Nokia
J. Salowey, Ed.
Cisco Systems
January 2006
Extensible Authentication Protocol Method for
Global System for Mobile Communications (GSM)
Subscriber Identity Modules (EAP-SIM)
Status of This Memo
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This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright Notice
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Copyright © The Internet Society (2006).
IESG Note
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The EAP-SIM protocol was developed by 3GPP. The documentation of EAP-SIM is provided as information to the Internet community. While the EAP WG has verified that EAP-SIM is compatible with EAP, as defined in RFC 3748, no other review has been done, including validation of the security claims. The IETF has also not reviewed the security of the cryptographic algorithms.
Abstract
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This document specifies an Extensible Authentication Protocol (EAP) mechanism for authentication and session key distribution using the Global System for Mobile Communications (GSM) Subscriber Identity Module (SIM). GSM is a second generation mobile network standard. The EAP-SIM mechanism specifies enhancements to GSM authentication and key agreement whereby multiple authentication triplets can be combined to create authentication responses and session keys of greater strength than the individual GSM triplets. The mechanism also includes network authentication, user anonymity support, result indications, and a fast re-authentication procedure.
Table of Contents
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1. Introduction ....................................................4 2. Terms ...........................................................5 3. Overview ........................................................8 4. Operation ......................................................10 4.1. Version Negotiation .......................................10 4.2. Identity Management .......................................11 4.2.1. Format, Generation and Usage of Peer Identities ....11 4.2.2. Communicating the Peer Identity to the Server ......17 4.2.3. Choice of Identity for the EAP-Response/Identity ...19 4.2.4. Server Operation in the Beginning of EAP-SIM Exchange ...................................19 4.2.5. Processing of EAP-Request/SIM/Start by the Peer ....20 4.2.6. Attacks Against Identity Privacy ...................21 4.2.7. Processing of AT_IDENTITY by the Server ............22 4.3. Message Sequence Examples (Informative) ...................23 4.3.1. Full Authentication ................................24 4.3.2. Fast Re-authentication .............................25 4.3.3. Fall Back to Full Authentication ...................26 4.3.4. Requesting the Permanent Identity 1 ................27 4.3.5. Requesting the Permanent Identity 2 ................28 4.3.6. Three EAP-SIM/Start Roundtrips .....................28 5. Fast Re-Authentication .........................................30 5.1. General ...................................................30 5.2. Comparison to UMTS AKA ....................................31 5.3. Fast Re-authentication Identity ...........................31 5.4. Fast Re-authentication Procedure ..........................33 5.5. Fast Re-authentication Procedure when Counter Is Too Small .................................................36 6. EAP-SIM Notifications ..........................................37 6.1. General ...................................................37 6.2. Result Indications ........................................39 6.3. Error Cases ...............................................40 6.3.1. Peer Operation .....................................40 6.3.2. Server Operation ...................................41 6.3.3. EAP-Failure ........................................42 6.3.4. EAP-Success ........................................42 7. Key Generation .................................................43 8. Message Format and Protocol Extensibility ......................45 8.1. Message Format ............................................45 8.2. Protocol Extensibility ....................................47 9. Messages .......................................................48 9.1. EAP-Request/SIM/Start .....................................48 9.2. EAP-Response/SIM/Start ....................................49 9.3. EAP-Request/SIM/Challenge .................................49 9.4. EAP-Response/SIM/Challenge ................................50 9.5. EAP-Request/SIM/Re-authentication .........................51 9.6. EAP-Response/SIM/Re-authentication ........................51 9.7. EAP-Response/SIM/Client-Error .............................52 9.8. EAP-Request/SIM/Notification ..............................52 9.9. EAP-Response/SIM/Notification .............................53 10. Attributes ....................................................53 10.1. Table of Attributes ......................................53 10.2. AT_VERSION_LIST ..........................................54 10.3. AT_SELECTED_VERSION ......................................55 10.4. AT_NONCE_MT ..............................................55 10.5. AT_PERMANENT_ID_REQ ......................................56 10.6. AT_ANY_ID_REQ ............................................56 10.7. AT_FULLAUTH_ID_REQ .......................................57 10.8. AT_IDENTITY ..............................................57 10.9. AT_RAND ..................................................58 10.10. AT_NEXT_PSEUDONYM .......................................59 10.11. AT_NEXT_REAUTH_ID .......................................59 10.12. AT_IV, AT_ENCR_DATA, and AT_PADDING .....................60 10.13. AT_RESULT_IND ...........................................62 10.14. AT_MAC ..................................................62 10.15. AT_COUNTER ..............................................63 10.16. AT_COUNTER_TOO_SMALL ....................................63 10.17. AT_NONCE_S ..............................................64 10.18. AT_NOTIFICATION .........................................64 10.19. AT_CLIENT_ERROR_CODE ....................................65 11. IANA Considerations ...........................................66 12. Security Considerations .......................................66 12.1. A3 and A8 Algorithms .....................................66 12.2. Identity Protection ......................................66 12.3. Mutual Authentication and Triplet Exposure ...............67 12.4. Flooding the Authentication Centre .......................69 12.5. Key Derivation ...........................................69 12.6. Cryptographic Separation of Keys and Session Independence .............................................70 12.7. Dictionary Attacks .......................................71 12.8. Credentials Re-use .......................................71 12.9. Integrity and Replay Protection, and Confidentiality .....72 12.10. Negotiation Attacks .....................................73 12.11. Protected Result Indications ............................73 12.12. Man-in-the-Middle Attacks ...............................74 12.13. Generating Random Numbers ...............................74 13. Security Claims ...............................................74 14. Acknowledgements and Contributions ............................75 14.1. Contributors .............................................75 14.2. Acknowledgements .........................................75 14.2.1. Contributors' Addresses ...........................77 15. References ....................................................78 15.1. Normative References .....................................78 15.2. Informative References ...................................79 Appendix A. Test Vectors .........................................81 A.1. EAP-Request/Identity .....................................81 A.2. EAP-Response/Identity ....................................81 A.3. EAP-Request/SIM/Start ....................................82 A.4. EAP-Response/SIM/Start ...................................82 A.5. EAP-Request/SIM/Challenge ................................83 A.6. EAP-Response/SIM/Challenge ...............................86 A.7. EAP-Success ..............................................86 A.8. Fast Re-authentication ...................................86 A.9. EAP-Request/SIM/Re-authentication ........................87 A.10. EAP-Response/SIM/Re-authentication ......................89 Appendix B. Pseudo-Random Number Generator .......................90
1. Introduction
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This document specifies an Extensible Authentication Protocol (EAP) [RFC3748] mechanism for authentication and session key distribution using the Global System for Mobile Communications (GSM) Subscriber Identity Module (SIM).
GSM is a second generation mobile network standard. Second generation mobile networks and third generation mobile networks use different authentication and key agreement mechanisms. EAP-AKA [EAP-AKA] specifies an EAP method that is based on the Authentication and Key Agreement (AKA) mechanism used in 3rd generation mobile networks.
GSM authentication is based on a challenge-response mechanism. The A3/A8 authentication and key derivation algorithms that run on the SIM can be given a 128-bit random number (RAND) as a challenge. The SIM runs operator-specific algorithms, which take the RAND and a secret key Ki (stored on the SIM) as input, and produce a 32-bit response (SRES) and a 64-bit long key Kc as output. The Kc key is originally intended to be used as an encryption key over the air interface, but in this protocol, it is used for deriving keying material and is not directly used. Hence, the secrecy of Kc is critical to the security of this protocol. For more information about GSM authentication, see [GSM-03.20]. See Section 12.1 for more discussion about the GSM algorithms used in EAP-SIM.
The lack of mutual authentication is a weakness in GSM authentication. The derived 64-bit cipher key (Kc) is not strong enough for data networks in which stronger and longer keys are required. Hence, in EAP-SIM, several RAND challenges are used for generating several 64-bit Kc keys, which are combined to constitute stronger keying material. In EAP-SIM, the client issues a random number NONCE_MT to the network in order to contribute to key derivation, and to prevent replays of EAP-SIM requests from previous exchanges. The NONCE_MT can be conceived as the client's challenge to the network. EAP-SIM also extends the combined RAND challenges and other messages with a message authentication code in order to provide message integrity protection along with mutual authentication.
EAP-SIM specifies optional support for protecting the privacy of subscriber identity using the same concept as the GSM, which uses pseudonyms/temporary identifiers. It also specifies an optional fast re-authentication procedure.
The security of EAP-SIM builds on underlying GSM mechanisms. The security properties of EAP-SIM are documented in Section 11 of this document. Implementers and users of EAP-SIM are advised to carefully study the security considerations in Section 11 in order to determine whether the security properties are sufficient for the environment in question, especially as the secrecy of Kc keys is essential to the security of EAP-SIM. In brief, EAP-SIM is in no sense weaker than the GSM mechanisms. In some cases EAP-SIM provides better security properties than the underlying GSM mechanisms, particularly if the SIM credentials are only used for EAP-SIM and are not re-used from GSM/GPRS. Many of the security features of EAP-SIM rely upon the secrecy of the Kc values in the SIM triplets, so protecting these values is key to the security of the EAP-SIM protocol.
The 3rd Generation Partnership Project (3GPP) has specified an enhanced Authentication and Key Agreement (AKA) architecture for the Universal Mobile Telecommunications System (UMTS). The 3rd generation AKA mechanism includes mutual authentication, replay protection, and derivation of longer session keys. EAP-AKA [EAP-AKA] specifies an EAP method that is based on the 3rd generation AKA. EAP-AKA, which is a more secure protocol, may be used instead of EAP-SIM, if 3rd generation identity modules and 3G network infrastructures are available.
2. Terms
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The terms and abbreviations "authenticator", "backend authentication server", "EAP server", "peer", "Silently Discard", "Master Session Key (MSK)", and "Extended Master Session Key (EMSK)" in this document are to be interpreted as described in [RFC3748].
This document frequently uses the following terms and abbreviations:
AAA protocol
Authentication, Authorization, and Accounting protocol
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AuC
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Authentication Centre. The GSM network element that provides the authentication triplets for authenticating the subscriber.
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Authentication vector
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GSM triplets can be alternatively called authentication vectors.
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EAP Extensible Authentication Protocol Fast re-authentication-
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An EAP-SIM authentication exchange that is based on keys derived upon a preceding full authentication exchange. The GSM authentication and key exchange algorithms are not used in the fast re-authentication procedure.
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Fast Re-authentication Identity
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A fast re-authentication identity of the peer, including an NAI realm portion in environments where a realm is used. Used on fast re-authentication only.
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Fast Re-authentication Username
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The username portion of fast re-authentication identity, i.e., not including any realm portions.
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Full authentication
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An EAP-SIM authentication exchange based on the GSM authentication and key agreement algorithms.
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GSM
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Global System for Mobile communications.
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GSM Triplet
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The tuple formed by the three GSM authentication values RAND, Kc, and SRES.
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IMSI
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International Mobile Subscriber Identifier, used in GSM to identify subscribers.
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MAC Message Authentication Code NAI Network Access Identifier Nonce-
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A value that is used at most once or that is never repeated within the same cryptographic context. In general, a nonce can be predictable (e.g., a counter) or unpredictable (e.g., a random value). Since some cryptographic properties may depend on the randomness of the nonce, attention should be paid to whether a nonce is required to be random or not. In this document, the term nonce is only used to denote random nonces, and it is not used to denote counters.
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Permanent Identity
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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.
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Permanent Username
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The username portion of permanent identity, i.e., not including any realm portions.
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Pseudonym Identity
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A pseudonym identity of the peer, including an NAI realm
portion in environments where a realm is used. Used on
full authentication only.
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Pseudonym Username
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The username portion of pseudonym identity, i.e., not including any realm portions.
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SIM
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Subscriber Identity Module. The SIM is traditionally a smart card distributed by a GSM operator.
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3. Overview
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Figure 1 shows an overview of the EAP-SIM full authentication procedure, wherein optional protected success indications are not used. The authenticator typically communicates with an EAP server that is located on a backend authentication server using an AAA protocol. The authenticator shown in the figure is often simply relaying EAP messages to and from the EAP server, but these backend AAA communications are not shown.
Peer Authenticator | EAP-Request/Identity | |<---------------------------------------------------------| | | | EAP-Response/Identity | |--------------------------------------------------------->| | | | EAP-Request/SIM/Start (AT_VERSION_LIST) | |<---------------------------------------------------------| | | | EAP-Response/SIM/Start (AT_NONCE_MT, AT_SELECTED_VERSION)| |--------------------------------------------------------->| | | | EAP-Request/SIM/Challenge (AT_RAND, AT_MAC) | |<---------------------------------------------------------| +-------------------------------------+ | | Peer runs GSM algorithms, verifies | | | AT_MAC and derives session keys | | +-------------------------------------+ | | EAP-Response/SIM/Challenge (AT_MAC) | |--------------------------------------------------------->| | | | EAP-Success | |<---------------------------------------------------------| | |
Figure 1: EAP-SIM full authentication procedure
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The first EAP Request issued by the authenticator is EAP-Request/Identity. On full authentication, the peer's response includes either the user's International Mobile Subscriber Identity (IMSI) or a temporary identity (pseudonym) if identity privacy is in effect, as specified in Section 4.2.
Following the peer's EAP-Response/Identity packet, the peer receives EAP Requests of Type 18 (SIM) from the EAP server and sends the corresponding EAP Responses. The EAP packets that are of the Type SIM also have a Subtype field. On full authentication, the first EAP-Request/SIM packet is of the Subtype 10 (Start). EAP-SIM packets encapsulate parameters in attributes, encoded in a Type, Length, Value format. The packet format and the use of attributes are specified in Section 8.
The EAP-Request/SIM/Start packet contains the list of EAP-SIM versions supported by the EAP server in the AT_VERSION_LIST attribute. This packet may also include attributes for requesting the subscriber identity, as specified in Section 4.2.
The peer responds to a EAP-Request/SIM/Start with the EAP-Response/SIM/Start packet, which includes the AT_NONCE_MT attribute that contains a random number NONCE_MT, chosen by the peer, and the AT_SELECTED_VERSION attribute that contains the version number selected by the peer. The version negotiation is protected by including the version list and the selected version in the calculation of keying material (Section 7).
After receiving the EAP Response/SIM/Start, the EAP server obtains n GSM triplets for use in authenticating the subscriber, where n = 2 or n = 3. From the triplets, the EAP server derives the keying material, as specified in Section 7. The triplets may be obtained by contacting an Authentication Centre (AuC) on the GSM network; per GSM specifications, between 1 and 5 triplets may be obtained at a time. Triplets may be stored in the EAP server for use at a later time, but triplets MUST NOT be re-used, except in some error cases that are specified in Section 10.9.
The next EAP Request the EAP Server issues is of the type SIM and subtype Challenge (11). It contains the RAND challenges and a message authentication code attribute AT_MAC to cover the challenges. The AT_MAC attribute is a general message authentication code attribute that is used in many EAP-SIM messages.
On receipt of the EAP-Request/SIM/Challenge message, the peer runs the GSM authentication algorithm and calculates a copy of the message authentication code. The peer then verifies that the calculated MAC equals the received MAC. If the MAC's do not match, then the peer sends the EAP-Response/SIM/Client-Error packet and the authentication exchange terminates.
Since the RANDs given to a peer are accompanied by the message authentication code AT_MAC, and since the peer's NONCE_MT value contributes to AT_MAC, the peer is able to verify that the EAP-SIM message is fresh (i.e., not a replay) and that the sender possesses valid GSM triplets for the subscriber.
If all checks out, the peer responds with the EAP-Response/SIM/Challenge, containing the AT_MAC attribute that covers the peer's SRES response values (Section 9.4). The EAP server verifies that the MAC is correct. Because protected success indications are not used in this example, the EAP server sends the EAP-Success packet, indicating that the authentication was successful. (Protected success indications are discussed in Section 6.2.) The EAP server may also include derived keying material in the message it sends to the authenticator. The peer has derived the same keying material, so the authenticator does not forward the keying material to the peer along with EAP-Success.
EAP-SIM also includes a separate fast re-authentication procedure that does not make use of the A3/A8 algorithms or the GSM infrastructure. Fast re-authentication is based on keys derived on full authentication. If the peer has maintained state information for fast re-authentication and wants to use fast re-authentication, then the peer indicates this by using a specific fast re-authentication identity instead of the permanent identity or a pseudonym identity. The fast re-authentication procedure is described in Section 5.
4. Operation
4.1. Version Negotiation
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EAP-SIM includes version negotiation so as to allow future developments in the protocol. The version negotiation is performed on full authentication and it uses two attributes, AT_VERSION_LIST, which the server always includes in EAP-Request/SIM/Start, and AT_SELECTED_VERSION, which the peer includes in EAP-Response/SIM/Start on full authentication.
AT_VERSION_LIST includes the EAP-SIM versions supported by the server. If AT_VERSION_LIST does not include a version that is implemented by the peer and allowed in the peer's security policy, then the peer MUST send the EAP-Response/SIM/Client-Error packet (Section 9.7) to the server with the error code "unsupported version". If a suitable version is included, then the peer includes the AT_SELECTED_VERSION attribute, containing the selected version in the EAP-Response/SIM/Start packet. The peer MUST only indicate a version that is included in the AT_VERSION_LIST. If several versions are acceptable, then the peer SHOULD choose the version that occurs first in the version list.
