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RFC 4187

 
 
 

Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA)

Part 2 of 4, p. 15 to 38
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4.  Operation

4.1.  Identity Management

4.1.1.  Format, Generation, and Usage of Peer Identities

4.1.1.1.  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 EAP-Response/Identity, which contains the
   user's identity.  The formats of these packets are specified in
   [RFC3748].

   Subscribers of mobile networks are identified with the International
   Mobile Subscriber Identity (IMSI) [TS23.003].  The IMSI is a string
   of not more than 15 digits.  It is composed of a Mobile Country Code
   (MCC) of 3 digits, a Mobile Network Code (MNC) of 2 or 3 digits, and
   a Mobile Subscriber Identification Number (MSIN) of not 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-AKA.  In
   this document, the term identity or peer identity refers to the whole
   identity string that is used to identify the peer.  The peer 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.

4.1.1.2.  Identity Privacy Support

   EAP-AKA 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

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   to but separate from the Temporary Mobile Subscriber Identities
   (TMSI) that are used on cellular networks.  Please see Section 12.1
   for security considerations regarding identity privacy.

4.1.1.3.  Username Types in EAP-AKA Identities

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

   (1) Permanent usernames.  For example,
   0123456789098765@myoperator.com might be a valid permanent identity.
   In this example, 0123456789098765 is the permanent username.

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

   (3) Fast re-authentication usernames.  For example,
   43953754@myoperator.com might be a valid fast re-authentication
   identity.  In this case, 43953754 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 type 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.

4.1.1.4.  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
   realm.  (The usage of an NAI realm portion is not considered to be
   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-AKA attributes.  Only the identity used in
   EAP-Response/Identity may be decorated.

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4.1.1.5.  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-AKA 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-AKA users.  This convention makes it easy to
   recognize that the NAI identifies an AKA subscriber.  Such a reserved
   NAI realm may be useful as a hint of 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 to how it selects 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, will be specified in [TS23.003].

   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 [TS23.003] 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 MSIN.  Hence, when configuring AAA networks for
   operators that have 2-digit MNC's, the network SHOULD also be
   prepared for realm names with incorrect 3-digit MNC's.

4.1.1.6.  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 "0" |
   IMSI, where the character "|" denotes concatenation.  In other words,
   the first character of the username is the digit zero (ASCII value 30
   hexadecimal), followed by the IMSI.  The IMSI is an ASCII string that
   consists of not more than 15 decimal digits (ASCII values between 30

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   and 39 hexadecimal), one character per IMSI digit, in the order as
   specified in [TS23.003].  For example, a permanent username derived
   from the IMSI 295023820005424 would be encoded as the ASCII string
   "0295023820005424" (byte values in hexadecimal notation: 30 32 39 35
   30 32 33 38 32 30 30 30 35 34 32 34)

   The EAP server MAY use the leading "0" as a hint to try EAP-AKA as
   the first authentication method during method negotiation, rather
   than using, for example, EAP-SIM.  The EAP-AKA server MAY propose
   EAP-AKA even if the leading character was not "0".

   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.

4.1.1.7.  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-AKA 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 peer to send the
   permanent identity.

   When issuing a fast re-authentication identity, the EAP server may
   include a realm name in the identity that will cause the fast
   re-authentication request to 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.  The random component

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   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
   the peer's fast re-authentication state information (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/AKA-Reauthentication packet.

   Transmitting pseudonyms and fast re-authentication identities from
   the server to the peer is discussed in Section 4.1.1.8.  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 in order to allow the
   server to include a server-specific realm.

   Regardless of construction method, the pseudonym username MUST
   conform to the grammar specified for the username portion of an NAI.
   Also, the fast re-authentication identity 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 that 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 usernames be recognizable from each other as an aid to the
   server when deciding 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 recognize if an NAI username is an EAP-AKA pseudonym username or
   an EAP-AKA 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 "2" 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.

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   Because the peer may fail to save a pseudonym username that was sent
   in an EAP-Request/AKA-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.

4.1.1.8.  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/AKA-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 to
   implement, 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/AKA-Reauthentication
   message.

