case, the peer is typically configured with the NAI realm of the home operator. Operators MAY reserve a specific realm name for EAP/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. When the peer is using a pseudonym username instead of the permanent username, the peer selects the realm name portion similarly as it select the realm portion when using the permanent username. If no configured realm name is available, the peer MAY derive the realm name from the MCC and MNC portions of the IMSI. A recommended way to derive the realm from the IMSI using the realm 3gppnetwork.org will be specified in [Draft 3GPP TS 23.234]. Alternatively, the realm name may be obtained by concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits of IMSI and ".owlan.org". For example, if the IMSI is 123456789098765, and the MNC is three digits long, then the derived realm name is "mnc456.mcc123.owlan.org". The IMSI is a string of digits without any explicit structure, so the peer may not be able to determine the length of the MNC portion. If the peer is not able to determine whether the MNC is two or three digits long, the peer MAY use a 3-digit MNC. If the correct length of the MNC is two, then the MNC used in the realm name includes the first digit of MSIN. Hence, when configuring AAA networks for operators that have 2-digit MNC's, the network SHOULD also be prepared for realm names with incorrect 3-digit MNC's. Format of the Permanent Username The non-pseudonym permanent username SHOULD be derived from the IMSI. In this case, the permanent username MUST be of the format "1" | IMSI, where the character "|" denotes concatenation. 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 [GSM 03.03]. Haverinen and Salowey Expires: 27 April, 2004 [Page 11] Internet Draft EAP SIM Authentication 27 October, 2003 The EAP server MAY use the leading "1" as a hint to try EAP/SIM as the first authentication method during method negotiation, rather than for example EAP/AKA. The EAP/SIM server MAY propose EAP/SIM even if the leading character was not "1". Alternatively, an implementation MAY choose a permanent username that is not based on the IMSI. In this case the selection of the username, its format, and its processing is out of the scope of this document. In this case, the peer implementation MUST NOT prepend any leading characters to the username. Generating Pseudonyms and Re-authentication Identities by the Server Pseudonym usernames and re-authentication identities are generated by the EAP server. The EAP server produces pseudonym usernames and re-authentication identities in an implementation-dependent manner. Only the EAP server needs to be able to map the pseudonym username to the permanent identity, or to recognize a re-authentication identity. Regardless of construction method, the pseudonym username MUST conform to the grammar specified for the username portion of an NAI. The re-authentication identity also MUST conform to the NAI grammar. The EAP servers that the subscribers of an operator can use MUST ensure that the pseudonym usernames and the username portions used in re-authentication identities they generate are unique. In any case, it is necessary that permanent usernames, pseudonym usernames and re-authentication usernames are separate and recognizable from each other. It is also desirable that EAP SIM and EAP AKA user names be recognizable from each other as an aid for the server to which method to offer. In general, it is the task of the EAP server and the policies of its administrator to ensure sufficient separation in the usernames. Pseudonym usernames and re-authentication usernames are both produced and used by the EAP server. The EAP server MUST compose pseudonym usernames and re-authentication usernames so that it can recognize if a NAI username is an EAP SIM pseudonym username or an EAP SIM re-authentication username. For instance, when the usernames have been derived from the IMSI, the server could use different leading characters in the pseudonym usernames and re-authentication usernames (e.g. the pseudonym could begin with a leading "3" character). When mapping a re-authentication identity to a permanent identity, the server SHOULD only examine the username portion of the re-authentication identity and ignore the realm portion of the identity. Because the peer may fail to save a pseudonym username sent to in an EAP-Request/SIM/Challenge, for example due to malfunction, the EAP server SHOULD maintain at least one old pseudonym username in addition to the most recent pseudonym username. Haverinen and Salowey Expires: 27 April, 2004 [Page 12] Internet Draft EAP SIM Authentication 27 October, 2003 Transmitting Pseudonyms and Re-authentication Identities to the Peer The server transmits pseudonym usernames and re-authentication identities to the peer in cipher, using the AT_ENCR_DATA attribute. The EAP-Request/SIM/Challenge message MAY include an encrypted pseudonym username and/or an encrypted re-authentication identity in the value field of the AT_ENCR_DATA attribute. Because identity privacy support and re-authentication are optional to implement, the peer MAY ignore the AT_ENCR_DATA attribute and always use the permanent identity. On re-authentication (discussed in Section 4.3), the server MAY include a new encrypted re-authentication identity in the EAP-Request/SIM/Re-authentication message. On receipt of the EAP-Request/SIM/Challenge, the peer MAY decrypt the encrypted data in AT_ENCR_DATA and if a pseudonym username is included, the peer may use the obtained pseudonym username on the next full authentication. If a re-authentication identity is included, then the peer MAY save it and other re-authentication state information, as discussed in Section 4.3, for the next re- authentication. If the peer does not receive a new pseudonym username in the EAP- Request/SIM/Challenge message, the peer MAY use an old pseudonym username instead of the permanent username on next full authentication. The username portions of re-authentication identities are one-time usernames, which the peer MUST NOT re-use. Usage of the Pseudonym by the Peer When the optional identity privacy support is used on full authentication, the peer MAY use the pseudonym username received as part of the previous full authentication sequence as the username portion of the NAI. The peer MUST NOT modify the pseudonym username received in AT_NEXT_PSEUDONYM. However, as discussed above, the peer MAY need to decorate the username in some environments by appending or prepending the username with a string that indicates supplementary AAA routing information. When using a pseudonym username in an environment where a realm portion is used, the peer concatenates the received pseudonym username with the "@" character and a NAI realm portion. The selection of the NAI realm is discussed above. Usage of the Re-authentication Identity by the Peer On re-authentication, the peer uses the re-authentication identity, received as part of the previous authentication sequence. A new re- authentication identity may be delivered as part of both full authentication and re-authentication. The peer MUST NOT modify the username part of the re-authentication identity received in AT_NEXT_REAUTH_ID, except in cases when username decoration is required. Even in these cases, the "root" re-authentication username Haverinen and Salowey Expires: 27 April, 2004 [Page 13] Internet Draft EAP SIM Authentication 27 October, 2003 must not be modified, but it may be appended or prepended with another string. 4.2.2 Communicating the Peer Identity to the Server General The peer identity MAY be communicated to the server with the EAP- Response/Identity message. This message MAY contain the permanent identity, a pseudonym identity, or a re-authentication identity. If the peer uses the permanent identity or a pseudonym identity, which the server is able to map to the permanent identity, then the authentication proceeds as discussed in the overview of Section 3. If the peer uses a re-authentication identity, and the server recognized the identity and agrees on using re-authentication, then a re-authentication exchange is performed, as described in 4.3. The peer identity can also be transmitted from the peer to the server using EAP/SIM messages instead of EAP-Response/Identity. In this case, the server includes an identity requesting attribute (AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ) in the EAP-Request/SIM/Start message, and the peer includes the AT_IDENTITY attribute, which contains the peer's identity, in the EAP- Response/SIM/Start message. The AT_ANY_ID_REQ attribute is a general identity requesting attribute, which the server uses if it does not specify which kind of an identity the peer should return in AT_IDENTITY. The server uses the AT_FULLAUTH_ID_REQ attribute to request either the permanent identity or a pseudonym identity. The server uses the AT_PERMANENT_ID_REQ attribute to request the peer to send its permanent identity. The identity format in the AT_IDENTITY attribute is the same as in the EAP-Response/Identity packet (except that identity decoration is not allowed). The AT_IDENTITY attribute contains a permanent identity, a pseudonym identity or a re-authentication identity. Obtaining the subscriber identity via EAP/SIM messages is useful if the server does not have any EAP/SIM peer identity at the beginning of the EAP/SIM exchange or does not recognize the identity the peer used in EAP-Response/Identity. This may happen if, for example, the EAP-Response/Identity has been issued by some EAP method other than EAP/SIM or if intermediate entities or software layers in the peer have modified the identity string in the EAP-Response/Identity packet. Also, some EAP layer implementations may cache the identity string from the first EAP authentication and do not obtain a new identity string from the EAP method implementation on subsequent authentication exchanges. As the identity string is used in key derivation, any of these cases will result in failed authentication unless the EAP server uses EAP/SIM attributes to obtain an unmodified copy of the identity string. Therefore, unless the EAP server can be certain that no intermediate element or software layer has modified the EAP- Haverinen and Salowey Expires: 27 April, 2004 [Page 14] Internet Draft EAP SIM Authentication 27 October, 2003 Response/Identity packet, the EAP server SHOULD always use the EAP/SIM attributes to obtain the identity, even if the identity received in EAP-Response/Identity was valid. Please note that the EAP/SIM peer 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 peer with the original identity from the EAP-Response/Identity packet regardless of whether the AT_IDENTITY attribute is used in EAP/SIM to transmit another identity. Choice of Identity for the EAP-Response/Identity If EAP/SIM peer is started upon receiving an EAP-Request/Identity message, then the peer performs the following steps. If the peer has maintained re-authentication state information and if the peer wants to use re-authentication, then the peer transmits the re-authentication identity in EAP-Response/Identity. Else, if the peer has a pseudonym username available, then the peer transmits the pseudonym identity in EAP-Response/Identity. In other cases, the peer transmits the permanent identity in EAP- Response/Identity. Server Operation in the Beginning of EAP/SIM Exchange If the EAP server has not received any identity (permanent identity, pseudonym identity or re-authentication identity) from the peer when sending the first EAP/SIM request, or if the EAP server has received an EAP-Response/Identity packet but the contents do not appear to be a valid permanent identity, pseudonym identity or a re- authentication identity, then the server MUST request an identity from the peer using one of the methods below. The server sends the EAP-Request/SIM/Start message with the AT_PERMANENT_ID_REQ message to indicate that the server wants the peer to include the permanent identity in the AT_IDENTITY attribute of the EAP-Response/SIM/Start message. This is done in the following cases: - The server does not support re-authentication or identity privacy. - The server received an identity that it recognizes as a pseudonym identity but the server is not able to map the pseudonym identity to a permanent identity. The server issues the EAP-Request/SIM/Start