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13 14 15 RFC 4187
Network Working Group J. Arkko
Internet Draft Ericsson
Document: draft-arkko-pppext-eap-aka-11.txt H. Haverinen
Expires: 27 April, 2004 Nokia
27 October, 2003
EAP AKA Authentication
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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Comments should be submitted to the eap@frascone.com mailing list.
Abstract
This document specifies an Extensible Authentication Protocol (EAP)
mechanism for authentication and session key distribution using the
Universal Mobile Telecommunications System (UMTS) Authentication and
Key Agreement (AKA) mechanism. UMTS AKA is based on symmetric keys,
and runs typically in a UMTS Subscriber Identity Module, a smart
card like device.
EAP AKA includes optional identity privacy support and an optional
re-authentication procedure.
Table of Contents
Status of this Memo................................................1
Abstract...........................................................1
1. Introduction and Motivation.....................................3
2. Terms and Conventions Used in This Document.....................4
3. Protocol Overview...............................................6
4. Operation......................................................11
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4.1. Identity Management..........................................11
4.2. Re-authentication............................................25
4.3. EAP/AKA Notifications........................................31
4.4. Error Cases..................................................32
4.5. Key Generation...............................................34
5. Message Format and Protocol Extensibility......................35
5.1. Message Format...............................................35
5.2. Protocol Extensibility.......................................37
6. Messages.......................................................37
6.1. EAP-Request/AKA-Identity.....................................37
6.2. EAP-Response/AKA-Identity....................................38
6.3. EAP-Request/AKA-Challenge....................................38
6.4. EAP-Response/AKA-Challenge...................................39
6.5. EAP-Response/AKA-Authentication-Reject.......................39
6.6. EAP-Response/AKA-Synchronization-Failure.....................39
6.7. EAP-Request/AKA-Reauthentication.............................39
6.8. EAP-Response/AKA-Reauthentication............................40
6.9. EAP-Response/AKA-Client-Error................................40
6.10. EAP-Request/AKA-Notification................................40
6.11. EAP-Response/AKA-Notification...............................41
7. Attributes.....................................................41
7.1. Table of Attributes..........................................41
7.2. AT_MAC.......................................................42
7.3. AT_IV, AT_ENCR_DATA and AT_PADDING...........................43
7.4. AT_CHECKCODE.................................................45
7.5. AT_PERMANENT_ID_REQ..........................................47
7.6. AT_ANY_ID_REQ................................................47
7.7. AT_FULLAUTH_ID_REQ...........................................47
7.8. AT_IDENTITY..................................................48
7.9. AT_RAND......................................................48
7.10. AT_AUTN.....................................................49
7.11. AT_RES......................................................49
7.12. AT_AUTS.....................................................49
7.13. AT_NEXT_PSEUDONYM...........................................50
7.14. AT_NEXT_REAUTH_ID...........................................50
7.15. AT_COUNTER..................................................51
7.16. AT_COUNTER_TOO_SMALL........................................51
7.17. AT_NONCE_S..................................................51
7.18. AT_NOTIFICATION.............................................52
7.19. AT_CLIENT_ERROR_CODE........................................53
8. IANA and Protocol Numbering Considerations.....................53
9. Security Considerations........................................54
9.1. Identity Protection..........................................55
9.2. Mutual Authentication........................................55
9.3. Key Derivation...............................................55
9.4. Brute-Force and Dictionary Attacks...........................55
9.5. Integrity Protection, Replay Protection and Confidentiality..55
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9.6. Negotiation Attacks..........................................56
9.7. Fast Reconnect...............................................56
9.8. Acknowledged Result Indications..............................56
9.9. Man-in-the-middle Attacks....................................57
9.10. Generating Random Numbers...................................57
10. Security Claims...............................................57
11. Intellectual Property Right Notices...........................58
Acknowledgements and Contributions................................58
Authors' Addresses................................................58
Annex A. Pseudo-Random Number Generator...........................59
1. Introduction and Motivation
This document specifies an Extensible Authentication Protocol (EAP)
mechanism for authentication and session key distribution using the
UMTS AKA authentication mechanism [TS 33.102]. UMTS is a global
third generation mobile network standard.
AKA is based on challenge-response mechanisms and symmetric
cryptography. AKA typically runs in a UMTS Subscriber Identity
Module (USIM). Compared to the GSM mechanism, UMTS AKA provides
substantially longer key lengths and mutual authentication.
The introduction of AKA inside EAP allows several new applications.
These include the following:
- The use of the AKA also as a secure PPP authentication method in
devices that already contain an USIM.
- The use of the third generation mobile network authentication
infrastructure in the context of wireless LANs
- Relying on AKA and the existing infrastructure in a seamless way
with any other technology that can use EAP.
AKA works in the following manner:
- The USIM and the home environment have agreed on a secret key
beforehand.
- The actual authentication process starts by having the home
environment produce an authentication vector, based on the secret
key and a sequence number. The authentication vector contains a
random part RAND, an authenticator part AUTN used for
authenticating the network to the USIM, an expected result part
XRES, a session key for integrity check IK, and a session key for
encryption CK.
- The RAND and the AUTN are delivered to the USIM.
- The USIM verifies the AUTN, again based on the secret key and the
sequence number. If this process is successful (the AUTN is valid
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and the sequence number used to generate AUTN is within the
correct range), the USIM produces an authentication result, RES
and sends this to the home environment.
