RFC 4186
Extensible Authentication Protocol Method for Global System for Mobile Communications (GSM) Subscriber Identity Modules (EAP-SIM)
9. Messages
This section specifies the messages used in EAP-SIM. It specifies when a message may be transmitted or accepted, which attributes are allowed in a message, which attributes are required in a message, and other message-specific details. The general message format is specified in Section 8.1.9.1. EAP-Request/SIM/Start
In full authentication the first SIM-specific EAP Request is EAP-Request/SIM/Start. The EAP/SIM/Start roundtrip is used for two purposes. In full authentication this packet is used to request the peer to send the AT_NONCE_MT attribute to the server. In addition, as specified in Section 4.2, the Start round trip may be used by the server for obtaining the peer identity. As discussed in Section 4.2, several Start rounds may be required to obtain a valid peer identity. The server MUST always include the AT_VERSION_LIST attribute. The server MAY include one of the following identity-requesting attributes: AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, or AT_ANY_ID_REQ. These three attributes are mutually exclusive, so the server MUST NOT include more than one of the attributes. If the server has received a response from the peer, it MUST NOT issue a new EAP-Request/SIM/Start packet if it has previously issued an EAP-Request/SIM/Start message either without any identity requesting attributes or with the AT_PERMANENT_ID_REQ attribute. If the server has received a response from the peer, it MUST NOT issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ attributes if it has previously issued an EAP-Request/SIM/Start message with the AT_FULLAUTH_ID_REQ attribute.
If the server has received a response from the peer, it MUST NOT issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ attribute if the server has previously issued an EAP-Request/SIM/Start message with the AT_ANY_ID_REQ attribute. This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.9.2. EAP-Response/SIM/Start
The peer sends EAP-Response/SIM/Start in response to a valid EAP-Request/SIM/Start from the server. If and only if the server's EAP-Request/SIM/Start includes one of the identity-requesting attributes, then the peer MUST include the AT_IDENTITY attribute. The usage of AT_IDENTITY is defined in Section 4.2. The AT_NONCE_MT attribute MUST NOT be included if the AT_IDENTITY with a fast re-authentication identity is present for fast re-authentication. AT_NONCE_MT MUST be included in all other cases (full authentication). The AT_SELECTED_VERSION attribute MUST NOT be included if the AT_IDENTITY attribute with a fast re-authentication identity is present for fast re-authentication. In all other cases, AT_SELECTED_VERSION MUST be included (full authentication). This attribute is used in version negotiation, as specified in Section 4.1. This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.9.3. EAP-Request/SIM/Challenge
The server sends the EAP-Request/SIM/Challenge after receiving a valid EAP-Response/SIM/Start that contains AT_NONCE_MT and AT_SELECTED_VERSION, and after successfully obtaining the subscriber identity. The AT_RAND attribute MUST be included. The AT_RESULT_IND attribute MAY be included. The usage of this attribute is discussed in Section 6.2. The AT_MAC attribute MUST be included. For EAP-Request/SIM/Challenge, the MAC code is calculated over the following data: EAP packet| NONCE_MT
The EAP packet is represented as specified in Section 8.1. It is followed by the 16-byte NONCE_MT value from the peer's AT_NONCE_MT attribute. The EAP-Request/SIM/Challenge packet MAY include encrypted attributes for identity privacy and for communicating the next fast re-authentication identity. In this case, the AT_IV and AT_ENCR_DATA attributes are included (Section 10.12). The plaintext of the AT_ENCR_DATA value field consists of nested attributes. The nested attributes MAY include AT_PADDING (as specified in Section 10.12). If the server supports identity privacy and wants to communicate a pseudonym to the peer for the next full authentication, then the nested encrypted attributes include the AT_NEXT_PSEUDONYM attribute. If the server supports re-authentication and wants to communicate a fast re-authentication identity to the peer, then the nested encrypted attributes include the AT_NEXT_REAUTH_ID attribute. When processing this message, the peer MUST process AT_RAND before processing other attributes. Only if AT_RAND is verified to be valid, the peer derives keys and verifies AT_MAC. The operation in case an error occurs is specified in Section 6.3.1.9.4. EAP-Response/SIM/Challenge
The peer sends EAP-Response/SIM/Challenge in response to a valid EAP-Request/SIM/Challenge. Sending this packet indicates that the peer has successfully authenticated the server and that the EAP exchange will be accepted by the peer's local policy. Hence, if these conditions are not met, then the peer MUST NOT send EAP-Response/SIM/Challenge, but the peer MUST send EAP-Response/SIM/Client-Error. The AT_MAC attribute MUST be included. For EAP- Response/SIM/Challenge, the MAC code is calculated over the following data: EAP packet| n*SRES The EAP packet is represented as specified in Section 8.1. The EAP packet bytes are immediately followed by the two or three SRES values concatenated, denoted above with the notation n*SRES. The SRES values are used in the same order as the corresponding RAND challenges in the server's AT_RAND attribute.