packet with the AT_FULLAUTH_ID_REQ attribute to indicate that the server wants the Haverinen and Salowey Expires: 27 April, 2004 [Page 15] Internet Draft EAP SIM Authentication 27 October, 2003 peer to include a full authentication identity (pseudonym identity or permanent identity) in the AT_IDENTITY attribute of the EAP- Response/SIM/Start message. This is done in the following cases: - The server does not support re-authentication and the server supports identity privacy - The server received an identity that it recognizes as a re- authentication identity but the server is not able to map the re- authentication identity to a permanent identity The server issues the EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ attribute to indicate that the server wants the peer to include an identity in the AT_IDENTITY attribute of the EAP- Response/SIM/Start message, and the server does not indicate any preferred type for the identity. This is done in other cases, such as when the server does not have any identity, or the server does not recognize the format of a received identity. Processing of EAP-Request/SIM/Start by the Peer Upon receipt of an EAP-Request/SIM/Start message, the peer MUST perform the following steps. If the EAP-Request/SIM/Start does not include any identity request attribute, then the peer responds with EAP-Response/SIM/Start without AT_IDENTITY. The peer includes the AT_SELECTED_VERSION and AT_NONCE_MT attributes, because the exchange is a full authentication exchange. If the EAP-Request/SIM/Start includes AT_PERMANENT_ID_REQ the peer MUST either respond with EAP-Response/SIM/Start and include the permanent identity in AT_IDENTITY or respond with EAP- Response/SIM/Client-Error packet with code "unable to process packet". If the EAP-Request/SIM/Start includes AT_FULL_AUTH_ID_REQ, and if the peer has a pseudonym available, then the peer SHOULD respond with EAP-Response/SIM/Start and includes the pseudonym identity in AT_IDENTITY. If the peer does not have a pseudonym when it receives this message, then the peer MUST either respond with EAP- Response/SIM/Start and include the permanent identity in AT_IDENTITY or respond with EAP-Response/SIM/Client-Error packet with code "unable to process packet." The Peer MUST NOT use a re- authentication identity in the AT_IDENTITY attribute. If the EAP-Request/SIM/Start includes AT_ANY_ID_REQ, and if the peer has maintained re-authentication state information and the peer wants to use re-authentication, then the peer responds with EAP- Response/SIM/Start and includes the re-authentication identity in AT_IDENTITY. Else, if the peer has a pseudonym identity available, then the peer responds with EAP-Response/SIM/Start and includes the pseudonym identity in AT_IDENTITY. Else, the peer responds with EAP- Haverinen and Salowey Expires: 27 April, 2004 [Page 16] Internet Draft EAP SIM Authentication 27 October, 2003 Response/SIM/Start and includes the permanent identity in AT_IDENTITY. An EAP/SIM exchange may include several EAP/SIM/Start rounds. The server may issue a second EAP-Request/SIM/Start, if it was not able to recognize the identity the peer used in the previous AT_IDENTITY attribute. At most three EAP/SIM/Start rounds can be used. AT_ANY_ID_REQ can only be used in the first EAP-Request/SIM/Start, in other words AT_ANY_ID_REQ MUST NOT be used in the second or third EAP-Request/SIM/Start. AT_FULLAUTH_ID_REQ MUST NOT be used if the previous EAP-Request/SIM/Start included AT_PERMANENT_ID_REQ. The peer operation in cases when it receives an unexpected attribute is specified in Section 4.5.1. Attacks against Identity Privacy The section above specifies two possible ways the peer can operate upon receipt of AT_PERMANENT_ID_REQ. This is because a received AT_PERMANENT_ID_REQ does not necessarily originate from the valid network, but an active attacker may transmit an EAP- Request/SIM/Start packet with an AT_PERMANENT_ID_REQ attribute to the peer, in an effort to find out the true identity of the user. If the peer does not want to reveal its permanent identity, then the peer sends the EAP-Response/SIM/Client-Error packet with the error code "unable to process packet", and the authentication exchange terminates. Basically, there are two different policies that the peer can employ with regard to AT_PERMANENT_ID_REQ. A "conservative" peer assumes that the network is able to maintain pseudonyms robustly. Therefore, if a conservative peer has a pseudonym username, the peer responds with EAP-Response/SIM/Client-Error to the EAP packet with AT_PERMANENT_ID_REQ, because the peer believes that the valid network is able to map the pseudonym identity to the peer's permanent identity. (Alternatively, the conservative peer may accept AT_PERMANENT_ID_REQ in certain circumstances, for example if the pseudonym was received a long time ago.) The benefit of this policy is that it protects the peer against active attacks on anonymity. On the other hand, a "liberal" peer always accepts the AT_PERMANENT_ID_REQ and responds with the permanent identity. The benefit of this policy is that it works even if the valid network sometimes loses pseudonyms and is not able to map them to the permanent identity. Processing of AT_IDENTITY by the Server When the server receives an EAP-Response/SIM/Start message with the AT_IDENTITY (in response to the server's identity requesting attribute), the server MUST operate as follows. If the server used AT_PERMANENT_ID_REQ, and if the AT_IDENTITY does not contain a valid permanent identity, then the server sends EAP Failure and the EAP exchange terminates. If the server recognizes Haverinen and Salowey Expires: 27 April, 2004 [Page 17] Internet Draft EAP SIM Authentication 27 October, 2003 the permanent identity and is able to continue, then the server proceeds with full authentication by sending EAP- Request/SIM/Challenge. If the server used AT_FULLAUTH_ID_REQ, and if AT_IDENTITY contains a valid permanent identity or a pseudonym identity that the server can map to a valid permanent identity, then the server proceeds with full authentication by sending EAP-Request/SIM/Challenge. If AT_IDENTITY contains a pseudonym identity that the server is not able to map to a valid permanent identity, or an identity that the server is not able to recognize or classify, then the server sends EAP-Request/SIM/Start with AT_PERMANENT_ID_REQ. If the server used AT_ANY_ID_REQ, and if the AT_IDENTITY contains a valid permanent identity or a pseudonym identity that the server can map to a valid permanent identity, then the server proceeds with full authentication by sending EAP-Request/SIM/Challenge. If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid re-authentication identity and the server agrees on using re- authentication, then the server proceeds with re-authentication by sending EAP-Request/SIM/Re-authentication (Section 4.3). If the server used AT_ANY_ID_REQ, and if the peer sent an EAP- Response/SIM/Start with only AT_IDENTITY (indicating re- authentication), but the server is not able to map the identity to a permanent identity, then the server sends EAP-Request/SIM/Start with AT_FULLAUTH_ID_REQ. If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid re-authentication identity, which the server is able to map to a permanent identity, and if the server does not want to use re- authentication, then the server sends EAP-Request/SIM/Start without any identity requesting attributes. If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an identity that the server recognizes as a pseudonym identity but the server is not able to map the pseudonym identity to a permanent identity, then the server sends EAP-Request/SIM/Start with AT_PERMANENT_ID_REQ. If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an identity that the server is not able to recognize or classify, then the server sends EAP-Request/SIM/Start with AT_FULLAUTH_ID_REQ. 4.2.3 Message Sequence Examples (Informative) This section contains non-normative message sequence examples to illustrate how the peer identity can be communicated to the server. Haverinen and Salowey Expires: 27 April, 2004 [Page 18] Internet Draft EAP SIM Authentication 27 October, 2003 Full Authentication 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. Peer 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 peer uses its full authentication identity and the AT_IDENTITY attribute contains a valid permanent identity or a valid pseudonym identity that the EAP server is able to map to the permanent identity, then the full authentication sequence proceeds as usual with the EAP Server issuing the EAP-Request/SIM/Challenge message. Re-authentication The case when the server uses the AT_ANY_ID_REQ and the peer wants to perform re-authentication is illustrated below. Haverinen and Salowey Expires: 27 April, 2004 [Page 19] Internet Draft EAP SIM Authentication 27 October, 2003 Peer 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) | |------------------------------------------------------>| | | 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 4.3. Fall Back to Full Authentication The case when the server does not recognize the re-authentication identity the peer used in AT_IDENTITY, and issues a second EAP- Request/SIM/Start message is illustrated below. Haverinen and Salowey Expires: 27 April, 2004 [Page 20] Internet Draft EAP SIM Authentication 27 October, 2003 Peer 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) | |------------------------------------------------------>| | | Requesting the Permanent Identity 1 The figure below illustrates the case when the EAP server fails to map the pseudonym identity included in the EAP-Response/Identity packet to a valid permanent identity. Haverinen and Salowey Expires: 27 April, 2004 [Page 21] Internet Draft EAP SIM Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes a pseudonym) | |------------------------------------------------------>| | | | +------------------------------+ | | Server fails to map the | | | Pseudonym to a permanent id. | | +------------------------------+ | | | EAP-Request/SIM/Start | | (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (AT_IDENTITY with permanent identity, AT_NONCE_MT, | | AT_SELECTED_VERSION) | |------------------------------------------------------>| | | 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. Requesting the Permanent Identity 2 The figure below illustrates the case when the EAP server fails to map the pseudonym included in the AT_IDENTITY attribute to a valid permanent identity. Haverinen and Salowey Expires: 27 April, 2004 [Page 22] Internet Draft EAP SIM Authentication 27 October, 2003 Peer 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 map the | | | Pseudonym in AT_IDENTITY | | | to a valid permanent identity | | +-------------------------------+ | | | EAP-Request/SIM/Start | | (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (AT_IDENTITY with permanent identity, | | AT_NONCE_MT, AT_SELECTED_VERSION) | |------------------------------------------------------>| | | Three EAP/SIM/Start Roundtrips In the worst case, there are three EAP/SIM/Start round trips before the server has obtained an acceptable identity. This case is illustrated below. Haverinen and Salowey Expires: 27 April, 2004 [Page 23] Internet Draft EAP SIM Authentication 27 October, 2003 Peer 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 map the | | | Pseudonym in AT_IDENTITY | | | to a valid permanent identity | | +-------------------------------+ | | | EAP-Request/SIM/Start | | (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (AT_IDENTITY with permanent identity, AT_NONCE_MT, | | AT_SELECTED_VERSION) | |------------------------------------------------------>| | | 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. Haverinen and Salowey Expires: 27 April, 2004 [Page 24] Internet Draft EAP SIM Authentication 27 October, 2003 4.3. Re-Authentication 4.3.1 General In some environments, EAP authentication may be performed frequently. Because the EAP SIM full authentication procedure makes use of the GSM SIM A3/A8 algorithms, and 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 peer. On each EAP authentication, either one of the entities may also fall back on full authentication if they do not want to use re-authentication. Re-authentication is based on the keys derived on the preceding full authentication. The same K_aut and K_encr