- The home environment verifies the correct result from the USIM. If
the result is correct, IK and CK can be used to protect further
communications between the USIM and the home environment.
When verifying AUTN, the USIM may detect that the sequence number
the network uses is not within the correct range. In this case, the
USIM calculates a sequence number synchronization parameter AUTS and
sends it to the network. AKA authentication may then be retried with
a new authentication vector generated using the synchronized
sequence number.
For a specification of the AKA mechanisms and how the cryptographic
values AUTN, RES, IK, CK and AUTS are calculated, see [TS 33.102].
In EAP AKA, the EAP server node obtains the authentication vectors,
compares RES and XRES, and uses CK and IK in key derivation.
In the third generation mobile networks, AKA is used both for radio
network authentication and IP multimedia service authentication
purposes. Different user identities and formats are used for these;
the radio network uses the International Mobile Subscriber
Identifier (IMSI), whereas the IP multimedia service uses the
Network Access Identifier (NAI) [RFC 2486].
2. Terms and Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
The terms and abbreviations "authenticator", "backend authentication
server", "EAP server", "Silently Discard", "Master Session Key
(MSK)", and "Extended Master Session Key (EMSK)" in this document
are to be interpreted as described in [EAP].
This document frequently uses the following terms and abbreviations:
AAA protocol
Authentication, Authorization and Accounting protocol
AKA
Authentication and Key Agreement
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AuC
Authentication Centre. The mobile network element that can
authenticate subscribers either in GSM or in UMTS networks.
EAP
Extensible Authentication Protocol [EAP].
GSM
Global System for Mobile communications.
NAI
Network Access Identifier [RFC 2486].
AUTN
Authentication value generated by the AuC which together with the
RAND authenticates the server to the peer, 128 bits [TS 33.102].
AUTS
A value generated by the peer upon experiencing a synchronization
failure, 112 bits.
Permanent Identity
The permanent identity of the peer, including an NAI realm
portion in environments where a realm is used. The permanent
identity is usually based on the IMSI. Used on full
authentication only.
Permanent Username
The username portion of permanent identity, ie. not including any
realm portions.
Pseudonym Identity
A pseudonym identity of the peer, including an NAI realm portion
in environments where a real is used. Used on full authentication
only.
Pseudonym Username
The username portion of pseudonym identity, ie. not including any
realm portions.
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Re-authentication Identity
A re-authentication identity of the peer, including an NAI realm
portion in environments where a real is used. Used on re-
authentication only.
Re-authentication Username
The username portion of re-authentication identity, ie. not
including any realm portions.
RAND
Random number generated by the AuC, 128 bits [TS 33.102].
RES
Authentication result from the peer, which together with the RAND
authenticates the peer to the server, 128 bits [TS 33.102].
SQN
Sequence number used in the authentication process, 48 bits [TS
33.102].
SIM
Subscriber Identity Module. The SIM is an application
traditionally resident on smart cards distributed by GSM
operators.
SRES
The authentication result parameter in GSM, corresponds to the
RES parameter in UMTS aka, 32 bits.
USIM
UMTS Subscriber Identity Module. USIM is an application that is
resident e.g. on smart cards distributed by UMTS operators.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC 2119]
3. Protocol Overview
The message flow below shows the basic successful full
authentication exchange in EAP AKA. At the minimum, EAP AKA uses two
roundtrips to authorize the user and generate session keys. As in
other EAP schemes, an identity request/response message pair is
usually exchanged first. On full authentication, the peer's identity
response includes either the user's International Mobile Subscriber
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Identity (IMSI), or a temporary identity (pseudonym) if identity
privacy is in effect, as specified in Section 4.1. (As specified in
[EAP], the initial identity request is not required, and MAY be
bypassed in cases where the network can presume the identity, such
as when using leased lines, dedicated dial-ups, etc. Please see also
Section 4.1.2 for specification how to obtain the identity via EAP
AKA messages.)
Next, the EAP server starts the actual AKA protocol by sending an
EAP-Request/AKA-Challenge message. EAP AKA packets encapsulate
parameters in attributes, encoded in a Type, Length, Value format.
The packet format and the use of attributes are specified in Section
5. The EAP-Request/AKA-Challenge message contains a random number
(AT_RAND) and a network authentication token (AT_AUTN), and a
message authentication code AT_MAC. The EAP-Request/AKA-Challenge
message MAY optionally contain encrypted data, which is used for
identity privacy and re-authentication support, as described in
Section 4.1. The AT_MAC attribute contains a message authentication
code covering the EAP packet. The encrypted data is not shown in the
figures of this section.
The peer runs the AKA algorithm (typically using a USIM) and
verifies the AUTN. If this is successful, the peer is talking to a
legitimate EAP server and proceeds to send the EAP-Response/AKA-
Challenge. This message contains a result parameter that allows the
EAP server in turn to authenticate the peer, and the AT_MAC
attribute to integrity protect the EAP message.
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Peer Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and AUTN. |
| +------------------------------+
| |
| EAP-Request/AKA-Challenge |
| (AT_RAND, AT_AUTN, AT_MAC) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Peer runs UMTS algorithms on USIM, | |
| verifies AUTN and MAC, derives RES | |
| and session key | |
+-------------------------------------+ |
| |
| EAP-Response/AKA-Challenge |
| (AT_RES, AT_MAC) |
|------------------------------------------------------>|
| |
| +--------------------------------+
| | Server checks the given RES, |
| | and MAC and finds them correct.|
| +--------------------------------+
| |
| EAP-Success |
|<------------------------------------------------------|
The second message flow shows how the EAP server rejects the Peer
due to a failed authentication. The same flow is also used in the
GSM compatible mode, except that the AT_AUTN attribute and AT_MAC
attribute are not used in the messages.
