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RFC 4186

 
 
 

Extensible Authentication Protocol Method for Global System for Mobile Communications (GSM) Subscriber Identity Modules (EAP-SIM)

Part 2 of 5, p. 10 to 30
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4.  Operation

4.1.  Version Negotiation

   EAP-SIM includes version negotiation so as to allow future
   developments in the protocol.  The version negotiation is performed
   on full authentication and it uses two attributes, AT_VERSION_LIST,
   which the server always includes in EAP-Request/SIM/Start, and
   AT_SELECTED_VERSION, which the peer includes in
   EAP-Response/SIM/Start on full authentication.

   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,
   1123456789098765@myoperator.com might be a valid permanent identity.
   In this example, 1123456789098765 is the permanent username.

   (2) Pseudonym usernames.  For example, 3s7ah6n9q@myoperator.com might
   be a valid pseudonym identity.  In this example, 3s7ah6n9q is the
   pseudonym username.

   (3) Fast re-authentication usernames.  For example,
   53953754@myoperator.com 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.

   In any case, it is necessary that permanent usernames, pseudonym
   usernames, and fast re-authentication usernames are separate and
   recognizable from each other.  It is also desirable that EAP-SIM and
   EAP-AKA [EAP-AKA] usernames be distinguishable from each other as an
   aid for the server on which method to offer.

   In general, it is the task of the EAP server and the policies of its
   administrator to ensure sufficient separation of the usernames.
   Pseudonym usernames and fast re-authentication usernames are both
   produced and used by the EAP server.  The EAP server MUST compose
   pseudonym usernames and fast re-authentication usernames so that it
   can determine if an NAI username is an EAP-SIM pseudonym username or

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   an EAP-SIM fast re-authentication username.  For instance, when the
   usernames have been derived from the IMSI, the server could use
   different leading characters in the pseudonym usernames and fast
   re-authentication usernames (e.g., the pseudonym could begin with a
   leading "3" character).  When mapping a fast re-authentication
   identity to a permanent identity, the server SHOULD only examine the
   username portion of the fast re-authentication identity and ignore
   the realm portion of the identity.

   Because the peer may fail to save a pseudonym username sent in an
   EAP-Request/SIM/Challenge, for example due to malfunction, the EAP
   server SHOULD maintain at least the most recently used pseudonym
   username in addition to the most recently issued pseudonym username.
   If the authentication exchange is not completed successfully, then
   the server SHOULD NOT overwrite the pseudonym username that was
   issued during the most recent successful authentication exchange.

4.2.1.8.  Transmitting Pseudonyms and Fast Re-authentication Identities
          to the Peer

   The server transmits pseudonym usernames and fast re-authentication
   identities to the peer in cipher, using the AT_ENCR_DATA attribute.

   The EAP-Request/SIM/Challenge message MAY include an encrypted
   pseudonym username and/or an encrypted fast re-authentication
   identity in the value field of the AT_ENCR_DATA attribute.  Because
   identity privacy support and fast re-authentication are optional
   implementations, the peer MAY ignore the AT_ENCR_DATA attribute and
   always use the permanent identity.  On fast re-authentication
   (discussed in Section 5), the server MAY include a new, encrypted
   fast re-authentication identity in the
   EAP-Request/SIM/Re-authentication message.

   On receipt of the EAP-Request/SIM/Challenge, the peer MAY decrypt the
   encrypted data in AT_ENCR_DATA.  If the authentication exchange is
   successful, and the encrypted data includes a pseudonym username,
   then the peer may use the obtained pseudonym username on the next
   full authentication.  If a fast re-authentication identity is
   included, then the peer MAY save it together with other fast
   re-authentication state information, as discussed in Section 5, for
   the next fast re-authentication.  If the authentication exchange does
   not complete successfully, the peer MUST ignore the received
   pseudonym username and the fast re-authentication identity.

   If the peer does not receive a new pseudonym username in the
   EAP-Request/SIM/Challenge message, the peer MAY use an old pseudonym
   username instead of the permanent username on the next full
   authentication.  The username portions of fast re-authentication

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   identities are one-time usernames, which the peer MUST NOT re-use.
   When the peer uses a fast re-authentication identity in an EAP
   exchange, the peer MUST discard the fast re-authentication identity
   and not re-use it in another EAP authentication exchange, even if the
   authentication exchange was not completed.

