| AT_MAC) |
|------------------------------------------------------>|
| +--------------------------------+
| | Server verifies AT_MAC and |
| | the counter |
| +--------------------------------+
| EAP-Success |
|<------------------------------------------------------|
| |
Figure 10: Reauthentication
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
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).
6. EAP-AKA Notifications
6.1. General
EAP-AKA does not prohibit the use of the EAP Notifications as
specified in [RFC3748]. EAP Notifications can be used at any time in
the EAP-AKA exchange. It should be noted that EAP-AKA does not
protect EAP Notifications. EAP-AKA also specifies method-specific
EAP-AKA notifications, which are protected in some cases.
The EAP server can use EAP-AKA notifications to convey notifications
and result indications (Section 6.2) to the peer.
The server MUST use notifications in cases discussed in
Section 6.3.2. When the EAP server issues an
EAP-Request/AKA-Notification packet to the peer, the peer MUST
process the notification packet. The peer MAY show a notification
message to the user and the peer MUST respond to the EAP server with
an EAP-Response/AKA-Notification packet, even if the peer did not
recognize the notification code.
An EAP-AKA full authentication exchange or a fast re-authentication
exchange MUST NOT include more than one EAP-AKA notification round.
The notification code is a 16-bit number. The most significant bit
is called the Success bit (S bit). The S bit specifies whether the
notification implies failure. The code values with the S bit set to
zero (code values 0...32767) are used on unsuccessful cases. The
receipt of a notification code from this range implies failed EAP
exchange, so the peer can use the notification as a failure
indication. After receiving the EAP-Response/AKA-Notification for
these notification codes, the server MUST send the EAP-Failure
packet.
The receipt of a notification code with the S bit set to one (values
32768...65536) does not imply failure. Notification code "Success"
(32768) has been reserved as a general notification code to indicate
successful authentication.
The second most significant bit of the notification code is called
the Phase bit (P bit). It specifies at which phase of the EAP-AKA
exchange the notification can be used. If the P bit is set to zero,
the notification can only be used after a successful EAP/AKA-
Challenge round in full authentication or a successful EAP/AKA-
Reauthentication round in re-authentication. A re-authentication
round is considered successful only if the peer has successfully
verified AT_MAC and AT_COUNTER attributes, and does not include the
AT_COUNTER_TOO_SMALL attribute in EAP-Response/AKA-Reauthentication.
If the P bit is set to one, the notification can only by used before
the EAP/AKA-Challenge round in full authentication or before the
EAP/AKA-Reauthentication round in reauthentication. These
notifications can only be used to indicate various failure cases. In
other words, if the P bit is set to one, then the S bit MUST be set
to zero.
Section 9.10 and Section 9.11 specify what other attributes must be
included in the notification packets.
Some of the notification codes are authorization related and hence
not usually considered as part of the responsibility of an EAP
method. However, they are included as part of EAP-AKA because there
are currently no other ways to convey this information to the user in
a localizable way, and the information is potentially useful for the
user. An EAP-AKA server implementation may decide never to send
these EAP-AKA notifications.
6.2. Result Indications
As discussed in Section 6.3, the server and the peer use explicit
error messages in all error cases. If the server detects an error
after successful authentication, the server uses an EAP-AKA
notification to indicate failure to the peer. In this case, the
result indication is integrity and replay protected.
By sending an EAP-Response/AKA-Challenge packet or an
EAP-Response/AKA-Reauthentication packet (without
AT_COUNTER_TOO_SMALL), the peer indicates that it has successfully
authenticated the server and that the peer’s local policy accepts the
EAP exchange. In other words, these packets are implicit success
indications from the peer to the server.
EAP-AKA also supports optional protected success indications from the
server to the peer. If the EAP server wants to use protected success
indications, it includes the AT_RESULT_IND attribute in the
EAP-Request/AKA-Challenge or the EAP-Request/AKA-Reauthentication
packet. This attribute indicates that the EAP server would like to
use result indications in both successful and unsuccessful cases. If
the peer also wants this, the peer includes AT_RESULT_IND in
EAP-Response/AKA-Challenge or EAP-Response/AKA-Reauthentication. The
peer MUST NOT include AT_RESULT_IND if it did not receive
AT_RESULT_IND from the server. If both the peer and the server used
AT_RESULT_IND, then the EAP exchange is not complete yet, but an
EAP-AKA notification round will follow. The following EAP-AKA
notification may indicate either failure or success.