keys as in full authentication are used to protect EAP 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 4.6. On re-authentication, the peer protects against replays with an unsigned 16-bit counter, included in the AT_COUNTER attribute. On full authentication, both the server and the peer initialize the counter to one. The counter value of at least one is used on the first re-authentication. On subsequent re-authentications, the counter MUST be greater than on any of the previous re- authentications. For example, on the second re-authentication, counter value is two or greater etc. The AT_COUNTER attribute is encrypted. The server includes an encrypted server nonce (AT_NONCE_S) in the re-authentication request. The AT_MAC attribute in the peer's response is calculated over NONCE_S to provide a challenge/response authentication scheme. The NONCE_S also contributes to the new Master Session Key. Both the peer and the server SHOULD have an upper limit for the number of subsequent re-authentications allowed before a full authentication needs to be performed. Because a 16-bit counter is used in re-authentication, the theoretical maximum number of re- authentications is reached when the counter value reaches 0xFFFF. In order to use re-authentication, the peer and the EAP server need to store the following values: Master Key, latest counter value and the next re-authentication identity. K_aut, K_encr may either be stored or derived again from MK. The server may also need to store the permanent identity of the user. Haverinen and Salowey Expires: 27 April, 2004 [Page 25] Internet Draft EAP SIM Authentication 27 October, 2003 4.3.2 Re-authentication Identity The re-authentication procedure makes use of separate re- authentication user identities. Pseudonyms and the permanent identity are reserved for full authentication only. If a re- authentication identity is lost and the network does not recognize it, the EAP server can fall back on full authentication. 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 6.3). This attribute contains a new re-authentication identity for the next re-authentication. The peer MAY ignore this attribute, in which case it MUST use full authentication next time. If the peer wants to use re- authentication, it uses this re-authentication identity on next authentication. Even if the peer has a re-authentication identity, the peer MAY discard the re-authentication identity and use a pseudonym or the permanent identity instead, in which case full authentication MUST be performed. In environments where a realm portion is needed in the peer identity, the re-authentication identity received in AT_NEXT_REAUTH_ID MUST contain both a username portion and a realm portion, as per the NAI format. The EAP Server can choose an appropriate realm part in order to have the AAA infrastructure route subsequent re-authentication related requests to the same AAA server. For example, the realm part MAY include a portion that is specific to the AAA server. Hence, it is sufficient to store the context required for re-authentication in the AAA server that performed the full authentication. The peer MAY use the re-authentication identity in the EAP- Response/Identity packet or, in response to server's AT_ANY_ID_REQ attribute, the peer MAY use the re-authentication identity in the AT_IDENTITY attribute of the EAP-Response/SIM/Start packet. The peer MUST NOT modify the username portion of the re-authentication identity, but the peer MAY modify the realm portion or replace it with another realm portion. Even if the peer uses a re-authentication identity, the server may want to fall back on full authentication, for example because the server does not recognize the re-authentication identity or does not want to use re-authentication. In this case, the server starts the full authentication procedure by issuing an EAP-Request/SIM/Start packet. This packet always starts a full authentication sequence if it does not include the AT_ANY_ID_REQ attribute. If the server was not able to recover the peer's identity from the re-authentication identity, the server includes either the AT_FULLAUTH_ID_REQ or the AT_PERMANENT_ID_REQ attribute in this EAP request. Haverinen and Salowey Expires: 27 April, 2004 [Page 26] Internet Draft EAP SIM Authentication 27 October, 2003 4.3.3 Re-authentication Procedure The following figure illustrates the re-authentication procedure. Encrypted attributes are denoted with '*'. The peer uses its re- authentication identity in the EAP-Response/Identity packet. As discussed above, an alternative way to communicate the re- authentication identity to the server is for the peer to use the AT_IDENTITY attribute in the EAP-Response/SIM/Start message. This latter case is not illustrated in the figure below, and it is only possible when the server requests the peer to send its identity by including the AT_ANY_ID_REQ attribute in the EAP-Request/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 peer. This packet MUST include the encrypted AT_COUNTER attribute, with a fresh counter value, the encrypted AT_NONCE_S attribute that contains a random number chosen by the server, the AT_ENCR_DATA and the AT_IV attributes used for encryption, and the AT_MAC attribute that contains a message authentication code over the packet. The packet MAY also include an encrypted AT_NEXT_REAUTH_ID attribute that contains the next re-authentication identity. Re-authentication identities are one-time identities. If the peer does not receive a new re-authentication identity, it MUST use either the permanent identity or a pseudonym identity on the next authentication to initiate full authentication. The peer verifies that the counter value is fresh (greater than any previously used value). The peer also verifies that AT_MAC is correct. The peer MAY save the next re-authentication identity from the encrypted AT_NEXT_REAUTH_ID for next time. If all checks are successful, the peer responds with the EAP-Response/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 peer. Haverinen and Salowey Expires: 27 April, 2004 [Page 27] Internet Draft EAP SIM Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes a re-authentication identity) | |------------------------------------------------------>| | | | +--------------------------------+ | | Server recognizes the identity | | | and agrees on using fast | | | re-authentication | | +--------------------------------+ | | | EAP-Request/SIM/Re-authentication | | (AT_IV, AT_ENCR_DATA, *AT_COUNTER, | | *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC) | |<------------------------------------------------------| | | | | +-----------------------------------------------+ | | Peer verifies AT_MAC and the freshness of | | | the counter. Peer MAY store the new 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 | |<------------------------------------------------------| | | 4.3.4 Re-authentication Procedure when Counter is Too Small If the peer does not accept the counter value of EAP-Request/SIM/Re- authentication, it indicates the counter synchronization problem by including the encrypted AT_COUNTER_TOO_SMALL in EAP-Response/SIM/Re- authentication. The server responds with EAP-Request/SIM/Start to initiate a normal full authentication procedure. This is illustrated in the following figure. Encrypted attributes are denoted with '*'. Haverinen and Salowey Expires: 27 April, 2004 [Page 28] Internet Draft EAP SIM Authentication 27 October, 2003 Peer 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 peer 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 peer detects that the counter value is not fresh, it includes the AT_COUNTER_TOO_SMALL attribute in EAP-Response/SIM/Re-authentication. This attribute doesn't contain any data but it is a request for the server to initiate full authentication. In this case, the peer MUST ignore the contents of the server's AT_NEXT_REAUTH_ID attribute. On receipt of AT_COUNTER_TOO_SMALL, the server verifies AT_MAC and verifies that AT_COUNTER contains the same as in the EAP- Request/SIM/Re-authentication packet. If not, the server terminates the authentication exchange and sends the EAP Failure 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 Haverinen and Salowey Expires: 27 April, 2004 [Page 29] Internet Draft EAP SIM Authentication 27 October, 2003 identity, it MUST NOT include AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ in the EAP-Request/SIM/Start. 4.4. EAP/SIM Notifications The EAP-Request/Notification, specified in [EAP], can be used to convey a displayable message from the EAP server to the peer. Because these messages are textual messages, it may be hard for the peer to present them in the user's preferred language. Therefore, EAP/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 peer. The peer MAY show a notification message to the user and the peer MUST respond to the EAP server with an EAP- Response/SIM/Notification packet, even if the peer did not recognize the notification code. The notification code is a 16-bit number. The most significant bit is called the Failure bit (F bit). The F bit specifies whether the notification implies failure. The code values with the F bit set to zero (code values 0...32767) are used on unsuccessful cases. The receipt of a notification code from this range implies failed authentication, so the peer can use the notification as a failure indication. After receiving the EAP-Response/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 peer MUST NOT change its state when it receives such a notification. (This version of the protocol does not specify any notification codes with the F bit set to one.) The second most significant bit of the notification code is called the Phase bit (P bit). It specifies at which phase of the EAP/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/Re-authentication 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/Re-authentication 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. (This version of the protocol does not specify any notification codes with the P bit set to one.) Haverinen and Salowey Expires: 27 April, 2004 [Page 30] Internet Draft EAP SIM Authentication 27 October, 2003 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. 4.5. Error Cases This section specifies the operation of the peer and the server in error cases. The subsections below require the EAP/SIM peer and server to send an error packet (EAP-Response/SIM/Client-Error or EAP Failure) in error cases. However, implementations SHOULD NOT rely upon the correct error reporting behavior of the peer, authenticator, or the server. It is possible for error and other messages to be lost in transit or for a malicious participant to attempt to consume resources by not issuing error messages. Both the peer and the EAP server SHOULD have a mechanism to clean up state even if an error message or EAP Success is not received after a timeout period. 4.5.1 Peer Operation In general, if an EAP/SIM peer detects an error in a received EAP/SIM packet, the EAP/SIM implementation responds with the EAP- Response/SIM/Client-Error packet. In response to the EAP- Response/SIM/Client-Error, the EAP server MUST issue the EAP Failure packet and the authentication exchange terminates. By default, the peer uses the client error code 0, "unable to process packet". This error code is used in the following cases: - the peer is not able to parse the EAP request, i.e. the EAP request is malformed - the peer encountered a malformed attribute - wrong attribute types or duplicate attributes have been included in the EAP request - a mandatory attribute is missing - unrecognized non-skippable attribute - unrecognized or unexpected EAP/SIM Subtype in the EAP request - A RAND challenge repeated in AT_RAND - invalid AT_MAC - invalid pad bytes in AT_PADDING Haverinen and Salowey Expires: 27 April, 2004 [Page 31] Internet Draft EAP SIM Authentication 27 October, 2003 - the peer does not want to process AT_PERMANENT_ID_REQ Separate error codes have been defined for the following error cases in Section 7.18: As specified in Section 4.1, when processing the AT_VERSION_LIST attribute, which lists the EAP/SIM versions supported by the server, if the attribute does not include a version that is implemented by the peer and allowed in the peer's security policy, then the peer MUST send the EAP-Response/SIM/Client-Error packet with the error code "unsupported version". When processing the AT_RAND attribute, the peer MUST send the EAP- Response/SIM/Client-Error packet with the error code "insufficient number of challenges", if the number of RAND challenges is smaller than what is required by peer's local policy. If the peer believes that the RAND challenges included in AT_RAND are not fresh e.g. because it is capable of remembering some previously used RANDs, the peer MUST send the EAP- Response/SIM/Client-Error packet with the error code "RANDs are not fresh". 