Network Working Group J. Arkko Internet Draft Ericsson Document: draft-arkko-pppext-eap-aka-11.txt H. Haverinen Expires: 27 April, 2004 Nokia 27 October, 2003 EAP AKA Authentication Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Comments should be submitted to the eap@frascone.com mailing list. Abstract This document specifies an Extensible Authentication Protocol (EAP) mechanism for authentication and session key distribution using the Universal Mobile Telecommunications System (UMTS) Authentication and Key Agreement (AKA) mechanism. UMTS AKA is based on symmetric keys, and runs typically in a UMTS Subscriber Identity Module, a smart card like device. EAP AKA includes optional identity privacy support and an optional re-authentication procedure. Table of Contents Status of this Memo................................................1 Abstract...........................................................1 1. Introduction and Motivation.....................................3 2. Terms and Conventions Used in This Document.....................4 3. Protocol Overview...............................................6 4. Operation......................................................11 Arkko and Haverinen Expires 27 April, 2004 [Page 1] EAP AKA Authentication 27 October, 2003 4.1. Identity Management..........................................11 4.2. Re-authentication............................................25 4.3. EAP/AKA Notifications........................................31 4.4. Error Cases..................................................32 4.5. Key Generation...............................................34 5. Message Format and Protocol Extensibility......................35 5.1. Message Format...............................................35 5.2. Protocol Extensibility.......................................37 6. Messages.......................................................37 6.1. EAP-Request/AKA-Identity.....................................37 6.2. EAP-Response/AKA-Identity....................................38 6.3. EAP-Request/AKA-Challenge....................................38 6.4. EAP-Response/AKA-Challenge...................................39 6.5. EAP-Response/AKA-Authentication-Reject.......................39 6.6. EAP-Response/AKA-Synchronization-Failure.....................39 6.7. EAP-Request/AKA-Reauthentication.............................39 6.8. EAP-Response/AKA-Reauthentication............................40 6.9. EAP-Response/AKA-Client-Error................................40 6.10. EAP-Request/AKA-Notification................................40 6.11. EAP-Response/AKA-Notification...............................41 7. Attributes.....................................................41 7.1. Table of Attributes..........................................41 7.2. AT_MAC.......................................................42 7.3. AT_IV, AT_ENCR_DATA and AT_PADDING...........................43 7.4. AT_CHECKCODE.................................................45 7.5. AT_PERMANENT_ID_REQ..........................................47 7.6. AT_ANY_ID_REQ................................................47 7.7. AT_FULLAUTH_ID_REQ...........................................47 7.8. AT_IDENTITY..................................................48 7.9. AT_RAND......................................................48 7.10. AT_AUTN.....................................................49 7.11. AT_RES......................................................49 7.12. AT_AUTS.....................................................49 7.13. AT_NEXT_PSEUDONYM...........................................50 7.14. AT_NEXT_REAUTH_ID...........................................50 7.15. AT_COUNTER..................................................51 7.16. AT_COUNTER_TOO_SMALL........................................51 7.17. AT_NONCE_S..................................................51 7.18. AT_NOTIFICATION.............................................52 7.19. AT_CLIENT_ERROR_CODE........................................53 8. IANA and Protocol Numbering Considerations.....................53 9. Security Considerations........................................54 9.1. Identity Protection..........................................55 9.2. Mutual Authentication........................................55 9.3. Key Derivation...............................................55 9.4. Brute-Force and Dictionary Attacks...........................55 9.5. Integrity Protection, Replay Protection and Confidentiality..55 Arkko and Haverinen Expires 27 April, 2004 [Page 2] EAP AKA Authentication 27 October, 2003 9.6. Negotiation Attacks..........................................56 9.7. Fast Reconnect...............................................56 9.8. Acknowledged Result Indications..............................56 9.9. Man-in-the-middle Attacks....................................57 9.10. Generating Random Numbers...................................57 10. Security Claims...............................................57 11. Intellectual Property Right Notices...........................58 Acknowledgements and Contributions................................58 Authors' Addresses................................................58 Annex A. Pseudo-Random Number Generator...........................59 1. Introduction and Motivation This document specifies an Extensible Authentication Protocol (EAP) mechanism for authentication and session key distribution using the UMTS AKA authentication mechanism [TS 33.102]. UMTS is a global third generation mobile network standard. AKA is based on challenge-response mechanisms and symmetric cryptography. AKA typically runs in a UMTS Subscriber Identity Module (USIM). Compared to the GSM mechanism, UMTS AKA provides substantially longer key lengths and mutual authentication. The introduction of AKA inside EAP allows several new applications. These include the following: - The use of the AKA also as a secure PPP authentication method in devices that already contain an USIM. - The use of the third generation mobile network authentication infrastructure in the context of wireless LANs - Relying on AKA and the existing infrastructure in a seamless way with any other technology that can use EAP. AKA works in the following manner: - The USIM and the home environment have agreed on a secret key beforehand. - The actual authentication process starts by having the home environment produce an authentication vector, based on the secret key and a sequence number. The authentication vector contains a random part RAND, an authenticator part AUTN used for authenticating the network to the USIM, an expected result part XRES, a session key for integrity check IK, and a session key for encryption CK. - The RAND and the AUTN are delivered to the USIM. - The USIM verifies the AUTN, again based on the secret key and the sequence number. If this process is successful (the AUTN is valid Arkko and Haverinen Expires 27 April, 2004 [Page 3] EAP AKA Authentication 27 October, 2003 and the sequence number used to generate AUTN is within the correct range), the USIM produces an authentication result, RES and sends this to the home environment. - The home environment verifies the correct result from the USIM. If the result is correct, IK and CK can be used to protect further communications between the USIM and the home environment. When verifying AUTN, the USIM may detect that the sequence number the network uses is not within the correct range. In this case, the USIM calculates a sequence number synchronization parameter AUTS and sends it to the network. AKA authentication may then be retried with a new authentication vector generated using the synchronized sequence number. For a specification of the AKA mechanisms and how the cryptographic values AUTN, RES, IK, CK and AUTS are calculated, see [TS 33.102]. In EAP AKA, the EAP server node obtains the authentication vectors, compares RES and XRES, and uses CK and IK in key derivation. In the third generation mobile networks, AKA is used both for radio network authentication and IP multimedia service authentication purposes. Different user identities and formats are used for these; the radio network uses the International Mobile Subscriber Identifier (IMSI), whereas the IP multimedia service uses the Network Access Identifier (NAI) [RFC 2486]. 2. Terms and Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119]. The terms and abbreviations "authenticator", "backend authentication server", "EAP server", "Silently Discard", "Master Session Key (MSK)", and "Extended Master Session Key (EMSK)" in this document are to be interpreted as described in [EAP]. This document frequently uses the following terms and abbreviations: AAA protocol Authentication, Authorization and Accounting protocol AKA Authentication and Key Agreement Arkko and Haverinen Expires 27 April, 2004 [Page 4] EAP AKA Authentication 27 October, 2003 AuC Authentication Centre. The mobile network element that can authenticate subscribers either in GSM or in UMTS networks. EAP Extensible Authentication Protocol [EAP]. GSM Global System for Mobile communications. NAI Network Access Identifier [RFC 2486]. AUTN Authentication value generated by the AuC which together with the RAND authenticates the server to the peer, 128 bits [TS 33.102]. AUTS A value generated by the peer upon experiencing a synchronization failure, 112 bits. Permanent Identity The permanent identity of the peer, including an NAI realm portion in environments where a realm is used. The permanent identity is usually based on the IMSI. Used on full authentication only. Permanent Username The username portion of permanent identity, ie. not including any realm portions. Pseudonym Identity A pseudonym identity of the peer, including an NAI realm portion in environments where a real is used. Used on full authentication only. Pseudonym Username The username portion of pseudonym identity, ie. not including any realm portions. Arkko and Haverinen Expires 27 April, 2004 [Page 5] EAP AKA Authentication 27 October, 2003 Re-authentication Identity A re-authentication identity of the peer, including an NAI realm portion in environments where a real is used. Used on re- authentication only. Re-authentication Username The username portion of re-authentication identity, ie. not including any realm portions. RAND Random number generated by the AuC, 128 bits [TS 33.102]. RES Authentication result from the peer, which together with the RAND authenticates the peer to the server, 128 bits [TS 33.102]. SQN Sequence number used in the authentication process, 48 bits [TS 33.102]. SIM Subscriber Identity Module. The SIM is an application traditionally resident on smart cards distributed by GSM operators. SRES The authentication result parameter in GSM, corresponds to the RES parameter in UMTS aka, 32 bits. USIM UMTS Subscriber Identity Module. USIM is an application that is resident e.g. on smart cards distributed by UMTS operators. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119] 3. Protocol Overview The message flow below shows the basic successful full authentication exchange in EAP AKA. At the minimum, EAP AKA uses two roundtrips to authorize the user and generate session keys. As in other EAP schemes, an identity request/response message pair is usually exchanged first. On full authentication, the peer's identity response includes either the user's International Mobile Subscriber Arkko and Haverinen Expires 27 April, 2004 [Page 6] EAP AKA Authentication 27 October, 2003 Identity (IMSI), or a temporary identity (pseudonym) if identity privacy is in effect, as specified in Section 4.1. (As specified in [EAP], the initial identity request is not required, and MAY be bypassed in cases where the network can presume the identity, such as when using leased lines, dedicated dial-ups, etc. Please see also Section 4.1.2 for specification how to obtain the identity via EAP AKA messages.) Next, the EAP server starts the actual AKA protocol by sending an EAP-Request/AKA-Challenge message. EAP AKA packets encapsulate parameters in attributes, encoded in a Type, Length, Value format. The packet format and the use of attributes are specified in Section 5. The EAP-Request/AKA-Challenge message contains a random number (AT_RAND) and a network authentication token (AT_AUTN), and a message authentication code AT_MAC. The EAP-Request/AKA-Challenge message MAY optionally contain encrypted data, which is used for identity privacy and re-authentication support, as described in Section 4.1. The AT_MAC attribute contains a message authentication code covering the EAP packet. The encrypted data is not shown in the figures of this section. The peer runs the AKA algorithm (typically using a USIM) and verifies the AUTN. If this is successful, the peer is talking to a legitimate EAP server and proceeds to send the EAP-Response/AKA- Challenge. This message contains a result parameter that allows the EAP server in turn to authenticate the peer, and the AT_MAC attribute to integrity protect the EAP message. Arkko and Haverinen Expires 27 April, 2004 [Page 7] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes user's NAI) | |------------------------------------------------------>| | | | +------------------------------+ | | Server runs UMTS algorithms, | | | generates RAND and AUTN. | | +------------------------------+ | | | EAP-Request/AKA-Challenge | | (AT_RAND, AT_AUTN, AT_MAC) | |<------------------------------------------------------| | | +-------------------------------------+ | | Peer runs UMTS algorithms on USIM, | | | verifies AUTN and MAC, derives RES | | | and session key | | +-------------------------------------+ | | | | EAP-Response/AKA-Challenge | | (AT_RES, AT_MAC) | |------------------------------------------------------>| | | | +--------------------------------+ | | Server checks the given RES, | | | and MAC and finds them correct.