The version number list of AT_VERSION_LIST and the selected version of AT_SELECTED_VERSION are included in the key derivation procedure (Section 7). If an attacker modifies either one of these attributes, then the peer and the server derive different keying material. Because K_aut keys are different, the server and peer calculate different AT_MAC values. Hence, the peer detects that AT_MAC, included in EAP-Request/SIM/Challenge, is incorrect and sends the EAP-Response/SIM/Client-Error packet. The authentication procedure terminates.
4.2. Identity Management
4.2.1. Format, Generation and Usage of Peer Identities
4.2.1.1. General
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In the beginning of EAP authentication, the Authenticator or the EAP server usually issues the EAP-Request/Identity packet to the peer. The peer responds with the EAP-Response/Identity, which contains the user's identity. The formats of these packets are specified in [RFC3748].
GSM subscribers are identified with the International Mobile Subscriber Identity (IMSI) [GSM-03.03]. The IMSI is a string of not more than 15 digits. It is composed of a three digit Mobile Country Code (MCC), a two or three digit Mobile Network Code (MNC), and a Mobile Subscriber Identification Number (MSIN) of no more than 10 digits. MCC and MNC uniquely identify the GSM operator and help identify the AuC from which the authentication vectors need to be retrieved for this subscriber.
Internet AAA protocols identify users with the Network Access Identifier (NAI) [RFC4282]. When used in a roaming environment, the NAI is composed of a username and a realm, separated with "@" (username@realm). The username portion identifies the subscriber within the realm.
This section specifies the peer identity format used in EAP-SIM. In this document, the term "identity" or "peer identity" refers to the whole identity string that is used to identify the peer. The peer identity may include a realm portion. "Username" refers to the portion of the peer identity that identifies the user, i.e., the username does not include the realm portion.
4.2.1.2. Identity Privacy Support
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EAP-SIM includes optional identity privacy (anonymity) support that can be used to hide the cleartext permanent identity and thereby make the subscriber's EAP exchanges untraceable to eavesdroppers. Because the permanent identity never changes, revealing it would help observers to track the user. The permanent identity is usually based on the IMSI, which may further help the tracking, because the same identifier may be used in other contexts as well. Identity privacy is based on temporary identities, or pseudonyms, which are equivalent to but separate from the Temporary Mobile Subscriber Identities (TMSI) that are used on cellular networks. Please see Section 12.2 for security considerations regarding identity privacy.
4.2.1.3. Username Types in EAP-SIM identities
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There are three types of usernames in EAP-SIM peer identities:
(1) Permanent usernames. For example, 1123456789098765@myoperator.com might be a valid permanent identity. In this example, 1123456789098765 is the permanent username.
(2) Pseudonym usernames. For example, 3s7ah6n9q@myoperator.com might be a valid pseudonym identity. In this example, 3s7ah6n9q is the pseudonym username.
(3) Fast re-authentication usernames. For example, 53953754@myoperator.com might be a valid fast re-authentication identity. In this case, 53953754 is the fast re-authentication username. Unlike permanent usernames and pseudonym usernames, fast re-authentication usernames are one-time identifiers, which are not re-used across EAP exchanges.
The first two types of identities are used only on full authentication and the last one only on fast re-authentication. When the optional identity privacy support is not used, the non-pseudonym permanent identity is used on full authentication. The fast re-authentication exchange is specified in Section 5.
4.2.1.4. Username Decoration
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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 an NAI realm portion is not considered decoration.) Username decoration is out of the scope of this document. However, it should be noted that username decoration might prevent the server from recognizing a valid username. Hence, although the peer MAY use username decoration in the identities that the peer includes in EAP-Response/Identity, and although the EAP server MAY accept a decorated peer username in this message, the peer or the EAP server MUST NOT decorate any other peer identities that are used in various EAP-SIM attributes. Only the identity used in the EAP-Response/Identity may be decorated.
4.2.1.5. NAI Realm Portion
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The peer MAY include a realm portion in the peer identity, as per the NAI format. The use of a realm portion is not mandatory.
If a realm is used, the realm MAY be chosen by the subscriber's home operator and it MAY be a configurable parameter in the EAP-SIM peer implementation. In this case, the peer is typically configured with the NAI realm of the home operator. Operators MAY reserve a specific realm name for EAP-SIM users. This convention makes it easy to recognize that the NAI identifies a GSM subscriber. Such a reserved NAI realm may be a useful hint as to the first authentication method to use during method negotiation. When the peer is using a pseudonym username instead of the permanent username, the peer selects the realm name portion similarly as it select the realm portion when using the permanent username.
If no configured realm name is available, the peer MAY derive the realm name from the MCC and MNC portions of the IMSI. A RECOMMENDED way to derive the realm from the IMSI using the realm 3gppnetwork.org is specified in [3GPP-TS-23.003].
Some old implementations derive the realm name from the IMSI by concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits of IMSI, and ".owlan.org". For example, if the IMSI is 123456789098765, and the MNC is three digits long, then the derived realm name is "mnc456.mcc123.owlan.org". As there are no DNS servers running at owlan.org, these realm names can only be used with manually configured AAA routing. New implementations SHOULD use the mechanism specified in [3GPP-TS-23.003] instead of owlan.org.
The IMSI is a string of digits without any explicit structure, so the peer may not be able to determine the length of the MNC portion. If the peer is not able to determine whether the MNC is two or three digits long, the peer MAY use a 3-digit MNC. If the correct length of the MNC is two, then the MNC used in the realm name includes the first digit of the MSIN. Hence, when configuring AAA networks for operators that have 2-digit MNCs, the network SHOULD also be prepared for realm names with incorrect, 3-digit MNCs.
4.2.1.6. Format of the Permanent Username
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The non-pseudonym permanent username SHOULD be derived from the IMSI. In this case, the permanent username MUST be of the format "1" | IMSI, where the character "|" denotes concatenation. In other words, the first character of the username is the digit one (ASCII value 31 hexadecimal), followed by the IMSI. The IMSI is encoded as an ASCII string that consists of not more than 15 decimal digits (ASCII values between 30 and 39 hexadecimal), one character per IMSI digit, in the order specified in [GSM-03.03]. For example, a permanent username derived from the IMSI 295023820005424 would be encoded as the ASCII string "1295023820005424" (byte values in hexadecimal notation: 31 32 39 35 30 32 33 38 32 30 30 30 35 34 32 34).
The EAP server MAY use the leading "1" as a hint to try EAP-SIM as the first authentication method during method negotiation, rather than, for example EAP/AKA. The EAP-SIM server MAY propose EAP-SIM, even if the leading character was not "1".
Alternatively, an implementation MAY choose a permanent username that is not based on the IMSI. In this case, the selection of the username, its format, and its processing is out of the scope of this document. In this case, the peer implementation MUST NOT prepend any leading characters to the username.
4.2.1.7. Generating Pseudonyms and Fast Re-authentication Identities by
the Server
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Pseudonym usernames and fast re-authentication identities are generated by the EAP server. The EAP server produces pseudonym usernames and fast re-authentication identities in an implementation-dependent manner. Only the EAP server needs to be able to map the pseudonym username to the permanent identity, or to recognize a fast re-authentication identity.
EAP-SIM includes no provisions to ensure that the same EAP server that generated a pseudonym username will be used on the authentication exchange when the pseudonym username is used. It is recommended that the EAP servers implement some centralized mechanism to allow all EAP servers of the home operator to map pseudonyms generated by other severs to the permanent identity. If no such mechanism is available, then the EAP server failing to understand a pseudonym issued by another server can request the that peer send the permanent identity.
When issuing a fast re-authentication identity, the EAP server may include a realm name in the identity to make the fast re-authentication request be forwarded to the same EAP server.
When generating fast re-authentication identities, the server SHOULD choose a fresh, new fast re-authentication identity that is different from the previous ones that were used after the same full authentication exchange. A full authentication exchange and the associated fast re-authentication exchanges are referred to here as the same "full authentication context". The fast re-authentication identity SHOULD include a random component. This random component works as a full authentication context identifier. A context-specific fast re-authentication identity can help the server to detect whether its fast re-authentication state information matches that of its peer (in other words, whether the state information is from the same full authentication exchange). The random component also makes the fast re-authentication identities unpredictable, so an attacker cannot initiate a fast re-authentication exchange to get the server's EAP-Request/SIM/ Re-authentication packet.
Transmitting pseudonyms and fast re-authentication identities from the server to the peer is discussed in Section 4.2.1.8. The pseudonym is transmitted as a username, without an NAI realm, and the fast re-authentication identity is transmitted as a complete NAI, including a realm portion if a realm is required. The realm is included in the fast re-authentication identity to allow the server to include a server-specific realm.
Regardless of the construction method, the pseudonym username MUST conform to the grammar specified for the username portion of an NAI. The fast re-authentication identity also MUST conform to the NAI grammar. The EAP servers that the subscribers of an operator can use MUST ensure that the pseudonym usernames and the username portions used in fast re-authentication identities they generate are unique.
In any case, it is necessary that permanent usernames, pseudonym usernames, and fast re-authentication usernames are separate and recognizable from each other. It is also desirable that EAP-SIM and EAP-AKA [EAP-AKA] usernames be distinguishable from each other as an aid for the server on which method to offer.
In general, it is the task of the EAP server and the policies of its administrator to ensure sufficient separation of the usernames. Pseudonym usernames and fast re-authentication usernames are both produced and used by the EAP server. The EAP server MUST compose pseudonym usernames and fast re-authentication usernames so that it can determine if an NAI username is an EAP-SIM pseudonym username or an EAP-SIM fast re-authentication username. For instance, when the usernames have been derived from the IMSI, the server could use different leading characters in the pseudonym usernames and fast re-authentication usernames (e.g., the pseudonym could begin with a leading "3" character). When mapping a fast re-authentication identity to a permanent identity, the server SHOULD only examine the username portion of the fast re-authentication identity and ignore the realm portion of the identity.
Because the peer may fail to save a pseudonym username sent in an EAP-Request/SIM/Challenge, for example due to malfunction, the EAP server SHOULD maintain at least the most recently used pseudonym username in addition to the most recently issued pseudonym username. If the authentication exchange is not completed successfully, then the server SHOULD NOT overwrite the pseudonym username that was issued during the most recent successful authentication exchange.
4.2.1.8. Transmitting Pseudonyms and Fast Re-authentication Identities
to the Peer
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The server transmits pseudonym usernames and fast re-authentication identities to the peer in cipher, using the AT_ENCR_DATA attribute.
The EAP-Request/SIM/Challenge message MAY include an encrypted pseudonym username and/or an encrypted fast re-authentication identity in the value field of the AT_ENCR_DATA attribute. Because identity privacy support and fast re-authentication are optional implementations, the peer MAY ignore the AT_ENCR_DATA attribute and always use the permanent identity. On fast re-authentication (discussed in Section 5), the server MAY include a new, encrypted fast re-authentication identity in the EAP-Request/SIM/Re-authentication message.
On receipt of the EAP-Request/SIM/Challenge, the peer MAY decrypt the encrypted data in AT_ENCR_DATA. If the authentication exchange is successful, and the encrypted data includes a pseudonym username, then the peer may use the obtained pseudonym username on the next full authentication. If a fast re-authentication identity is included, then the peer MAY save it together with other fast re-authentication state information, as discussed in Section 5, for the next fast re-authentication. If the authentication exchange does not complete successfully, the peer MUST ignore the received pseudonym username and the fast re-authentication identity.
If the peer does not receive a new pseudonym username in the EAP-Request/SIM/Challenge message, the peer MAY use an old pseudonym username instead of the permanent username on the next full authentication. The username portions of fast re-authentication identities are one-time usernames, which the peer MUST NOT re-use. When the peer uses a fast re-authentication identity in an EAP exchange, the peer MUST discard the fast re-authentication identity and not re-use it in another EAP authentication exchange, even if the authentication exchange was not completed.
4.2.1.9. Usage of the Pseudonym by the Peer
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When the optional identity privacy support is used on full authentication, the peer MAY use a pseudonym username received as part of a previous full authentication sequence as the username portion of the NAI. The peer MUST NOT modify the pseudonym username received in AT_NEXT_PSEUDONYM. However, as discussed above, the peer MAY need to decorate the username in some environments by appending or prepending the username with a string that indicates supplementary AAA routing information.
When using a pseudonym username in an environment where a realm portion is used, the peer concatenates the received pseudonym username with the "@" character and an NAI realm portion. The selection of the NAI realm is discussed above. The peer can select the realm portion similarly, regardless of whether it uses the permanent username or a pseudonym username.
4.2.1.10. Usage of the Fast Re-authentication Identity by the Peer
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On fast re-authentication, the peer uses the fast re-authentication identity that was received as part of the previous authentication sequence. A new re-authentication identity may be delivered as part of both full authentication and fast re-authentication. The peer MUST NOT modify the username part of the fast re-authentication identity received in AT_NEXT_REAUTH_ID, except in cases when username decoration is required. Even in these cases, the "root" fast re-authentication username must not be modified, but it may be appended or prepended with another string.
4.2.2. Communicating the Peer Identity to the Server
4.2.2.1. General
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The peer identity MAY be communicated to the server with the EAP-Response/Identity message. This message MAY contain the permanent identity, a pseudonym identity, or a fast re-authentication identity. If the peer uses the permanent identity or a pseudonym identity, which the server is able to map to the permanent identity, then the authentication proceeds as discussed in the overview of Section 3. If the peer uses a fast re-authentication identity, and if the fast re-authentication identity matches with a valid fast re-authentication identity maintained by the server, and if the server agrees to use fast re-authentication, then a fast re-authentication exchange is performed, as described in Section 5.
The peer identity can also be transmitted from the peer to the server using EAP-SIM messages instead of the EAP-Response/Identity. In this case, the server includes an identity-requesting attribute (AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ) in the EAP-Request/SIM/Start message, and the peer includes the AT_IDENTITY attribute, which contains the peer's identity, in the EAP-Response/SIM/Start message. The AT_ANY_ID_REQ attribute is a general identity-requesting attribute, which the server uses if it does not specify which kind of an identity the peer should return in AT_IDENTITY. The server uses the AT_FULLAUTH_ID_REQ attribute to request either the permanent identity or a pseudonym identity. The server uses the AT_PERMANENT_ID_REQ attribute to request that the peer send its permanent identity.
The identity format in the AT_IDENTITY attribute is the same as in the EAP-Response/Identity packet (except that identity decoration is not allowed). The AT_IDENTITY attribute contains a permanent identity, a pseudonym identity, or a fast re-authentication identity.
Please note that the EAP-SIM peer and the EAP-SIM server only process the AT_IDENTITY attribute; entities that only pass through EAP packets do not process this attribute. Hence, the authenticator and other intermediate AAA elements (such as possible AAA proxy servers) will continue to refer to the peer with the original identity from the EAP-Response/Identity packet unless the identity authenticated in the AT_IDENTITY attribute is communicated to them in another way within the AAA protocol.
4.2.2.2. Relying on EAP-Response/Identity Discouraged
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The EAP-Response/Identity packet is not method-specific, so in many implementations it may be handled by an EAP Framework. This introduces an additional layer of processing between the EAP peer and EAP server. The extra layer of processing may cache identity responses or add decorations to the identity. A modification of the identity response will cause the EAP peer and EAP server to use different identities in the key derivation, which will cause the protocol to fail.
For this reason, it is RECOMMENDED that the EAP peer and server use the method-specific identity attributes in EAP-SIM, and the server is strongly discouraged from relying upon the EAP-Response/Identity.
In particular, if the EAP server receives a decorated identity in EAP-Response/Identity, then the EAP server MUST use the identity-requesting attributes to request that the peer send an unmodified and undecorated copy of the identity in AT_IDENTITY.
4.2.3. Choice of Identity for the EAP-Response/Identity
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If EAP-SIM peer is started upon receiving an EAP-Request/Identity message, then the peer MAY use an EAP-SIM identity in the EAP- Response/Identity packet. In this case, the peer performs the following steps.
If the peer has maintained fast re-authentication state information and wants to use fast re-authentication, then the peer transmits the fast re-authentication identity in EAP-Response/Identity.
Else, if the peer has a pseudonym username available, then the peer transmits the pseudonym identity in EAP-Response/Identity.
In other cases, the peer transmits the permanent identity in EAP-Response/Identity.
4.2.4. Server Operation in the Beginning of EAP-SIM Exchange
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As discussed in Section 4.2.2.2, the server SHOULD NOT rely on an identity string received in EAP-Response/Identity. Therefore, the RECOMMENDED way to start an EAP-SIM exchange is to ignore any received identity strings. The server SHOULD begin the EAP-SIM exchange by issuing the EAP-Request/SIM/Start packet with an identity-requesting attribute to indicate that the server wants the peer to include an identity in the AT_IDENTITY attribute of the EAP- Response/SIM/Start message. Three methods to request an identity from the peer are discussed below.
If the server chooses not to ignore the contents of EAP- Response/Identity, then the server may have already received an EAP- SIM identity in this packet. However, if the EAP server has not received any EAP-SIM peer identity (permanent identity, pseudonym identity, or fast re-authentication identity) from the peer when sending the first EAP-SIM request, or if the EAP server has received an EAP-Response/Identity packet but the contents do not appear to be a valid permanent identity, pseudonym identity or a re-authentication identity, then the server MUST request an identity from the peer using one of the methods below.
