Point-to-Point Extensions Working Group H. Haverinen (editor) Internet Draft Nokia June 2002 EAP SIM Authentication draft-haverinen-pppext-eap-sim-05.txt 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. This document is an individual submission for the Point-to-Point Extensions Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the ietf-ppp@merit.edu mailing list. Distribution of this memo is unlimited. Abstract This document specifies an Extensible Authentication Protocol (EAP) mechanism for authentication and session key distribution using the GSM Subscriber Identity Module (SIM). The mechanism specifies enhancements to GSM authentication and key agreement whereby multiple authentication triplets can be combined to create authentication responses and encryption keys of greater strength than the individual GSM triplets. The mechanism also includes network authentication. Haverinen Expires in six months [Page 1] Internet Draft EAP SIM Authentication June 2002 Table of Contents Status of this Memo.........................................1 Abstract....................................................1 Table of Contents...........................................2 1. Introduction.............................................2 2. Terms....................................................3 3. Overview.................................................4 4. Obtaining Subscriber Identity via EAP/SIM Messages.......5 5. Identity Privacy Support.................................6 6. Message Format...........................................9 7. Message Integrity and Privacy Protection................11 7.1. AT_MAC Attribute......................................11 7.2. AT_IV and AT_ENCR_DATA Attributes.....................11 8. EAP-Response/Identity...................................12 9. EAP-Request/SIM/Start...................................14 10. EAP-Response/SIM/Start.................................15 11. EAP-Request/SIM/Challenge..............................17 12. EAP-Response/SIM/Challenge.............................19 13. Unsuccessful Cases.....................................21 14. EAP/SIM Notifications..................................21 15. Calculation of Cryptographic Values....................23 16. IANA Considerations....................................25 17. Security Considerations................................26 18. Intellectual Property Right Notice.....................27 19. Acknowledgements and Contributions.....................27 References.................................................27 Editor's Address...........................................29 1. Introduction This document specifies an Extensible Authentication Protocol (EAP) [1] mechanism for authentication and session key distribution using the GSM Subscriber Identity Module (SIM). GSM authentication is based on a challenge-response mechanism. The authentication algorithm that runs on the SIM can be given a 128-bit random number (RAND) as a challenge. The SIM runs an operator- specific confidential algorithm which takes the RAND and a secret key Ki stored on the SIM as input, and produces a 32-bit response (SRES) and a 64-bit long key Kc as output. The Kc key is originally intended to be used as an encryption key over the air interface. Please find more information about GSM authentication in [2]. In EAP/SIM, several RAND challenges are used for generating several 64-bit Kc keys, which are combined to constitute a longer session key. EAP/SIM also enhances the basic GSM authentication mechanism by accompanying the RAND challenges with a message authentication code in order to provide mutual authentication. EAP/SIM specifies optional support for protecting the privacy of subscriber identity. Haverinen Expires in six months [Page 2] Internet Draft EAP SIM Authentication June 2002 2. Terms The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [3]. This document frequently uses the following terms and abbreviations: AAA protocol Authentication, Authorization and Accounting protocol AAA server In this document, AAA server refers to the network element that resides on the border of Internet AAA network and GSM network. Cf. EAP server AuC Authentication Centre. The GSM network element that can authenticate the subscriber. EAP Extensible Authentication Protocol. 