| | +--------------------------------+ | | | EAP-Success | |<------------------------------------------------------| The second message flow shows how the EAP server rejects the Peer due to a failed authentication. The same flow is also used in the GSM compatible mode, except that the AT_AUTN attribute and AT_MAC attribute are not used in the messages. Arkko and Haverinen Expires 27 April, 2004 [Page 8] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes user's NAI) | |------------------------------------------------------>| | | | +------------------------------+ | | Server runs UMTS algorithms, | | | generates RAND and AUTN. | | +------------------------------+ | | | EAP-Request/AKA-Challenge | | (AT_RAND, AT_AUTN, AT_MAC) | |<------------------------------------------------------| | | +-------------------------------------+ | | Peer runs UMTS algorithms on USIM, | | | possibly verifies AUTN, and sends an| | | invalid response | | +-------------------------------------+ | | | | EAP-Response/AKA-Challenge | | (AT_RES, AT_MAC) | |------------------------------------------------------>| | | | +------------------------------------------+ | | Server checks the given RES and the MAC, | | | and finds one of them incorrct. | | +------------------------------------------+ | | | EAP-Failure | |<------------------------------------------------------| The next message flow shows the peer rejecting the AUTN of the EAP server. The peer sends an explicit error message (EAP-Response/AKA- Authentication-Reject) to the EAP server, as usual in AKA when AUTN is incorrect. This allows the EAP server to produce the same error statistics as AKA in general produces in UMTS. Arkko and Haverinen Expires 27 April, 2004 [Page 9] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes user's NAI) | |------------------------------------------------------>| | | | +------------------------------+ | | Server runs UMTS algorithms, | | | generates RAND and a bad AUTN| | +------------------------------+ | | | EAP-Request/AKA-Challenge | | (AT_RAND, AT_AUTN, AT_MAC) | |<------------------------------------------------------| | | +-------------------------------------+ | | Peer runs UMTS algorithms on USIM | | | and discovers AUTN that can not be | | | verified | | +-------------------------------------+ | | | | EAP-Response/AKA-Authentication-Reject | |------------------------------------------------------>| | | | | | EAP-Failure | |<------------------------------------------------------| The AKA uses shared secrets between the Peer and the Peer's home operator together with a sequence number to actually perform an authentication. In certain circumstances it is possible for the sequence numbers to get out of sequence. Here's what happens then: Arkko and Haverinen Expires 27 April, 2004 [Page 10] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes user's NAI) | |------------------------------------------------------>| | | | +------------------------------+ | | Server runs UMTS algorithms, | | | generates RAND and AUTN. | | +------------------------------+ | | | EAP-Request/AKA-Challenge | | (AT_RAND, AT_AUTN, AT_MAC) | |<------------------------------------------------------| | | +-------------------------------------+ | | Peer runs UMTS algorithms on USIM | | | and discovers AUTN that contains an | | | inappropriate sequence number | | +-------------------------------------+ | | | | EAP-Response/AKA-Synchronization-Failure | | (AT_AUTS) | |------------------------------------------------------>| | | | +---------------------------+ | | Perform resynchronization | | | Using AUTS and | | | the sent RAND | | +---------------------------+ | | After the resynchronization process has taken place in the server and AAA side, the process continues by the server side sending a new EAP-Request/AKA-Challenge message. In addition to the full authentication scenarios described above, EAP AKA includes a re-authentication procedure, which is specified in Section 4.2. Re-authentication is based on keys derived on full authentication. If the peer has maintained state information for re- authentication and wants to use re-authentication, then the peer indicates this by using a specific re-authentication identity instead of the permanent identity or a pseudonym identity. The re- authentication procedure is described in Section 4.2. 4. Operation 4.1. Identity Management 4.1.1. Format, Generation and Usage of Peer Identities Arkko and Haverinen Expires 27 April, 2004 [Page 11] EAP AKA Authentication 27 October, 2003 General In the beginning of EAP authentication, the Authenticator or the EAP server usually issues the EAP-Request/Identity packet to the peer. The peer responds with EAP-Response/Identity, which contains the user's identity. The formats of these packets are specified in [EAP]. UMTS subscribers are identified with the International Mobile Subscriber Identity (IMSI) [TS 23.003]. The IMSI is composed of a three digit Mobile Country Code (MCC), a two or three digit Mobile Network Code (MNC) and a not more than 10 digit Mobile Subscriber Identification Number (MSIN). In other words, the IMSI is a string of not more than 15 digits. MCC and MNC uniquely identify the GSM operator and help identify the AuC from which the authentication vectors need to be retrieved for this subscriber. Internet AAA protocols identify users with the Network Access Identifier (NAI) [RFC 2486]. When used in a roaming environment, the NAI is composed of a username and a realm, separated with "@" (username@realm). The username portion identifies the subscriber within the realm. This section specifies the peer identity format used in EAP/AKA. In this document, the term identity or peer identity refers to the whole identity string that is used to identify the peer. The peer identity may include a realm portion. "Username" refers to the portion of the peer identity that identifies the user, i.e. the username does not include the realm portion. Identity Privacy Support EAP/AKA includes optional identity privacy (anonymity) support that can be used to hide the cleartext permanent identity and thereby to make the subscriber's EAP exchanges untraceable to eavesdroppers. Because the permanent identity never changes, revealing it would help observers to track the user. The permanent identity is usually based on the IMSI, which may further help the tracking, because the same identifier may used in other contexts as well. Identity privacy is based on temporary identities, or pseudonyms, which are equivalent to but separate from the Temporary Mobile Subscriber Identities (TMSI) that are used on cellular networks. Please see Section 9.1 for security considerations regarding identity privacy. Username Types in EAP/AKA Identities There are three types of usernames in EAP/AKA peer identities: (1) Permanent usernames. For example, 0123456789098765@myoperator.com might be a valid permanent identity. In this example, 0123456789098765 is the permanent username. Arkko and Haverinen Expires 27 April, 2004 [Page 12] EAP AKA Authentication 27 October, 2003 (2) Pseudonym usernames. For example, 2s7ah6n9q@myoperator.com might be a valid pseudonym identity. In this example, 2s7ah6n9q is the pseudonym username. (3) Re-authentication usernames. For example, 43953754a@myoperator.com might be a valid re-authentication identity. In this case, 43953754 is the re-authentication username. The first two types of identities are only used on full authentication and the last one only on re-authentication. When the optional identity privacy support is not used, the non-pseudonym permanent identity is used on full authentication. The re- authentication exchange is specified in Section 4.2. sername Decoration In some environments, the peer may need to decorate the identity by prepending or appending the username with a string, in order to indicate supplementary AAA routing information in addition to the NAI realm. (The usage of a NAI realm portion is not considered to be decoration.) Username decoration is out of the scope of this document. However, it should be noted that username decoration might prevent the server from recognizing a valid username. Hence, although the peer MAY use username decoration in the identities the peer includes in EAP-Response/Identity, and the EAP server MAY accept a decorated peer username in this message, the peer or the EAP server MUST NOT decorate any other peer identities that are used in various EAP/AKA attributes. Only the identity used in EAP- Response/Identity may be decorated. NAI Realm Portion The peer MAY include a realm portion in the peer identity, as per the NAI format. The use of a realm portion is not mandatory. If a realm is used, the realm MAY be chosen by the operator and it MAY a configurable parameter in the EAP/SIM peer implementation. In this case, the peer is typically configured with the NAI realm of the home operator. Operators MAY reserve a specific realm name for EAP/AKA users. This convention makes it easy to recognize that the NAI identifies a UMTS subscriber. Such reserved NAI realm may be useful as a hint as to the first authentication method to use during method negotiation. When the peer is using a pseudonym username instead of the permanent username, the peer selects the realm name portion similarly as it select the realm portion when using the permanent username. If no configured realm name is available, the peer MAY derive the realm name from the MCC and MNC portions of the IMSI. A recommended way to derive the realm from the IMSI using the realm 3gppnetwork.org will be specified in [Draft 3GPP TS 23.234]. Alternatively, the realm name may be obtained by concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits of IMSI and ".owlan.org". For example, if the IMSI is 123456789098765, and the Arkko and Haverinen Expires 27 April, 2004 [Page 13] EAP AKA Authentication 27 October, 2003 MNC is three digits long, then the derived realm name is "mnc456.mcc123.owlan.org". The IMSI is a string of digits without any explicit structure, so the peer may not be able to determine the length of the MNC portion. If the peer is not able to determine whether the MNC is two or three digits long, the peer MAY use a 3-digit MNC. If the correct length of the MNC is two, then the MNC used in the realm name includes the first digit of MSIN. Hence, when configuring AAA networks for operators that have 2-digit MNC's, the network SHOULD also be prepared for realm names with incorrect 3-digit MNC's. Format of the Permanent Username The non-pseudonym permanent username SHOULD be derived from the IMSI. In this case, the permanent username MUST be of the format "0" | IMSI, where the character "|" denotes concatenation. In other words, the first character of the username is the digit zero (ASCII value 0x30), followed by the IMSI. The IMSI is an ASCII string that consists of not more than 15 decimal digits (ASCII values between 0x30 and 0x39) as specified in [TS 23.003]. The EAP server MAY use the leading "0" as a hint to try EAP/AKA as the first authentication method during method negotiation, rather than for example EAP/SIM. The EAP/AKA server MAY propose EAP/AKA even if the leading character was not "0". Alternatively, an implementation MAY choose a permanent username that is not based on the IMSI. In this case the selection of the username, its format, and its processing is out of the scope of this document. In this case, the peer implementation MUST NOT prepend any leading characters to the username. Generating Pseudonyms and Re-authentication Identities by the Server Pseudonym usernames and re-authentication identities are generated by the EAP server. The EAP server produces pseudonym usernames and re-authentication identities in an implementation-dependent manner. Only the EAP server needs to be able to map the pseudonym username to the permanent identity, or to recognize a re-authentication identity. Regardless of construction method, the pseudonym username MUST conform to the grammar specified for the username portion of an NAI. The re-authentication identity also MUST conform to the NAI grammar. The EAP servers that the subscribers of an operator can use MUST ensure that the pseudonym usernames and the username portions used in re-authentication identities they generate are unique. In any case, it is necessary that permanent usernames, pseudonym usernames and re-authentication usernames are separate and recognizable from each other. It is also desirable that EAP SIM and EAP AKA user names be recognizable from each other as an aid for the server to which method to offer. Arkko and Haverinen Expires 27 April, 2004 [Page 14] EAP AKA Authentication 27 October, 2003 In general, it is the task of the EAP server and the policies of its administrator to ensure sufficient separation in the usernames. Pseudonym usernames and re-authentication usernames are both produced and used by the EAP server. The EAP server MUST compose pseudonym usernames and re-authentication usernames so that it can recognize if a NAI username is an EAP AKA pseudonym username or an EAP AKA re-authentication username. For instance, when the usernames have been derived from the IMSI, the server could use different leading characters in the pseudonym usernames and re-authentication usernames (e.g. the pseudonym could begin with a leading "2" character). When mapping a re-authentication identity to a permanent identity, the server SHOULD only examine the username portion of the re-authentication identity and ignore the realm portion of the identity. Because the peer may fail to save a pseudonym username sent to in an EAP-Request/AKA-Challenge, for example due to malfunction, the EAP server SHOULD maintain at least one old pseudonym username in addition to the most recent pseudonym username. Transmitting Pseudonyms and Re-authentication Identities to the Peer The server transmits pseudonym usernames and re-authentication identities to the peer in cipher, using the AT_ENCR_DATA attribute. The EAP-Request/AKA-Challenge message MAY include an encrypted pseudonym username and/or an encrypted re-authentication identity in the value field of the AT_ENCR_DATA attribute. Because identity privacy support and re-authentication are optional to implement, the peer MAY ignore the AT_ENCR_DATA attribute and always use the permanent identity. On re-authentication (discussed in Section 4.2), the server MAY include a new encrypted re-authentication identity in the EAP-Request/AKA-Reauthentication message. On receipt of the EAP-Request/AKA-Challenge, the peer MAY decrypt the encrypted data in AT_ENCR_DATA and if a pseudonym username is included, the peer may use the obtained pseudonym username on the next full authentication. If a re-authentication identity is included, then the peer MAY save it and other re-authentication state information, as discussed in Section 4.2, for the next re- authentication. If the peer does not receive a new pseudonym username in the EAP- Request/AKA-Challenge message, the peer MAY use an old pseudonym username instead of the permanent username on next full authentication. The username portions of re-authentication identities are one-time usernames, which the peer MUST NOT re-use. Usage of the Pseudonym by the Peer When the optional identity privacy support is used on full authentication, the peer MAY use the pseudonym username received as part of the previous full authentication sequence as the username portion of the NAI. The peer MUST NOT modify the pseudonym username Arkko and Haverinen Expires 27 April, 2004 [Page 15] EAP AKA Authentication 27 October, 2003 received in AT_NEXT_PSEUDONYM. However, as discussed above, the peer MAY need to decorate the username in some environments by appending or prepending the username with a string that indicates supplementary AAA routing information. When using a pseudonym username in an environment where a realm portion is used, the peer concatenates the received pseudonym username with the "@" character and a NAI realm portion. The selection of the NAI realm is discussed above. Usage of the Re-authentication Identity by the Peer On re-authentication, the peer uses the re-authentication identity, received as part of the previous authentication sequence. A new re- authentication identity may be delivered as part of both full authentication and re-authentication. The peer MUST NOT modify the username part of the re-authentication identity received in AT_NEXT_REAUTH_ID, except in cases when username decoration is required. Even in these cases, the "root" re-authentication username must not be modified, but it may be appended or prepended with another string. 