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Peer Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and AUTN. |
| +------------------------------+
| |
| EAP-Request/AKA-Challenge |
| (AT_RAND, AT_AUTN, AT_MAC) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Peer runs UMTS algorithms on USIM, | |
| possibly verifies AUTN, and sends an| |
| invalid response | |
+-------------------------------------+ |
| |
| EAP-Response/AKA-Challenge |
| (AT_RES, AT_MAC) |
|------------------------------------------------------>|
| |
| +------------------------------------------+
| | Server checks the given RES and the MAC, |
| | and finds one of them incorrct. |
| +------------------------------------------+
| |
| EAP-Failure |
|<------------------------------------------------------|
The next message flow shows the peer rejecting the AUTN of the EAP
server.
The peer sends an explicit error message (EAP-Response/AKA-
Authentication-Reject) to the EAP server, as usual in AKA when AUTN
is incorrect. This allows the EAP server to produce the same error
statistics as AKA in general produces in UMTS.
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Peer Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and a bad AUTN|
| +------------------------------+
| |
| EAP-Request/AKA-Challenge |
| (AT_RAND, AT_AUTN, AT_MAC) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Peer runs UMTS algorithms on USIM | |
| and discovers AUTN that can not be | |
| verified | |
+-------------------------------------+ |
| |
| EAP-Response/AKA-Authentication-Reject |
|------------------------------------------------------>|
| |
| |
| EAP-Failure |
|<------------------------------------------------------|
The AKA uses shared secrets between the Peer and the Peer's home
operator together with a sequence number to actually perform an
authentication. In certain circumstances it is possible for the
sequence numbers to get out of sequence. Here's what happens then:
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Peer Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and AUTN. |
| +------------------------------+
| |
| EAP-Request/AKA-Challenge |
| (AT_RAND, AT_AUTN, AT_MAC) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Peer runs UMTS algorithms on USIM | |
| and discovers AUTN that contains an | |
| inappropriate sequence number | |
+-------------------------------------+ |
| |
| EAP-Response/AKA-Synchronization-Failure |
| (AT_AUTS) |
|------------------------------------------------------>|
| |
| +---------------------------+
| | Perform resynchronization |
| | Using AUTS and |
| | the sent RAND |
| +---------------------------+
| |
After the resynchronization process has taken place in the server
and AAA side, the process continues by the server side sending a new
EAP-Request/AKA-Challenge message.
In addition to the full authentication scenarios described above,
EAP AKA includes a re-authentication procedure, which is specified
in Section 4.2. Re-authentication is based on keys derived on full
authentication. If the peer has maintained state information for re-
authentication and wants to use re-authentication, then the peer
indicates this by using a specific re-authentication identity
instead of the permanent identity or a pseudonym identity. The re-
authentication procedure is described in Section 4.2.
4. Operation
4.1. Identity Management
4.1.1. Format, Generation and Usage of Peer Identities
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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
[EAP].
UMTS subscribers are identified with the International Mobile
Subscriber Identity (IMSI) [TS 23.003]. The IMSI is composed of a
three digit Mobile Country Code (MCC), a two or three digit Mobile
Network Code (MNC) and a not more than 10 digit Mobile Subscriber
Identification Number (MSIN). In other words, the IMSI is a string
of not more than 15 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) [RFC 2486]. 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.
Identity Privacy Support
EAP/AKA includes optional identity privacy (anonymity) support that
can be used to hide the cleartext permanent identity and thereby to
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 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 9.1 for security considerations regarding identity privacy.
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.
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(2) Pseudonym usernames. For example, 2s7ah6n9q@myoperator.com might
be a valid pseudonym identity. In this example, 2s7ah6n9q is the
pseudonym username.
(3) Re-authentication usernames. For example,
43953754a@myoperator.com might be a valid re-authentication
identity. In this case, 43953754 is the re-authentication username.
The first two types of identities are only used on full
authentication and the last one only on re-authentication. When the
optional identity privacy support is not used, the non-pseudonym
permanent identity is used on full authentication. The re-
authentication exchange is specified in Section 4.2.
sername 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 a 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 the
peer includes in EAP-Response/Identity, and 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.
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 operator and it
MAY 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/AKA users. This convention makes it easy to recognize that the
NAI identifies a UMTS subscriber. Such reserved NAI realm may be
useful as a 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 will be specified in [Draft 3GPP TS 23.234].
Alternatively, the realm name may be obtained 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
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MNC is three digits long, then the derived realm name is
"mnc456.mcc123.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.
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 0x30), followed by the IMSI. The IMSI is an ASCII string that
consists of not more than 15 decimal digits (ASCII values between
0x30 and 0x39) as specified in [TS 23.003].
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 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.
Generating Pseudonyms and Re-authentication Identities by the Server
Pseudonym usernames and re-authentication identities are generated
by the EAP server. The EAP server produces pseudonym usernames and
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 re-authentication
identity. Regardless of construction method, the pseudonym username
MUST conform to the grammar specified for the username portion of an
NAI. The 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 re-authentication identities they generate are unique.