4.5.2 Server Operation If an EAP/SIM server detects an error in a received EAP/SIM response, the server MUST issue the EAP Failure packet and the authentication exchange terminates. The errors cases when the server issues an EAP Failure include the following: - the server is not able to parse the peer's EAP response - the server encounters a malformed attribute, a non-recognized non- skippable attribute, or a duplicate attribute - a mandatory attribute is missing or an invalid attribute was included - unrecognized or unexpected EAP/SIM Subtype in the EAP Response - invalid AT_MAC - invalid AT_COUNTER 4.5.3 EAP Failure As normally in EAP, the EAP server sends the EAP-Failure packet to the peer when the authentication procedure fails on the EAP Server. In EAP/SIM, this may occur for example if the EAP server does not recognize the peer identity, or if the EAP server is not able to obtain the GSM triplets for the subscriber or the authentication exchange times out. The server may also send EAP Failure if there is Haverinen and Salowey Expires: 27 April, 2004 [Page 32] Internet Draft EAP SIM Authentication 27 October, 2003 an error in the received EAP/SIM response, as discussed in Section 4.5.2. The server can send EAP-Failure at any time in the EAP exchange. The peer MUST process EAP-Failure. .5.4 EAP Success On full authentication, the server can only send EAP-Success after the EAP/SIM/Challenge round. The peer MUST silently discard any EAP- Success packets if they are received before the peer has successfully authenticated the server and sent the EAP- Response/SIM/Challenge packet. On re-authentication, EAP-Success can only be sent after the EAP/SIM/Re-authentication round. The peer MUST silently discard any EAP-Success packets if they are received before the peer has successfully authenticated the server and sent the EAP- Response/SIM/Re-authentication packet. If the peer receives an EAP/SIM notification (section 4.4) that indicates failure, then the peer MUST no longer accept the EAP- Success packet even if the server authentication was successfully completed. 4.6. Key Generation This section specifies how keying material is generated. On EAP SIM full authentication, a Master Key (MK) is derived from the underlying GSM authentication values (Kc keys), the NONCE_MT and other relevant context as follows. MK = SHA1(Identity|n*Kc| NONCE_MT| Version List| Selected Version) In the formula above, the "|" character denotes concatenation. Identity denotes the peer identity string without any terminating null characters. It is the identity from the 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 identity string is included as-is, without any changes and including the possible identity decoration. 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 SHA-1 is specified in [SHA-1]. If several EAP/SIM/Start roundtrips are used in an EAP/SIM exchange, then the NONCE_MT, Version List and Selected version from the last EAP/SIM/Start round are used, and the previous EAP/SIM/Start rounds are ignored. Haverinen and Salowey Expires: 27 April, 2004 [Page 33] Internet Draft EAP SIM Authentication 27 October, 2003 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 MUST be used for protecting EAP packets, but a new MSK and a new EMSK MUST be derived from the original MK and new values exchanged in the re- authentication. EAP 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. Key derivation is based on the random number generation specified in NIST Federal Information Processing Standards (FIPS) Publication 186-2 [PRF]. The pseudo-random number generator is specified in the change notice 1 (2001 October 5) of [PRF] (Algorithm 1). As specified in the change notice (page 74), when Algorithm 1 is used as a general-purpose pseudo-random number generator, the "mod q" term in step 3.3 is omitted. The function G used in the algorithm is constructed via Secure Hash Standard as specified in Appendix 3.3 of the standard. It should be noted that the function G is very similar to SHA-1, but the message padding is different. Please refer to [PRF] for full details. For convenience, the random number algorithm with the correct modification is cited in Annex 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 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 Haverinen and Salowey Expires: 27 April, 2004 [Page 34] Internet Draft EAP SIM Authentication 27 October, 2003 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 [RFC 2548] are used to transport keying material, then the first 32 bytes of the MSK correspond to MS-MPPE-RECV-KEY and the second 32 bytes to MS-MPPE- SEND-KEY. In this case, only 64 bytes of keying material (the MSK) are used. When generating the initial Master Key, the hash function is used as a mixing function to combine several session keys (Kc's) generated by the GSM authentication procedure and the random number NONCE_MT into a single session key. There are several reasons for this. The current GSM session keys are at most 64 bits, so two or more of them are needed to generate a longer key. By using a one-way function to combine the keys, we are assured that even if an attacker managed to learn one of the EAP/SIM session keys, it wouldn't help him in learning the original GSM Kc's. In addition, since we include the random number NONCE_MT in the calculation, the peer is able to verify that the EAP SIM packets it receives from the network are fresh and not a replay. (Please see also Section 9.) 5. Message Format and Protocol Extensibility 5.1. Message Format As specified in [EAP], EAP packets begin with the Code, Identifiers, Length, and Type fields, which are followed by EAP method specific Type-Data. The Code field in the EAP header is set to 1 for EAP requests, and to 2 for EAP Responses. The usage of the Length and Identifier fields in the EAP header are also specified in [EAP]. In EAP/SIM, the Type field is set to 18. In EAP/SIM, the Type-Data begins with an EAP/SIM header that consists of a 1-octet Subtype field and a 2-octet reserved field. The Subtype values used in EAP/SIM are defined in Section 8. The formats of the EAP header and the EAP/SIM header are shown below. Haverinen and Salowey Expires: 27 April, 2004 [Page 35] Internet Draft EAP SIM Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The rest of the Type-Data, immediately following the EAP/SIM header, consists of attributes that are encoded in Type, Length, Value format. The figure below shows the generic format of an attribute. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Attribute Type Indicates the particular type of attribute. The attribute type values are listed in Section 8. 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. Attributes numbered within the range 0 through 127 are called non- skippable attributes. When an EAP/SIM peer encounters a non- skippable attribute that the peer does not recognize, the peer MUST send the EAP-Response/SIM/Client-Error packet which terminates the authentication exchange. If an EAP/SIM server encounters a non- skippable attribute that the server does not recognize, then the server sends the EAP Failure packet which terminates the authentication exchange. Attributes within the range of 128 through 255 are called skippable attributes. When a skippable attribute is encountered that is not recognized it is ignored. The rest of the attributes and message data MUST still be processed. The Length field of the attribute is Haverinen and Salowey Expires: 27 April, 2004 [Page 36] Internet Draft EAP SIM Authentication 27 October, 2003 used to skip the attribute value in searching for the next attribute. Unless otherwise specified, the order of the attributes in an EAP/SIM message is insignificant and an EAP/SIM implementation should not assume a certain order to be used. Attributes can be encapsulated within other attributes. In other words, the value field of an attribute type can be specified to contain other attributes. 5.2. Protocol Extensibility EAP/SIM can be extended by specifying new attribute types. If skippable attributes are used, it is possible to extend the protocol without breaking old implementations. However, any new attributes added to the EAP-Request/SIM/Start or EAP-Response/SIM/Start packets would not be integrity protected. Therefore, these messages MUST NOT be extended in the current version of EAP/SIM. When specifying new attributes, it should be noted that EAP/SIM does not support message fragmentation. Hence, the sizes of the new extensions MUST be limited so that the maximum transfer unit (MTU) of the underlying lower layer is not exceeded. According to [EAP], lower layers must provide an EAP MTU of 1020 bytes or greater, so any extensions to EAP/SIM SHOULD NOT exceed the EAP MTU of 1020 bytes. Because EAP/SIM supports version negotiation, new versions of the protocol can also be specified by using a new version number. 6. Messages This section specifies the messages used in EAP/SIM. It specifies when a message may be transmitted or accepted, which attributes are allowed in a message, which attributes are required in a message, and other message specific details. The general message format is specified in Section 5.1. 6.1. EAP-Request/SIM/Start In full authentication the first SIM specific EAP Request is EAP- Request/SIM/Start. The EAP/SIM/Start roundtrip is used for two purposes. In full authentication this packet is used to request the peer to send the AT_NONCE_MT attribute to the server. In addition, as specified in Section 4.2, the Start round trip may be used by the server for obtaining the peer identity. As discussed in Section 4.2, several Start rounds may be required in order to obtain a valid peer identity. The server MUST always include the AT_VERSION_LIST attribute. Haverinen and Salowey Expires: 27 April, 2004 [Page 37] Internet Draft EAP SIM Authentication 27 October, 2003 The server MAY include one of the following identity requesting attributes: AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, and AT_ANY_ID_REQ. These three attributes are mutually exclusive, so the server MUST NOT include more than one of the attributes. If the server has received a response from the peer, it MUST NOT issue a new EAP-Request/SIM/Start packet if it has either previously issued an EAP-Request/SIM/Start message without any identity requesting attributes or with the AT_PERMANENT_ID_REQ attribute. If the server has received a response from the peer, it MUST NOT issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ attributes if it has previously issued an EAP- Request/SIM/Start message with the AT_FULLAUTH_ID_REQ attribute If the server has received a response from the peer, it MUST NOT issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ attribute if the server has previously issued an EAP- Request/SIM/Start message with the AT_ANY_ID_REQ attribute. This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA. 6.2. EAP-Response/SIM/Start The peer sends EAP-Response/SIM/Start in response to a valid EAP- Request/SIM/Start from the server. If and only if the server's EAP-Request/SIM/Start includes one of the identity requesting attributes, then the peer MUST include the AT_IDENTITY attribute. The usage of AT_IDENITY is defined in Section 4.2. The AT_NONCE_MT attribute MUST NOT be included if the AT_IDENTITY with a re-authentication identity is present for re-authentication. AT_NONCE_MT MUST be included in all other cases (full authentication). The AT_SELECTED_VERSION attribute MUST NOT be included if the AT_IDENTITY attribute with a re-authentication identity is present for re-authentication. In all other cases, AT_SELECTED_VERSION MUST be included (full authentication). This attribute is used in version negotiation, as specified in Section 4.1. This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA. 6.3. EAP-Request/SIM/Challenge The server sends the EAP-Request/SIM/Challenge after receiving a valid EAP-Response/SIM/Start, containing AT_NONCE_MT and AT_SELECTED_VERSION, and after successfully obtaining the subscriber identity. The AT_RAND attribute MUST be included. Haverinen and Salowey Expires: 27 April, 2004 [Page 38] Internet Draft EAP SIM Authentication 27 October, 2003 The AT_MAC attribute MUST be included. For EAP- Request/SIM/Challenge, the MAC code is calculated over the following data: EAP packet| NONCE_MT The EAP packet is represented as specified in Section 5.1. It is followed by the 16-byte NONCE_MT value from the peer's AT_NONCE_MT attribute. The EAP-Request/SIM/Challenge packet MAY include encrypted attributes for identity privacy and for communicating the next re- authentication identity. In this case, the AT_IV and AT_ENCR_DATA attributes are included (Section 7.3). The plaintext of the AT_ENCR_DATA value field consists of nested attributes. The nested attributes MAY include AT_PADDING (as specified in Section 7.3). If the server supports identity privacy and wants to communicate a pseudonym to the peer for the next full authentication, then the nested encrypted attributes include the AT_NEXT_PSEUDONYM attribute. If the server supports re- authentication and wants to communicate a re-authentication identity to the peer, then the nested encrypted attributes include the AT_NEXT_REAUTH_ID attribute. 