J. Arkko Internet Draft Ericsson Document: draft-arkko-pppext-eap-aka-04.txt H. Haverinen Expires: December 2002 Nokia June 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 UMTS AKA authentication 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 GSM authentication, making it possible to use EAP AKA for authenticating both GSM and UMTS subscribers. Table of Contents Status of this Memo................................................1 Abstract...........................................................1 1. Introduction and Motivation.....................................2 2. Conventions used in this document...............................3 3. Protocol Overview...............................................5 4. Obtaining Subscriber Identity via EAP AKA Messages.............10 5. Identity Privacy Support.......................................11 Arkko and Haverinen Expires in six months [Page 1]
EAP AKA Authentication June 2002 6. Message Format.................................................14 7. Message Integrity and Privacy Protection.......................16 7.1. AT_MAC Attribute.............................................16 7.2. AT_IV and AT_ENCR_DATA Attributes............................16 8. Messages.......................................................17 8.1. EAP-Response/Identity........................................18 8.2. EAP-Request/AKA-Challenge....................................19 8.3. EAP-Response/AKA-Challenge...................................22 8.4. EAP-Response/AKA-Authentication-Reject.......................24 8.5. EAP-Response/AKA-Synchronization-Failure.....................24 8.6. EAP-Request/AKA-Identity.....................................25 8.7. EAP-Response/AKA-Identity....................................26 9. Key Derivation.................................................28 10. Interoperability with GSM.....................................29 11. IANA and Protocol Numbering Considerations....................30 12. Security Considerations.......................................31 13. Intellectual Property Right Notices...........................31 Acknowledgements and Contributions................................31 Authors' Addresses................................................31 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]. The Universal Mobile Telecommunications System (UMTS) is a global third generation mobile network standard. AKA is based on challenge-response mechanisms and symmetric cryptography. AKA typically runs in a UMTS Subscriber Identity Module (USIM), a smart card like device. However, the applicability of AKA is not limited to client devices with smart cards, but the AKA mechanisms could also be implemented in host software, for example. AKA also provides backward compatibility to the GSM authentication mechanism [2]. Compared to the GSM mechanism, AKA provides substantially longer key lengths and the authentication of the server side as well as the client side. 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 801.1x 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. Arkko and Haverinen Expires in six months [Page 2]
EAP AKA Authentication June 2002 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]. 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. Conventions used in this document The following terms will be used through this document: Arkko and Haverinen Expires in six months [Page 3]
EAP AKA Authentication June 2002 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]. 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]. Arkko and Haverinen Expires in six months [Page 4]
EAP AKA Authentication June 2002 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. 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 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 below message flow shows the basic successful 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 Arkko and Haverinen Expires in six months [Page 5]
EAP AKA Authentication June 2002 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 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. This message contains a random number (RAND) and an authorization vector (AUTN). The EAP- Request/AKA-Challenge message MAY optionally contain encrypted data, which is used for IMSI privacy support, as described in Section 5. 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 verify that the client is a legitimate one. Client Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes user's NAI) | |------------------------------------------------------>| | | | +------------------------------+ | | Server runs UMTS algorithms, | | | generates RAND and AUTN. | | +------------------------------+ | | | EAP-Request/AKA-Challenge | | (RAND, AUTN) | |<------------------------------------------------------| | | +-------------------------------------+ | | Client runs UMTS algorithms on USIM,| | | verifies AUTN, derives RES | | | and session key | | +-------------------------------------+ | | | | EAP-Response/AKA-Challenge | | (RES) | |------------------------------------------------------>| | | | +------------------------------+ | | Server checks the given RES, | | | and finds it correct. | | +------------------------------+ | | | EAP-Success | |<------------------------------------------------------| Arkko and Haverinen Expires in six months [Page 6]