   On receipt of the EAP-Request/AKA-Challenge, the peer MAY decrypt the
   encrypted data in AT_ENCR_DATA; and if a pseudonym username is
   included, 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 peer does not receive a new pseudonym username in the
   EAP-Request/AKA-Challenge message, the peer MAY use an old pseudonym
   username instead of the permanent username on next full
   authentication.  The username portions of fast re-authentication
   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.

4.1.1.9.  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

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   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.

4.1.1.10.  Usage of the Fast Re-Authentication Identity by the Peer

   On fast re-authentication, the peer uses the fast re-authentication
   identity received as part of the previous authentication sequence.  A
   new fast 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.

4.1.2.  Communicating the Peer Identity to the Server

4.1.2.1.  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
   re-authentication identity maintained by the server, 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-AKA messages instead of 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/AKA-Identity message; and the peer includes the
   AT_IDENTITY attribute, which contains the peer's identity, in the
   EAP-Response/AKA-Identity message.  The AT_ANY_ID_REQ attribute is a
   general identity requesting attribute, which the server uses if it

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   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 EAP-Request/AKA-Challenge,
   EAP-Response/AKA-Challenge, or the packets used on fast re-
   authentication may optionally include the AT_CHECKCODE attribute,
   which enables the protocol peers to ensure the integrity of the
   AKA-Identity packets.  AT_CHECKCODE is specified in Section 10.13.

   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 only the EAP-AKA peer and the EAP-AKA server process
   the AT_IDENTITY attribute and entities that 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.

4.1.2.2.  Relying on EAP-Response/Identity Discouraged

   The EAP-Response/Identity packet is not method specific; therefore,
   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-AKA, and the server is
   strongly discouraged from relying upon the EAP-Response/Identity.

   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 the peer to send an
   unmodified and undecorated copy of the identity in AT_IDENTITY.

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

   If EAP-AKA peer is started upon receiving an EAP-Request/Identity
   message, then the peer MAY use an EAP-AKA 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 if the peer 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.

4.1.4.  Server Operation in the Beginning of EAP-AKA Exchange

   As discussed in Section 4.1.2.2, the server SHOULD NOT rely on an
   identity string received in EAP-Response/Identity.  Therefore, the
   RECOMMENDED way to start an EAP-AKA exchange is to ignore any
   received identity strings.  The server SHOULD begin the EAP-AKA
   exchange by issuing the EAP-Request/AKA-Identity 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/AKA-Identity message.  Three methods to request an identity
   from the peer are discussed below.

   If the server chooses to not ignore the contents of
   EAP-Response/Identity, then the server may already receive an EAP-AKA
   identity in this packet.  However, if the EAP server has not received
   any EAP-AKA peer identity (permanent identity, pseudonym identity, or
   fast re-authentication identity) from the peer when sending the first
   EAP-AKA 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/AKA-Identity 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
   of the EAP-Response/AKA-Identity message.  This is done in the
   following cases:

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   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/AKA-Identity 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/AKA-Identity 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/AKA-Identity 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/AKA-Identity 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,
   when the server does not have any identity, or when the server does
   not recognize the format of a received identity.

4.1.5.  Processing of EAP-Request/AKA-Identity by the Peer

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

   If the EAP-Request/AKA-Identity includes AT_PERMANENT_ID_REQ, and if
   the peer does not have a pseudonym available, then the peer MUST
   respond with EAP-Response/AKA-Identity and include the permanent
   identity in 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/AKA-Identity and
   include the permanent identity in AT_IDENTITY or respond with
   EAP-Response/AKA-Client-Error packet with code "unable to process
   packet".

   If the EAP-Request/AKA-Identity includes AT_FULL_AUTH_ID_REQ, and if
   the peer has a pseudonym available, then the peer SHOULD respond with
   EAP-Response/AKA-Identity and include the pseudonym identity in

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   AT_IDENTITY.  If the peer does not have a pseudonym when it receives
   this message, then the peer MUST respond with EAP-Response/
   AKA-Identity and include the permanent identity in AT_IDENTITY.  The
   Peer MUST NOT use a fast re-authentication identity in the
   AT_IDENTITY attribute.