In any case, it is necessary that permanent usernames, pseudonym
usernames and re-authentication usernames are separate and
recognizable from each other. It is also desirable that EAP SIM and
EAP AKA user names be recognizable from each other as an aid for the
server to which method to offer.
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In general, it is the task of the EAP server and the policies of its
administrator to ensure sufficient separation in the usernames.
Pseudonym usernames and re-authentication usernames are both
produced and used by the EAP server. The EAP server MUST compose
pseudonym usernames and re-authentication usernames so that it can
recognize if a NAI username is an EAP AKA pseudonym username or an
EAP AKA 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 re-authentication
usernames (e.g. the pseudonym could begin with a leading "2"
character). When mapping a re-authentication identity to a permanent
identity, the server SHOULD only examine the username portion of the
re-authentication identity and ignore the realm portion of the
identity.
Because the peer may fail to save a pseudonym username sent to in an
EAP-Request/AKA-Challenge, for example due to malfunction, the EAP
server SHOULD maintain at least one old pseudonym username in
addition to the most recent pseudonym username.
Transmitting Pseudonyms and Re-authentication Identities to the Peer
The server transmits pseudonym usernames and 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 re-authentication identity in
the value field of the AT_ENCR_DATA attribute. Because identity
privacy support and re-authentication are optional to implement, the
peer MAY ignore the AT_ENCR_DATA attribute and always use the
permanent identity. On re-authentication (discussed in Section 4.2),
the server MAY include a new encrypted 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 re-authentication identity is
included, then the peer MAY save it and other re-authentication
state information, as discussed in Section 4.2, for the next 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 re-authentication
identities are one-time usernames, which the peer MUST NOT re-use.
Usage of the Pseudonym by the Peer
When the optional identity privacy support is used on full
authentication, the peer MAY use the pseudonym username received as
part of the previous full authentication sequence as the username
portion of the NAI. The peer MUST NOT modify the pseudonym username
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EAP AKA Authentication 27 October, 2003
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 a NAI realm portion. The
selection of the NAI realm is discussed above.
Usage of the Re-authentication Identity by the Peer
On re-authentication, the peer uses the re-authentication identity,
received as part of the previous authentication sequence. A new re-
authentication identity may be delivered as part of both full
authentication and re-authentication. The peer MUST NOT modify the
username part of the re-authentication identity received in
AT_NEXT_REAUTH_ID, except in cases when username decoration is
required. Even in these cases, the "root" 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
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 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 re-authentication identity, and the server
recognized the identity and agrees on using re-authentication, then
a re-authentication exchange is performed, as described in Section
4.2.
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
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 the peer to
send its permanent identity. The EAP-Request/AKA-Challenge, EAP-
Response/AKA-Challenge, or the packets used on 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 0.
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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 re-authentication identity.
Obtaining the subscriber identity via EAP/AKA messages is useful if
the server does not have any EAP/AKA peer identity at the beginning
of the EAP/AKA exchange or does not recognize the identity the peer
used in EAP-Response/Identity. This may happen if, for example, the
EAP-Response/Identity has been issued by some EAP method other than
EAP/AKA or if intermediate entities or software layers in the peer
have modified the identity string in the EAP-Response/Identity
packet. Also, some EAP layer implementations may cache the identity
string from the first EAP authentication and do not obtain a new
identity string from the EAP method implementation on subsequent
authentication exchanges.
As the identity string is used in key derivation, any of these cases
will result in failed authentication unless the EAP server uses
EAP/AKA attributes to obtain an unmodified copy of the identity
string. Therefore, unless the EAP server can be certain that no
intermediate element or software layer has modified the EAP-
Response/Identity packet, the EAP server SHOULD always use the
EAP/AKA attributes to obtain the identity, even if the identity
received in EAP-Response/Identity was valid.
Please note that the EAP/AKA peer and the EAP/AKA server only
process the AT_IDENTITY attribute and entities that only pass
through EAP packets do not process this attribute. Hence, if the EAP
server is not co-located in the authenticator, then 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 regardless
of whether the AT_IDENTITY attribute is used in EAP/AKA to transmit
another identity.
Choice of Identity for the EAP-Response/Identity
If EAP/AKA peer is started upon receiving an EAP-Request/Identity
message, then the peer performs the following steps.
If the peer has maintained re-authentication state information and
if the peer wants to use re-authentication, then the peer transmits
the 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.
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Server Operation in the Beginning of EAP/AKA Exchange
If the EAP server has not received any identity (permanent identity,
pseudonym identity or 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 message 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:
- The server does not support re-authentication or identity privacy.
- The server received an identity that it recognizes 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:
- The server does not support re-authentication and the server
supports identity privacy
- The server received an identity that it recognizes 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/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 does not have any identity, or the server does
not recognize the format of a received identity.
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 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".
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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 includes the pseudonym identity
in AT_IDENTITY. If the peer does not have a pseudonym when it
receives this message, then 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." The Peer MUST NOT use a 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 re-authentication state information and the peer
wants to use re-authentication, then the peer responds with EAP-
Response/AKA-Identity and includes the 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. 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 is specified in Section 4.4.1.
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/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,
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
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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.
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 EAP
Failure 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/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 re-authentication identity and the server agrees on using re-
authentication, then the server proceeds with re-authentication by
sending EAP-Request/ AKA-Reauthentication (Section 4.2).