6.4. EAP-Response/SIM/Challenge The peer sends EAP-Response/SIM/Challenge in response to a valid EAP-Request/SIM/Challenge. The AT_MAC attribute MUST be included. For EAP- Response/SIM/Challenge, the MAC code is calculated over the following data: EAP packet| n*SRES The EAP packet is represented as specified in Section 5.1. The EAP packet bytes are immediately followed by the two or three SRES values concatenated, denoted above with the notation n*SRES. The SRES values are used in the same order as the corresponding RAND challenges in server's AT_RAND attribute. Later versions of this protocol MAY make use of the AT_ENCR_DATA and AT_IV attributes in this message to include encrypted (skippable) attributes. The EAP server MUST process EAP-Response/SIM/Challenge messages that include these attributes even if the server did not implement these optional attributes. Haverinen and Salowey Expires: 27 April, 2004 [Page 39] Internet Draft EAP SIM Authentication 27 October, 2003 6.5. EAP-Request/SIM/Re-authentication The server sends the EAP-Request/SIM/Re-authentication message if it wants to use re-authentication, and if it has received a valid re- authentication identity in EAP-Response/Identity or EAP- Response/SIM/Start. AT_MAC MUST be included. No message-specific data is included in the MAC calculation. See Section 7.2. The AT_IV and AT_ENCR_DATA attributes MUST be included. The plaintext consists of the following nested encrypted attributes, which MUST be included: AT_COUNTER and AT_NONCE_S. In addition, the nested encrypted attributes MAY include the following attributes: AT_NEXT_REAUTH_ID and AT_PADDING. 6.6. EAP-Response/SIM/Re-authentication The client sends the EAP-Response/SIM/Re-authentication packet in response to a valid EAP-Request/SIM/Re-authentication. The AT_MAC attribute MUST be included. For EAP-Response/SIM/Re- authentication, the MAC code is calculated over the following data: EAP packet| NONCE_S The EAP packet is represented as specified in Section 5.1. It is followed by the 16-byte NONCE_S value from the server's AT_NONCE_S attribute. The AT_IV and AT_ENCR_DATA attributes MUST be included. The nested encrypted attributes MUST include the AT_COUNTER attribute. The AT_COUNTER_TOO_SMALL attribute MAY be included in the nested encrypted attributes, and it is included in cases specified in Section 4.3. The AT_PADDING attribute MAY be included. 6.7. EAP-Response/SIM/Client-Error The peer sends EAP-Response/SIM/Client-Error in error cases, as specified in Section 4.5.1. The AT_CLIENT_ERROR_CODE attribute MUST be included. The AT_MAC, AT_IV, or AT_ENCR_DATA attributes MUST NOT be used with this packet. 6.8. EAP-Request/SIM/Notification The usage of this message is specified in Section 4.4. The AT_NOTIFICATION attribute MUST be included. Haverinen and Salowey Expires: 27 April, 2004 [Page 40] Internet Draft EAP SIM Authentication 27 October, 2003 The AT_MAC attribute is included in cases discussed in Section 4.4. No message-specific data is included in the MAC calculation. See Section 7.2. Later versions of this protocol MAY make use of the AT_ENCR_DATA and AT_IV attributes in this message to include encrypted (skippable) attributes. These attributes MAY be included only if the P bit of the notification code in AT_NOTIFICATION is set to zero. 6.9. EAP-Response/SIM/Notification The usage of this message is specified in Section 4.4. Because this packet is only an acknowledgement of EAP-Request/SIM/Notification, it does not contain any mandatory attributes. The AT_MAC attribute is included in cases described in Section 4.4. No message-specific data is included in the MAC calculation, see Section 7.2. Later versions of this protocol MAY make use of the AT_ENCR_DATA and AT_IV attributes in this message to include encrypted (skippable) attributes. These attributes MAY be included only if the P bit of the notification code in the AT_NOTIFICATION attribute of the server's EAP-Request/SIM/Notification packet is set to zero. 7. Attributes This section specifies the format of message attributes. The attribute type numbers are specified in Section 8. 7.1. Table of Attributes The following table provides a guide to which attributes may be found in which kinds of messages, and in what quantity. Messages are denoted with numbers in parentheses as follows: (1) EAP- Request/SIM/Start, (2) EAP-Response/SIM/Start, (3) EAP- Request/SIM/Challenge, (4) EAP-Response/SIM/Challenge, (5) EAP- Request/SIM/Notification, (6) EAP-Response/SIM/Notification, (7) EAP-Response/SIM/Client-Error (8) EAP-Request/SIM/Re-authentication, and (9) EAP-Response/SIM/Re-authentication. The column denoted with "Encr" indicates whether the attribute is a nested attribute that MUST be included within AT_ENCR_DATA, and the column denoted with "Skip" indicates whether the attribute is a skippable attribute. "0" indicates that the attribute MUST NOT be included in the message, "1" indicates that the attribute MUST be included in the message, "0-1" indicates that the attribute is sometimes included in the message, and "0*" indicates that the attribute is not included in the message in cases specified in this document, but MAY be included in the future versions of the protocol. Haverinen and Salowey Expires: 27 April, 2004 [Page 41] Internet Draft EAP SIM Authentication 27 October, 2003 Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) Encr Skip AT_MAC 0 0 1 1 0-1 0-1 0 1 1 N N AT_IV 0 0 0-1 0* 0* 0* 0 1 1 N Y AT_ENCR_DATA 0 0 0-1 0* 0* 0* 0 1 1 N Y AT_PADDING 0 0 0-1 0* 0* 0* 0 0-1 0-1 Y N AT_VERSION_LIST 1 0 0 0 0 0 0 0 0 N N AT_SELECTED_VERSION 0 0-1 0 0 0 0 0 0 0 N N AT_NONCE_MT 0 0-1 0 0 0 0 0 0 0 N N AT_PERMANENT_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N AT_ANY_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N AT_FULLAUTH_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N AT_IDENTITY 0 0-1 0 0 0 0 0 0 0 N N AT_RAND 0 0 1 0 0 0 0 0 0 N N AT_NEXT_PSEUDONYM 0 0 0-1 0 0 0 0 0 0 Y Y AT_NEXT_REAUTH_ID 0 0 0-1 0 0 0 0 0-1 0 Y Y AT_COUNTER 0 0 0 0 0 0 0 1 1 Y N AT_COUNTER_TOO_SMALL 0 0 0 0 0 0 0 0 0-1 Y N AT_NONCE_S 0 0 0 0 0 0 0 1 0 Y N AT_NOTIFICATION 0 0 0 0 1 0 0 0 0 N N AT_CLIENT_ERROR_CODE 0 0 0 0 0 0 1 0 0 N N It should be noted that attributes AT_PERMANENT_ID_REQ, AT_ANY_ID_REQ and AT_FULLAUTH_ID_REQ are mutually exclusive, so that only one of them can be included at the same time. If one of the attributes AT_IV and AT_ENCR_DATA is included, then both of the attributes MUST be included. 7.2. AT_MAC The AT_MAC attribute is used for EAP/SIM message authentication. Section 6 specifies which messages AT_MAC MUST be included. The value field of the AT_MAC attribute contains two reserved bytes followed by a keyed message authentication code (MAC). The MAC is calculated over the whole EAP packet concatenated with optional message-specific data, with the exception that the value field of the MAC attribute is set to zero when calculating the MAC. The EAP packet includes the EAP header that begins with the Code field, the EAP/SIM header that begins with the Subtype field, and all the attributes, as specified in Section 5.1. The reserved bytes in AT_MAC are set to zero when sending and ignored on reception. The contents of the message-specific data that may be included in the MAC calculation are specified separately for each EAP/SIM message in Section 6. The format of the AT_MAC attribute is shown below. Haverinen and Salowey Expires: 27 April, 2004 [Page 42] Internet Draft EAP SIM Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_MAC | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | MAC | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The MAC algorithm is HMAC-SHA1-128 [RFC 2104] keyed hash value. (The HMAC-SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by truncating the output to 16 bytes. Hence, the length of the MAC is 16 bytes.) The derivation of the authentication key (K_aut) used in the calculation of the MAC is specified in Section 4.6. When the AT_MAC attribute is included in an EAP/SIM message, the recipient MUST process the AT_MAC attribute before looking at any other attributes. If the message authentication code is invalid, then the recipient MUST ignore all other attributes in the message and operate as specified in Section 4.5. 7.3. AT_IV, AT_ENCR_DATA and AT_PADDING AT_IV and AT_ENCR_DATA attributes can be used to transmit encrypted information between the EAP/SIM peer and server. The value field of AT_IV contains two reserved bytes followed by a 16-byte initialization vector required by the AT_ENCR_DATA attribute. The reserved bytes are set to zero when sending and ignored on reception. The AT_IV attribute MUST be included if and only if the AT_ENCR_DATA is included. Section 4.5 specifies the operation if a packet that does not meet this condition is encountered. The sender of the AT_IV attribute chooses the initialization vector by random. The sender MUST NOT reuse the initialization vector value from previous EAP SIM packets and the sender MUST choose it freshly for each AT_IV attribute. The sender SHOULD use a good source of randomness to generate the initialization vector. Please see [RFC 1750] for more information about generating random numbers for security applications. The format of AT_IV is shown below. Haverinen and Salowey Expires: 27 April, 2004 [Page 43] Internet Draft EAP SIM Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IV | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Initialization Vector | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the AT_ENCR_DATA attribute consists of two reserved bytes followed by cipher text bytes encrypted using the Advanced Encryption Standard (AES) [AES] in the Cipher Block Chaining (CBC) mode of operation using the initialization vector from the AT_IV attribute. The reserved bytes are set to zero when sending and ignored on reception. Please see [CBC] for a description of the CBC mode. The format of the AT_ENCR_DATA attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_ENCR_DATA | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Encrypted Data . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The derivation of the encryption key (K_encr) is specified in Section 4.6. 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. If this verification fails on the peer, then it MUST send the EAP-Response/SIM/Client- Error packet with the error code "unable to process packet" to terminate the authentication exchange. If this verification fails on Haverinen and Salowey Expires: 27 April, 2004 [Page 44] Internet Draft EAP SIM Authentication 27 October, 2003 the server, then the server sends EAP Failure to terminate the authentication exchange. The format of the AT_PADDING attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_PADDING | Length | Padding... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 7.4. AT_VERSION_LIST The format of the AT_VERSION_LIST attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_VERSION_L..