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

   An EAP-AKA exchange may include several EAP/AKA-Identity rounds.  The
   server may issue a second EAP-Request/AKA-Identity, if it was not
   able to recognize the identity the peer used in the previous
   AT_IDENTITY attribute.  At most three EAP/AKA-Identity rounds can be
   used, so the peer MUST NOT respond to more than three
   EAP-Request/AKA-Identity messages within an EAP exchange.  The peer
   MUST verify that the sequence of EAP-Request/AKA-Identity packets 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/AKA-Identity; in other words, AT_ANY_ID_REQ MUST NOT be
   used in the second or third EAP-Request/AKA-Identity.
   AT_FULLAUTH_ID_REQ MUST NOT be used if the previous
   EAP-Request/AKA-Identity 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.

4.1.6.  Attacks against Identity Privacy

   The section above specifies two possible ways the peer can operate
   upon receipt of AT_PERMANENT_ID_REQ because a received
   AT_PERMANENT_ID_REQ does not necessarily originate from the valid
   network.  However, an active attacker may transmit an
   EAP-Request/AKA-Identity 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 EAP-Response/AKA-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,

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   if a conservative peer has a pseudonym username, the peer responds
   with EAP-Response/AKA-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.

4.1.7.  Processing of AT_IDENTITY by the Server

   When the server receives an EAP-Response/AKA-Identity 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 an
   EAP-Request/AKA-Notification packet with AT_NOTIFICATION code
   "General failure" (16384) to terminate the EAP exchange.  If the
   server recognizes the permanent identity and is able to continue,
   then the server proceeds with full authentication by sending
   EAP-Request/AKA-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/AKA-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/
   AKA-Identity 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/ AKA-Challenge.

   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/AKA-Reauthentication
   (Section 5).

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   If the server used AT_ANY_ID_REQ, and if the peer sent an EAP-
   Response/AKA-Identity with AT_IDENTITY that contains an identity that
   the server recognizes as a fast re-authentication identity, but the
   server is not able to map the identity to a permanent identity, then
   the server sends EAP-Request/AKA-Identity with AT_FULLAUTH_ID_REQ.

   If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid
   fast re-authentication identity, which 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 proceeds with full authentication
   by sending EAP-Request/AKA-Challenge.

   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/AKA-Identity 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/AKA-Identity with AT_FULLAUTH_ID_REQ.

4.2.  Message Sequence Examples (Informative)

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

4.2.1.  Usage of AT_ANY_ID_REQ

   Obtaining the peer identity with EAP-AKA attributes is illustrated in
   Figure 5 below.

       Peer                                             Authenticator
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not have any     |
          |                            | Subscriber identity available|
          |                            | When starting EAP-AKA        |
          |                            +------------------------------+
          |          EAP-Request/AKA-Identity                     |
          |          (AT_ANY_ID_REQ)                              |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY)                                         |
          |------------------------------------------------------>|
          |                                                       |
                     Figure 5: Usage of AT_ANY_ID_REQ

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4.2.2.  Fall Back on Full Authentication

   Figure 6 illustrates the case when the server does not recognize the
   fast re-authentication identity the peer used in AT_IDENTITY.

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

                Figure 6: Fall back on full authentication

   If the server recognizes the fast re-authentication identity, but
   still wants to fall back on full authentication, the server may issue
   the EAP-Request/AKA-Challenge packet.  In this case, the full
   authentication procedure proceeds as usual.

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4.2.3.  Requesting the Permanent Identity 1

   Figure 7 illustrates the case when the EAP server fails to decode a
   pseudonym identity included in the EAP-Response/Identity packet.

       Peer                                             Authenticator
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          | EAP-Response/Identity                                 |
          | (Includes a pseudonym)                                |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server fails to decode the   |
          |                            | Pseudonym.                   |
          |                            +------------------------------+
          |  EAP-Request/AKA-Identity                             |
          |  (AT_PERMANENT_ID_REQ)                                |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with permanent identity)                 |
          |------------------------------------------------------>|
          |                                                       |

               Figure 7: Requesting the permanent identity 1

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

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4.2.4.  Requesting the Permanent Identity 2

   Figure 8 illustrates the case when the EAP server fails to decode the
   pseudonym included in the AT_IDENTITY attribute.