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. IMSI International Mobile Subscriber Identifier, used in GSM to identify subscribers. NAI Network Access Identifier SIM Subscriber Identity Module. The SIM is an application traditionally resident on smart cards distributed by GSM operators. Haverinen Expires in six months [Page 3] Internet Draft EAP SIM Authentication June 2002 3. Overview Figure 1 shows an overview of the EAP/SIM authentication procedure. This version of EAP/SIM exchange uses three roundtrips to authenticate the user and generate session keys. In this document, the term EAP Server refers to the network element that terminates the EAP protocol. The Authenticator typically communicates with the user's EAP server using an AAA protocol. The AAA communications is not shown in the figure. The first EAP Request issued by the Authenticator is EAP- Request/Identity. The clients response includes either the user's International Mobile Subscriber Identity (IMSI) or a temporary identity (pseudonym), as specified in Section 8. Following the client's EAP-Response/Identity packet, the client receives EAP Requests of type 18 (SIM) from the Authenticator and sends the corresponding EAP Responses. The EAP packets that are of the Type SIM also have a Subtype field. The first EAP-Request/SIM packet is of the Subtype 10 (Start). Usually this packet contains no attributes. (However, see Section 5 for an exception.) The client responds with the EAP-Response/SIM/Start packet, which includes the AT_NONCE_MT attribute that contains a random number NONCE_MT, chosen by the client. The client MUST NOT reuse the NONCE_MT value from previous sessions but the client MUST choose it freshly for each EAP/SIM authentication exchange. The client SHOULD use a good source of randomness to generate NONCE_MT. In this document, we assume that the EAP server has an interface to the GSM network and it operates as a gateway between the Internet AAA network and the GSM authentication infrastructure. After receiving the EAP Response/SIM/Start, the EAP server obtains n GSM triplets from the user's home operator's Authentication Centre (AuC) on the GSM network, where n = 2 or n = 3. From the triplets, the EAP server derives the keying material. Section 15 specifies how these cryptographic values are calculated. The next EAP Request the Authenticator issues is of the type SIM and subtype Challenge (11). It contains the RAND challenges and a message authentication code attribute AT_MAC to cover the challenges. On receipt of this message, the client runs the GSM authentication algorithm and calculates a copy of the message authentication code. The client then verifies that the calculated MAC equals the received MAC. If the MAC's do not match, then the client silently ignores the EAP packet and does not send any authentication values to the network. Eventually, if another EAP- Request/SIM/Challenge packet with a valid AT_MAC is not received, the connection establishment will time out. Since the RAND's given to a client are accompanied with the message authentication code AT_MAC, the client is able to verify that the RAND's are fresh and they have been generated by the GSM network. Haverinen Expires in six months [Page 4] Internet Draft EAP SIM Authentication June 2002 If all checks out, the client responds with the EAP- Response/SIM/Challenge, containing the client's response MAC_SRES (Section 15). The EAP server verifies that the MAC_SRES is correct and sends the EAP-Success packet, indicating that the authentication was successful. The EAP server may also include derived keying material in the message it sends to the Authenticator. Client Authenticator | | | EAP-Request/Identity | |<---------------------------------------------------------| | | | EAP-Response/Identity | |--------------------------------------------------------->| | | | EAP-Request/SIM/Start | |<---------------------------------------------------------| | | | EAP-Response/SIM/Start | | (AT_NONCE_MT) | |--------------------------------------------------------->| | | | EAP-Request/SIM/Challenge | | (AT_RAND, AT_MAC) | |<---------------------------------------------------------| | | +-------------------------------------+ | | Client runs GSM algorithms, | | | verifies AT_MAC, derives AT_MAC_SRES| | | and session key | | +-------------------------------------+ | | | | EAP-Response/SIM/Challenge | | (AT_MAC_SRES) | |--------------------------------------------------------->| | | | | | EAP-Success | |<---------------------------------------------------------| | | Figure 1 EAP/SIM authentication procedure 4. Obtaining Subscriber Identity via EAP/SIM 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/SIM as the second method since it is not specified how to deal with a new EAP Request/Identity. Haverinen Expires in six months [Page 5] Internet Draft EAP SIM Authentication June 2002 If the EAP server does not have any identity (IMSI or pseudonym) available when sending the first EAP/SIM request (EAP- Request/SIM/Start), then the EAP server includes the AT_IDENTITY_REQ attribute (specified in Section 9) in the EAP-Request/SIM/Start packet. This attribute does not contain any data. It requests the client to include the AT_IDENTITY attribute (specified in Section 10) in the EAP-Response/SIM/Start packet. 