4.2.1.9.  Usage of the Pseudonym by the Peer

   When the optional identity privacy support is used on full
   authentication, the peer MAY use a pseudonym username received as
   part of a previous full authentication sequence as the username
   portion of the NAI.  The peer MUST NOT modify the pseudonym username
   received in AT_NEXT_PSEUDONYM.  However, as discussed above, the peer
   MAY need to decorate the username in some environments by appending
   or prepending the username with a string that indicates supplementary
   AAA routing information.

   When using a pseudonym username in an environment where a realm
   portion is used, the peer concatenates the received pseudonym
   username with the "@" character and an NAI realm portion.  The
   selection of the NAI realm is discussed above.  The peer can select
   the realm portion similarly, regardless of whether it uses the
   permanent username or a pseudonym username.

4.2.1.10.  Usage of the Fast Re-authentication Identity by the Peer

   On fast re-authentication, the peer uses the fast re-authentication
   identity that was received as part of the previous authentication
   sequence.  A new re-authentication identity may be delivered as part
   of both full authentication and fast re-authentication.  The peer
   MUST NOT modify the username part of the fast re-authentication
   identity received in AT_NEXT_REAUTH_ID, except in cases when username
   decoration is required.  Even in these cases, the "root" fast
   re-authentication username must not be modified, but it may be
   appended or prepended with another string.

4.2.2.  Communicating the Peer Identity to the Server

4.2.2.1.  General

   The peer identity MAY be communicated to the server with the
   EAP-Response/Identity message.  This message MAY contain the
   permanent identity, a pseudonym identity, or a fast re-authentication
   identity.  If the peer uses the permanent identity or a pseudonym
   identity, which the server is able to map to the permanent identity,
   then the authentication proceeds as discussed in the overview of
   Section 3.  If the peer uses a fast re-authentication identity, and
   if the fast re-authentication identity matches with a valid fast

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   re-authentication identity maintained by the server, and if the
   server agrees to use fast re-authentication, then a fast
   re-authentication exchange is performed, as described in Section 5.

   The peer identity can also be transmitted from the peer to the server
   using EAP-SIM messages instead of the EAP-Response/Identity.  In this
   case, the server includes an identity-requesting attribute
   (AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ) in the
   EAP-Request/SIM/Start message, and the peer includes the AT_IDENTITY
   attribute, which contains the peer's identity, in the
   EAP-Response/SIM/Start message.  The AT_ANY_ID_REQ attribute is a
   general identity-requesting attribute, which the server uses if it
   does not specify which kind of an identity the peer should return in
   AT_IDENTITY.  The server uses the AT_FULLAUTH_ID_REQ attribute to
   request either the permanent identity or a pseudonym identity.  The
   server uses the AT_PERMANENT_ID_REQ attribute to request that the
   peer send its permanent identity.

   The identity format in the AT_IDENTITY attribute is the same as in
   the EAP-Response/Identity packet (except that identity decoration is
   not allowed).  The AT_IDENTITY attribute contains a permanent
   identity, a pseudonym identity, or a fast re-authentication identity.

   Please note that the EAP-SIM peer and the EAP-SIM server only process
   the AT_IDENTITY attribute; entities that only pass through EAP
   packets do not process this attribute.  Hence, the authenticator and
   other intermediate AAA elements (such as possible AAA proxy servers)
   will continue to refer to the peer with the original identity from
   the EAP-Response/Identity packet unless the identity authenticated in
   the AT_IDENTITY attribute is communicated to them in another way
   within the AAA protocol.

4.2.2.2.  Relying on EAP-Response/Identity Discouraged

   The EAP-Response/Identity packet is not method-specific, so in many
   implementations it may be handled by an EAP Framework.  This
   introduces an additional layer of processing between the EAP peer and
   EAP server.  The extra layer of processing may cache identity
   responses or add decorations to the identity.  A modification of the
   identity response will cause the EAP peer and EAP server to use
   different identities in the key derivation, which will cause the
   protocol to fail.

