RFC 4186
Extensible Authentication Protocol Method for Global System for Mobile Communications (GSM) Subscriber Identity Modules (EAP-SIM)
5. Fast Re-Authentication
5.1. General
In some environments, EAP authentication may be performed frequently. Because the EAP-SIM full authentication procedure makes use of the GSM SIM A3/A8 algorithms, and therefore requires 2 or 3 fresh triplets from the Authentication Centre, the full authentication procedure is not very well suited for frequent use. Therefore, EAP-SIM includes a more inexpensive fast re-authentication procedure that does not make use of the SIM A3/A8 algorithms and does not need new triplets from the Authentication Centre. Re-authentication can be performed in fewer roundtrips than the full authentication. Fast re-authentication is optional to implement for both the EAP-SIM server and peer. On each EAP authentication, either one of the entities may also fall back on full authentication if it does not want to use fast re-authentication. Fast re-authentication is based on the keys derived on the preceding full authentication. The same K_aut and K_encr keys that were used in full authentication are used to protect EAP-SIM packets and attributes, and the original Master Key from full authentication is used to generate a fresh Master Session Key, as specified in Section 7. The fast re-authentication exchange makes use of an unsigned 16-bit counter, included in the AT_COUNTER attribute. The counter has three goals: 1) it can be used to limit the number of successive reauthentication exchanges without full authentication 2) it contributes to the keying material, and 3) it protects the peer and the server from replays. On full authentication, both the server and the peer initialize the counter to one. The counter value of at least one is used on the first fast re-authentication. On subsequent fast re-authentications, the counter MUST be greater than on any of the previous re-authentications. For example, on the second fast re-authentication, the counter value is two or greater. The AT_COUNTER attribute is encrypted. Both the peer and the EAP server maintain a copy of the counter. The EAP server sends its counter value to the peer in the fast re-authentication request. The peer MUST verify that its counter value is less than or equal to the value sent by the EAP server.
The server includes an encrypted server random nonce (AT_NONCE_S) in the fast re-authentication request. The AT_MAC attribute in the peer's response is calculated over NONCE_S to provide a challenge/response authentication scheme. The NONCE_S also contributes to the new Master Session Key. Both the peer and the server SHOULD have an upper limit for the number of subsequent fast re-authentications allowed before a full authentication needs to be performed. Because a 16-bit counter is used in fast re-authentication, the theoretical maximum number of re-authentications is reached when the counter value reaches FFFF hexadecimal. In order to use fast re-authentication, the peer and the EAP server need to store the following values: Master Key, latest counter value and the next fast re-authentication identity. K_aut, K_encr may either be stored or derived again from MK. The server may also need to store the permanent identity of the user.5.2. Comparison to UMTS AKA
When analyzing the fast re-authentication exchange, it may be helpful to compare it with the UMTS Authentication and Key Agreement (AKA) exchange, which it resembles closely. The counter corresponds to the UMTS AKA sequence number, NONCE_S corresponds to RAND, AT_MAC in EAP-Request/SIM/Re-authentication corresponds to AUTN, the AT_MAC in EAP-Response/SIM/Re-authentication corresponds to RES, AT_COUNTER_TOO_SMALL corresponds to AUTS, and encrypting the counter corresponds to the usage of the Anonymity Key. Also, the key generation on fast re-authentication, with regard to random or fresh material, is similar to UMTS AKA -- the server generates the NONCE_S and counter values, and the peer only verifies that the counter value is fresh. It should also be noted that encrypting the AT_NONCE_S, AT_COUNTER, or AT_COUNTER_TOO_SMALL attributes is not important to the security of the fast re-authentication exchange.5.3. Fast Re-authentication Identity
The fast re-authentication procedure makes use of separate re-authentication user identities. Pseudonyms and the permanent identity are reserved for full authentication only. If a re-authentication identity is lost and the network does not recognize it, the EAP server can fall back on full authentication.
