J. Arkko Internet Draft Ericsson Document: draft-arkko-pppext-eap-aka-06.txt H. Haverinen Expires: March 2003 Nokia November 2002 EAP AKA Authentication Status of this Memo This document is an Internet-Draft and is in full conformance with 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. 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. AKA is based on symmetric keys, and runs typically in a UMTS Subscriber Identity Module, a smart card like device. AKA provides also backward compatibility to Global System for Mobile Communications (GSM) authentication, making it possible to use EAP AKA for authenticating both GSM and UMTS subscribers. 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.....................................2 Arkko and Haverinen Expires in six months [Page 1]
EAP AKA Authentication November 2002 2. Terms and Conventions Used in This Document.....................4 3. Protocol Overview...............................................6 4. Identity Management............................................10 4.1. User Identity in EAP-Response/Identity.......................10 4.2. Obtaining Subscriber Identity via EAP AKA Messages...........12 4.3. Identity Privacy Support.....................................14 5. Re-authentication..............................................20 6. Message Format.................................................25 7. Message Authentication and Encryption..........................26 7.1. AT_MAC Attribute.............................................26 7.2. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes................27 8. Messages.......................................................28 8.1. EAP-Request/AKA-Challenge....................................28 8.2. EAP-Response/AKA-Challenge...................................32 8.3. EAP-Response/AKA-Authentication-Reject.......................33 8.4. EAP-Response/AKA-Synchronization-Failure.....................34 8.5. EAP-Request/AKA-Identity.....................................35 8.6. EAP-Response/AKA-Identity....................................36 8.7. EAP-Request/AKA-Reauthentication.............................37 8.8. EAP-Response/AKA-Reauthentication............................40 9. Unsuccessful Cases.............................................42 10. Key Derivation................................................42 11. Interoperability with GSM.....................................44 12. IANA and Protocol Numbering Considerations....................45 13. Security Considerations.......................................46 13.1. Identity Protection and Privacy.............................46 13.2. Mutual Authentication and Triplet Exposure..................46 13.3. Key Derivation Considerations...............................47 13.4. Brute-Force and Dictionary Attacks..........................47 13.5. Packet Modification Attacks.................................47 13.6. Negotiation Attacks.........................................47 13.7. Fast Reconnect..............................................47 13.8. Unreliable Media............................................47 13.9. Man-in-the-middle Attacks...................................48 13.10. Generating Random Numbers..................................48 14. Intellectual Property Right Notices...........................48 Acknowledgements and Contributions................................48 Authors' Addresses................................................49 Annex A. Key Derivation for IEEE 802.11...........................50 Annex B. Pseudo-Random Number Generator...........................51 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 [1].UMTS is a global third generation mobile network standard. Arkko and Haverinen Expires in six months [Page 2]
EAP AKA Authentication November 2002 AKA is based on challenge-response mechanisms and symmetric cryptography. AKA typically runs in a UMTS Subscriber Identity Module (USIM). AKA also provides backward compatibility to the GSM authentication mechanism [2]. 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 and IEEE 802.1x technology through EAP over Wireless [3, 4]. - 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 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 reference [1]. Arkko and Haverinen Expires in six months [Page 3]
EAP AKA Authentication November 2002 It is also possible that the home environment delegates the actual authentication task to an intermediate node. In this case the authentication vector or parts of it are delivered to the intermediate node, enabling it to perform the comparison between RES and XRES, and possibly also use CK and IK. Such delivery MUST be done in a secure manner. In EAP AKA, the EAP server node is such an intermediate node. 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) [5]. 2. Terms and Conventions Used in This Document The following terms will be used through this document: AAA protocol Authentication, Authorization and Accounting protocol AAA server The AAA server is responsible for storing shared secrets and other credential information necessary for the authentication of users. Cf. EAP server AKA Authentication and Key Agreement AuC Authentication Centre. The mobile network element that can authenticate subscribers either in GSM or in UMTS networks. Authenticator The entity that terminates the protocol carrying EAP used by the client, such as a Network Access Server (NAS) terminating the PPP link. The EAP server may be co-located in the Authenticator. In this case, the Authenticator may actually authenticate the user based on information received from the AAA server. EAP Extensible Authentication Protocol [6]. Arkko and Haverinen Expires in six months [Page 4]
EAP AKA Authentication November 2002 EAP server The network element that terminates the EAP protocol. Typically, the EAP server functionality is implemented in a AAA server. GSM Global System for Mobile communications. NAI Network Access Identifier [5]. AUTN Authentication value generated by the AuC which together with the RAND authenticates the server to the client, 128 bits [1]. AUTS A value generated by the client upon experiencing a synchronization failure, 112 bits. RAND Random number generated by the AuC, 128 bits [1]. RES Authentication result from the client, which together with the RAND authenticates the client to the server, 128 bits [1]. SQN Sequence number used in the authentication process, 48 bits [1]. 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. Arkko and Haverinen Expires in six months [Page 5]
EAP AKA Authentication November 2002 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 [7] 3. Protocol Overview In this document, the term EAP Server refers to the network element that terminates the EAP protocol. Usually the EAP server is separate from the authenticator device, which is the network element closest to the client, such as a Network Access Server (NAS) or an IEEE 802.1X bridge. Alternatively, the EAP server functionality may be co-located in the authenticator although typically, the EAP server functionality is implemented on a separate AAA server with whom the authenticator communicates using an AAA protocol. (The exact AAA communications are outside the scope of this document, however.) The message flow below shows the basic successful full authentication case with the EAP AKA. The EAP AKA uses two roundtrips to authorize the user and generate session keys. As in other EAP schemes, first an identity request/response message pair is exchanged. (As specified in [6], the initial identity request is not required, and MAY be bypassed in cases where the authenticator can presume the identity, such as when using leased lines, dedicated dial-ups, etc. Please see also Section 4.2 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 6. The EAP-Request/AKA-Challenge message contains a random number (AT_RAND) and an authorization vector (AT_AUTN), and a message authentication code AT_MAC. The EAP-Request/AKA-Challenge message MAY optionally contain encrypted data, which is used for IMSI privacy support, as described in Section 4.3. 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 client runs the AKA algorithm (perhaps inside an USIM) and verifies the AUTN. If this is successful, the client 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 client, and the AT_MAC attribute to integrity protect the EAP message. Arkko and Haverinen Expires in six months [Page 6]
EAP AKA Authentication November 2002 Client 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) | |<------------------------------------------------------| | | +-------------------------------------+ | | Client 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 | |<------------------------------------------------------| When EAP AKA is run in the GSM compatible mode, the message flow is otherwise identical to the message flow above except that the AT_AUTN attribute is not included in the EAP-Request/AKA-Challenge packet and AT_MAC attribute is not included in any attribute. The second message flow shows how the EAP server rejects the Client 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 in six months [Page 7]