EAP AKA Authentication June 2002 When EAP AKA is run in the GSM compatible mode, the message flow is otherwise identical to the message flow below except that the AUTN attribute is not included in EAP-Request/AKA-Challenge packet. The second message flow shows how the EAP server rejects the Client due to failed authentication. The same flow is also used in the GSM compatible mode, except that the AUTN parameter is not included in the EAP-Request/AKA-Challenge packet. Client Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes user's NAI) | |------------------------------------------------------>| | | | +------------------------------+ | | Server runs UMTS algorithms, | | | generates RAND and AUTN. | | +------------------------------+ | | | EAP-Request/AKA-Challenge | | (RAND, AUTN) | |<------------------------------------------------------| | | +-------------------------------------+ | | Client runs UMTS algorithms on USIM,| | | possibly verifies AUTN, and sends an| | | invalid response | | +-------------------------------------+ | | | | EAP-Response/AKA-Challenge | | (RES) | |------------------------------------------------------>| | | | +------------------------------+ | | Server checks the given RES, | | | and finds it incorrect. | | +------------------------------+ | | | 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 Arkko and Haverinen Expires in six months [Page 7]
EAP AKA Authentication June 2002 that this behavior is different from other EAP/AKA error cases, such as when encountering an incorrect AT_MAC attribute, when the client silently discards the EAP/AKA message. 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 | | (RAND, AUTN) | |<------------------------------------------------------| | | +-------------------------------------+ | | 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 with the EAP-Response/Nak message [6], as shown in the following figure: Arkko and Haverinen Expires in six months [Page 8]
EAP AKA Authentication June 2002 Client Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes user's NAI) | |------------------------------------------------------>| | | | +------------------------------+ | | Server runs GSM algorithms, | | | generates RAND | | +------------------------------+ | | | EAP-Request/AKA-Challenge | | (RAND) | |<------------------------------------------------------| | | +-------------------------------------+ | | Client does not accept the GSM | | | compatible version of this protocol.| | +-------------------------------------+ | | | | EAP-Response/Nak | |------------------------------------------------------>| | | | | | EAP-Failure | |<------------------------------------------------------| 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. HereÆs what happens then: Arkko and Haverinen Expires in six months [Page 9]
EAP AKA Authentication June 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 | | (RAND, AUTN) | |<------------------------------------------------------| | | +-------------------------------------+ | | Client runs UMTS algorithms on USIM | | | and discovers AUTN that contains an | | | inappropriate sequence number | | +-------------------------------------+ | | | | EAP-Response/AKA-Synchronization-Failure | | (AUTS) | |------------------------------------------------------>| | | | +---------------------------+ | | Perform resynchronization | | | Using AUTS and | | | the sent RAND | | +---------------------------+ | | After the resynchronization process takes place in the server and AAA side, the process continues by the server side sending a new EAP-Request/AKA-Challenge message. 4. 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 it is not specified how to deal with a new EAP Request/Identity. If the EAP server does not have any identity (IMSI or pseudonym) available when sending the first EAP/AKA request (usually EAP- Request/AKA-Challenge), then the EAP server issues the EAP- Request/AKA-Identity as the first message and includes the AT_IDENTITY_REQ attribute (Section 8.6). This attribute does not contain any data. It requests the client to include the AT_IDENTITY Arkko and Haverinen Expires in six months [Page 10]
EAP AKA Authentication June 2002 attribute (specified in Section 8.7) in the EAP-Response/AKA- Identity. The AT_IDENTITY attribute contains the current identity of the subscriber (IMSI or pseudonym). The use of pseudonyms for anonymity is specified in Section 5. This case is illustrated in the figure below. Client Authenticator | | | +------------------------------+ | | Server does not have any | | | Subscriber identity available| | | When starting EAP/AKA | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (Includes AT_IDENTITY_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/AKA-Idenity | | (Includes AT_IDENTITY) | |------------------------------------------------------>| | | If the AT_IDENTITY attribute contains a valid cleartext identity or a pseudonym identity that the EAP server is able to decode to the cleartext identity, then the authentication sequence proceeds as usual with the EAP Server issuing the EAP-Request/AKA-Challenge message. The operation in the case when the AT_IDENTITY attribute contains a pseudonym that the EAP server fails to decode is specified in Section 5. 