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 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 re-authentication identity, which the server is able to map to
a permanent identity, and if the server does not want to use 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
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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.
4.1.3. Message Sequence Examples (Informative)
This section contains non-normative message sequence examples to
illustrate how the peer identity can be communicated to the server.
sage of AT_ANY_ID_REQ
Obtaining the peer identity with EAP/AKA attributes is illustrated
in the figure 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) |
|------------------------------------------------------>|
| |
all Back on Full Authentication
The figure below illustrates the case when the server does not
recognize the re-authentication identity the peer used in
AT_IDENTITY.
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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 containing a re-authentication identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server does not recognize |
| | The re-authentication |
| | Identity |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_FULLAUTH_ID_REQ) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/AKA-Identity |
| (AT_IDENTITY with a full-auth. Identity) |
|------------------------------------------------------>|
| |
If the server recognizes the 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.
Requesting the Permanent Identity 1
The figure below illustrates the case when the EAP server fails to
decode a pseudonym identity included in the EAP-Response/Identity
packet.
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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) |
|------------------------------------------------------>|
| |
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.
Requesting the Permanent Identity 2
The figure below illustrates the case when the EAP server fails to
decode the pseudonym included in the AT_IDENTITY attribute.
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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 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) |
|------------------------------------------------------>|
| |
Three EAP/AKA-Identity Round Trips
The figure below 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 re-authentication identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server does not accept |
| | The 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) |
|------------------------------------------------------>|
| |
After the last EAP-Response/AKA-Identity message, the full
authentication sequence proceeds as usual.
4.2. Re-authentication
4.2.1. General
In some environments, EAP authentication may be performed
frequently. Because the EAP AKA full authentication procedure makes
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EAP AKA Authentication 27 October, 2003
use of the UMTS AKA algorithms, and it 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
re-authentication procedure that does not make use of the UMTS AKA
algorithms and does not need new vectors from the Authentication
Centre.
Re-authentication is optional to implement for both the EAP AKA
server and peer. On each EAP authentication, either one of the
entities may also fall back on full authentication if they do not
want to use re-authentication.
Re-authentication is based on the keys derived on the preceding full
authentication. The same K_aut and K_encr keys as 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 4.5.
On re-authentication, the peer protects against replays with an
unsigned 16-bit counter, included in the AT_COUNTER attribute. 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 re-authentication. On subsequent re-authentications, the
counter MUST be greater than on any of the previous re-
authentications. For example, on the second re-authentication,
counter value is two or greater etc. The AT_COUNTER attribute is
encrypted.
The server includes an encrypted server nonce (AT_NONCE_S) in the
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.
Both the peer and the server SHOULD have an upper limit for the
number of subsequent re-authentications allowed before a full
authentication needs to be performed. Because a 16-bit counter is
used in re-authentication, the theoretical maximum number of re-
authentications is reached when the counter value reaches 0xFFFF.
In order to use re-authentication, the peer and the EAP server need
to store the following values: Master Key, latest counter value and
the next re-authentication identity. K_aut, K_encr may either be
stored or derived again from MK. The server may also need to store
the permanent identity of the user.
4.2.2. Re-authentication Identity
The re-authentication procedure makes use of separate re-
authentication user identities. Pseudonyms and the permanent
identity are reserved for full authentication only. If a re-
authentication identity is lost and the network does not recognize
it, the EAP server can fall back on full authentication.
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If the EAP server supports 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 re-authentication. The peer MAY
ignore this attribute, in which case it will use full authentication
next time. If the peer wants to use re-authentication, it uses this
re-authentication identity on next authentication. Even if the peer
has a re-authentication 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.
In environments where a real portion is needed in the peer identity,
the 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 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 re-authentication in
the AAA server that performed the full authentication.
The peer MAY use the re-authentication identity in the EAP-
Response/Identity packet or, in response to server's AT_ANY_ID_REQ
attribute, the peer MAY use the 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 re-authentication
identity, but the peer MAY modify the realm portion or replace it
with another realm portion.
Even if the peer uses a re-authentication identity, the server may
want to fall back on full authentication, for example because the
server does not recognize the re-authentication identity or does not
want to use re-authentication. If the server was able to decode the
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 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.
4.2.3. Re-authentication Procedure
The following figure illustrates the re-authentication procedure.
Encrypted attributes are denoted with '*'. The peer uses its re-
authentication identity in the EAP-Response/Identity packet. As
discussed above, an alternative way to communicate the 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 the peer to send its identity by
including the AT_ANY_ID_REQ attribute in the EAP-Request/AKA-
Identity packet.
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If the server recognizes the re-authentication identity and agrees
on using 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 re-authentication identity.
Re-authentication identities are one-time identities. If the peer
does not receive a new 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 the counter value is fresh (greater than any
previously used value). The peer also verifies that AT_MAC is
correct. The peer MAY save the next 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 re-
authentication has succeeded and the server sends the EAP-Success
packet to the peer.
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Peer Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a 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 |
|<------------------------------------------------------|
| |
4.2.4. 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 the following figure. Encrypted attributes are
denoted with '*'.
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Peer Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a 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. |
+---------------------------------------------------------------+
| |
In the figure above, the first three messages are similar to the
basic 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 as in the EAP-
Request/AKA-Reauthentication packet. If not, the server silently
discards the EAP-Response/AKA-Reauthentication packet. If all checks
on the packet are successful, the server transmits a EAP-
Request/AKA-Challenge packet and the full authentication procedure
is performed as usual. Since the server already knows the subscriber
identity, it MUST NOT use the EAP-Request/AKA-Identity packet to
request the identity.