The AT_RESULT_IND attribute MAY be included if it was included in EAP-Request/SIM/Challenge. The usage of this attribute is discussed in Section 6.2. Later versions of this protocol MAY make use of the AT_ENCR_DATA and AT_IV attributes in this message to include encrypted (skippable) attributes. The EAP server MUST process EAP-Response/SIM/Challenge messages that include these attributes even if the server did not implement these optional attributes.9.5. EAP-Request/SIM/Re-authentication
The server sends the EAP-Request/SIM/Re-authentication message if it wants to use fast re-authentication, and if it has received a valid fast re-authentication identity in EAP-Response/Identity or EAP-Response/SIM/Start. AT_MAC MUST be included. No message-specific data is included in the MAC calculation. See Section 10.14. The AT_RESULT_IND attribute MAY be included. The usage of this attribute is discussed in Section 6.2. The AT_IV and AT_ENCR_DATA attributes MUST be included. The plaintext consists of the following nested encrypted attributes, which MUST be included: AT_COUNTER and AT_NONCE_S. In addition, the nested encrypted attributes MAY include the following attributes: AT_NEXT_REAUTH_ID and AT_PADDING.9.6. EAP-Response/SIM/Re-authentication
The client sends the EAP-Response/SIM/Re-authentication packet in response to a valid EAP-Request/SIM/Re-authentication. The AT_MAC attribute MUST be included. For EAP-Response/SIM/Re-authentication, the MAC code is calculated over the following data: EAP packet| NONCE_S The EAP packet is represented as specified in Section 8.1. It is followed by the 16-byte NONCE_S value from the server's AT_NONCE_S attribute. The AT_IV and AT_ENCR_DATA attributes MUST be included. The nested encrypted attributes MUST include the AT_COUNTER attribute. The AT_COUNTER_TOO_SMALL attribute MAY be included in the nested
encrypted attributes, and it is included in cases specified in Section 5. The AT_PADDING attribute MAY be included. The AT_RESULT_IND attribute MAY be included if it was included in EAP-Request/SIM/Re-authentication. The usage of this attribute is discussed in Section 6.2. Sending this packet without AT_COUNTER_TOO_SMALL indicates that the peer has successfully authenticated the server and that the EAP exchange will be accepted by the peer's local policy. Hence, if these conditions are not met, then the peer MUST NOT send EAP-Response/SIM/Re-authentication, but the peer MUST send EAP-Response/SIM/Client-Error.9.7. EAP-Response/SIM/Client-Error
The peer sends EAP-Response/SIM/Client-Error in error cases, as specified in Section 6.3.1. The AT_CLIENT_ERROR_CODE attribute MUST be included. The AT_MAC, AT_IV, or AT_ENCR_DATA attributes MUST NOT be used with this packet.9.8. EAP-Request/SIM/Notification
The usage of this message is specified in Section 6. The AT_NOTIFICATION attribute MUST be included. The AT_MAC attribute MUST be included if the P bit of the notification code in AT_NOTIFICATION is set to zero, and MUST NOT be included in cases when the P bit is set to one. The P bit is discussed in Section 6. No message-specific data is included in the MAC calculation. See Section 10.14. If EAP-Request/SIM/Notification is used on a fast re-authentication exchange, and if the P bit in AT_NOTIFICATION is set to zero, then AT_COUNTER is used for replay protection. In this case, the AT_ENCR_DATA and AT_IV attributes MUST be included, and the encapsulated plaintext attributes MUST include the AT_COUNTER attribute. The counter value included in AT_COUNTER MUST be the same as in the EAP-Request/SIM/Re-authentication packet on the same fast re-authentication exchange.
9.9. EAP-Response/SIM/Notification
The usage of this message is specified in Section 6. This packet is an acknowledgement of EAP-Request/SIM/Notification. The AT_MAC attribute MUST be included in cases when the P bit of the notification code in AT_NOTIFICATION of EAP-Request/SIM/Notification is set to zero, and MUST NOT be included in cases when the P bit is set to one. The P bit is discussed in Section 6. No message-specific data is included in the MAC calculation, see Section 10.14. If EAP-Request/SIM/Notification is used on a fast re-authentication exchange, and if the P bit in AT_NOTIFICATION is set to zero, then AT_COUNTER is used for replay protection. In this case, the AT_ENCR_DATA and AT_IV attributes MUST be included, and the encapsulated plaintext attributes MUST include the AT_COUNTER attribute. The counter value included in AT_COUNTER MUST be the same as in the EAP-Request/SIM/Re-authentication packet on the same fast re-authentication exchange.10. Attributes
This section specifies the format of message attributes. The attribute type numbers are specified in the IANA considerations section of the EAP-AKA specification [EAP-AKA].10.1. Table of Attributes
The following table provides a guide to which attributes may be found in which kinds of messages, and in what quantity. Messages are denoted with numbers in parentheses as follows: (1) EAP-Request/SIM/Start, (2) EAP-Response/SIM/Start, (3) EAP-Request/SIM/Challenge, (4) EAP-Response/SIM/Challenge, (5) EAP-Request/SIM/Notification, (6) EAP-Response/SIM/Notification, (7) EAP-Response/SIM/Client-Error, (8) EAP-Request/SIM/Re-authentication, and (9) EAP-Response/SIM/Re-authentication. The column denoted with "Encr" indicates whether the attribute is a nested attribute that MUST be included within AT_ENCR_DATA, and the column denoted with "Skip" indicates whether the attribute is a skippable attribute. "0" indicates that the attribute MUST NOT be included in the message, "1" indicates that the attribute MUST be included in the message, "0-1" indicates that the attribute is sometimes included in the message, and "0*" indicates that the attribute is not included in the message in cases specified in this document, but MAY be included in future versions of the protocol.
Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) Encr Skip
AT_VERSION_LIST 1 0 0 0 0 0 0 0 0 N N
AT_SELECTED_VERSION 0 0-1 0 0 0 0 0 0 0 N N
AT_NONCE_MT 0 0-1 0 0 0 0 0 0 0 N N
AT_PERMANENT_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N
AT_ANY_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N
AT_FULLAUTH_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N
AT_IDENTITY 0 0-1 0 0 0 0 0 0 0 N N
AT_RAND 0 0 1 0 0 0 0 0 0 N N
AT_NEXT_PSEUDONYM 0 0 0-1 0 0 0 0 0 0 Y Y
AT_NEXT_REAUTH_ID 0 0 0-1 0 0 0 0 0-1 0 Y Y
AT_IV 0 0 0-1 0* 0-1 0-1 0 1 1 N Y
AT_ENCR_DATA 0 0 0-1 0* 0-1 0-1 0 1 1 N Y
AT_PADDING 0 0 0-1 0* 0-1 0-1 0 0-1 0-1 Y N
AT_RESULT_IND 0 0 0-1 0-1 0 0 0 0-1 0-1 N Y
AT_MAC 0 0 1 1 0-1 0-1 0 1 1 N N
AT_COUNTER 0 0 0 0 0-1 0-1 0 1 1 Y N
AT_COUNTER_TOO_SMALL 0 0 0 0 0 0 0 0 0-1 Y N
AT_NONCE_S 0 0 0 0 0 0 0 1 0 Y N
AT_NOTIFICATION 0 0 0 0 1 0 0 0 0 N N
AT_CLIENT_ERROR_CODE 0 0 0 0 0 0 1 0 0 N N
It should be noted that attributes AT_PERMANENT_ID_REQ,
AT_ANY_ID_REQ, and AT_FULLAUTH_ID_REQ are mutually exclusive; only
one of them can be included at the same time. If one of the
attributes AT_IV and AT_ENCR_DATA is included, then both of the
attributes MUST be included.