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
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
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
In the figure above, the first three messages are similar to the basic fast re-authentication case. When the peer detects that the counter value is not fresh, it includes the AT_COUNTER_TOO_SMALL attribute in EAP-Response/SIM/Re-authentication. This attribute doesn't contain any data, but it is a request for the server to initiate full authentication. In this case, the peer MUST ignore the contents of the server's AT_NEXT_REAUTH_ID attribute. On receipt of AT_COUNTER_TOO_SMALL, the server verifies AT_MAC and verifies that AT_COUNTER contains the same counter value as in the EAP-Request/SIM/Re-authentication packet. If not, the server terminates the authentication exchange by sending the EAP-Request/SIM/Notification with AT_NOTIFICATION code "General failure" (16384). If all checks on the packet are successful, the server transmits a new EAP-Request/SIM/Start packet and the full authentication procedure is performed as usual. Since the server already knows the subscriber identity, it MUST NOT include AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ, or AT_PERMANENT_ID_REQ in the EAP-Request/SIM/Start. It should be noted that in this case, peer identity is only transmitted in the AT_IDENTITY attribute at the beginning of the whole EAP exchange. The fast re-authentication identity used in this AT_IDENTITY attribute will be used in key derivation (see Section 7).6. EAP-SIM Notifications
6.1. General
EAP-SIM does not prohibit the use of the EAP Notifications as specified in [RFC3748]. EAP Notifications can be used at any time in the EAP-SIM exchange. It should be noted that EAP-SIM does not protect EAP Notifications. EAP-SIM also specifies method-specific EAP-SIM notifications that are protected in some cases. The EAP server can use EAP-SIM notifications to convey notifications and result indications (Section 6.2) to the peer. The server MUST use notifications in cases discussed in Section 6.3.2. When the EAP server issues an EAP-Request/SIM/Notification packet to the peer, the peer MUST process the notification packet. The peer MAY show a notification message to the user and the peer MUST respond to the EAP server with an EAP-Response/SIM/Notification packet, even if the peer did not recognize the notification code.
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
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
This section specifies the operation of the peer and the server in error cases. The subsections below require the EAP-SIM peer and server to send an error packet (EAP-Response/SIM/Client-Error from the peer or EAP-Request/SIM/Notification from the server) in error cases. However, implementations SHOULD NOT rely upon the correct error reporting behavior of the peer, authenticator, or the server. It is possible for error and other messages to be lost in transit or for a malicious participant to attempt to consume resources by not issuing error messages. Both the peer and the EAP server SHOULD have a mechanism to clean up state, even if an error message or EAP-Success is not received after a timeout period.6.3.1. Peer Operation
In general, if an EAP-SIM peer detects an error in a received EAP-SIM packet, the EAP-SIM implementation responds with the EAP-Response/SIM/Client-Error packet. In response to the EAP-Response/SIM/Client-Error, the EAP server MUST issue the EAP-Failure packet and the authentication exchange terminates. By default, the peer uses the client error code 0, "unable to process packet". This error code is used in the following cases: o EAP exchange is not acceptable according to the peer's local policy. o the peer is not able to parse the EAP request, i.e., the EAP request is malformed. o the peer encountered a malformed attribute. o wrong attribute types or duplicate attributes have been included in the EAP request. o a mandatory attribute is missing. o unrecognized, non-skippable attribute. o unrecognized or unexpected EAP-SIM Subtype in the EAP request. o A RAND challenge repeated in AT_RAND. o invalid AT_MAC. The peer SHOULD log this event. o invalid pad bytes in AT_PADDING.