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/SIM | | +------------------------------+ | | | EAP-Request/SIM/Start | | (Includes AT_IDENTITY_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (Includes AT_IDENTITY and AT_NONCE_MT) | |------------------------------------------------------>| | | 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/SIM/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 (anonymity) support can be used to hide the IMSI and to make the connections unlinkable to a passive eavesdropper. The EAP-Request/SIM/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 11. 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 Haverinen Expires in six months [Page 6] Internet Draft EAP SIM Authentication June 2002 IMSI in the EAP-Response/Identity packet and in the AT_IDENTITY attribute. On receipt of the EAP-Request/SIM/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 used in the next authentication. The EAP server produces pseudonyms in an implementation-dependent manner. Please see [4] 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 SIM server MAY produce pseudonyms that begin with a leading "1" 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 successfully decodes the pseudonym received in the EAP-Response/Identity packet to a known client identity (IMSI), the authentication proceeds with the EAP- Request/SIM/Start message as usual. If the EAP server requests the client to include the AT_IDENTITY attribute in the EAP-Response/SIM/Start packet, as specified in Section 4, the client MAY transmit the received 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/SIM/Challenge packet as usual. If the EAP server fails to decode the pseudonym to a known identity, then the EAP server requests the regular IMSI (non-pseudonym identity) by including the AT_PERMANENT_IDENTITY_REQ attribute (Section 9) in the EAP-Request/SIM/Start message. The EAP server issues the EAP-Request/SIM/Start message also in the case when it received the undecodable pseudonym in AT_IDENTITY included the EAP-Response/SIM/Start packet. In this case, there are two EAP/SIM/Start round trips. The authentication sequence proceeds similarly in both cases. For example, AT_NONCE_MT is always included in the EAP-Response/SIM/Start packet, even if it was already transmitted in the previous EAP-Response/SIM/Start. The first EAP/SIM/Start round trip is ignored. The NONCE_MT value included in the second EAP-Response/SIM/Start packet is used in all calculations. The EAP/SIM client MAY use the same NONCE_MT value in both EAP-Response/SIM/Start packets. Haverinen Expires in six months [Page 7] Internet Draft EAP SIM Authentication June 2002 The value field of the AT_PERMANENT_IDENTITY_REQ does not contain any data but the attribute is included to request the client to include the AT_PERMANENT_IDENTITY attribute (Section 10) in the EAP- Response/SIM/Start message. The AT_PERMANENT_IDENTITY attribute contains the client's identity in the clear. Please note that the EAP/SIM client and the EAP/SIM server only process the AT_PERMANENT_IDENTITY_REQ and AT_PERMANENT_IDENTITY attributes and entities that only pass through EAP packets do not process these attributes. 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 identity from the EAP-Response/Identity packet regardless if the decoding fails in the EAP server. 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/SIM/Start | | (Includes AT_PERMANENT_IDENTITY_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (Includes AT_PERMANENT_IDENTITY and AT_NONCE_MT) | |------------------------------------------------------>| | | After the EAP-Response/SIM/Start message, the authentication sequence proceeds as usual with the EAP Server issuing the EAP- Request/SIM/Challenge message. The figure below illustrates the case when the EAP server fails to decode the pseudonym included in the AT_IDENTITY attribute. Haverinen Expires in six months [Page 8] Internet Draft EAP SIM Authentication June 2002 Client Authenticator | | | +------------------------------+ | | Server does not have any | | | Subscriber identity available| | | When starting EAP/SIM | | +------------------------------+ | | | EAP-Request/SIM/Start | | (Includes AT_IDENTITY_REQ) | |<------------------------------------------------------| | | | | |EAP-Response/SIM/Start | |(Includes a pseudonym AT_IDENTITY and AT_NONCE_MT) | |------------------------------------------------------>| | | | | | +------------------------------+ | | Server fails to decode the | | | Pseudonym in AT_IDENTITY | | +------------------------------+ | | | EAP-Request/SIM/Start | | (Includes AT_PERMANENT_IDENTITY_REQ) | |<------------------------------------------------------| | | | | | EAP-Response/SIM/Start | | (Includes AT_PERMANENT_IDENTITY and AT_NONCE_MT) | |------------------------------------------------------>| | | After the latter EAP-Response/SIM/Start message, the authentication sequence proceeds as usual with the EAP Server issuing the EAP- Request/SIM/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/SIM/Start 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. 