EAP AKA Authentication November 2002 Client 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) | |<------------------------------------------------------| | | +-------------------------------------+ | | Client 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 client rejecting the AUTN of the EAP server. This flow is not used in the GSM compatible mode. The client sends an explicit error message (EAP-Response/AKA- Authentication-Reject) to the Authenticator, 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. Please note that this behavior is different from other EAP/AKA error cases, such as when encountering an incorrect AT_MAC attribute, the client silently discards the EAP/AKA message. Arkko and Haverinen Expires in six months [Page 8]
EAP AKA Authentication November 2002 Client 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) | |<------------------------------------------------------| | | +-------------------------------------+ | | Client runs UMTS algorithms on USIM | | | and discovers AUTN that can not be | | | verified | | +-------------------------------------+ | | | | EAP-Response/AKA-Authentication-Reject | |------------------------------------------------------>| | | | | | EAP-Failure | |<------------------------------------------------------| Networks that are not UMTS aware use the GSM compatible version of this protocol even for UMTS subscribers. In this case, the AUTN parameter is not included in the EAP-Request/AKA-Challenge packet. If a UMTS capable client does not want to accept the use of the GSM compatible mode, the client can reject the authentication by silently ignoring any EAP-Request/AKA-Challenge packets that do not include the AUTN parameter. The AKA uses shared secrets between the Client and the Client'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 in six months [Page 9]
EAP AKA Authentication November 2002 Client 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) | |<------------------------------------------------------| | | +-------------------------------------+ | | Client 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 5. 4. Identity Management This section specifies user identity management and identity privacy support. 4.1. User Identity in EAP-Response/Identity In the beginning of an EAP authentication, the Authenticator issues the EAP-Request/Identity packet to the client. The client responds with EAP-Response/Identity, which contains the user's identity. The formats of these packets are specified in [6]. Arkko and Haverinen Expires in six months [Page 10]
EAP AKA Authentication November 2002 UMTS and GSM subscribers are identified with the International Mobile Subscriber Identity (IMSI) [8]. 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 operator. Internet AAA protocols identify users with the Network Access Identifier (NAI) [5]. When used in a roaming environment, the NAI is composed of a username and a realm, separated with "@" (username@realm). The username portion identifies the subscriber within the realm. The AAA nodes use the realm portion of the NAI to route AAA requests to the correct AAA server. The realm name used in this protocol MAY be chosen by the operator and it MAY be a configurable parameter in the EAP/AKA client implementation. In this case, the client is typically configured with the NAI realm of the home operator. Operators MAY reserve a specific realm name for EAP/AKA users. This convention makes it easy to recognize that the NAI identifies a subscriber that uses EAP/AKA. Such a reserved NAI realm may be a useful hint to the first authentication method to use during method negotiation. There are three types of NAI username portions in EAP/AKA: non- pseudonym permanent usernames that are based on the IMSI, pseudonym usernames and re-authentication usernames. The first two are only used on full authentication and the last one only on re- authentication. When the optional IMSI privacy support is not used, the non-pseudonym permanent username is used. The non-pseudonym permanent username is of the format "0imsi". 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 [8] The EAP server MAY use the leading "0" as a hint to try EAP/AKA as the first authentication method during method negotiation. The EAP/AKA server MAY propose EAP/AKA even if the leading character was not "0". When the optional identity privacy support is used on full authentication, the client MAY use the pseudonym received upon the previous full authentication sequence as the username portion of the NAI, as specified in Section 4.3. The client MUST NOT modify the pseudonym received in AT_NEXT_PSEUDONYM. For example, the client MUST NOT append any leading characters in the pseudonym. On re-authentication, the client uses the re-authentication identity received upon the previous authentication sequence as the NAI. A new re-authentication identity may be delivered as part of both full authentication and re-authentication. The client MUST NOT modify the re-authentication identity received in AT_NEXT_REAUTH_ID but the client must use the re-authentication identity as it is. For Arkko and Haverinen Expires in six months [Page 11]
EAP AKA Authentication November 2002 example, the client MUST NOT append any leading characters in the re-authentication identity. If no configured realm name is available, the client MAY derive the realm name from the MCC and MNC portions of the IMSI. In this case, the realm name is obtained by concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits of IMSI and ".owlan.org". For example, if the IMSI is 123456789098765, and the MNC is three digits long, then the derived realm name is "mnc456.mcc123.owlan.org". If the client is not able to determine whether the MNC is two or three digits long, the client MAY use a 3-digit MNC. If the correct length of the MNC is two, then the MNC used in the realm name will include the first digit of MSIN. Hence, when configuring AAA networks for operators that have 2-digit MNCs, the network SHOULD also be prepared for realm names with incorrect 3-digit MNCs. 4.2. Obtaining Subscriber Identity via EAP AKA Messages It may be useful to obtain the identity of the subscriber through means other than EAP Request/Identity. This can eliminate the need for an identity request when using EAP method negotiation. If this was not possible then it might not be possible to negotiate EAP/AKA as the second method since not all EAP implementations support multiple EAP Identity requests.. If the EAP server has not received any identity (IMSI, pseudonym or re-authentication identity) from the client when sending the first EAP/AKA request, then the EAP server may issue the EAP-Request/AKA- Identity packet and includes the AT_ANY_ID_REQ attribute (specified in Section 8.5). This attribute does not contain any data. The AT_ANY_ID_REQ attribute requests the client to include the AT_IDENTITY attribute (specified in Section 8.6) in the EAP- Response/AKA-Identity packet. The identity format in the AT_IDENTITY attribute is the same as in the Type-Data field of the EAP- Response/Identity packet. The AT_IDENTITY attribute contains an IMSI-based permanent identity, a pseudonym identity or a re- authentication identity. If the server does not support re- authentication, it uses the AT_FULLAUTH_ID_REQ attribute instead of the AT_ANY_ID_REQ attribute to directly request for a full authentication identity (either the permanent identity or a pseudonym identity). If the server uses the AT_FULLAUTH_ID_REQ attribute, the client MUST NOT use a re-authentication identity in the AT_IDENTITY attribute. The use of pseudonyms for anonymity is specified in Section 4.3. The use of re-authentication identities is specified in Section 5. The full authentication case is illustrated in the figure below. In this case, AT_IDENTITY contains either the permanent identity or a pseudonym identity. The same sequence is also used in case the server uses the AT_FULLAUTH_ID_REQ in EAP-Request/AKA-Identity Arkko and Haverinen Expires in six months [Page 12]
EAP AKA Authentication November 2002 Client 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) | |------------------------------------------------------>| | | If the client wants to perform full authentication, it includes the permanent identity or a pseudonym identity in the AT_IDENTITY attribute. The client may use these identities in response to either AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ. If the server uses the AT_ANY_ID_REQ and the client wants to perform re-authentication, then the client includes a re-authentication identity in the AT_IDENTITY attribute. If the client uses its full authentication identity and the AT_IDENTITY attribute contains a valid permanent identity or a valid pseudonym identity that the EAP server is able to decode to the permanent identity, then the full authentication sequence proceeds as usual with the EAP Server issuing the EAP-Request/AKA-Challenge message. On re-authentication, if the AT_IDENTITY attribute contains a valid re-authentication identity and the server agrees on using re- authentication, then the server proceeds with the re-authentication sequence and issues the EAP-Request/AKA-Reauthentication packet, as specified in Section 5. If the server does not recognize the re- authentication identity, then it issues a second EAP-Request/AKA- Identity message and includes the AT_FULLAUTH_ID_REQ attribute. In this case, a second EAP/AKA-Identity round trip is required. The messages used on the first roundtrip are ignored. This is illustrated below. Arkko and Haverinen Expires in six months [Page 13]