5. Identity Privacy Support In the very first connection to an EAP server, the client always transmits the cleartext identity (IMSI) in the EAP-Response/Identity packet or in the AT_IDENTITY attribute. In subsequent connections, the optional identity privacy support can be used to hide the identity and to make the connections unlinkable to a passive eavesdropper. 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 8.2. Because the identity privacy support is optional to implement, the client MAY ignore the AT_IV, AT_ENCR_DATA, and AT_MAC attributes and always transmit the cleartext 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_AUTN attribute before looking at the AT_ENCR_DATA or AT_MAC attributes. If the AUTN is invalid, then the client MUST ignore the Arkko and Haverinen Expires in six months [Page 11]
EAP AKA Authentication June 2002 AT_IV, AT_ENCR_DATA and AT_MAC attributes. If AUTN is valid, then the client MAY derive the K_encr and K_int keys as described in Section 9 and verify the AT_MAC attribute. If the AT_MAC attribute is valid, then the client MAY decrypt the encrypted data and use the pseudonym in the next authentication. If the MAC is invalid, then the encrypted data MUST be ignored and the whole EAP packet MAY be silently ignored. The EAP server produces pseudonyms in an implementation-dependent manner. Please see [8] for examples on how to produce pseudonyms. Only the EAP server needs to be able to map the pseudonym to the cleartext 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. On the next connection to the EAP server, the client MAY transmit the received pseudonym in the first EAP-Response/Identity packet. The client concatenates the received pseudonym with the "@" character and the NAI realm portion. The client MUST use the same realm portion that it used in the connection when it received the pseudonym. If the EAP server issues the EAP-Request/AKA-Identity packet and requests the client to include the AT_IDENTITY attribute in the EAP- Response/AKA-Identity packet, as specified in Section 4, the client MAY transmit a pseudonym in the AT_IDENTITY packet. If the EAP server successfully decodes the pseudonym to a 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 client name, then the EAP server requests the cleartext identity (non- pseudonym identity) by issuing the EAP-Request/AKA-Identity packet to the client. In this case, the EAP request packet includes AT_PERMANENT_IDENTITY_REQ to request the client to send its non- pseudonym identity. The client responds with the EAP-Response/AKA- Identity, which includes the client's identity in the clear in the AT_PERMANENT_IDENTITY attribute. The EAP server issues the EAP-Request/AKA-Identity message also in the case when it received the undecodable pseudonym in AT_IDENTITY included the EAP-Response/AKA-Identity. In this case, there are two EAP/AKA-Identity round trips. The authentication sequence proceeds similarly in both cases. Please note that the EAP/AKA client and the EAP/AKA server only process the AKA-Identity packets and entities that only pass through EAP packets do not process these packets. 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 pseudonym Arkko and Haverinen Expires in six months [Page 12]
EAP AKA Authentication June 2002 identity from the EAP-Response/Identity packet regardless if the decoding fails in the EAP server. The figure below illustrates the case when an undecodable pseudonym is received in EAP-Response/Identity. Client Authenticator | | | EAP-Request/Identity | |<------------------------------------------------------| | | | EAP-Response/Identity | | (Includes a pseudonym) | |------------------------------------------------------>| | | | +------------------------------+ | | Server fails to decode the | | | Pseudonym. | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (Includes AT_PERMANENT_IDENTITY_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/AKA-Identity | | (Includes cleartext identity in AT_PERMANENT_IDENTITY)| |------------------------------------------------------>| | | After receiving the EAP-Response/AKA-Identity packet, the EAP server issues the EAP-Request/AKA-Challenge and the authentication proceeds as usual. 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 13]
EAP AKA Authentication June 2002 Client Authenticator | | | +------------------------------+ | | Server does not have any | | | Subscriber identity available| | | When starting EAP/AKA | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (Includes AT_IDENTITY_REQ) | |<------------------------------------------------------| | | | | |EAP-Response/AKA-Identity | |(Includes a pseudonym AT_IDENTITY) | |------------------------------------------------------>| | | | | | +------------------------------+ | | Server fails to decode the | | | Pseudonym in AT_IDENTITY | | +------------------------------+ | | | EAP-Request/AKA-Identity | | (Includes AT_PERMANENT_IDENTITY_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/AKA-Identity | | (Includes AT_PERMANENT_IDENTITY) | |------------------------------------------------------>| | | After the latter EAP-Response/AKA-Identity message, the authentication sequence proceeds as usual with the EAP Server issuing the EAP-Request/AKA-Challenge message. If the client believes that the server should be able to decode the pseudonym identity, the client MAY refuse to send a clear text identity. In this case, the client silently ignores the EAP- Request/AKA-Identity packet that contains AT_PERMANENT_IDENTITY_REQ. This is necessary in some environments to prevent Man-in-the-Middle attackers from claiming to be servers that do not recognize the pseudonym, in an effort to find out the true identity of the user. 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. 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 Arkko and Haverinen Expires in six months [Page 14]