6. Message Format The Type-Data of the EAP/SIM 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. Haverinen Expires in six months [Page 9] Internet Draft EAP SIM 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Attribute Type Indicates the particular type of attribute. The attribute type values are listed in Section 16. Length Indicates the length of this attribute in multiples of four bytes. The maximum length of an attribute is 1024 bytes. The length includes the Attribute Type and Length bytes. Value The particular data associated with this attribute. This field is always included and it may be two or more bytes in length. The type and length fields determine the format and length of the value field. When an attribute numbered within the range 0 through 127 is encountered but not recognized, the EAP/SIM 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/SIM 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/SIM versions in a backward-compatible manner. Unless otherwise specified, the order of the attributes in an EAP/SIM message is insignificant, and an EAP/SIM implementation should not assume a certain order to be used. Attributes can be encapsulated within other attributes. In other words, the value field of an attribute type can be specified to contain other attributes. Haverinen Expires in six months [Page 10] Internet Draft EAP SIM Authentication June 2002 7. Message Integrity and Privacy Protection This section specifies EAP/SIM attributes for attribute encryption and EAP/SIM message integrity protection. Because the K_encr and K_int keys derived from the RAND challenges (as specified in Section 15) are required to process the integrity protection and encryption attributes, these attributes can only be used in the EAP-Request/SIM/Challenge message and any EAP/SIM messages sent after EAP-Requets/SIM/Challenge. For example, these attributes cannot be used in EAP-Request/SIM/Start. 7.1. AT_MAC Attribute The AT_MAC attribute can be used for EAP/SIM 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 [11] keyed hash value. (The HMAC- SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by truncating the output to 16 bytes. Hence, the length of the MAC is 16 bytes.) The derivation of the integrity protection key (K_int) used in the calculation of the MAC is specified in Section 15. 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/SIM 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 Haverinen Expires in six months [Page 11] Internet Draft EAP SIM Authentication June 2002 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 SIM 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) [5] 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 [6] for a description of the CBC mode. The format of the AT_ENCR_DATA attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_ENCR_DATA | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Encrypted Data . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The derivation of the encryption key (K_encr) is specified in Section 15. The plaintext consists of nested EAP/SIM attributes. 8. EAP-Response/Identity In the beginning of EAP authentication, the Authenticator issues the EAP-Request/Identity packet to the client. The client responds with Haverinen Expires in six months [Page 12] Internet Draft EAP SIM Authentication June 2002 EAP-Response/Identity, which contains the user's identity. The formats of these packets are specified in [1]. GSM subscribers are identified with the International Mobile Subscriber Identity (IMSI) [7]. The IMSI is composed of a three digit Mobile Country Code (MCC), a two or three digit Mobile Network Code (MNC) and a not more than 10 digit Mobile Subscriber Identification Number (MSIN). In other words, the IMSI is a string of not more than 15 digits. MCC and MNC uniquely identify the GSM operator. Internet AAA protocols identify users with the Network Access Identifier (NAI) [8]. 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 a configurable parameter in the EAP/SIM 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/SIM users. This convention makes it easy to recognize that the NAI identifies a GSM subscriber. Such reserved NAI realm may be useful as a hint as to the first authentication method to use during method negotiation. There are two types of NAI username portions in EAP/SIM: non- pseudonym permanent usernames and pseudonym usernames. 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 "1imsi". In other words, the first character of the username is the digit one (ASCII value 0x31), 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 [7]. The EAP server MAY use the leading "1" as a hint to try EAP/SIM as the first authentication method during method negotiation, rather than for example EAP/AKA. The EAP/SIM server MAY propose EAP/SIM even if the leading character was not "1". 