The server sends the EAP-Request/SIM/Start message with the AT_PERMANENT_ID_REQ attribute to indicate that the server wants the peer to include the permanent identity in the AT_IDENTITY attribute of the EAP-Response/SIM/Start message. This is done in the following cases:
- The server does not support fast re-authentication or identity privacy.
- The server decided to process a received identity, and the server recognizes the received identity as a pseudonym identity but the server is not able to map the pseudonym identity to a permanent identity.
The server issues the EAP-Request/SIM/Start packet with the AT_FULLAUTH_ID_REQ attribute to indicate that the server wants the peer to include a full authentication identity (pseudonym identity or permanent identity) in the AT_IDENTITY attribute of the EAP-Response/SIM/Start message. This is done in the following cases:
- The server does not support fast re-authentication and the server supports identity privacy.
- The server decided to process a received identity, and the server recognizes the received identity as a re-authentication identity but the server is not able to map the re-authentication identity to a permanent identity.
The server issues the EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ attribute to indicate that the server wants the peer to include an identity in the AT_IDENTITY attribute of the EAP-Response/SIM/Start message, and the server does not indicate any preferred type for the identity. This is done in other cases, such as when the server ignores a received EAP-Response/Identity, the server does not have any identity, or the server does not recognize the format of a received identity.
4.2.5. Processing of EAP-Request/SIM/Start by the Peer
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Upon receipt of an EAP-Request/SIM/Start message, the peer MUST perform the following steps.
If the EAP-Request/SIM/Start does not include an identity request attribute, then the peer responds with EAP-Response/SIM/Start without AT_IDENTITY. The peer includes the AT_SELECTED_VERSION and AT_NONCE_MT attributes, because the exchange is a full authentication exchange.
If the EAP-Request/SIM/Start includes AT_PERMANENT_ID_REQ, and if the peer does not have a pseudonym available, then the peer MUST respond with EAP-Response/SIM/Start and include the permanent identity in
AT_IDENTITY. If the peer has a pseudonym available, then the peer MAY refuse to send the permanent identity; hence, in this case the peer MUST either respond with EAP-Response/SIM/Start and include the permanent identity in AT_IDENTITY or respond with EAP-Response/SIM/ Client-Error packet with the code "unable to process packet".
If the EAP-Request/SIM/Start includes AT_FULL_AUTH_ID_REQ, and if the peer has a pseudonym available, then the peer SHOULD respond with EAP-Response/SIM/Start and include the pseudonym identity in AT_IDENTITY. If the peer does not have a pseudonym when it receives this message, then the peer MUST respond with EAP-Response/SIM/Start and include the permanent identity in AT_IDENTITY. The Peer MUST NOT use a re-authentication identity in the AT_IDENTITY attribute.
If the EAP-Request/SIM/Start includes AT_ANY_ID_REQ, and if the peer has maintained fast re-authentication state information and the peer wants to use fast re-authentication, then the peer responds with EAP-Response/SIM/Start and includes the fast re-authentication identity in AT_IDENTITY. Else, if the peer has a pseudonym identity available, then the peer responds with EAP-Response/SIM/Start and includes the pseudonym identity in AT_IDENTITY. Else, the peer responds with EAP-Response/SIM/Start and includes the permanent identity in AT_IDENTITY.
An EAP-SIM exchange may include several EAP/SIM/Start rounds. The server may issue a second EAP-Request/SIM/Start if it was not able to recognize the identity that the peer used in the previous AT_IDENTITY attribute. At most, three EAP/SIM/Start rounds can be used, so the peer MUST NOT respond to more than three EAP-Request/SIM/Start messages within an EAP exchange. The peer MUST verify that the sequence of EAP-Request/SIM/Start packets that the peer receives comply with the sequencing rules defined in this document. That is, AT_ANY_ID_REQ can only be used in the first EAP-Request/SIM/Start; in other words, AT_ANY_ID_REQ MUST NOT be used in the second or third EAP-Request/SIM/Start. AT_FULLAUTH_ID_REQ MUST NOT be used if the previous EAP-Request/SIM/Start included AT_PERMANENT_ID_REQ. The peer operation, in cases when it receives an unexpected attribute or an unexpected message, is specified in Section 6.3.1.
4.2.6. Attacks Against Identity Privacy
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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.
4.2.7. Processing of AT_IDENTITY by the Server
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When the server receives an EAP-Response/SIM/Start message with the AT_IDENTITY (in response to the server's identity requesting attribute), the server MUST operate as follows.
If the server used AT_PERMANENT_ID_REQ, and if the AT_IDENTITY does not contain a valid permanent identity, then the server sends EAP-Request/SIM/Notification with AT_NOTIFICATION code "General failure" (16384), and the EAP exchange terminates. If the server recognizes the permanent identity and is able to continue, then the server proceeds with full authentication by sending EAP-Request/SIM/ Challenge.
If the server used AT_FULLAUTH_ID_REQ, and if AT_IDENTITY contains a valid permanent identity or a pseudonym identity that the server can map to a valid permanent identity, then the server proceeds with full authentication by sending EAP-Request/SIM/Challenge. If AT_IDENTITY contains a pseudonym identity that the server is not able to map to a valid permanent identity, or an identity that the server is not able to recognize or classify, then the server sends EAP-Request/SIM/Start with AT_PERMANENT_ID_REQ.
If the server used AT_ANY_ID_REQ, and if the AT_IDENTITY contains a valid permanent identity or a pseudonym identity that the server can map to a valid permanent identity, then the server proceeds with full authentication by sending EAP-Request/SIM/Challenge.
If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid fast re-authentication identity and the server agrees on using re-authentication, then the server proceeds with fast re-authentication by sending EAP-Request/SIM/Re-authentication (Section 5).
If the server used AT_ANY_ID_REQ, and if the peer sent an EAP-Response/SIM/Start with only AT_IDENTITY (indicating re-authentication), but the server is not able to map the identity to a permanent identity, then the server sends EAP-Request/SIM/Start with AT_FULLAUTH_ID_REQ.
If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid fast re-authentication identity that the server is able to map to a permanent identity, and if the server does not want to use fast re-authentication, then the server sends EAP-Request/SIM/Start without any identity requesting attributes.
If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an identity that the server recognizes as a pseudonym identity but the server is not able to map the pseudonym identity to a permanent identity, then the server sends EAP-Request/SIM/Start with AT_PERMANENT_ID_REQ.
If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an identity that the server is not able to recognize or classify, then the server sends EAP-Request/SIM/Start with AT_FULLAUTH_ID_REQ.
4.3. Message Sequence Examples (Informative)
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This section contains non-normative message sequence examples to illustrate how the peer identity can be communicated to the server.
4.3.1. Full Authentication
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This case for full authentication is illustrated below in Figure 2. In this case, AT_IDENTITY contains either the permanent identity or a pseudonym identity. The same sequence is also used in case the server uses the AT_FULLAUTH_ID_REQ in EAP-Request/SIM/Start.
Peer Authenticator | | | +------------------------------+ | | Server does not have a | | | Subscriber identity available| | | When starting EAP-SIM | | +------------------------------+ | | | EAP-Request/SIM/Start | | (AT_ANY_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (AT_IDENTITY, AT_NONCE_MT, | | AT_SELECTED_VERSION) | |------------------------------------------------------>| | |
Figure 2: Requesting any identity, full authentication
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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.
4.3.2. Fast Re-authentication
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The case when the server uses the AT_ANY_ID_REQ and the peer wants to perform fast re-authentication is illustrated below in Figure 3.
Peer Authenticator | | | +------------------------------+ | | Server does not have a | | | Subscriber identity available| | | When starting EAP-SIM | | +------------------------------+ | | | EAP-Request/SIM/Start | | (AT_ANY_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (AT_IDENTITY containing a fast re-auth. identity) | |------------------------------------------------------>| | | Figure 3: Requesting any identity, fast re-authenticationOn fast re-authentication, if the AT_IDENTITY attribute contains a valid fast re-authentication identity and the server agrees on using fast re-authentication, then the server proceeds with the fast re-authentication sequence and issues the EAP-Request/SIM/ Re-authentication packet, as specified in Section 5.
4.3.3. Fall Back to Full Authentication
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Figure 4 illustrates cases in which the server does not recognize the fast re-authentication identity the peer used in AT_IDENTITY, and issues a second EAP-Request/SIM/Start message.
Peer Authenticator | | | +------------------------------+ | | Server does not have a | | | Subscriber identity available| | | When starting EAP-SIM | | +------------------------------+ | | | EAP-Request/SIM/Start | | (AT_ANY_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (AT_IDENTITY containing a fast re-auth. identity) | |------------------------------------------------------>| | | | +------------------------------+ | | Server does not recognize | | | The fast re-auth. | | | Identity | | +------------------------------+ | | | EAP-Request/SIM/Start | | (AT_FULLAUTH_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (AT_IDENTITY with a full-auth. identity, AT_NONCE_MT, | | AT_SELECTED_VERSION) | |------------------------------------------------------>| | |
Figure 4: Fall back to full authentication
4.3.4. Requesting the Permanent Identity 1
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Figure 5 illustrates the case in which the EAP server fails to map the pseudonym identity included in the EAP-Response/Identity packet to a valid permanent identity.
Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes a pseudonym) | |------------------------------------------------------>| | | | +------------------------------+ | | Server fails to map the | | | Pseudonym to a permanent id. | | +------------------------------+ | EAP-Request/SIM/Start | | (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | EAP-Response/SIM/Start | | (AT_IDENTITY with permanent identity, AT_NONCE_MT, | | AT_SELECTED_VERSION) | |------------------------------------------------------>| | |
Figure 5: Requesting the permanent identity
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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.
4.3.5. Requesting the Permanent Identity 2
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Figure 6 illustrates the case in which the EAP server fails to map the pseudonym included in the AT_IDENTITY attribute to a valid permanent identity.
Peer Authenticator | | | +------------------------------+ | | Server does not have a | | | Subscriber identity available| | | When starting EAP-SIM | | +------------------------------+ | EAP-Request/SIM/Start | | (AT_ANY_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | |EAP-Response/SIM/Start | |(AT_IDENTITY with a pseudonym identity, AT_NONCE_MT, | | AT_SELECTED_VERSION) | |------------------------------------------------------>| | +-------------------------------+ | | Server fails to map the | | | Pseudonym in AT_IDENTITY | | | to a valid permanent identity | | +-------------------------------+ | | | EAP-Request/SIM/Start | | (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | EAP-Response/SIM/Start | | (AT_IDENTITY with permanent identity, | | AT_NONCE_MT, AT_SELECTED_VERSION) | |------------------------------------------------------>| | |Figure 6: Requesting a permanent identity (two EAP-SIM Start rounds)
4.3.6. Three EAP-SIM/Start Roundtrips
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In the worst case, there are three EAP/SIM/Start round trips before the server obtains an acceptable identity. This case is illustrated in Figure 7.
Peer Authenticator | | | +------------------------------+ | | Server does not have a | | | Subscriber identity available| | | When starting EAP-SIM | | +------------------------------+ | EAP-Request/SIM/Start | | (Includes AT_ANY_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | EAP-Response/SIM/Start | | (AT_IDENTITY with fast re-auth. identity) | |------------------------------------------------------>| | | | +------------------------------+ | | Server does not accept | | | The fast re-auth. | | | Identity | | +------------------------------+ | EAP-Request/SIM/Start | | (AT_FULLAUTH_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | : : : : : : : : |EAP-Response/SIM/Start | |(AT_IDENTITY with a pseudonym identity, AT_NONCE_MT, | | AT_SELECTED_VERSION) | |------------------------------------------------------>| | | | +-------------------------------+ | | Server fails to map the | | | Pseudonym in AT_IDENTITY | | | to a valid permanent identity | | +-------------------------------+ | EAP-Request/SIM/Start | | (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | EAP-Response/SIM/Start | | (AT_IDENTITY with permanent identity, AT_NONCE_MT, | | AT_SELECTED_VERSION) | |------------------------------------------------------>| | | Figure 7: Three EAP-SIM Start roundsAfter the last EAP-Response/SIM/Start message, the full authentication sequence proceeds as usual. If the EAP Server recognizes the permanent identity and is able to proceed, the server issues the EAP-Request/SIM/Challenge message.
5. Fast Re-Authentication
5.1. General
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In some environments, EAP authentication may be performed frequently. Because the EAP-SIM full authentication procedure makes use of the GSM SIM A3/A8 algorithms, and therefore requires 2 or 3 fresh triplets from the Authentication Centre, the full authentication procedure is not very well suited for frequent use. Therefore, EAP-SIM includes a more inexpensive fast re-authentication procedure that does not make use of the SIM A3/A8 algorithms and does not need new triplets from the Authentication Centre. Re-authentication can be performed in fewer roundtrips than the full authentication.
Fast re-authentication is optional to implement for both the EAP-SIM server and peer. On each EAP authentication, either one of the entities may also fall back on full authentication if it does not want to use fast re-authentication.
Fast re-authentication is based on the keys derived on the preceding full authentication. The same K_aut and K_encr keys that were used in full authentication are used to protect EAP-SIM packets and attributes, and the original Master Key from full authentication is used to generate a fresh Master Session Key, as specified in Section 7.
The fast re-authentication exchange makes use of an unsigned 16-bit counter, included in the AT_COUNTER attribute. The counter has three goals: 1) it can be used to limit the number of successive reauthentication exchanges without full authentication 2) it contributes to the keying material, and 3) it protects the peer and the server from replays. On full authentication, both the server and the peer initialize the counter to one. The counter value of at least one is used on the first fast re-authentication. On subsequent fast re-authentications, the counter MUST be greater than on any of the previous re-authentications. For example, on the second fast re-authentication, the counter value is two or greater. The AT_COUNTER attribute is encrypted.
Both the peer and the EAP server maintain a copy of the counter. The EAP server sends its counter value to the peer in the fast re-authentication request. The peer MUST verify that its counter value is less than or equal to the value sent by the EAP server.
The server includes an encrypted server random nonce (AT_NONCE_S) in the fast re-authentication request. The AT_MAC attribute in the peer's response is calculated over NONCE_S to provide a challenge/response authentication scheme. The NONCE_S also contributes to the new Master Session Key.
Both the peer and the server SHOULD have an upper limit for the number of subsequent fast re-authentications allowed before a full authentication needs to be performed. Because a 16-bit counter is used in fast re-authentication, the theoretical maximum number of re-authentications is reached when the counter value reaches FFFF hexadecimal.
In order to use fast re-authentication, the peer and the EAP server need to store the following values: Master Key, latest counter value and the next fast re-authentication identity. K_aut, K_encr may either be stored or derived again from MK. The server may also need to store the permanent identity of the user.
5.2. Comparison to UMTS AKA
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When analyzing the fast re-authentication exchange, it may be helpful to compare it with the UMTS Authentication and Key Agreement (AKA) exchange, which it resembles closely. The counter corresponds to the UMTS AKA sequence number, NONCE_S corresponds to RAND, AT_MAC in EAP-Request/SIM/Re-authentication corresponds to AUTN, the AT_MAC in EAP-Response/SIM/Re-authentication corresponds to RES, AT_COUNTER_TOO_SMALL corresponds to AUTS, and encrypting the counter corresponds to the usage of the Anonymity Key. Also, the key generation on fast re-authentication, with regard to random or fresh material, is similar to UMTS AKA -- the server generates the NONCE_S and counter values, and the peer only verifies that the counter value is fresh.
It should also be noted that encrypting the AT_NONCE_S, AT_COUNTER, or AT_COUNTER_TOO_SMALL attributes is not important to the security of the fast re-authentication exchange.
5.3. Fast Re-authentication Identity
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The fast re-authentication procedure makes use of separate re-authentication user identities. Pseudonyms and the permanent identity are reserved for full authentication only. If a re-authentication identity is lost and the network does not recognize it, the EAP server can fall back on full authentication.
If the EAP server supports fast re-authentication, it MAY include the skippable AT_NEXT_REAUTH_ID attribute in the encrypted data of EAP-Request/SIM/Challenge message (Section 9.3). This attribute contains a new fast re-authentication identity for the next fast re-authentication. The attribute also works as a capability flag that, indicating that the server supports fast re-authentication, and that the server wants to continue using fast re-authentication within the current context. The peer MAY ignore this attribute, in which case it MUST use full authentication next time. If the peer wants to use re-authentication, it uses this fast re-authentication identity on next authentication. Even if the peer has a fast re-authentication identity, the peer MAY discard the fast re-authentication identity and use a pseudonym or the permanent identity instead, in which case full authentication MUST be performed. If the EAP server does not include the AT_NEXT_REAUTH_ID in the encrypted data of EAP-Request/SIM/Challenge or EAP-Request/SIM/ Re-authentication, then the peer MUST discard its current fast re-authentication state information and perform a full authentication next time.
In environments where a realm portion is needed in the peer identity, the fast re-authentication identity received in AT_NEXT_REAUTH_ID MUST contain both a username portion and a realm portion, as per the NAI format. The EAP Server can choose an appropriate realm part in order to have the AAA infrastructure route subsequent fast re-authentication related requests to the same AAA server. For example, the realm part MAY include a portion that is specific to the AAA server. Hence, it is sufficient to store the context required for fast re-authentication in the AAA server that performed the full authentication.