o the peer does not want to process AT_PERMANENT_ID_REQ. Separate error codes have been defined for the following error cases in Section 10.19: As specified in Section 4.1, when processing the AT_VERSION_LIST attribute, which lists the EAP-SIM versions supported by the server, if the attribute does not include a version that is implemented by the peer and allowed in the peer's security policy, then the peer MUST send the EAP-Response/SIM/Client-Error packet with the error code "unsupported version". If the number of RAND challenges is smaller than what is required by peer's local policy when processing the AT_RAND attribute, the peer MUST send the EAP-Response/SIM/Client-Error packet with the error code "insufficient number of challenges". If the peer believes that the RAND challenges included in AT_RAND are not fresh e.g., because it is capable of remembering some previously used RANDs, the peer MUST send the EAP-Response/SIM/Client-Error packet with the error code "RANDs are not fresh".6.3.2. Server Operation
If an EAP-SIM server detects an error in a received EAP-SIM response, the server MUST issue the EAP-Request/SIM/Notification packet with an AT_NOTIFICATION code that implies failure. By default, the server uses one of the general failure codes ("General failure after authentication" (0), or "General failure" (16384)). The choice between these two codes depends on the phase of the EAP-SIM exchange, see Section 6. When the server issues an EAP- Request/SIM/Notification that implies failure, the error cases include the following: o the server is not able to parse the peer's EAP response o the server encounters a malformed attribute, a non-recognized non-skippable attribute, or a duplicate attribute o a mandatory attribute is missing or an invalid attribute was included o unrecognized or unexpected EAP-SIM Subtype in the EAP Response o invalid AT_MAC. The server SHOULD log this event. o invalid AT_COUNTER
6.3.3. EAP-Failure
The EAP-SIM server sends EAP-Failure in two cases: 1) In response to an EAP-Response/SIM/Client-Error packet the server has received from the peer, or 2) Following an EAP-SIM notification round, when the AT_NOTIFICATION code implies failure. The EAP-SIM server MUST NOT send EAP-Failure in cases other than these two. However, it should be noted that even though the EAP-SIM server would not send an EAP-Failure, an authorization decision that happens outside EAP-SIM, such as in the AAA server or in an intermediate AAA proxy, may result in a failed exchange. The peer MUST accept the EAP-Failure packet in case 1) and case 2), above. The peer SHOULD silently discard the EAP-Failure packet in other cases.6.3.4. EAP-Success
On full authentication, the server can only send EAP-Success after the EAP/SIM/Challenge round. The peer MUST silently discard any EAP-Success packets if they are received before the peer has successfully authenticated the server and sent the EAP-Response/SIM/Challenge packet. If the peer did not indicate that it wants to use protected success indications with AT_RESULT_IND (as discussed in Section 6.2) on full authentication, then the peer MUST accept EAP-Success after a successful EAP/SIM/Challenge round. If the peer indicated that it wants to use protected success indications with AT_RESULT_IND (as discussed in Section 6.2), then the peer MUST NOT accept EAP-Success after a successful EAP/SIM/Challenge round. In this case, the peer MUST only accept EAP-Success after receiving an EAP-SIM Notification with the AT_NOTIFICATION code "Success" (32768). On fast re-authentication, EAP-Success can only be sent after the EAP/SIM/Re-authentication round. The peer MUST silently discard any EAP-Success packets if they are received before the peer has successfully authenticated the server and sent the EAP-Response/SIM/Re-authentication packet. If the peer did not indicate that it wants to use protected success indications with AT_RESULT_IND (as discussed in Section 6.2) on fast
re-authentication, then the peer MUST accept EAP-Success after a successful EAP/SIM/Re-authentication round. If the peer indicated that it wants to use protected success indications with AT_RESULT_IND (as discussed in Section 6.2), then the peer MUST NOT accept EAP-Success after a successful EAP/SIM/Re- authentication round. In this case, the peer MUST only accept EAP-Success after receiving an EAP-SIM Notification with the AT_NOTIFICATION code "Success" (32768). If the peer receives an EAP-SIM notification (Section 6) that indicates failure, then the peer MUST no longer accept the EAP-Success packet, even if the server authentication was successfully completed.7. Key Generation