Success indications with the AT_NOTIFICATION code "Success" (32768)
can only be used if both the server and the peer indicate they want
to use them with AT_RESULT_IND. If the server did not include
AT_RESULT_IND in the EAP-Request/AKA-Challenge or
EAP-Request/AKA-Reauthentication packet, or if the peer did not
include AT_RESULT_IND in the corresponding response packet, then the
server MUST NOT use protected success indications.
Because the server uses the AT_NOTIFICATION code "Success" (32768) to
indicate that the EAP exchange has completed successfully, the EAP
exchange cannot fail when the server processes the EAP-AKA response
to this notification. Hence, the server MUST ignore the contents of
the EAP-AKA response it receives to the EAP-Request/AKA-Notification
with this code. Regardless of the contents of the EAP-AKA response,
the server MUST send EAP-Success as the next packet.
6.3. Error Cases
This section specifies the operation of the peer and the server in
error cases. The subsections below require the EAP-AKA peer and
server to send an error packet (EAP-Response/AKA-Client-Error,
EAP-Response/AKA-Authentication-Reject or
EAP-Response/AKA-Synchronization-Failure from the peer and
EAP-Request/AKA-Notification from the server) in error cases.
However, implementations SHOULD NOT rely upon the correct error
reporting behavior of the peer, authenticator, or server. It is
possible for error messages and other messages to be lost in transit,
or for a malicious participant to attempt to consume resources by not
issuing error messages. Both the peer and the EAP server SHOULD have
a mechanism to clean up state even if an error message or EAP-Success
is not received after a timeout period.
6.3.1. Peer Operation
Two special error messages have been specified for error cases that
are related to the processing of the AKA AUTN parameter, as described
in Section 3: (1) if the peer does not accept AUTN, the peer responds
with EAP-Response/AKA-Authentication-Reject (Section 9.5), and the
server issues EAP-Failure, and (2) if the peer detects that the
sequence number in AUTN is not correct, the peer responds with
EAP-Response/AKA-Synchronization-Failure (Section 9.6), and the
server proceeds with a new EAP-Request/AKA-Challenge.
In other error cases, when an EAP-AKA peer detects an error in a
received EAP-AKA packet, the EAP-AKA peer responds with the
EAP-Response/AKA-Client-Error packet. In response to the
EAP-Response/AKA-Client-Error, the EAP server MUST issue the
EAP-Failure packet, and the authentication exchange terminates.
By default, the peer uses the client error code 0, "unable to process
packet". This error code is used in the following cases:
o EAP exchange is not acceptable according to the peer’s local
policy.
o The peer is not able to parse the EAP request, i.e., the EAP
request is malformed.
o The peer encountered a malformed attribute.
o Wrong attribute types or duplicate attributes have been included
in the EAP request.
o A mandatory attribute is missing.
o Unrecognized non-skippable attribute.
o Unrecognized or unexpected EAP-AKA Subtype in the EAP request.
o Invalid AT_MAC. The peer SHOULD log this event.
o Invalid AT_CHECKCODE. The peer SHOULD log this event.
o Invalid pad bytes in AT_PADDING.
o The peer does not want to process AT_PERMANENT_ID_REQ.
6.3.2. Server Operation
If an EAP-AKA server detects an error in a received EAP-AKA response,
the server MUST issue the EAP-Request/AKA-Notification packet with an
AT_NOTIFICATION code that implies failure. By default, the server
uses one of the general failure codes ("General failure after
authentication" (0) or "General failure" (16384)). The choice
between these two codes depends on the phase of the EAP-AKA exchange,
see Section 6. The error cases when the server issues an
EAP-Request/AKA-Notification that implies failure include the
following:
o The server is not able to parse the peer’s EAP response.
o The server encounters a malformed attribute, a non-recognized
non-skippable attribute, or a duplicate attribute.
o A mandatory attribute is missing or an invalid attribute was
included.
o Unrecognized or unexpected EAP-AKA Subtype in the EAP Response.
o Invalid AT_MAC. The server SHOULD log this event.
o Invalid AT_CHECKCODE. The server SHOULD log this event.
o Invalid AT_COUNTER.