10.2. AT_VERSION_LIST
The format of the AT_VERSION_LIST 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_VERSION_L..| Length | Actual Version List Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Supported Version 1 | Supported Version 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Supported Version N | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This attribute is used in version negotiation, as specified in
Section 4.1. The attribute contains the version numbers supported by
the EAP-SIM server. The server MUST only include versions that it
implements and that are allowed in its security policy. The server SHOULD list the versions in the order of preference, with the most preferred versions listed first. At least one version number MUST be included. The version number for the protocol described in this document is one (0001 hexadecimal). The value field of this attribute begins with 2-byte Actual Version List Length, which specifies the length of the Version List in bytes, not including the Actual Version List Length attribute length. This field is followed by the list of the versions supported by the server, which each have a length of 2 bytes. For example, if there is only one supported version, then the Actual Version List Length is 2. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the value field with zero bytes when necessary.10.3. AT_SELECTED_VERSION
The format of the AT_SELECTED_VERSION 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_SELECTED...| Length = 1 | Selected Version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This attribute is used in version negotiation, as specified in Section 4.1. The value field of this attribute contains a two-byte version number, which indicates the EAP-SIM version that the peer wants to use.10.4. AT_NONCE_MT
The format of the AT_NONCE_MT 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_NONCE_MT | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | NONCE_MT | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value field of the NONCE_MT attribute contains two reserved bytes followed by a random number freshly generated by the peer (16 bytes long) for this EAP-SIM authentication exchange. 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. The peer MUST NOT re-use the NONCE_MT value from a previous EAP-SIM authentication exchange. If an EAP-SIM exchange includes several EAP/SIM/Start rounds, then the peer SHOULD use the same NONCE_MT value in all EAP-Response/SIM/Start packets. The peer SHOULD use a good source of randomness to generate NONCE_MT. Please see [RFC4086] for more information about generating random numbers for security applications.10.5. AT_PERMANENT_ID_REQ
The format of the AT_PERMANENT_ID_REQ attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_PERM..._REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The use of the AT_PERMANENT_ID_REQ is defined in Section 4.2. The value field contains only two reserved bytes, which are set to zero on sending and ignored on reception.10.6. AT_ANY_ID_REQ
The format of the AT_ANY_ID_REQ attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_ANY_ID_REQ | Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The use of the AT_ANY_ID_REQ is defined in Section 4.2. The value field contains only two reserved bytes, which are set to zero on sending and ignored on reception.
10.7. AT_FULLAUTH_ID_REQ
The format of the AT_FULLAUTH_ID_REQ attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_FULLAUTH_...| Length = 1 | Reserved | +---------------+---------------+-------------------------------+ The use of the AT_FULLAUTH_ID_REQ is defined in Section 4.2. The value field contains only two reserved bytes, which are set to zero on sending and ignored on reception.10.8. AT_IDENTITY
The format of the AT_IDENTITY attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_IDENTITY | Length | Actual Identity Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Identity (optional) . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The use of the AT_IDENTITY is defined in Section 4.2. The value field of this attribute begins with a 2-byte actual identity length, which specifies the length of the identity in bytes. This field is followed by the subscriber identity of the indicated actual length. The identity is the permanent identity, a pseudonym identity, or a fast re-authentication identity. The identity format is specified in Section 4.2.1. The same identity format is used in the AT_IDENTITY attribute and the EAP-Response/Identity packet, with the exception that the peer MUST NOT decorate the identity it includes in AT_IDENTITY. The identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the identity with zero bytes when necessary.
10.9. AT_RAND
The format of the AT_RAND attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_RAND | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . n*RAND . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains two reserved bytes followed by n GSM RANDs, each 16 bytes long. The value of n can be determined by the attribute length. The reserved bytes are set to zero upon sending and ignored upon reception. The number of RAND challenges (n) MUST be two or three. The peer MUST verify that the number of RAND challenges is sufficient according to the peer's policy. The server MUST use different RAND values. In other words, a RAND value can only be included once in AT_RAND. When processing the AT_RAND attribute, the peer MUST check that the RANDs are different. The EAP server MUST obtain fresh RANDs for each EAP-SIM full authentication exchange. More specifically, the server MUST consider RANDs it included in AT_RAND to be consumed if the server receives an EAP-Response/SIM/Challenge packet with a valid AT_MAC, or an EAP-Response/SIM/Client-Error with the code "insufficient number of challenges" or "RANDs are not fresh". However, in other cases (if the server does not receive a response to its EAP-Request/SIM/Challenge packet, or if the server receives a response other than the cases listed above), the server does not need to consider the RANDs to be consumed, and the server MAY re-use the RANDs in the AT_RAND attribute of the next full authentication attempt.