   For this reason, it is RECOMMENDED that the EAP peer and server use
   the method-specific identity attributes in EAP-SIM, and the server is
   strongly discouraged from relying upon the EAP-Response/Identity.

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   In particular, if the EAP server receives a decorated identity in
   EAP-Response/Identity, then the EAP server MUST use the
   identity-requesting attributes to request that the peer send an
   unmodified and undecorated copy of the identity in AT_IDENTITY.

4.2.3.  Choice of Identity for the EAP-Response/Identity

   If EAP-SIM peer is started upon receiving an EAP-Request/Identity
   message, then the peer MAY use an EAP-SIM identity in the EAP-
   Response/Identity packet.  In this case, the peer performs the
   following steps.

   If the peer has maintained fast re-authentication state information
   and wants to use fast re-authentication, then the peer transmits the
   fast re-authentication identity in EAP-Response/Identity.

   Else, if the peer has a pseudonym username available, then the peer
   transmits the pseudonym identity in EAP-Response/Identity.

   In other cases, the peer transmits the permanent identity in
   EAP-Response/Identity.

4.2.4.  Server Operation in the Beginning of EAP-SIM Exchange

   As discussed in Section 4.2.2.2, the server SHOULD NOT rely on an
   identity string received in EAP-Response/Identity.  Therefore, the
   RECOMMENDED way to start an EAP-SIM exchange is to ignore any
   received identity strings.  The server SHOULD begin the EAP-SIM
   exchange by issuing the EAP-Request/SIM/Start packet with an
   identity-requesting attribute to indicate that the server wants the
   peer to include an identity in the AT_IDENTITY attribute of the EAP-
   Response/SIM/Start message.  Three methods to request an identity
   from the peer are discussed below.

   If the server chooses not to ignore the contents of EAP-
   Response/Identity, then the server may have already received an EAP-
   SIM identity in this packet.  However, if the EAP server has not
   received any EAP-SIM peer identity (permanent identity, pseudonym
   identity, or fast re-authentication identity) from the peer when
   sending the first EAP-SIM request, or if the EAP server has received
   an EAP-Response/Identity packet but the contents do not appear to be
   a valid permanent identity, pseudonym identity or a re-authentication
   identity, then the server MUST request an identity from the peer
   using one of the methods below.

   The server sends the EAP-Request/SIM/Start message with the
   AT_PERMANENT_ID_REQ attribute to indicate that the server wants the
   peer to include the permanent identity in the AT_IDENTITY attribute

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   of the EAP-Response/SIM/Start message.  This is done in the following
   cases:

   o  The server does not support fast re-authentication or identity
      privacy.

   o  The server decided to process a received identity, and the server
      recognizes the received identity as a pseudonym identity but the
      server is not able to map the pseudonym identity to a permanent
      identity.

   The server issues the EAP-Request/SIM/Start packet with the
   AT_FULLAUTH_ID_REQ attribute to indicate that the server wants the
   peer to include a full authentication identity (pseudonym identity or
   permanent identity) in the AT_IDENTITY attribute of the
   EAP-Response/SIM/Start message.  This is done in the following cases:

   o  The server does not support fast re-authentication and the server
      supports identity privacy.

   o  The server decided to process a received identity, and the server
      recognizes the received identity as a re-authentication identity
      but the server is not able to map the re-authentication identity
      to a permanent identity.

   The server issues the EAP-Request/SIM/Start packet with the
   AT_ANY_ID_REQ attribute to indicate that the server wants the peer to
   include an identity in the AT_IDENTITY attribute of the
   EAP-Response/SIM/Start message, and the server does not indicate any
   preferred type for the identity.  This is done in other cases, such
   as when the server ignores a received EAP-Response/Identity, the
   server does not have any identity, or the server does not recognize
   the format of a received identity.

4.2.5.  Processing of EAP-Request/SIM/Start by the Peer

   Upon receipt of an EAP-Request/SIM/Start message, the peer MUST
   perform the following steps.