6.3.3. EAP-Failure
The EAP-AKA server sends EAP-Failure in three cases:
1. In response to an EAP-Response/AKA-Client-Error packet the server
has received from the peer, or
2. In response to an EAP-Response/AKA-Authentication-Reject packet
the server has received from the peer, or
3. Following an EAP-AKA notification round, when the AT_NOTIFICATION
code implies failure.
The EAP-AKA server MUST NOT send EAP-Failure in other cases than
these three. However, it should be noted that even though the
EAP-AKA server would not send an EAP-Failure, an authorization
decision that happens outside EAP-AKA, such as in the AAA server or
in an intermediate AAA proxy, may result in a failed exchange.
The peer MUST accept the EAP-Failure packet in case 1), case 2), and
case 3) above. The peer SHOULD silently discard the EAP-Failure
packet in other cases.
6.3.4. EAP-Success
On full authentication, the server can only send EAP-Success after
the EAP/AKA-Challenge round. The peer MUST silently discard any
EAP-Success packets if they are received before the peer has
successfully authenticated the server and sent the
EAP-Response/AKA-Challenge packet.
If the peer did not indicate that it wants to use protected success
indications with AT_RESULT_IND (as discussed in Section 6.2) on full
authentication, then the peer MUST accept EAP-Success after a
successful EAP/AKA-Challenge round.
If the peer indicated that it wants to use protected success
indications with AT_RESULT_IND (as discussed in Section 6.2), then
the peer MUST NOT accept EAP-Success after a successful EAP/
AKA-Challenge round. In this case, the peer MUST only accept
EAP-Success after receiving an EAP-AKA Notification with the
AT_NOTIFICATION code "Success" (32768).
On fast re-authentication, EAP-Success can only be sent after the
EAP/AKA-Reauthentication round. The peer MUST silently discard any
EAP-Success packets if they are received before the peer has
successfully authenticated the server and sent the
EAP-Response/AKA-Reauthentication packet.
If the peer did not indicate that it wants to use protected success
indications with AT_RESULT_IND (as discussed in Section 6.2) on fast
re-authentication, then the peer MUST accept EAP-Success after a
successful EAP/AKA-Reauthentication round.
If the peer indicated that it wants to use protected success
indications with AT_RESULT_IND (as discussed in Section 6.2), then
the peer MUST NOT accept EAP-Success after a successful EAP/AKA-
Reauthentication round. In this case, the peer MUST only accept
EAP-Success after receiving an EAP-AKA Notification with the
AT_NOTIFICATION code "Success" (32768).
If the peer receives an EAP-AKA notification (Section 6) that
indicates failure, then the peer MUST no longer accept the
EAP-Success packet, even if the server authentication was
successfully completed.
7. Key Generation
This section specifies how keying material is generated.
On EAP-AKA full authentication, a Master Key (MK) is derived from the
underlying AKA values (CK and IK keys), and the identity, as follows.
MK = SHA1(Identity|IK|CK)
In the formula above, the "|" character denotes concatenation.
Identity denotes the peer identity string without any terminating
null characters. It is the identity from the last AT_IDENTITY
attribute sent by the peer in this exchange, or, if AT_IDENTITY was
not used, the identity from the EAP-Response/Identity packet. The
identity string is included as-is, without any changes. As discussed
in Section 4.1.2.2, relying on EAP-Response/Identity for conveying
the EAP-AKA peer identity is discouraged, and the server SHOULD use
the EAP-AKA method-specific identity attributes. The hash function
SHA-1 is specified in [SHA-1].
The Master Key is fed into a Pseudo-Random number Function (PRF),
which generates separate Transient EAP Keys (TEKs) for protecting
EAP-AKA packets, as well as a Master Session Key (MSK) for link layer
security and an Extended Master Session Key (EMSK) for other
purposes. On fast re-authentication, the same TEKs MUST be used for
protecting EAP packets, but a new MSK and a new EMSK MUST be derived
from the original MK and from new values exchanged in the fast
re-authentication.