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. 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, Haverinen Expires in six months [Page 13] Internet Draft EAP SIM Authentication June 2002 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. 9. EAP-Request/SIM/Start The first SIM specific EAP Request is of subtype Start. The format of the EAP Request/SIM/Start 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_ID..._REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 1 for Request Identifier See [1]. Length The length of the EAP packet. Type 18 Subtype 10 Reserved Set to zero on sending, ignored on reception Haverinen Expires in six months [Page 14] Internet Draft EAP SIM Authentication June 2002 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. 10. EAP-Response/SIM/Start The format of the EAP Response/SIM/Start 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_NONCE_MT | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | NONCE_MT | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_PERM... | Length | Actual Identity Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Cleartext Identity (optional) . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IDENTITY | Length | Actual Identity Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Current Identity (optional) . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 2 for Response Haverinen Expires in six months [Page 15] Internet Draft EAP SIM Authentication June 2002 Identifier See [1]. Length The length of the EAP packet. Type 18 Subtype 10 Reserved Set to zero when sending, ignored on reception. AT_NONCE_MT The AT_NONCE_MT attribute MUST be included. The value field contains two reserved bytes followed by a random number generated by the client (16 bytes) freshly for this EAP/SIM authentication. The random number is used as a seed value for the new keying material. The reserved bytes are set to zero upon sending and ignored upon reception. AT_PERMANENT_IDENTITY The AT_PERMANENT_IDENTITY attribute is optional and it is included in cases defined 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 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 username format is specified in Section 8. 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. The username is either the non-pseudonym permanent Haverinen Expires in six months [Page 16] Internet Draft EAP SIM Authentication June 2002 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. 11. EAP-Request/SIM/Challenge The format of the EAP-Request/SIM/Challenge 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_RAND | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . n*RAND . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IV | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Initialization Vector (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_ENCR_DATA | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Encrypted Data (optional) . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_MAC | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | MAC | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 1 for Request Identifier See [1] Haverinen Expires in six months [Page 17] Internet Draft EAP SIM Authentication June 2002 Length The length of the EAP packet. Type 18 Subtype 11 Reserved Set to zero when sending, ignored on reception. AT_RAND The AT_RAND attribute MUST be included. The value field of this attribute contains two reserved bytes followed by n GSM RANDs (each 16 bytes long). The reserved bytes are set to zero upon sending and ignored upon reception. The number of RAND challenges MUST be two or three. The client MAY silently ignore the EAP-Request/SIM/Challenge message, if the number of RAND challenges is two while the client's local policy requires three challenges to be used. AT_IV The AT_IV attribute is optional. See section 7.2. AT_ENCR_DATA The AT_ENCR_DATA attribute is optional. See section 7.2. The plaintext consists of nested attributes as described below. AT_MAC AT_MAC MUST be included in EAP-Request/SIM/Challenge for network authentication. See Section 7.1. The AT_IV, AT_ENCR_DATA and AT_MAC attributes are used for identity privacy. The plaintext of the AT_ENCR_DATA value field consists of nested attributes, which are shown below. Haverinen Expires in six months [Page 18] Internet Draft EAP SIM 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 The AT_PSEUDONYM 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. 