The peer MAY use the fast re-authentication identity in the EAP-Response/Identity packet or, in response to the server's AT_ANY_ID_REQ attribute, the peer MAY use the fast re-authentication identity in the AT_IDENTITY attribute of the EAP-Response/SIM/Start packet.
The peer MUST NOT modify the username portion of the fast re-authentication identity, but the peer MAY modify the realm portion or replace it with another realm portion. The peer might need to modify the realm in order to influence the AAA routing, for example, to make sure that the correct server is reached. It should be noted that sharing the same fast re-authentication key among several servers may have security risks, so changing the realm portion of the NAI in order to change the EAP server is not desirable.
Even if the peer uses a fast re-authentication identity, the server may want to fall back on full authentication, for example because the server does not recognize the fast re-authentication identity or does not want to use fast re-authentication. In this case, the server starts the full authentication procedure by issuing an EAP-Request/SIM/Start packet. This packet always starts a full authentication sequence if it does not include the AT_ANY_ID_REQ attribute. If the server was not able to recover the peer's identity from the fast re-authentication identity, the server includes either the AT_FULLAUTH_ID_REQ or the AT_PERMANENT_ID_REQ attribute in this EAP request.
5.4. Fast Re-authentication Procedure
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Figure 8 illustrates the fast re-authentication procedure. In this example, the optional protected success indication is not used. Encrypted attributes are denoted with '*'. The peer uses its re-authentication identity in the EAP-Response/Identity packet. As discussed above, an alternative way to communicate the re-authentication identity to the server is for the peer to use the AT_IDENTITY attribute in the EAP-Response/SIM/Start message. This latter case is not illustrated in the figure below, and it is only possible when the server requests that the peer send its identity by including the AT_ANY_ID_REQ attribute in the EAP-Request/SIM/Start packet.
If the server recognizes the identity as a valid fast re-authentication identity, and if the server agrees to use fast re-authentication, then the server sends the EAP-Request/SIM/ Re-authentication packet to the peer. This packet MUST include the encrypted AT_COUNTER attribute, with a fresh counter value, the encrypted AT_NONCE_S attribute that contains a random number chosen by the server, the AT_ENCR_DATA and the AT_IV attributes used for encryption, and the AT_MAC attribute that contains a message authentication code over the packet. The packet MAY also include an encrypted AT_NEXT_REAUTH_ID attribute that contains the next fast re-authentication identity.
Fast re-authentication identities are one-time identities. If the peer does not receive a new fast re-authentication identity, it MUST use either the permanent identity or a pseudonym identity on the next authentication to initiate full authentication.
The peer verifies that AT_MAC is correct, and that the counter value is fresh (greater than any previously used value). The peer MAY save the next fast re-authentication identity from the encrypted AT_NEXT_REAUTH_ID for next time. If all checks are successful, the peer responds with the EAP-Response/SIM/Re-authentication packet, including the AT_COUNTER attribute with the same counter value and AT_MAC attribute.
The server verifies the AT_MAC attribute and also verifies that the counter value is the same that it used in the EAP-Request/SIM/ Re-authentication packet. If these checks are successful, the re-authentication has succeeded and the server sends the EAP-Success packet to the peer.
If protected success indications (Section 6.2) were used, the EAP-Success packet would be preceded by an EAP-SIM notification round.
Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes a fast re-authentication identity) | |------------------------------------------------------>| | | | +--------------------------------+ | | Server recognizes the identity | | | and agrees to use fast | | | re-authentication | | +--------------------------------+ | | : : : : : : : : | EAP-Request/SIM/Re-authentication | | (AT_IV, AT_ENCR_DATA, *AT_COUNTER, | | *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC) | |<------------------------------------------------------| | | +-----------------------------------------------+ | | Peer verifies AT_MAC and the freshness of | | | the counter. Peer MAY store the new fast re- | | | authentication identity for next re-auth. | | +-----------------------------------------------+ | | | | EAP-Response/SIM/Re-authentication | | (AT_IV, AT_ENCR_DATA, *AT_COUNTER with same value, | | AT_MAC) | |------------------------------------------------------>| | +--------------------------------+ | | Server verifies AT_MAC and | | | the counter | | +--------------------------------+ | | | EAP-Success | |<------------------------------------------------------| | |
Figure 8: Fast Re-authentication
5.5. Fast Re-authentication Procedure when Counter Is Too Small
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If the peer does not accept the counter value of EAP-Request/SIM/ Re-authentication, it indicates the counter synchronization problem by including the encrypted AT_COUNTER_TOO_SMALL in EAP-Response/SIM/ Re-authentication. The server responds with EAP-Request/SIM/Start to initiate a normal full authentication procedure. This is illustrated in Figure 9. Encrypted attributes are denoted with '*'.
Peer Authenticator | EAP-Request/SIM/Start | | (AT_ANY_ID_REQ, AT_VERSION_LIST) | |<------------------------------------------------------| | | | EAP-Response/SIM/Start | | (AT_IDENTITY) | | (Includes a fast re-authentication identity) | |------------------------------------------------------>| | | | EAP-Request/SIM/Re-authentication | | (AT_IV, AT_ENCR_DATA, *AT_COUNTER, | | *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC) | |<------------------------------------------------------| +-----------------------------------------------+ | | AT_MAC is valid but the counter is not fresh. | | +-----------------------------------------------+ | | | | EAP-Response/SIM/Re-authentication | | (AT_IV, AT_ENCR_DATA, *AT_COUNTER_TOO_SMALL, | | *AT_COUNTER, AT_MAC) | |------------------------------------------------------>| | +----------------------------------------------+ | | Server verifies AT_MAC but detects | | | That peer has included AT_COUNTER_TOO_SMALL | | +----------------------------------------------+ | | | EAP-Request/SIM/Start | | (AT_VERSION_LIST) | |<------------------------------------------------------| +---------------------------------------------------------------+ | Normal full authentication follows. | +---------------------------------------------------------------+ | |
Figure 9: Fast Re-authentication, counter is not fresh
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In the figure above, the first three messages are similar to the basic fast re-authentication case. When the peer detects that the counter value is not fresh, it includes the AT_COUNTER_TOO_SMALL attribute in EAP-Response/SIM/Re-authentication. This attribute doesn't contain any data, but it is a request for the server to initiate full authentication. In this case, the peer MUST ignore the contents of the server's AT_NEXT_REAUTH_ID attribute.
On receipt of AT_COUNTER_TOO_SMALL, the server verifies AT_MAC and verifies that AT_COUNTER contains the same counter value as in the EAP-Request/SIM/Re-authentication packet. If not, the server terminates the authentication exchange by sending the EAP-Request/SIM/Notification with AT_NOTIFICATION code "General failure" (16384). If all checks on the packet are successful, the server transmits a new EAP-Request/SIM/Start packet and the full authentication procedure is performed as usual. Since the server already knows the subscriber identity, it MUST NOT include AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ, or AT_PERMANENT_ID_REQ in the EAP-Request/SIM/Start.
It should be noted that in this case, peer identity is only transmitted in the AT_IDENTITY attribute at the beginning of the whole EAP exchange. The fast re-authentication identity used in this AT_IDENTITY attribute will be used in key derivation (see Section 7).
6. EAP-SIM Notifications
6.1. General
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EAP-SIM does not prohibit the use of the EAP Notifications as specified in [RFC3748]. EAP Notifications can be used at any time in the EAP-SIM exchange. It should be noted that EAP-SIM does not protect EAP Notifications. EAP-SIM also specifies method-specific EAP-SIM notifications that are protected in some cases.
The EAP server can use EAP-SIM notifications to convey notifications and result indications (Section 6.2) to the peer.
The server MUST use notifications in cases discussed in Section 6.3.2. When the EAP server issues an EAP-Request/SIM/Notification packet to the peer, the peer MUST process the notification packet. The peer MAY show a notification message to the user and the peer MUST respond to the EAP server with an EAP-Response/SIM/Notification packet, even if the peer did not recognize the notification code.
An EAP-SIM full authentication exchange or a fast re-authentication exchange MUST NOT include more than one EAP-SIM notification round.
The notification code is a 16-bit number. The most significant bit is called the Success bit (S bit). The S bit specifies whether the notification implies failure. The code values with the S bit set to zero (code values 0...32767) are used on unsuccessful cases. The receipt of a notification code from this range implies a failed EAP exchange, so the peer can use the notification as a failure indication. After receiving the EAP-Response/SIM/Notification for these notification codes, the server MUST send the EAP-Failure packet.
The receipt of a notification code with the S bit set to one (values 32768...65536) does not imply failure. Notification code "Success" (32768) has been reserved as a general notification code to indicate successful authentication.
The second most significant bit of the notification code is called the Phase bit (P bit). It specifies at which phase of the EAP-SIM exchange the notification can be used. If the P bit is set to zero, the notification can only be used after a successful EAP/SIM/Challenge round in full authentication or a successful EAP/SIM/Re-authentication round in reauthentication. A re-authentication round is considered successful only if the peer has successfully verified AT_MAC and AT_COUNTER attributes, and does not include the AT_COUNTER_TOO_SMALL attribute in EAP-Response/SIM/Re-authentication.
If the P bit is set to one, the notification can only by used before the EAP/SIM/Challenge round in full authentication, or before the EAP/SIM/Re-authentication round in reauthentication. These notifications can only be used to indicate various failure cases. In other words, if the P bit is set to one, then the S bit MUST be set to zero.
Section 9.8 and Section 9.9 specify what other attributes must be included in the notification packets.
Some of the notification codes are authorization related and, hence, are not usually considered part of the responsibility of an EAP method. However, they are included as part of EAP-SIM because there are currently no other ways to convey this information to the user in a localizable way, and the information is potentially useful for the user. An EAP-SIM server implementation may decide never to send these EAP-SIM notifications.
6.2. Result Indications
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As discussed in Section 6.3, the server and the peer use explicit error messages in all error cases. If the server detects an error after successful authentication, the server uses an EAP-SIM notification to indicate failure to the peer. In this case, the result indication is integrity and replay protected.
By sending an EAP-Response/SIM/Challenge packet or an EAP-Response/SIM/Re-authentication packet (without AT_COUNTER_TOO_SMALL), the peer indicates that it has successfully authenticated the server and that the peer's local policy accepts the EAP exchange. In other words, these packets are implicit success indications from the peer to the server.
EAP-SIM also supports optional protected success indications from the server to the peer. If the EAP server wants to use protected success indications, it includes the AT_RESULT_IND attribute in the EAP-Request/SIM/Challenge or the EAP-Request/SIM/Re-authentication packet. This attribute indicates that the EAP server would like to use result indications in both successful and unsuccessful cases. If the peer also wants this, the peer includes AT_RESULT_IND in EAP-Response/SIM/Challenge or EAP-Response/SIM/Re-authentication. The peer MUST NOT include AT_RESULT_IND if it did not receive AT_RESULT_IND from the server. If both the peer and the server used AT_RESULT_IND, then the EAP exchange is not complete yet, but an EAP-SIM notification round will follow. The following EAP-SIM notification may indicate either failure or success.
Success indications with the AT_NOTIFICATION code "Success" (32768) can only be used if both the server and the peer indicate they want to use them with AT_RESULT_IND. If the server did not include AT_RESULT_IND in the EAP-Request/SIM/Challenge or EAP-Request/SIM/Re-authentication packet, or if the peer did not include AT_RESULT_IND in the corresponding response packet, then the server MUST NOT use protected success indications.
Because the server uses the AT_NOTIFICATION code "Success" (32768) to indicate that the EAP exchange has completed successfully, the EAP exchange cannot fail when the server processes the EAP-SIM response to this notification. Hence, the server MUST ignore the contents of the EAP-SIM response it receives from the EAP-Request/SIM/Notification with this code. Regardless of the contents of the EAP-SIM response, the server MUST send EAP-Success as the next packet.
6.3. Error Cases
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This section specifies the operation of the peer and the server in error cases. The subsections below require the EAP-SIM peer and server to send an error packet (EAP-Response/SIM/Client-Error from the peer or EAP-Request/SIM/Notification from the server) in error cases. However, implementations SHOULD NOT rely upon the correct error reporting behavior of the peer, authenticator, or the server. It is possible for error and other messages to be lost in transit or for a malicious participant to attempt to consume resources by not issuing error messages. Both the peer and the EAP server SHOULD have a mechanism to clean up state, even if an error message or EAP-Success is not received after a timeout period.
6.3.1. Peer Operation
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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:
- EAP exchange is not acceptable according to the peer's local policy.
- 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. The peer SHOULD log this event.
- invalid pad bytes in AT_PADDING.
- the peer does not want to process AT_PERMANENT_ID_REQ.
Separate error codes have been defined for the following error cases in Section 10.19:
As specified in Section 4.1, when processing the AT_VERSION_LIST attribute, which lists the EAP-SIM versions supported by the server, if the attribute does not include a version that is implemented by the peer and allowed in the peer's security policy, then the peer MUST send the EAP-Response/SIM/Client-Error packet with the error code "unsupported version".
If the number of RAND challenges is smaller than what is required by peer's local policy when processing the AT_RAND attribute, the peer MUST send the EAP-Response/SIM/Client-Error packet with the error code "insufficient number of challenges".
If the peer believes that the RAND challenges included in AT_RAND are not fresh e.g., because it is capable of remembering some previously used RANDs, the peer MUST send the EAP-Response/SIM/Client-Error packet with the error code "RANDs are not fresh".
6.3.2. Server Operation
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If an EAP-SIM server detects an error in a received EAP-SIM response, the server MUST issue the EAP-Request/SIM/Notification packet with an AT_NOTIFICATION code that implies failure. By default, the server uses one of the general failure codes ("General failure after authentication" (0), or "General failure" (16384)). The choice between these two codes depends on the phase of the EAP-SIM exchange, see Section 6. When the server issues an EAP- Request/SIM/Notification that implies failure, the error cases include the following:
- 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. The server SHOULD log this event.
- invalid AT_COUNTER
6.3.3. EAP-Failure
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The EAP-SIM server sends EAP-Failure in two cases:
1) In response to an EAP-Response/SIM/Client-Error packet the server has received from the peer, or- Following an EAP-SIM notification round, when the AT_NOTIFICATION code implies failure.
The EAP-SIM server MUST NOT send EAP-Failure in cases other than these two. However, it should be noted that even though the EAP-SIM server would not send an EAP-Failure, an authorization decision that happens outside EAP-SIM, such as in the AAA server or in an intermediate AAA proxy, may result in a failed exchange.
The peer MUST accept the EAP-Failure packet in case 1) and case 2), above. The peer SHOULD silently discard the EAP-Failure packet in other cases.
6.3.4. EAP-Success
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On full authentication, the server can only send EAP-Success after the EAP/SIM/Challenge round. The peer MUST silently discard any EAP-Success packets if they are received before the peer has successfully authenticated the server and sent the EAP-Response/SIM/Challenge packet.
If the peer did not indicate that it wants to use protected success indications with AT_RESULT_IND (as discussed in Section 6.2) on full authentication, then the peer MUST accept EAP-Success after a successful EAP/SIM/Challenge round.
If the peer indicated that it wants to use protected success indications with AT_RESULT_IND (as discussed in Section 6.2), then the peer MUST NOT accept EAP-Success after a successful EAP/SIM/Challenge round. In this case, the peer MUST only accept EAP-Success after receiving an EAP-SIM Notification with the AT_NOTIFICATION code "Success" (32768).
On fast re-authentication, EAP-Success can only be sent after the EAP/SIM/Re-authentication round. The peer MUST silently discard any EAP-Success packets if they are received before the peer has successfully authenticated the server and sent the EAP-Response/SIM/Re-authentication packet.
If the peer did not indicate that it wants to use protected success indications with AT_RESULT_IND (as discussed in Section 6.2) on fast re-authentication, then the peer MUST accept EAP-Success after a successful EAP/SIM/Re-authentication round.
If the peer indicated that it wants to use protected success indications with AT_RESULT_IND (as discussed in Section 6.2), then the peer MUST NOT accept EAP-Success after a successful EAP/SIM/Re- authentication round. In this case, the peer MUST only accept EAP-Success after receiving an EAP-SIM Notification with the AT_NOTIFICATION code "Success" (32768).
If the peer receives an EAP-SIM notification (Section 6) that indicates failure, then the peer MUST no longer accept the EAP-Success packet, even if the server authentication was successfully completed.
7. Key Generation
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This section specifies how keying material is generated.
On EAP-SIM full authentication, a Master Key (MK) is derived from the underlying GSM authentication values (Kc keys), the NONCE_MT, and other relevant context as follows.
MK = SHA1(Identity|n*Kc| NONCE_MT| Version List| Selected Version)
In the formula above, the "|" character denotes concatenation. "Identity" denotes the peer identity string without any terminating null characters. It is the identity from the last AT_IDENTITY attribute sent by the peer in this exchange, or, if AT_IDENTITY was not used, it is the identity from the EAP-Response/Identity packet. The identity string is included as-is, without any changes. As discussed in Section 4.2.2.2, relying on EAP-Response/Identity for conveying the EAP-SIM peer identity is discouraged, and the server SHOULD use the EAP-SIM method-specific identity attributes.