EAP AKA Authentication November 2002 Client 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. An extra EAP/AKA-Identity round trip is also required in cases when the AT_IDENTITY attribute contains a pseudonym identity that the EAP server fails to decode. The operation in this case is specified in Section 4.3. 4.3. Identity Privacy Support EAP/AKA includes optional identity privacy (anonymity) support that can be used to hide the cleartext IMSI and to make the subscriber's connections unlinkable to eavesdroppers. 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 13.1 for security considerations concerning identity privacy. Arkko and Haverinen Expires in six months [Page 14]
EAP AKA Authentication November 2002 If identity privacy is not used or if the client does not have any pseudonyms or re-authentication identities available, the client transmits the permanent identity (based on IMSI) in the EAP- Response/Identity packet or in the AT_IDENTITY attribute. The EAP-Request/AKA-Challenge message MAY include an encrypted pseudonym in the value field of the AT_ENCR_DATA attribute. The AT_IV and AT_MAC attributes are also used to transport the pseudonym to the client, as described in Section 8.1. Because the identity privacy support is optional to implement, the client MAY ignore the AT_IV and AT_ENCR_DATA attributes and always transmit the IMSI-based permanent identity in the EAP-Response/Identity packet and in the AT_IDENTITY attribute. On receipt of the EAP-Request/AKA-Challenge, the client verifies the AT_MAC attribute before looking at the AT_ENCR_DATA attribute. If the AT_MAC is invalid, then the client MUST silently discard the EAP packet. If the AT_MAC attribute is valid, then the client MAY decrypt the encrypted data in AT_ENCR_DATA and use the obtained pseudonym on the next full authentication. If the client does not receive a new pseudonym in the EAP- Request/AKA-Challenge message, the client MAY use an old pseudonym instead of the permanent identity on next full authentication. The EAP server produces pseudonyms in an implementation-dependent manner. Only the EAP server needs to be able to map the pseudonym to the permanent identity. Regardless of construction method, the pseudonym MUST conform to the grammar specified for the username portion of an NAI. The EAP AKA server MAY produce pseudonyms that begin with a leading "0" character in order to be able to use the leading character as a hint in EAP method negotiation during next authentication. The client MAY transmit the received pseudonym in the first EAP- Response/Identity packet of the next full authentication with the EAP server. The client concatenates the received pseudonym with the "@" character and the NAI realm portion. The client selects the realm name portion similarly as it select the realm name portion when using the permanent identity. If the EAP server successfully decodes the pseudonym received in the EAP-Response/Identity packet to a known client identity (IMSI), the authentication proceeds with the EAP-Request/AKA-Challenge message as usual. Because the client may fail to save a pseudonym sent to in an EAP- Request/AKA-Challenge, for example due to malfunction, the EAP server SHOULD maintain at least one old pseudonym in addition to the most recent pseudonym. If the EAP server requests the client to include its identity in the EAP-Response/AKA-Identity packet, as specified in Section 4.2, the client MAY transmit the received pseudonym in the AT_IDENTITY attribute. If the EAP server successfully decodes the pseudonym to a Arkko and Haverinen Expires in six months [Page 15]
EAP AKA Authentication November 2002 known identity, then the authentication proceeds with the EAP- Request/AKA-Challenge packet as usual. If the EAP server fails to decode the pseudonym to a known identity, then the EAP server requests the permanent identity (non-pseudonym identity) by including the AT_PERMANENT_ID_REQ attribute (Section 8.5) in the EAP-Request/AKA-Challenge message. The EAP server issues the EAP-Request/AKA-Identity message also in the case when it received the undecodable pseudonym in AT_IDENTITY included in the EAP-Response/AKA-Identity packet. In this case, a second EAP/AKA-Identity round trip is required. A received AT_PERMANENT_ID_REQ does not necessarily originate from the valid network, but an active attacker may transmit an EAP- Request/AKA-Identity packet with an AT_PERMANENT_ID_REQ attribute to the client, in an effort to find out the true identity of the user. On receipt of EAP-Request/AKA-Identity that includes AT_PERMANENT_ID_REQ, the client MAY delay the processing of the message for a while in order to wait for another EAP AKA message that does not include the AT_PERMANENT_ID_REQ attribute. Basically, there are two different policies that the client can employ with regard to AT_PERMANENT_ID_REQ. A "conservative" client assumes that the network is able to maintain pseudonyms robustly. Therefore, if a conservative client has a pseudonym, the client silently ignores the EAP packet with AT_PERMANENT_ID_REQ, because the client believes that the valid network is able to decode the pseudonym. (Alternatively, the conservative client may respond to AT_PERMANENT_ID_REQ in certain circumstances, for example if the pseudonym was received a long time ago.) The benefit of this policy is that it protects the client against active attacks on anonymity. On the other hand, a "liberal" client always accepts the AT_PERMANENT_ID_REQ and responds with the IMSI-based permanent identity. The benefit of this policy is that it works even if the valid network sometimes loses pseudonyms and is not able to decode them to the permanent identity. The value field of the AT_PERMANENT_ID_REQ does not contain any data but the attribute is included to request the client to include the AT_IDENTITY attribute (Section 8.6) with the permanent authentication identity in the EAP-Response/AKA-Identity message. In this case, the AT_IDENTITY attribute contains the client's permanent identity in the clear. Please note that the EAP/AKA client and the EAP/AKA server only process the AT_IDENTITY attribute. Entities that only pass EAP packets through do not process this attribute. Hence, if the EAP server is not co-located in the authenticator, then the authenticator and other intermediate AAA elements (such as possible AAA proxy servers) will continue to refer to the client with the original identity from the EAP-Response/Identity packet regardless if the decoding fails in the EAP server. Arkko and Haverinen Expires in six months [Page 16]
EAP AKA Authentication November 2002 The figure below illustrates the case when the EAP server fails to decode the pseudonym included in the EAP-Response/Identity packet. Client 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) | |------------------------------------------------------>| | | After the EAP-Response/AKA-Identity message, the authentication sequence proceeds as usual with the EAP Server issuing the EAP- Request/AKA-Challenge message. 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 in six months [Page 17]
EAP AKA Authentication November 2002 Client 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) | |------------------------------------------------------>| | | In the worst case, there are three EAP/AKA-Identity round trips before the server has obtained an acceptable identity: on the first round, the client sends its re-authentication identity in AT_IDENTITY. The server fails to accept it and request a full authentication identity with a second EAP-Request/AKA-Identity. The client responds with a pseudonym identity in AT_IDENTITY. The server fails to decode the pseudonym and has to issue a third EAP- Request/AKA-Identity, including AT_PERMANENT_ID_REQ. Finally, the server accepts the client's EAP-Response/AKA-Identity with the AT_IDENTITY attribute and proceeds with full authentication. This is illustrated in the figure below. Arkko and Haverinen Expires in six months [Page 18]
EAP AKA Authentication November 2002 Client 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 with the EAP Server issuing the EAP-Request/AKA-Challenge message. Because the keys that are used to protect the pseudonym are derived from the AKA cipher key (CK) and the AKA integrity key (IK), the identity privacy support is not available when EAP AKA is used in the GSM compatible mode. Arkko and Haverinen Expires in six months [Page 19]