| Length | Actual Version List Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Supported Version 1 | Supported Version 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Supported Version N | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This attribute is used in version negotiation, as specified in Section 4.1. The attribute contains the version numbers supported by the EAP/SIM server. The server MUST only include versions that it implements and that are allowed in its security policy. The server SHOULD list the versions 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). The value field of this attribute begins with 2-byte Actual Version List Length, which specifies the length of the Version List in bytes, not including the Actual Version List Length attribute length. This field is followed by the list of the versions supported by the server, which each have a length of 2 bytes. For example, if there is only one supported version, then the Actual Version List Length is 2. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the value field with zero bytes when necessary. Haverinen and Salowey Expires: 27 April, 2004 [Page 45] Internet Draft EAP SIM Authentication 27 October, 2003 7.5. AT_SELECTED_VERSION The format of the AT_SELECTED_VERSION attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_SELECTED...| Length = 1 | Selected Version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This attribute is used in version negotiation, as specified in Section 4.1. The value field of this attribute contains a two-byte version number, which indicates the EAP/SIM version that the peer wants to use. 7.6. AT_NONCE_MT The format of the AT_NONCE_MT attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_NONCE_MT | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | NONCE_MT | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the NONCE_MT attribute contains two reserved bytes followed by a random number generated by the peer (16 bytes long) freshly for this EAP/SIM authentication exchange. The random number is used as a seed value for the new keying material. The reserved bytes are set to zero upon sending and ignored upon reception. The peer MUST choose the NONCE_MT value freshly for each EAP/SIM authentication exchange. If an EAP/SIM exchange includes several EAP/SIM/Start rounds, then the peer MAY use the same NONCE_MT value in all EAP-Response/SIM/Start packets. The peer SHOULD use a good source of randomness to generate NONCE_MT. Please see [RFC 1750] for more information about generating random numbers for security applications. 7.7. AT_PERMANENT_ID_REQ The format of the AT_PERMANENT_ID_REQ attribute is shown below. Haverinen and Salowey Expires: 27 April, 2004 [Page 46] Internet Draft EAP SIM Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_PERM..._REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The use of the AT_PERMANENT_ID_REQ is defined in Section 4.2. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. 7.8. AT_ANY_ID_REQ The format of the AT_ANY_ID_REQ attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_ANY_ID_REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The use of the AT_ANY_ID_REQ is defined in Section 4.2. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. 7.9. AT_FULLAUTH_ID_REQ The format of the AT_FULLAUTH_ID_REQ attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_ANY_ID_REQ | Length = 1 | Reserved | +---------------+---------------+-------------------------------+ The use of the AT_FULLAUTH_ID_REQ is defined in Section 4.2. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. 7.10. AT_IDENTITY The format of the AT_IDENTITY attribute is shown below. Haverinen and Salowey Expires: 27 April, 2004 [Page 47] Internet Draft EAP SIM Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IDENTITY | Length | Actual Identity Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Identity (optional) . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The use of the AT_IDENTITY is defined in Section 4.2. The value field of this attribute begins with 2-byte actual identity length, which specifies the length of the identity in bytes. This field is followed by the subscriber identity of the indicated actual length. The identity is the permanent identity, a pseudonym identity or a re-authentication identity. The identity format is specified in Section 4.2.1. The same identity format is used in the AT_IDENTITY attribute and the EAP-Response/Identity packet, with the exception that the peer MUST NOT decorate the identity it includes in AT_IDENTITY. The identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the identity with zero bytes when necessary. 7.11. AT_RAND The format of the AT_RAND attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_RAND | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . n*RAND . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains two reserved bytes followed by n GSM RANDs (each 16 bytes long). The reserved bytes are set to zero upon sending and ignored upon reception. The number of RAND challenges (n) MUST be two or three. The peer MUST verify that the number of RAND challenges is sufficient according to the peer's policy. The server MUST use different RAND values. In other words, a RAND value can only be included once in AT_RAND. When processing the AT_RAND attribute, the peer MUST check that the RANDs are different. The EAP server MUST obtain fresh RANDs for each EAP/SIM full authentication exchange. More specifically, the server MUST consider Haverinen and Salowey Expires: 27 April, 2004 [Page 48] Internet Draft EAP SIM Authentication 27 October, 2003 RANDs it included in AT_RAND to be consumed if the server receives an EAP-Response/SIM/Challenge packet with a valid AT_MAC, or an EAP- Response/SIM/Client-Error with the code "insufficient number of challenges" or "RANDs are not fresh". However, in other cases (if the server does not receive any response to its EAP- Request/SIM/Challenge packet, or if the server receives some other kind of response than the cases listed above), the server does not need to consider the RANDs to be consumed, and the server MAY re-use the RANDs in the AT_RAND attribute of the next full authentication attempt. 7.12. AT_NEXT_PSEUDONYM The format of the AT_NEXT_PSEUDONYM attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_PSEU..| Length | Actual Pseudonym Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Pseudonym . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute begins with 2-byte actual pseudonym length, which specifies the length of the following pseudonym in bytes. This field is followed by a pseudonym username that the peer can use in the next authentication. The username MUST NOT include any realm portion. The username does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the pseudonym with zero bytes when necessary. The username encoding MUST follow the UTF-8 transformation format [RFC2279]. 7.13. AT_NEXT_REAUTH_ID The format of the AT_NEXT_REAUTH_ID attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_REAU..| Length | Actual Re-Auth Identity Length| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Re-authentication Username . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute begins with 2-byte actual re- authentication identity length which specifies the length of the Haverinen and Salowey Expires: 27 April, 2004 [Page 49] Internet Draft EAP SIM Authentication 27 October, 2003 following re-authentication identity in bytes. This field is followed by a re-authentication identity that the peer can use in the next re-authentication, as described in Section 4.3. In environments where a realm portion is required, the re- authentication identity includes both a username portion and a realm name portion. The re-authentication identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the re-authentication identity with zero bytes when necessary. The identity encoding MUST follow the UTF-8 transformation format [RFC2279]. 7.14. AT_COUNTER The format of the AT_COUNTER attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER | Length = 1 | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the AT_COUNTER attribute consists of a 16-bit unsigned integer counter value, represented in network byte order. 7.15. AT_COUNTER_TOO_SMALL The format of the AT_COUNTER_TOO_SMALL attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER...| Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute consists of two reserved bytes, which are set to zero upon sending and ignored upon reception. 7.16. AT_NONCE_S The format of the AT_NONCE_S attribute is shown below. Haverinen and Salowey Expires: 27 April, 2004 [Page 50] Internet Draft EAP SIM Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER | Length = 1 | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NONCE_S | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | NONCE_S | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the AT_NONCE_S attribute contains two reserved bytes followed by a random number generated by the server (16 bytes) freshly for this EAP/SIM re-authentication. The random number is used as challenge for the peer and also a seed value for the new keying material. The reserved bytes are set to zero upon sending and ignored upon reception. The server MUST choose the NONCE_S value freshly for each EAP/SIM re-authentication exchange. The server SHOULD use a good source of randomness to generate NONCE_S. Please see [RFC 1750] for more information about generating random numbers for security applications. 7.17. AT_NOTIFICATION The format of the AT_NOTIFICATION attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_NOTIFICATION| Length = 1 |F|P| Notification Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains a two-byte notification code. The first and second bit (F and P) of the notification code are interpreted as described in Section 4.4. The notification code values listed below have been reserved. The descriptions below illustrate the semantics of the notifications. The peer implementation MAY use different wordings when presenting the notifications to the user. The "requested service" depends on the environment where EAP/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) Haverinen and Salowey Expires: 27 April, 2004 [Page 51] Internet Draft EAP SIM Authentication 27 October, 2003 7.18. AT_CLIENT_ERROR_CODE The format of the AT_CLIENT_ERROR_CODE attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_CLIENT_ERR..| Length = 1 | Client Error Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains a two-byte client error code. The following error code values have been reserved. 0 "unable to process packet": a general error code 1 "unsupported version": the peer does not support any of the versions listed in AT_VERSION_LIST 2 "insufficient number of challenges": the peer's policy requires more triplets than the server included in AT_RAND 3 "RANDs are not fresh": the peer believes that the RAND challenges included in AT_RAND were not fresh 8. 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 Client-Error...................................14 Haverinen and Salowey Expires: 27 April, 2004 [Page 52] Internet Draft EAP SIM Authentication 27 October, 2003 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_CLIENT_ERROR_CODE...........................22 AT_IV.........................................129 AT_ENCR_DATA..................................130 AT_NEXT_PSEUDONYM.............................132 AT_NEXT_REAUTH_ID.............................133 The AT_NOTIFICATION attribute contains a notification code value. Values 1024, 1026 and 1031 have been specified in Section 7.17 of this document. The AT_VERSION_LIST and AT_SELECTED_VERSION attributes contain version numbers. Version 1 has been specified in Section 7.4 of this document. The AT_CLIENT_ERROR_CODE attribute contains a client error code. Values 0, 1, 2 and 3 have been specified in Section 7.18 of this document. All requests for value assignment from the various number spaces described in this document require proper documentation, according to the "Specification Required" policy described in [RFC 2434]. Requests must be specified in sufficient detail so that interoperability between independent implementations is possible. Possible forms of documentation include, but are not limited to, RFCs, the products of another standards body (e.g. 3GPP), or permanently and readily available vendor design notes. EAP SIM and EAP AKA [EAP AKA] are "sister" protocols with similar message structure and protocol numbering spaces. Many attributes and message Subtypes have the same protocol numbers in these two protocols. Hence, it is recommended that the same protocol number value SHOULD NOT be allocated for two different purposes in EAP AKA and EAP SIM. Haverinen and Salowey Expires: 27 April, 2004 [Page 53] Internet Draft EAP SIM Authentication 27 October, 2003 9. Security Considerations The EAP base protocol [EAP] 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. 