   If the EAP-Request/SIM/Start does not include an identity request
   attribute, then the peer responds with EAP-Response/SIM/Start without
   AT_IDENTITY.  The peer includes the AT_SELECTED_VERSION and
   AT_NONCE_MT attributes, because the exchange is a full authentication
   exchange.

   If the EAP-Request/SIM/Start includes AT_PERMANENT_ID_REQ, and if the
   peer does not have a pseudonym available, then the peer MUST respond
   with EAP-Response/SIM/Start and include the permanent identity in

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   AT_IDENTITY.  If the peer has a pseudonym available, then the peer
   MAY refuse to send the permanent identity; hence, in this case the
   peer MUST either respond with EAP-Response/SIM/Start and include the
   permanent identity in AT_IDENTITY or respond with EAP-Response/SIM/
   Client-Error packet with the code "unable to process packet".

   If the EAP-Request/SIM/Start includes AT_FULL_AUTH_ID_REQ, and if the
   peer has a pseudonym available, then the peer SHOULD respond with
   EAP-Response/SIM/Start and include the pseudonym identity in
   AT_IDENTITY.  If the peer does not have a pseudonym when it receives
   this message, then the peer MUST respond with EAP-Response/SIM/Start
   and include the permanent identity in AT_IDENTITY.  The Peer MUST NOT
   use a re-authentication identity in the AT_IDENTITY attribute.

   If the EAP-Request/SIM/Start includes AT_ANY_ID_REQ, and if the peer
   has maintained fast re-authentication state information and the peer
   wants to use fast re-authentication, then the peer responds with
   EAP-Response/SIM/Start and includes the fast re-authentication
   identity in AT_IDENTITY.  Else, if the peer has a pseudonym identity
   available, then the peer responds with EAP-Response/SIM/Start and
   includes the pseudonym identity in AT_IDENTITY.  Else, the peer
   responds with EAP-Response/SIM/Start and includes the permanent
   identity in AT_IDENTITY.

   An EAP-SIM exchange may include several EAP/SIM/Start rounds.  The
   server may issue a second EAP-Request/SIM/Start if it was not able to
   recognize the identity that the peer used in the previous AT_IDENTITY
   attribute.  At most, three EAP/SIM/Start rounds can be used, so the
   peer MUST NOT respond to more than three EAP-Request/SIM/Start
   messages within an EAP exchange.  The peer MUST verify that the
   sequence of EAP-Request/SIM/Start packets that the peer receives
   comply with the sequencing rules defined in this document.  That is,
   AT_ANY_ID_REQ can only be used in the first EAP-Request/SIM/Start; in
   other words, AT_ANY_ID_REQ MUST NOT be used in the second or third
   EAP-Request/SIM/Start.  AT_FULLAUTH_ID_REQ MUST NOT be used if the
   previous EAP-Request/SIM/Start included AT_PERMANENT_ID_REQ.  The
   peer operation, in cases when it receives an unexpected attribute or
   an unexpected message, is specified in Section 6.3.1.

4.2.6.  Attacks Against Identity Privacy

   The section above specifies two possible ways the peer can operate
   upon receipt of AT_PERMANENT_ID_REQ.  This is because a received
   AT_PERMANENT_ID_REQ does not necessarily originate from the valid
   network, but an active attacker may transmit an EAP-Request/SIM/
   Start packet with an AT_PERMANENT_ID_REQ attribute to the peer, in an
   effort to find out the true identity of the user.  If the peer does
   not want to reveal its permanent identity, then the peer sends the

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   EAP-Response/SIM/Client-Error packet with the error code "unable to
   process packet", and the authentication exchange terminates.

   Basically, there are two different policies that the peer can employ
   with regard to AT_PERMANENT_ID_REQ.  A "conservative" peer assumes
   that the network is able to maintain pseudonyms robustly.  Therefore,
   if a conservative peer has a pseudonym username, the peer responds
   with EAP-Response/SIM/Client-Error to the EAP packet with
   AT_PERMANENT_ID_REQ, because the peer believes that the valid network
   is able to map the pseudonym identity to the peer's permanent
   identity.  (Alternatively, the conservative peer may accept
   AT_PERMANENT_ID_REQ in certain circumstances, for example, if the
   pseudonym was received a long time ago.)  The benefit of this policy
   is that it protects the peer against active attacks on anonymity.  On
   the other hand, a "liberal" peer always accepts the
   AT_PERMANENT_ID_REQ and responds with the permanent identity.  The
   benefit of this policy is that it works even if the valid network
   sometimes loses pseudonyms and is not able to map them to the
   permanent identity.