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4.3. EAP/AKA Notifications
The EAP-Request/Notification, specified in [EAP], can be used to
convey a displayable message from the EAP server to the peer.
Because these messages are textual messages, it may be hard for the
peer to present them in the user's preferred language. Therefore,
EAP/AKA uses a separate EAP/AKA message subtype to transmit
localizable notification codes instead of the EAP-
Request/Notification packet.
The EAP server MAY issue an EAP-Request/AKA-Notification packet to
the peer. 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.
The notification code is a 16-bit number. The most significant bit
is called the Failure bit (F bit). The F bit specifies whether the
notification implies failure. The code values with the F bit set to
zero (code values 0...32767) are used on unsuccessful cases. The
receipt of a notification code from this range implies failed
authentication, 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 F bit set to one (values
32768...65536) does not imply failure, so the peer MUST NOT change
its state when it receives such a notification. (This version of the
protocol does not specify any notification codes with the F bit set
to one.)
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 the EAP/AKA-Challenge round
in full authentication or the EAP/AKA-Reauthentication round in
reautentication. For these notifications, the AT_MAC attribute MUST
be included in both EAP-Request/AKA-Notification and EAP-
Response/AKA-Notification.
If the P bit is set to one, the notification can only by used before
the EAP/AKA-Challenge round in full authentication or the EAP/AKA-
Reauthentication round in reauthentication. For these notifications,
the AT_MAC attribute MUST NOT be included in either EAP-Request/AKA-
Notification or EAP-Response/AKA-Notification. (This version of the
protocol does not specify any notification codes with the P bit set
to one.)
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
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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.
4.4. 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 or EAP
Failure) in error cases. However, implementations SHOULD NOT rely
upon the correct error reporting behavior of the peer,
authenticator, or the server. It is possible for error 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.
4.4.1. Peer Operation
Two special error messages have been specified for error cases that
are related to the processing of the UMTS 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
6.5), 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 6.6), 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:
- the peer is not able to parse the EAP request, i.e. the EAP
request is malformed
- the peer encountered a malformed attribute
- wrong attribute types or duplicate attributes have been included
in the EAP request
- a mandatory attribute is missing
- unrecognized non-skippable attribute
- unrecognized or unexpected EAP/AKA Subtype in the EAP request
- invalid AT_MAC
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- invalid AT_CHECKCODE
- invalid pad bytes in AT_PADDING
- the peer does not want to process AT_PERMANENT_ID_REQ
4.4.2. Server Operation
If an EAP/AKA server detects an error in a received EAP/AKA
response, the server MUST issue the EAP Failure packet and the
authentication exchange terminates. The errors cases when the server
issues an EAP Failure include the following:
- the server is not able to parse the peer's EAP response
- the server encounters a malformed attribute, a non-recognized non-
skippable attribute, or a duplicate attribute
- a mandatory attribute is missing or an invalid attribute was
included
- unrecognized or unexpected EAP/AKA Subtype in the EAP Response
- invalid AT_MAC
- invalid AT_CHECKCODE
- invalid AT_COUNTER
4.4.3. Failure
As normally in EAP, the EAP server sends the EAP-Failure packet to
the peer when the authentication procedure fails on the EAP Server.
In EAP/AKA, this may occur for example if the EAP server does not
recognize the peer identity, or if the EAP server is not able to
obtain the authentication vectors for the subscriber or the
authentication exchange times out. The server may also send EAP
Failure if there is an error in the received EAP/AKA response, as
discussed in Section 4.4.2.
The server can send EAP-Failure at any time in the EAP exchange. The
peer MUST process EAP-Failure.
4.4.4. 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.
On 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
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successfully authenticated the server and sent the EAP-Response/AKA-
Reauthentication packet.
If the peer receives an EAP/AKA notification (section 4.3) that
indicates failure, then the peer MUST no longer accept the EAP-
Success packet even if the server authentication was successfully
completed.
4.5. Key Generation
This section specifies how keying material is generated.
On EAP AKA full authentication, a Master Key (MK) is derived from
the underlying UMTS 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 AT_IDENTITY attribute
from the last EAP-Response/AKA-Identity packet, 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 and
including the possible identity decoration. 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 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 new values exchanged in the 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
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.
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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.
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 re-authentication, the same pseudo-random number generator can be
used to generate a new Master Session Key and new Initialization
Vectors. The seed value XKEY' is calculated as follows:
XKEY' = SHA1(Identity|counter|NONCE_S| MK)
In the formula above, the Identity denotes the 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 re-authentication, the
identity string from the EAP-Response/Identity packet. The counter
denotes the counter value from 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 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. 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.
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 [RFC 2548] 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.
5. Message Format and Protocol Extensibility
5.1. Message Format
As specified in [EAP], 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 [EAP]. In
EAP/AKA, the Type field is set to 23.
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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 8. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The rest of the Type-Data, immediately following the EAP/AKA header,
consists of attributes that are encoded in Type, Length, Value
format. The figure below shows the generic format of an attribute.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Attribute Type | Length | Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute Type
Indicates the particular type of attribute. The attribute type
values are listed in Section 8.
Length
Indicates the length of this attribute in multiples of 4 bytes.