9.1. 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 EAP exchange with a given server, because 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 to attempt to learn the subscriber's permanent identity. However, as discussed in Section 4.2.2, the terminal can refuse to send the cleartext permanent identity if it believes that the network should be able to recognize the pseudonym. If the peer and server cannot guarantee that the pseudonym will be maintained reliably and identity privacy is required then additional protection from an external security mechanism such as Protected Extensible Authentication Protocol (PEAP) [PEAP] may be used. If an external security mechanism is in use the identity privacy features of EAP-SIM may not be useful. The security considerations of using an external security mechanism with EAP-SIM are beyond the scope of this document. 9.2. Mutual Authentication and Triplet Exposure EAP/SIM provides mutual authentication. The peer believes that the network is authentic because the network can calculate a correct AT_MAC value in the EAP-Request/SIM/Challenge packet. To calculate AT_MAC it is sufficient to know the RAND and Kc values from the GSM triplets (RAND, SRES, Kc) used in the authentication. Because the network selects the RAND challenges and the triplets, an attacker that knows n (2 or 3) GSM triplets for the subscriber is able to impersonate a valid network to the peer. Given physical access to the SIM card, it is easy to obtain any number of GSM triplets. Another way to obtain triplets is to mount an attack on the peer platform via a virus or other malicious piece of software. The peer SHOULD be protected against triplet querying attacks by malicious software. If the same SIM credentials are also used for GSM traffic, the triplets could be revealed in the GSM network; see Section 9.5. 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 Haverinen and Salowey Expires: 27 April, 2004 [Page 54] Internet Draft EAP SIM Authentication 27 October, 2003 should be taken to limit the transport, storage and handling of these values outside a protected environment. These considerations are important at both the peer and EAP server implementations. In GSM, the network is allowed to reuse the RAND challenge in consecutive authentication exchanges. This is not allowed in EAP/SIM. The EAP/SIM server is mandated to use fresh triplets (RAND challenges) in consecutive authentication exchanges, as specified in Section 3. However, EAP SIM does not mandate any means for the peer to check if the RANDs are fresh, so the security of the scheme leans on the secrecy of the triplets. (However, the peer MAY employ implementation-specific mechanisms to remember some of the previously used RANDs, and the peer MAY check the freshness of the server's RANDs. The operation in cases when the peer detects that the RANDs are not fresh is specified in Section 4.5.1.) Preventing the re-use of authentication vectors has been taken into account in the design of the UMTS Authentication and Key Agreement (AKA), which is used in EAP AKA [EAP AKA]. In cases when the triplet re-use considerations of EAP SIM are not considered sufficient, it is advised to use EAP AKA. 9.3. Key Derivation EAP/SIM supports key derivation. The key hierarchy is specified in Section 4.6. 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 some operator specific parameters including authentication algorithms, the strength of the Ki key, and the quality of the RAND challenges. For example, some SIM cards generate Kc keys with 10 bits set to zero. Such restrictions may prevent the concatenation technique from yielding strong session keys. Because the strength of the Ki key is 128 bits, the ultimate strength of any derived secret key material is never more than 128 bits. It should also be noted that a security policy that allows n=2 to be used may compromise the security of a future policy that requires three triplets, because adversaries may be able to exploit the messages exchanged when the weaker policy was applied. There is no known way to obtain complete GSM triplets by mounting an attack against EAP/SIM. A passive eavesdropper can learn n*RAND and AT_MAC and may be able to link this information to the subscriber identity. An active attacker that impersonates a GSM subscriber can easily obtain n*RAND and AT_MAC values from the EAP server for any given subscriber identity. However, calculating the Kc and SRES values from AT_MAC would require the attacker to reverse the keyed message authentication code function HMAC-SHA1-128. As EAP SIM does not expose any values calculated from an individual GSM Kc keys, it is not possible to mount a brute force attack on just one of the Kc keys in EAP SIM. Therefore, when considering Haverinen and Salowey Expires: 27 April, 2004 [Page 55] Internet Draft EAP SIM Authentication 27 October, 2003 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. For additional considerations see Section 9.5. 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 peer 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 9.2 for more information about RAND reuse. 9.4. Dictionary Attacks Because EAP/SIM is not a password protocol, it is not vulnerable to dictionary attacks. (The pre-shared symmetric secret stored on the SIM card shall not be a weak password.) 9.5. 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 2-3 Kc keys, he can then impersonate a valid network to an EAP-SIM peer. 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 peer to use the weaker A5/2 algorithm that can be broken in less than a second). Haverinen and Salowey Expires: 27 April, 2004 [Page 56] Internet Draft EAP SIM Authentication 27 October, 2003 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. 9.6. Integrity and 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. 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 RAND values from the underlying GSM authentication scheme 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. 9.7. 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 to ensure that the protocol cannot be downgraded and that the peer and server use the same version of EAP/SIM. Haverinen and Salowey Expires: 27 April, 2004 [Page 57] Internet Draft EAP SIM Authentication 27 October, 2003 As described in Section 5, 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. 9.8. Fast Reconnect EAP/SIM includes an optional re-authentication ("fast reconnect") procedure, as recommended in [EAP] for EAP types that are intended for physically insecure networks. 9.9. 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 EAP server may end up having a different interpretation of the state of the authentication conversation. On physically insecure networks, an attacker may mount denial of service attacks by sending false EAP Success or EAP Failure indications. However, the attacker cannot force the peer or the EAP server to believe successful authentication has occurred when mutual authentication failed or has not happened yet. 9.10. 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 is beyond the scope of this document. Haverinen and Salowey Expires: 27 April, 2004 [Page 58] Internet Draft EAP SIM Authentication 27 October, 2003 9.11. 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 [RFC 1750] for more information on generating random numbers for security applications. 10. Security Claims This section provides the security claims required by [EAP]. [a] Intended use. EAP SIM is intended for use over both physically insecure networks and physically or otherwise secure networks. Applicable media include but are not limited to PPP, IEEE 802 wired networks and IEEE 802.11. [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 peer challenge to provide mutual authentication. [c] Security claims. The security properties of the method are discussed in Section 9. [d] Key strength. EAP SIM supports key derivation with 128-bit effective key strength. However, as discussed in Section 9, 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 4.6. [f] Indication of vulnerabilities. Vulnerabilities are discussed in Section 9. 11. 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. 12. Acknowledgements and Contributions 12.1. Contributors In addition to the editors, Nora Dabbous, Jose Puthenkulam, and Prasanna Satarasinghe were significant contributors to this document. Pasi Eronen and Jukka-Pekka Honkanen contributed Annex A. Haverinen and Salowey Expires: 27 April, 2004 [Page 59] Internet Draft EAP SIM Authentication 27 October, 2003 12.2. Acknowledgements 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 contributed many original ideas and concepts to this protocol. N. Asokan and Jukka-Pekka Honkanen 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. Thanks to Greg Rose for his most valuable comments [S3-020125]. 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, Tom Porcher, Michael Richardson, Stefan Schr÷der, Jesse Walker and Thomas Wieland for their contributions and critiques. Special thanks to Max for proposing improvements to the MAC calculation. Thanks to Glen Zorn for reviewing this document and for providing most useful comments on the protocol. The identity privacy support is based on the identity privacy support of [EAP SRP]. The attribute format is based on the extension format of Mobile IPv4 [RFC 3344]. This protocol has been partly developed in parallel with EAP AKA [EAP AKA], and hence this specification incorporates many ideas from Jari Arkko. Normative References [GSM 03.20] 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. [RFC 2119] S. Bradner, "Key words for use in RFCs to indicate Requirement Levels", RFC 2119, March 1997. [GSM 03.03] 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. Haverinen and Salowey Expires: 27 April, 2004 [Page 60] Internet Draft EAP SIM Authentication 27 October, 2003 [RFC 2486] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC 2486, January 1999. [RFC 2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [AES] Federal Information Processing Standards (FIPS) Publication 197, "Advanced Encryption Standard (AES)", National Institute of Standards and Technology, November 26, 2001. http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf [CBC] NIST Special Publication 800-38A, "Recommendation for Block Cipher Modes of Operation - Methods and Techniques", National Institute of Standards and Technology, December 2001. http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf [SHA-1] Federal Information Processing Standard (FIPS) Publication 180-1, "Secure Hash Standard," National Institute of Standards and Technology, U.S. Department of Commerce, April 17, 1995. [PRF] Federal Information Processing Standards (FIPS) Publication 186-2 (with change notice), "Digital Signature Standard (DSS)", National Institute of Standards and Technology, January 27, 2000. Available on-line at: http://csrc.nist.gov/publications/fips/fips186-2/fips186-2- change1.pdf [RFC 2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998. [RFC2279] F. Yergeau, "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. [EAP] L. Blunk et al., "Extensible Authentication Protocol (EAP)", draft-ietf-eap-rfc2284bis-05.txt, work-in-progress, September 2003. Informative References [Draft 3GPP TS 23.234] Draft 3GPP Technical Specification 3GPP TS 23.234 V 1.4.0: "Technical Specification Group Services and System Aspects; 3GPP system to Wireless Local Area Network (WLAN) Interworking; System Description", 3rd Generation Partnership Project, work in progress, January 2003. [PEAP] H. Andersson, S. Josefsson, G. Zorn, D. Simon, A. Palekar, "Protected EAP Protocol (PEAP)", draft-josefsson-pppext-eap-tls-eap- 05.txt, work-in-progress, September 2002. [RFC 1750] D. Eastlake, 3rd, S. Crocker, J. Schiller, "Randomness Recommendations for Security", RFC 1750 (Informational), December 1994. Haverinen