EAP AKA Authentication November 2002 5. Re-authentication In some environments, EAP authentication may be performed frequently. Because the EAP AKA full authentication procedure makes 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 client. On each EAP authentication, either one of the entities may also fall back on full authentication if they do not want to use re-authentication. Re-authentication is based on the keys derived on the preceding full authentication. The same K_aut and K_encr keys as in full authentication are used to protect EAP AKA packets and attributes, and the original XKEY seed value from full authentication is used to generate fresh application specific keys, as specified in Section 10. On re-authentication, the client protects against replays with an unsigned 16-bit counter, included in the AT_COUNTER attribute. On full authentication, both the server and the client initialize the counter to one. The counter value of at least one is used on the first re-authentication. On subsequent re-authentications, the counter MUST be greater than on any of the previous re- authentications. For example, on the second re-authentication, counter value is two or greater etc. The AT_COUNTER attribute is encrypted. The server includes an encrypted server nonce (AT_NONCE_S) in the re-authentication request. The AT_MAC attribute in the client's response is calculated over NONCE_S to provide a challenge/response authentication scheme. The NONCE_S also contributes to the new application specific keys. As discussed in Section 4.3, in some environments the client may assume that the network can reliably store pseudonyms and therefore the client may fail to respond to the AT_PERMANENT_ID_REQ attribute. The network SHOULD store pseudonyms on a reliable database. Because one of the objectives of the re-authentication procedure is to reduce load on the network, the re-authentication procedure does not require the EAP server to contact a reliable database. Therefore, the re-authentication procedure makes use of separate re- authentication user identities. Pseudonyms and the permanent IMSI- based identity are reserved for full authentication only. The network does not need to store re-authentication identities as carefully as pseudonyms. If a re-authentication identity is lost and the network does not recognize it, the EAP server can fall back on full authentication. Arkko and Haverinen Expires in six months [Page 20]
EAP AKA Authentication November 2002 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 client MAY ignore this attribute, in which case it will use full authentication next time. If the client wants to use re- authentication, it uses this re-authentication identity on next authentication. Even if the client has a re-authentication identity, the client MAY discard the re-authentication identity and use a pseudonym or the IMSI-based permanent identity instead, in which case full authentication will be performed. The re-authentication identity received in AT_NEXT_REAUTH_ID contains both the username portion and the realm portion of the Network Access Identifier. The EAP Server can choose an appropriate realm part in order to have the AAA infrastructure route subsequent re-authentication related requests to the same AAA server. For example, the realm part MAY include a portion that is specific to the AAA server. Hence, it is sufficient to store the context required for re-authentication in the AAA server that performed the full authentication. The client MAY use the re-authentication identity in the EAP- Response/Identity packet or, in response to server's AT_ANY_ID_REQ attribute, the client MAY use the re-authentication identity in the AT_IDENTITY attribute of the EAP-Response/AKA-Identity packet. Even if the client uses a re-authentication identity, the server may want to fall back on full authentication, for example because the server does not recognize the re-authentication identity or does not want to use re-authentication. 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 client'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. (As specified in Sections 4.2 and 4.3, the server MAY use AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ attributes if it does not know the client's identity.) Both the client and the server SHOULD have an upper limit for the number of subsequent re-authentications allowed before a full authentication needs to be performed. Because a 16-bit counter is used in re-authentication, the theoretical maximum number of re- authentications is reached when the counter value reaches 0xFFFF. In order to use re-authentication, the client and the server need to store the following values: original XKEY, K_aut, K_encr, latest counter value and the next re-authentication identity. Arkko and Haverinen Expires in six months [Page 21]
EAP AKA Authentication November 2002 The following figure illustrates the re-authentication procedure. Encrypted attributes are denoted with '*'. The client uses its re- authentication identity in the EAP-Response/Identity packet. As discussed above, an alternative way to communicate the re- authentication identity to the server is for the client to use the AT_IDENTITY attribute in the EAP-Response/AKA-Identity message. This latter case is not illustrated in the figure below, and it is only possible when the server requests the client to send its identity by including the AT_ANY_ID_REQ attribute in the EAP-Request/AKA- Identity packet. 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 client. This packet MUST include the encrypted AT_COUNTER attribute, with a fresh counter value, the encrypted AT_NONCE_S attribute that contains a random number chosen by the server, the AT_ENCR_DATA and the AT_IV attributes used for encryption, and the AT_MAC attribute that contains a message authentication code over the packet. The packet MAY also include an encrypted AT_NEXT_REAUTH_ID attribute that contains the next re-authentication identity. Re-authentication identities are one-time identities. If the client does not receive a new re-authentication identity, it MUST use either the permanent identity or a pseudonym identity on the next authentication to initiate full authentication. The client verifies that the counter value is fresh (greater than any previously used value). The client also verifies that AT_MAC is correct. The client MAY save the next re-authentication identity from the encrypted AT_NEXT_REAUTH_ID for next time. If all checks are successful, the client responds with the EAP-Response/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 client. Arkko and Haverinen Expires in six months [Page 22]
EAP AKA Authentication November 2002 Client Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes a re-authentication identity) | |------------------------------------------------------>| | | | +--------------------------------+ | | Server recognizes the identity | | | and agrees on using fast | | | re-authentication | | +--------------------------------+ | | | EAP-Request/AKA-Reauthentication | | (AT_IV, AT_ENCR_DATA, *AT_COUNTER, | | *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC) | |<------------------------------------------------------| | | | | +-----------------------------------------------+ | | Client verifies AT_MAC and the freshness of | | | the counter. Client MAY store the new re- | | | authentication identity for next re-auth. | | +-----------------------------------------------+ | | | | EAP-Response/AKA-Reauthentication | | (AT_IV, AT_ENCR_DATA, *AT_COUNTER with same value, | | AT_MAC) | |------------------------------------------------------>| | | | +--------------------------------+ | | Server verifies AT_MAC and | | | the counter | | +--------------------------------+ | | | EAP-Success | |<------------------------------------------------------| | | If the client does not accept the counter value of EAP-Request/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 in six months [Page 23]
EAP AKA Authentication November 2002 Client 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 client 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 client 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 client MUST ignore the contents of the server's AT_NEXT_REAUTH_ID attribute. On receipt of AT_COUNTER_TOO_SMALL, the server verifies AT_MAC and verifies that AT_COUNTER contains the same as in the EAP- Request/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 in six months [Page 24]
EAP AKA Authentication November 2002 6. Message Format The Type-Data of the EAP AKA packets begins with a 1-octet Subtype field, which is followed by a 2-octet reserved field. The rest of the Type-Data consists of attributes that are encoded in Type, Length, Value format. The figure below shows the generic format of an attribute. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Attribute Type | Length | Value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Attribute Type Indicates the particular type of attribute. The attribute type values are listed in Section 12. 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. When an attribute numbered within the range 0 through 127 is encountered but not recognized, the EAP/AKA message containing that attribute MUST be silently discarded. These attributes are called non-skippable attributes. When an attribute numbered in the range 128 through 255 is encountered but not recognized that particular attribute is ignored, but the rest of the attributes and message data MUST still be processed. The Length field of the attribute is used to skip the attribute value when searching for the next attribute. These attributes are called skippable attributes. 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. 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. Arkko and Haverinen Expires in six months [Page 25]