10.10. AT_NEXT_PSEUDONYM
The format of the AT_NEXT_PSEUDONYM attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_PSEU..| Length | Actual Pseudonym Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Pseudonym . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute begins with the 2-byte actual pseudonym length, which specifies the length of the following pseudonym in bytes. This field is followed by a pseudonym username that the peer can use in the next authentication. The username MUST NOT include any realm portion. The username does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the pseudonym with zero bytes when necessary. The username encoding MUST follow the UTF-8 transformation format [RFC3629]. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute.10.11. AT_NEXT_REAUTH_ID
The format of the AT_NEXT_REAUTH_ID attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_NEXT_REAU..| Length | Actual Re-Auth Identity Length| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Next Fast Re-authentication Username . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute begins with the 2-byte actual re-authentication identity length which specifies the length of the following fast re-authentication identity in bytes. This field is followed by a fast re-authentication identity that the peer can use in the next fast re-authentication, as described in Section 5. In environments where a realm portion is required, the fast re-authentication identity includes both a username portion and a
realm name portion. The fast re-authentication identity does not include any terminating null characters. Because the length of the attribute must be a multiple of 4 bytes, the sender pads the fast re-authentication identity with zero bytes when necessary. The identity encoding MUST follow the UTF-8 transformation format [RFC3629]. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute.10.12. AT_IV, AT_ENCR_DATA, and AT_PADDING
AT_IV and AT_ENCR_DATA attributes can be used to transmit encrypted information between the EAP-SIM peer and server. The value field of AT_IV contains two reserved bytes followed by a 16-byte initialization vector required by the AT_ENCR_DATA attribute. The reserved bytes are set to zero when sending and ignored on reception. The AT_IV attribute MUST be included if and only if the AT_ENCR_DATA is included. Section 6.3 specifies the operation if a packet that does not meet this condition is encountered. The sender of the AT_IV attribute chooses the initialization vector at random. The sender MUST NOT re-use the initialization vector value from previous EAP-SIM packets. The sender SHOULD use a good source of randomness to generate the initialization vector. Please see [RFC4086] for more information about generating random numbers for security applications. 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 cipher text bytes encrypted using the Advanced Encryption Standard (AES) [AES] with a 128-bit key in the Cipher Block Chaining (CBC) mode of operation using the initialization vector from the AT_IV attribute. The reserved bytes are set to zero when sending and ignored on reception. Please see [CBC] for a description of the CBC mode. The format of the AT_ENCR_DATA attribute is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_ENCR_DATA | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Encrypted Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The derivation of the encryption key (K_encr) is specified in Section
7.
The plaintext consists of nested EAP-SIM attributes.
The encryption algorithm requires the length of the plaintext to be a
multiple of 16 bytes. The sender may need to include the AT_PADDING
attribute as the last attribute within AT_ENCR_DATA. The AT_PADDING
attribute is not included if the total length of other nested
attributes within the AT_ENCR_DATA attribute is a multiple of 16
bytes. As usual, the Length of the Padding attribute includes the
Attribute Type and Attribute Length fields. The length of the
Padding attribute is 4, 8, or 12 bytes. It is chosen so that the
length of the value field of the AT_ENCR_DATA attribute becomes a
multiple of 16 bytes. The actual pad bytes in the value field are
set to zero (00 hexadecimal) on sending. The recipient of the
message MUST verify that the pad bytes are set to zero. If this
verification fails on the peer, then it MUST send the
EAP-Response/SIM/Client-Error packet with the error code "unable to
process packet" to terminate the authentication exchange. If this
verification fails on the server, then the server sends the peer the
EAP-Request/SIM/Notification packet with an AT_NOTIFICATION code that
implies failure to terminate the authentication exchange. The format
of the AT_PADDING attribute is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_PADDING | Length | Padding... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
10.13. AT_RESULT_IND
The format of the AT_RESULT_IND 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_RESULT_...| Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute consists of two reserved bytes, which are set to zero upon sending and ignored upon reception. This attribute is always sent unencrypted, so it MUST NOT be encapsulated within the AT_ENCR_DATA attribute.10.14. AT_MAC
The AT_MAC attribute is used for EAP-SIM message authentication. Section 8 specifies in which messages AT_MAC MUST be included. The value field of the AT_MAC attribute contains two reserved bytes followed by a keyed message authentication code (MAC). The MAC is calculated over the whole EAP packet and concatenated with optional message-specific data, with the exception that the value field of the MAC attribute is set to zero when calculating the MAC. The EAP packet includes the EAP header that begins with the Code field, the EAP-SIM header that begins with the Subtype field, and all the attributes, as specified in Section 8.1. The reserved bytes in AT_MAC are set to zero when sending and ignored on reception. The contents of the message-specific data that may be included in the MAC calculation are specified separately for each EAP-SIM message in Section 9. 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 an HMAC-SHA1-128 [RFC2104] keyed hash value. (The HMAC-SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by truncating the output to the first 16 bytes. Hence, the length of the MAC is 16 bytes. The derivation of the authentication key (K_aut) used in the calculation of the MAC is specified in Section 7. When the AT_MAC attribute is included in an EAP-SIM message, the recipient MUST process the AT_MAC attribute before looking at any other attributes, except when processing EAP-Request/SIM/Challenge. The processing of EAP-Request/SIM/Challenge is specified in Section 9.3. If the message authentication code is invalid, then the recipient MUST ignore all other attributes in the message and operate as specified in Section 6.3.10.15. AT_COUNTER
The format of the AT_COUNTER attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER | Length = 1 | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the AT_COUNTER attribute consists of a 16-bit unsigned integer counter value, represented in network byte order. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute.10.16. AT_COUNTER_TOO_SMALL
The format of the AT_COUNTER_TOO_SMALL attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AT_COUNTER...| Length = 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute consists of two reserved bytes, which are set to zero upon sending and ignored upon reception. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute.