and Salowey Expires: 27 April, 2004 [Page 61] Internet Draft EAP SIM Authentication 27 October, 2003 [S3-020125] Qualcomm, "Comments on draft EAP/SIM", 3rd Generation Partnership Project document 3GPP TSG SA WG3 Security S3#22, S3- 020125, February 2002. (INFORMATIVE) [RFC 3344] C. Perkins (editor), "IP Mobility Support", RFC 3344, August 2002. [EAP AKA] J. Arkko, H. Haverinen, "EAP AKA Authentication", draft- arkko-pppext-eap-aka-10.txt, June 2003 (work in progress). [RFC 2548] G. Zorn, "Microsoft Vendor-specific RADIUS Attributes", RFC 2548, March 1999 [EAP SRP] J. Carlson, B. Aboba, H. Haverinen, "EAP SRP-SHA1 Authentication Protocol", draft-ietf-pppext-eap-srp-03.txt, July 2001 (work-in-progress). Haverinen and Salowey Expires: 27 April, 2004 [Page 62] Internet Draft EAP SIM Authentication 27 October, 2003 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 Haverinen and Salowey Expires: 27 April, 2004 [Page 63] Internet Draft EAP SIM Authentication 27 October, 2003 Annex A. Test Vectors Test vectors for the NIST FIPS 186-2 pseudo-random number generator [PRF] are available at the following URL: http://csrc.nist.gov/encryption/dss/Examples-1024bit.pdf The following examples show the contents of EAP/SIM packets on full authentication and re-authentication. A.1 EAP-Request/Identity The first packet is a plain Identity Request: 01 ; Code: Request 00 ; Identifier: 0 00 05 ; Length: 5 octets 01 ; Type: Identity A.2 EAP-Response/Identity The client's identity is "1244070100000001@eapsim.foo", so it responds with the following packet: 02 ; Code: Response 00 ; Identifier: 0 00 20 ; Length: 32 octets 01 ; Type: Identity 31 32 34 34 ; "1244070100000001@eapsim.foo" 30 37 30 31 30 30 30 30 30 30 30 31 40 65 61 70 73 69 6d 2e 66 6f 6f A.3 EAP-Request/SIM/Start The server's first packet looks like this: 01 ; Code: Request 01 ; Identifier: 1 00 10 ; Length: 16 octets 12 ; Type: EAP-SIM 0a ; EAP-SIM subtype: Start 00 00 ; (reserved) 0f ; Attribute type: AT_VERSION_LIST 02 ; Attribute length: 8 octets (2*4) 00 02 ; Actual version list length: 2 octets 00 01 ; Version: 1 00 00 ; (attribute padding) Haverinen and Salowey Expires: 27 April, 2004 [Page 64] Internet Draft EAP SIM Authentication 27 October, 2003 A.4 EAP-Response/SIM/Start The client selects a nonce and responds with the following packet: 02 ; Code: Response 01 ; Identifier: 1 00 20 ; Length: 32 octets 12 ; Type: EAP-SIM 0a ; EAP-SIM subtype: Start 00 00 ; (reserved) 07 ; Attribute type: AT_NONCE_MT 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) 01 23 45 67 ; NONCE_MT value 89 ab cd ef fe dc ba 98 76 54 32 10 10 ; Attribute type: AT_SELECTED_VERSION 01 ; Attribute length: 4 octets (1*4) 00 01 ; Version: 1 A.5 EAP-Request/SIM/Challenge Next, the server selects three authentication triplets (RAND1,SRES1,Kc1) = (10111213 14151617 18191a1b 1c1d1e1f, d1d2d3d4, a0a1a2a3 a4a5a6a7) (RAND2,SRES2,Kc2) = (20212223 24252627 28292a2b 2c2d2e2f, e1e2e3e4, b0b1b2b3 b4b5b6b7) (RAND3,SRES3,Kc3) = (30313233 34353637 38393a3b 3c3d3e3f, f1f2f3f4, c0c1c2c3 c4c5c6c7) Next, the MK is calculated as specified in Section 4.6. MK = e576d5ca 332e9930 018bf1ba ee2763c7 95b3c712 And the other keys are derived using the PRNG: K_encr = 536e5ebc 4465582a a6a8ec99 86ebb620 K_aut = 25af1942 efcbf4bc 72b39434 21f2a974 MSK = 39d45aea f4e30601 983e972b 6cfd46d1 c3637733 65690d09 cd44976b 525f47d3 a60a985e 955c53b0 90b2e4b7 3719196a 40254296 8fd14a88 8f46b9a7 886e4488 EMSK = 5949eab0 fff69d52 315c6c63 4fd14a7f 0d52023d 56f79698 fa6596ab eed4f93f bb48eb53 4d985414 ceed0d9a 8ed33c38 7c9dfdab 92ffbdf2 40fcecf6 5a2c93b9 Haverinen and Salowey Expires: 27 April, 2004 [Page 65] Internet Draft EAP SIM Authentication 27 October, 2003 Next, the server selects a pseudonym and a re-authentication identity (in this case, "w8w49PexCazWJ&xCIARmxuMKht5S1sxR DqXSEFBEg3DcZP9cIxTe5J4OyIwNGVzxeJOU1G" and "Y24fNSrz8BP274jOJaF17WfxI8YO7QX0 0pMXk9XMMVOw7broaNhTczuFq53aEpOkk3L0dm@eapsim.foo", respectively). The following plaintext will be encrypted and stored in the AT_ENCR_DATA attribute: 84 ; Attribute type: AT_NEXT_PSEUDONYM 13 ; Attribute length: 76 octets (19*4) 00 46 ; Actual pseudonym length: 70 octets 77 38 77 34 39 50 65 78 43 61 7a 57 4a 26 78 43 49 41 52 6d 78 75 4d 4b 68 74 35 53 31 73 78 52 44 71 58 53 45 46 42 45 67 33 44 63 5a 50 39 63 49 78 54 65 35 4a 34 4f 79 49 77 4e 47 56 7a 78 65 4a 4f 55 31 47 00 00 ; (attribute padding) 85 ; Attribute type: AT_NEXT_REAUTH_ID 16 ; Attribute length: 88 octets (22*4) 00 51 ; Actual re-auth identity length: 81 octets 59 32 34 66 4e 53 72 7a 38 42 50 32 37 34 6a 4f 4a 61 46 31 37 57 66 78 49 38 59 4f 37 51 58 30 30 70 4d 58 6b 39 58 4d 4d 56 4f 77 37 62 72 6f 61 4e 68 54 63 7a 75 46 71 35 33 61 45 70 4f 6b 6b 33 4c 30 64 6d 40 65 61 70 73 69 6d 2e 66 6f 6f 00 00 00 ; (attribute padding) 06 ; Attribute type: AT_PADDING 03 ; Attribute length: 12 octets (3*4) 00 00 00 00 00 00 00 00 00 00 The EAP packet looks like this: 01 ; Code: Request 02 ; Identifier: 2 01 18 ; Length: 280 octets 12 ; Type: EAP-SIM 0b ; EAP-SIM subtype: Challenge 00 00 ; (reserved) 01 ; Attribute type: AT_RAND 0d ; Attribute length: 52 octets (13*4) 00 00 ; (reserved) 10 11 12 13 ; first RAND 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 ; second RAND 24 25 26 27 28 29 2a 2b Haverinen and Salowey Expires: 27 April, 2004 [Page 66] Internet Draft EAP SIM Authentication 27 October, 2003 2c 2d 2e 2f 30 31 32 33 ; third RAND 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f 81 ; Attribute type: AT_IV 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) 9e 18 b0 c2 ; IV value 9a 65 22 63 c0 6e fb 54 dd 00 a8 95 82 ; Attribute type: AT_ENCR_DATA 2d ; Attribute length: 180 octets (45*4) 00 00 ; (reserved) 55 f2 93 9b bd b1 b1 9e a1 b4 7f c0 b3 e0 be 4c ab 2c f7 37 2d 98 e3 02 3c 6b b9 24 15 72 3d 58 ba d6 6c e0 84 e1 01 b6 0f 53 58 35 4b d4 21 82 78 ae a7 bf 2c ba ce 33 10 6a ed dc 62 5b 0c 1d 5a a6 7a 41 73 9a e5 b5 79 50 97 3f c7 ff 83 01 07 3c 6f 95 31 50 fc 30 3e a1 52 d1 e1 0a 2d 1f 4f 52 26 da a1 ee 90 05 47 22 52 bd b3 b7 1d 6f 0c 3a 34 90 31 6c 46 92 98 71 bd 45 cd fd bc a6 11 2f 07 f8 be 71 79 90 d2 5f 6d d7 f2 b7 b3 20 bf 4d 5a 99 2e 88 03 31 d7 29 94 5a ec 75 ae 5d 43 c8 ed a5 fe 62 33 fc ac 49 4e e6 7a 0d 50 4d 0b ; Attribute type: AT_MAC 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) fe f3 24 ac ; MAC value 39 62 b5 9f 3b d7 82 53 ae 4d cb 6a The MAC is calculated over the EAP packet above (with MAC value set to zero), followed by the NONCE_MT value (a total of 296 bytes). A.6 EAP-Response/SIM/Challenge The client's response looks like this: 02 ; Code: Response 02 ; Identifier: 2 00 1c ; Length: 28 octets 12 ; Type: EAP-SIM 0b ; EAP-SIM subtype: Challenge 00 00 ; (reserved) 0b ; Attribute type: AT_MAC 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) f5 6d 64 33 ; MAC value e6 8e d2 97 Haverinen and Salowey Expires: 27 April, 2004 [Page 67] Internet Draft EAP SIM Authentication 27 October, 2003 6a c1 19 37 fc 3d 11 54 The MAC is calculated over the EAP packet above (with MAC value set to zero), followed by the SRES values (a total of 40 bytes). A.7 EAP-Success The last packet is an EAP Success: 03 ; Code: Success 03 ; Identifier: 3 00 04 ; Length: 4 octets A.8 Re-authentication When performing re-authentication, the EAP-Request/Identity packet is the same as usual. The EAP-Response/Identity contains the re-authentication identity (from AT_ENCR_DATA attribute above): 02 ; Code: Response 00 ; Identifier: 0 00 56 ; Length: 86 octets 01 ; Type: Identity 59 32 34 66 4e 53 72 7a 38 42 50 32 37 34 6a 4f 4a 61 46 31 37 57 66 78 49 38 59 4f 37 51 58 30 30 70 4d 58 6b 39 58 4d 4d 56 4f 77 37 62 72 6f 61 4e 68 54 63 7a 75 46 71 35 33 61 45 70 4f 6b 6b 33 4c 30 64 6d 40 65 61 70 73 69 6d 2e 66 6f A.9 EAP-Request/SIM/Re-authentication The server recognizes the reauthentication identity, so it will respond with EAP-Request/SIM/Re-authentication. It retrieves the associated counter value, generates a nonce, and picks a new reauthentication identity (in this case, "uta0M0iyIsMwWp5TTdSdnOLvg2XDVf21OYt1vnfiMcs5dnIDHOIFVavIRzMR yzW6vFzdHW@eapsim.foo"). The following plaintext will be encrypted and stored in the AT_ENCR_DATA attribute: 13 ; Attribute type: AT_COUNTER 01 ; Attribute length: 4 octets (1*4) 00 01 ; Counter value 15 ; Attribute type: AT_NONCE_S 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) 01 23 45 67 ; NONCE_S value 89 ab cd ef fe dc ba 98 Haverinen and Salowey Expires: 27 April, 2004 [Page 68] Internet Draft EAP SIM Authentication 27 October, 2003 76 54 32 10 85 ; Attribute type: AT_NEXT_REAUTH_ID 16 ; Attribute length: 88 octets (22*4) 00 51 ; Actual re-auth identity length: 81 octets 75 74 61 30 4d 30 69 79 49 73 4d 77 57 70 35 54 54 64 53 64 6e 4f 4c 76 67 32 58 44 56 66 32 31 4f 59 74 31 76 6e 66 69 4d 63 73 35 64 6e 49 44 48 4f 49 46 56 61 76 49 52 7a 4d 52 79 7a 57 36 76 46 7a 64 48 57 40 65 61 70 73 69 6d 2e 66 6f 6f 00 00 00 ; (attribute padding) 06 ; Attribute type: AT_PADDING 04 ; Attribute length: 16 octets (4*4) 00 00 00 00 00 00 00 00 00 00 00 00 00 00 The EAP packet looks like this: 01 ; Code: Request 01 ; Identifier: 1 00 b4 ; Length: 180 octets 12 ; Type: EAP-SIM 0d ; EAP-SIM subtype: Re-authentication 00 00 ; (reserved) 81 ; Attribute type: AT_IV 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) d5 85 ac 77 ; IV value 86 b9 03 36 65 7c 77 b4 65 75 b9 c4 82 ; Attribute type: AT_ENCR_DATA 21 ; Attribute length: 132 octets (33*4) 00 00 ; (reserved) 68 62 91 a9 d2 ab c5 8c aa 32 94 b6 e8 5b 44 84 6c 44 e5 dc b2 de 8b 9e 80 d6 9d 49 85 8a 5d b8 4c dc 1c 9b c9 5c 01 b9 6b 6e ca 31 34 74 ae a6 d3 14 16 e1 9d aa 9d f7 0f 05 00 88 41 ca 80 14 96 4d 3b 30 a4 9b cf 43 e4 d3 f1 8e 86 29 5a 4a 2b 38 d9 6c 97 05 c2 bb b0 5c 4a ac e9 7d 5e af f5 64 04 6c 8b d3 0b c3 9b e5 e1 7a ce 2b 10 a6 fe b9 98 e6 c5 35 3f ea ab 59 a7 4c 84 60 45 9f 0b ; Attribute type: AT_MAC 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) 39 73 65 a3 ; MAC value b3 d3 da dc 22 7a 7c 05 1d 80 56 6f The MAC is calculated over the EAP packet above (with MAC value Haverinen and Salowey Expires: 27 April, 2004 [Page 69] Internet Draft EAP SIM Authentication 27 October, 2003 set to zero; a total of 180 bytes). Finally, the server derives new keys. The XKEY' is calculated as described in Section 17: XKEY' = 863dc120 32e08343 c1a2308d b48377f6 801f58d4 The new MSK and EMSK are derived using the PRNG (note that K_encr and K_aut stay the same). MSK = 756d9e4c ed6d5ed6 40eb3fe3 8565ca07 6e7fb8a8 17cfe8d9 adbce441 d47c4f5e 3d8ff786 3a630b2b 06e2cf20 9684c13f 6b82f992 f2b06f1b 54bf51ef 237f2a40 EMSK = 1ef5e0d7 e098a34c 533eaebf 34578854 b7721526 20a777f0 e0340884 a294fb73 af7102ff cd27f692 fd672be9 a55f0cd1 2a4a5106 78fff62a b4c76023 6ff0163d A.10 EAP-Response/SIM/Re-authentication The client's response includes the counter as well. The following plaintext will be encrypted and stored in the AT_ENCR_DATA attribute: 13 ; Attribute type: AT_COUNTER 01 ; Attribute length: 4 octets (1*4) 00 01 ; Counter value 06 ; Attribute type: AT_PADDING 03 ; Attribute length: 12 octets (3*4) 00 00 00 00 00 00 00 00 00 00 The EAP packet looks like this: 02 ; Code: Response 01 ; Identifier: 1 00 44 ; Length: 68 octets 12 ; Type: EAP-SIM 0d ; EAP-SIM subtype: Re-authentication 00 00 ; (reserved) 81 ; Attribute type: AT_IV 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) cd f7 ff a6 ; IV value 5d e0 4c 02 6b 56 c8 6b 76 b1 02 ea 82 ; Attribute type: AT_ENCR_DATA 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) b6 ed d3 82 Haverinen and Salowey Expires: 27 April, 2004 [Page 70] Internet Draft EAP SIM Authentication 27 October, 2003 79 e2 a1 42 3c 1a fc 5c 45 5c 7d 56 0b ; Attribute type: AT_MAC 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) fa f7 6b 71 ; MAC value fb e2 d2 55 b9 6a 35 66 c9 15 c6 17 The MAC is calculated over the EAP packet above (with MAC value set to zero), followed by the NONCE_S value (a total of 84 bytes). The next packet will be EAP Success, same as above. Haverinen and Salowey Expires: 27 April, 2004 [Page 71] Internet Draft EAP SIM Authentication 27 October, 2003 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 [SHA-1] Step 3: For j = 0 to m - 1 do 3.1 XSEED_j = 0 /* no optional user input */ 3.2 For i = 0 to 1 do a. XVAL = (XKEY + XSEED_j) mod 2^b b. w_i = G(t, XVAL) c. XKEY = (1 + XKEY + w_i) mod 2^b 3.3 x_j = w_0|w_1 Haverinen and Salowey Expires: 27 April, 2004 [Page 72]