rcf4186.txt

RCF4186 about EAP-SIM

   AT_VERSION_LIST includes the EAP-SIM versions supported by the
   server.  If AT_VERSION_LIST does not include a version that is
   implemented by the peer and allowed in the peer's security policy,
   then the peer MUST send the EAP-Response/SIM/Client-Error packet
   (Section 9.7) to the server with the error code "unsupported
   version".  If a suitable version is included, then the peer includes



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   the AT_SELECTED_VERSION attribute, containing the selected version in
   the EAP-Response/SIM/Start packet.  The peer MUST only indicate a
   version that is included in the AT_VERSION_LIST.  If several versions
   are acceptable, then the peer SHOULD choose the version that occurs
   first in the version list.

   The version number list of AT_VERSION_LIST and the selected version
   of AT_SELECTED_VERSION are included in the key derivation procedure
   (Section 7).  If an attacker modifies either one of these attributes,
   then the peer and the server derive different keying material.
   Because K_aut keys are different, the server and peer calculate
   different AT_MAC values.  Hence, the peer detects that AT_MAC,
   included in EAP-Request/SIM/Challenge, is incorrect and sends the
   EAP-Response/SIM/Client-Error packet.  The authentication procedure
   terminates.

4.2.  Identity Management

4.2.1.  Format, Generation and Usage of Peer Identities

4.2.1.1.  General

   In the beginning of EAP authentication, the Authenticator or the EAP
   server usually issues the EAP-Request/Identity packet to the peer.
   The peer responds with the EAP-Response/Identity, which contains the
   user's identity.  The formats of these packets are specified in
   [RFC3748].

   GSM subscribers are identified with the International Mobile
   Subscriber Identity (IMSI) [GSM-03.03].  The IMSI is a string of not
   more than 15 digits.  It is composed of a three digit Mobile Country
   Code (MCC), a two or three digit Mobile Network Code (MNC), and a
   Mobile Subscriber Identification Number (MSIN) of no more than 10
   digits.  MCC and MNC uniquely identify the GSM operator and help
   identify the AuC from which the authentication vectors need to be
   retrieved for this subscriber.

   Internet AAA protocols identify users with the Network Access
   Identifier (NAI) [RFC4282].  When used in a roaming environment, the
   NAI is composed of a username and a realm, separated with "@"
   (username@realm).  The username portion identifies the subscriber
   within the realm.

   This section specifies the peer identity format used in EAP-SIM.  In
   this document, the term "identity" or "peer identity" refers to the
   whole identity string that is used to identify the peer.  The peer





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   identity may include a realm portion.  "Username" refers to the
   portion of the peer identity that identifies the user, i.e., the
   username does not include the realm portion.

4.2.1.2.  Identity Privacy Support

   EAP-SIM includes optional identity privacy (anonymity) support that
   can be used to hide the cleartext permanent identity and thereby make
   the subscriber's EAP exchanges untraceable to eavesdroppers.  Because
   the permanent identity never changes, revealing it would help
   observers to track the user.  The permanent identity is usually based
   on the IMSI, which may further help the tracking, because the same
   identifier may be used in other contexts as well.  Identity privacy
   is based on temporary identities, or pseudonyms, which are equivalent
   to but separate from the Temporary Mobile Subscriber Identities
   (TMSI) that are used on cellular networks.  Please see Section 12.2
   for security considerations regarding identity privacy.

4.2.1.3.  Username Types in EAP-SIM identities

   There are three types of usernames in EAP-SIM peer identities:

   (1) Permanent usernames.  For example,
   [e-mail] might be a valid permanent identity.
   In this example, 1123456789098765 is the permanent username.

   (2) Pseudonym usernames.  For example, [e-mail] might
   be a valid pseudonym identity.  In this example, 3s7ah6n9q is the
   pseudonym username.

   (3) Fast re-authentication usernames.  For example,
   [e-mail] might be a valid fast re-authentication
   identity.  In this case, 53953754 is the fast re-authentication
   username.  Unlike permanent usernames and pseudonym usernames, fast
   re-authentication usernames are one-time identifiers, which are not
   re-used across EAP exchanges.

   The first two types of identities are used only on full
   authentication and the last one only on fast re-authentication.  When
   the optional identity privacy support is not used, the non-pseudonym
   permanent identity is used on full authentication.  The fast
   re-authentication exchange is specified in Section 5.

4.2.1.4.  Username Decoration

   In some environments, the peer may need to decorate the identity by
   prepending or appending the username with a string, in order to
   indicate supplementary AAA routing information in addition to the NAI



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   realm.  (The usage of an NAI realm portion is not considered
   decoration.)  Username decoration is out of the scope of this
   document.  However, it should be noted that username decoration might
   prevent the server from recognizing a valid username.  Hence,
   although the peer MAY use username decoration in the identities that
   the peer includes in EAP-Response/Identity, and although the EAP
   server MAY accept a decorated peer username in this message, the peer
   or the EAP server MUST NOT decorate any other peer identities that
   are used in various EAP-SIM attributes.  Only the identity used in
   the EAP-Response/Identity may be decorated.

4.2.1.5.  NAI Realm Portion

   The peer MAY include a realm portion in the peer identity, as per the
   NAI format.  The use of a realm portion is not mandatory.