       Peer                                             Authenticator
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not have any     |
          |                            | Subscriber identity available|
          |                            | When starting EAP-AKA        |
          |                            +------------------------------+
          |        EAP-Request/AKA-Identity                       |
          |        (AT_ANY_ID_REQ)                                |
          |<------------------------------------------------------|
          |                                                       |
          |EAP-Response/AKA-Identity                              |
          |(AT_IDENTITY with a pseudonym identity)                |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server fails to decode the   |
          |                            | Pseudonym in AT_IDENTITY     |
          |                            +------------------------------+
          |                EAP-Request/AKA-Identity               |
          |                (AT_PERMANENT_ID_REQ)                  |
          |<------------------------------------------------------|
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with permanent identity)                 |
          |------------------------------------------------------>|
          |                                                       |

               Figure 8: Requesting the permanent identity 2

4.2.5.  Three EAP/AKA-Identity Round Trips

   Figure 9 illustrates the case with three EAP/AKA-Identity round
   trips.

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       Peer                                             Authenticator
          |                                                       |
          |                            +------------------------------+
          |                            | Server does not have any     |
          |                            | Subscriber identity available|
          |                            | When starting EAP-AKA        |
          |                            +------------------------------+
          |        EAP-Request/AKA-Identity                       |
          |        (AT_ANY_ID_REQ)                                |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with fast re-auth. identity)             |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server does not accept       |
          |                            | The fast re-authentication   |
          |                            | Identity                     |
          |                            +------------------------------+
          |                                                       |
          :                                                       :
          :                                                       :


          :                                                       :
          :                                                       :
          |     EAP-Request/AKA-Identity                          |
          |     (AT_FULLAUTH_ID_REQ)                              |
          |<------------------------------------------------------|
          |EAP-Response/AKA-Identity                              |
          |(AT_IDENTITY with a pseudonym identity)                |
          |------------------------------------------------------>|
          |                            +------------------------------+
          |                            | Server fails to decode the   |
          |                            | Pseudonym in AT_IDENTITY     |
          |                            +------------------------------+
          |           EAP-Request/AKA-Identity                    |
          |           (AT_PERMANENT_ID_REQ)                       |
          |<------------------------------------------------------|
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY with permanent identity)                 |
          |------------------------------------------------------>|
          |                                                       |

                   Figure 9: Three EAP-AKA Start rounds

   After the last EAP-Response/AKA-Identity message, the full
   authentication sequence proceeds as usual.

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5.  Fast Re-Authentication

5.1.  General

   In some environments, EAP authentication may be performed frequently.
   Because the EAP-AKA full authentication procedure uses the AKA
   algorithms, and therefore requires fresh authentication vectors from
   the Authentication Centre, the full authentication procedure may
   result in many network operations when used very frequently.
   Therefore, EAP-AKA includes a more inexpensive fast re-authentication
   procedure that does not make use of the AKA algorithms and does not
   need new vectors from the Authentication Centre.

   Fast re-authentication is optional to implement for both the EAP-AKA
   server and peer.  On each EAP authentication, either one of the
   entities may fall back on full authentication if is does not want to
   use fast re-authentication.

   Fast re-authentication is based on the keys derived on the preceding
   full authentication.  The same K_aut and K_encr keys used in full
   authentication are used to protect EAP-AKA packets and attributes,
   and the original Master Key from full authentication is used to
   generate a fresh Master Session Key, as specified in Section 7.

   The fast re-authentication exchange makes use of an unsigned 16-bit
   counter, included in the AT_COUNTER attribute.  The counter has three
   goals: 1) it can be used to limit the number of successive
   reauthentication exchanges without full-authentication 2) it
   contributes to the keying material, and 3) it protects the peer and
   the server from replays.  On full authentication, both the server and
   the peer initialize the counter to one.  The counter value of at
   least one is used on the first fast re-authentication.  On subsequent
   fast re-authentications, the counter MUST be greater than on any of
   the previous fast re-authentications.  For example, on the second
   fast re-authentication, counter value is two or greater, etc.  The
   AT_COUNTER attribute is encrypted.