10.17. AT_NONCE_S
The format of the AT_NONCE_S 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_NONCE_S | Length = 5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | NONCE_S | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of the AT_NONCE_S attribute contains two reserved bytes followed by a random number freshly generated by the server (16 bytes) for this EAP-SIM fast re-authentication. The random number is used as a challenge for the peer and also as a seed value for the new keying material. The reserved bytes are set to zero upon sending and ignored upon reception. This attribute MUST always be encrypted by encapsulating it within the AT_ENCR_DATA attribute. The server MUST NOT re-use the NONCE_S value from any previous EAP-SIM fast re-authentication exchange. The server SHOULD use a good source of randomness to generate NONCE_S. Please see [RFC4086] for more information about generating random numbers for security applications.10.18. AT_NOTIFICATION
The format of the AT_NOTIFICATION attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_NOTIFICATION| Length = 1 |S|P| Notification Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains a two-byte notification code. The first and second bit (S and P) of the notification code are interpreted as described in Section 6. The notification code values listed below have been reserved. The descriptions below illustrate the semantics of the notifications.
The peer implementation MAY use different wordings when presenting the notifications to the user. The "requested service" depends on the environment where EAP-SIM is applied. 0 - General failure after authentication. (Implies failure, used after successful authentication.) 16384 - General failure. (Implies failure, used before authentication.) 32768 - Success. User has been successfully authenticated. (Does not imply failure, used after successful authentication). The usage of this code is discussed in Section 6.2. 1026 - User has been temporarily denied access to the requested service. (Implies failure, used after successful authentication.) 1031 - User has not subscribed to the requested service. (Implies failure, used after successful authentication.)10.19. AT_CLIENT_ERROR_CODE
The format of the AT_CLIENT_ERROR_CODE attribute is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AT_CLIENT_ERR..| Length = 1 | Client Error Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value field of this attribute contains a two-byte client error code. The following error code values have been reserved. 0 "unable to process packet": a general error code 1 "unsupported version": the peer does not support any of the versions listed in AT_VERSION_LIST 2 "insufficient number of challenges": the peer's policy requires more triplets than the server included in AT_RAND 3 "RANDs are not fresh": the peer believes that the RAND challenges included in AT_RAND were not fresh
11. IANA Considerations
IANA has assigned the EAP type number 18 for this protocol. EAP-SIM shares most of the protocol design, such as attributes and message Subtypes, with EAP-AKA [EAP-AKA]. EAP-SIM protocol numbers should be administered in the same IANA registry as EAP-AKA. The initial values are listed in [EAP-AKA] for both protocols, so this document does not require any new registries or parameter allocation. As a common registry is used for EAP-SIM and EAP-AKA, the protocol number allocation policy for both protocols is specified in [EAP-AKA].12. Security Considerations
The EAP specification [RFC3748] describes the security vulnerabilities of EAP, which does not include its own security mechanisms. This section discusses the claimed security properties of EAP-SIM, as well as vulnerabilities and security recommendations.12.1. A3 and A8 Algorithms
The GSM A3 and A8 algorithms are used in EAP-SIM. [GSM-03.20] specifies the general GSM authentication procedure and the external interface (inputs and outputs) of the A3 and A8 algorithms. The operation of these functions falls completely within the domain of an individual operator, and therefore, the functions are specified by each operator rather than being fully standardised. The GSM-MILENAGE algorithm, specified publicly in [3GPP-TS-55.205], is an example algorithm set for A3 and A8 algorithms. The security of the A3 and A8 algorithms is important to the security of EAP-SIM. Some A3/A8 algorithms have been compromised; see [GSM- Cloning] for discussion about the security of COMP-128 version 1. Note that several revised versions of the COMP-128 A3/A8 algorithm have been devised after the publication of these weaknesses and that the publicly specified GSM-MILENAGE algorithm is not vulnerable to any known attacks.12.2. Identity Protection
EAP-SIM includes optional identity privacy support that protects the privacy of the subscriber identity against passive eavesdropping. This document only specifies a mechanism to deliver pseudonyms from the server to the peer as part of an EAP-SIM exchange. Hence, a peer that has not yet performed any EAP-SIM exchanges does not typically have a pseudonym available. If the peer does not have a pseudonym available, then the privacy mechanism cannot be used, but the
permanent identity will have to be sent in the clear. The terminal SHOULD store the pseudonym in a non-volatile memory so that it can be maintained across reboots. An active attacker that impersonates the network may use the AT_PERMANENT_ID_REQ attribute to attempt to learn the subscriber's permanent identity. However, as discussed in Section 4.2.2, the terminal can refuse to send the cleartext permanent identity if it believes that the network should be able to recognize the pseudonym. If the peer and server cannot guarantee that the pseudonym will be maintained reliably, and identity privacy is required, then additional protection from an external security mechanism (such as Protected Extensible Authentication Protocol (PEAP) [PEAP]) may be used. If an external security mechanism is in use, the identity privacy features of EAP-SIM may not be useful. The security considerations of using an external security mechanism with EAP-SIM are beyond the scope of this document.12.3. Mutual Authentication and Triplet Exposure
EAP-SIM provides mutual authentication. The peer 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 RAND and Kc values from the GSM triplets (RAND, SRES, Kc) used in the authentication. Because the network selects the RAND challenges and the triplets, an attacker that knows n (2 or 3) GSM triplets for the subscriber is able to impersonate a valid network to the peer. (Some peers MAY employ an implementation-specific counter-measure against impersonating a valid network by re-using a previously used RAND; see below.) In other words, the security of EAP-SIM is based on the secrecy of Kc keys, which are considered secret intermediate results in the EAP-SIM cryptographic calculations. Given physical access to the SIM card, it is easy to obtain any number of GSM triplets. Another way to obtain triplets is to mount an attack on the peer platform via a virus or other malicious piece of software. The peer SHOULD be protected against triplet querying attacks by malicious software. Care should be taken not to expose Kc keys to attackers when they are stored or handled by the peer, or transmitted between subsystems of the peer. Steps should be taken to limit the transport, storage, and handling of these values outside a protected environment within the peer. However, the virus protection of the peer and the security capabilities of the peer's operating system are outside the scope of this document.