EAP AKA Authentication November 2002 Attributes can be encapsulated within other attributes. In other words, the value field of an attribute type can be specified to contain other attributes. 7. Message Authentication and Encryption This section specifies EAP/AKA attributes for attribute encryption and EAP/AKA message authentication. Encryption and integrity protection are based on the AKA session keys CK and IK. Because the CK and IK keys are derived from the RAND challenge, these attributes can only be used in the EAP-Request/AKA- Challenge message and any EAP/AKA messages sent after it. For example, these attributes cannot be used in EAP-Request/AKA- Identity, because the RAND challenge has not yet been transmitted at that point. Integrity protection with AT_MAC MUST be used in all messages when keys have been derived. As there is no key derivation specification for the GSM mode, attribute encryption and message integrity protection are not available in the GSM mode. 7.1. AT_MAC Attribute The AT_MAC attribute can optionally be used for EAP/AKA message integrity protection. Whenever AT_ENCR_DATA (Section 7.2) is included in an EAP message, it MUST be followed (not necessarily immediately) by an AT_MAC attribute. Messages that do not meet this condition MUST be silently discarded. The value field of the AT_MAC attribute contains two reserved bytes followed by a message authentication code (MAC). The MAC is calculated over the whole EAP packet, concatenated with optional message-specific data, with the exception that the value field of the MAC attribute is set to zero when calculating the MAC. The reserved bytes are set to zero when sending and ignored on reception. The contents of the message-specific data, if present, are specified separately for each EAP/AKA message. The message-specific data is included in order to protect data that is not transmitted with the EAP packet. The format of the AT_MAC attribute is shown below. Arkko and Haverinen Expires in six months [Page 26]
EAP AKA Authentication November 2002 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 [9] 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 message authentication key (K_aut) used in the calculation of the MAC is derived from the AKA integrity key (IK) and cipher key (CK), as specified in Section 10. 7.2. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes AT_IV and AT_ENCR_DATA attributes can be optionally used to transmit encrypted information between the EAP/AKA client and server. The value field of AT_IV contains two reserved bytes followed by a 16-byte initialization vector required by the AT_ENCR_DATA attribute. The reserved bytes are set to zero when sending and ignored on reception. The AT_IV attribute MUST be included if and only if the AT_ENCR_DATA is included. Messages that do not meet this condition MUST be silently discarded. The sender of the AT_IV attribute chooses the initialization vector by random. The sender MUST NOT reuse the initialization vector value from previous EAP AKA packets but the sender MUST choose it freshly for each AT_IV attribute. The sends SHOULD use a good source of randomness to generate the initialization vector. The format of AT_IV is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IV | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Initialization Vector | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the AT_ENCR_DATA attribute consists of two reserved bytes followed by bytes encrypted using the Advanced Encryption Standard (AES) [10] 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 Arkko and Haverinen Expires in six months [Page 27]
EAP AKA Authentication November 2002 ignored on reception. Please see [11] 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 encryption key (K_encr) is derived is derived from the AKA integrity key (IK) and cipher key (CK), as specified in Section10. 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, and silently drop the message if this verification fails. The format of the AT_PADDING attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_PADDING | Length | Padding... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 8. Messages 8.1. EAP-Request/AKA-Challenge The format of the EAP-Request/AKA-Challenge packet is shown below. Arkko and Haverinen Expires in six months [Page 28]
EAP AKA Authentication November 2002 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_RAND | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | RAND | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_AUTN | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | AUTN (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IV | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Initialization Vector (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_ENCR_DATA | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Encrypted Data (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_MAC | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | MAC (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The semantics of the fields is described below: Code 1 for Request Identifier See [6] Arkko and Haverinen Expires in six months [Page 29]
EAP AKA Authentication November 2002 Length The length of the EAP Request packet. Type 23 Subtype 1 for AKA-Challenge Reserved Set to zero when sending, ignored on reception. AT_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. The AT_RAND attribute MUST be present in EAP- Request/AKA-Challenge. AT_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. The AT_AUTN attribute MUST NOT be included in the GSM compatible mode of this protocol; otherwise it MUST be included. AT_IV See Section 7.2. AT_ENCR_DATA See Section 7.2. The nested attributes that are included in the plaintext of AT_ENCR_DATA are described below. AT_MAC AT_MAC MUST NOT be included in GSM compatible mode; otherwise it MUST be included. In EAP-Request/AKA-Challenge, there is no message-specific data covered by the MAC. See Section 7.1. In the EAP-Request/AKA-Challege message, the AT_IV, AT_ENCR_DATA and AT_MAC attributes are used for IMSI privacy and for communicating the next re-authentication identity. The plaintext of the AT_ENCR_DATA value field consists of nested attributes, which are shown below. Later versions of this protocol MAY specify additional attributes to be included within the encrypted data. Arkko and Haverinen Expires in six months [Page 30]
EAP AKA Authentication November 2002 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_PS... | Length | Actual Pseudonym Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Pseudonym . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_REAU..| Length | Actual Re-Auth Identity Length| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Re-authentication Username . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_PADDING | Length | Padding... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AT_NEXT_PSEUDONYM This attribute is optional. The value field of this attribute begins with a 2-byte actual pseudonym length, which specifies the length of the pseudonym in bytes. This field is followed by a pseudonym user name, of the indicated actual length, that the client can use in the next authentication, as described in Section 4.3. The user name does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the pseudonym with zero bytes when necessary. AT_NEXT_REAUTH_ID The AT_NEXT_REAUTH_ID attribute is optional to include. The value field of this attribute begins with a 2-byte actual re- authentication identity length, which specifies the length of the re-authentication identity in bytes. This field is followed by a re-authentication identity, of the indicated actual length, that the client can use in the next re-authentication, as described in Section 5. The re-authentication identity includes both a username portion and a realm name portion. The re-authentication identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the re-authentication identity with zero bytes when necessary. AT_PADDING AT_PADDING is optional to include. See Section 7.2. Arkko and Haverinen Expires in six months [Page 31]