EAP-AKA requires two TEKs for its own purposes: the authentication
key K_aut, to be used with the AT_MAC attribute, and the encryption
key K_encr, to be used with the AT_ENCR_DATA attribute. The same
K_aut and K_encr keys are used in full authentication and subsequent
fast re-authentications.
Key derivation is based on the random number generation specified in
NIST Federal Information Processing Standards (FIPS) Publication
186-2 [PRF]. The pseudo-random number generator is specified in the
change notice 1 (2001 October 5) of [PRF] (Algorithm 1). As
specified in the change notice (page 74), when Algorithm 1 is used as
a general-purpose pseudo-random number generator, the "mod q" term in
step 3.3 is omitted. The function G used in the algorithm is
constructed via Secure Hash Standard as specified in Appendix 3.3 of
the standard. It should be noted that the function G is very similar
to SHA-1, but the message padding is different. Please refer to
[PRF] for full details. For convenience, the random number algorithm
with the correct modification is cited in Annex A.
160-bit XKEY and XVAL values are used, so b = 160. On each full
authentication, the Master Key is used as the initial secret seed-key
XKEY. The optional user input values (XSEED_j) in step 3.1 are set
to zero.
On full authentication, the resulting 320-bit random numbers x_0,
x_1, ..., x_m-1 are concatenated and partitioned into suitable-sized
chunks and used as keys in the following order: K_encr (128 bits),
K_aut (128 bits), Master Session Key (64 bytes), Extended Master
Session Key (64 bytes).
On fast re-authentication, the same pseudo-random number generator
can be used to generate a new Master Session Key and a new Extended
Master Session Key. The seed value XKEY’ is calculated as follows:
XKEY’ = SHA1(Identity|counter|NONCE_S| MK)
In the formula above, the Identity denotes the fast re-authentication
identity, without any terminating null characters, from the
AT_IDENTITY attribute of the EAP-Response/AKA-Identity packet, or, if
EAP-Response/AKA-Identity was not used on fast re-authentication, it
denotes the identity string from the EAP-Response/Identity packet.
The counter denotes the counter value from the AT_COUNTER attribute
used in the EAP-Response/AKA-Reauthentication packet. The counter is
used in network byte order. NONCE_S denotes the 16-byte random
NONCE_S value from the AT_NONCE_S attribute used in the
EAP-Request/AKA-Reauthentication packet. The MK is the Master Key
derived on the preceding full authentication.
On fast re-authentication, the pseudo-random number generator is run
with the new seed value XKEY’, and the resulting 320-bit random
numbers x_0, x_1, ..., x_m-1 are concatenated and partitioned into
64-byte chunks and used as the new 64-byte Master Session Key and the
new 64-byte Extended Master Session Key. Note that because K_encr
and K_aut are not derived on fast re-authentication, the Master
Session Key and the Extended Master Session key are obtained from the
beginning of the key stream x_0, x_1, ....
The first 32 bytes of the MSK can be used as the Pairwise Master Key
(PMK) for IEEE 802.11i.
When the RADIUS attributes specified in [RFC2548] are used to
transport keying material, then the first 32 bytes of the MSK
correspond to MS-MPPE-RECV-KEY and the second 32 bytes to
MS-MPPE-SEND-KEY. In this case, only 64 bytes of keying material
(the MSK) are used.
8. Message Format and Protocol Extensibility
8.1. Message Format
As specified in [RFC3748], EAP packets begin with the Code,
Identifiers, Length, and Type fields, which are followed by
EAP-method-specific Type-Data. The Code field in the EAP header is
set to 1 for EAP requests, and to 2 for EAP Responses. The usage of
the Length and Identifier fields in the EAP header is also specified
in [RFC3748]. In EAP-AKA, the Type field is set to 23.
In EAP-AKA, the Type-Data begins with an EAP-AKA header that consists
of a 1-octet Subtype field, and a 2-octet reserved field. The
Subtype values used in EAP-AKA are defined in Section 11. The
formats of the EAP header and the EAP-AKA header are shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+