4.2.7.  Processing of AT_IDENTITY by the Server

   When the server receives an EAP-Response/SIM/Start message with the
   AT_IDENTITY (in response to the server's identity requesting
   attribute), the server MUST operate as follows.

   If the server used AT_PERMANENT_ID_REQ, and if the AT_IDENTITY does
   not contain a valid permanent identity, then the server sends
   EAP-Request/SIM/Notification with AT_NOTIFICATION code "General
   failure" (16384), and the EAP exchange terminates.  If the server
   recognizes the permanent identity and is able to continue, then the
   server proceeds with full authentication by sending EAP-Request/SIM/
   Challenge.

   If the server used AT_FULLAUTH_ID_REQ, and if AT_IDENTITY contains a
   valid permanent identity or a pseudonym identity that the server can
   map to a valid permanent identity, then the server proceeds with full
   authentication by sending EAP-Request/SIM/Challenge.  If AT_IDENTITY
   contains a pseudonym identity that the server is not able to map to a
   valid permanent identity, or an identity that the server is not able
   to recognize or classify, then the server sends EAP-Request/SIM/Start
   with AT_PERMANENT_ID_REQ.

   If the server used AT_ANY_ID_REQ, and if the AT_IDENTITY contains a
   valid permanent identity or a pseudonym identity that the server can
   map to a valid permanent identity, then the server proceeds with full
   authentication by sending EAP-Request/SIM/Challenge.

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   If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid
   fast re-authentication identity and the server agrees on using
   re-authentication, then the server proceeds with fast
   re-authentication by sending EAP-Request/SIM/Re-authentication
   (Section 5).

   If the server used AT_ANY_ID_REQ, and if the peer sent an
   EAP-Response/SIM/Start with only AT_IDENTITY (indicating
   re-authentication), but the server is not able to map the identity to
   a permanent identity, then the server sends EAP-Request/SIM/Start
   with AT_FULLAUTH_ID_REQ.

   If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid
   fast re-authentication identity that the server is able to map to a
   permanent identity, and if the server does not want to use fast
   re-authentication, then the server sends EAP-Request/SIM/Start
   without any identity requesting attributes.

   If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an
   identity that the server recognizes as a pseudonym identity but the
   server is not able to map the pseudonym identity to a permanent
   identity, then the server sends EAP-Request/SIM/Start with
   AT_PERMANENT_ID_REQ.

   If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an
   identity that the server is not able to recognize or classify, then
   the server sends EAP-Request/SIM/Start with AT_FULLAUTH_ID_REQ.

4.3.  Message Sequence Examples (Informative)

   This section contains non-normative message sequence examples to
   illustrate how the peer identity can be communicated to the server.

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4.3.1.  Full Authentication

   This case for full authentication is illustrated below in Figure 2.
   In this case, AT_IDENTITY contains either the permanent identity or a
   pseudonym identity.  The same sequence is also used in case the
   server uses the AT_FULLAUTH_ID_REQ in EAP-Request/SIM/Start.

      Peer                                             Authenticator
        |                                                       |
        |                            +------------------------------+
        |                            | Server does not have a       |
        |                            | Subscriber identity available|
        |                            | When starting EAP-SIM        |
        |                            +------------------------------+
        |                                                       |
        |          EAP-Request/SIM/Start                        |
        |          (AT_ANY_ID_REQ, AT_VERSION_LIST)             |
        |<------------------------------------------------------|
        |                                                       |
        |                                                       |
        | EAP-Response/SIM/Start                                |
        | (AT_IDENTITY, AT_NONCE_MT,                            |
        |  AT_SELECTED_VERSION)                                 |
        |------------------------------------------------------>|
        |                                                       |

         Figure 2: Requesting any identity, full authentication

   If the peer uses its full authentication identity and the AT_IDENTITY
   attribute contains a valid permanent identity or a valid pseudonym
   identity that the EAP server is able to map to the permanent
   identity, then the full authentication sequence proceeds as usual
   with the EAP Server issuing the EAP-Request/SIM/Challenge message.