4.1.2. Communicating the Peer Identity to the Server General The peer identity MAY be communicated to the server with the EAP- Response/Identity message. This message MAY contain the permanent identity, a pseudonym identity, or a re-authentication identity. If the peer uses the permanent identity or a pseudonym identity, which the server is able to map to the permanent identity, then the authentication proceeds as discussed in the overview of Section 3. If the peer uses a re-authentication identity, and the server recognized the identity and agrees on using re-authentication, then a re-authentication exchange is performed, as described in Section 4.2. The peer identity can also be transmitted from the peer to the server using EAP/AKA messages instead of EAP-Response/Identity. In this case, the server includes an identity requesting attribute (AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ) in the EAP-Request/AKA-Identity message, and the peer includes the AT_IDENTITY attribute, which contains the peer's identity, in the EAP-Response/AKA-Identity message. The AT_ANY_ID_REQ attribute is a general identity requesting attribute, which the server uses if it does not specify which kind of an identity the peer should return in AT_IDENTITY. The server uses the AT_FULLAUTH_ID_REQ attribute to request either the permanent identity or a pseudonym identity. The server uses the AT_PERMANENT_ID_REQ attribute to request the peer to send its permanent identity. The EAP-Request/AKA-Challenge, EAP- Response/AKA-Challenge, or the packets used on re-authentication may optionally include the AT_CHECKCODE attribute, which enables the protocol peers to ensure the integrity of the AKA-Identity packets. AT_CHECKCODE is specified in Section 0. Arkko and Haverinen Expires 27 April, 2004 [Page 16] EAP AKA Authentication 27 October, 2003 The identity format in the AT_IDENTITY attribute is the same as in the EAP-Response/Identity packet (except that identity decoration is not allowed). The AT_IDENTITY attribute contains a permanent identity, a pseudonym identity or a re-authentication identity. Obtaining the subscriber identity via EAP/AKA messages is useful if the server does not have any EAP/AKA peer identity at the beginning of the EAP/AKA exchange or does not recognize the identity the peer used in EAP-Response/Identity. This may happen if, for example, the EAP-Response/Identity has been issued by some EAP method other than EAP/AKA or if intermediate entities or software layers in the peer have modified the identity string in the EAP-Response/Identity packet. Also, some EAP layer implementations may cache the identity string from the first EAP authentication and do not obtain a new identity string from the EAP method implementation on subsequent authentication exchanges. As the identity string is used in key derivation, any of these cases will result in failed authentication unless the EAP server uses EAP/AKA attributes to obtain an unmodified copy of the identity string. Therefore, unless the EAP server can be certain that no intermediate element or software layer has modified the EAP- Response/Identity packet, the EAP server SHOULD always use the EAP/AKA attributes to obtain the identity, even if the identity received in EAP-Response/Identity was valid. Please note that the EAP/AKA peer and the EAP/AKA server only process the AT_IDENTITY attribute and entities that only pass through EAP packets do not process this attribute. Hence, if the EAP server is not co-located in the authenticator, then the authenticator and other intermediate AAA elements (such as possible AAA proxy servers) will continue to refer to the peer with the original identity from the EAP-Response/Identity packet regardless of whether the AT_IDENTITY attribute is used in EAP/AKA to transmit another identity. Choice of Identity for the EAP-Response/Identity If EAP/AKA peer is started upon receiving an EAP-Request/Identity message, then the peer performs the following steps. If the peer has maintained re-authentication state information and if the peer wants to use re-authentication, then the peer transmits the re-authentication identity in EAP-Response/Identity. Else, if the peer has a pseudonym username available, then the peer transmits the pseudonym identity in EAP-Response/Identity. In other cases, the peer transmits the permanent identity in EAP- Response/Identity. Arkko and Haverinen Expires 27 April, 2004 [Page 17] EAP AKA Authentication 27 October, 2003 Server Operation in the Beginning of EAP/AKA Exchange If the EAP server has not received any identity (permanent identity, pseudonym identity or re-authentication identity) from the peer when sending the first EAP/AKA 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/AKA-Identity message with the AT_PERMANENT_ID_REQ message to indicate that the server wants the peer to include the permanent identity in the AT_IDENTITY attribute of the EAP-Response/AKA-Identity message. This is done in the following cases: - The server does not support re-authentication or identity privacy. - The server received an identity that it recognizes as a pseudonym identity but the server is not able to map the pseudonym identity to a permanent identity. The server issues the EAP-Request/AKA-Identity 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/AKA-Identity message. This is done in the following cases: - The server does not support re-authentication and the server supports identity privacy - The server received an identity that it recognizes as a re- authentication identity but the server is not able to map the re- authentication identity to a permanent identity The server issues the EAP-Request/AKA-Identity packet with the AT_ANY_ID_REQ attribute to indicate that the server wants the peer to include an identity in the AT_IDENTITY attribute of the EAP- Response/SIM/Start message, and the server does not indicate any preferred type for the identity. This is done in other cases, such as when the server does not have any identity, or the server does not recognize the format of a received identity. Processing of EAP-Request/AKA-Identity by the Peer Upon receipt of an EAP-Request/AKA-Identity message, the peer MUST perform the following steps. If the EAP-Request/AKA-Identity includes AT_PERMANENT_ID_REQ the peer MUST either respond with EAP-Response/AKA-Identity and include the permanent identity in AT_IDENTITY or respond with EAP- Response/AKA-Client-Error packet with code "unable to process packet". Arkko and Haverinen Expires 27 April, 2004 [Page 18] EAP AKA Authentication 27 October, 2003 If the EAP-Request/AKA-Identity includes AT_FULL_AUTH_ID_REQ, and if the peer has a pseudonym available, then the peer SHOULD respond with EAP-Response/AKA-Identity and includes the pseudonym identity in AT_IDENTITY. If the peer does not have a pseudonym when it receives this message, then the peer MUST either respond with EAP- Response/AKA-Identity and include the permanent identity in AT_IDENTITY or respond with EAP-Response/AKA-Client-Error packet with code "unable to process packet." The Peer MUST NOT use a re- authentication identity in the AT_IDENTITY attribute. If the EAP-Request/AKA-Identity includes AT_ANY_ID_REQ, and if the peer has maintained re-authentication state information and the peer wants to use re-authentication, then the peer responds with EAP- Response/AKA-Identity and includes the re-authentication identity in AT_IDENTITY. Else, if the peer has a pseudonym identity available, then the peer responds with EAP-Response/AKA-Identity and includes the pseudonym identity in AT_IDENTITY. Else, the peer responds with EAP-Response/AKA-Identity and includes the permanent identity in AT_IDENTITY. An EAP/AKA exchange may include several EAP/AKA-Identity rounds. The server may issue a second EAP-Request/AKA-Identity, if it was not able to recognize the identity the peer used in the previous AT_IDENTITY attribute. At most three EAP/AKA-Identity rounds can be used. AT_ANY_ID_REQ can only be used in the first EAP-Request/AKA- Identity, in other words AT_ANY_ID_REQ MUST NOT be used in the second or third EAP-Request/AKA-Identity. AT_FULLAUTH_ID_REQ MUST NOT be used if the previous EAP-Request/AKA-Identity included AT_PERMANENT_ID_REQ. The peer operation in cases when it receives an unexpected attribute is specified in Section 4.4.1. Attacks against Identity Privacy The section above specifies two possible ways the peer can operate upon receipt of AT_PERMANENT_ID_REQ. This is because a received AT_PERMANENT_ID_REQ does not necessarily originate from the valid network, but an active attacker may transmit an EAP-Request/AKA- Identity 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/AKA-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/AKA-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 Arkko and Haverinen Expires 27 April, 2004 [Page 19] EAP AKA Authentication 27 October, 2003 the other hand, a "liberal" peer always accepts the AT_PERMANENT_ID_REQ and responds with the permanent identity. The benefit of this policy is that it works even if the valid network sometimes loses pseudonyms and is not able to map them to the permanent identity. Processing of AT_IDENTITY by the Server When the server receives an EAP-Response/AKA-Identity message with the AT_IDENTITY (in response to the server's identity requesting attribute), the server MUST operate as follows. If the server used AT_PERMANENT_ID_REQ, and if the AT_IDENTITY does not contain a valid permanent identity, then the server sends EAP Failure and the EAP exchange terminates. If the server recognizes the permanent identity and is able to continue, then the server proceeds with full authentication by sending EAP-Request/AKA- 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/AKA-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/ AKA-Identity 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/ AKA-Challenge. If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid re-authentication identity and the server agrees on using re- authentication, then the server proceeds with re-authentication by sending EAP-Request/ AKA-Reauthentication (Section 4.2). If the server used AT_ANY_ID_REQ, and if the peer sent an EAP- Response/AKA-Identity with AT_IDENTITY that contains an identity that the server recognizes as a re-authentication identity, but the server is not able to map the identity to a permanent identity, then the server sends EAP-Request/AKA-Identity with AT_FULLAUTH_ID_REQ. If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid re-authentication identity, which the server is able to map to a permanent identity, and if the server does not want to use re- authentication, then the server proceeds with full authentication by sending EAP-Request/AKA-Challenge. 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 Arkko and Haverinen Expires 27 April, 2004 [Page 20] EAP AKA Authentication 27 October, 2003 identity, then the server sends EAP-Request/AKA-Identity 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/AKA-Identity with AT_FULLAUTH_ID_REQ. 4.1.3. Message Sequence Examples (Informative) This section contains non-normative message sequence examples to illustrate how the peer identity can be communicated to the server. sage of AT_ANY_ID_REQ Obtaining the peer identity with EAP/AKA attributes is illustrated in the figure below. Peer Authenticator | | | +------------------------------+ | | Server does not have any | | | Subscriber identity available| | | When starting EAP/AKA | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (AT_ANY_ID_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/AKA-Identity | | (AT_IDENTITY) | |------------------------------------------------------>| | | all Back on Full Authentication The figure below illustrates the case when the server does not recognize the re-authentication identity the peer used in AT_IDENTITY. Arkko and Haverinen Expires 27 April, 2004 [Page 21] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | +------------------------------+ | | Server does not have any | | | Subscriber identity available| | | When starting EAP/AKA | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (AT_ANY_ID_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/AKA-Identity | | (AT_IDENTITY containing a re-authentication identity) | |------------------------------------------------------>| | | | +------------------------------+ | | Server does not recognize | | | The re-authentication | | | Identity | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (AT_FULLAUTH_ID_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/AKA-Identity | | (AT_IDENTITY with a full-auth. Identity) | |------------------------------------------------------>| | | If the server recognizes the re-authentication identity, but still wants to fall back on full authentication, the server may issue the EAP-Request/AKA-Challenge packet. In this case, the full authentication procedure proceeds as usual. Requesting the Permanent Identity 1 The figure below illustrates the case when the EAP server fails to decode a pseudonym identity included in the EAP-Response/Identity packet. Arkko and Haverinen Expires 27 April, 2004 [Page 22] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes a pseudonym) | |------------------------------------------------------>| | | | +------------------------------+ | | Server fails to decode the | | | Pseudonym. | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (AT_PERMANENT_ID_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/AKA-Identity | | (AT_IDENTITY with permanent identity) | |------------------------------------------------------>| | | If the server recognizes the permanent identity, then the authentication sequence proceeds as usual with the EAP Server issuing the EAP-Request/AKA-Challenge message. Requesting the Permanent Identity 2 The figure below illustrates the case when the EAP server fails to decode the pseudonym included in the AT_IDENTITY attribute. Arkko and Haverinen Expires 27 April, 2004 [Page 23] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | +------------------------------+ | | Server does not have any | | | Subscriber identity available| | | When starting EAP/AKA | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (AT_ANY_ID_REQ) | |<------------------------------------------------------| | | | | |EAP-Response/AKA-Identity | |(AT_IDENTITY with a pseudonym identity) | |------------------------------------------------------>| | | | | | +------------------------------+ | | Server fails to decode the | | | Pseudonym in AT_IDENTITY | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (AT_PERMANENT_ID_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/AKA-Identity | | (AT_IDENTITY with permanent identity) | |------------------------------------------------------>| | | Three EAP/AKA-Identity Round Trips The figure below illustrates the case with three EAP/AKA-Identity round trips. Arkko and Haverinen Expires 27 April, 2004 [Page 24] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | +------------------------------+ | | Server does not have any | | | Subscriber identity available| | | When starting EAP/AKA | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (AT_ANY_ID_REQ) | |<------------------------------------------------------| | | | EAP-Response/AKA-Identity | | (AT_IDENTITY with re-authentication identity) | |------------------------------------------------------>| | | | +------------------------------+ | | Server does not accept | | | The re-authentication | | | Identity | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (AT_FULLAUTH_ID_REQ) | |<------------------------------------------------------| | | |EAP-Response/AKA-Identity | |(AT_IDENTITY with a pseudonym identity) | |------------------------------------------------------>| | | | +------------------------------+ | | Server fails to decode the | | | Pseudonym in AT_IDENTITY | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (AT_PERMANENT_ID_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/AKA-Identity | | (AT_IDENTITY with permanent identity) | |------------------------------------------------------>| | | After the last EAP-Response/AKA-Identity message, the full authentication sequence proceeds as usual. 4.2. Re-authentication 4.2.1. General In some environments, EAP authentication may be performed frequently. Because the EAP AKA full authentication procedure makes Arkko and Haverinen Expires 27 April, 2004 [Page 25] EAP AKA Authentication 27 October, 2003 use of the UMTS AKA algorithms, and it therefore requires fresh authentication vectors from the Authentication Centre, the full authentication procedure may result in many network operations when used very frequently. Therefore, EAP AKA includes a more inexpensive re-authentication procedure that does not make use of the UMTS AKA algorithms and does not need new vectors from the Authentication Centre. Re-authentication is optional to implement for both the EAP AKA server and peer. On each EAP authentication, either one of the entities may also fall back on full authentication if they do not want to use re-authentication. Re-authentication is based on the keys derived on the preceding full authentication. The same K_aut and K_encr keys as in full authentication are used to protect EAP AKA packets and attributes, and the original Master Key from full authentication is used to generate a fresh Master Session Key, as specified in Section 4.5. On re-authentication, the peer protects against replays with an unsigned 16-bit counter, included in the AT_COUNTER attribute. On full authentication, both the server and the peer initialize the counter to one. The counter value of at least one is used on the first re-authentication. On subsequent re-authentications, the counter MUST be greater than on any of the previous re- authentications. For example, on the second re-authentication, counter value is two or greater etc. The AT_COUNTER attribute is encrypted. The server includes an encrypted server nonce (AT_NONCE_S) in the re-authentication request. The AT_MAC attribute in the peer's response is calculated over NONCE_S to provide a challenge/response authentication scheme. The NONCE_S also contributes to the new Master Session Key. Both the peer and the server SHOULD have an upper limit for the number of subsequent re-authentications allowed before a full authentication needs to be performed. Because a 16-bit counter is used in re-authentication, the theoretical maximum number of re- authentications is reached when the counter value reaches 0xFFFF. In order to use re-authentication, the peer and the EAP server need to store the following values: Master Key, latest counter value and the next re-authentication identity. K_aut, K_encr may either be stored or derived again from MK. The server may also need to store the permanent identity of the user. 4.2.2. Re-authentication Identity The re-authentication procedure makes use of separate re- authentication user identities. Pseudonyms and the permanent identity are reserved for full authentication only. If a re- authentication identity is lost and the network does not recognize it, the EAP server can fall back on full authentication. Arkko and Haverinen Expires 27 April, 2004 [Page 26] EAP AKA Authentication 27 October, 2003 If the EAP server supports re-authentication, it MAY include the skippable AT_NEXT_REAUTH_ID attribute in the encrypted data of EAP- Request/AKA-Challenge message. This attribute contains a new re- authentication identity for the next re-authentication. The peer MAY ignore this attribute, in which case it will use full authentication next time. If the peer wants to use re-authentication, it uses this re-authentication identity on next authentication. Even if the peer has a re-authentication identity, the peer MAY discard the re- authentication identity and use a pseudonym or the permanent identity instead, in which case full authentication MUST be performed. In environments where a real portion is needed in the peer identity, the re-authentication identity received in AT_NEXT_REAUTH_ID MUST contain both a username portion and a realm portion, as per the NAI format. The EAP Server can choose an appropriate realm part in order to have the AAA infrastructure route subsequent re-authentication related requests to the same AAA server. For example, the realm part MAY include a portion that is specific to the AAA server. Hence, it is sufficient to store the context required for re-authentication in the AAA server that performed the full authentication. The peer MAY use the re-authentication identity in the EAP- Response/Identity packet or, in response to server's AT_ANY_ID_REQ attribute, the peer MAY use the re-authentication identity in the AT_IDENTITY attribute of the EAP-Response/AKA-Identity packet. The peer MUST NOT modify the username portion of the re-authentication identity, but the peer MAY modify the realm portion or replace it with another realm portion. Even if the peer uses a re-authentication identity, the server may want to fall back on full authentication, for example because the server does not recognize the re-authentication identity or does not want to use re-authentication. If the server was able to decode the re-authentication identity to the permanent identity, the server issues the EAP-Request/AKA-Challenge packet to initiate full authentication. If the server was not able to recover the peer's identity from the re-authentication identity, the server starts the full authentication procedure by issuing an EAP-Request/AKA-Identity packet. This packet always starts a full authentication sequence if it does not include the AT_ANY_ID_REQ attribute. 4.2.3. Re-authentication Procedure The following figure illustrates the re-authentication procedure. Encrypted attributes are denoted with '*'. The peer uses its re- authentication identity in the EAP-Response/Identity packet. As discussed above, an alternative way to communicate the re- authentication identity to the server is for the peer to use the AT_IDENTITY attribute in the EAP-Response/AKA-Identity message. This latter case is not illustrated in the figure below, and it is only possible when the server requests the peer to send its identity by including the AT_ANY_ID_REQ attribute in the EAP-Request/AKA- Identity packet. Arkko and Haverinen Expires 27 April, 2004 [Page 27] EAP AKA Authentication 27 October, 2003 If the server recognizes the re-authentication identity and agrees on using re-authentication, then the server sends the EAP- Request/AKA-Reauthentication packet to the peer. This packet MUST include the encrypted AT_COUNTER attribute, with a fresh counter value, the encrypted AT_NONCE_S attribute that contains a random number chosen by the server, the AT_ENCR_DATA and the AT_IV attributes used for encryption, and the AT_MAC attribute that contains a message authentication code over the packet. The packet MAY also include an encrypted AT_NEXT_REAUTH_ID attribute that contains the next re-authentication identity. Re-authentication identities are one-time identities. If the peer does not receive a new re-authentication identity, it MUST use either the permanent identity or a pseudonym identity on the next authentication to initiate full authentication. The peer verifies that the counter value is fresh (greater than any previously used value). The peer also verifies that AT_MAC is correct. The peer MAY save the next re-authentication identity from the encrypted AT_NEXT_REAUTH_ID for next time. If all checks are successful, the peer responds with the EAP-Response/AKA- Reauthentication packet, including the AT_COUNTER attribute with the same counter value and the AT_MAC attribute. The server verifies the AT_MAC attribute and also verifies that the counter value is the same that it used in the EAP-Request/AKA- Reauthentication packet. If these checks are successful, the re- authentication has succeeded and the server sends the EAP-Success packet to the peer. Arkko and Haverinen Expires 27 April, 2004 [Page 28] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes a re-authentication identity) | |------------------------------------------------------>| | | | +--------------------------------+ | | Server recognizes the identity | | | and agrees on using fast | | | re-authentication | | +--------------------------------+ | | | EAP-Request/AKA-Reauthentication | | (AT_IV, AT_ENCR_DATA, *AT_COUNTER, | | *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC) | |<------------------------------------------------------| | | | | +-----------------------------------------------+ | | Peer verifies AT_MAC and the freshness of | | | the counter. Peer MAY store the new re- | | | authentication identity for next re-auth. | | +-----------------------------------------------+ | | | | EAP-Response/AKA-Reauthentication | | (AT_IV, AT_ENCR_DATA, *AT_COUNTER with same value, | | AT_MAC) | |------------------------------------------------------>| | | | +--------------------------------+ | | Server verifies AT_MAC and | | | the counter | | +--------------------------------+ | | | EAP-Success | |<------------------------------------------------------| | | 4.2.4. Re-authentication Procedure when Counter is Too Small If the peer does not accept the counter value of EAP-Request/AKA- Reauthentication, it indicates the counter synchronization problem by including the encrypted AT_COUNTER_TOO_SMALL in EAP-Response/AKA- Reauthentication. The server responds with EAP-Request/AKA-Challenge to initiate a normal full authentication procedure. This is illustrated in the following figure. Encrypted attributes are denoted with '*'. Arkko and Haverinen Expires 27 April, 2004 [Page 29] EAP AKA Authentication 27 October, 2003 Peer Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes a re-authentication identity) | |------------------------------------------------------>| | | | EAP-Request/AKA-Reauthentication | | (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/AKA-Reauthentication | | (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/AKA-Challenge | |<------------------------------------------------------| | | +---------------------------------------------------------------+ | Normal full authentication follows. | +---------------------------------------------------------------+ | | In the figure above, the first three messages are similar to the basic re-authentication case. When the peer detects that the counter value is not fresh, it includes the AT_COUNTER_TOO_SMALL attribute in EAP-Response/AKA-Reauthentication. This attribute doesn't contain any data but it is a request for the server to initiate full authentication. In this case, the peer MUST ignore the contents of the server's AT_NEXT_REAUTH_ID attribute. On receipt of AT_COUNTER_TOO_SMALL, the server verifies AT_MAC and verifies that AT_COUNTER contains the same as in the EAP- Request/AKA-Reauthentication packet. If not, the server silently discards the EAP-Response/AKA-Reauthentication packet. If all checks on the packet are successful, the server transmits a EAP- Request/AKA-Challenge packet and the full authentication procedure is performed as usual. Since the server already knows the subscriber identity, it MUST NOT use the EAP-Request/AKA-Identity packet to request the identity. Arkko and Haverinen Expires 27 April, 2004 [Page 30] EAP AKA Authentication 27 October, 2003 4.3. EAP/AKA Notifications The EAP-Request/Notification, specified in [EAP], can be used to convey a displayable message from the EAP server to the peer. Because these messages are textual messages, it may be hard for the peer to present them in the user's preferred language. Therefore, EAP/AKA uses a separate EAP/AKA message subtype to transmit localizable notification codes instead of the EAP- Request/Notification packet. The EAP server MAY issue an EAP-Request/AKA-Notification packet to the peer. The peer MAY show a notification message to the user and the peer MUST respond to the EAP server with an EAP-Response/AKA- Notification packet, even if the peer did not