   Both the peer and the EAP server maintain a copy of the counter.  The
   EAP server sends its counter value to the peer in the fast
   re-authentication request.  The peer MUST verify that its counter
   value is less than or equal to the value sent by the EAP server.

   The server includes an encrypted server random nonce (AT_NONCE_S) in
   the fast re-authentication request.  The AT_MAC attribute in the
   peer's response is calculated over NONCE_S to provide a
   challenge/response authentication scheme.  The NONCE_S also
   contributes to the new Master Session Key.

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   Both the peer and the server SHOULD have an upper limit for the
   number of subsequent fast re-authentications allowed before a full
   authentication needs to be performed.  Because a 16-bit counter is
   used in fast re-authentication, the theoretical maximum number of
   re-authentications is reached when the counter value reaches FFFF
   hexadecimal.  In order to use fast re-authentication, the peer and
   the EAP server need to store the following values: Master Key, latest
   counter value and the next fast re-authentication identity.  K_aut
   and K_encr may either be stored or derived again from MK.  The server
   may also need to store the permanent identity of the user.

5.2.  Comparison to AKA

   When analyzing the fast re-authentication exchange, it may be helpful
   to compare it with the 3rd generation Authentication and Key
   Agreement (AKA) exchange used on full authentication.  The counter
   corresponds to the AKA sequence number, NONCE_S corresponds to RAND,
   the AT_MAC in EAP-Request/AKA-Reauthentication corresponds to AUTN,
   the AT_MAC in EAP-Response/AKA-Reauthentication corresponds to RES,
   AT_COUNTER_TOO_SMALL corresponds to AUTS, and encrypting the counter
   corresponds to the usage of the Anonymity Key.  Also, the key
   generation on fast re-authentication, with regard to random or fresh
   material, is similar to AKA -- the server generates the NONCE_S and
   counter values, and the peer only verifies that the counter value is
   fresh.

   It should also be noted that encrypting the AT_NONCE_S, AT_COUNTER,
   or AT_COUNTER_TOO_SMALL attributes is not important to the security
   of the fast re-authentication exchange.

5.3.  Fast Re-Authentication Identity

   The fast re-authentication procedure makes use of separate
   re-authentication user identities.  Pseudonyms and the permanent
   identity are reserved for full authentication only.  If a fast
   re-authentication identity is lost and the network does not recognize
   it, the EAP server can fall back on full authentication.  If the EAP
   server supports fast re-authentication, it MAY include the skippable
   AT_NEXT_REAUTH_ID attribute in the encrypted data of EAP- Request/-
   AKA-Challenge message.  This attribute contains a new
   re-authentication identity for the next fast re-authentication.  The
   attribute also works as a capability flag that indicates that the
   server supports fast re-authentication and that the server wants to
   continue using fast re-authentication within the current context.
   The peer MAY ignore this attribute, in which case it will use full
   authentication next time.  If the peer wants to use fast
   re-authentication, it uses this fast re-authentication identity on
   next authentication.  Even if the peer has a fast re-authentication

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   identity, the peer MAY discard the re-authentication identity and use
   a pseudonym or the permanent identity instead, in which case full
   authentication MUST be performed.  If the EAP server does not include
   the AT_NEXT_REAUTH_ID in the encrypted data of
   EAP-Request/AKA-Challenge or EAP-Request/AKA-Reauthentication, then
   the peer MUST discard its current fast re-authentication state
   information and perform a full authentication next time.

   In environments where a realm portion is needed in the peer identity,
   the fast re-authentication identity received in AT_NEXT_REAUTH_ID
   MUST contain both a username portion and a realm portion, as per the
   NAI format.  The EAP Server can choose an appropriate realm part in
   order to have the AAA infrastructure route subsequent fast
   re-authentication-related requests to the same AAA server.  For
   example, the realm part MAY include a portion that is specific to the
   AAA server.  Hence, it is sufficient to store the context required
   for fast re-authentication in the AAA server that performed the full
   authentication.

   The peer MAY use the fast re-authentication identity in the
   EAP-Response/Identity packet or, in response to the server's
   AT_ANY_ID_REQ attribute, the peer MAY use the fast re-authentication
   identity in the AT_IDENTITY attribute of the EAP-Response/
   AKA-Identity packet.