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4.3.2.  Fast Re-authentication

   The case when the server uses the AT_ANY_ID_REQ and the peer wants to
   perform fast re-authentication is illustrated below in Figure 3.

      Peer                                             Authenticator
        |                                                       |
        |                            +------------------------------+
        |                            | Server does not have a       |
        |                            | Subscriber identity available|
        |                            | When starting EAP-SIM        |
        |                            +------------------------------+
        |                                                       |
        |        EAP-Request/SIM/Start                          |
        |        (AT_ANY_ID_REQ, AT_VERSION_LIST)               |
        |<------------------------------------------------------|
        |                                                       |
        |                                                       |
        | EAP-Response/SIM/Start                                |
        | (AT_IDENTITY containing a fast re-auth. identity)     |
        |------------------------------------------------------>|
        |                                                       |

       Figure 3: Requesting any identity, fast re-authentication

   On fast re-authentication, if the AT_IDENTITY attribute contains a
   valid fast re-authentication identity and the server agrees on using
   fast re-authentication, then the server proceeds with the fast
   re-authentication sequence and issues the EAP-Request/SIM/
   Re-authentication packet, as specified in Section 5.

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4.3.3.  Fall Back to Full Authentication

   Figure 4 illustrates cases in which the server does not recognize the
   fast re-authentication identity the peer used in AT_IDENTITY, and
   issues a second EAP-Request/SIM/Start message.

      Peer                                             Authenticator
        |                                                       |
        |                            +------------------------------+
        |                            | Server does not have a       |
        |                            | Subscriber identity available|
        |                            | When starting EAP-SIM        |
        |                            +------------------------------+
        |                                                       |
        |        EAP-Request/SIM/Start                          |
        |        (AT_ANY_ID_REQ, AT_VERSION_LIST)               |
        |<------------------------------------------------------|
        |                                                       |
        |                                                       |
        | EAP-Response/SIM/Start                                |
        | (AT_IDENTITY containing a fast re-auth. identity)     |
        |------------------------------------------------------>|
        |                                                       |
        |                            +------------------------------+
        |                            | Server does not recognize    |
        |                            | The fast re-auth.            |
        |                            | Identity                     |
        |                            +------------------------------+
        |                                                       |
        |     EAP-Request/SIM/Start                             |
        |     (AT_FULLAUTH_ID_REQ, AT_VERSION_LIST)             |
        |<------------------------------------------------------|
        |                                                       |
        |                                                       |
        | EAP-Response/SIM/Start                                |
        | (AT_IDENTITY with a full-auth. identity, AT_NONCE_MT, |
        |  AT_SELECTED_VERSION)                                 |
        |------------------------------------------------------>|
        |                                                       |

              Figure 4: Fall back to full authentication

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4.3.4.  Requesting the Permanent Identity 1

   Figure 5 illustrates the case in which the EAP server fails to map
   the pseudonym identity included in the EAP-Response/Identity packet
   to a valid permanent identity.

      Peer                                             Authenticator
         |                                                       |
         |                               EAP-Request/Identity    |
         |<------------------------------------------------------|
         |                                                       |
         | EAP-Response/Identity                                 |
         | (Includes a pseudonym)                                |
         |------------------------------------------------------>|
         |                                                       |
         |                            +------------------------------+
         |                            | Server fails to map the      |
         |                            | Pseudonym to a permanent id. |
         |                            +------------------------------+
         |  EAP-Request/SIM/Start                                |
         |  (AT_PERMANENT_ID_REQ, AT_VERSION_LIST)               |
         |<------------------------------------------------------|
         |                                                       |
         | EAP-Response/SIM/Start                                |
         | (AT_IDENTITY with permanent identity, AT_NONCE_MT,    |
         |  AT_SELECTED_VERSION)                                 |
         |------------------------------------------------------>|
         |                                                       |

              Figure 5: Requesting the permanent identity

   If the server recognizes the permanent identity, then the
   authentication sequence proceeds as usual with the EAP Server issuing
   the EAP-Request/SIM/Challenge message.