EAP AKA Authentication November 2002 8.2. EAP-Response/AKA-Challenge The format of the EAP-Response/AKA-Challenge packet is shown below. Later versions of this protocol MAY make use of the AT_ENCR_DATA and AT_IV attributes in this message to include encrypted (skippable) attributes. AT_MAC, AT_ENCR_DATA and AT_IV attributes are not shown in the figure below. If present, they are processed as in EAP- Request/AKA-Challenge packet. The EAP server MUST process EAP- Response/AKA-Challenge messages that include these attributes even if the server did not implement these optional attributes. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_RES | Length | RES Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | | RES | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_MAC | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | MAC (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The semantics of the fields is described below: Code 2 for Response Identifier See [6] Length The length of the EAP Response packet. Type 23 Arkko and Haverinen Expires in six months [Page 32]
EAP AKA Authentication November 2002 Subtype 1 for AKA-Challenge Reserved Set to zero when sending, ignored on reception. AT_RES This attribute MUST be included in EAP-Response/AKA-Challenge. The value field of this attribute begins with the 2-byte RES Length, which is identifies the exact length of the RES (or SRES) in bits. The RES length is followed by the UMTS AKA RES or GSM SRES parameter. According to the specification [12] the length of the AKA RES can vary between 32 and 128 bits. The GSM SRES parameter is always 32 bits long. 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. AT_MAC AT_MAC MUST NOT be included in GSM compatible mode; otherwise it MUST be included. In EAP-Response/AKA-Challenge, there is no message-specific data covered by the MAC. See Section 7.1. 8.3. EAP-Response/AKA-Authentication-Reject The format of the EAP-Response/AKA-Authentication-Reject packet is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The semantics of the fields is described below: Code 2 for Response Identifier See [6] Length The length of the EAP Response packet. Arkko and Haverinen Expires in six months [Page 33]
EAP AKA Authentication November 2002 Type 23 Subtype 2 for AKA-Authentication-Reject Reserved Set to zero on sending, ignored on reception. 8.4. EAP-Response/AKA-Synchronization-Failure The format of the EAP-Response/AKA-Synchronization-Failure packet is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| | AT_AUTS | Length = 4 | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | AUTS | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The semantics of the fields is described below: Code 2 for Response Identifier See [6] Length The length of the EAP Response packet, 20. Type 23 Subtype 4 for AKA-Synchronization-Failure Arkko and Haverinen Expires in six months [Page 34]
EAP AKA Authentication November 2002 AT_AUTS This attribute MUST be included in EAP-Response/AKA- Synchronization-Failure. The value field of this attribute contains the AKA AUTS parameter, 112 bits (14 bytes). 8.5. EAP-Request/AKA-Identity The format of the EAP-Request/AKA-Identity packet is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_PERM..._REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_FULL..._REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_ANY_ID_REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The semantics of the fields is described below: Code 1 for Request Identifier See [6] Length The length of the EAP Request packet. Type 23 Subtype 5 for AKA-Identity Reserved Set to zero on sending, ignored on reception. AT_PERMANENT_ID_REQ The AT_PERMANENT_ID_REQ attribute is optional to include and it is included in the cases defined in Section 4.3. It MUST NOT be Arkko and Haverinen Expires in six months [Page 35]
EAP AKA Authentication November 2002 included if AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ is included. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. AT_FULLAUTH_ID_REQ The AT_FULLAUTH_ID_REQ attribute is optional to include and it is included in the cases defined in Section 4.2. It MUST NOT be included if AT_ANY_ID_REQ or AT_PERMANENT_ID_REQ is included. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. AT_ANY_ID_REQ The AT_ANY_ID_REQ attribute is optional and it is included in the cases defined in Section 4.2. It MUST NOT be included if AT_PERMANENT_ID_REQ or AT_FULLAUTH_ID_REQ is included. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. 8.6. EAP-Response/AKA-Identity The format of the EAP-Response/AKA-Identity packet is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IDENTITY | Length | Actual Identity Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Current Identity . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The semantics of the fields is described below: Code 2 for Response Identifier See [6] Length The length of the EAP Response packet. Arkko and Haverinen Expires in six months [Page 36]
EAP AKA Authentication November 2002 Type 23 Subtype 5 for AKA-Identity Reserved Set to zero on sending, ignored on reception. AT_IDENTITY The AT_IDENTITY attribute is optional to include and it is included in cases defined in Section 4.2 and 4.3. The value field of this attribute begins with 2-byte actual identity length, which specifies the length of the identity in bytes. This field is followed by the subscriber identity of the indicated actual length, in the same Network Access Identifier format that is used in EAP-Response/Identity, i.e. including the NAI realm portion. The identity is the permanent IMSI-based identity, a pseudonym identity or a re-authentication identity. The identity format is specified in Section 4.1. The identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the identity with zero bytes when necessary. 8.7. EAP-Request/AKA-Reauthentication The format of the EAP-Request/AKA-Reauthentication packet is shown below. Arkko and Haverinen Expires in six months [Page 37]
EAP AKA Authentication November 2002 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IV | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Initialization Vector | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_ENCR_DATA | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Encrypted Data . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_MAC | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | MAC | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 1 for Request Identifier See [6]. Length The length of the EAP packet. Type 23 Subtype 13 Reserved Set to zero when sending, ignored on reception. Arkko and Haverinen Expires in six months [Page 38]
EAP AKA Authentication November 2002 AT_IV The AT_IV attribute is MUST be included. See Section 7.2. AT_ENCR_DATA The AT_ENCR_DATA attribute MUST be included. See Section 7.2. The plaintext consists of nested attributes as described below. AT_MAC AT_MAC MUST be included. No message-specific data is included in the MAC calculation. See Section 7.1. The AT_IV and AT_ENCR_DATA attributes are used for communicating encrypted attributes. The plaintext of the AT_ENCR_DATA value field consists of nested attributes, which are shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER | Length = 1 | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NONCE_S | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | NONCE_S | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_REAU..| Length | Actual Re-Auth Identity Length| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Re-authentication Username . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_PADDING | Length | Padding... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AT_COUNTER The AT_COUNTER attribute MUST be included. The value field consists of a 16-bit unsigned integer counter value, represented in network byte order. AT_NONCE_S The AT_NONCE_S attribute MUST be included. The value field contains two reserved bytes followed by a random number generated Arkko and Haverinen Expires in six months [Page 39]
EAP AKA Authentication November 2002 by the server (16 bytes) freshly for this EAP/AKA re- authentication. The random number is used as challenge for the client and also a seed value for the new keying material. The reserved bytes are set to zero upon sending and ignored upon reception. AT_NEXT_REAUTH_ID The AT_NEXT_REAUTH_ID attribute is optional to include. The attribute is described in Section 8.1. AT_PADDING The AT_PADDING attribute is optional to include. See section 7.2 8.8. EAP-Response/AKA-Reauthentication The format of the EAP-Response/AKA-Reauthentication packet is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Subtype | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IV | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Initialization Vector | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_ENCR_DATA | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Encrypted Data . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_MAC | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | MAC | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 2 for Response Arkko and Haverinen Expires in six months [Page 40]