The maximum length of an attribute is 1024 bytes. The length
includes the Attribute Type and Length bytes.
Value
The particular data associated with this attribute. This field is
always included and it is two or more bytes in length. The type
and length fields determine the format and length of the value
field.
Attributes numbered within the range 0 through 127 are called non-
skippable attributes. When an EAP/AKA peer encounters a non-
skippable attribute type that the peer does not recognize, the peer
MUST send the EAP-Response/AKA-Client-Error packet, and the
authentication exchange terminates. If an EAP/AKA server encounters
a non-skippable attribute that the server does not recognize, then
the server sends the EAP Failure packet and the authentication
exchange terminates.
When an attribute numbered in the range 128 through 255 is
encountered but not recognized that particular attribute is ignored,
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but the rest of the attributes and message data MUST still be
processed. The Length field of the attribute is used to skip the
attribute value when searching for the next attribute. These
attributes are called skippable attributes.
Unless otherwise specified, the order of the attributes in an EAP
AKA message is insignificant, and an EAP AKA implementation should
not assume a certain order to be used.
Attributes can be encapsulated within other attributes. In other
words, the value field of an attribute type can be specified to
contain other attributes.
5.2. Protocol Extensibility
EAP/AKA can be extended by specifying new attribute types. If
skippable attributes are used, it is possible to extend the protocol
without breaking old implementations. As specified in Section 7.4,
if new attributes are specified for EAP-Request/AKA-Identity or EAP-
Response/AKA-Identity, then the AT_CHECKCODE MUST be used to
integrity protect the new attributes.
When specifying new attributes, it should be noted that EAP/AKA does
not support message fragmentation. Hence, the sizes of the new
extensions MUST be limited so that the maximum transfer unit (MTU)
of the underlying lower layer is not exceeded. According to [EAP],
lower layers must provide an EAP MTU of 1020 bytes or greater, so
any extensions to EAP/AKA SHOULD NOT exceed the EAP MTU of 1020
bytes.
EAP/AKA packets do not include a version field. However, should
there be a reason to revise this protocol in the future, new non-
skippable or skippable attributes could be specified in order to
implement revised EAP/AKA versions in a backward-compatible manner.
It is possible to introduce version negotiation in the EAP-
Request/AKA-Identity and EAP-Response/AKA-Identity messages by
specifying new skippable attributes.
6. Messages
This section specifies the messages used in EAP/AKA. It specifies
when a message may be transmitted or accepted, which attributes are
allowed in a message, which attributes are required in a message,
and other message specific details. Message format is specified in
Section 5.1.
6.1. EAP-Request/AKA-Identity
The EAP/AKA-Identity roundtrip MAY used for obtaining the peer
identity to the server. As discussed in Section 4.1, several AKA-
Identity rounds may be required in order to obtain a valid peer
identity.
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The server MUST include one of the following identity requesting
attributes: AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, AT_ANY_ID_REQ.
These three attributes are mutually exclusive, so the server MUST
NOT include more than one of the attributes.
If the server has previously issued an EAP-Request/AKA-Identity
message with the AT_PERMANENT_ID_REQ attribute, and if the server
has received a response from the peer, then the server MUST NOT
issue a new EAP-Request/AKA-Identity packet.
If the server has previously issued an EAP-Request/AKA-Identity
message with the AT_FULLAUTH_ID_REQ attribute, and if the server has
received a response from the peer, then the server MUST NOT issue a
new EAP-Request/AKA-Identity packet with the AT_ANY_ID_REQ or
AT_FULLAUTH_ID_REQ attributes.
If the server has previously issued an EAP-Request/AKA-Identity
message with the AT_ANY_ID_REQ attribute, and if the server has
received a response from the peer, then the server MUST NOT issue a
new EAP-Request/AKA-Identity packet with the AT_ANY_ID_REQ.
This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.
6.2. EAP-Response/AKA-Identity
The peer sends EAP-Response/AKA-Identity in response to a valid EAP-
Request/AKA-Identity from the server.
The peer MUST include the AT_IDENTITY attribute. The usage of
AT_IDENITY is defined in Section 4.1.
This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.
6.3. EAP-Request/AKA-Challenge
The server sends the EAP-Request/AKA-Challenge on full
authentication after successfully obtaining the subscriber identity.
The AT_RAND attribute MUST be included.
AT_MAC MUST be included. In EAP-Request/AKA-Challenge, there is no
message-specific data covered by the MAC, see Section 7.2.
The AT_CHECKCODE attribute MAY be included, and in certain cases
specified in Section 7.4, it MUST be included.
The EAP-Request/AKA-Challenge packet MAY include encrypted
attributes for identity privacy and for communicating the next re-
authentication identity. In this case, the AT_IV and AT_ENCR_DATA
attributes are included (Section 7.3).
The plaintext of the AT_ENCR_DATA value field consist of nested
attributes. The nested attributes MAY include AT_PADDING (as
specified in Section 7.3). If the server supports identity privacy
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and wants to communicate a pseudonym to the peer for the next full
authentication, then the nested encrypted attributes include the
AT_NEXT_PSEUDONYM attribute. If the server supports re-
authentication and wants to communicate a re-authentication identity
to the peer, then the nested encrypted attributes include the
AT_NEXT_REAUTH_ID attribute. Later versions of this protocol MAY
specify additional attributes to be included within the encrypted
data.