The notation n*Kc in the formula above denotes the n Kc values concatenated. The Kc keys are used in the same order as the RAND challenges in AT_RAND attribute. NONCE_MT denotes the NONCE_MT value (not the AT_NONCE_MT attribute, but only the nonce value). The Version List includes the 2-byte-supported version numbers from AT_VERSION_LIST, in the same order as in the attribute. The Selected Version is the 2-byte selected version from AT_SELECTED_VERSION. Network byte order is used, just as in the attributes. The hash function SHA-1 is specified in [SHA-1]. If several EAP/SIM/Start roundtrips are used in an EAP-SIM exchange, then the NONCE_MT, Version List and Selected version from the last EAP/SIM/Start round are used, and the previous EAP/SIM/Start rounds are ignored.
The Master Key is fed into a Pseudo-Random number Function (PRF) which generates separate Transient EAP Keys (TEKs) for protecting EAP-SIM packets, as well as a Master Session Key (MSK) for link layer security, and an Extended Master Session Key (EMSK) for other purposes. On fast re-authentication, the same TEKs MUST be used for protecting EAP packets, but a new MSK and a new EMSK MUST be derived from the original MK and from new values exchanged in the fast re-authentication.
EAP-SIM requires two TEKs for its own purposes; the authentication key K_aut is to be used with the AT_MAC attribute, and the encryption key K_encr is to be used with the AT_ENCR_DATA attribute. The same K_aut and K_encr keys are used in full authentication and subsequent fast re-authentications.
Key derivation is based on the random number generation specified in NIST Federal Information Processing Standards (FIPS) Publication 186-2 [PRF]. The pseudo-random number generator is specified in the change notice 1 (2001 October 5) of [PRF] (Algorithm 1). As specified in the change notice (page 74), when Algorithm 1 is used as a general-purpose pseudo-random number generator, the "mod q" term in step 3.3 is omitted. The function G used in the algorithm is constructed via the Secure Hash Standard, as specified in Appendix 3.3 of the standard. It should be noted that the function G is very similar to SHA-1, but the message padding is different. Please refer to [PRF] for full details. For convenience, the random number algorithm with the correct modification is cited in Appendix B.
160-bit XKEY and XVAL values are used, so b = 160. On each full authentication, the Master Key is used as the initial secret seed-key XKEY. The optional user input values (XSEED_j) in step 3.1 are set to zero.
On full authentication, the resulting 320-bit random numbers (x_0, x_1, ..., x_m-1) are concatenated and partitioned into suitable-sized chunks and used as keys in the following order: K_encr (128 bits), K_aut (128 bits), Master Session Key (64 bytes), Extended Master Session Key (64 bytes).
On fast re-authentication, the same pseudo-random number generator can be used to generate a new Master Session Key and a new Extended Master Session Key. The seed value XKEY' is calculated as follows:
XKEY' = SHA1(Identity|counter|NONCE_S| MK)
In the formula above, the Identity denotes the fast re-authentication identity, without any terminating null characters, from the AT_IDENTITY attribute of the EAP-Response/SIM/Start packet, or, if
EAP-Response/SIM/Start was not used on fast re-authentication, it denotes the identity string from the EAP-Response/Identity packet. The counter denotes the counter value from the AT_COUNTER attribute used in the EAP-Response/SIM/Re-authentication packet. The counter is used in network byte order. NONCE_S denotes the 16-byte NONCE_S value from the AT_NONCE_S attribute used in the EAP-Request/SIM/Re-authentication packet. The MK is the Master Key derived on the preceding full authentication.
On fast re-authentication, the pseudo-random number generator is run with the new seed value XKEY', and the resulting 320-bit random numbers (x_0, x_1, ..., x_m-1) are concatenated and partitioned into two 64-byte chunks and used as the new 64-byte Master Session Key and the new 64-byte Extended Master Session Key. Note that because K_encr and K_aut are not derived on fast re-authentication, the Master Session Key and the Extended Master Session key are obtained from the beginning of the key stream (x_0, x_1, ...).
The first 32 bytes of the MSK can be used as the Pairwise Master Key (PMK) for IEEE 802.11i.
When the RADIUS attributes specified in [RFC2548] are used to transport keying material, then the first 32 bytes of the MSK correspond to MS-MPPE-RECV-KEY and the second 32 bytes to MS-MPPE-SEND-KEY. In this case, only 64 bytes of keying material (the MSK) are used.
When generating the initial Master Key, the hash function is used as a mixing function to combine several session keys (Kc's) generated by the GSM authentication procedure and the random number NONCE_MT into a single session key. There are several reasons for this. The current GSM session keys are, at most, 64 bits, so two or more of them are needed to generate a longer key. By using a one-way function to combine the keys, we are assured that, even if an attacker managed to learn one of the EAP-SIM session keys, it wouldn't help him in learning the original GSM Kc's. In addition, since we include the random number NONCE_MT in the calculation, the peer is able to verify that the EAP-SIM packets it receives from the network are fresh and not replays (also see Section 11).
8. Message Format and Protocol Extensibility
8.1. Message Format
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As specified in [RFC3748], EAP packets begin with the Code, Identifiers, Length, and Type fields, which are followed by EAP- method-specific Type-Data. The Code field in the EAP header is set to 1 for EAP requests, and to 2 for EAP Responses. The usage of the
Length and Identifier fields in the EAP header are also specified in [RFC3748]. In EAP-SIM, the Type field is set to 18.
In EAP-SIM, the Type-Data begins with an EAP-SIM header that consists of a 1-octet Subtype field and a 2-octet reserved field. The Subtype values used in EAP-SIM are defined in the IANA considerations section of the EAP-AKA specification [EAP-AKA]. The formats of the EAP header and the EAP-SIM header are shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The rest of the Type-Data that immediately follows the EAP-SIM header consists of attributes that are encoded in Type, Length, Value format. The figure below shows the generic format of an attribute.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Attribute Type-
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Indicates the particular type of attribute. The attribute type values are listed in the IANA considerations section of the EAP-AKA specification [EAP-AKA].
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Length
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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.
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Value
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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.
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Attributes numbered within the range 0 through 127 are called non-skippable attributes. When an EAP-SIM peer encounters a non-skippable attribute that the peer does not recognize, the peer MUST send the EAP-Response/SIM/Client-Error packet, which terminates the authentication exchange. If an EAP-SIM server encounters a non-skippable attribute that the server does not recognize, then the server sends the EAP-Request/SIM/Notification packet with an AT_NOTIFICATION code, which implies general failure ("General failure after authentication" (0), or "General failure" (16384), depending on the phase of the exchange), which terminates the authentication exchange.
Attributes within the range of 128 through 255 are called skippable attributes. When a skippable attribute is encountered and is not recognized, it is ignored. The rest of the attributes and message data MUST still be processed. The Length field of the attribute is used to skip the attribute value in searching for the next attribute.
Unless otherwise specified, the order of the attributes in an EAP-SIM message is insignificant and an EAP-SIM implementation should not assume a certain order to be used.
Attributes can be encapsulated within other attributes. In other words, the value field of an attribute type can be specified to contain other attributes.
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8.2. Protocol Extensibility
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EAP-SIM can be extended by specifying new attribute types. If skippable attributes are used, it is possible to extend the protocol without breaking old implementations.
However, any new attributes added to the EAP-Request/SIM/Start or EAP-Response/SIM/Start packets would not be integrity-protected. Therefore, these messages MUST NOT be extended in the current version of EAP-SIM. If the list of supported EAP-SIM versions in the AT_VERSION_LIST does not include versions other than 1, then the server MUST NOT include attributes other than those specified in this document in the EAP-Request/SIM/Start message. Note that future versions of this protocol might specify new attributes for EAP-Request/SIM/Start and still support version 1 of the protocol. In this case, the server might send an EAP-Request/SIM/Start message that includes new attributes and indicates support for protocol version 1 and other versions in the AT_VERSION_LIST attribute. If the peer selects version 1, then the peer MUST ignore any other attributes included in EAP-Request/SIM/Start, other than those specified in this document. If the selected EAP-SIM version in peer's AT_SELECTED_VERSION is 1, then the peer MUST NOT include other attributes aside from those specified in this document in the EAP-Response/SIM/Start message.
When specifying new attributes, it should be noted that EAP-SIM does not support message fragmentation. Hence, the sizes of the new extensions MUST be limited so that the maximum transfer unit (MTU) of the underlying lower layer is not exceeded. According to [RFC3748], lower layers must provide an EAP MTU of 1020 bytes or greater, so any extensions to EAP-SIM SHOULD NOT exceed the EAP MTU of 1020 bytes.
Because EAP-SIM supports version negotiation, new versions of the protocol can also be specified by using a new version number.
9. Messages
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This section specifies the messages used in EAP-SIM. It specifies when a message may be transmitted or accepted, which attributes are allowed in a message, which attributes are required in a message, and other message-specific details. The general message format is specified in Section 8.1.
9.1. EAP-Request/SIM/Start
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In full authentication the first SIM-specific EAP Request is EAP-Request/SIM/Start. The EAP/SIM/Start roundtrip is used for two purposes. In full authentication this packet is used to request the peer to send the AT_NONCE_MT attribute to the server. In addition, as specified in Section 4.2, the Start round trip may be used by the server for obtaining the peer identity. As discussed in Section 4.2, several Start rounds may be required to obtain a valid peer identity.
The server MUST always include the AT_VERSION_LIST attribute.
The server MAY include one of the following identity-requesting attributes: AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, or AT_ANY_ID_REQ. These three attributes are mutually exclusive, so the server MUST NOT include more than one of the attributes.
If the server has received a response from the peer, it MUST NOT issue a new EAP-Request/SIM/Start packet if it has previously issued an EAP-Request/SIM/Start message either without any identity requesting attributes or with the AT_PERMANENT_ID_REQ attribute.
If the server has received a response from the peer, it MUST NOT issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ attributes if it has previously issued an EAP-Request/SIM/Start message with the AT_FULLAUTH_ID_REQ attribute.
If the server has received a response from the peer, it MUST NOT issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ attribute if the server has previously issued an EAP-Request/SIM/Start message with the AT_ANY_ID_REQ attribute.
This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.
9.2. EAP-Response/SIM/Start
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The peer sends EAP-Response/SIM/Start in response to a valid EAP-Request/SIM/Start from the server.
If and only if the server's EAP-Request/SIM/Start includes one of the identity-requesting attributes, then the peer MUST include the AT_IDENTITY attribute. The usage of AT_IDENTITY is defined in Section 4.2.
The AT_NONCE_MT attribute MUST NOT be included if the AT_IDENTITY with a fast re-authentication identity is present for fast re-authentication. AT_NONCE_MT MUST be included in all other cases (full authentication).
The AT_SELECTED_VERSION attribute MUST NOT be included if the AT_IDENTITY attribute with a fast re-authentication identity is present for fast re-authentication. In all other cases, AT_SELECTED_VERSION MUST be included (full authentication). This attribute is used in version negotiation, as specified in Section 4.1.
This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.
9.3. EAP-Request/SIM/Challenge
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The server sends the EAP-Request/SIM/Challenge after receiving a valid EAP-Response/SIM/Start that contains AT_NONCE_MT and AT_SELECTED_VERSION, and after successfully obtaining the subscriber identity.
The AT_RAND attribute MUST be included.
The AT_RESULT_IND attribute MAY be included. The usage of this attribute is discussed in Section 6.2.
The AT_MAC attribute MUST be included. For EAP-Request/SIM/Challenge, the MAC code is calculated over the following data:
EAP packet| NONCE_MT
The EAP packet is represented as specified in Section 8.1. It is followed by the 16-byte NONCE_MT value from the peer's AT_NONCE_MT attribute.
The EAP-Request/SIM/Challenge packet MAY include encrypted attributes for identity privacy and for communicating the next fast re-authentication identity. In this case, the AT_IV and AT_ENCR_DATA attributes are included (Section 10.12).
The plaintext of the AT_ENCR_DATA value field consists of nested attributes. The nested attributes MAY include AT_PADDING (as specified in Section 10.12). If the server supports identity privacy and wants to communicate a pseudonym to the peer for the next full authentication, then the nested encrypted attributes include the AT_NEXT_PSEUDONYM attribute. If the server supports re-authentication and wants to communicate a fast re-authentication identity to the peer, then the nested encrypted attributes include the AT_NEXT_REAUTH_ID attribute.
When processing this message, the peer MUST process AT_RAND before processing other attributes. Only if AT_RAND is verified to be valid, the peer derives keys and verifies AT_MAC. The operation in case an error occurs is specified in Section 6.3.1.
9.4. EAP-Response/SIM/Challenge
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The peer sends EAP-Response/SIM/Challenge in response to a valid EAP-Request/SIM/Challenge.
Sending this packet indicates that the peer has successfully authenticated the server and that the EAP exchange will be accepted by the peer's local policy. Hence, if these conditions are not met, then the peer MUST NOT send EAP-Response/SIM/Challenge, but the peer MUST send EAP-Response/SIM/Client-Error.
The AT_MAC attribute MUST be included. For EAP- Response/SIM/Challenge, the MAC code is calculated over the following data:
EAP packet| n*SRES
The EAP packet is represented as specified in Section 8.1. The EAP packet bytes are immediately followed by the two or three SRES values concatenated, denoted above with the notation n*SRES. The SRES values are used in the same order as the corresponding RAND challenges in the server's AT_RAND attribute.
The AT_RESULT_IND attribute MAY be included if it was included in EAP-Request/SIM/Challenge. The usage of this attribute is discussed in Section 6.2.
Later versions of this protocol MAY make use of the AT_ENCR_DATA and AT_IV attributes in this message to include encrypted (skippable) attributes. The EAP server MUST process EAP-Response/SIM/Challenge messages that include these attributes even if the server did not implement these optional attributes.
9.5. EAP-Request/SIM/Re-authentication
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The server sends the EAP-Request/SIM/Re-authentication message if it wants to use fast re-authentication, and if it has received a valid fast re-authentication identity in EAP-Response/Identity or EAP-Response/SIM/Start.
AT_MAC MUST be included. No message-specific data is included in the MAC calculation. See Section 10.14.
The AT_RESULT_IND attribute MAY be included. The usage of this attribute is discussed in Section 6.2.
The AT_IV and AT_ENCR_DATA attributes MUST be included. The plaintext consists of the following nested encrypted attributes, which MUST be included: AT_COUNTER and AT_NONCE_S. In addition, the nested encrypted attributes MAY include the following attributes: AT_NEXT_REAUTH_ID and AT_PADDING.
9.6. EAP-Response/SIM/Re-authentication
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The client sends the EAP-Response/SIM/Re-authentication packet in response to a valid EAP-Request/SIM/Re-authentication.
The AT_MAC attribute MUST be included. For EAP-Response/SIM/Re-authentication, the MAC code is calculated over the following data:
EAP packet| NONCE_S
The EAP packet is represented as specified in Section 8.1. It is followed by the 16-byte NONCE_S value from the server's AT_NONCE_S attribute.
The AT_IV and AT_ENCR_DATA attributes MUST be included. The nested encrypted attributes MUST include the AT_COUNTER attribute. The AT_COUNTER_TOO_SMALL attribute MAY be included in the nested encrypted attributes, and it is included in cases specified in Section 5. The AT_PADDING attribute MAY be included.
The AT_RESULT_IND attribute MAY be included if it was included in EAP-Request/SIM/Re-authentication. The usage of this attribute is discussed in Section 6.2.
Sending this packet without AT_COUNTER_TOO_SMALL indicates that the peer has successfully authenticated the server and that the EAP exchange will be accepted by the peer's local policy. Hence, if these conditions are not met, then the peer MUST NOT send EAP-Response/SIM/Re-authentication, but the peer MUST send EAP-Response/SIM/Client-Error.
9.7. EAP-Response/SIM/Client-Error
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The peer sends EAP-Response/SIM/Client-Error in error cases, as specified in Section 6.3.1.
The AT_CLIENT_ERROR_CODE attribute MUST be included.
The AT_MAC, AT_IV, or AT_ENCR_DATA attributes MUST NOT be used with this packet.
9.8. EAP-Request/SIM/Notification
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The usage of this message is specified in Section 6. The AT_NOTIFICATION attribute MUST be included.
The AT_MAC attribute MUST be included if the P bit of the notification code in AT_NOTIFICATION is set to zero, and MUST NOT be included in cases when the P bit is set to one. The P bit is discussed in Section 6.
No message-specific data is included in the MAC calculation. See Section 10.14.
If EAP-Request/SIM/Notification is used on a fast re-authentication exchange, and if the P bit in AT_NOTIFICATION is set to zero, then AT_COUNTER is used for replay protection. In this case, the AT_ENCR_DATA and AT_IV attributes MUST be included, and the encapsulated plaintext attributes MUST include the AT_COUNTER attribute. The counter value included in AT_COUNTER MUST be the same as in the EAP-Request/SIM/Re-authentication packet on the same fast re-authentication exchange.
9.9. EAP-Response/SIM/Notification
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The usage of this message is specified in Section 6. This packet is an acknowledgement of EAP-Request/SIM/Notification.
The AT_MAC attribute MUST be included in cases when the P bit of the notification code in AT_NOTIFICATION of EAP-Request/SIM/Notification is set to zero, and MUST NOT be included in cases when the P bit is set to one. The P bit is discussed in Section 6.
No message-specific data is included in the MAC calculation, see Section 10.14.