   The peer MUST NOT modify the username portion of the fast
   re-authentication identity, but the peer MAY modify the realm portion
   or replace it with another realm portion.  The peer might need to
   modify the realm in order to influence the AAA routing, for example,
   to make sure that the correct server is reached.  It should be noted
   that sharing the same fast re-authentication key among several
   servers may have security risks, so changing the realm portion of the
   NAI in order to change the EAP server is not desirable.

   Even if the peer uses a fast re-authentication identity, the server
   may want to fall back on full authentication, for example, because
   the server does not recognize the fast re-authentication identity or
   does not want to use fast re-authentication.  If the server was able
   to decode the fast re-authentication identity to the permanent
   identity, the server issues the EAP-Request/AKA-Challenge packet to
   initiate full authentication.  If the server was not able to recover
   the peer's identity from the fast re-authentication identity, the
   server starts the full authentication procedure by issuing an
   EAP-Request/AKA-Identity packet.  This packet always starts a full
   authentication sequence if it does not include the AT_ANY_ID_REQ
   attribute.

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5.4.  Fast Re-Authentication Procedure

   Figure 10 illustrates the fast re-authentication procedure.  In this
   example, the optional protected success indication is not used.
   Encrypted attributes are denoted with '*'.  The peer uses its fast
   re-authentication identity in the EAP-Response/Identity packet.  As
   discussed above, an alternative way to communicate the fast
   re-authentication identity to the server is for the peer to use the
   AT_IDENTITY attribute in the EAP-Response/AKA-Identity message.  This
   latter case is not illustrated in the figure below, and it is only
   possible when the server requests that the peer send its identity by
   including the AT_ANY_ID_REQ attribute in the EAP-Request/AKA-Identity
   packet.

   If the server recognizes the identity as a valid fast
   re-authentication identity, and if the server agrees to use fast
   re-authentication, then the server sends the EAP- Request/AKA-
   Reauthentication packet to the peer.  This packet MUST include the
   encrypted AT_COUNTER attribute, with a fresh counter value, the
   encrypted AT_NONCE_S attribute that contains a random number chosen
   by the server, the AT_ENCR_DATA and the AT_IV attributes used for
   encryption, and the AT_MAC attribute that contains a message
   authentication code over the packet.  The packet MAY also include an
   encrypted AT_NEXT_REAUTH_ID attribute that contains the next fast
   re-authentication identity.

   Fast re-authentication identities are one-time identities.  If the
   peer does not receive a new fast re-authentication identity, it MUST
   use either the permanent identity or a pseudonym identity on the next
   authentication to initiate full authentication.

   The peer verifies that AT_MAC is correct and that the counter value
   is fresh (greater than any previously used value).  The peer MAY save
   the next fast re-authentication identity from the encrypted
   AT_NEXT_REAUTH_ID for next time.  If all checks are successful, the
   peer responds with the EAP-Response/AKA-Reauthentication packet,
   including the AT_COUNTER attribute with the same counter value and
   the AT_MAC attribute.

   The server verifies the AT_MAC attribute and also verifies that the
   counter value is the same that it used in the
   EAP-Request/AKA-Reauthentication packet.  If these checks are
   successful, the fast re-authentication has succeeded and the server
   sends the EAP-Success packet to the peer.

   If protected success indications (Section 6.2) were used, the
   EAP-Success packet would be preceded by an EAP-AKA notification
   round.

Top      Up      ToC       Page 36 
        Peer                                             Authenticator
          |                                                       |
          |                               EAP-Request/Identity    |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/Identity                                 |
          | (Includes a fast re-authentication identity)          |
          |------------------------------------------------------>|
          |                          +--------------------------------+
          |                          | Server recognizes the identity |
          |                          | and agrees on using fast       |
          |                          | re-authentication              |
          |                          +--------------------------------+
          |  EAP-Request/AKA-Reauthentication                     |
          |  (AT_IV, AT_ENCR_DATA, *AT_COUNTER,                   |
          |   *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC)            |
          |<------------------------------------------------------|
          |                                                       |
          :                                                       :
          :                                                       :