12. EAP-Response/SIM/Challenge The format of the EAP-Response/SIM/Challenge packet is shown below. As specified in Section 7, EAP-Response/SIM/Challenge MAY include the AT_MAC attribute to integrity protect the EAP packet. Later Haverinen Expires in six months [Page 19] Internet Draft EAP SIM Authentication June 2002 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/SIM/Challenge packet. The EAP server MUST process EAP- Response/SIM/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_MAC_SRES | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | MAC_SRES | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 2 for Response Identifier See [1]. Length The length of the EAP packet. Type 18 Subtype 11 Reserved Set to zero when sending, ignored on reception. AT_MAC_SRES The AT_MAC_SRES attribute MUST be included. The value field of this attribute contains two reserved bytes followed by the Haverinen Expires in six months [Page 20] Internet Draft EAP SIM Authentication June 2002 MAC_SRES response calculated by the client (Section 15), 16 bytes. The reserved bytes are set to zero upon sending and ignored upon reception. 13. Unsuccessful Cases As normally in EAP, the client is sent the EAP-Failure packet when the authentication procedure fails on the EAP Server. In EAP/SIM, this may occur for example if the EAP server is not able to obtain the GSM triplets for the subscriber or the EAP server receives an incorrect MAC_SRES. In general, if an error occurs on the client while processing a received EAP-Request packet, the client silently ignores the EAP packet and does not send any EAP messages to the network. Examples of such errors, specified in detail elsewhere in this document, are an invalid AT_MAC value, insufficient number of RAND challenges included in AT_RAND, and an unrecognized non-skippable attribute. As specified in [1], the EAP client must respond with EAP- Response/Nak when it receives an EAP Request of an undesired or unrecognized authentication type. 14. EAP/SIM Notifications The EAP-Request/Notification, specified in [1], can be used to convey a displayable message from the authenticator to the client. Because these messages are textual messages, it may be hard for the client to present them in the userÆs preferred language. Therefore, EAP/SIM uses a separate EAP/SIM message subtype to transmit localizable notification codes instead of the EAP- Request/Notification packet. The EAP server MAY issue an EAP-Request/SIM/Notification packet to the client. The client MAY delay the processing of EAP- Request/SIM/Notification and wait for other EAP/SIM requests. If a valid EAP/SIM request of another subtype is received, the client MAY silently ignore the EAP-Request/SIM notification and process the other EAP/SIM request instead. If the client decides to process the EAP-Request/SIM/Notification, then the client MAY show a notification message to the user and the client MUST respond to the EAP server with an EAP-Response/SIM/Notification packet. Some of the notification codes are authorization related and hence not usually considered as part of the responsibility of an EAP method. However, they are included as part of EAP/SIM because there are currently no other ways to convey this information to the user in a localizable way, and the information is potentially useful for the user. An EAP/SIM server implementation may decide never to send these EAP/SIM notifications. The format of the EAP-Request/SIM/Notification packet is shown below. Haverinen Expires in six months [Page 21] Internet Draft EAP SIM 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_NOTIFICATION| Length = 1 | Notification Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 1 for Request Identifier See [1]. Length The length of the EAP packet. Type 18 Subtype 12 Reserved Set to zero when sending, ignored on reception. AT_NOTIFICATION The AT_NOTIFICATION attribute MUST be included. The value field of this attribute contains a two-byte notification code. The following code values have been reserved. The descriptions below illustrate the semantics of the notifications. The client implementation MAY use different wordings when presenting the notifications to the user. The "requested service" depends on the environment where EAP/SIM is applied. 1024 - Visited network does not have a roaming agreement with user's home operator or a suitable roaming broker 1026 û User has been temporarily denied access to the requested service 1031 - User has not subscribed to the requested service Haverinen Expires in six months [Page 22] Internet Draft EAP SIM Authentication June 2002 The format of the EAP-Response/SIM/Notification packet is shown below. Because this packet is only an acknowledgement of EAP- Request/SIM/Notification, it does not contain any mandatory 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 2 for Response Identifier See [1]. Length The length of the EAP packet. Type 18 Subtype 12 Reserved Set to zero when sending, ignored on reception. 