EAP AKA Authentication November 2002 Identifier See [6]. Length The length of the EAP packet. Type 23 Subtype 13 Reserved Set to zero when sending, ignored on reception. AT_IV The AT_IV attribute is MUST be included. See Section 7.2. AT_ENCR_DATA The AT_ENCR_DATA attribute MUST be included. See Section 7.2. The plaintext consists of nested attributes as described below. AT_MAC 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 7.1. It is followed by the 16-byte NONCE_S value from the client's AT_NONCE_S attribute. The AT_IV and AT_ENCR_DATA attributes are used for communicating encrypted attributes. The plaintext of the AT_ENCR_DATA value field consists of nested attributes, which are shown below. Arkko and Haverinen Expires in six months [Page 41]
EAP AKA Authentication November 2002 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER | Length = 1 | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER...| Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_PADDING | Length | Padding... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AT_COUNTER The AT_COUNTER attribute MUST be included. The format of this attribute is specified in Section 8.7. AT_COUNTER_TOO_SMALL The AT_COUNTER_TOO_SMALL attribute is optional to include, and it is included in cases specified in Section 5. AT_PADDING The AT_PADDING attribute is optional to include. See section 7.2 9. Unsuccessful Cases In general, if an EAP/AKA client or server implementation detects an error in a received EAP/AKA packet, the EAP/AKA implementation silently ignores the EAP packet, does not change its state and does not send any EAP messages to its peer. Examples of such errors, specified in detail elsewhere in this document, are an invalid AT_MAC value, a mandatory attribute is missing, illegal attributes included and an unrecognized non-skippable attribute. If no valid packets are received, the authentication exchange will eventually time out. As normally in EAP, the EAP server sends the EAP-Failure packet to the client when the authentication procedure fails on the EAP Server. In EAP/AKA, this may occur for example if the EAP server is not able to obtain authentication vectors for the subscriber or the authentication exchange times out. 10. Key Derivation This section specifies how EAP AKA keying material is derived from the IK and CK keys. Because IK and CK are not available in the GSM mode, this key derivation specification can only be applied in the UMTS AKA mode. Arkko and Haverinen Expires in six months [Page 42]
EAP AKA Authentication November 2002 EAP AKA requires two keys for its own purposes, a message authentication key K_aut and an encryption key K_encr, to be used with the AT_MAC and AT_ENCR_DATA attributes. The same K_aut and K_encr keys are used in full authentication and subsequent re- authentications. In addition, it is possible to derive additional application specific key material, such as a master key to be used with IEEE 802.11i. Key derivation is based on the pseudo-random number generator specified in NIST Federal Information Processing Standards Publication 186-2 [13]. The pseudo-random number generator is specified in the change notice 1 (2001 October 5)of [13] (Algorithm 1). As specified in the change notice (page 74), when Algorithm 1 is used as a general-purpose random number generator, the "mod q" term in step 3.2 is omitted. The function G used in the algorithm is constructed via Secure Hash Standard as specified in Appendix 3.3 of the standard. For convenience, the pseudo-random number algorithm with the correct modification is cited in Annex B. 160-bit XKEY and XVAL values are used, so b = 160. The initial secret seed value XKEY is computed from the AKA integrity key IK and cipher key CK with the following formula: XKEY = SHA1(Identity|IK|CK) In the formula above, the "|" character denotes concatenation. Identity denotes the user identity string without any terminating null characters. It is the identity from the AT_IDENTITY attribute from the last EAP-Response/AKA-Identity packet, or, if AT_IDENTITY was not used, the identity from the EAP-Response/Identity packet. 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), EAP application specific keys. The number of pseudo-random number generator iterations (m) depends on the amount of required keying material. The EAP application specific material immediately follows K_aut. On re-authentication, the same pseudo-random number generator can be used to generate new application specific keys. The seed value XKEYÆ is calculated as follows: XKEYÆ = SHA1(Identity|counter|NONCE_S|original XKEY) In the formula above, the Identity denotes the re-authentication user identity, without any terminating null characters, from the AT_IDENTITY attribute of the EAP-Response/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- Arkko and Haverinen Expires in six months [Page 43]
EAP AKA Authentication November 2002 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 original XKEY is the XKEY value from the preceding full authentication. The pseudo-random number generator is run with the new seed value XKEYÆ, and the resulting 320-bit random numbers x_0, x_1, ..., x_m-1 are concatenated and partitioned into suitable-sized chunks and used as new application specific keys. For example, the EAP application specific material can be used for packet security between the client and the authenticator. Because the required keying material depends on the EAP application and the EAP key derivation standardization has not been finalized yet, rules of key derivation cannot be given here. ). However, please see Annex A for a specification of how keys for IEEE 802.11 are derived. If a master session key is required, then the master session key is the first EAP application specific key. On full authentication, the master session key immediately follows K_aut in the key stream resulting from the key expansion scheme. On re-authentication, the master session key is the first new application specific key that is derived. 11. Interoperability with GSM The EAP AKA protocol is able to authenticate both UMTS and GSM users, if the subscriber's operator's network is UMTS aware. This is because the home network will be able to determine from the subscriber records whether the subscriber is equipped with a UMTS USIM or a GSM SIM. A UMTS aware home network will hence always use UMTS AKA with UMTS subscribers and GSM authentication with GSM subscribers. With GSM subscribers, the EAP AKA protocol is always used in the GSM compatible mode. It is not possible to use a GSM AuC to authenticate UMTS subscribers. (Note that if the home network doesn't support an authentication method it should not distribute SIMs for that method.) However, it is possible that the node actually terminating EAP and the node that stores the authentication keys (AuC) are separate, and support different authentication types. If the node terminating EAP is GSM-only but AuC is UMTS-aware, then authentication can still be achieved using the GSM compatible version of EAP AKA. This authentication will be weaker, since the GSM compatible mode does not provide for mutual authentication. Section 6.8.1.1 in [1] specifies how the GSM SRES parameter and the Kc key can be calculated on the USIM and the AuC. If a UMTS terminal does not want to accept the GSM compatible version of this protocol, then it can reject GSM authentication by silently ignoring the GSM mode EAP- Request/AKA-Challenge packet. In conclusion, the following table shows which variant of the EAP AKA protocol should be run under different conditions: Arkko and Haverinen Expires in six months [Page 44]
EAP AKA Authentication November 2002 SIM EAP node AuC EAP AKA mode ---------------------------------------------------- GSM (any) (any) GSM UMTS (any) GSM (illegal) UMTS GSM GSM+UMTS GSM UMTS GSM+UMTS GSM+UMTS UMTS The GSM mode and the UMTS mode provide a different level of security. Section 13 discusses security considerations for both modes. 12. 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-Reauthentication...........................13 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_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_IV.........................................129 AT_ENCR_DATA..................................130 AT_NEXT_PSEUDONYM.............................132 AT_NEXT_REAUTH_ID.............................133 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 [14]. Requests must be specified in sufficient detail so that interoperability Arkko and Haverinen Expires in six months [Page 45]