If EAP-Request/SIM/Notification is used on a fast re-authentication exchange, and if the P bit in AT_NOTIFICATION is set to zero, then AT_COUNTER is used for replay protection. In this case, the AT_ENCR_DATA and AT_IV attributes MUST be included, and the encapsulated plaintext attributes MUST include the AT_COUNTER attribute. The counter value included in AT_COUNTER MUST be the same as in the EAP-Request/SIM/Re-authentication packet on the same fast re-authentication exchange.
10. Attributes
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This section specifies the format of message attributes. The attribute type numbers are specified in the IANA considerations section of the EAP-AKA specification [EAP-AKA].
10.1. Table of Attributes
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The following table provides a guide to which attributes may be found in which kinds of messages, and in what quantity. Messages are denoted with numbers in parentheses as follows: (1) EAP-Request/SIM/Start, (2) EAP-Response/SIM/Start, (3) EAP-Request/SIM/Challenge, (4) EAP-Response/SIM/Challenge, (5) EAP-Request/SIM/Notification, (6) EAP-Response/SIM/Notification, (7) EAP-Response/SIM/Client-Error, (8) EAP-Request/SIM/Re-authentication, and (9) EAP-Response/SIM/Re-authentication. The column denoted with "Encr" indicates whether the attribute is a nested attribute that MUST be included within AT_ENCR_DATA, and the column denoted with "Skip" indicates whether the attribute is a skippable attribute.
"0" indicates that the attribute MUST NOT be included in the message, "1" indicates that the attribute MUST be included in the message, "0-1" indicates that the attribute is sometimes included in the message, and "0*" indicates that the attribute is not included in the message in cases specified in this document, but MAY be included in future versions of the protocol.
Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) Encr Skip AT_VERSION_LIST 1 0 0 0 0 0 0 0 0 N N AT_SELECTED_VERSION 0 0-1 0 0 0 0 0 0 0 N N AT_NONCE_MT 0 0-1 0 0 0 0 0 0 0 N N AT_PERMANENT_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N AT_ANY_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N AT_FULLAUTH_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N AT_IDENTITY 0 0-1 0 0 0 0 0 0 0 N N AT_RAND 0 0 1 0 0 0 0 0 0 N N AT_NEXT_PSEUDONYM 0 0 0-1 0 0 0 0 0 0 Y Y AT_NEXT_REAUTH_ID 0 0 0-1 0 0 0 0 0-1 0 Y Y AT_IV 0 0 0-1 0* 0-1 0-1 0 1 1 N Y AT_ENCR_DATA 0 0 0-1 0* 0-1 0-1 0 1 1 N Y AT_PADDING 0 0 0-1 0* 0-1 0-1 0 0-1 0-1 Y N AT_RESULT_IND 0 0 0-1 0-1 0 0 0 0-1 0-1 N Y AT_MAC 0 0 1 1 0-1 0-1 0 1 1 N N AT_COUNTER 0 0 0 0 0-1 0-1 0 1 1 Y N AT_COUNTER_TOO_SMALL 0 0 0 0 0 0 0 0 0-1 Y N AT_NONCE_S 0 0 0 0 0 0 0 1 0 Y N AT_NOTIFICATION 0 0 0 0 1 0 0 0 0 N N AT_CLIENT_ERROR_CODE 0 0 0 0 0 0 1 0 0 N NIt should be noted that attributes AT_PERMANENT_ID_REQ, AT_ANY_ID_REQ, and AT_FULLAUTH_ID_REQ are mutually exclusive; only one of them can be included at the same time. If one of the attributes AT_IV and AT_ENCR_DATA is included, then both of the attributes MUST be included.
10.2. AT_VERSION_LIST
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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, with the most preferred versions listed first. At least one version number MUST be included. The version number for the protocol described in this document is one (0001 hexadecimal).
The value field of this attribute begins with 2-byte Actual Version List Length, which specifies the length of the Version List in bytes, not including the Actual Version List Length attribute length. This field is followed by the list of the versions supported by the server, which each have a length of 2 bytes. For example, if there is only one supported version, then the Actual Version List Length is 2. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the value field with zero bytes when necessary.
10.3. AT_SELECTED_VERSION
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The format of the AT_SELECTED_VERSION attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_SELECTED...| Length = 1 | Selected Version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+This attribute is used in version negotiation, as specified in Section 4.1. The value field of this attribute contains a two-byte version number, which indicates the EAP-SIM version that the peer wants to use.
10.4. AT_NONCE_MT
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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 freshly generated by the peer (16 bytes long) for this EAP-SIM authentication exchange. The random number is used as a seed value for the new keying material. The reserved bytes are set to zero upon sending and ignored upon reception.
The peer MUST NOT re-use the NONCE_MT value from a previous EAP-SIM authentication exchange. If an EAP-SIM exchange includes several EAP/SIM/Start rounds, then the peer SHOULD use the same NONCE_MT value in all EAP-Response/SIM/Start packets. The peer SHOULD use a good source of randomness to generate NONCE_MT. Please see [RFC4086] for more information about generating random numbers for security applications.
10.5. AT_PERMANENT_ID_REQ
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The format of the AT_PERMANENT_ID_REQ attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_PERM..._REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The use of the AT_PERMANENT_ID_REQ is defined in Section 4.2. The value field contains only two reserved bytes, which are set to zero on sending and ignored on reception.
10.6. AT_ANY_ID_REQ
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The format of the AT_ANY_ID_REQ attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_ANY_ID_REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The use of the AT_ANY_ID_REQ is defined in Section 4.2. The value field contains only two reserved bytes, which are set to zero on sending and ignored on reception.
10.7. AT_FULLAUTH_ID_REQ
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The format of the AT_FULLAUTH_ID_REQ attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_FULLAUTH_...| Length = 1 | Reserved | +---------------+---------------+-------------------------------+The use of the AT_FULLAUTH_ID_REQ is defined in Section 4.2. The value field contains only two reserved bytes, which are set to zero on sending and ignored on reception.
10.8. AT_IDENTITY
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The format of the AT_IDENTITY attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IDENTITY | Length | Actual Identity Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Identity (optional) . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The use of the AT_IDENTITY is defined in Section 4.2. The value field of this attribute begins with a 2-byte actual identity length, which specifies the length of the identity in bytes. This field is followed by the subscriber identity of the indicated actual length. The identity is the permanent identity, a pseudonym identity, or a fast re-authentication identity. The identity format is specified in Section 4.2.1. The same identity format is used in the AT_IDENTITY attribute and the EAP-Response/Identity packet, with the exception that the peer MUST NOT decorate the identity it includes in AT_IDENTITY. The identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the identity with zero bytes when necessary.
10.9. AT_RAND
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The format of the AT_RAND attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_RAND | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . n*RAND . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of this attribute contains two reserved bytes followed by n GSM RANDs, each 16 bytes long. The value of n can be determined by the attribute length. The reserved bytes are set to zero upon sending and ignored upon reception.
The number of RAND challenges (n) MUST be two or three. The peer MUST verify that the number of RAND challenges is sufficient according to the peer's policy. The server MUST use different RAND values. In other words, a RAND value can only be included once in AT_RAND. When processing the AT_RAND attribute, the peer MUST check that the RANDs are different.
The EAP server MUST obtain fresh RANDs for each EAP-SIM full authentication exchange. More specifically, the server MUST consider RANDs it included in AT_RAND to be consumed if the server receives an EAP-Response/SIM/Challenge packet with a valid AT_MAC, or an EAP-Response/SIM/Client-Error with the code "insufficient number of challenges" or "RANDs are not fresh". However, in other cases (if the server does not receive a response to its EAP-Request/SIM/Challenge packet, or if the server receives a response other than the cases listed above), the server does not need to consider the RANDs to be consumed, and the server MAY re-use the RANDs in the AT_RAND attribute of the next full authentication attempt.
10.10. AT_NEXT_PSEUDONYM
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The format of the AT_NEXT_PSEUDONYM attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_PSEU..| Length | Actual Pseudonym Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Pseudonym . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of this attribute begins with the 2-byte actual pseudonym length, which specifies the length of the following pseudonym in bytes. This field is followed by a pseudonym username that the peer can use in the next authentication. The username MUST NOT include any realm portion. The username does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the pseudonym with zero bytes when necessary. The username encoding MUST follow the UTF-8 transformation format [RFC3629]. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute.
10.11. AT_NEXT_REAUTH_ID
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The format of the AT_NEXT_REAUTH_ID attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_REAU..| Length | Actual Re-Auth Identity Length| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Fast Re-authentication Username . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of this attribute begins with the 2-byte actual re-authentication identity length which specifies the length of the following fast re-authentication identity in bytes. This field is followed by a fast re-authentication identity that the peer can use in the next fast re-authentication, as described in Section 5. In environments where a realm portion is required, the fast re-authentication identity includes both a username portion and a realm name portion. The fast re-authentication identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the fast re-authentication identity with zero bytes when necessary. The identity encoding MUST follow the UTF-8 transformation format [RFC3629]. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute.
10.12. AT_IV, AT_ENCR_DATA, and AT_PADDING
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AT_IV and AT_ENCR_DATA attributes can be used to transmit encrypted information between the EAP-SIM peer and server.
The value field of AT_IV contains two reserved bytes followed by a 16-byte initialization vector required by the AT_ENCR_DATA attribute. The reserved bytes are set to zero when sending and ignored on reception. The AT_IV attribute MUST be included if and only if the AT_ENCR_DATA is included. Section 6.3 specifies the operation if a packet that does not meet this condition is encountered.
The sender of the AT_IV attribute chooses the initialization vector at random. The sender MUST NOT re-use the initialization vector value from previous EAP-SIM packets. The sender SHOULD use a good source of randomness to generate the initialization vector. Please see [RFC4086] for more information about generating random numbers for security applications. The format of AT_IV is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IV | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Initialization Vector | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of the AT_ENCR_DATA attribute consists of two reserved bytes followed by cipher text bytes encrypted using the Advanced Encryption Standard (AES) [AES] with a 128-bit key in the Cipher Block Chaining (CBC) mode of operation using the initialization vector from the AT_IV attribute. The reserved bytes are set to zero when sending and ignored on reception. Please see [CBC] for a description of the CBC mode. The format of the AT_ENCR_DATA attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_ENCR_DATA | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Encrypted Data . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The derivation of the encryption key (K_encr) is specified in Section 7.
The plaintext consists of nested EAP-SIM attributes.
The encryption algorithm requires the length of the plaintext to be a multiple of 16 bytes. The sender may need to include the AT_PADDING attribute as the last attribute within AT_ENCR_DATA. The AT_PADDING attribute is not included if the total length of other nested attributes within the AT_ENCR_DATA attribute is a multiple of 16 bytes. As usual, the Length of the Padding attribute includes the Attribute Type and Attribute Length fields. The length of the Padding attribute is 4, 8, or 12 bytes. It is chosen so that the length of the value field of the AT_ENCR_DATA attribute becomes a multiple of 16 bytes. The actual pad bytes in the value field are set to zero (00 hexadecimal) on sending. The recipient of the message MUST verify that the pad bytes are set to zero. If this verification fails on the peer, then it MUST send the EAP-Response/SIM/Client-Error packet with the error code "unable to process packet" to terminate the authentication exchange. If this verification fails on the server, then the server sends the peer the EAP-Request/SIM/Notification packet with an AT_NOTIFICATION code that implies failure to terminate the authentication exchange. The format of the AT_PADDING attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_PADDING | Length | Padding... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
10.13. AT_RESULT_IND
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The format of the AT_RESULT_IND attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_RESULT_...| Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of this attribute consists of two reserved bytes, which are set to zero upon sending and ignored upon reception. This attribute is always sent unencrypted, so it MUST NOT be encapsulated within the AT_ENCR_DATA attribute.
10.14. AT_MAC
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The AT_MAC attribute is used for EAP-SIM message authentication. Section 8 specifies in which messages AT_MAC MUST be included.
The value field of the AT_MAC attribute contains two reserved bytes followed by a keyed message authentication code (MAC). The MAC is calculated over the whole EAP packet and concatenated with optional message-specific data, with the exception that the value field of the MAC attribute is set to zero when calculating the MAC. The EAP packet includes the EAP header that begins with the Code field, the EAP-SIM header that begins with the Subtype field, and all the attributes, as specified in Section 8.1. The reserved bytes in AT_MAC are set to zero when sending and ignored on reception. The contents of the message-specific data that may be included in the MAC calculation are specified separately for each EAP-SIM message in Section 9.
The format of the AT_MAC attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_MAC | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | MAC | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The MAC algorithm is an HMAC-SHA1-128 [RFC2104] keyed hash value. (The HMAC-SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by truncating the output to the first 16 bytes. Hence, the length of the MAC is 16 bytes. The derivation of the authentication key (K_aut) used in the calculation of the MAC is specified in Section 7.
When the AT_MAC attribute is included in an EAP-SIM message, the recipient MUST process the AT_MAC attribute before looking at any other attributes, except when processing EAP-Request/SIM/Challenge. The processing of EAP-Request/SIM/Challenge is specified in Section 9.3. If the message authentication code is invalid, then the recipient MUST ignore all other attributes in the message and operate as specified in Section 6.3.
10.15. AT_COUNTER
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The format of the AT_COUNTER attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER | Length = 1 | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of the AT_COUNTER attribute consists of a 16-bit unsigned integer counter value, represented in network byte order. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute.
10.16. AT_COUNTER_TOO_SMALL
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The format of the AT_COUNTER_TOO_SMALL attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER...| Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of this attribute consists of two reserved bytes, which are set to zero upon sending and ignored upon reception. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute.
10.17. AT_NONCE_S
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The format of the AT_NONCE_S attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NONCE_S | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | NONCE_S | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of the AT_NONCE_S attribute contains two reserved bytes followed by a random number freshly generated by the server (16 bytes) for this EAP-SIM fast re-authentication. The random number is used as a challenge for the peer and also as a seed value for the new keying material. The reserved bytes are set to zero upon sending and ignored upon reception. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute.
The server MUST NOT re-use the NONCE_S value from any previous EAP-SIM fast re-authentication exchange. The server SHOULD use a good source of randomness to generate NONCE_S. Please see [RFC4086] for more information about generating random numbers for security applications.
10.18. AT_NOTIFICATION
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The format of the AT_NOTIFICATION attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_NOTIFICATION| Length = 1 |S|P| Notification Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of this attribute contains a two-byte notification code. The first and second bit (S and P) of the notification code are interpreted as described in Section 6.
The notification code values listed below have been reserved. The descriptions below illustrate the semantics of the notifications.
The peer implementation MAY use different wordings when presenting the notifications to the user. The "requested service" depends on the environment where EAP-SIM is applied.
0 - General failure after authentication. (Implies failure, used after successful authentication.)
16384 - General failure. (Implies failure, used before authentication.)
32768 - Success. User has been successfully authenticated. (Does not imply failure, used after successful authentication). The usage of this code is discussed in Section 6.2.
1026 - User has been temporarily denied access to the requested service. (Implies failure, used after successful authentication.)
1031 - User has not subscribed to the requested service. (Implies failure, used after successful authentication.)
10.19. AT_CLIENT_ERROR_CODE
-
The format of the AT_CLIENT_ERROR_CODE attribute is shown below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_CLIENT_ERR..| Length = 1 | Client Error Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The value field of this attribute contains a two-byte client error code. The following error code values have been reserved.
0 "unable to process packet": a general error code 1 "unsupported version": the peer does not support any of the versions listed in AT_VERSION_LIST 2 "insufficient number of challenges": the peer's policy requires more triplets than the server included in AT_RAND 3 "RANDs are not fresh": the peer believes that the RAND challenges included in AT_RAND were not fresh
11. IANA Considerations
-
IANA has assigned the EAP type number 18 for this protocol.
EAP-SIM shares most of the protocol design, such as attributes and message Subtypes, with EAP-AKA [EAP-AKA]. EAP-SIM protocol numbers should be administered in the same IANA registry as EAP-AKA. The initial values are listed in [EAP-AKA] for both protocols, so this document does not require any new registries or parameter allocation. As a common registry is used for EAP-SIM and EAP-AKA, the protocol number allocation policy for both protocols is specified in [EAP-AKA].
12. Security Considerations
-
The EAP specification [RFC3748] describes the security vulnerabilities of EAP, which does not include its own security mechanisms. This section discusses the claimed security properties of EAP-SIM, as well as vulnerabilities and security recommendations.
12.1. A3 and A8 Algorithms
-
The GSM A3 and A8 algorithms are used in EAP-SIM. [GSM-03.20] specifies the general GSM authentication procedure and the external interface (inputs and outputs) of the A3 and A8 algorithms. The operation of these functions falls completely within the domain of an individual operator, and therefore, the functions are specified by each operator rather than being fully standardised. The GSM-MILENAGE algorithm, specified publicly in [3GPP-TS-55.205], is an example algorithm set for A3 and A8 algorithms.
The security of the A3 and A8 algorithms is important to the security of EAP-SIM. Some A3/A8 algorithms have been compromised; see [GSM- Cloning] for discussion about the security of COMP-128 version 1. Note that several revised versions of the COMP-128 A3/A8 algorithm have been devised after the publication of these weaknesses and that the publicly specified GSM-MILENAGE algorithm is not vulnerable to any known attacks.