recognize the notification code. The notification code is a 16-bit number. The most significant bit is called the Failure bit (F bit). The F bit specifies whether the notification implies failure. The code values with the F bit set to zero (code values 0...32767) are used on unsuccessful cases. The receipt of a notification code from this range implies failed authentication, so the peer can use the notification as a failure indication. After receiving the EAP-Response/AKA-Notification for these notification codes, the server MUST send the EAP-Failure packet. The receipt of a notification code with the F bit set to one (values 32768...65536) does not imply failure, so the peer MUST NOT change its state when it receives such a notification. (This version of the protocol does not specify any notification codes with the F bit set to one.) The second most significant bit of the notification code is called the Phase bit (P bit). It specifies at which phase of the EAP/AKA exchange the notification can be used. If the P bit is set to zero, the notification can only be used after the EAP/AKA-Challenge round in full authentication or the EAP/AKA-Reauthentication round in reautentication. For these notifications, the AT_MAC attribute MUST be included in both EAP-Request/AKA-Notification and EAP- Response/AKA-Notification. If the P bit is set to one, the notification can only by used before the EAP/AKA-Challenge round in full authentication or the EAP/AKA- Reauthentication round in reauthentication. For these notifications, the AT_MAC attribute MUST NOT be included in either EAP-Request/AKA- Notification or EAP-Response/AKA-Notification. (This version of the protocol does not specify any notification codes with the P bit set to one.) Some of the notification codes are authorization related and hence not usually considered as part of the responsibility of an EAP method. However, they are included as part of EAP/AKA because there are currently no other ways to convey this information to the user Arkko and Haverinen Expires 27 April, 2004 [Page 31] EAP AKA Authentication 27 October, 2003 in a localizable way, and the information is potentially useful for the user. An EAP/AKA server implementation may decide never to send these EAP/AKA notifications. 4.4. Error Cases This section specifies the operation of the peer and the server in error cases. The subsections below require the EAP/AKA peer and server to send an error packet (EAP-Response/AKA-Client-Error or EAP Failure) in error cases. However, implementations SHOULD NOT rely upon the correct error reporting behavior of the peer, authenticator, or the server. It is possible for error and other messages to be lost in transit or for a malicious participant to attempt to consume resources by not issuing error messages. Both the peer and the EAP server SHOULD have a mechanism to clean up state even if an error message or EAP Success is not received after a timeout period. 4.4.1. Peer Operation Two special error messages have been specified for error cases that are related to the processing of the UMTS AKA AUTN parameter, as described in Section 3: (1) if the peer does not accept AUTN, the peer responds with EAP-Response/AKA-Authentication-Reject (Section 6.5), and the server issues EAP Failure, and (2) if the peer detects that the sequence number in AUTN is not correct, the peer responds with EAP-Response/AKA-Synchronization-Failure (Section 6.6), and the server proceeds with a new EAP-Request/AKA-Challenge. In other error cases, when an EAP/AKA peer detects an error in a received EAP/AKA packet, the EAP/AKA peer responds with the EAP- Response/AKA-Client-Error packet. In response to the EAP- Response/AKA-Client-Error, the EAP server MUST issue the EAP Failure packet and the authentication exchange terminates. By default, the peer uses the client error code 0, "unable to process packet". This error code is used in the following cases: - the peer is not able to parse the EAP request, i.e. the EAP request is malformed - the peer encountered a malformed attribute - wrong attribute types or duplicate attributes have been included in the EAP request - a mandatory attribute is missing - unrecognized non-skippable attribute - unrecognized or unexpected EAP/AKA Subtype in the EAP request - invalid AT_MAC Arkko and Haverinen Expires 27 April, 2004 [Page 32] EAP AKA Authentication 27 October, 2003 - invalid AT_CHECKCODE - invalid pad bytes in AT_PADDING - the peer does not want to process AT_PERMANENT_ID_REQ 4.4.2. Server Operation If an EAP/AKA server detects an error in a received EAP/AKA response, the server MUST issue the EAP Failure packet and the authentication exchange terminates. The errors cases when the server issues an EAP Failure include the following: - the server is not able to parse the peer's EAP response - the server encounters a malformed attribute, a non-recognized non- skippable attribute, or a duplicate attribute - a mandatory attribute is missing or an invalid attribute was included - unrecognized or unexpected EAP/AKA Subtype in the EAP Response - invalid AT_MAC - invalid AT_CHECKCODE - invalid AT_COUNTER 4.4.3. Failure As normally in EAP, the EAP server sends the EAP-Failure packet to the peer when the authentication procedure fails on the EAP Server. In EAP/AKA, this may occur for example if the EAP server does not recognize the peer identity, or if the EAP server is not able to obtain the authentication vectors for the subscriber or the authentication exchange times out. The server may also send EAP Failure if there is an error in the received EAP/AKA response, as discussed in Section 4.4.2. The server can send EAP-Failure at any time in the EAP exchange. The peer MUST process EAP-Failure. 4.4.4. EAP Success On full authentication, the server can only send EAP-Success after the EAP/AKA-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/AKA- Challenge packet. On re-authentication, EAP-Success can only be sent after the EAP/AKA-Reauthentication round. The peer MUST silently discard any EAP-Success packets if they are received before the peer has Arkko and Haverinen Expires 27 April, 2004 [Page 33] EAP AKA Authentication 27 October, 2003 successfully authenticated the server and sent the EAP-Response/AKA- Reauthentication packet. If the peer receives an EAP/AKA notification (section 4.3) that indicates failure, then the peer MUST no longer accept the EAP- Success packet even if the server authentication was successfully completed. 4.5. Key Generation This section specifies how keying material is generated. On EAP AKA full authentication, a Master Key (MK) is derived from the underlying UMTS AKA values (CK and IK keys), and the identity as follows. MK = SHA1(Identity|IK|CK) In the formula above, the "|" character denotes concatenation. Identity denotes the peer identity string without any terminating null characters. It is the identity from the AT_IDENTITY attribute from the last EAP-Response/AKA-Identity packet, or, if AT_IDENTITY was not used, the identity from the EAP-Response/Identity packet. The identity string is included as-is, without any changes and including the possible identity decoration. The hash function SHA-1 is specified in [SHA-1]. The Master Key is fed into a Pseudo-Random number Function (PRF), which generates separate Transient EAP Keys (TEKs) for protecting EAP AKA packets, as well as a Master Session Key (MSK) for link layer security and an Extended Master Session Key (EMSK) for other purposes. On re-authentication, the same TEKs MUST be used for protecting EAP packets, but a new MSK and a new EMSK MUST be derived from the original MK and new values exchanged in the re- authentication. EAP AKA requires two TEKs for its own purposes, the authentication key K_aut to be used with the AT_MAC attribute, and the encryption key K_encr, to be used with the AT_ENCR_DATA attribute. The same K_aut and K_encr keys are used in full authentication and subsequent re-authentications. Key derivation is based on the random number generation specified in NIST Federal Information Processing Standards (FIPS) Publication 186-2 [PRF]. The pseudo-random number generator is specified in the change notice 1 (2001 October 5) of [PRF] (Algorithm 1). As specified in the change notice (page 74), when Algorithm 1 is used as a general-purpose pseudo-random number generator, the "mod q" term in step 3.3 is omitted. The function G used in the algorithm is constructed via Secure Hash Standard as specified in Appendix 3.3 of the standard. It should be noted that the function G is very similar to SHA-1, but the message padding is different. Please refer to [PRF] for full details. For convenience, the random number algorithm with the correct modification is cited in Annex A. Arkko and Haverinen Expires 27 April, 2004 [Page 34] EAP AKA Authentication 27 October, 2003 160-bit XKEY and XVAL values are used, so b = 160. On each full authentication, the Master Key is used as the initial secret seed- key XKEY. The optional user input values (XSEED_j) in step 3.1 are set to zero. The resulting 320-bit random numbers x_0, x_1, ..., x_m-1 are concatenated and partitioned into suitable-sized chunks and used as keys in the following order: K_encr (128 bits), K_aut (128 bits), Master Session Key (64 bytes), Extended Master Session Key (64 bytes). On re-authentication, the same pseudo-random number generator can be used to generate a new Master Session Key and new Initialization Vectors. The seed value XKEY' is calculated as follows: XKEY' = SHA1(Identity|counter|NONCE_S| MK) In the formula above, the Identity denotes the re-authentication identity, without any terminating null characters, from the AT_IDENTITY attribute of the EAP-Response/AKA-Identity packet, or, if EAP-Response/AKA-Identity was not used on re-authentication, the identity string from the EAP-Response/Identity packet. The counter denotes the counter value from AT_COUNTER attribute used in the EAP- Response/AKA-Reauthentication 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/AKA-Reauthentication packet. The MK is the Master Key derived on the preceding full authentication. The pseudo-random number generator is run with the new seed value XKEY', and the resulting 320-bit random numbers x_0, x_1, ..., x_m-1 are concatenated and partitioned into 64-byte chunks and used as the new 64-byte Master Session Key and the new 64-byte Extended Master Session Key. The first 32 bytes of the MSK can be used as the Pairwise Master Key (PMK) for IEEE 802.11i. When the RADIUS attributes specified in [RFC 2548] are used to transport keying material, then the first 32 bytes of the MSK correspond to MS-MPPE-RECV-KEY and the second 32 bytes to MS-MPPE- SEND-KEY. In this case, only 64 bytes of keying material (the MSK) are used. 5. Message Format and Protocol Extensibility 5.1. Message Format As specified in [EAP], EAP packets begin with the Code, Identifiers, Length, and Type fields, which are followed by EAP method specific Type-Data. The Code field in the EAP header is set to 1 for EAP requests, and to 2 for EAP Responses. The usage of the Length and Identifier fields in the EAP header is also specified in [EAP]. In EAP/AKA, the Type field is set to 23. Arkko and Haverinen Expires 27 April, 2004 [Page 35] EAP AKA Authentication 27 October, 2003 In EAP/AKA, the Type-Data begins with an EAP/AKA header that consists of a 1-octet Subtype field, and a 2-octet reserved field. The Subtype values used in EAP/AKA are defined in Section 8. The formats of the EAP header and the EAP/AKA 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, immediately following the EAP/AKA 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Attribute Type | Length | Value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Attribute Type Indicates the particular type of attribute. The attribute type values are listed in Section 8. Length Indicates the length of this attribute in multiples of 4 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 is 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/AKA peer encounters a non- skippable attribute type that the peer does not recognize, the peer MUST send the EAP-Response/AKA-Client-Error packet, and the authentication exchange terminates. If an EAP/AKA server encounters a non-skippable attribute that the server does not recognize, then the server sends the EAP Failure packet and the authentication exchange terminates. When an attribute numbered in the range 128 through 255 is encountered but not recognized that particular attribute is ignored, Arkko and Haverinen Expires 27 April, 2004 [Page 36] EAP AKA Authentication 27 October, 2003 but the rest of the attributes and message data MUST still be processed. The Length field of the attribute is used to skip the attribute value when searching for the next attribute. These attributes are called skippable attributes. Unless otherwise specified, the order of the attributes in an EAP AKA message is insignificant, and an EAP AKA implementation should not assume a certain order to be used. Attributes can be encapsulated within other attributes. In other words, the value field of an attribute type can be specified to contain other attributes. 5.2. Protocol Extensibility EAP/AKA can be extended by specifying new attribute types. If skippable attributes are used, it is possible to extend the protocol without breaking old implementations. As specified in Section 7.4, if new attributes are specified for EAP-Request/AKA-Identity or EAP- Response/AKA-Identity, then the AT_CHECKCODE MUST be used to integrity protect the new attributes. When specifying new attributes, it should be noted that EAP/AKA does not support message fragmentation. Hence, the sizes of the new extensions MUST be limited so that the maximum transfer unit (MTU) of the underlying lower layer is not exceeded. According to [EAP], lower layers must provide an EAP MTU of 1020 bytes or greater, so any extensions to EAP/AKA SHOULD NOT exceed the EAP MTU of 1020 bytes. EAP/AKA packets do not include a version field. However, should there be a reason to revise this protocol in the future, new non- skippable or skippable attributes could be specified in order to implement revised EAP/AKA versions in a backward-compatible manner. It is possible to introduce version negotiation in the EAP- Request/AKA-Identity and EAP-Response/AKA-Identity messages by specifying new skippable attributes. 6. Messages This section specifies the messages used in EAP/AKA. 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. Message format is specified in Section 5.1. 