EAP AKA Authentication November 2002 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. 13. Security Considerations The protocol in this document is intended to provide the appropriate level of security to operate Extensible Authentication Protocol using the UMTS AKA procedure in both physically insecure environments and physically or otherwise secure environments, and the GSM AKA procedure in physically or otherwise secure environments. Implementations running the EAP AKA protocol will rely on the security of the AKA scheme, and the secrecy of the symmetric keys stored in the USIM and the AuC. This section highlights any known vulnerabilities and the ways to address them. The revised EAP base protocol [15] highlights several attacks that are possible against the EAP protocol. Therefore it specifies security requirements needed for standardizing EAP methods. This section discusses how the security requirements are addressed in EAP/AKA. 13.1. Identity Protection and Privacy The UMTS mode of EAP/AKA includes optional IMSI privacy support that protects the privacy of the subscriber identity against passive eavesdropping. The mechanism cannot be used on the first connection with a given server, when the 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 4.3) to learn the subscriber's IMSI. However, as discussed in Section 4.3, the terminal can refuse to send the cleartext IMSI if it believes that the network should be able to recognize the pseudonym. If the client and server cannot guarantee that the pseudonym will be maintained reliably and IMSI privacy is required then additional protection from an external security mechanism such as Protected Extensible Authentication Protocol (PEAP) [16] 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. The GSM mode of EAP/AKA does not provide identity privacy. 13.2. Mutual Authentication and Triplet Exposure The UMTS mode of EAP/AKA provides mutual authentication via the UMTS AKA mechanisms. The GSM mode does not provide mutual authentication. Arkko and Haverinen Expires in six months [Page 46]
EAP AKA Authentication November 2002 13.3. Key Derivation Considerations The UMTS mode of EAP/AKA supports key derivation with 128-bit entropy. The key hierarchy is specified in Section 10. In the GSM mode, EAP/AKA derives the 64-bit GSM Kc encryption key. 13.4. Brute-Force and Dictionary Attacks The effective strength of EAP/AKA values in the UMTS mode is 128- bits, and there are no known computationally feasible brute-force attacks. As the GSM mode is not intended for insecure invironments, brute-force attacks should not be possible. Because neither GSM or UMTS AKA are password protocols, neither mode of EAP/AKA is vulnerable to dictionary attacks. 13.5. Packet Modification Attacks In the UMTS mode, 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 header. In UMTS mode, the contents of the EAP-Response/Identity packet are implicitly integrity protected by including them in key derivation. The GSM mode does not protect against packet modification attacks. Because EAP/AKA is not a tunneling method, EAP Notification, EAP Success or EAP Failure packets are not 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. 13.6. 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. EAP/AKA does not support ciphersuite negotiation or EAP/AKA protocol version negotiation. 13.7. Fast Reconnect The UMTS mode of EAP/AKA includes an optional re-authentication ("fast reconnect") procedure, as recommended in [15] for EAP types that are intended for physically insecure networks. 13.8. Unreliable Media Arkko and Haverinen Expires in six months [Page 47]
EAP AKA Authentication November 2002 EAP/AKA does not provide acknowledged Success or Failure indications. If a Success or 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 authenticator may end up having a different interpretation of the state of the authentication conversation. 13.9. 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 UMTS mode application specific keys or keys derived from them 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. EAP/AKA 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 application-specific keys 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. 13.10. 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 [17] for more information on generating random numbers for security applications. 14. 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, 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 [18]. Arkko and Haverinen Expires in six months [Page 48]
EAP AKA Authentication November 2002 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 in six months [Page 49]
EAP AKA Authentication November 2002 Annex A. Key Derivation for IEEE 802.11 As specified in Section 12, application specific keying material can be derived with the pseudo-random function. The key hierarchy in IEEE 802.11i currently assumes that EAP methods produce a 256-bit long Pairwise Master Key (PMK) with 128 bits entropy. When a Pairwise Master Key is required, it is the first EAP application specific key that is derived. On full authentication, the PMK immediately follows K_aut in the key stream resulting from the key expansion scheme. On re-authentication, the PMK is the first new application specific key that is derived. For pre 802.11i networks, the signature key used to authenticate broadcast keys in IEEE 802.1x is selected as the first 256 bits of the EAP application specific keys immediately after K_aut. (On re- authentication, the first 256 application specific key bits are used as the signature key.) The next 256 bits are used as the WEP session key. The full 256-bit key is not usually used during WEP encryption, unused bits at then end should be ignored by the implementation. When the keys are transmitted from the authenticator to the access point using the RADIUS protocol the session key is placed in an MS-MPPE-RECV-KEY attribute and the signature key is placed in an MS-MPPE-SEND-KEY attribute. These attributes are defined in RFC 2548. Arkko and Haverinen Expires in six months [Page 50]
EAP AKA Authentication November 2002 Annex B. Pseudo-Random Number Generator The "|" character denotes concatenation, and "^" denotes involution. Step 1: Choose a new, secret value for the seed-key, XKEY Step 2: In hexadecimal notation let t = 67452301 EFCDAB89 98BADCFE 10325476 C3D2E1F0 This is the initial value for H0|H1|H2|H3|H4 in the FIPS SHS [12] Step 3: For j = 0 to m û 1 do 3.1 XSEED_j = optional user input 3.2 For i = 0 to 1 do a. XVAL = (XKEY + XSEED_j) mod 2^b b. w_i = G(t, XVAL) c. XKEY = (1 + XKEY + w_i) mod 2^b 3.3 x_j = w_0|w_1 Arkko and Haverinen Expires in six months [Page 51]
EAP AKA Authentication November 2002 References [1] 3GPP Technical Specification 3GPP TS 33.102 V3.6.0: "Technical Specification Group Services and System Aspects; 3G Security; Security Architecture (Release 1999)", 3rd Generation Partnership Project, November 2000. (NORMATIVE) [2] GSM Technical Specification GSM 03.20 (ETS 300 534): "Digital cellular telecommunication system (Phase 2); Security related network functions", European Telecommunications Standards, Institute, August 1997. (NORMATIVE) [3] IEEE P802.1X/D11, "Standards for Local Area and Metropolitan Area Networks: Standard for Port Based Network Access Control", March 2001. (INFORMATIVE) [4] IEEE Draft 802.11eS/D1, "Draft Supplement to STANDARD FOR Telecommunications and Information Exchange between Systems - LAN/MAN Specific Requirements - Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: Specification for Enhanced Security", March 2001. (INFORMATIVE) [5] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC 2486, January 1999. (NORMATIVE) [6] L. Blunk, J. Vollbrecht, "PPP Extensible Authentication Protocol (EAP)", RFC 2284, March 1998. (NORMATIVE) [7] S. Bradner, "Key words for use in RFCs to indicate Requirement Levels", RFC 2119, March 1997. (NORMATIVE) [8] GSM Technical Specification GSM 03.03 (ETS 300 523): "Digital cellular telecommunication system (Phase 2); Numbering, addressing and identification", European Telecommunications Standards Institute, April 1997. (NORMATIVE) [9] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC2104, February 1997. (NORMATIVE) [10] Federal Information Processing Standard (FIPS) draft standard, "Advanced Encryption Standard (AES)", http://csrc.nist.gov/publications/drafts/dfips-AES.pdf, September 2001. (NORMATIVE) [11] US National Bureau of Standards, "DES Modes of Operation", Federal Information Processing Standard (FIPS) Publication 81, December 1980. (NORMATIVE) Arkko and Haverinen Expires in six months [Page 52]
EAP AKA Authentication November 2002 [12] 3GPP Technical Specification 3GPP TS 33.105 V3.5.0: "Technical Specification Group Services and System Aspects; 3G Security; Cryptographic Algorithm Requirements (Release 1999)", 3rdGeneration Partnership Project, October 2000 (NORMATIVE) [13] Federal Information Processing Standards (FIPS) Publication 186-2 (with change notice), "Digital Signature Standard (DSS)", National Institute of Standards and Technology, January 27, 2000, (NORMATIVE) Available on-line at: http://csrc.nist.gov/publications/fips/fips186-2/ fips186-2-change1.pdf [14] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998. (NORMATIVE) [15] L. Blunk, J. Vollbrecht, B. Aboba, "Extensible Authentication Protocol (EAP)", draft-ietf-pppext-rfc2284bis-07.txt, work-in- progress, October 2002. (NORMATIVE) [16] H. Andersson, S. Josefsson, G. Zorn, D. Simon, A. Palekar, "Protected EAP Protocol (PEAP)", draft-josefsson-pppext-eap- tls-eap-05.txt, work-in-progress, September 2002. (IMFORMATIVE) [17] D. Eastlake, 3rd, S. Crocker, J. Schiller, "Randomness Recommendations for Security", RFC 1750 (Informational), December 1994. (INFORMATIVE) [18] C. Perkins (editor), "IP Mobility Support", RFC 2002, October 1996. (INFORMATIVE) Arkko and Haverinen Expires in six months [Page 53]