6.4. EAP-Response/AKA-Challenge
The peer sends EAP-Response/AKA-Challenge in response to a valid
EAP-Request/AKA-Challenge.
The AT_MAC attribute MUST be included. In EAP-Response/AKA-
Challenge, there is no message-specific data covered by the MAC, see
Section 7.2.
The AT_RES attribute MUST be included.
The AT_CHECKCODE attribute MAY be included, and in certain cases
specified in Section 7.4, it MUST be included.
Later versions of this protocol MAY make use of the AT_ENCR_DATA and
AT_IV attributes in this message to include encrypted (skippable)
attributes. The EAP server MUST process EAP-Response/AKA-Challenge
messages that include these attributes even if the server did not
implement these optional attributes.
6.5. EAP-Response/AKA-Authentication-Reject
The peer sends the EAP-Response/AKA-Authentication-Reject packet if
it does not accept the AUTN parameter. This version of the protocol
does not specify any attributes for this message. Future versions of
the protocol MAY specify attributes for this message.
The AT_MAC, AT_ENCR_DATA, or AT_IV attributes MUST NOT be used in
this message.
6.6. EAP-Response/AKA-Synchronization-Failure
The peer sends the EAP-Response/AKA-Synchronization-Failure, when
the sequence number in the AUTN parameter is incorrect.
The peer MUST include the AT_AUTS attribute. Future versions of the
protocol MAY specify other additional attributes for this message.
The AT_MAC, AT_ENCR_DATA, or AT_IV attributes MUST NOT be used in
this message.
6.7. EAP-Request/AKA-Reauthentication
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The server sends the EAP-Request/AKA-Reauthentication message if it
wants to use re-authentication, and if it has received a valid re-
authentication identity in EAP-Response/Identity or EAP-
Response/AKA-Identity.
The AT_MAC attribute MUST be included. No message-specific data is
included in the MAC calculation, see Section 7.2.
The AT_CHECKCODE attribute MAY be included, and in certain cases
specified in Section 7.4, it MUST be included.
The AT_IV and AT_ENCR_DATA attributes MUST be included. The
plaintext consists of the following nested encrypted attributes,
which MUST be included: AT_COUNTER and AT_NONCE_S. In addition, the
nested encrypted attributes MAY include the following attributes:
AT_NEXT_REAUTH_ID and AT_PADDING.
6.8. EAP-Response/AKA-Reauthentication
The client sends the EAP-Response/AKA-Reauthentication packet in
response to a valid EAP-Request/AKA-Reauthentication.
The AT_MAC attribute MUST be included. For EAP-Response/AKA-
Reauthentication, the MAC code is calculated over the following
data: EAP packet| NONCE_S. The EAP packet is represented as
specified in Section 5.1. It is followed by the 16-byte NONCE_S
value from the server's AT_NONCE_S attribute.
The AT_CHECKCODE attribute MAY be included, and in certain cases
specified in Section 7.4, it MUST be included.
The AT_IV and AT_ENCR_DATA attributes MUST be included. The nested
encrypted attributes MUST include the AT_COUNTER attribute. The
AT_COUNTER_TOO_SMALL attribute MAY be included in the nested
encrypted attributes, and it is included in cases specified in
Section 4.2. The AT_PADDING attribute MAY be included.
6.9. EAP-Response/AKA-Client-Error
The peer sends EAP-Response/AKA-Client-Error in error cases, as
specified in Section 4.4.1.
The AT_CLIENT_ERROR_CODE attribute MUST be included.
The AT_MAC, AT_IV, or AT_ENCR_DATA attributes MUST NOT be used with
this packet.
6.10. EAP-Request/AKA-Notification
The usage of this message is specified in Section 4.3.
The AT_NOTIFICATION attribute MUST be included.
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The AT_MAC attribute is included in cases discussed in Section 4.3.
No message-specific data is included in the MAC calculation. See
Section 7.2.
Later versions of this protocol MAY make use of the AT_ENCR_DATA and
AT_IV attributes in this message to include encrypted (skippable)
attributes. These attributes MAY be included only if the P bit of
the notification code in AT_NOTIFICATION is set to zero.
6.11. EAP-Response/AKA-Notification
The usage of this message is specified in Section 4.3. Because this
packet is only an acknowledgement of EAP-Request/AKA-Notification,
it does not contain any mandatory attributes.
The AT_MAC attribute is included in cases described in Section 4.3.
No message-specific data is included in the MAC calculation. See
Section 7.2.
Later versions of this protocol MAY make use of the AT_ENCR_DATA and
AT_IV attributes in this message to include encrypted (skippable)
attributes. These attributes MAY be included only if the P bit of
the notification code in the AT_NOTIFICATION attribute of the
server's EAP-Request/AKA-Notification packet is set to zero.
7. Attributes
This section specifies the format of message attributes. The
attribute type numbers are specified in Section 8.
7.1. Table of Attributes
The following table provides a guide to which attributes may be
found in which kinds of messages, and in what quantity. Messages are
denoted with numbers in parentheses as follows: (1) EAP-Request/AKA-
Identity, (2) EAP-Response/AKA-Identity, (3) EAP-Request/AKA-
Challenge, (4) EAP-Response/AKA-Challenge, (5) EAP-Request/AKA-
Notification, (6) EAP-Response/AKA-Notification, (7) EAP-
Response/AKA-Client-Error (8) EAP-Request/AKA-Reauthentication, (9)