6.1. EAP-Request/AKA-Identity The EAP/AKA-Identity roundtrip MAY used for obtaining the peer identity to the server. As discussed in Section 4.1, several AKA- Identity rounds may be required in order to obtain a valid peer identity. Arkko and Haverinen Expires 27 April, 2004 [Page 37] EAP AKA Authentication 27 October, 2003 The server MUST include one of the following identity requesting attributes: AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, 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 previously issued an EAP-Request/AKA-Identity message with the AT_PERMANENT_ID_REQ attribute, and if the server has received a response from the peer, then the server MUST NOT issue a new EAP-Request/AKA-Identity packet. If the server has previously issued an EAP-Request/AKA-Identity message with the AT_FULLAUTH_ID_REQ attribute, and if the server has received a response from the peer, then the server MUST NOT issue a new EAP-Request/AKA-Identity packet with the AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ attributes. If the server has previously issued an EAP-Request/AKA-Identity message with the AT_ANY_ID_REQ attribute, and if the server has received a response from the peer, then the server MUST NOT issue a new EAP-Request/AKA-Identity packet with the AT_ANY_ID_REQ. This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA. 6.2. EAP-Response/AKA-Identity The peer sends EAP-Response/AKA-Identity in response to a valid EAP- Request/AKA-Identity from the server. The peer MUST include the AT_IDENTITY attribute. The usage of AT_IDENITY is defined in Section 4.1. This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA. 6.3. EAP-Request/AKA-Challenge The server sends the EAP-Request/AKA-Challenge on full authentication after successfully obtaining the subscriber identity. The AT_RAND attribute MUST be included. AT_MAC MUST be included. In EAP-Request/AKA-Challenge, there is no message-specific data covered by the MAC, see Section 7.2. The AT_CHECKCODE attribute MAY be included, and in certain cases specified in Section 7.4, it MUST be included. The EAP-Request/AKA-Challenge packet MAY include encrypted attributes for identity privacy and for communicating the next re- authentication identity. In this case, the AT_IV and AT_ENCR_DATA attributes are included (Section 7.3). The plaintext of the AT_ENCR_DATA value field consist of nested attributes. The nested attributes MAY include AT_PADDING (as specified in Section 7.3). If the server supports identity privacy Arkko and Haverinen Expires 27 April, 2004 [Page 38] EAP AKA Authentication 27 October, 2003 and wants to communicate a pseudonym to the peer for the next full authentication, then the nested encrypted attributes include the AT_NEXT_PSEUDONYM attribute. If the server supports re- authentication and wants to communicate a re-authentication identity to the peer, then the nested encrypted attributes include the AT_NEXT_REAUTH_ID attribute. Later versions of this protocol MAY specify additional attributes to be included within the encrypted data. 6.4. EAP-Response/AKA-Challenge The peer sends EAP-Response/AKA-Challenge in response to a valid EAP-Request/AKA-Challenge. The AT_MAC attribute MUST be included. In EAP-Response/AKA- Challenge, there is no message-specific data covered by the MAC, see Section 7.2. The AT_RES attribute MUST be included. The AT_CHECKCODE attribute MAY be included, and in certain cases specified in Section 7.4, it MUST be included. 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/AKA-Challenge messages that include these attributes even if the server did not implement these optional attributes. 6.5. EAP-Response/AKA-Authentication-Reject The peer sends the EAP-Response/AKA-Authentication-Reject packet if it does not accept the AUTN parameter. This version of the protocol does not specify any attributes for this message. Future versions of the protocol MAY specify attributes for this message. The AT_MAC, AT_ENCR_DATA, or AT_IV attributes MUST NOT be used in this message. 6.6. EAP-Response/AKA-Synchronization-Failure The peer sends the EAP-Response/AKA-Synchronization-Failure, when the sequence number in the AUTN parameter is incorrect. The peer MUST include the AT_AUTS attribute. Future versions of the protocol MAY specify other additional attributes for this message. The AT_MAC, AT_ENCR_DATA, or AT_IV attributes MUST NOT be used in this message. 6.7. EAP-Request/AKA-Reauthentication Arkko and Haverinen Expires 27 April, 2004 [Page 39] EAP AKA Authentication 27 October, 2003 The server sends the EAP-Request/AKA-Reauthentication message if it wants to use re-authentication, and if it has received a valid re- authentication identity in EAP-Response/Identity or EAP- Response/AKA-Identity. The AT_MAC attribute MUST be included. No message-specific data is included in the MAC calculation, see Section 7.2. The AT_CHECKCODE attribute MAY be included, and in certain cases specified in Section 7.4, it MUST be included. The AT_IV and AT_ENCR_DATA attributes MUST be included. The plaintext consists of the following nested encrypted attributes, which MUST be included: AT_COUNTER and AT_NONCE_S. In addition, the nested encrypted attributes MAY include the following attributes: AT_NEXT_REAUTH_ID and AT_PADDING. 6.8. EAP-Response/AKA-Reauthentication The client sends the EAP-Response/AKA-Reauthentication packet in response to a valid EAP-Request/AKA-Reauthentication. The AT_MAC attribute MUST be included. For EAP-Response/AKA- Reauthentication, the MAC code is calculated over the following data: EAP packet| NONCE_S. The EAP packet is represented as specified in Section 5.1. It is followed by the 16-byte NONCE_S value from the server's AT_NONCE_S attribute. The AT_CHECKCODE attribute MAY be included, and in certain cases specified in Section 7.4, it MUST be included. The AT_IV and AT_ENCR_DATA attributes MUST be included. The nested encrypted attributes MUST include the AT_COUNTER attribute. The AT_COUNTER_TOO_SMALL attribute MAY be included in the nested encrypted attributes, and it is included in cases specified in Section 4.2. The AT_PADDING attribute MAY be included. 6.9. EAP-Response/AKA-Client-Error The peer sends EAP-Response/AKA-Client-Error in error cases, as specified in Section 4.4.1. The AT_CLIENT_ERROR_CODE attribute MUST be included. The AT_MAC, AT_IV, or AT_ENCR_DATA attributes MUST NOT be used with this packet. 6.10. EAP-Request/AKA-Notification The usage of this message is specified in Section 4.3. The AT_NOTIFICATION attribute MUST be included. Arkko and Haverinen Expires 27 April, 2004 [Page 40] EAP AKA Authentication 27 October, 2003 The AT_MAC attribute is included in cases discussed in Section 4.3. No message-specific data is included in the MAC calculation. See Section 7.2. Later versions of this protocol MAY make use of the AT_ENCR_DATA and AT_IV attributes in this message to include encrypted (skippable) attributes. These attributes MAY be included only if the P bit of the notification code in AT_NOTIFICATION is set to zero. 6.11. EAP-Response/AKA-Notification The usage of this message is specified in Section 4.3. Because this packet is only an acknowledgement of EAP-Request/AKA-Notification, it does not contain any mandatory attributes. The AT_MAC attribute is included in cases described in Section 4.3. No message-specific data is included in the MAC calculation. See Section 7.2. Later versions of this protocol MAY make use of the AT_ENCR_DATA and AT_IV attributes in this message to include encrypted (skippable) attributes. These attributes MAY be included only if the P bit of the notification code in the AT_NOTIFICATION attribute of the server's EAP-Request/AKA-Notification packet is set to zero. 7. Attributes This section specifies the format of message attributes. The attribute type numbers are specified in Section 8. 7.1. Table of Attributes The following table provides a guide to which attributes may be found in which kinds of messages, and in what quantity. Messages are denoted with numbers in parentheses as follows: (1) EAP-Request/AKA- Identity, (2) EAP-Response/AKA-Identity, (3) EAP-Request/AKA- Challenge, (4) EAP-Response/AKA-Challenge, (5) EAP-Request/AKA- Notification, (6) EAP-Response/AKA-Notification, (7) EAP- Response/AKA-Client-Error (8) EAP-Request/AKA-Reauthentication, (9) EAP-Response/AKA-Re-authentication, (10) EAP-Response/AKA- Authentication-Reject, and (11) EAP-Response/AKA-Synchronization- Failure. The column denoted with "E" indicates whether the attribute is a nested attribute that MUST be included within AT_ENCR_DATA. "0" indicates that the attribute MUST NOT be included in the message, "1" indicates that the attribute MUST be included in the message, "0-1" indicates that the attribute is sometimes included in the message, and "0*" indicates that the attribute is not included in the message in cases specified in this document, but MAY be included in the future versions of the protocol. Arkko and Haverinen Expires 27 April, 2004 [Page 41] EAP AKA Authentication 27 October, 2003 Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)(11) E AT_MAC 0 0 1 1 0-1 0-1 0 1 1 0 0 N AT_IV 0 0 0-1 0* 0* 0* 0 1 1 0 0 N AT_ENCR_DATA 0 0 0-1 0* 0* 0* 0 1 1 0 0 N AT_PADDING 0 0 0-1 0* 0* 0* 0 0-1 0-1 0 0 Y AT_CHECKCODE 0 0 0-1 0-1 0 0 0 0-1 0-1 0 0 N AT_PERMANENT_ID_REQ 0-1 0 0 0 0 0 0 0 0 0 0 N AT_ANY_ID_REQ 0-1 0 0 0 0 0 0 0 0 0 0 N AT_FULLAUTH_ID_REQ 0-1 0 0 0 0 0 0 0 0 0 0 N AT_IDENTITY 0 0-1 0 0 0 0 0 0 0 0 0 N AT_RAND 0 0 1 0 0 0 0 0 0 0 0 N AT_AUTN 0 0 1 0 0 0 0 0 0 0 0 N AT_RES 0 0 0 1 0 0 0 0 0 0 0 N AT_AUTS 0 0 0 0 0 0 0 0 0 0 1 N AT_NEXT_PSEUDONYM 0 0 0-1 0 0 0 0 0 0 0 0 Y AT_NEXT_REAUTH_ID 0 0 0-1 0 0 0 0 0-1 0 0 0 Y AT_COUNTER 0 0 0 0 0 0 0 1 1 0 0 Y AT_COUNTER_TOO_SMALL 0 0 0 0 0 0 0 0 0-1 0 0 Y AT_NONCE_S 0 0 0 0 0 0 0 1 0 0 0 Y AT_NOTIFICATION 0 0 0 0 1 0 0 0 0 0 0 N AT_CLIENT_ERROR_CODE 0 0 0 0 0 0 1 0 0 0 0 N It should be noted that attributes AT_PERMANENT_ID_REQ, AT_ANY_ID_REQ and AT_FULLAUTH_ID_REQ are mutually exclusive, so that only one of them can be included at the same time. If one of the attributes AT_IV and AT_ENCR_DATA is included, then both of the attributes MUST be included. 7.2. AT_MAC The AT_MAC attribute is used for EAP/AKA message authentication. Section 6 specifies which messages AT_MAC MUST be included. The value field of the AT_MAC attribute contains two reserved bytes followed by a keyed message authentication code (MAC). The MAC is calculated over the whole EAP packet, concatenated with optional message-specific data, with the exception that the value field of the MAC attribute is set to zero when calculating the MAC. The EAP packet includes the EAP header that begins with the Code field, the EAP/AKA header that begins with the Subtype field, and all the attributes, as specified in Section 5.1. The reserved bytes in AT_MAC are set to zero when sending and ignored on reception. The contents of the message-specific data that may be included in the MAC calculation are specified separately for each EAP/AKA message in Section 6. The format of the AT_MAC attribute is shown below. Arkko and Haverinen Expires 27 April, 2004 [Page 42] EAP AKA Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_MAC | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | MAC | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The MAC algorithm is HMAC-SHA1-128 [RFC 2104] keyed hash value. (The HMAC-SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by truncating the output to 16 bytes. Hence, the length of the MAC is 16 bytes.) The derivation of the authentication key (K_aut) used in the calculation of the MAC is specified in Section 4.5. When the AT_MAC attribute is included in an EAP/AKA message, the recipient MUST process the AT_MAC attribute before looking at any other attributes. If the message authentication code is invalid, then the recipient MUST ignore all other attributes in the message and operate as specified in Section 4.4. 7.3. AT_IV, AT_ENCR_DATA and AT_PADDING AT_IV and AT_ENCR_DATA attributes can be used to transmit encrypted information between the EAP/SIM peer and server. The value field of AT_IV contains two reserved bytes followed by a 16-byte initialization vector required by the AT_ENCR_DATA attribute. The reserved bytes are set to zero when sending and ignored on reception. The AT_IV attribute MUST be included if and only if the AT_ENCR_DATA is included. Section 4.4 specifies the operation if a packet that does not meet this condition is encountered. The sender of the AT_IV attribute chooses the initialization vector by random. The sender MUST NOT reuse the initialization vector value from previous EAP AKA packets and the sender MUST choose it freshly for each AT_IV attribute. The sender SHOULD use a good source of randomness to generate the initialization vector. Please see [RFC 1750] for more information about generating random numbers for security applications. The format of AT_IV is shown below. Arkko and Haverinen Expires 27 April, 2004 [Page 43] EAP AKA Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IV | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Initialization Vector | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the AT_ENCR_DATA attribute consists of two reserved bytes followed by cipher text bytes encrypted using the Advanced Encryption Standard (AES) [AES] in the Cipher Block Chaining (CBC) mode of operation using the initialization vector from the AT_IV attribute. The reserved bytes are set to zero when sending and ignored on reception. Please see [CBC] for a description of the CBC mode. The format of the AT_ENCR_DATA attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_ENCR_DATA | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Encrypted Data . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The derivation of the encryption key (K_encr) is specified in Section 4.5. The plaintext consists of nested EAP/AKA attributes. The encryption algorithm requires the length of the plaintext to be a multiple of 16 bytes. The sender may need to include the AT_PADDING attribute as the last attribute within AT_ENCR_DATA. The AT_PADDING attribute is not included if the total length of other nested attributes within the AT_ENCR_DATA attribute is a multiple of 16 bytes. As usual, the Length of the Padding attribute includes the Attribute Type and Attribute Length fields. The length of the Padding attribute is 4, 8 or 12 bytes. It is chosen so that the length of the value field of the AT_ENCR_DATA attribute becomes a multiple of 16 bytes. The actual pad bytes in the value field are set to zero (0x00) on sending. The recipient of the message MUST verify that the pad bytes are set to zero. If this verification fails on the peer, then it MUST send the EAP-Response/AKA-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 EAP Failure, and the authentication exchange terminates. The format of the AT_PADDING attribute is shown below. Arkko and Haverinen Expires 27 April, 2004 [Page 44] EAP AKA Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_PADDING | Length | Padding... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 7.4. AT_CHECKCODE The AT_MAC attribute is not used in the very first EAP/AKA messages during the AKA-Identity round, because keying material has not been derived yet. The peer and the server may exchange one or more pairs of EAP/AKA messages of the Subtype AKA-Identity before keys are derived and before the AT_MAC attribute can be applied. The EAP/AKA- Identity messages may also be used upon re-authentication. The AT_CHECKCODE attribute MAY be used to protect the EAP/AKA- Identity messages. AT_CHECKCODE is included in EAP-Request/AKA- Challenge and/or EAP-Response/AKA-Challenge upon full authentication. In re-authentication, AT_CHECKCODE MAY be included in EAP-Request/AKA-Reauthentication and/or EAP-Response/AKA- Reauthentication. Because the AT_MAC attribute is used in these messages, AT_CHECKCODE will be integrity protected with AT_MAC. The format of the AT_CHECKCODE attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_CHECKCODE | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Checkcode (0 or 20 bytes) | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of AT_CHECKCODE begins with two reserved bytes, which may be followed by a 20-byte checkcode. If the checkcode is not included in AT_CHECKCODE, then the attribute indicates that no EAP/AKA-Identity messages were exchanged. This may occur in both full authentication and re-authentication. The reserved bytes are set to zero when sending and ignored on reception. The checkcode is a hash value, calculated with SHA1 [SHA-1], over all EAP-Request/AKA-Identity and EAP-Response/ AKA-Identity packets exchanged in this authentication exchange. The packets are included in the order that they were transmitted, that is, starting with the first EAP-Request/ AKA-Identity message, followed by the Arkko and Haverinen Expires 27 April, 2004 [Page 45] EAP AKA Authentication 27 October, 2003 corresponding EAP-Response/ AKA-Identity, followed by the second EAP-Request/ AKA-Identity (if used) etc. EAP packets are included in the hash calculation "as-is", as they were transmitted or received. All reserved bytes, padding bytes etc. that are specified for various attributes are included as such, and the receiver must not reset them to zero. No delimiter bytes, padding or any other framing are included between the EAP packets when calculating the checkcode. Messages are included in request/response pairs; in other words only full "round trips" are included. Packets that are silently discarded are not included. The EAP server must only include an EAP- Request/AKA-Identity in the calculation once it has received a corresponding response, with the same Identifier value. Retransmissions or requests to which the server does not receive response are not included. The peer must include the EAP-Request/AKA-Identity and the corresponding response in the calculation only if the peer receives a subsequent EAP-Request/AKA-Challenge, or a follow-up EAP- Request/AKA-Identity with different attributes (attribute types) than in the first EAP-Request/AKA-Identity. After sending EAP- Response/AKA-Identity, if the peer receives another EAP-Request/AKA- Identity with the same attributes as in the previous request, then the peer's response to the first request must have been lost. In this case the peer must not include the first request and its response in the calculation of the checkcode. The AT_CHECKCODE attribute is optional to implement. It is specified in order to allow protecting the EAP/ AKA-Identity messages and any future extensions to them. The implementation of AT_CHECKCODE is RECOMMENDED. If the receiver of AT_CHECKCODE implements this attribute, then the receiver MUST check that the checkcode is correct. If the checkcode is invalid, the receiver must operate as specified in Section 4.4. If the EAP/AKA-Identity messages are extended with new attributes then AT_CHECKCODE MUST be implemented and used. More specifically, if the server includes any other attributes than AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ or AT_ANY_ID_REQ in the EAP- Request/AKA-Identity packet, then the server MUST include AT_CHECKCODE in EAP-Request/AKA-Challenge or EAP-Request/AKA- Reauthentication. If the peer includes any other attributes than AT_IDENTITY in the EAP-Response/AKA-Identity message, then the peer MUST include AT_CHECKCODE in EAP-Response/AKA-Challenge or EAP- Response/AKA-Reauthentication. If the server implements the processing of any other attribute than AT_IDENTITY for the EAP-Response/AKA-Identity message, then the server MUST implement AT_CHECKCODE. In this case, if the server receives any other attribute than AT_IDENTITY in the EAP- Response/AKA-Identity message, then the server MUST check that Arkko and Haverinen Expires 27 April, 2004 [Page 46] EAP AKA Authentication 27 October, 2003 AT_CHECKCODE is present in EAP-Response/AKA-Challenge or EAP- Response/AKA-Reauthentication. The operation when a mandatory attribute is missing is specified in Section 4.4. Similarly, if the peer implements the processing of any other attribute than AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ or AT_ANY_ID_REQ for the EAP-Request/AKA-Identity packet, then the peer MUST implement AT_CHECKCODE. In this case, if the peer receives any other attribute than AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ or AT_ANY_ID_REQ in the EAP-Request/AKA-Identity packet, then the peer MUST check that AT_CHECKCODE is present in EAP-Request/AKA-Challenge or EAP-Request/AKA-Reauthentication. The operation when a mandatory attribute is missing is specified in Section 4.4. 7.5. AT_PERMANENT_ID_REQ The format of the AT_PERMANENT_ID_REQ attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_PERM..._REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The use of the AT_PERMANENT_ID_REQ is defined in Section 4.1. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. 7.6. AT_ANY_ID_REQ The format of the AT_ANY_ID_REQ attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_ANY_ID_REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The use of the AT_ANY_ID_REQ is defined in Section 4.1. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. 7.7. AT_FULLAUTH_ID_REQ The format of the AT_FULLAUTH_ID_REQ attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_ANY_ID_REQ | Length = 1 | Reserved | +---------------+---------------+-------------------------------+ Arkko and Haverinen Expires 27 April, 2004 [Page 47] EAP AKA Authentication 27 October, 2003 The use of the AT_FULLAUTH_ID_REQ is defined in Section 4.1. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. 7.8. AT_IDENTITY The format of the AT_IDENTITY attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IDENTITY | Length | Actual Identity Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Identity . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The use of the AT_IDENTITY is defined in Section 4.1. The value field of this attribute begins with 2-byte actual identity length, which specifies the length of the identity in bytes. This field is followed by the subscriber identity of the indicated actual length. The identity is the permanent identity, a pseudonym identity or a re-authentication identity. The identity format is specified in Section 4.1.1. The same identity format is used in the AT_IDENTITY attribute and the EAP-Response/Identity packet, with the exception that the peer MUST NOT decorate the identity it includes in AT_IDENTITY. The identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the identity with zero bytes when necessary. 7.9. AT_RAND The format of the AT_RAND attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_RAND | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | RAND | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains two reserved bytes followed by the AKA RAND parameter, 16 bytes (128 bits). The reserved bytes are set to zero when sending and ignored on reception. Arkko and Haverinen Expires 27 April, 2004 [Page 48] EAP AKA Authentication 27 October, 2003 7.10. AT_AUTN The format of the AT_AUTN 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_AUTN | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | AUTN | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains two reserved bytes followed by the AKA AUTN parameter, 16 bytes (128 bits). The reserved bytes are set to zero when sending and ignored on reception. 7.11. AT_RES The format of the AT_RES 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_RES | Length | RES Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | | RES | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute begins with the 2-byte RES Length, which is identifies the exact length of the RES in bits. The RES length is followed by the UMTS AKA RES parameter. According to [TS 33.105] the length of the AKA RES can vary between 32 and 128 bits. Because the length of the AT_RES attribute must be a multiple of 4 bytes, the sender pads the RES with zero bits where necessary. 7.12. AT_AUTS The format of the AT_AUTS attribute is shown below. Arkko and Haverinen Expires 27 April, 2004 [Page 49] EAP AKA Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| | AT_AUTS | Length = 4 | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | AUTS | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains the AKA AUTS parameter, 112 bits (14 bytes). 7.13. AT_NEXT_PSEUDONYM The format of the AT_NEXT_PSEUDONYM attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_PSEU..| Length | Actual Pseudonym Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Pseudonym . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute begins with 2-byte actual pseudonym length which specifies the length of the following pseudonym in bytes. This field is followed by a pseudonym username that the peer can use in the next authentication. The username MUST NOT include any realm portion. The username does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the pseudonym with zero bytes when necessary. The username encoding MUST follow the UTF-8 transformation format [RFC2279]. 7.14. AT_NEXT_REAUTH_ID The format of the AT_NEXT_REAUTH_ID attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_REAU..| Length | Actual Re-Auth Identity Length| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Re-authentication Username . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Arkko and Haverinen Expires 27 April, 2004 [Page 50] EAP AKA Authentication 27 October, 2003 The value field of this attribute begins with 2-byte actual re- authentication identity length which specifies the length of the following re-authentication identity in bytes. This field is followed by a re-authentication identity that the peer can use in the next re-authentication, as described in Section 4.2. In environments where a realm portion is required, the re- authentication identity includes both a username portion and a realm name portion. The re-authentication identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the re-authentication identity with zero bytes when necessary. The identity encoding MUST follow the UTF-8 transformation format [RFC2279]. 7.15. AT_COUNTER The format of the AT_COUNTER attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER | Length = 1 | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the AT_COUNTER attribute consists of a 16-bit unsigned integer counter value, represented in network byte order. 7.16. AT_COUNTER_TOO_SMALL The format of the AT_COUNTER_TOO_SMALL attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER...| Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute consists of two reserved bytes, which are set to zero upon sending and ignored upon reception. 7.17. AT_NONCE_S The format of the AT_NONCE_S attribute is shown below. Arkko and Haverinen Expires 27 April, 2004 [Page 51] EAP AKA Authentication 27 October, 2003 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER | Length = 1 | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NONCE_S | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | NONCE_S | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the AT_NONCE_S attribute contains two reserved bytes followed by a random number generated by the server (16 bytes) freshly for this EAP/AKA re-authentication. The random number is used as challenge for the peer and also a seed value for the new keying material. The reserved bytes are set to zero upon sending and ignored upon reception. The server MUST choose the NONCE_S value freshly for each EAP/AKA re-authentication exchange. The server SHOULD use a good source of randomness to generate NONCE_S. Please see [RFC 1750] for more information about generating random numbers for security applications. 7.18. AT_NOTIFICATION The format of the AT_NOTIFICATION attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_NOTIFICATION| Length = 1 |F|P| Notification Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains a two-byte notification code. The first and second bit (F and P) of the notification code are interpreted as described in Section 4.3. 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/AKA is applied. 1026 - User has been temporarily denied access to the requested service. (Implies failure, used after the challenge round) 1031 - User has not subscribed to the requested service (implies failure, used after the challenge round) Arkko and Haverinen Expires 27 April, 2004 [Page 52] EAP AKA Authentication 27 October, 2003 7.19. 