12.2. Identity Protection
-
EAP-SIM includes optional identity privacy support that protects the privacy of the subscriber identity against passive eavesdropping. This document only specifies a mechanism to deliver pseudonyms from the server to the peer as part of an EAP-SIM exchange. Hence, a peer that has not yet performed any EAP-SIM exchanges does not typically have a pseudonym available. If the peer does not have a pseudonym available, then the privacy mechanism cannot be used, but the permanent identity will have to be sent in the clear. The terminal SHOULD store the pseudonym in a non-volatile memory so that it can be maintained across reboots. An active attacker that impersonates the network may use the AT_PERMANENT_ID_REQ attribute to attempt to learn the subscriber's permanent identity. However, as discussed in Section 4.2.2, the terminal can refuse to send the cleartext permanent identity if it believes that the network should be able to recognize the pseudonym.
If the peer and server cannot guarantee that the pseudonym will be maintained reliably, and identity privacy is required, then additional protection from an external security mechanism (such as Protected Extensible Authentication Protocol (PEAP) [PEAP]) may be used. If an external security mechanism is in use, the identity privacy features of EAP-SIM may not be useful. The security considerations of using an external security mechanism with EAP-SIM are beyond the scope of this document.
12.3. Mutual Authentication and Triplet Exposure
-
EAP-SIM provides mutual authentication. The peer believes that the network is authentic because the network can calculate a correct AT_MAC value in the EAP-Request/SIM/Challenge packet. To calculate AT_MAC it is sufficient to know the RAND and Kc values from the GSM triplets (RAND, SRES, Kc) used in the authentication. Because the network selects the RAND challenges and the triplets, an attacker that knows n (2 or 3) GSM triplets for the subscriber is able to impersonate a valid network to the peer. (Some peers MAY employ an implementation-specific counter-measure against impersonating a valid network by re-using a previously used RAND; see below.) In other words, the security of EAP-SIM is based on the secrecy of Kc keys, which are considered secret intermediate results in the EAP-SIM cryptographic calculations.
Given physical access to the SIM card, it is easy to obtain any number of GSM triplets.
Another way to obtain triplets is to mount an attack on the peer platform via a virus or other malicious piece of software. The peer SHOULD be protected against triplet querying attacks by malicious software. Care should be taken not to expose Kc keys to attackers when they are stored or handled by the peer, or transmitted between subsystems of the peer. Steps should be taken to limit the transport, storage, and handling of these values outside a protected environment within the peer. However, the virus protection of the peer and the security capabilities of the peer's operating system are outside the scope of this document.
The EAP-SIM server typically obtains the triplets from the Home Location Register (HLR). An attacker might try to obtain triplets by attacking against the network used between the EAP-SIM server and the HLR. Care should be taken not to expose Kc keys to attackers when they are stored or handled by the EAP-SIM server, or transmitted between the EAP server and the HLR. Steps should be taken to limit the transport, storage, and handling of these values outside a protected environment. However, the protection of the communications between the EAP-SIM server and the HLR is outside the scope of this document.
If the same SIM credentials are also used for GSM traffic, the triplets could be revealed in the GSM network; see Section 12.8.
In GSM, the network is allowed to re-use the RAND challenge in consecutive authentication exchanges. This is not allowed in EAP-SIM. The EAP-SIM server is mandated to use fresh triplets (RAND challenges) in consecutive authentication exchanges, as specified in Section 3. EAP-SIM does not mandate any means for the peer to check if the RANDs are fresh, so the security of the scheme leans on the secrecy of the triplets. However, the peer MAY employ implementation-specific mechanisms to remember some of the previously used RANDs, and the peer MAY check the freshness of the server's RANDs. The operation in cases when the peer detects that the RANDs are not fresh is specified in Section 6.3.1.
Preventing the re-use of authentication vectors has been taken into account in the design of the UMTS Authentication and Key Agreement (AKA), which is used in EAP-AKA [EAP-AKA]. In cases when the triplet re-use properties of EAP-SIM are not considered sufficient, it is advised to use EAP-AKA.
Note that EAP-SIM mutual authentication is done with the EAP server. In general, EAP methods do not authenticate the identity or services provided by the EAP authenticator (if distinct from the EAP server) unless they provide the so-called channel bindings property. The vulnerabilities related to this have been discussed in [RFC3748], [EAP-Keying], [Service-Identity].
EAP-SIM does not provide the channel bindings property, so it only authenticates the EAP server. However, ongoing work such as [Service-Identity] may provide such support as an extension to popular EAP methods such as EAP-TLS, EAP-SIM, or EAP-AKA.
12.4. Flooding the Authentication Centre
-
The EAP-SIM server typically obtains authentication vectors from the Authentication Centre (AuC). EAP-SIM introduces a new usage for the AuC. The protocols between the EAP-SIM server and the AuC are out of the scope of this document. However, it should be noted that a malicious EAP-SIM peer may generate a lot of protocol requests to mount a denial of service attack. The EAP-SIM server implementation SHOULD take this into account and SHOULD take steps to limit the traffic that it generates towards the AuC, preventing the attacker from flooding the AuC and from extending the denial of service attack from EAP-SIM to other users of the AuC.
12.5. Key Derivation
-
EAP-SIM supports key derivation. The key hierarchy is specified in Section 7. EAP-SIM combines several GSM triplets in order to generate stronger keying material and stronger AT_MAC values. The actual strength of the resulting keys depends, among other things, on operator-specific parameters including authentication algorithms, the strength of the Ki key, and the quality of the RAND challenges. For example, some SIM cards generate Kc keys with 10 bits set to zero. Such restrictions may prevent the concatenation technique from yielding strong session keys. Because the strength of the Ki key is 128 bits, the ultimate strength of any derived secret key material is never more than 128 bits.
It should also be noted that a security policy that allows n=2 to be used may compromise the security of a future policy that requires three triplets, because adversaries may be able to exploit the messages exchanged when the weaker policy is applied.
There is no known way to obtain complete GSM triplets by mounting an attack against EAP-SIM. A passive eavesdropper can learn n*RAND and AT_MAC and may be able to link this information to the subscriber identity. An active attacker that impersonates a GSM subscriber can easily obtain n*RAND and AT_MAC values from the EAP server for any given subscriber identity. However, calculating the Kc and SRES values from AT_MAC would require the attacker to reverse the keyed message authentication code function HMAC-SHA1-128.
As EAP-SIM does not expose any values calculated from an individual GSM Kc keys, it is not possible to mount a brute force attack on only one of the Kc keys in EAP-SIM. Therefore, when considering brute force attacks on the values exposed in EAP-SIM, the effective length of EAP-SIM session keys is not compromised by the fact that they are combined from several shorter keys, i.e., the effective length of 128 bits may be achieved. For additional considerations, see Section 12.8.
12.6. Cryptographic Separation of Keys and Session Independence
-
The EAP Transient Keys used to protect EAP-SIM packets (K_encr, K_aut), the Master Session Key, and the Extended Master Session Key are cryptographically separate in EAP-SIM. An attacker cannot derive any non-trivial information about any of these keys based on the other keys. An attacker also cannot calculate the pre-shared secret (Ki) from the GSM Kc keys, from EAP-SIM K_encr, from EAP-SIM K_aut, from the Master Session Key, or from the Extended Master Session Key.
Each EAP-SIM exchange generates fresh keying material, and the keying material exported from the method upon separate EAP-SIM exchanges is cryptographically separate. The EAP-SIM peer contributes to the keying material with the NONCE_MT parameter, which must be chosen freshly for each full authentication exchange. The EAP server is mandated to choose the RAND challenges freshly for each full authentication exchange. If either the server or the peer chooses its random value (NONCE_MT or RAND challenges) freshly, even if the other entity re-used its value from a previous exchange, then the EAP Transient Keys, the Master Session Key, and the Extended Master Session Key will be different and cryptographically separate from the corresponding values derived upon the previous full authentication exchange.
On fast re-authentication, freshness of the Master Session Key and the Extended Master Session Key is provided with a counter (AT_COUNTER). The same EAP Transient Keys (K_encr, K_aut) that were used in the full authentication exchange are used to protect the EAP negotiation. However, replay and integrity protection across all the fast re-authentication exchanges that use the same EAP Transient Keys is provided with AT_COUNTER.
[RFC3748] defines session independence as the "demonstration that passive attacks (such as capture of the EAP conversation) or active attacks (including compromise of the MSK or EMSK) do not enable compromise of subsequent or prior MSKs or EMSKs". Because the MSKs and EMSKs are separate between EAP exchanges, EAP-SIM supports this security claim.
It should be noted that [Patel-2003], which predates [RFC3748], uses a slightly different meaning for session independence. The EAP-SIM protocol does not allow the peer to ensure that different Kc key values would be used in different exchanges. Only the server is able to ensure that fresh RANDs, and therefore, fresh Kc keys are used.
Hence, the peer cannot guarantee EAP-SIM sessions to be independent with regard to the internal Kc values. However, in EAP-SIM, the Kc keys are considered to be secret intermediate results, which are not exported outside the method. See Section 12.3 for more information about RAND re-use.
12.7. Dictionary Attacks
-
Because EAP-SIM is not a password protocol, it is not vulnerable to dictionary attacks. (The pre-shared symmetric secret stored on the SIM card is not a passphrase, nor is it derived from a passphrase.)
12.8. Credentials Re-use
-
EAP-SIM cannot prevent attacks over the GSM or GPRS radio networks. If the same SIM credentials are also used in GSM or GPRS, it is possible to mount attacks over the cellular interface.
A passive attacker can eavesdrop GSM or GPRS traffic and obtain RAND, SRES pairs. He can then use a brute force attack or other cryptanalysis techniques to obtain the 64-bit Kc keys used to encrypt the GSM or GPRS data. This makes it possible to attack each 64-bit key separately.
An active attacker can mount a "rogue GSM/GPRS base station attack", replaying previously seen RAND challenges to obtain SRES values. He can then use a brute force attack to obtain the Kc keys. If successful, the attacker can impersonate a valid network or decrypt previously seen traffic, because EAP-SIM does not provide perfect forward secrecy (PFS).
Due to several weaknesses in the GSM encryption algorithms, the effective key strength of the Kc keys is much less than the expected 64 bits (no more than 40 bits if the A5/1 GSM encryption algorithm is used; as documented in [Barkan-2003], an active attacker can force the peer to use the weaker A5/2 algorithm that can be broken in less than a second).
Because the A5 encryption algorithm is not used in EAP-SIM, and because EAP-SIM does not expose any values calculated from individual Kc keys, it should be noted that these attacks are not possible if the SIM credentials used in EAP-SIM are not shared in GSM/GPRS.
At the time this document was written, the 3rd Generation Partnership Project (3GPP) has started to work on fixes to these A5 vulnerabilities. One of the solution proposals discussed in 3GPP is integrity-protected A5 version negotiation, which would require the base station to prove knowledge of the Kc key before the terminal sends any values calculated from the Kc to the network. Another proposal is so-called special RANDs, where some bits of the RAND challenge would be used for cryptographic separation by indicating the allowed use of the triplet, such as the allowed A5 algorithm in GSM or the fact that the triplet is intended for EAP-SIM. This is currently a work in progress, and the mechanisms have not been selected yet.
12.9. Integrity and Replay Protection, and Confidentiality
-
AT_MAC, AT_IV, AT_ENCR_DATA, and AT_COUNTER attributes are used to provide integrity, replay and confidentiality protection for EAP-SIM requests and responses. Integrity protection with AT_MAC includes the EAP header. These attributes cannot be used during the EAP/SIM/Start roundtrip. However, the protocol values (user identity string, NONCE_MT, and version negotiation parameters) are (implicitly) protected by later EAP-SIM messages by including them in key derivation.
Integrity protection (AT_MAC) is based on a keyed message authentication code. Confidentiality (AT_ENCR_DATA and AT_IV) is based on a block cipher.
Confidentiality protection is applied only to a part of the protocol fields. The table of attributes in Section 10.1 summarizes which fields are confidentiality-protected. It should be noted that the error and notification code attributes AT_CLIENT_ERROR_CODE and AT_NOTIFICATION are not confidential, but they are transmitted in the clear. Identity protection is discussed in Section 12.2.
On full authentication, replay protection of the EAP exchange is provided by the RAND values from the underlying GSM authentication scheme and the use of the NONCE_MT value. Protection against replays of EAP-SIM messages is also based on the fact that messages that can include AT_MAC can only be sent once with a certain EAP-SIM Subtype, and on the fact that a different K_aut key will be used for calculating AT_MAC in each full authentication exchange.
On fast re-authentication, a counter included in AT_COUNTER and a server random nonce is used to provide replay protection. The AT_COUNTER attribute is also included in EAP-SIM notifications if it is used after successful authentication in order to provide replay protection between re-authentication exchanges.
Because EAP-SIM is not a tunneling method, EAP-Request/Notification, EAP-Response/Notification, EAP-Success, or EAP-Failure packets are not confidential, integrity-protected, or replay-protected in EAP-SIM. On physically insecure networks, this may enable an
attacker to send false notifications to the peer and to mount denial of service attacks by spoofing these packets. As discussed in Section 6.3, the peer will only accept EAP-Success after the peer successfully authenticates the server. Hence, the attacker cannot force the peer to believe successful mutual authentication has occurred until the peer successfully authenticates the server or after the peer fails to authenticate the server.
The security considerations of EAP-SIM result indications are covered in Section 12.11
An eavesdropper will see the EAP-Request/Notification, EAP-Response/Notification, EAP-Success, and EAP-Failure packets sent in the clear. With EAP-SIM, confidential information MUST NOT be transmitted in EAP Notification packets.
12.10. Negotiation Attacks
-
EAP-SIM does not protect the EAP-Response/Nak packet. Because EAP-SIM does not protect the EAP method negotiation, EAP method downgrading attacks may be possible, especially if the user uses the same identity with EAP-SIM and other EAP methods.
EAP-SIM includes a version negotiation procedure. In EAP-SIM the keying material derivation includes the version list and selected version to ensure that the protocol cannot be downgraded and that the peer and server use the same version of EAP-SIM.
EAP-SIM does not support ciphersuite negotiation.
12.11. Protected Result Indications
-
EAP-SIM supports optional protected success indications and acknowledged failure indications. If a failure occurs after successful authentication, then the EAP-SIM failure indication is integrity- and replay-protected.
Even if an EAP-Failure packet is lost when using EAP-SIM over an unreliable medium, then the EAP-SIM failure indications will help ensure that the peer and EAP server will know the other party's authentication decision. If protected success indications are used, then the loss of Success packet will also be addressed by the acknowledged, integrity- and replay-protected EAP-SIM success indication. If the optional success indications are not used, then the peer may end up believing that the server succeeded authentication, when it actually failed. Since access will not be granted in this case, protected result indications are not needed unless the client is not able to realize it does not have access for an extended period of time.
12.12. Man-in-the-Middle Attacks
-
In order to avoid man-in-the-middle attacks and session hijacking, user data SHOULD be integrity-protected on physically insecure networks. The EAP-SIM Master Session Key, or keys derived from it, MAY be used as the integrity protection keys, or, if an external security mechanism such as PEAP is used, then the link integrity protection keys MAY be derived by the external security mechanism.
There are man-in-the-middle attacks associated with the use of any EAP method within a tunneled protocol. For instance, an early version of PEAP [PEAP-02] was vulnerable to this attack. This specification does not address these attacks. If EAP-SIM is used with a tunneling protocol, there should be cryptographic binding provided between the protocol and EAP-SIM to prevent man-in-the-middle attacks through rogue authenticators being able to setup one-way authenticated tunnels. For example, newer versions of PEAP include such cryptographic binding. The EAP-SIM Master Session Key MAY be used to provide the cryptographic binding. However, the mechanism by which the binding is provided depends on the tunneling protocol and is beyond the scope of this document.
12.13. Generating Random Numbers
-
An EAP-SIM implementation SHOULD use a good source of randomness to generate the random numbers required in the protocol. Please see [RFC4086] for more information on generating random numbers for security applications.
13. Security Claims
-
This section provides the security claims required by [RFC3748].
Auth. mechanism: EAP-SIM is based on the GSM SIM mechanism, which is a challenge/response authentication and key agreement mechanism based on a symmetric 128-bit pre-shared secret. EAP-SIM also makes use of a peer challenge to provide mutual authentication.
Ciphersuite negotiation: No Mutual authentication: Yes (Section 12.3) Integrity protection: Yes (Section 12.9) Replay protection: Yes (Section 12.9)
Confidentiality: Yes, except method-specific success and failure indications (Section 12.2, Section 12.9)
Key derivation: Yes
Key strength: EAP-SIM supports key derivation with 128-bit effective key strength (Section 12.5). However, as discussed in Section 11, if the same credentials are used in GSM/GPRS and in EAP-SIM, then the key strength may be reduced considerably, basically to the same level as in GSM, by mounting attacks over GSM/GPRS. For example an active attack using a false GSM/GPRS base station reduces the effective key strength to almost zero.
Description of key hierarchy: Please see Section 7.
Dictionary attack protection: N/A (Section 12.7) Fast reconnect: Yes Cryptographic binding: N/A Session independence: Yes (Section 12.6)
Fragmentation:
No Channel binding: No
Indication of vulnerabilities: Vulnerabilities are discussed in Section 12.
14. Acknowledgements and Contributions
14.1. Contributors
-
In addition to the editors, Nora Dabbous, Jose Puthenkulam, and Prasanna Satarasinghe were significant contributors to this document.