EAP AKA Authentication June 2002 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 11. 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 may be 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 in searching for the next attribute. These attributes are called skippable attributes. EAP/AKA packets do not include a version field. However, should there be 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. Attributes can be encapsulated within other attributes. In other words, the value field of an attribute type can be specified to contain other attributes. Arkko and Haverinen Expires in six months [Page 15]
EAP AKA Authentication June 2002 7. Message Integrity and Privacy Protection This section specifies EAP/AKA attributes for attribute encryption and EAP/AKA message integrity protection. 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 EAP- Requets/AKA-Challenge. For example, these attributes cannot be used in EAP-Request/AKA-Identity, because the RAND challenge has not yet been transmitted at that point. 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 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 format of the AT_MAC attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_MAC | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | MAC | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The MAC algorithm is 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 integrity protection key (K_int) used in the calculation of the MAC is derived from the AKA integrity key (IK) and cipher key (CK), as specified in Section 9. 7.2. AT_IV and AT_ENCR_DATA Attributes AT_IV and AT_ENCR_DATA attributes can be optionally used to transmit encrypted information between the EAP/AKA client and server. Arkko and Haverinen Expires in six months [Page 16]
EAP AKA Authentication June 2002 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 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 Section 9. The plaintext consists of nested EAP/AKA attributes. 8. Messages Arkko and Haverinen Expires in six months [Page 17]
EAP AKA Authentication June 2002 8.1. EAP-Response/Identity In the beginning of 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]. The EAP AKA mechanism uses the NAI format [5] as the identity. In order to facilitate the use of the existing cellular roaming infrastructure, the subscriber's IMSI is used as the client identifier. 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. There are two types of NAI username portions in EAP AKA: non- pseudonym permanent usernames and pseudonym usernames. When identity privacy 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 [13]. The EAP server MAY use the leading "0" as a hint to try EAP/AKA as the first authentication method during method negotiation, rather than for example EAP/SIM. The EAP/AKA server MAY propose EAP/AKA even if the leading character was not "0". When the optional identity privacy support is used, the client MAY use the pseudonym received as part of the previous authentication sequence as the username portion of the NAI, as specified in Section 5. The client MUST NOT modify the pseudonym received in AT_PSEUDONYM. For example, the client MUST NOT append any leading characters in the pseudonym. The AAA network routes AAA requests to the correct AAA server using the realm part of the NAI. The realm part MAY be decided 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. Because cellular roaming can be used with EAP AKA, the AAA request can be routed to an AAA server in the visited network instead of the server indicated in the NAI realm. Network operators that wish to apply this approach must make the necessary arrangements before this special routing can be enabled. Operators MAY reserve a specific realm portion of NAI for EAP AKA users. This convention makes it easy to recognize that the NAI identifies a UMTS or GSM subscriber. Such reserved NAI realm may be useful as a hint as to the first authentication method to use during method negotiation. If no configured realm name is available in the client, the client MAY derive the realm name from the IMSI. 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 Arkko and Haverinen Expires in six months [Page 18]
EAP AKA Authentication June 2002 Identification Number (MSIN). In other words, the IMSI is a string of not more than 15 digits. MCC and MNC uniquely identify the operator. A NAI realm name can be derived from the IMSI by concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits of IMSI and ".owlan.org". For example, if the IMSI is 123456789098765, and the MNC is three digits long, then the derived realm name is "mnc456.mcc123.owlan.org". 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 MNC's, the network SHOULD also be prepared for realm names with incorrect 3-digit MNC's. 8.2. EAP-Request/AKA-Challenge The format of the EAP-Request/AKA-Challenge packet is shown below. Arkko and Haverinen Expires in six months [Page 19]