15. Calculation of Cryptographic Values This section specifies how keying material is generated and how the message authentication code MAC_SRES is calculated. EAP SIM requires three keys for its own purposes, the integrity protection keys K_sres and K_int, and the encryption key K_encr, to be used with the AT_MAC_SRES, 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 (FIPS) Publication 186-2 [9]. The random number generator is specified in the change notice 1 (2001 October 5) of [9] (Algorithm 1). As specified in the Haverinen Expires in six months [Page 23] Internet Draft EAP SIM Authentication June 2002 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 n GSM Kc keys and the NONCE_MT with the following formula: XKEY = SHA1(n*Kc| NONCE_MT) The notation n*Kc| NONCE_MT denotes the n Kc keys and the NONCE_MT value concatenated. The hash function SHA1 is specified in [10]. The optional user input values (XSEED_j) are set to zero. 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_sres (128 bits), 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). K_sres is used in the calculation of MAC_SRES as follows: MAC_SRES HMAC-SHA1-128 (K_sres, n*SRES | Message Subtype) The keyed message authentication code function HMAC-SHA1-128 is specified in [11]. Message subtype above contains the contents of Haverinen Expires in six months [Page 24] Internet Draft EAP SIM Authentication June 2002 the Subtype field of the EAP/SIM message (one octet), in which MAC_SRES parameter is included. When generating the initial seed value XKEY, the hash function is used as a mixing function to combine several session keys (Kc's) generated by the GSM authentication procedure and the random number NONCE_MT into a single session key. There are several reasons for this. The current GSM session keys are at most 64 bits, so two or more of them are needed to generate a longer key. By using a one-way function to combine the keys, we are assured that even if an attacker managed to learn one of the EAP/SIM session keys, it wouldnÆt help him in learning the original GSM Kc's. In addition, since we include the random number NONCE_MT in the calculation, the client is able to verify that the SIM authentication values it receives from the network are fresh and not a replay. (Please see also Section 17.) 16. IANA Considerations The realm name "owlan.org" has been reserved for NAI realm names generated from the IMSI. IANA has assigned the EAP type number 18 for this protocol. EAP/SIM messages include a Subtype field. The following Subtypes are specified: Start..........................................10 Challenge......................................11 Notification...................................12 The Subtype-specific data is composed of attributes, which have attribute type numbers. The following attribute types are specified: AT_RAND.........................................1 AT_PERMANENT_IDENTITY...........................5 AT_PADDING......................................6 AT_NONCE_MT.....................................7 AT_MAC_SRES.....................................9 AT_PERMANENT_IDENTITY_REQ......................10 AT_MAC.........................................11 AT_NOTIFICATION................................12 AT_IDENTITY_REQ................................13 AT_IDENTITY....................................14 AT_IV.........................................129 AT_ENCR_DATA..................................130 AT_PSEUDONYM..................................132 The AT_NOTIFICATION attribute contains a notification code value. Values 1024, 1026 and 1031 have been specified in Section 14 of this document. Haverinen Expires in six months [Page 25] Internet Draft EAP SIM Authentication June 2002 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 [12]. Requests must be specified in sufficient detail so that interoperability between independent implementations is possible. Possible forms of documentation include, but are not limited to, RFCs, the products of another standards body (e.g. 3GPP), or permanently and readily available vendor design notes. 17. Security Considerations The protocol in this document is intended to provide the appropriate level of security to operate Extensible Authentication Protocol using the GSM SIM application. EAP/SIM 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. EAP/SIM does not protect the privacy of the IMSI against active attacks. An active attacker that impersonates the network can easily learn the subscriber's IMSI. This is the same level of protection as in the GSM and UMTS cellular networks. In EAP/SIM, the client believes that the network is authentic because the network can calculate a correct AT_MAC value in the EAP- Request/SIM/Challenge packet. To calculate AT_MAC, it is sufficient to know the complete GSM triplets (RAND, SRES, Kc) used in the authentication. Because the network selects the RAND challenges and hereby the triplets, an attacker that knows two or three GSM triplets for the