          :                                                       :
          :                                                       :
          |                                                       |
     +-----------------------------------------------+            |
     | Peer verifies AT_MAC and the freshness of     |            |
     | the counter. Peer MAY store the new re-       |            |
     | authentication identity for next re-auth.     |            |
     +-----------------------------------------------+            |
          |                                                       |
          | EAP-Response/AKA-Reauthentication                     |
          | (AT_IV, AT_ENCR_DATA, *AT_COUNTER with same value,    |
          |  AT_MAC)                                              |
          |------------------------------------------------------>|
          |                          +--------------------------------+
          |                          | Server verifies AT_MAC and     |
          |                          | the counter                    |
          |                          +--------------------------------+
          |                                          EAP-Success  |
          |<------------------------------------------------------|
          |                                                       |

                        Figure 10: Reauthentication

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5.5.  Fast Re-Authentication Procedure when Counter is Too Small

   If the peer does not accept the counter value of EAP-Request/
   AKA-Reauthentication, it indicates the counter synchronization
   problem by including the encrypted AT_COUNTER_TOO_SMALL in
   EAP-Response/AKA-Reauthentication.  The server responds with
   EAP-Request/AKA-Challenge to initiate a normal full authentication
   procedure.  This is illustrated in Figure 11.  Encrypted attributes
   are denoted with '*'.

        Peer                                             Authenticator
          |          EAP-Request/AKA-Identity                     |
          |          (AT_ANY_ID_REQ)                              |
          |<------------------------------------------------------|
          |                                                       |
          | EAP-Response/AKA-Identity                             |
          | (AT_IDENTITY)                                         |
          | (Includes a fast re-authentication identity)          |
          |------------------------------------------------------>|
          |                                                       |
          |  EAP-Request/AKA-Reauthentication                     |
          |  (AT_IV, AT_ENCR_DATA, *AT_COUNTER,                   |
          |   *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC)            |
          |<------------------------------------------------------|
     +-----------------------------------------------+            |
     | AT_MAC is valid but the counter is not fresh. |            |
     +-----------------------------------------------+            |
          | EAP-Response/AKA-Reauthentication                     |
          | (AT_IV, AT_ENCR_DATA, *AT_COUNTER_TOO_SMALL,          |
          |  *AT_COUNTER, AT_MAC)                                 |
          |------------------------------------------------------>|
          |            +----------------------------------------------+
          |            | Server verifies AT_MAC but detects           |
          |            | That peer has included AT_COUNTER_TOO_SMALL|
          |            +----------------------------------------------+
          |                        EAP-Request/AKA-Challenge      |
          |<------------------------------------------------------|
     +---------------------------------------------------------------+
     |                Normal full authentication follows.            |
     +---------------------------------------------------------------+
          |                                                       |

            Figure 11: Fast re-authentication counter too small

   In the figure above, the first three messages are similar to the
   basic fast re-authentication case.  When the peer detects that the
   counter value is not fresh, it includes the AT_COUNTER_TOO_SMALL
   attribute in EAP-Response/AKA-Reauthentication.  This attribute

Top      Up      ToC       Page 38 
   doesn't contain any data but it is a request for the server to
   initiate full authentication.  In this case, the peer MUST ignore the
   contents of the server's AT_NEXT_REAUTH_ID attribute.

   On receipt of AT_COUNTER_TOO_SMALL, the server verifies AT_MAC and
   verifies that AT_COUNTER contains the same counter value as in the
   EAP-Request/AKA-Reauthentication packet.  If not, the server
   terminates the authentication exchange by sending the
   EAP-Request/AKA-Notification packet with AT_NOTIFICATION code
   "General failure" (16384).  If all checks on the packet are
   successful, the server transmits an EAP-Request/AKA-Challenge packet
   and the full authentication procedure is performed as usual.  Because
   the server already knows the subscriber identity, it MUST NOT use the
   EAP-Request/AKA-Identity packet to request the identity.

   It should be noted that in this case, peer identity is only
   transmitted in the AT_IDENTITY attribute at the beginning of the
   whole EAP exchange.  The fast re-authentication identity used in this
   AT_IDENTITY attribute will be used in key derivation (see Section 7).



(page 38 continued on part 3)

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