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 8. IANA and Protocol Numbering Considerations The realm name "owlan.org" has been reserved for NAI realm names generated from the IMSI. IANA has assigned the number 23 for EAP AKA authentication. EAP AKA messages include a Subtype field. The following Subtypes are specified: AKA-Challenge...................................1 AKA-Authentication-Reject.......................2 AKA-Synchronization-Failure.....................4 AKA-Identity....................................5 AKA-Notification...............................12 AKA-Reauthentication...........................13 AKA-Client-Error...............................14 Arkko and Haverinen Expires 27 April, 2004 [Page 53] EAP AKA Authentication 27 October, 2003 The Subtype-specific data is composed of attributes, which have attribute type numbers. The following attribute types are specified: AT_RAND.........................................1 AT_AUTN.........................................2 AT_RES..........................................3 AT_AUTS.........................................4 AT_PADDING......................................6 AT_PERMANENT_ID_REQ............................10 AT_MAC.........................................11 AT_NOTIFICATION................................12 AT_ANY_ID_REQ..................................13 AT_IDENTITY....................................14 AT_FULLAUTH_ID_REQ.............................17 AT_COUNTER.....................................19 AT_COUNTER_TOO_SMALL...........................20 AT_NONCE_S.....................................21 AT_CLIENT_ERROR_CODE...........................22 AT_IV.........................................129 AT_ENCR_DATA..................................130 AT_NEXT_PSEUDONYM.............................132 AT_NEXT_REAUTH_ID.............................133 AT_CHECKCODE..................................134 The AT_NOTIFICATION attribute contains a notification code value. Values 1024, 1026 and 1031 have been specified in Section 7.18 of this document. The AT_CLIENT_ERROR_CODE attribute contains a client error code. Value 0 has been specified in Section 7.19 of this document. All requests for value assignment from the various number spaces described in this document require proper documentation, according to the "Specification Required" policy described in [RFC 2434]. Requests must be specified in sufficient detail so that interoperability between independent implementations is possible. Possible forms of documentation include, but are not limited to, RFCs, the products of another standards body (e.g. 3GPP), or permanently and readily available vendor design notes. EAP AKA and EAP SIM [EAP SIM] are "sister" protocols with similar message structure and protocol numbering spaces. Many attributes and message Subtypes have the same protocol numbers in these two protocols. Hence, it is recommended that the same protocol number value SHOULD NOT be allocated for two different purposes in EAP AKA and EAP SIM. 9. Security Considerations The EAP base protocol specification [EAP] highlights several attacks that are possible against the EAP protocol. This section discusses Arkko and Haverinen Expires 27 April, 2004 [Page 54] EAP AKA Authentication 27 October, 2003 the claimed security properties of EAP AKA as well as vulnerabilities and security recommendations. 9.1. Identity Protection EAP/AKA includes optional Identity privacy support that protects the privacy of the subscriber identity against passive eavesdropping. The mechanism cannot be used on the first exchange with a given server, when the IMSI will have to be sent in the clear. The terminal SHOULD store the pseudonym in a non-volatile memory so that it can be maintained across reboots. An active attacker that impersonates the network may use the AT_PERMANENT_ID_REQ attribute (Section 1.1) to learn the subscriber's IMSI. However, as discussed in Section 1.1, the terminal can refuse to send the cleartext IMSI 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. The benefits and the security considerations of using an external security mechanism with EAP/AKA are beyond the scope of this document. 9.2. Mutual Authentication EAP/AKA provides mutual authentication via the UMTS AKA mechanisms. 9.3. Key Derivation EAP/AKA supports key derivation with 128-bit effective key strength. The key hierarchy is specified in Section 0. The Transient EAP Keys used to protect EAP AKA packets (K_encr, K_aut) and the Master Session Keys are cryptographically separate. An attacker cannot derive any non-trivial information from K_encr or K_aut based on the Master Session Key or vice versa. An attacker also cannot calculate the pre-shared secret from the UMTS AKA IK, UMTS AKA CK, EAP AKA K_encr, EAP AKA K_aut or from the Master Session Key. 9.4. Brute-Force and Dictionary Attacks The effective strength of EAP/AKA values is 128 bits, and there are no known computationally feasible brute-force attacks. Because UMTS AKA is not a password protocol (the pre-shared secret must not be a weak password), EAP/AKA is not vulnerable to dictionary attacks. 9.5. Integrity Protection, Replay Protection and Confidentiality AT_MAC, AT_IV and AT_ENCR_DATA attributes are used to provide integrity, replay and confidentiality protection for EAP/AKA Requests and Responses. Integrity protection includes the EAP Arkko and Haverinen Expires 27 April, 2004 [Page 55] EAP AKA Authentication 27 October, 2003 header. 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. Because keys are not available in the beginning of the EAP methods, the AT_MAC attribute cannot be used for protecting EAP/AKA-Identity messages. However, the AT_CHECKCODE attribute can optionally be used to protect the integrity of the EAP/AKA-Identity roundtrip. On full authentication, replay protection is provided by RAND and AUTN values from the underlying UMTS AKA scheme. On re- authentication, a counter and a server nonce is used to provide replay protection. The contents of the EAP-Response/Identity packet are implicitly integrity protected by including them in key derivation. Because EAP/AKA is not a tunneling method, EAP Notification, EAP Success or EAP Failure packets are not confidential, integrity protected or replay protected. On physically insecure networks, this may enable an attacker to mount denial of service attacks by sending false EAP Notification, EAP Success or EAP Failure packets. However, the attacker cannot force the peers to believe successful authentication has occurred when mutual authentication failed or has not happened yet. An eavesdropper will see the EAP Notification, EAP Success and EAP Failure packets sent in the clear. With EAP AKA, confidential information MUST NOT be transmitted in EAP Notification packets. 9.6. Negotiation Attacks EAP/AKA does not protect the EAP-Response/Nak packet. Because EAP/AKA does not protect the EAP method negotiation, EAP method downgrading attacks may be possible, especially if the user uses the same identity with EAP/AKA and other EAP methods. As described in Section 5, EAP/AKA allows the protocol to be extended by defining new attribute types. When defining such attributes, it should be noted that any extra attributes included in EAP-Request/AKA-Identity or EAP-Response/AKA-Identity packets are not included in the MACs later on, and thus some other precautions must be taken to avoid modifications to them. EAP/AKA does not support ciphersuite negotiation or EAP/AKA protocol version negotiation. 9.7. Fast Reconnect EAP/AKA includes an optional re-authentication ("fast reconnect") procedure, as recommended in [EAP] for EAP types that are intended for physically insecure networks. 9.8. Acknowledged Result Indications Arkko and Haverinen Expires 27 April, 2004 [Page 56] EAP AKA Authentication 27 October, 2003 EAP/AKA does not provide acknowledged or integrity protected Success or Failure indications. If an EAP Success or an EAP Failure packet is lost when using EAP/AKA over an unreliable medium, and if the protocol over which EAP/AKA is transported does not address the possible loss of Success or Failure, then the peer and EAP server may end up having a different interpretation of the state of the authentication conversation. On physically insecure networks, an attacker may mount denial of service attacks by sending false EAP Success or EAP Failure indications. However, the attacker cannot force the peer or the EAP server to believe successful authentication has occurred when mutual authentication failed or has not happened yet. 9.9. 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/AKA Master Session Key or keys derived from it MAY be used as the integrity protection keys, or, if an external security mechanism such as PEAP is used, then the link integrity protection keys MAY be derived by the external security mechanism. There are man-in-the-middle attacks associated with the use of any EAP method within a tunneled protocol such as PEAP, or within a sequence of EAP methods followed by each other. This specification does not address these attacks. If EAP/AKA is used with a tunneling protocol or as part of a sequence of methods, there should be cryptographic binding provided between the protocols and EAP/AKA to prevent man-in-the-middle attacks through rogue authenticators being able to setup one-way authenticated tunnels. EAP/AKA Master Session Key MAY be used to provide the cryptographic binding. However the mechanism how the binding is provided depends on the tunneling or sequencing protocol, and it is beyond the scope of this document. 9.10. Generating Random Numbers An EAP/AKA implementation SHOULD use a good source of randomness to generate the random numbers required in the protocol. Please see [RFC 1750] for more information on generating random numbers for security applications. 10. Security Claims This section provides the security claims required by [EAP]. [a] Intended use. EAP AKA is intended for use over both physically insecure networks and physically or otherwise secure networks. Applicable media include but are not limited to PPP, IEEE 802 wired networks and IEEE 802.11. Arkko and Haverinen Expires 27 April, 2004 [Page 57] EAP AKA Authentication 27 October, 2003 [b] Mechanism. EAP AKA is based on the UMTS AKA mechanism, which is an authentication and key agreement mechanism based on a symmetric 128-bit pre-shared secret. [c] Security claims. The security properties of the method are discussed in Section 9. [d] Key strength. EAP/AKA supports key derivation with 128-bit effective key strength. [e] Description of key hierarchy. Please see Section 0. [f] Indication of vulnerabilities. Vulnerabilities are discussed in Section 9. 11. Intellectual Property Right Notices On IPR related issues, Nokia and Ericsson refer to the their respective statements on patent licensing. Please see http://www.ietf.org/ietf/IPR/NOKIA and http://www.ietf.org/ietf/IPR/ERICSSON-General Acknowledgements and Contributions The authors wish to thank Rolf Blom of Ericsson, Bernard Aboba of Microsoft, Arne Norefors of Ericsson, N.Asokan of Nokia, Valtteri Niemi of Nokia, Kaisa Nyberg of Nokia, Jukka-Pekka Honkanen of Nokia, Pasi Eronen of Nokia, Olivier Paridaens of Alcatel and Ilkka Uusitalo of Ericsson for interesting discussions in this problem space. The attribute format is based on the extension format of Mobile IPv4 [RFC 3344]. Authors' Addresses Jari Arkko Ericsson 02420 Jorvas Phone: +358 40 5079256 Finland Email: jari.arkko@ericsson.com Henry Haverinen Nokia Mobile Phones P.O. Box 88 33721 Tampere Phone: +358 50 594 4899 Finland E-mail: henry.haverinen@nokia.com Arkko and Haverinen Expires 27 April, 2004 [Page 58] EAP AKA Authentication 27 October, 2003 Annex A. Pseudo-Random Number Generator The "|" character denotes concatenation, and "^" denotes involution. Step 1: Choose a new, secret value for the seed-key, XKEY Step 2: In hexadecimal notation let t = 67452301 EFCDAB89 98BADCFE 10325476 C3D2E1F0 This is the initial value for H0|H1|H2|H3|H4 in the FIPS SHS [SHA-1] Step 3: For j = 0 to m - 1 do 3.1 XSEED_j = 0 /* no optional user input */ 3.2 For i = 0 to 1 do a. XVAL = (XKEY + XSEED_j) mod 2^b b. w_i = G(t, XVAL) c. XKEY = (1 + XKEY + w_i) mod 2^b 3.3 x_j = w_0|w_1 Arkko and Haverinen Expires 27 April, 2004 [Page 59] EAP AKA Authentication 27 October, 2003 Normative References [TS 33.102] 3GPP Technical Specification 3GPP TS 33.102 V5.1.0: "Technical Specification Group Services and System Aspects; 3G Security; Security Architecture (Release 5)", 3rd Generation Partnership Project, December 2002. [RFC 2486] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC 2486, January 1999. [EAP] L. Blunk et al., "Extensible Authentication Protocol (EAP)", draft-ietf-eap-rfc2284bis-05.txt, work-in-progress, September 2003. [RFC 2119] S. Bradner, "Key words for use in RFCs to indicate Requirement Levels", RFC 2119, March 1997. [TS 23.003] 3GPP Technical Specification 3GPP TS 23.003 V5.5.1: "3rd Generation Parnership Project; Technical Specification Group Core Network; Numbering, addressing and identification (Release 5)", 3rd Generation Partnership Project, January 2003 [RFC 2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC2104, February 1997. [SHA-1] Federal Information Processing Standard (FIPS) Publication 180-1, "Secure Hash Standard," National Institute of Standards and Technology, U.S. Department of Commerce, April 17, 1995. [AES] Federal Information Processing Standards (FIPS) Publication 197, "Advanced Encryption Standard (AES)", National Institute of Standards and Technology, November 26, 2001. http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf [CBC] NIST Special Publication 800-38A, "Recommendation for Block Cipher Modes of Operation - Methods and Techniques", National Institute of Standards and Technology, December 2001. http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf [TS 33.105] 3GPP Technical Specification 3GPP TS 33.105 4.1.0: "Technical Specification Group Services and System Aspects; 3G Security; Cryptographic Algorithm Requirements (Release 4)", 3rd Generation Partnership Project, June 2001 [PRF] Federal Information Processing Standards (FIPS) Publication 186-2 (with change notice), "Digital Signature Standard (DSS)", National Institute of Standards and Technology, January 27, 2000 Available on-line at: http://csrc.nist.gov/publications/fips/fips186-2/fips186-2- change1.pdf Arkko and Haverinen Expires 27 April, 2004 [Page 60] EAP AKA Authentication 27 October, 2003 [RFC 2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998. Informative References [RFC 2548] G. Zorn, "Microsoft Vendor-specific RADIUS Attributes", RFC 2548, March 1999 [PEAP] H. Andersson, S. Josefsson, G. Zorn, D. Simon, A. Palekar, "Protected EAP Protocol (PEAP)", draft-josefsson-pppext-eap-tls-eap- 05.txt, work-in-progress, September 2002. [RFC 1750] D. Eastlake, 3rd, S. Crocker, J. Schiller, "Randomness Recommendations for Security", RFC 1750 (Informational), December 1994. [RFC 3344] C. Perkins (editor), "IP Mobility Support", RFC 3344, August 2002. [EAP SIM] H. Haverinen, J. Salowey, "EAP SIM Authentication", draft- haverinen-pppext-eap-sim-12.txt, October 2003, work in progress [TS 23.234] Draft 3GPP Technical Specification 3GPP TS 23.234 V 1.4.0: "Technical Specification Group Services and System Aspects; 3GPP system to Wireless Local Area Network (WLAN) Interworking; System Description", 3rd Generation Partnership Project, work in progress, January 2003. Arkko and Haverinen Expires 27 April, 2004 [Page 61]