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4.3.5.  Requesting the Permanent Identity 2

   Figure 6 illustrates the case in which the EAP server fails to map
   the pseudonym included in the AT_IDENTITY attribute to a valid
   permanent identity.

      Peer                                             Authenticator
         |                                                       |
         |                            +------------------------------+
         |                            | Server does not have a       |
         |                            | Subscriber identity available|
         |                            | When starting EAP-SIM        |
         |                            +------------------------------+
         |        EAP-Request/SIM/Start                          |
         |        (AT_ANY_ID_REQ, AT_VERSION_LIST)               |
         |<------------------------------------------------------|
         |                                                       |
         |EAP-Response/SIM/Start                                 |
         |(AT_IDENTITY with a pseudonym identity, AT_NONCE_MT,   |
         | AT_SELECTED_VERSION)                                  |
         |------------------------------------------------------>|
         |                           +-------------------------------+
         |                           | Server fails to map the       |
         |                           | Pseudonym in AT_IDENTITY      |
         |                           | to a valid permanent identity |
         |                           +-------------------------------+
         |                                                       |
         |                EAP-Request/SIM/Start                  |
         |                (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) |
         |<------------------------------------------------------|
         |                                                       |
         | EAP-Response/SIM/Start                                |
         | (AT_IDENTITY with permanent identity,                 |
         |  AT_NONCE_MT, AT_SELECTED_VERSION)                    |
         |------------------------------------------------------>|
         |                                                       |

   Figure 6: Requesting a permanent identity (two EAP-SIM Start rounds)

4.3.6.  Three EAP-SIM/Start Roundtrips

   In the worst case, there are three EAP/SIM/Start round trips before
   the server obtains an acceptable identity.  This case is illustrated
   in Figure 7.

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      Peer                                             Authenticator
       |                                                       |
       |                            +------------------------------+
       |                            | Server does not have a       |
       |                            | Subscriber identity available|
       |                            | When starting EAP-SIM        |
       |                            +------------------------------+
       |        EAP-Request/SIM/Start                          |
       |        (Includes AT_ANY_ID_REQ, AT_VERSION_LIST)      |
       |<------------------------------------------------------|
       |                                                       |
       | EAP-Response/SIM/Start                                |
       | (AT_IDENTITY with fast re-auth. identity)             |
       |------------------------------------------------------>|
       |                                                       |
       |                            +------------------------------+
       |                            | Server does not accept       |
       |                            | The fast re-auth.            |
       |                            | Identity                     |
       |                            +------------------------------+
       |     EAP-Request/SIM/Start                             |
       |     (AT_FULLAUTH_ID_REQ, AT_VERSION_LIST)             |
       |<------------------------------------------------------|
       |                                                       |
       :                                                       :
       :                                                       :
       :                                                       :
       :                                                       :
       |EAP-Response/SIM/Start                                 |
       |(AT_IDENTITY with a pseudonym identity, AT_NONCE_MT,   |
       | AT_SELECTED_VERSION)                                  |
       |------------------------------------------------------>|
       |                                                       |
       |                           +-------------------------------+
       |                           | Server fails to map the       |
       |                           | Pseudonym in AT_IDENTITY      |
       |                           | to a valid permanent identity |
       |                           +-------------------------------+
       |           EAP-Request/SIM/Start                       |
       |           (AT_PERMANENT_ID_REQ, AT_VERSION_LIST)      |
       |<------------------------------------------------------|
       |                                                       |
       | EAP-Response/SIM/Start                                |
       | (AT_IDENTITY with permanent identity, AT_NONCE_MT,    |
       |  AT_SELECTED_VERSION)                                 |
       |------------------------------------------------------>|
       |                                                       |
                Figure 7: Three EAP-SIM Start rounds

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   After the last EAP-Response/SIM/Start message, the full
   authentication sequence proceeds as usual.  If the EAP Server
   recognizes the permanent identity and is able to proceed, the server
   issues the EAP-Request/SIM/Challenge message.



(page 30 continued on part 3)

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