EAP AKA Authentication June 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 20]
EAP AKA Authentication June 2002 Length The length of the EAP Request packet. Type TBD 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 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. 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 21]
EAP AKA Authentication June 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_PSEUDONYM | Length | Actual Pseudonym Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Pseudonym . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_PADDING | Length | Padding... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AT_PSEUDONYM This attribute is optional. The value field of this attribute begins with 2-byte actual pseudonym length, which specifies the length of the pseudonym in bytes. This field is followed by a pseudonym username, of the indicated actual length, that the client can use in the next authentication, as described in Section 5. The username does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the pseudonym with zero bytes when necessary. AT_PADDING 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. 8.3. EAP-Response/AKA-Challenge The format of the EAP-Response/AKA-Challenge packet is shown below. As specified in Section 7, EAP-Response/AKA-Challenge MAY include the AT_MAC attribute to integrity protect the EAP packet. 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 Arkko and Haverinen Expires in six months [Page 22]
EAP AKA Authentication June 2002 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 | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The semantics of the fields is described below: Code 2 for Response Identifier See [6] Length The length of the EAP Response packet. Type TBD 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 [14] the length of the AKA RES can vary between 32 and 128 bits. The GSM SRES Arkko and Haverinen Expires in six months [Page 23]
EAP AKA Authentication June 2002 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. 8.4. 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. Type TBD Subtype 2 for AKA-Authentication-Reject Reserved Set to zero on sending, ignored on reception. 8.5. EAP-Response/AKA-Synchronization-Failure The format of the EAP-Response/AKA-Synchronization-Failure packet is shown below. Arkko and Haverinen Expires in six months [Page 24]
EAP AKA Authentication June 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_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 TBD Subtype 4 for AKA-Synchronization-Failure 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). 8.6. 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_ID..._REQ | Length = 1 | Reserved | Arkko and Haverinen Expires in six months [Page 25]
EAP AKA Authentication June 2002 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_PERM..._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 TBD Subtype 5 for AKA-Identity Reserved Set to zero on sending, ignored on reception. AT_PERMANENT_IDENTITY_REQ The AT_PERMANENT_IDENTITY_REQ attribute is optional and it is included in the cases defined in Section 5. It MUST NOT be included if AT_IDENTITY_REQ is included. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. AT_IDENTITY_REQ The AT_IDENTITY_REQ attribute is optional and it is included in the cases defined in Section 4. It MUST NOT be included if AT_PERMANENT_IDENTITY_REQ is included. The value field only contains two reserved bytes, which are set to zero on sending and ignored on reception. 8.7. EAP-Response/AKA-Identity The format of the EAP-Response/AKA-Identity packet is shown below. Arkko and Haverinen Expires in six months [Page 26]
EAP AKA Authentication June 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_PERM... | Length | Actual Identity Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Cleartext Identity (optional) . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IDENTITY | Length | Actual Identity Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Current Identity (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 TBD Subtype 5 for AKA-Identity Reserved Set to zero on sending, ignored on reception. AT_PERMANENT_IDENTITY This attribute is optional and it is included in EAP- Response/AKA-Identity in cases specified in Section 5. It MUST NOT be included if AT_IDENTITY is included. The value field of this attribute begins with 2-byte actual identity length, which Arkko and Haverinen Expires in six months [Page 27]