This section specifies how keying material is generated. On EAP-SIM full authentication, a Master Key (MK) is derived from the underlying GSM authentication values (Kc keys), the NONCE_MT, and other relevant context as follows. MK = SHA1(Identity|n*Kc| NONCE_MT| Version List| Selected Version) In the formula above, the "|" character denotes concatenation. "Identity" denotes the peer identity string without any terminating null characters. It is the identity from the last AT_IDENTITY attribute sent by the peer in this exchange, or, if AT_IDENTITY was not used, it is the identity from the EAP-Response/Identity packet. The identity string is included as-is, without any changes. As discussed in Section 4.2.2.2, relying on EAP-Response/Identity for conveying the EAP-SIM peer identity is discouraged, and the server SHOULD use the EAP-SIM method-specific identity attributes. The notation n*Kc in the formula above denotes the n Kc values concatenated. The Kc keys are used in the same order as the RAND challenges in AT_RAND attribute. NONCE_MT denotes the NONCE_MT value (not the AT_NONCE_MT attribute, but only the nonce value). The Version List includes the 2-byte-supported version numbers from AT_VERSION_LIST, in the same order as in the attribute. The Selected Version is the 2-byte selected version from AT_SELECTED_VERSION. Network byte order is used, just as in the attributes. The hash function SHA-1 is specified in [SHA-1]. If several EAP/SIM/Start roundtrips are used in an EAP-SIM exchange, then the NONCE_MT, Version List and Selected version from the last EAP/SIM/Start round are used, and the previous EAP/SIM/Start rounds are ignored.
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
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 Indicates the particular type of attribute. The attribute type values are listed in the IANA considerations section of the EAP-AKA specification [EAP-AKA]. Length Indicates the length of this attribute in multiples of four bytes. The maximum length of an attribute is 1024 bytes. The length includes the Attribute Type and Length bytes. Value The particular data associated with this attribute. This field is always included and it may be two or more bytes in length. The type and length fields determine the format and length of the value field.
Attributes numbered within the range 0 through 127 are called
non-skippable attributes. When an EAP-SIM peer encounters a
non-skippable attribute that the peer does not recognize, the peer
MUST send the EAP-Response/SIM/Client-Error packet, which terminates
the authentication exchange. If an EAP-SIM server encounters a
non-skippable attribute that the server does not recognize, then the
server sends the EAP-Request/SIM/Notification packet with an
AT_NOTIFICATION code, which implies general failure ("General failure
after authentication" (0), or "General failure" (16384), depending on
the phase of the exchange), which terminates the authentication
exchange.
Attributes within the range of 128 through 255 are called skippable
attributes. When a skippable attribute is encountered and is not
recognized, it is ignored. The rest of the attributes and message
data MUST still be processed. The Length field of the attribute is
used to skip the attribute value in searching for the next attribute.
Unless otherwise specified, the order of the attributes in an EAP-SIM
message is insignificant and an EAP-SIM implementation should not
assume a certain order to be used.
Attributes can be encapsulated within other attributes. In other
words, the value field of an attribute type can be specified to
contain other attributes.
8.2. Protocol Extensibility
EAP-SIM can be extended by specifying new attribute types. If
skippable attributes are used, it is possible to extend the protocol
without breaking old implementations.
However, any new attributes added to the EAP-Request/SIM/Start or
EAP-Response/SIM/Start packets would not be integrity-protected.
Therefore, these messages MUST NOT be extended in the current version
of EAP-SIM. If the list of supported EAP-SIM versions in the
AT_VERSION_LIST does not include versions other than 1, then the
server MUST NOT include attributes other than those specified in this
document in the EAP-Request/SIM/Start message. Note that future
versions of this protocol might specify new attributes for
EAP-Request/SIM/Start and still support version 1 of the protocol.
In this case, the server might send an EAP-Request/SIM/Start message
that includes new attributes and indicates support for protocol
version 1 and other versions in the AT_VERSION_LIST attribute. If
the peer selects version 1, then the peer MUST ignore any other
attributes included in EAP-Request/SIM/Start, other than those
specified in this document. If the selected EAP-SIM version in
peer's AT_SELECTED_VERSION is 1, then the peer MUST NOT include other
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.
(page 48 continued on part 4)