The EAP-SIM server typically obtains the triplets from the Home Location Register (HLR). An attacker might try to obtain triplets by attacking against the network used between the EAP-SIM server and the HLR. Care should be taken not to expose Kc keys to attackers when they are stored or handled by the EAP-SIM server, or transmitted between the EAP server and the HLR. Steps should be taken to limit the transport, storage, and handling of these values outside a protected environment. However, the protection of the communications between the EAP-SIM server and the HLR is outside the scope of this document. If the same SIM credentials are also used for GSM traffic, the triplets could be revealed in the GSM network; see Section 12.8. In GSM, the network is allowed to re-use the RAND challenge in consecutive authentication exchanges. This is not allowed in EAP-SIM. The EAP-SIM server is mandated to use fresh triplets (RAND challenges) in consecutive authentication exchanges, as specified in Section 3. EAP-SIM does not mandate any means for the peer to check if the RANDs are fresh, so the security of the scheme leans on the secrecy of the triplets. However, the peer MAY employ implementation-specific mechanisms to remember some of the previously used RANDs, and the peer MAY check the freshness of the server's RANDs. The operation in cases when the peer detects that the RANDs are not fresh is specified in Section 6.3.1. Preventing the re-use of authentication vectors has been taken into account in the design of the UMTS Authentication and Key Agreement (AKA), which is used in EAP-AKA [EAP-AKA]. In cases when the triplet re-use properties of EAP-SIM are not considered sufficient, it is advised to use EAP-AKA. Note that EAP-SIM mutual authentication is done with the EAP server. In general, EAP methods do not authenticate the identity or services provided by the EAP authenticator (if distinct from the EAP server) unless they provide the so-called channel bindings property. The vulnerabilities related to this have been discussed in [RFC3748], [EAP-Keying], [Service-Identity]. EAP-SIM does not provide the channel bindings property, so it only authenticates the EAP server. However, ongoing work such as [Service-Identity] may provide such support as an extension to popular EAP methods such as EAP-TLS, EAP-SIM, or EAP-AKA.
12.4. Flooding the Authentication Centre
The EAP-SIM server typically obtains authentication vectors from the Authentication Centre (AuC). EAP-SIM introduces a new usage for the AuC. The protocols between the EAP-SIM server and the AuC are out of the scope of this document. However, it should be noted that a malicious EAP-SIM peer may generate a lot of protocol requests to mount a denial of service attack. The EAP-SIM server implementation SHOULD take this into account and SHOULD take steps to limit the traffic that it generates towards the AuC, preventing the attacker from flooding the AuC and from extending the denial of service attack from EAP-SIM to other users of the AuC.12.5. Key Derivation
EAP-SIM supports key derivation. The key hierarchy is specified in Section 7. EAP-SIM combines several GSM triplets in order to generate stronger keying material and stronger AT_MAC values. The actual strength of the resulting keys depends, among other things, on operator-specific parameters including authentication algorithms, the strength of the Ki key, and the quality of the RAND challenges. 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. It should also be noted that a security policy that allows n=2 to be used may compromise the security of a future policy that requires three triplets, because adversaries may be able to exploit the messages exchanged when the weaker policy is applied. There is no known way to obtain complete GSM triplets by mounting an attack against EAP-SIM. A passive eavesdropper can learn n*RAND and AT_MAC 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 would require the attacker to reverse the keyed message authentication code function HMAC-SHA1-128. As EAP-SIM does not expose any values calculated from an individual GSM Kc keys, it is not possible to mount a brute force attack on only one of the Kc keys in EAP-SIM. Therefore, when considering brute force attacks on the values exposed in EAP-SIM, the effective length of EAP-SIM session keys is not compromised by the fact that they are
combined from several shorter keys, i.e., the effective length of 128 bits may be achieved. For additional considerations, see Section 12.8.12.6. Cryptographic Separation of Keys and Session Independence
The EAP Transient Keys used to protect EAP-SIM packets (K_encr, K_aut), the Master Session Key, and the Extended Master Session Key are cryptographically separate in EAP-SIM. An attacker cannot derive any non-trivial information about any of these keys based on the other keys. An attacker also cannot calculate the pre-shared secret (Ki) from the GSM Kc keys, from EAP-SIM K_encr, from EAP-SIM K_aut, from the Master Session Key, or from the Extended Master Session Key. Each EAP-SIM exchange generates fresh keying material, and the keying material exported from the method upon separate EAP-SIM exchanges is cryptographically separate. The EAP-SIM peer contributes to the keying material with the NONCE_MT parameter, which must be chosen freshly for each full authentication exchange. The EAP server is mandated to choose the RAND challenges freshly for each full authentication exchange. If either the server or the peer chooses its random value (NONCE_MT or RAND challenges) freshly, even if the other entity re-used its value from a previous exchange, then the EAP Transient Keys, the Master Session Key, and the Extended Master Session Key will be different and cryptographically separate from the corresponding values derived upon the previous full authentication exchange. On fast re-authentication, freshness of the Master Session Key and the Extended Master Session Key is provided with a counter (AT_COUNTER). The same EAP Transient Keys (K_encr, K_aut) that were used in the full authentication exchange are used to protect the EAP negotiation. However, replay and integrity protection across all the fast re-authentication exchanges that use the same EAP Transient Keys is provided with AT_COUNTER. [RFC3748] defines session independence as the "demonstration that passive attacks (such as capture of the EAP conversation) or active attacks (including compromise of the MSK or EMSK) do not enable compromise of subsequent or prior MSKs or EMSKs". Because the MSKs and EMSKs are separate between EAP exchanges, EAP-SIM supports this security claim. It should be noted that [Patel-2003], which predates [RFC3748], uses a slightly different meaning for session independence. The EAP-SIM protocol does not allow the peer to ensure that different Kc key values would be used in different exchanges. Only the server is able to ensure that fresh RANDs, and therefore, fresh Kc keys are used.
Hence, the peer cannot guarantee EAP-SIM sessions to be independent with regard to the internal Kc values. However, in EAP-SIM, the Kc keys are considered to be secret intermediate results, which are not exported outside the method. See Section 12.3 for more information about RAND re-use.12.7. Dictionary Attacks
Because EAP-SIM is not a password protocol, it is not vulnerable to dictionary attacks. (The pre-shared symmetric secret stored on the SIM card is not a passphrase, nor is it derived from a passphrase.)12.8. Credentials Re-use
EAP-SIM cannot prevent attacks over the GSM or GPRS radio networks. If the same SIM credentials are also used in GSM or GPRS, it is possible to mount attacks over the cellular interface. A passive attacker can eavesdrop GSM or GPRS traffic and obtain RAND, SRES pairs. He can then use a brute force attack or other cryptanalysis techniques to obtain the 64-bit Kc keys used to encrypt the GSM or GPRS data. This makes it possible to attack each 64-bit key separately. An active attacker can mount a "rogue GSM/GPRS base station attack", replaying previously seen RAND challenges to obtain SRES values. He can then use a brute force attack to obtain the Kc keys. If successful, the attacker can impersonate a valid network or decrypt previously seen traffic, because EAP-SIM does not provide perfect forward secrecy (PFS). Due to several weaknesses in the GSM encryption algorithms, the effective key strength of the Kc keys is much less than the expected 64 bits (no more than 40 bits if the A5/1 GSM encryption algorithm is used; as documented in [Barkan-2003], an active attacker can force the peer to use the weaker A5/2 algorithm that can be broken in less than a second). Because the A5 encryption algorithm is not used in EAP-SIM, and because EAP-SIM does not expose any values calculated from individual Kc keys, it should be noted that these attacks are not possible if the SIM credentials used in EAP-SIM are not shared in GSM/GPRS. At the time this document was written, the 3rd Generation Partnership Project (3GPP) has started to work on fixes to these A5 vulnerabilities. One of the solution proposals discussed in 3GPP is integrity-protected A5 version negotiation, which would require the base station to prove knowledge of the Kc key before the terminal
sends any values calculated from the Kc to the network. Another proposal is so-called special RANDs, where some bits of the RAND challenge would be used for cryptographic separation by indicating the allowed use of the triplet, such as the allowed A5 algorithm in GSM or the fact that the triplet is intended for EAP-SIM. This is currently a work in progress, and the mechanisms have not been selected yet.12.9. Integrity and Replay Protection, and Confidentiality
AT_MAC, AT_IV, AT_ENCR_DATA, and AT_COUNTER attributes are used to provide integrity, replay and confidentiality protection for EAP-SIM requests and responses. Integrity protection with AT_MAC includes the EAP header. These attributes cannot be used during the EAP/SIM/Start roundtrip. However, the protocol values (user identity string, NONCE_MT, and version negotiation parameters) are (implicitly) protected by later EAP-SIM messages by including them in key derivation. Integrity protection (AT_MAC) is based on a keyed message authentication code. Confidentiality (AT_ENCR_DATA and AT_IV) is based on a block cipher. Confidentiality protection is applied only to a part of the protocol fields. The table of attributes in Section 10.1 summarizes which fields are confidentiality-protected. It should be noted that the error and notification code attributes AT_CLIENT_ERROR_CODE and AT_NOTIFICATION are not confidential, but they are transmitted in the clear. Identity protection is discussed in Section 12.2. On full authentication, replay protection of the EAP exchange is provided by the RAND values from the underlying GSM authentication scheme and the use of the NONCE_MT value. Protection against replays of EAP-SIM messages is also based on the fact that messages that can include AT_MAC can only be sent once with a certain EAP-SIM Subtype, and on the fact that a different K_aut key will be used for calculating AT_MAC in each full authentication exchange. On fast re-authentication, a counter included in AT_COUNTER and a server random nonce is used to provide replay protection. The AT_COUNTER attribute is also included in EAP-SIM notifications if it is used after successful authentication in order to provide replay protection between re-authentication exchanges. Because EAP-SIM is not a tunneling method, EAP-Request/Notification, EAP-Response/Notification, EAP-Success, or EAP-Failure packets are not confidential, integrity-protected, or replay-protected in EAP-SIM. On physically insecure networks, this may enable an
attacker to send false notifications to the peer and to mount denial of service attacks by spoofing these packets. As discussed in Section 6.3, the peer will only accept EAP-Success after the peer successfully authenticates the server. Hence, the attacker cannot force the peer to believe successful mutual authentication has occurred until the peer successfully authenticates the server or after the peer fails to authenticate the server. The security considerations of EAP-SIM result indications are covered in Section 12.11 An eavesdropper will see the EAP-Request/Notification, EAP-Response/Notification, EAP-Success, and EAP-Failure packets sent in the clear. With EAP-SIM, confidential information MUST NOT be transmitted in EAP Notification packets.12.10. Negotiation Attacks
EAP-SIM does not protect the EAP-Response/Nak packet. Because EAP-SIM does not protect the EAP method negotiation, EAP method downgrading attacks may be possible, especially if the user uses the same identity with EAP-SIM and other EAP methods. EAP-SIM includes a version negotiation procedure. In EAP-SIM the keying material derivation includes the version list and selected version to ensure that the protocol cannot be downgraded and that the peer and server use the same version of EAP-SIM. EAP-SIM does not support ciphersuite negotiation.12.11. Protected Result Indications
EAP-SIM supports optional protected success indications and acknowledged failure indications. If a failure occurs after successful authentication, then the EAP-SIM failure indication is integrity- and replay-protected. Even if an EAP-Failure packet is lost when using EAP-SIM over an unreliable medium, then the EAP-SIM failure indications will help ensure that the peer and EAP server will know the other party's authentication decision. If protected success indications are used, then the loss of Success packet will also be addressed by the acknowledged, integrity- and replay-protected EAP-SIM success indication. If the optional success indications are not used, then the peer may end up believing that the server succeeded authentication, when it actually failed. Since access will not be
granted in this case, protected result indications are not needed unless the client is not able to realize it does not have access for an extended period of time.12.12. Man-in-the-Middle Attacks
In order to avoid man-in-the-middle attacks and session hijacking, user data SHOULD be integrity-protected on physically insecure networks. The EAP-SIM Master Session Key, or keys derived from it, MAY be used as the integrity protection keys, or, if an external security mechanism such as PEAP is used, then the link integrity protection keys MAY be derived by the external security mechanism. There are man-in-the-middle attacks associated with the use of any EAP method within a tunneled protocol. For instance, an early version of PEAP [PEAP-02] was vulnerable to this attack. This specification does not address these attacks. If EAP-SIM is used with a tunneling protocol, there should be cryptographic binding provided between the protocol and EAP-SIM to prevent man-in-the-middle attacks through rogue authenticators being able to setup one-way authenticated tunnels. For example, newer versions of PEAP include such cryptographic binding. The EAP-SIM Master Session Key MAY be used to provide the cryptographic binding. However, the mechanism by which the binding is provided depends on the tunneling protocol and is beyond the scope of this document.12.13. Generating Random Numbers
An EAP-SIM implementation SHOULD use a good source of randomness to generate the random numbers required in the protocol. Please see [RFC4086] for more information on generating random numbers for security applications.13. Security Claims
This section provides the security claims required by [RFC3748]. Auth. mechanism: EAP-SIM is based on the GSM SIM mechanism, which is a challenge/response authentication and key agreement mechanism based on a symmetric 128-bit pre-shared secret. EAP-SIM also makes use of a peer challenge to provide mutual authentication. Ciphersuite negotiation: No Mutual authentication: Yes (Section 12.3) Integrity protection: Yes (Section 12.9)
Replay protection: Yes (Section 12.9) Confidentiality: Yes, except method-specific success and failure indications (Section 12.2, Section 12.9) Key derivation: Yes Key strength: EAP-SIM supports key derivation with 128-bit effective key strength (Section 12.5). However, as discussed in Section 11, if the same credentials are used in GSM/GPRS and in EAP-SIM, then the key strength may be reduced considerably, basically to the same level as in GSM, by mounting attacks over GSM/GPRS. For example an active attack using a false GSM/GPRS base station reduces the effective key strength to almost zero. Description of key hierarchy: Please see Section 7. Dictionary attack protection: N/A (Section 12.7) Fast reconnect: Yes Cryptographic binding: N/A Session independence: Yes (Section 12.6) Fragmentation: No Channel binding: No Indication of vulnerabilities: Vulnerabilities are discussed in Section 12.14. Acknowledgements and Contributions
14.1. Contributors
In addition to the editors, Nora Dabbous, Jose Puthenkulam, and Prasanna Satarasinghe were significant contributors to this document. Pasi Eronen and Jukka-Pekka Honkanen contributed Appendix A.14.2. Acknowledgements
Juha Ala-Laurila, N. Asokan, Jan-Erik Ekberg, Patrik Flykt, Jukka-Pekka Honkanen, Antti Kuikka, Jukka Latva, Lassi Lehtinen, Jyri Rinnemaa, Timo Takamaki, and Raimo Vuonnala contributed many original ideas and concepts to this protocol.
N. Asokan, Pasi Eronen, and Jukka-Pekka Honkanen contributed and helped in innumerable ways during the development of the protocol. Valtteri Niemi and Kaisa Nyberg contributed substantially to the design of the key derivation and the fast re-authentication procedure, and have also provided their cryptographic expertise in many discussions related to this protocol. Simon Blake-Wilson provided very helpful comments on key derivation and version negotiation. Thanks to Greg Rose for his very valuable comments to an early version of this specification [S3-020125], and for reviewing and providing very useful comments on version 12. Thanks to Bernard Aboba, Vladimir Alperovich, Florent Bersani, Jacques Caron, Gopal Dommety, Augustin Farrugia, Mark Grayson, Max de Groot, Prakash Iyer, Nishi Kant, Victor Lortz, Jouni Malinen, Sarvar Patel, Tom Porcher, Michael Richardson, Stefan Schroeder, Uma Shankar, Jesse Walker, and Thomas Wieland for their contributions and critiques. Special thanks to Max for proposing improvements to the MAC calculation. Thanks to Glen Zorn for reviewing this document and for providing very useful comments on the protocol. Thanks to Sarvar Patel for his review of the protocol [Patel-2003]. Thanks to Bernard Aboba for reviewing this document for RFC 3748 compliance. The identity privacy support is based on the identity privacy support of [EAP-SRP]. The attribute format is based on the extension format of Mobile IPv4 [RFC3344]. This protocol has been partly developed in parallel with EAP-AKA [EAP-AKA], and hence this specification incorporates many ideas from Jari Arkko.
14.2.1. Contributors' Addresses
Nora Dabbous Gemplus 34 rue Guynemer 92447 Issy les Moulineaux France Phone: +33 1 4648 2000 EMail: nora.dabbous@gemplus.com Jose Puthenkulam Intel Corporation 2111 NE 25th Avenue, JF2-58 Hillsboro, OR 97124 USA Phone: +1 503 264 6121 EMail: jose.p.puthenkulam@intel.com Prasanna Satarasinghe Transat Technologies 180 State Street, Suite 240 Southlake, TX 76092 USA Phone: + 1 817 4814412 EMail: prasannas@transat-tech.com