EAP AKA Authentication June 2002 specifies the length of the identity in bytes. This field is followed by the non-pseudonym permanent Network Access Identifier username portion of the indicated actual length. The EAP/AKA username format is specified in Section 8.1. The username does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the identity with zero bytes when necessary. AT_IDENTITY The AT_IDENTITY attribute is optional and it is included in cases defined in Section 4. It MUST NOT be included if AT_PERMANENT_IDENTITY is included. 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 Network Access Identifier username portion of the indicated actual length. The username format is specified in Section 8.1. The username is either the non-pseudonym permanent username or a pseudonym username. The username does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the identity with zero bytes when necessary. 9. 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. EAP AKA requires two keys for its own purposes, an integrity protection key K_int and an encryption key K_encr, to be used with the AT_MAC and AT_ENCR_DATA attributes. In addition, it is possible to derive additional key material, such as a master key to be used with IEEE 802.11i. Key derivation is based on the random number generation specified in NIST Federal Information Processing Standards Publication 186-2 [15]. The random number generator is specified in the change notice 1 (2001 October 5)of [15] (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 3.3 is omitted. The function G used in the algorithm is constructed via Secure Hash Standard as specified in Appendix 3.3 of the standard. 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(IK|CK) The notation IK|CK denotes IK concatenated with CK. The optional user input values (XSEED_j) are set to zero. Arkko and Haverinen Expires in six months [Page 28]
EAP AKA Authentication June 2002 The resulting 160-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_int (128 bits), EAP application specific keys. The number of random number generator iterations (m) depends on the amount of required keying material. Even if K_encr or K_int were not used in the particular authentication sequence, they are derived and the EAP application specific material begins after K_int. 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, exact rules of key derivation cannot be given here. As a guideline, the EAP application specific keys resulting from the key expansion scheme is used in the following order: any master session keys required, any encryption keys required, any integrity protection keys required, any initialization vectors required If separate keys or IV's are required for each direction, then the downlink material (to protect traffic to user) is taken before the uplink material (to protect traffic from user). 10. 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 Arkko and Haverinen Expires in six months [Page 29]
EAP AKA Authentication June 2002 reject the authentication with the EAP-Response/AKA-GSM- Authentication-Reject packet. In conclusion, the following table shows which variant of the EAP AKA protocol should be run under different conditions: 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 11. 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 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_PERMANENT_IDENTITY...........................5 AT_PADDING......................................6 AT_PERMANENT_IDENTITY_REQ.......................7 AT_IDENTITY_REQ.................................8 AT_IDENTITY.....................................9 AT_IV.........................................129 AT_ENCR_DATA..................................130 AT_MAC........................................131 AT_PSEUDONYM..................................132 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 [16]. Requests must be specified in sufficient detail so that interoperability between independent implementations is possible. Possible forms of documentation include, but are not limited to, RFCs, the products of Arkko and Haverinen Expires in six months [Page 30]
EAP AKA Authentication June 2002 another standards body (e.g. 3GPP), or permanently and readily available vendor design notes. 12. Security Considerations 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. 13. 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 and Olivier Paridaens of Alcatel for interesting discussions in this problem space. The identiy privacy support is based on the identity privacy support of [8]. The attribute format is based on the extension format of Mobile IPv4 [17]. 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 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) Arkko and Haverinen Expires in six months [Page 31]
EAP AKA Authentication June 2002 [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] J. Carlson, B. Aboba, H. Haverinen, "EAP SRP-SHA1 Authentication Protocol", draft-ietf-pppext-eap-srp-03.txt, July 2001 (work-in-progress). (INFORMATIVE) [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) [12] Federal Information Processing Standard (FIPS) Publication 180-1, "Secure Hash Standard," National Institute of Standards and Technology, U.S. Department of Commerce, April 17, 1995. (NORMATIVE) [13] 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) [14] 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) Arkko and Haverinen Expires in six months [Page 32]
EAP AKA Authentication June 2002 [15] 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 [16] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998. (NORMATIVE) [17] C. Perkins (editor), "IP Mobility Support", RFC 2002, October 1996. (INFORMATIVE) Arkko and Haverinen Expires in six months [Page 33]