   If a realm is used, the realm MAY be chosen by the subscriber's home
   operator and it MAY be a configurable parameter in the EAP-SIM peer
   implementation.  In this case, the peer is typically configured with
   the NAI realm of the home operator.  Operators MAY reserve a specific
   realm name for EAP-SIM users.  This convention makes it easy to
   recognize that the NAI identifies a GSM subscriber.  Such a reserved
   NAI realm may be a useful hint as to the first authentication method
   to use during method negotiation.  When the peer is using a pseudonym
   username instead of the permanent username, the peer selects the
   realm name portion similarly as it select the realm portion when
   using the permanent username.

   If no configured realm name is available, the peer MAY derive the
   realm name from the MCC and MNC portions of the IMSI.  A RECOMMENDED
   way to derive the realm from the IMSI using the realm 3gppnetwork.org
   is specified in [3GPP-TS-23.003].

   Some old implementations derive the realm name from the IMSI by
   concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits
   of IMSI, and ".owlan.org".  For example, if the IMSI is
   123456789098765, and the MNC is three digits long, then the derived
   realm name is "mnc456.mcc123.owlan.org".  As there are no DNS servers
   running at owlan.org, these realm names can only be used with
   manually configured AAA routing.  New implementations SHOULD use the
   mechanism specified in [3GPP-TS-23.003] instead of owlan.org.

   The IMSI is a string of digits without any explicit structure, so the
   peer may not be able to determine the length of the MNC portion.  If
   the peer is not able to determine whether the MNC is two or three
   digits long, the peer MAY use a 3-digit MNC.  If the correct length
   of the MNC is two, then the MNC used in the realm name includes the
   first digit of the MSIN.  Hence, when configuring AAA networks for



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   operators that have 2-digit MNCs, the network SHOULD also be prepared
   for realm names with incorrect, 3-digit MNCs.

4.2.1.6.  Format of the Permanent Username

   The non-pseudonym permanent username SHOULD be derived from the IMSI.
   In this case, the permanent username MUST be of the format "1" |
   IMSI, where the character "|" denotes concatenation.  In other words,
   the first character of the username is the digit one (ASCII value 31
   hexadecimal), followed by the IMSI.  The IMSI is encoded as an ASCII
   string that consists of not more than 15 decimal digits (ASCII values
   between 30 and 39 hexadecimal), one character per IMSI digit, in the
   order specified in [GSM-03.03].  For example, a permanent username
   derived from the IMSI 295023820005424 would be encoded as the ASCII
   string "1295023820005424" (byte values in hexadecimal notation: 31 32
   39 35 30 32 33 38 32 30 30 30 35 34 32 34).

   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".

   Alternatively, an implementation MAY choose a permanent username that
   is not based on the IMSI.  In this case, the selection of the
   username, its format, and its processing is out of the scope of this
   document.  In this case, the peer implementation MUST NOT prepend any
   leading characters to the username.

4.2.1.7.  Generating Pseudonyms and Fast Re-authentication Identities by
          the Server

   Pseudonym usernames and fast re-authentication identities are
   generated by the EAP server.  The EAP server produces pseudonym
   usernames and fast re-authentication identities in an
   implementation-dependent manner.  Only the EAP server needs to be
   able to map the pseudonym username to the permanent identity, or to
   recognize a fast re-authentication identity.

   EAP-SIM includes no provisions to ensure that the same EAP server
   that generated a pseudonym username will be used on the
   authentication exchange when the pseudonym username is used.  It is
   recommended that the EAP servers implement some centralized mechanism
   to allow all EAP servers of the home operator to map pseudonyms
   generated by other severs to the permanent identity.  If no such
   mechanism is available, then the EAP server failing to understand a
   pseudonym issued by another server can request the that peer send the
   permanent identity.




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   When issuing a fast re-authentication identity, the EAP server may
   include a realm name in the identity to make the fast
   re-authentication request be forwarded to the same EAP server.

   When generating fast re-authentication identities, the server SHOULD
   choose a fresh, new fast re-authentication identity that is different
   from the previous ones that were used after the same full
   authentication exchange.  A full authentication exchange and the
   associated fast re-authentication exchanges are referred to here as
   the same "full authentication context".  The fast re-authentication
   identity SHOULD include a random component.  This random component
   works as a full authentication context identifier.  A
   context-specific fast re-authentication identity can help the server
   to detect whether its fast re-authentication state information
   matches that of its peer (in other words, whether the state
   information is from the same full authentication exchange).  The
   random component also makes the fast re-authentication identities
   unpredictable, so an attacker cannot initiate a fast
   re-authentication exchange to get the server's EAP-Request/SIM/
   Re-authentication packet.

   Transmitting pseudonyms and fast re-authentication identities from
   the server to the peer is discussed in Section 4.2.1.8.  The
   pseudonym is transmitted as a username, without an NAI realm, and the
   fast re-authentication identity is transmitted as a complete NAI,
   including a realm portion if a realm is required.  The realm is
   included in the fast re-authentication identity to allow the server
   to include a server-specific realm.

   Regardless of the construction method, the pseudonym username MUST
   conform to the grammar specified for the username portion of an NAI.
   The fast re-authentication identity also MUST conform to the NAI
   grammar.  The EAP servers that the subscribers of an operator can use
   MUST ensure that the pseudonym usernames and the username portions
   used in fast re-authentication identities they generate are unique.