subscriber is able to impersonate a valid network to the client. Given physical access to the SIM card, it is easy to obtain any number of GSM triplets. Another way to obtain a RAND challenge and the corresponding SRES response of a GSM triplet is to eavesdrop on the GSM network. For these reasons, network authentication of EAP/SIM SHOULD NOT be used exclusively if strong network authentication is a concern. There is no known way to obtain complete GSM triplets by mounting an attack against EAP/SIM. A passive eavesdropper can learn n*RAND, AT_MAC and AT_MAC_SRES, and may be able to link this information to the subscriber identity. An active attacker that impersonates a GSM subscriber can easily obtain n*RAND and AT_MAC values from the EAP server for any given subscriber identity. However, calculating the Kc and SRES values from AT_MAC and MAC_SRES would require the attacker to reverse the keyed message authentication code function HMAC-SHA1-128. EAP/SIM combines several GSM triplets in order to generate a stronger session key and stronger AT_MAC and AT_MAC_SRES values. The actual strength of the resulting key depends, among other things, on the operator-specific authentication algorithms, the strength of the Ki key, and the quality of the RAND challenges, which is also Haverinen Expires in six months [Page 26] Internet Draft EAP SIM Authentication June 2002 operator specific. For example, some SIM cards generate Kc keys with 10 bits set to zero. Such restrictions may prevent the concatenation technique from yielding strong session keys. Because the strength of the Ki key is 128 bits, the ultimate strength of any derived secret key material is never more than 128 bits. An EAP/SIM implementation SHOULD use a good source of randomness to generate the random numbers required in the protocol. Please see [13] for more information on generating random numbers for security applications. 18. Intellectual Property Right Notice On IPR related issues, Nokia refers to the Nokia Statement on Patent licensing, see http://www.ietf.org/ietf/IPR/NOKIA. 19. Acknowledgements and Contributions The editor thanks Juha Ala-Laurila, N. Asokan, Simon Blake-Wilson, Jan-Erik Ekberg, Patrik Flykt, Mark Grayson, Jukka-Pekka Honkanen, Antti Kuikka, Jukka Latva, Lassi Lehtinen, Valtteri Niemi, Kaisa Nyberg, Jyri Rinnemaa, Joe Salowey, Timo Takam„ki and Raimo Vuonnala for theirs contributions and critiques. Thanks to Greg Rose of Qualcomm for his most valuable comments [14]. The IMSI privacy support is based on the identity privacy support of [4]. The attribute format is based on the extension format of Mobile IPv4 [15]. This protocol has been partly developed in parallel with EAP AKA [16], and hence this specification incorporates many ideas from Jari Arkko. References [1] L. Blunk, J. Vollbrecht, "PPP Extensible Authentication Protocol (EAP)", RFC 2284, March 1998. (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] S. Bradner, "Key words for use in RFCs to indicate Requirement Levels", RFC 2119, March 1997. (NORMATIVE) [4] J. Carlson, B. Aboba, H. Haverinen, "EAP SRP-SHA1 Authentication Protocol", draft-ietf-pppext-eap-srp-03.txt, July 2001 (work-in-progress). (INFORMATIVE) Haverinen Expires in six months [Page 27] Internet Draft EAP SIM Authentication June 2002 [5] Federal Information Processing Standard (FIPS) draft standard, "Advanced Encryption Standard (AES)", http://csrc.nist.gov/publications/drafts/dfips-AES.pdf, September 2001. (NORMATIVE) [6] US National Bureau of Standards, "DES Modes of Operation", Federal Information Processing Standard (FIPS) Publication 81, December 1980. (NORMATIVE) [7] 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) [8] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC 2486, January 1999. (NORMATIVE) [9] 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 [10] 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) [11] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. (NORMATIVE) [12] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998. (NORMATIVE) [13] D. Eastlake, 3rd, S. Crocker, J. Schiller, "Randomness Recommendations for Security", RFC 1750 (Informational), December 1994. (INFORMATIVE) [14] Qualcomm, "Comments on draft EAP/SIM", 3rd Generation Partnership Project document 3GPP TSG SA WG3 Security ù S3#22, S3-020125, February 2002. (INFORMATIVE) [15] C. Perkins (editor), "IP Mobility Support", RFC 2002, October 1996. (INFORMATIVE) Haverinen Expires in six months [Page 28] Internet Draft EAP SIM Authentication June 2002 [16] J. Arkko, H. Haverinen, "EAP AKA Authentication", draft-arkko- pppext-eap-aka-04.txt, June 2002 (work in progress). (INFORMATIVE) Editor's Address Henry Haverinen Nokia Mobile Phones P.O. Box 88 FIN-33721 Tampere Finland E-mail: henry.haverinen@nokia.com Phone: +358 50 594 4899 Fax: +358 3 318 3690 Haverinen Expires in six months [Page 29]