Pasi Eronen and Jukka-Pekka Honkanen contributed Appendix A.
14.2. Acknowledgements
-
Juha Ala-Laurila, N. Asokan, Jan-Erik Ekberg, Patrik Flykt, Jukka-Pekka Honkanen, Antti Kuikka, Jukka Latva, Lassi Lehtinen, Jyri Rinnemaa, Timo Takamaki, and Raimo Vuonnala contributed many original ideas and concepts to this protocol.
N. Asokan, Pasi Eronen, and Jukka-Pekka Honkanen contributed and helped in innumerable ways during the development of the protocol.
Valtteri Niemi and Kaisa Nyberg contributed substantially to the design of the key derivation and the fast re-authentication procedure, and have also provided their cryptographic expertise in many discussions related to this protocol.
Simon Blake-Wilson provided very helpful comments on key derivation and version negotiation.
Thanks to Greg Rose for his very valuable comments to an early version of this specification [S3-020125], and for reviewing and providing very useful comments on version 12.
Thanks to Bernard Aboba, Vladimir Alperovich, Florent Bersani, Jacques Caron, Gopal Dommety, Augustin Farrugia, Mark Grayson, Max de Groot, Prakash Iyer, Nishi Kant, Victor Lortz, Jouni Malinen, Sarvar Patel, Tom Porcher, Michael Richardson, Stefan Schroeder, Uma Shankar, Jesse Walker, and Thomas Wieland for their contributions and critiques. Special thanks to Max for proposing improvements to the MAC calculation.
Thanks to Glen Zorn for reviewing this document and for providing very useful comments on the protocol.
Thanks to Sarvar Patel for his review of the protocol [Patel-2003].
Thanks to Bernard Aboba for reviewing this document for RFC 3748 compliance.
The identity privacy support is based on the identity privacy support of [EAP-SRP]. The attribute format is based on the extension format of Mobile IPv4 [RFC3344].
This protocol has been partly developed in parallel with EAP-AKA [EAP-AKA], and hence this specification incorporates many ideas from Jari Arkko.
14.2.1. Contributors' Addresses
-
Nora Dabbous
Gemplus
34 rue Guynemer
92447 Issy les Moulineaux
FrancePhone: +33 1 4648 2000 EMail: nora.dabbous@gemplus.com
Jose Puthenkulam
Intel Corporation
2111 NE 25th Avenue, JF2-58
Hillsboro, OR 97124
USAPhone: +1 503 264 6121 EMail: jose.p.puthenkulam@intel.com
Prasanna Satarasinghe
Transat Technologies
180 State Street, Suite 240
Southlake, TX 76092
USAPhone: + 1 817 4814412 EMail: prasannas@transat-tech.com
15. References
15.1. Normative References
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[GSM-03.20] European Telecommunications Standards Institute, "GSM Technical Specification GSM 03.20 (ETS 300 534): "Digital cellular telecommunication system (Phase 2); Security related network functions"", August 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [GSM-03.03] European Telecommunications Standards Institute, "GSM Technical Specification GSM 03.03 (ETS 300 523): "Digital cellular telecommunication system (Phase 2); Numbering, addressing and identification"", April 1997. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The Network Access Identifier", RFC 4282, December 2005. [AES] National Institute of Standards and Technology, "Federal Information Processing Standards (FIPS) Publication 197, "Advanced Encryption Standard (AES)"", November 2001. http://csrc.nist.gov/publications/fips/fips197/ fips-197.pdf [CBC] National Institute of Standards and Technology, "NIST Special Publication 800-38A, "Recommendation for Block Cipher Modes of Operation - Methods and Techniques"", December 2001. http://csrc.nist.gov/publications/nistpubs/ 800-38a/sp800-38a.pdf [SHA-1] National Institute of Standards and Technology, U.S. Department of Commerce, "Federal Information Processing Standard (FIPS) Publication 180-1, "Secure Hash Standard"", April 1995. [PRF] National Institute of Standards and Technology, "Federal Information Processing Standards (FIPS) Publication 186-2 (with change notice); Digital Signature Standard (DSS)", January 2000. Available on-line at: http://csrc.nist.gov/publications/ fips/fips186-2/fips186-2-change1.pdf [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [EAP-AKA] Arkko, J. and H. Haverinen, "Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA)", RFC 4187, January 2006.
15.2. Informative References
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[3GPP-TS-23.003] 3rd Generation Partnership Project, "3GPP Technical Specification 3GPP TS 23.003 V6.8.0: "3rd Generation Parnership Project; Technical Specification Group Core Network; Numbering, addressing and identification (Release 6)"", December 2005. [3GPP-TS-55.205] 3rd Generation Partnership Project, "3GPP Technical Specification 3GPP TS 55.205 V 6.0.0: "3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Specification of the GSM-MILENAGE Algorithms: An example algorithm set for the GSM Authentication and Key Generation functions A3 and A8 (Release 6)"", December 2002. [PEAP] Palekar, A., Simon, D., Zorn, G., Salowey, J., Zhou, H., and S. Josefsson, "Protected EAP Protocol (PEAP) Version 2", Work in Progress, October 2004. [PEAP-02] Anderson, H., Josefsson, S., Zorn, G., Simon, D., and A. Palekar, "Protected EAP Protocol (PEAP)", Work in Progress, February 2002. [EAP-Keying] Aboba, B., Simon, D., Arkko, J., Eronen, P., and H. Levkowetz, "Extensible Authentication Protocol (EAP) Key Management Framework", Work in Progress, October 2005. [Service-Identity] Arkko, J. and P. Eronen, "Authenticated Service Information for the Extensible Authentication Protocol (EAP)", Work in Progress, October 2004. [RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [S3-020125] Qualcomm, "Comments on draft EAP/SIM, 3rd Generation Partnership Project document 3GPP TSG SA WG3 Security S3#22, S3-020125", February 2002. [RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August 2002. [RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes ", RFC 2548, March 1999. [EAP-SRP] Carlson, J., Aboba, B., and H. Haverinen, "EAP SRP-SHA1 Authentication Protocol", Work in Progress, July 2001. [GSM-Cloning] Wagner, D., "GSM Cloning". Web page about COMP-128 version 1 vulnerabilities, available at http://www.isaac.cs.berkeley.edu/isaac/gsm.html [Barkan-2003] Barkan, E., Biham, E., and N. Keller, "Instant Ciphertext-Only Cryptanalysis of GSM Encrypted Communications". available on-line at http://cryptome.org/gsm-crack-bbk.pdf [Patel-2003] Patel, S., "Analysis of EAP-SIM Session Key Agreement". Posted to the EAP mailing list 29 May,2003. http:// mail.frascone.com/pipermail/public/eap/2003-May/ 001267.html
Appendix A. Test Vectors
-
Test vectors for the NIST FIPS 186-2 pseudo-random number generator [PRF] are available at the following URL: http://csrc.nist.gov/encryption/dss/Examples-1024bit.pdf
The following examples show the contents of EAP-SIM packets on full authentication and fast re-authentication.
A.1. EAP-Request/Identity
-
The first packet is a plain Identity Request:
01 ; Code: Request 00 ; Identifier: 0 00 05 ; Length: 5 octets 01 ; Type: Identity
A.2. EAP-Response/Identity
-
The client's identity is "1244070100000001@eapsim.foo", so it responds with the following packet:
02 ; Code: Response 00 ; Identifier: 0 00 20 ; Length: 32 octets 01 ; Type: Identity 31 32 34 34 ; "1244070100000001@eapsim.foo" 30 37 30 31 30 30 30 30 30 30 30 31 40 65 61 70 73 69 6d 2e 66 6f 6f
A.3. EAP-Request/SIM/Start
-
The server's first packet looks like this:
01 ; Code: Request 01 ; Identifier: 1 00 10 ; Length: 16 octets 12 ; Type: EAP-SIM 0a ; EAP-SIM subtype: Start 00 00 ; (reserved) 0f ; Attribute type: AT_VERSION_LIST 02 ; Attribute length: 8 octets (2*4) 00 02 ; Actual version list length: 2 octets 00 01 ; Version: 1 00 00 ; (attribute padding)
A.4. EAP-Response/SIM/Start
-
The client selects a nonce and responds with the following packet:
02 ; Code: Response 01 ; Identifier: 1 00 20 ; Length: 32 octets 12 ; Type: EAP-SIM 0a ; EAP-SIM subtype: Start 00 00 ; (reserved) 07 ; Attribute type: AT_NONCE_MT 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) 01 23 45 67 ; NONCE_MT value 89 ab cd ef fe dc ba 98 76 54 32 10 10 ; Attribute type: AT_SELECTED_VERSION 01 ; Attribute length: 4 octets (1*4) 00 01 ; Version: 1
A.5. EAP-Request/SIM/Challenge
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Next, the server selects three authentication triplets
(RAND1,SRES1,Kc1) = (10111213 14151617 18191a1b 1c1d1e1f, d1d2d3d4, a0a1a2a3 a4a5a6a7) (RAND2,SRES2,Kc2) = (20212223 24252627 28292a2b 2c2d2e2f, e1e2e3e4, b0b1b2b3 b4b5b6b7) (RAND3,SRES3,Kc3) = (30313233 34353637 38393a3b 3c3d3e3f, f1f2f3f4, c0c1c2c3 c4c5c6c7)Next, the MK is calculated as specified in Section 7*.
MK = e576d5ca 332e9930 018bf1ba ee2763c7 95b3c712
And the other keys are derived using the PRNG:
K_encr = 536e5ebc 4465582a a6a8ec99 86ebb620 K_aut = 25af1942 efcbf4bc 72b39434 21f2a974 MSK = 39d45aea f4e30601 983e972b 6cfd46d1 c3637733 65690d09 cd44976b 525f47d3 a60a985e 955c53b0 90b2e4b7 3719196a 40254296 8fd14a88 8f46b9a7 886e4488 EMSK = 5949eab0 fff69d52 315c6c63 4fd14a7f 0d52023d 56f79698 fa6596ab eed4f93f bb48eb53 4d985414 ceed0d9a 8ed33c38 7c9dfdab 92ffbdf2 40fcecf6 5a2c93b9Next, the server selects a pseudonym and a fast 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 00The EAP packet looks like this:
01 ; Code: Request 02 ; Identifier: 2 01 18 ; Length: 280 octets 12 ; Type: EAP-SIM 0b ; EAP-SIM subtype: Challenge 00 00 ; (reserved) 01 ; Attribute type: AT_RAND 0d ; Attribute length: 52 octets (13*4) 00 00 ; (reserved) 10 11 12 13 ; first RAND 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 ; second RAND 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 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 6aThe 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
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The client's response looks like this:
02 ; Code: Response 02 ; Identifier: 2 00 1c ; Length: 28 octets 12 ; Type: EAP-SIM 0b ; EAP-SIM subtype: Challenge 00 00 ; (reserved) 0b ; Attribute type: AT_MAC 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) f5 6d 64 33 ; MAC value e6 8e d2 97 6a c1 19 37 fc 3d 11 54The 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
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The last packet is an EAP-Success:
03 ; Code: Success 02 ; Identifier: 2 00 04 ; Length: 4 octets
A.8. Fast Re-authentication
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When performing fast re-authentication, the EAP-Request/Identity packet is the same as usual. The EAP-Response/Identity contains the fast re-authentication identity (from AT_ENCR_DATA attribute above):
02 ; Code: Response 00 ; Identifier: 0 00 56 ; Length: 86 octets 01 ; Type: Identity 59 32 34 66 4e 53 72 7a 38 42 50 32 37 34 6a 4f 4a 61 46 31 37 57 66 78 49 38 59 4f 37 51 58 30 30 70 4d 58 6b 39 58 4d 4d 56 4f 77 37 62 72 6f 61 4e 68 54 63 7a 75 46 71 35 33 61 45 70 4f 6b 6b 33 4c 30 64 6d 40 65 61 70 73 69 6d 2e 66 6f 6f
A.9. EAP-Request/SIM/Re-authentication
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The server recognizes the reauthentication identity, so it will respond with EAP-Request/SIM/Re-authentication. It retrieves the associated counter value, generates a nonce, and picks a new reauthentication identity (in this case, "uta0M0iyIsMwWp5TTdSdnOLvg2XDVf21OYt1vnfiMcs5dnIDHOIFVavIRzMR yzW6vFzdHW@eapsim.foo").
The following plaintext will be encrypted and stored in the AT_ENCR_DATA attribute. Note that AT_PADDING is not used because the length of the plaintext is a multiple of 16 bytes.
13 ; Attribute type: AT_COUNTER 01 ; Attribute length: 4 octets (1*4) 00 01 ; Counter value 15 ; Attribute type: AT_NONCE_S 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) 01 23 45 67 ; NONCE_S value 89 ab cd ef fe dc ba 98 76 54 32 10 85 ; Attribute type: AT_NEXT_REAUTH_ID 16 ; Attribute length: 88 octets (22*4) 00 51 ; Actual re-auth identity length: 81 octets 75 74 61 30 4d 30 69 79 49 73 4d 77 57 70 35 54 54 64 53 64 6e 4f 4c 76 67 32 58 44 56 66 32 31 4f 59 74 31 76 6e 66 69 4d 63 73 35 64 6e 49 44 48 4f 49 46 56 61 76 49 52 7a 4d 52 79 7a 57 36 76 46 7a 64 48 57 40 65 61 70 73 69 6d 2e 66 6f 6f 00 00 00 ; (attribute padding)The EAP packet looks like this:
01 ; Code: Request 01 ; Identifier: 1 00 a4 ; Length: 164 octets 12 ; Type: EAP-SIM 0d ; EAP-SIM subtype: Re-authentication 00 00 ; (reserved) 81 ; Attribute type: AT_IV 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) d5 85 ac 77 ; IV value 86 b9 03 36 65 7c 77 b4 65 75 b9 c4 82 ; Attribute type: AT_ENCR_DATA 1d ; Attribute length: 116 octets (29*4) 00 00 ; (reserved) 68 62 91 a9 d2 ab c5 8c aa 32 94 b6 e8 5b 44 84 6c 44 e5 dc b2 de 8b 9e 80 d6 9d 49 85 8a 5d b8 4c dc 1c 9b c9 5c 01 b9 6b 6e ca 31 34 74 ae a6 d3 14 16 e1 9d aa 9d f7 0f 05 00 88 41 ca 80 14 96 4d 3b 30 a4 9b cf 43 e4 d3 f1 8e 86 29 5a 4a 2b 38 d9 6c 97 05 c2 bb b0 5c 4a ac e9 7d 5e af f5 64 04 6c 8b d3 0b c3 9b e5 e1 7a ce 2b 10 a6 0b ; Attribute type: AT_MAC 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) 48 3a 17 99 ; MAC value b8 3d 7c d3 d0 a1 e4 01 d9 ee 47 70The MAC is calculated over the EAP packet above (with MAC value set to zero; a total of 164 bytes).
Finally, the server derives new keys. The XKEY' is calculated as described in Section 7*:
XKEY' = 863dc120 32e08343 c1a2308d b48377f6 801f58d4
The new MSK and EMSK are derived using the PRNG (note that K_encr and K_aut stay the same).
MSK = 6263f614 973895e1 335f7e30 cff028ee 2176f519 002c9abe 732fe0ef 00cf167c 756d9e4c ed6d5ed6 40eb3fe3 8565ca07 6e7fb8a8 17cfe8d9 adbce441 d47c4f5e EMSK = 3d8ff786 3a630b2b 06e2cf20 9684c13f 6b82f992 f2b06f1b 54bf51ef 237f2a40 1ef5e0d7 e098a34c 533eaebf 34578854 b7721526 20a777f0 e0340884 a294fb73
A.10. EAP-Response/SIM/Re-authentication
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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 00The EAP packet looks like this:
02 ; Code: Response 01 ; Identifier: 1 00 44 ; Length: 68 octets 12 ; Type: EAP-SIM 0d ; EAP-SIM subtype: Re-authentication 00 00 ; (reserved) 81 ; Attribute type: AT_IV 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) cd f7 ff a6 ; IV value 5d e0 4c 02 6b 56 c8 6b 76 b1 02 ea 82 ; Attribute type: AT_ENCR_DATA 05 ; Attribute length: 20 octets (5*4) 00 00 ; (reserved) b6 ed d3 82 79 e2 a1 42 3c 1a fc 5c 45 5c 7d 56 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 17The MAC is calculated over the EAP packet above (with MAC value set to zero), followed by the NONCE_S value (a total of 84 bytes).
The next packet will be EAP-Success:
03 ; Code: Success 01 ; Identifier: 1 00 04 ; Length: 4 octets
Appendix B. Pseudo-Random Number Generator
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The "|" character denotes concatenation, and "^" denotes exponentiation.
Step 1: Choose a new, secret value for the seed-key, XKEY
Step 2: In hexadecimal notation let
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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
Authors' Addresses
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Henry Haverinen (editor)
Nokia Enterprise Solutions
P.O. Box 12
FIN-40101 Jyvaskyla
FinlandEMail: henry.haverinen@nokia.com Joseph Salowey (editor) Cisco Systems 2901 Third Avenue Seattle, WA 98121 USA Phone: +1 206 256 3380 EMail: jsalowey@cisco.com
Full Copyright Statement
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Acknowledgement
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Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA).