wdiff draft-arkko-pppext-eap-aka-10.txt draft-arkko-pppext-eap-aka-11.txt

Network Working Group J. Arkko
Internet Draft Ericsson
Document: draft-arkko-pppext-eap-aka-10.txt draft-arkko-pppext-eap-aka-11.txt H. Haverinen
Expires: December 2003 27 April, 2004 Nokia
June
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.

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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.....................................2 Motivation.....................................3
2. Terms and Conventions Used in This Document.....................4
3. Protocol Overview...............................................6
4. Identity Management............................................10
4.1. User Identity in EAP-Response/Identity.......................10 Operation......................................................11

EAP AKA Authentication June 27 October, 2003

4.2. Obtaining Subscriber

4.1. Identity via EAP AKA Messages...........12 Management..........................................11
4.2. Re-authentication............................................25
4.3. Identity Privacy Support.....................................15 EAP/AKA Notifications........................................31
4.4. Error Cases..................................................32
4.5. Key Generation...............................................34
5. Re-authentication..............................................21
6. Message Format.................................................26
7. Message Authentication Format and Encryption..........................27 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. AT_MAC Attribute.............................................27 Table of Attributes..........................................41
7.2. AT_CHECKCODE Attribute.......................................28 AT_MAC.......................................................42
7.3. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes................30 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. Messages.......................................................31
8.1. EAP-Request/AKA-Challenge....................................31
8.2. EAP-Response/AKA-Challenge...................................35
8.3. EAP-Response/AKA-Authentication-Reject.......................36
8.4. EAP-Response/AKA-Synchronization-Failure.....................37
8.5. EAP-Request/AKA-Identity.....................................38
8.6. EAP-Response/AKA-Identity....................................39
8.7. EAP-Request/AKA-Reauthentication.............................41
8.8. EAP-Response/AKA-Reauthentication............................43
8.9. EAP/AKA Notifications........................................46
9. Error Cases and the Usage of EAP-Failure and EAP-Success.......49
9.1. Processing Erroneous Packets.................................49
9.2. EAP-Failure..................................................49
9.3. EAP-Success..................................................50
10. Key Derivation................................................50
11. IANA and Protocol Numbering Considerations....................52
12. Considerations.....................53
9. Security Considerations.......................................53
12.1. Considerations........................................54
9.1. Identity Protection.........................................53
12.2. Protection..........................................55
9.2. Mutual Authentication.......................................53
12.3. Authentication........................................55
9.3. Key Derivation..............................................53
12.4. Derivation...............................................55
9.4. Brute-Force and Dictionary Attacks..........................53
12.5. Attacks...........................55
9.5. Integrity Protection, Replay Protection and Confidentiality.54
12.6. Confidentiality..55

EAP AKA Authentication 27 October, 2003

9.6. Negotiation Attacks.........................................54
12.7. Attacks..........................................56
9.7. Fast Reconnect..............................................55
12.8. Reconnect...............................................56
9.8. Acknowledged Result Indications.............................55
12.9. Indications..............................56
9.9. Man-in-the-middle Attacks...................................55
12.10. Attacks....................................57
9.10. Generating Random Numbers..................................55
13. Numbers...................................57
10. Security Claims...............................................55
14. Claims...............................................57
11. Intellectual Property Right Notices...........................56 Notices...........................58
Acknowledgements and Contributions................................56 Contributions................................58
Authors' Addresses................................................56 Addresses................................................58
Annex A. Pseudo-Random Number Generator...........................57 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 [1]. [TS 33.102]. UMTS is a global
third generation mobile network standard.

EAP AKA Authentication June 2003

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 and IEEE 802.1x
technology through EAP over Wireless [2, 3].

- 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

EAP AKA Authentication 27 October, 2003

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 reference [1]. [TS 33.102].

In EAP AKA Authentication June 2003

It is also possible that the home environment delegates AKA, the actual
authentication task to an intermediate node. In this case EAP server node obtains the authentication vector or parts of it are delivered to the
intermediate node, enabling it to perform the comparison between vectors,
compares RES and XRES, and possibly also use uses CK and IK. Such delivery MUST be
done IK in a secure manner. In EAP AKA, the EAP server node is such an
intermediate node. 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) [4]. [RFC 2486].

2. Terms and Conventions Used in This Document

The following terms will key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be used through 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: 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

AAA server

The AAA server is responsible for storing shared secrets and
other credential information necessary for the authentication of
users. Cf. EAP server

AKA

Authentication and Key Agreement

EAP AKA Authentication 27 October, 2003

AuC

Authentication Centre. The mobile network element that can
authenticate subscribers either in GSM or in UMTS networks.

Authenticator

The entity that terminates the protocol carrying EAP used by the
client, such as a Network Access Server (NAS) terminating the PPP
link. The EAP server may be co-located in the Authenticator. In
this case, the Authenticator may actually authenticate the user
based on information received from the AAA server.

EAP

Extensible Authentication Protocol [5].

EAP AKA Authentication June 2003

EAP server

The network element that terminates the EAP protocol. Typically,
the EAP server functionality is implemented in a AAA server. [EAP].

GSM

Global System for Mobile communications.

NAI

Network Access Identifier [4]. [RFC 2486].

AUTN

Authentication value generated by the AuC which together with the
RAND authenticates the server to the client, peer, 128 bits [1]. [TS 33.102].

AUTS

A value generated by the client 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.

EAP AKA Authentication 27 October, 2003

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 [1]. [TS 33.102].

RES

Authentication result from the client, peer, which together with the RAND
authenticates the client peer to the server, 128 bits [1]. [TS 33.102].

SQN

Sequence number used in the authentication process, 48 bits [1]. [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.

EAP AKA Authentication June 2003

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 [6] [RFC 2119]

3. Protocol Overview

In this document, the term EAP Server refers to the network element
that terminates the EAP protocol. Usually the EAP server is separate
from the authenticator device, which is the network element closest
to the client, such as a Network Access Server (NAS) or an IEEE
802.1X bridge. Alternatively, the EAP server functionality may be
co-located in the authenticator although typically, the EAP server
functionality is implemented on a separate AAA server with whom the
authenticator communicates using an AAA protocol. (The exact AAA
communications are outside the scope of this document, however.)

The message flow below shows the basic successful full
authentication case with the exchange in EAP AKA. The At the minimum, EAP AKA uses two
roundtrips to authorize the user and generate session keys. As in
other EAP schemes, first an identity request/response message pair is exchanged.
usually exchanged first. On full authentication, the peer's identity
response includes either the user's International Mobile Subscriber

EAP AKA Authentication 27 October, 2003

Identity (IMSI), or a temporary identity (pseudonym) if identity
privacy is in effect, as specified in Section 4.1. (As specified in [5],
[EAP], the initial identity request is not required, and MAY be
bypassed in cases where the authenticator network can presume the identity, such
as when using leased lines, dedicated dial-ups, etc. Please see also
Section 4.2 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
6.
5. The EAP-Request/AKA-Challenge message contains a random number
(AT_RAND) and an authorization vector 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
identity privacy and re-authentication support, as described in
Section 4.3. 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 client peer runs the AKA algorithm (perhaps inside an (typically using a USIM) and
verifies the AUTN. If this is successful, the client 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 client, peer, and the AT_MAC
attribute to integrity protect the EAP message.

EAP AKA Authentication June 27 October, 2003

Client

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) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client Peer runs UMTS algorithms on USIM,| 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 Client 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.

EAP AKA Authentication June 27 October, 2003

Client

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) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client Peer runs UMTS algorithms on USIM,| 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 client peer rejecting the AUTN of the EAP
server.

The client peer sends an explicit error message (EAP-Response/AKA-
Authentication-Reject) to the Authenticator, 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. Please note
that this behavior is different from other EAP/AKA error cases, such
as when encountering an incorrect AT_MAC attribute, the client
silently discards the EAP/AKA message.

EAP AKA Authentication June 27 October, 2003

Client

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) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client 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 Client Peer and the Client's 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:

EAP AKA Authentication June 27 October, 2003

Client

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) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client 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 5.

4. Identity Management

This section specifies user identity management 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 privacy
support.
instead of the permanent identity or a pseudonym identity. The re-
authentication procedure is described in Section 4.2.

4. Operation

4.1. User Identity in EAP-Response/Identity Management

4.1.1. Format, Generation and Usage of Peer Identities

EAP AKA Authentication 27 October, 2003

General

In the beginning of an EAP authentication, the Authenticator or the EAP
server usually issues the EAP-Request/Identity packet to the client. peer.
The client peer responds with EAP-Response/Identity, which contains the
user's identity. The formats of these packets are specified in [5].

EAP AKA Authentication June 2003
[EAP].

UMTS subscribers are identified with the International Mobile
Subscriber Identity (IMSI) [7]. [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
operator. 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) [4]. [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. The AAA nodes use

This section specifies the realm portion of peer identity format used in EAP/AKA. In
this document, the NAI term identity or peer identity refers to
route AAA requests the
whole identity string that is used to identify the correct AAA server. peer. The peer
identity may include a realm name used in
this protocol MAY be chosen by portion. "Username" refers to the operator and it MAY be a
configurable parameter in
portion of the EAP/AKA client implementation. In this
case, peer identity that identifies the client is typically configured with user, i.e. the NAI realm of
username does not include the
home operator. Operators MAY reserve a specific realm name for portion.

Identity Privacy Support

EAP/AKA users. This convention makes 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 easy would
help observers to recognize that track the
NAI identifies a subscriber that uses EAP/AKA. Such a reserved NAI
realm user. The permanent identity is usually
based on the IMSI, which may be a useful hint to further help the first authentication method 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 use
during method negotiation. 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 NAI username portions usernames in EAP/AKA: non-
pseudonym EAP/AKA peer identities:

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

EAP AKA Authentication 27 October, 2003

(2) Pseudonym usernames. For example, 2s7ah6n9q@myoperator.com might
be a valid pseudonym usernames and re-
authentication 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 username identity is used. used on full authentication. The non-pseudonym permanent username MAY be derived from the IMSI. re-
authentication exchange is specified in Section 4.2.

sername Decoration

In this case, some environments, the peer may need to decorate the identity by
prepending or appending the permanent username MUST 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 format "0imsi".
In other words, the first character scope of the this
document. However, it should be noted that username is the digit
zero (ASCII value 0x30), followed by decoration might
prevent 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 [7]

The EAP server from recognizing a valid username. Hence,
although the peer MAY use username decoration in the leading "0" as a hint to try EAP/AKA as identities the first authentication method during method negotiation. The
EAP/AKA
peer includes in EAP-Response/Identity, and the EAP server MAY propose EAP/AKA even if the leading character was
not "0".

Alternatively, an implementation may choose
accept a permanent decorated peer username
that is not based on the IMSI. In in this case the selection of message, the
username, its format, and its processing is a local matter. In this
case, peer or the client implementation
EAP server MUST NOT prepend decorate any leading
characters to the username.

When other peer identities that are used
in various EAP/AKA attributes. Only the optional identity privacy support is used on full
authentication, the client in EAP-
Response/Identity may be decorated.

NAI Realm Portion

The peer MAY use the pseudonym received upon the
previous full authentication sequence as the username include a realm portion of in the
NAI, peer identity, as specified in Section 4.3. per
the NAI format. The client MUST NOT modify use of a realm portion is not mandatory.

If a realm is used, the

EAP AKA Authentication June 2003

pseudonym received in AT_NEXT_PSEUDONYM. For example, realm MAY be chosen by the client
MUST NOT prepend any leading characters operator and it
MAY a configurable parameter in the pseudonym.

On re-authentication, EAP/SIM peer implementation. In
this case, the client uses peer is typically configured with the re-authentication identity
received upon NAI realm of
the previous authentication sequence as home operator. Operators MAY reserve a specific realm name for
EAP/AKA users. This convention makes it easy to recognize that the NAI. A new
re-authentication identity
NAI identifies a UMTS subscriber. Such reserved NAI realm may be delivered
useful as part of both full a hint as to the first authentication and re-authentication. The client MUST NOT modify method to use during
method negotiation. When the
re-authentication identity received in AT_NEXT_REAUTH_ID but peer is using a pseudonym username
instead of the
client must use permanent username, the re-authentication identity peer selects the realm name
portion similarly as it is. For
example, select the client MUST NOT prepend any leading characters in realm portion when using the
re-authentication identity.
permanent username.

If no configured realm name is available, the client 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 [8]. [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

EAP AKA Authentication 27 October, 2003

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 client peer is not able to determine whether the MNC is two or three
digits long, the client peer MAY use a 3-digit MNC. If the correct length
of the MNC is two, then the MNC used in the realm name will
include includes the
first digit of MSIN. Hence, when configuring AAA networks for
operators that have 2-digit MNCs, MNC's, the network SHOULD also be
prepared for realm names with incorrect 3-digit MNCs.

4.2. Obtaining Subscriber Identity via EAP AKA Messages

It may be useful to obtain the identity MNC's.

Format of the subscriber through
means other than EAP Request/Identity. This can eliminate the need
for an identity request when using EAP method negotiation. If this
was not possible then it might not be possible to negotiate EAP/AKA
as the second method since not all EAP implementations support
multiple EAP Identity requests.

EAP-Request/AKA-Identity and EAP-Response/AKA-Identity packets may
be used for obtaining the subscriber identity. Permanent Username

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 7.2.

If the EAP server has not received any identity (permanent identity,
pseudonym or re-authentication identity) from the client when
sending the first EAP/AKA request, then the EAP server SHOULD issue
the EAP-Request/AKA-Identity packet and includes the AT_ANY_ID_REQ
attribute (specified in Section 8.5). This attribute does not
contain any data.

If the EAP server has received an EAP-Response/Identity packet but
the contents do not appear to be a valid non-pseudonym permanent identity,
pseudonym or a re-authentication identity, the EAP server username SHOULD

EAP AKA Authentication June 2003

issue an EAP-Request/AKA-Identity packet with be derived from the AT_ANY_ID_REQ
attribute.
IMSI. In some environments the intermediate entities or software layers in
the client may modify this case, the identity string in permanent username MUST be of the EAP-
Response/Identity packet. For example, some EAP layer
implementations may cache format "0"
| IMSI, where the identity string from character "|" denotes concatenation. In other
words, the first
authentication and do not obtain a new identity string from the EAP
method implementation on subsequent authentication exchanges.
Because the identity string is used in key derivation, such
modifications will result in failed authentication unless the EAP
server uses the AT_ANY_ID_REQ attribute to obtain an unmodified copy character of the identity string. Therefore, in cases when there username is a
possibility that an intermediate element or software layer may
modify the EAP-Response/Identity packet, the EAP server SHOULD
always use the EAP-Request/AKA-Identity packet with the
AT_ANY_ID_REQ attribute, even if the identity received in EAP-
Response/Identity was valid.

The AT_ANY_ID_REQ attribute requests the client to include the
AT_IDENTITY attribute (specified in Section 8.6) in digit zero (ASCII
value 0x30), followed by the EAP-
Response/AKA-Identity packet. IMSI. The identity format in the AT_IDENTITY
attribute IMSI is the same as in the Type-Data field an ASCII string that
consists of the EAP-
Response/Identity packet. The AT_IDENTITY attribute contains a
permanent identity, a pseudonym identity or a re-authentication
identity. If the server does not support re-authentication, it uses
the AT_FULLAUTH_ID_REQ attribute instead of the AT_ANY_ID_REQ
attribute to directly request for a full authentication identity
(either the permanent identity or a pseudonym identity). If the
server uses the AT_FULLAUTH_ID_REQ attribute, the client MUST NOT
use a re-authentication identity in the AT_IDENTITY attribute.

The use of pseudonyms for anonymity is more than 15 decimal digits (ASCII values between
0x30 and 0x39) as specified in Section 4.3. [TS 23.003].

The EAP server MAY use of re-authentication identities is specified in Section 5.

The full authentication case is illustrated in the figure below. In
this case, AT_IDENTITY contains either the permanent identity or leading "0" as a
pseudonym identity. The same sequence is also used in case the
server uses the AT_FULLAUTH_ID_REQ in EAP-Request/AKA-Identity

EAP AKA Authentication June 2003

If the client wants hint to perform full authentication, it includes the
permanent identity or a pseudonym identity in try EAP/AKA as
the AT_IDENTITY
attribute. first authentication method during method negotiation, rather
than for example EAP/SIM. The client may use these identities in response to either
AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ. If the EAP/AKA server uses the
AT_ANY_ID_REQ and the client wants to perform re-authentication,
then the client includes a re-authentication identity in the
AT_IDENTITY attribute.

If the client uses its full authentication identity and MAY propose EAP/AKA
even if the
AT_IDENTITY attribute contains leading character was not "0".

Alternatively, an implementation MAY choose a valid permanent identity or a valid
pseudonym identity username
that the EAP server is able to decode to the
permanent identity, then the full authentication sequence proceeds
as usual with the EAP Server issuing the EAP-Request/AKA-Challenge
message.

On re-authentication, if the AT_IDENTITY attribute contains a valid
re-authentication identity and the server agrees on using re-
authentication, then the server proceeds with the re-authentication
sequence and issues the EAP-Request/AKA-Reauthentication packet, as
specified in Section 5. If the server does not recognize the re-
authentication identity, then it issues a second EAP-Request/AKA-
Identity message and includes based on the AT_FULLAUTH_ID_REQ attribute. IMSI. In this case, a second EAP/AKA-Identity round trip is required. The
messages used on the first roundtrip are ignored. (However all AKA-
Identity round trips are included in case the calculation selection of the
AT_CHECKCODE attribute, as specified in Section 7.2). This
username, its format, and its processing is
illustrated below.

EAP AKA Authentication June 2003

Client 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 out of the
EAP-Request/AKA-Challenge packet. scope of this
document. In this case, the full
authentication procedure proceeds as usual.

An extra EAP/AKA-Identity round trip is also required in cases when
the AT_IDENTITY attribute contains a pseudonym identity that the EAP
server fails to decode. The operation in this case is specified in
Section 4.3.

4.3. Identity Privacy Support

EAP/AKA includes optional identity privacy (anonymity) support that
can be used peer implementation MUST NOT prepend any
leading characters to hide the cleartext permanent identity username.

Generating Pseudonyms and to make the
subscriber's connections unlinkable to eavesdroppers. Identity
privacy is based on temporary identities, or pseudonyms, which are
equivalent to but separate from the Temporary Mobile Subscriber Re-authentication Identities (TMSI) that are used on cellular networks. Please see
Section 12.1 for security considerations concerning identity
privacy.

EAP AKA Authentication June 2003

If identity privacy is not used or if by the client does not have any
pseudonyms or Server

Pseudonym usernames and re-authentication identities available, the client
transmits the permanent identity in the EAP-Response/Identity packet
or in the AT_IDENTITY attribute.

The EAP-Request/AKA-Challenge message MAY include an encrypted
pseudonym in the value field of the AT_ENCR_DATA attribute. The
AT_IV and AT_MAC attributes are also used to transport the pseudonym
to the client, as described in Section 8.1. Because the identity
privacy support is optional to implement, the client MAY ignore the
AT_IV and AT_ENCR_DATA attributes and always transmit the permanent
identity in the EAP-Response/Identity packet and in the AT_IDENTITY
attribute.

On receipt of the EAP-Request/AKA-Challenge, the client verifies the
AT_MAC attribute before looking at the AT_ENCR_DATA attribute. If
the AT_MAC is invalid, then the client MUST silently discard generated
by the EAP
packet. If the AT_MAC attribute is valid, then the client MAY
decrypt the encrypted data in AT_ENCR_DATA and use the obtained
pseudonym on the next full authentication.

If the client does not receive a new pseudonym in the EAP-
Request/AKA-Challenge message, the client MAY use an old pseudonym
instead of the permanent identity on next full authentication. server. The EAP server produces pseudonyms 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 pseudonyms re-authentication usernames are separate and
recognizable from each other. It is also desirable that EAP SIM and
EAP AKA usernames user names be recognizable from each other as an aid for the
server to which method to offer.

EAP AKA Authentication 27 October, 2003

In general, it is the task of the EAP server and the policies of its
administrator to ensure sufficient separation in the usernames.
Pseudonyms, for instance,
Pseudonym usernames and re-authentication usernames are both
produced and used by the EAP server. The EAP server MUST compose pseudonyms
pseudonym usernames and re-authentication usernames so that it can
recognize if a NAI username is an EAP AKA pseudonym. 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.

The client MAY transmit
character). When mapping a re-authentication identity to a permanent
identity, the received pseudonym in server SHOULD only examine the first EAP-
Response/Identity packet username portion of the next full authentication with the
EAP server. The client concatenates the received pseudonym with the
"@" character
re-authentication identity and the NAI realm portion. The client selects the
realm name portion similarly as it select ignore the realm name portion
when using of the permanent
identity. If the EAP server successfully
decodes the pseudonym received in the EAP-Response/Identity packet
to a known client permanent identity, the authentication proceeds
with the EAP-Request/AKA-Challenge message as usual.

EAP AKA Authentication June 2003

Because the client peer may fail to save a pseudonym username sent to in an EAP-
Request/AKA-Challenge,
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.

If pseudonym username.

Transmitting Pseudonyms and Re-authentication Identities to the EAP Peer

The server requests the client transmits pseudonym usernames and re-authentication
identities to include its identity in the
EAP-Response/AKA-Identity packet, as specified peer in Section 4.2, cipher, using the
client AT_ENCR_DATA attribute.

The EAP-Request/AKA-Challenge message MAY transmit the received include an encrypted
pseudonym username and/or an encrypted re-authentication identity in
the AT_IDENTITY
attribute. If the EAP server successfully decodes value field of the pseudonym AT_ENCR_DATA attribute. Because identity
privacy support and re-authentication are optional to a
known identity, then implement, the authentication proceeds with
peer MAY ignore the EAP-
Request/AKA-Challenge packet as usual.
If AT_ENCR_DATA attribute and always use the EAP server fails to decode
permanent identity. On re-authentication (discussed in Section 4.2),
the pseudonym to server MAY include a known identity,
then new encrypted re-authentication identity in
the EAP server requests EAP-Request/AKA-Reauthentication message.

On receipt of the permanent identity (non-pseudonym
identity) by including EAP-Request/AKA-Challenge, the AT_PERMANENT_ID_REQ attribute (Section
8.5) peer MAY decrypt
the encrypted data in AT_ENCR_DATA and if a pseudonym username is
included, the EAP-Request/AKA-Identity message. Because another EAP
server peer may have generated use the obtained pseudonym using username on the
next full authentication. If a different coding
scheme, re-authentication identity is
included, then the EAP server SHOULD use AT_PERMANENT_ID_REQ also in cases
when 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 recognize receive a new pseudonym username in the format EAP-
Request/AKA-Challenge message, the peer MAY use an old pseudonym
username instead of the client identity. permanent username on next full
authentication. The EAP server issues username portions of re-authentication
identities are one-time usernames, which the EAP-Request/AKA-Identity message also in peer MUST NOT re-use.

Usage of the case when it received Pseudonym by the undecodable pseudonym in AT_IDENTITY
included in Peer

When the EAP-Response/AKA-Identity packet. In this case, a
second EAP/AKA-Identity round trip optional identity privacy support is required.

A received AT_PERMANENT_ID_REQ does not necessarily originate from used on full
authentication, the valid network, but an active attacker may transmit an EAP-
Request/AKA-Identity packet with an AT_PERMANENT_ID_REQ attribute to peer MAY use the client, in an effort to find out pseudonym username received as
part of the true identity previous full authentication sequence as the username
portion of the user. NAI. The client peer MUST NOT modify the pseudonym username

EAP AKA Authentication 27 October, 2003

received in AT_NEXT_PSEUDONYM. However, as discussed above, the peer
MAY silently discard any EAP-Request/AKA-Identity
messages need to decorate the username in some environments by appending
or prepending the username with a string that include AT_PERMANENT_ID_REQ for indicates
supplementary AAA routing information.

When using a while pseudonym username in order to
wait for an EAP-Request/AKA-Identity packet without
AT_PERMANENT_ID_REQ. If environment where a realm
portion is used, the valid network sent peer concatenates the message, received pseudonym
username with the
message will "@" 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 retransmitted, so delivered as part of both full
authentication and re-authentication. The peer MUST NOT modify the client can reconsider replying
to
username part of the message re-authentication identity received in
AT_NEXT_REAUTH_ID, except in cases when it receives a retransmission.

Basically, there are two different policies that username decoration is
required. Even in these cases, the client can
employ "root" re-authentication username
must not be modified, but it may be appended or prepended with regard
another string.

4.1.2. Communicating the Peer Identity to AT_PERMANENT_ID_REQ. A "conservative" client
assumes that the network is able Server

General

The peer identity MAY be communicated to maintain pseudonyms robustly.
Therefore, if the server with the EAP-
Response/Identity message. This message MAY contain the permanent
identity, a conservative client has pseudonym identity, or a pseudonym, the client
silently ignores re-authentication identity. If
the EAP packet with AT_PERMANENT_ID_REQ, because peer uses the client believes that permanent identity or a pseudonym identity, which
the valid network server is able to decode map to the
pseudonym. (Alternatively, permanent identity, then the conservative client may respond to
AT_PERMANENT_ID_REQ
authentication proceeds as discussed in certain circumstances, for example if the
pseudonym was received a long time ago.) The benefit overview of this policy
is that it protects the client against active attacks on anonymity.
On Section 3.
If the other hand, peer uses a "liberal" client always accepts the
AT_PERMANENT_ID_REQ re-authentication identity, and responds with the permanent identity. server
recognized the identity and agrees on using re-authentication, then
a re-authentication exchange is performed, as described in Section
4.2.

The
benefit peer identity can also be transmitted from the peer to the
server using EAP/AKA messages instead of EAP-Response/Identity. In
this policy is that it works even if case, the valid network
sometimes loses pseudonyms 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 able to decode them 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 value field of
server uses the AT_PERMANENT_ID_REQ does not contain any data
but the attribute is included to request the client 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_IDENTITY attribute (Section 8.6) with AT_CHECKCODE attribute, which enables the permanent
protocol peers to ensure the integrity of the AKA-Identity packets.
AT_CHECKCODE is specified in Section 0.

EAP AKA Authentication June 27 October, 2003

authentication

The identity format in the EAP-Response/AKA-Identity message. In
this case, 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 the client's 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 clear. 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 client peer and the EAP/AKA server only
process the AT_IDENTITY attribute. Entities attribute and entities that only pass
through EAP packets through 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 client peer with the
original identity from the EAP-Response/Identity packet regardless
if
of whether the decoding fails AT_IDENTITY attribute is used in EAP/AKA to transmit
another identity.

Choice of Identity for the EAP server.

The figure below illustrates EAP-Response/Identity

If EAP/AKA peer is started upon receiving an EAP-Request/Identity
message, then the case when peer performs the EAP server fails following steps.

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

Else, if the EAP-Response/Identity packet.

Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes peer has a pseudonym) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server fails to decode 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.

EAP AKA Authentication 27 October, 2003

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 |
| | Pseudonym. |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_PERMANENT_ID_REQ) |
|<------------------------------------------------------|
| |
| |
| 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 |
| (AT_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) |
|------------------------------------------------------>|
| |

If 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".

EAP AKA Authentication 27 October, 2003

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 sequence proceeds as usual 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

EAP AKA Authentication 27 October, 2003

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
issuing

When the EAP-Request/AKA-Challenge message. 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 recognize 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 if an identity that the
server is not able to continue 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 exchange 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
client after receiving 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
issues 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

EAP Failure packet AKA Authentication 27 October, 2003

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.

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.

EAP AKA Authentication 27 October, 2003

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 exchange
terminates. 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.

EAP AKA Authentication 27 October, 2003

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.

EAP AKA Authentication June 27 October, 2003

Client

In the worst case, there are three

Three EAP/AKA-Identity round trips
before the server has obtained an acceptable identity: on the first
round, the client sends its re-authentication identity in
AT_IDENTITY. The server fails to accept it and request a full
authentication identity with a second EAP-Request/AKA-Identity. Round Trips

The
client responds with a pseudonym identity in AT_IDENTITY. The server
fails to decode the pseudonym and has to issue a third EAP-
Request/AKA-Identity, including AT_PERMANENT_ID_REQ. Finally, the
server accepts the client's EAP-Response/AKA-Identity with figure below illustrates the
AT_IDENTITY attribute and proceeds case with full authentication. This is
illustrated in the figure below. three EAP/AKA-Identity
round trips.

EAP AKA Authentication June 27 October, 2003

Client

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. If the EAP Server
recognizes the permanent identity and is able to proceed, the server
issues the EAP-Request/AKA-Challenge message. If the server does not
recognize the permanent identity, or if the server is not able to
continue the authentication exchange with the client after receiving
the permanent identity, then the server issues the EAP Failure
packet and the authentication exchange terminates.

EAP AKA Authentication June 2003

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

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 client. 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 10. 4.5.

On re-authentication, the client peer protects against replays with an
unsigned 16-bit counter, included in the AT_COUNTER attribute. On
full authentication, both the server and the client 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 client's 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.

As discussed in Section 4.3, in some environments the client may
assume that

Both the network can reliably store pseudonyms peer and therefore
the client may fail to respond to the AT_PERMANENT_ID_REQ attribute.
The network server SHOULD store pseudonyms on have an upper limit for the
number of subsequent re-authentications allowed before a reliable database. full
authentication needs to be performed. Because
one of a 16-bit counter is
used in re-authentication, the objectives theoretical maximum number of the re-authentication procedure re-
authentications is to
reduce load on reached when the network, counter value reaches 0xFFFF.
In order to use re-authentication, the re-authentication procedure does not
require peer and the EAP server need
to contact a reliable database. Therefore, 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. The network does
not need to store re-authentication identities as carefully as
pseudonyms. If a re-authentication re-
authentication identity is lost and the network does not recognize
it, the EAP server can fall back on full authentication.

EAP AKA Authentication June 27 October, 2003

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 client peer MAY
ignore this attribute, in which case it will use full authentication
next time. If the client peer wants to use re-
authentication, re-authentication, it uses this
re-authentication identity on next authentication. Even if the client peer
has a re-authentication identity, the client peer MAY discard the re-authentication re-
authentication identity and use a pseudonym or the permanent
identity instead, in which case full authentication will MUST be
performed.

The

In environments where a real portion is needed in the peer identity,
the re-authentication identity received in AT_NEXT_REAUTH_ID
contains MUST
contain both the a username portion and the a realm portion of portion, as per the
Network Access Identifier. 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 client 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 client 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 client 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 client's 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. (As specified in
Sections 4.2 and 4.3, the server MAY use AT_ANY_ID_REQ,
AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ attributes if it does not
know the client's identity.)

Both the client 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 client and the server need to
store the following values: original Master Key, K_aut, K_encr,
latest counter value and the next re-authentication identity.

4.2.3. Re-authentication Procedure

The following figure illustrates the re-authentication procedure.
Encrypted attributes are denoted with '*'. The client peer uses its re-

EAP AKA Authentication June 2003
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 client 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 client peer to send its identity by
including the AT_ANY_ID_REQ attribute in the EAP-Request/AKA-
Identity packet.

EAP AKA Authentication 27 October, 2003

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 client. 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 client 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 client peer verifies that the counter value is fresh (greater than any
previously used value). The client peer also verifies that AT_MAC is
correct. The client peer MAY save the next re-authentication identity from
the encrypted AT_NEXT_REAUTH_ID for next time. If all checks are
successful, the client 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 client. peer.

EAP AKA Authentication June 27 October, 2003

Client

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) |
|<------------------------------------------------------|
| |
| |
+-----------------------------------------------+ |
| Client Peer verifies AT_MAC and the freshness of | |
| the counter. Client 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 client 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 '*'.

EAP AKA Authentication June 27 October, 2003

Client

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 client 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 client 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 client 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.

EAP AKA Authentication June 27 October, 2003

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

EAP AKA Authentication 27 October, 2003

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

EAP AKA Authentication 27 October, 2003

- 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

EAP AKA Authentication 27 October, 2003

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.

EAP AKA Authentication 27 October, 2003

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 Type-Data 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.

EAP AKA packets Authentication 27 October, 2003

In EAP/AKA, the Type-Data begins with an EAP/AKA header that
consists of a 1-octet Subtype field, which is followed by 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.

The rest of the Type-Data 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 11. 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.

When an attribute

Attributes numbered within the range 0 through 127 is
encountered but not recognized, the are called non-
skippable attributes. When an EAP/AKA message containing that peer encounters a non-
skippable attribute type that the peer does not recognize, the peer
MUST be silently discarded. These attributes are called send the EAP-Response/AKA-Client-Error packet, and the
authentication exchange terminates. If an EAP/AKA server encounters
a non-skippable attributes. 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,

EAP AKA Authentication 27 October, 2003

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.

Unless otherwise specified,
It is possible to introduce version negotiation in the order of 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 an EAP
AKA a message, which attributes are required in a message,
and other message specific details. Message format is insignificant, and an EAP AKA implementation should
not assume a certain order 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 used. required in order to obtain a valid peer
identity.

EAP AKA Authentication June 27 October, 2003

Attributes can be encapsulated within other attributes. In other
words,

The server MUST include one of the value field 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 attribute type can be specified to
contain other 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.

7. Message Authentication

If the server has previously issued an EAP-Request/AKA-Identity
message with the AT_ANY_ID_REQ attribute, and Encryption 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 section specifies EAP/AKA 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 attribute encryption identity privacy and EAP/AKA message authentication.

Encryption for communicating the next re-
authentication identity. In this case, the AT_IV and integrity protection AT_ENCR_DATA
attributes are based on 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

EAP AKA session
keys CK Authentication 27 October, 2003

and IK. Because wants to communicate a pseudonym to the CK peer for the next full
authentication, then the nested encrypted attributes include the
AT_NEXT_PSEUDONYM attribute. If the server supports re-
authentication and IK keys are derived from wants to communicate a re-authentication identity
to the RAND
challenge, these peer, then the nested encrypted attributes can only include the
AT_NEXT_REAUTH_ID attribute. Later versions of this protocol MAY
specify additional attributes to be used 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 EAP-Request/AKA-
Challenge message MAC, see
Section 7.2.

The AT_RES attribute MUST be included.

The AT_CHECKCODE attribute MAY be included, and any EAP/AKA 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 sent after it. For
example, that include these attributes cannot 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 EAP-Request/AKA-
Identity, because
this message.

6.6. EAP-Response/AKA-Synchronization-Failure

The peer sends the RAND challenge has not yet been transmitted at
that point. Integrity protection with AT_MAC 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 all
messages when keys have been derived.

7.1. AT_MAC Attribute
this message.

6.7. EAP-Request/AKA-Reauthentication

EAP AKA Authentication 27 October, 2003

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 can MUST be used for EAP/AKA message integrity
protection. Whenever AT_ENCR_DATA (Section 7.3) included. No message-specific data is
included in an
EAP message, 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 (not necessarily immediately) 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.

EAP AKA Authentication 27 October, 2003

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. attribute is included in cases described in Section 4.3.
No message-specific data is included in the MAC calculation. See
Section 7.2.

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)
EAP-Response/AKA-Re-authentication, (10) EAP-Response/AKA-
Authentication-Reject, and (11) EAP-Response/AKA-Synchronization-
Failure. The column denoted with "E" indicates whether the attribute
is a nested attribute that do MUST be included within AT_ENCR_DATA.

"0" indicates that the attribute MUST NOT be included in the
message, "1" indicates that the attribute MUST be included in the
message, "0-1" indicates that the attribute is sometimes included in
the message, and "0*" indicates that the attribute is not meet included
in the message in cases specified in this condition document, but MAY be
included in the future versions of the protocol.

EAP AKA Authentication 27 October, 2003

Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)(11) E
AT_MAC 0 0 1 1 0-1 0-1 0 1 1 0 0 N
AT_IV 0 0 0-1 0* 0* 0* 0 1 1 0 0 N
AT_ENCR_DATA 0 0 0-1 0* 0* 0* 0 1 1 0 0 N
AT_PADDING 0 0 0-1 0* 0* 0* 0 0-1 0-1 0 0 Y
AT_CHECKCODE 0 0 0-1 0-1 0 0 0 0-1 0-1 0 0 N
AT_PERMANENT_ID_REQ 0-1 0 0 0 0 0 0 0 0 0 0 N
AT_ANY_ID_REQ 0-1 0 0 0 0 0 0 0 0 0 0 N
AT_FULLAUTH_ID_REQ 0-1 0 0 0 0 0 0 0 0 0 0 N
AT_IDENTITY 0 0-1 0 0 0 0 0 0 0 0 0 N
AT_RAND 0 0 1 0 0 0 0 0 0 0 0 N
AT_AUTN 0 0 1 0 0 0 0 0 0 0 0 N
AT_RES 0 0 0 1 0 0 0 0 0 0 0 N
AT_AUTS 0 0 0 0 0 0 0 0 0 0 1 N
AT_NEXT_PSEUDONYM 0 0 0-1 0 0 0 0 0 0 0 0 Y
AT_NEXT_REAUTH_ID 0 0 0-1 0 0 0 0 0-1 0 0 0 Y
AT_COUNTER 0 0 0 0 0 0 0 1 1 0 0 Y
AT_COUNTER_TOO_SMALL 0 0 0 0 0 0 0 0 0-1 0 0 Y
AT_NONCE_S 0 0 0 0 0 0 0 1 0 0 0 Y
AT_NOTIFICATION 0 0 0 0 1 0 0 0 0 0 0 N
AT_CLIENT_ERROR_CODE 0 0 0 0 0 0 1 0 0 0 0 N

It should be noted that attributes AT_PERMANENT_ID_REQ,
AT_ANY_ID_REQ and AT_FULLAUTH_ID_REQ are mutually exclusive, so that
only one of them can be included at the same time. If one of the
attributes AT_IV and AT_ENCR_DATA is included, then both of the
attributes MUST be
silently discarded. included.

7.2. AT_MAC

The AT_MAC attribute is used for EAP/AKA message authentication.
Section 6 specifies which messages AT_MAC MUST be included.

The value field of the AT_MAC attribute contains two reserved bytes
followed by a keyed message authentication code (MAC). The MAC is
calculated over the whole EAP packet, concatenated with optional
message-specific data, with the exception that the value field of
the MAC attribute is set to zero when calculating the MAC. The EAP
packet includes the EAP header that begins with the Code field, the
EAP/AKA header that begins with the Subtype field, and all the
attributes, as specified in Section 5.1. The reserved bytes in
AT_MAC are set to zero when sending and ignored on reception. The
contents of the message-specific data, if present, data that may be included in the
MAC calculation are specified separately for each EAP/AKA message. The message-specific data is
included message in order to protect data that is not transmitted with the
EAP packet.
Section 6.

The format of the AT_MAC attribute is shown below.

EAP AKA Authentication 27 October, 2003

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The MAC algorithm is HMAC-SHA1-128 [9] [RFC 2104] keyed hash value. (The HMAC-
SHA1-128
HMAC-SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by

EAP AKA Authentication June 2003
truncating the output to 16 bytes. Hence, the length of the MAC is
16 bytes.) The message derivation of the authentication key (K_aut) used in
the calculation of the MAC is derived from specified in Section 4.5.

When the AT_MAC attribute is included in an EAP/AKA message, the
recipient MUST process the AT_MAC attribute before looking at any
other attributes. If the message authentication code is invalid,
then the recipient MUST ignore all other attributes in the message
and operate as specified in Section 4.4.

7.3. AT_IV, AT_ENCR_DATA and AT_PADDING

AT_IV and AT_ENCR_DATA attributes can be used to transmit encrypted
information between the EAP/SIM peer and server.

The value field of AT_IV contains two reserved bytes followed by a
16-byte initialization vector required by the AT_ENCR_DATA
attribute. The reserved bytes are set to zero when sending and
ignored on reception. The AT_IV attribute MUST be included if and
only if the AT_ENCR_DATA is included. Section 4.4 specifies the
operation if a packet that does not meet this condition is
encountered.

The sender of the AT_IV attribute chooses the initialization vector
by random. The sender MUST NOT reuse the initialization vector value
from previous EAP AKA integrity key (IK) packets and the sender MUST choose it freshly
for each AT_IV attribute. The sender SHOULD use a good source of
randomness to generate the initialization vector. Please see [RFC
1750] for more information about generating random numbers for
security applications. The format of AT_IV is shown below.

EAP AKA Authentication 27 October, 2003

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IV | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Initialization Vector |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The value field of the AT_ENCR_DATA attribute consists of two
reserved bytes followed by cipher text bytes encrypted using the
Advanced Encryption Standard (AES) [AES] in the Cipher Block
Chaining (CBC) mode of operation using the initialization vector
from the AT_IV attribute. The reserved bytes are set to zero when
sending and ignored on reception. Please see [CBC] for a description
of the CBC mode. The format of the AT_ENCR_DATA attribute is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_ENCR_DATA | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Encrypted Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The derivation of the encryption key (CK), as (K_encr) is specified in
Section 10.

7.2. AT_CHECKCODE 4.5.

The plaintext consists of nested EAP/AKA attributes.

The encryption algorithm requires the length of the plaintext to be
a multiple of 16 bytes. The sender may need to include the
AT_PADDING attribute as the last attribute within AT_ENCR_DATA. The
AT_PADDING attribute is not included if the total length of other
nested attributes within the AT_ENCR_DATA attribute is a multiple of
16 bytes. As usual, the Length of the Padding attribute includes the
Attribute Type and Attribute Length fields. The length of the
Padding attribute is 4, 8 or 12 bytes. It is chosen so that the
length of the value field of the AT_ENCR_DATA attribute becomes a
multiple of 16 bytes. The actual pad bytes in the value field are
set to zero (0x00) on sending. The recipient of the message MUST
verify that the pad bytes are set to zero. If this verification
fails on the peer, then it MUST send the EAP-Response/AKA-Client-
Error packet with the error code "unable to process packet" to
terminate the authentication exchange. If this verification fails on
the server, then the server sends EAP Failure, and the
authentication exchange terminates. The format of the AT_PADDING
attribute is shown below.

EAP AKA Authentication 27 October, 2003

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_PADDING | Length | Padding... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.4. AT_CHECKCODE

The AT_MAC attribute is not used in the very first EAP/AKA messages, messages
during the AKA-Identity round, because keying material has not been
derived yet. The client peer and the server may exchange one or more pairs
of EAP/AKA messages of the Subtype AKA-Identity before keys are
derived and before the AT_MAC attribute can be applied. The EAP/AKA-Identity EAP/AKA-
Identity messages may also be used upon re-authentication.

The AT_CHECKCODE attribute MAY be used to protect the EAP/AKA-
Identity messages. AT_CHECKCODE is included in EAP-Request/AKA-
Challenge and/or EAP-Response/AKA-Challenge upon full
authentication. In re-authentication, AT_CHECKCODE can MAY be included
in EAP-Request/AKA-Reauthentication and/or EAP-Response/AKA-
Reauthentication. Because the AT_MAC attribute is used in these
messages, AT_CHECKCODE will be integrity protected with AT_MAC.
The format of the AT_CHECKCODE attribute is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_CHECKCODE | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Checkcode (0 or 20 bytes) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The value field of AT_CHECKCODE begins with two reserved bytes,
which may be followed by a 20-byte checkcode. If the checkcode is
not included in AT_CHECKCODE, then the attribute indicates that no
EAP/AKA-Identity messages were exchanged. This may occur in both
full authentication and re-authentication. The reserved bytes are
set to zero when sending and ignored on reception.

The checkcode is a hash value, calculated with SHA1 [10], [SHA-1], over
all EAP-Request/AKA-Identity and EAP-Response/ AKA-Identity packets
exchanged in this authentication exchange. The packets are included
in the order that they were transmitted, that is, starting with the
first EAP-Request/ AKA-Identity message, followed by the

EAP AKA Authentication 27 October, 2003

corresponding EAP-Response/ AKA-Identity, followed by the second
EAP-Request/ AKA-Identity (if used) etc.

EAP packets are included in the hash calculation "as-is", as they
were transmitted or received. All reserved bytes, padding bytes etc.
that are specified for various attributes are included as such, and
the receiver must not reset them to zero. No delimiter bytes,

EAP AKA Authentication June 2003
padding or any other framing are included between the EAP packets
when calculating the checkcode.

Messages are included in request/response pairs; in other words only
full "round trips" are included. Packets that are silently discarded
are not included. The EAP server must only include an EAP-
Request/AKA-Identity in the calculation once it has received a
corresponding response, with the same Identifier value.
Retransmissions or requests to which the server does not receive
response are not included.

The client peer must include the EAP-Request/AKA-Identity and the
corresponding response in the calculation only if the client peer receives
a subsequent EAP-Request/AKA-Challenge, or a follow-up EAP-
Request/AKA-Identity with different attributes (attribute types)
than in the first EAP-Request/AKA-Identity. After sending EAP-
Response/AKA-Identity, if the client peer receives another EAP-
Request/AKA-Identity EAP-Request/AKA-
Identity with the same attributes as in the previous request, then
the client's peer's response to the first request must have been lost. In
this case the client peer must not include the first request and its
response in the calculation of the checkcode.

The AT_CHECKCODE attribute is optional to implement. It is specified
in order to allow protecting the EAP/ AKA-Identity messages and any
future extensions to them. The implementation of AT_CHECKCODE is
recommended.
RECOMMENDED.

If the receiver of AT_CHECKCODE implements this attribute, then the
receiver MUST check that the checkcode is correct. If the checkcode
is invalid, the receiver must terminate the authentication exchange. operate as specified in Section 4.4.

If the EAP/AKA-Identity messages are extended with new attributes
then AT_CHECKCODE must MUST be implemented and used. More specifically,
if the server includes any other attributes than
AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ or AT_ANY_ID_REQ in the EAP-
Request/AKA-Identity packet, then the server MUST include
AT_CHECKCODE in EAP-Request/AKA-Challenge or EAP-Request/AKA-
Reauthentication. If the client peer includes any other attributes than
AT_IDENTITY in the EAP-Response/AKA-Identity message, then the
client peer
MUST include AT_CHECKCODE in EAP-Response/AKA-Challenge or
EAP-Response/AKA-Reauthentication. EAP-
Response/AKA-Reauthentication.

If the server implements the processing of any other attribute than
AT_IDENTITY for the EAP-Response/AKA-Identity message, then the
server MUST implement AT_CHECKCODE. In this case, if the server
receives any other attribute than AT_IDENTITY in the EAP-
Response/AKA-Identity message, then the server MUST check that

EAP AKA Authentication 27 October, 2003

AT_CHECKCODE is present in EAP-Response/AKA-Challenge or EAP-
Response/AKA-Reauthentication. If AT_CHECKCODE The operation when a mandatory
attribute is not included, the
server must terminate the authentication exchange. missing is specified in Section 4.4.

Similarly, if the client peer implements the processing of any other
attribute than AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ or
AT_ANY_ID_REQ for the EAP-Request/AKA-Identity packet, then the

EAP AKA Authentication June 2003

client peer
MUST implement AT_CHECKCODE. In this case, if the client peer receives any
other attribute than AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ or
AT_ANY_ID_REQ in the EAP-Request/AKA-Identity packet, then the client peer
MUST check that AT_CHECKCODE is present in EAP-Request/AKA-Challenge
or EAP-Request/AKA-Reauthentication. If
the attribute was not included, the client must terminate the
authentication exchange.

7.3. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes

AT_IV and AT_ENCR_DATA attributes can be optionally used to transmit
encrypted information between the EAP/AKA client and server. The value field of AT_IV contains two reserved bytes followed by a
16-byte initialization vector required by the AT_ENCR_DATA
attribute. The reserved bytes are set to zero operation when sending and
ignored on reception. The AT_IV a mandatory
attribute MUST be included if and
only if the AT_ENCR_DATA is included. Messages that do not meet this
condition MUST be silently discarded.

The sender of the AT_IV attribute chooses the initialization vector
by random. The sender MUST NOT reuse the initialization vector value
from previous EAP AKA packets but the sender MUST choose it freshly
for each AT_IV attribute. The sends SHOULD use a good source of
randomness to generate the initialization vector. The format of
AT_IV missing is shown below.

The value field of the AT_ENCR_DATA attribute consists of two
reserved bytes followed by bytes encrypted using the Advanced
Encryption Standard (AES) [11] specified in the Cipher Block Chaining (CBC)
mode of operation, using the initialization vector from the AT_IV
attribute. The reserved bytes are set to zero when sending and
ignored on reception. Please see [12] for a description of the CBC
mode. Section 4.4.

7.5. AT_PERMANENT_ID_REQ

The format of the AT_ENCR_DATA AT_PERMANENT_ID_REQ attribute is shown below.

EAP AKA Authentication June 2003

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_ENCR_DATA
|AT_PERM..._REQ | Length = 1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Encrypted Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The encryption key (K_encr) is derived is derived from the AKA
integrity key (IK) and cipher key (CK), as specified in Section10.
The plaintext consists of nested EAP/AKA attributes.

The encryption algorithm requires the length of the plaintext to be
a multiple of 16 bytes. The sender may need to include the
AT_PADDING attribute as the last attribute within AT_ENCR_DATA. The
AT_PADDING attribute is not included if the total length of other
nested attributes within the AT_ENCR_DATA attribute is a multiple of
16 bytes. As usual, the Length of the Padding attribute includes the
Attribute Type and Attribute Length fields. The Length use of the
Padding attribute is 4, 8 or 12 bytes. It AT_PERMANENT_ID_REQ is chosen so that the
length of the value field of the AT_ENCR_DATA attribute becomes a
multiple of 16 bytes. The actual pad bytes defined in the Section 4.1. The
value field only contains two reserved bytes, which are set to zero (0x00)
on sending. The recipient of the message MUST
verify that the pad bytes are set to zero, sending and silently drop the
message if this verification fails. ignored on reception.

7.6. AT_ANY_ID_REQ

The format of the AT_PADDING AT_ANY_ID_REQ attribute is shown below.

8. Messages

8.1. EAP-Request/AKA-Challenge

The format of the EAP-Request/AKA-Challenge packet is shown below.

EAP AKA Authentication June 2003

The semantics use of the fields AT_ANY_ID_REQ is described below:

EAP AKA Authentication June 2003

Code

1 for Request

Identifier

See [5]

Length

The length of the EAP Request packet.

Type

Subtype

1 for AKA-Challenge

Reserved

Set to zero when sending, ignored on reception.

AT_RAND defined in Section 4.1. The value
field of this attribute only contains two reserved bytes
followed by the AKA RAND parameter, 16 bytes (128 bits). The
reserved bytes bytes, which are set to zero when on
sending and ignored on reception.

7.7. AT_FULLAUTH_ID_REQ

The AT_RAND format of the AT_FULLAUTH_ID_REQ attribute MUST be present is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_ANY_ID_REQ | Length = 1 | Reserved |
+---------------+---------------+-------------------------------+

EAP AKA Authentication 27 October, 2003

The use of the AT_FULLAUTH_ID_REQ is defined in EAP-
Request/AKA-Challenge.

AT_AUTN Section 4.1. The
value field of this attribute only contains two reserved bytes
followed by the AKA AUTN parameter, 16 bytes (128 bits). The
reserved bytes bytes, which are set to zero when
on sending and ignored on reception.

7.8. AT_IDENTITY

The AT_AUTN attribute MUST be included.

AT_IV

See Section 7.3.

AT_ENCR_DATA

See Section 7.3. The nested attributes that are included in the
plaintext format of AT_ENCR_DATA are described below.

AT_CHECKCODE

The AT_CHECKCODE the AT_IDENTITY attribute is optional to include. See section
7.2

EAP AKA Authentication June 2003

AT_MAC

AT_MAC MUST be included. In EAP-Request/AKA-Challenge, there is
no message-specific data covered by the MAC. See Section 7.1.

In the EAP-Request/AKA-Challege message, the AT_IV, AT_ENCR_DATA and
AT_MAC attributes are used for Identity privacy and for
communicating the next re-authentication identity. The plaintext of
the AT_ENCR_DATA value field consists of nested attributes, which
are shown below. Later versions of this protocol MAY specify
additional attributes to be included within the encrypted data.

AT_NEXT_PSEUDONYM

This attribute

The use of the AT_IDENTITY is optional. defined in Section 4.1. The value
field of this attribute begins with a 2-byte actual pseudonym identity length,
which specifies the length of the pseudonym identity in bytes. This field is
followed by a
pseudonym user name, the subscriber identity of the indicated actual length, that the
client can use in the next authentication, as described in
Section 4.3. length.
The user name does not include any terminating null
characters. Because the length of identity is the attribute must be permanent identity, a
multiple of 4 bytes, the sender pads the pseudonym with zero
bytes when necessary.

AT_NEXT_REAUTH_ID identity or a
re-authentication identity. The AT_NEXT_REAUTH_ID attribute identity format is optional to include. specified in
Section 4.1.1. The value
field of this attribute begins with a 2-byte actual re-
authentication same identity length, which specifies format is used in the length of AT_IDENTITY
attribute and the
re-authentication identity in bytes. This field is followed by a
re-authentication identity, of EAP-Response/Identity packet, with the indicated actual length, exception
that

EAP AKA Authentication June 2003 the client can use in peer MUST NOT decorate the next re-authentication, as described in
Section 5. The re-authentication identity it includes both a
username portion and a realm name portion. in
AT_IDENTITY. The re-authentication identity does not include any terminating null
characters. Because the length of the attribute must be a multiple
of 4 bytes, the sender pads the re-authentication identity with zero bytes when
necessary.

AT_PADDING

AT_PADDING is optional to include. See Section 7.3.

8.2. EAP-Response/AKA-Challenge

7.9. AT_RAND

The format of the EAP-Response/AKA-Challenge packet AT_RAND attribute is shown below.

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. AT_MAC, AT_ENCR_DATA and AT_IV attributes are not shown
in the figure below. If present, they are processed as in EAP-
Request/AKA-Challenge packet. The EAP server MUST process EAP-
Response/AKA-Challenge messages that include these attributes even
if the server did not implement these optional attributes.

EAP AKA Authentication June 2003

The semantics of the fields is described below:

Code

2 for Response

Identifier

See [5]

Length

The length of the EAP Response packet.

Type

Subtype

1 for AKA-Challenge

Reserved

Set to zero when sending, ignored on reception.

AT_RES

This attribute MUST be included in EAP-Response/AKA-Challenge.

The value field of this attribute begins with the 2-byte RES
Length, which is identifies the exact length of the RES in bits.
The RES length is contains two reserved bytes
followed by the UMTS AKA RES parameter.
According to the specification [13] the length of the AKA RES can
vary between 32 and 128 bits. Because the length of the AT_RES
attribute must be a multiple of 4 bytes, the sender pads the RES
with zero bits where necessary.

AT_CHECKCODE RAND parameter, 16 bytes (128 bits). The AT_CHECKCODE attribute is optional
reserved bytes are set to include. See section
7.2

AT_MAC

AT_MAC MUST be included. In EAP-Response/AKA-Challenge, there is
no message-specific data covered by the MAC. See Section 7.1.

8.3. EAP-Response/AKA-Authentication-Reject zero when sending and ignored on
reception.

EAP AKA Authentication 27 October, 2003

7.10. AT_AUTN

The format of the EAP-Response/AKA-Authentication-Reject packet AT_AUTN attribute is shown below.

EAP AKA Authentication June 2003

The semantics value field of this attribute contains two reserved bytes
followed by the fields is described below:

Code

2 for Response

Identifier

See [5]

Length AKA AUTN parameter, 16 bytes (128 bits). The length of the EAP Response packet.

Type

Subtype

2 for AKA-Authentication-Reject

Reserved

Set
reserved bytes are set to zero on sending, when sending and ignored on
reception.

8.4. EAP-Response/AKA-Synchronization-Failure

7.11. AT_RES

The format of the EAP-Response/AKA-Synchronization-Failure packet AT_RES attribute is 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 AT_RES | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
| AT_AUTS | RES Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| RES |
| AUTS |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

EAP AKA Authentication June 2003

The semantics value field of this attribute begins with the fields 2-byte RES Length,
which is described below:

Code

2 for Response

Identifier

See [5]

Length

The identifies the exact length of the EAP Response packet, 20.

Type

Subtype

4 for AKA-Synchronization-Failure

AT_AUTS

This attribute MUST be included RES in EAP-Response/AKA-
Synchronization-Failure. bits. The value field RES
length is followed by the UMTS AKA RES parameter. According to [TS
33.105] the length of this attribute
contains the AKA AUTS parameter, 112 RES can vary between 32 and 128 bits.
Because the length of the AT_RES attribute must be a multiple of 4
bytes, the sender pads the RES with zero bits (14 bytes).

8.5. EAP-Request/AKA-Identity where necessary.

7.12. AT_AUTS

The format of the EAP-Request/AKA-Identity packet AT_AUTS attribute is shown below.

EAP AKA Authentication 27 October, 2003

The semantics of the fields is described below:

Code

1 for Request

EAP AKA Authentication June 2003

Identifier

See [5]

Length

The length of the EAP Request packet.

Type

Subtype

5 for AKA-Identity

Reserved

Set to zero on sending, ignored on reception.

AT_PERMANENT_ID_REQ

The AT_PERMANENT_ID_REQ attribute is optional to include and it
is included in the cases defined in Section 4.3. It MUST NOT be
included if AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ is included. The value field only contains two reserved bytes, which are set to
zero on sending and ignored on reception.

AT_FULLAUTH_ID_REQ

The AT_FULLAUTH_ID_REQ of this attribute is optional to include and it is
included in the cases defined in Section 4.2. It MUST NOT be
included if AT_ANY_ID_REQ or AT_PERMANENT_ID_REQ is included. The
value field only contains two reserved bytes, which are set to
zero on sending and ignored on reception.

AT_ANY_ID_REQ

The AT_ANY_ID_REQ attribute is optional and it is included in the
cases defined in Section 4.2. It MUST NOT be included if
AT_PERMANENT_ID_REQ or AT_FULLAUTH_ID_REQ is included. The value
field only contains two reserved bytes, which are set to zero on
sending and ignored on reception.

8.6. EAP-Response/AKA-Identity AKA AUTS parameter,
112 bits (14 bytes).

7.13. AT_NEXT_PSEUDONYM

The format of the EAP-Response/AKA-Identity packet AT_NEXT_PSEUDONYM attribute is shown below.

EAP AKA Authentication June 2003

The semantics of the fields is described below:

Code

2 for Response

Identifier

See [5]

Length

The length of the EAP Response packet.

Type

Subtype

5 for AKA-Identity

Reserved

Set to zero on sending, ignored on reception.

AT_IDENTITY

The AT_IDENTITY attribute is optional to include and it is
included in cases defined in Section 4.2 and 4.3. The value field of this attribute begins with 2-byte actual identity length,
pseudonym length which specifies the length of the identity following
pseudonym in bytes. This field is followed by the subscriber identity of the indicated actual
length, in the same Network Access Identifier format a pseudonym username
that is used the peer can use in EAP-Response/Identity, i.e. including the NAI next authentication. The username MUST
NOT include any realm portion. The identity is the permanent identity, a pseudonym identity or a
re-authentication identity. The identity format is specified in
Section 4.1. The identity username does not include any
terminating null characters. Because the length of the attribute
must be a

EAP AKA Authentication June 2003 multiple of 4 bytes, the sender pads the identity pseudonym with
zero bytes when necessary.

8.7. EAP-Request/AKA-Reauthentication The username encoding MUST follow the
UTF-8 transformation format [RFC2279].

7.14. AT_NEXT_REAUTH_ID

The format of the EAP-Request/AKA-Reauthentication packet AT_NEXT_REAUTH_ID attribute is shown below.

Code

1 for Request

Identifier

See [5].

EAP AKA Authentication June 27 October, 2003

Length

The value field of this attribute begins with 2-byte actual re-
authentication identity length which specifies the length of the EAP packet.

Type

Subtype

Reserved

Set to zero when sending, ignored on reception.

AT_IV

The AT_IV attribute
following re-authentication identity in bytes. This field is MUST be included. See
followed by a re-authentication identity that the peer can use in
the next re-authentication, as described in Section 7.3.

AT_ENCR_DATA 4.2. In
environments where a realm portion is required, the re-
authentication identity includes both a username portion and a realm
name portion. The AT_ENCR_DATA re-authentication identity does not include any
terminating null characters. Because the length of the attribute MUST
must be included. See Section 7.3. The
plaintext consists a multiple of nested attributes as described below.

AT_CHECKCODE 4 bytes, the sender pads the re-authentication
identity with zero bytes when necessary. The AT_CHECKCODE attribute is optional to include. See section
7.2

AT_MAC

AT_MAC identity encoding MUST be included. No message-specific data is included in
follow the MAC calculation. See Section 7.1.

The AT_IV and AT_ENCR_DATA attributes are used for communicating
encrypted attributes. UTF-8 transformation format [RFC2279].

7.15. AT_COUNTER

The plaintext format of the AT_ENCR_DATA value field
consists of nested attributes, which are AT_COUNTER attribute is shown below.

EAP AKA Authentication June 2003

AT_COUNTER

The AT_COUNTER attribute MUST be included.

The value field of the AT_COUNTER attribute consists of a 16-bit
unsigned integer counter value, represented in network byte order.

AT_NONCE_S

The AT_NONCE_S attribute MUST be included. The value field
contains two reserved bytes followed by a random number generated
by the server (16 bytes) freshly for this EAP/AKA re-
authentication. The random number is used as challenge for the
client and also a seed value for the new keying material. The
reserved bytes are set to zero upon sending and ignored upon
reception.

AT_NEXT_REAUTH_ID

The AT_NEXT_REAUTH_ID attribute is optional to include. The
attribute is described in Section 8.1.

AT_PADDING

The AT_PADDING attribute is optional to include. See section 7.3

8.8. EAP-Response/AKA-Reauthentication

EAP AKA Authentication June 2003

7.16. AT_COUNTER_TOO_SMALL

The format of the EAP-Response/AKA-Reauthentication packet AT_COUNTER_TOO_SMALL attribute is shown below.

Code

2 for Response

Identifier

See [5].

Length

The length value field of the EAP packet.

EAP AKA Authentication June 2003

Type

Subtype

Reserved

Set to zero when sending, ignored on reception.

AT_IV

The AT_IV attribute is MUST be included. See Section 7.3.

AT_ENCR_DATA

The AT_ENCR_DATA this attribute MUST be included. See Section 7.3. The
plaintext consists of nested attributes as described below.

AT_CHECKCODE

The AT_CHECKCODE attribute is optional two reserved bytes,
which are set to include. See section
7.2

AT_MAC

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 7.1. It is
followed by the 16-byte NONCE_S value from the server's
AT_NONCE_S attribute.

The AT_IV zero upon sending and AT_ENCR_DATA attributes are used for communicating
encrypted attributes. ignored upon reception.

7.17. AT_NONCE_S

The plaintext format of the AT_ENCR_DATA value field
consists of nested attributes, which are AT_NONCE_S attribute is shown below.

EAP AKA Authentication 27 October, 2003

EAP AKA Authentication June 2003

AT_COUNTER

The AT_COUNTER attribute MUST be included.

The format value field of this
attribute is specified in Section 8.7.

AT_COUNTER_TOO_SMALL

The AT_COUNTER_TOO_SMALL attribute is optional to include, and it
is included in cases specified in Section 5.

AT_PADDING

The AT_PADDING the AT_NONCE_S attribute is optional to include. See section 7.3

8.9. EAP/AKA Notifications

The EAP-Request/Notification, specified in [5], can be used to
convey contains two reserved
bytes followed by a displayable message from the authenticator to random number generated by the client.
Because these messages are textual messages, it may be hard server (16 bytes)
freshly for the
client to present them in the user's preferred language. Therefore,
EAP/AKA uses a separate this 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 client. re-authentication. The client MAY show a notification message to random number is
used as challenge for the user peer and the client MUST respond to the EAP server with an EAP-
Response/AKA-Notification packet, even if the client did not
recognize the notification code.

The notification code is also a 16-bit number. The most significant bit
is called the Failure bit (F bit). The F bit specifies whether seed value for the
notification implies failure. new
keying material. The code values with the F bit reserved bytes are set to zero (code values 0...32767) are used on unsuccessful cases. upon sending and
ignored upon reception.

The
receipt of a notification code from this range implies failed
authentication, so the client can use the notification as a failure
indication. After receiving the EAP-Response/AKA-Notification for
these notification codes, the server MUST send choose the EAP-Failure
packet. NONCE_S value freshly for each EAP/AKA
re-authentication exchange. The receipt of a notification code with the F bit set to one (values
32768...65536) does not imply failure, so the client MUST NOT change
its state when it receives such server SHOULD use a notification.

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 re-
autentication. For these notifications, the AT_MAC attribute MUST be
included in both EAP-Request/AKA-Notification and EAP-Response/AKA-
Notification.

EAP AKA Authentication June 2003

If the P bit of the notification code 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.

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 good source of EAP/AKA because there
are currently no other ways to convey this information
randomness to the user
in a localizable way, and the generate NONCE_S. Please see [RFC 1750] for more
information is potentially useful about generating random numbers for
the user. An EAP/AKA server implementation may decide never to send
these EAP/AKA notifications. security
applications.

7.18. AT_NOTIFICATION

The format of the EAP-Request/AKA-Notification packet AT_NOTIFICATION attribute is shown below.

Code

1 for Request

Identifier

See [5].

Length

The length of the EAP packet.

Type

EAP AKA Authentication June 2003

Subtype

Reserved

Set to zero when sending, ignored on reception.

AT_NOTIFICATION

The AT_NOTIFICATION attribute MUST be included.

The value field of this attribute contains a two-byte notification
code. The first and second bit (F and P) of the notification code
are interpreted as described above. in Section 4.3.

The following notification code values listed below have been reserved. The
descriptions below illustrate the semantics of the notifications.
The client peer implementation MAY use different wordings when presenting
the notifications to the user. The "requested service" depends on
the environment where EAP/AKA is applied.

1026 - User has been temporarily denied access to the requested
service
service. (Implies failure, used after the challenge round)

1031 - User has not subscribed to the requested service (Implies (implies
failure, used after the challenge round)

AT_MAC

AT_MAC is included in cases described above. No message-specific
data is included in the MAC calculation. See Section 7.1.

EAP AKA Authentication 27 October, 2003

7.19. AT_CLIENT_ERROR_CODE

The format of the EAP-Response/AKA-Notification packet AT_CLIENT_ERROR_CODE attribute is shown below. Because this packet is only an acknowledgement of EAP-
Request/AKA-Notification, it does not contain any mandatory
attributes.

EAP AKA Authentication June 2003

Code

2 for Response

Identifier

See [5].

Length

The length of the EAP packet.

Type

Subtype

Reserved

Set to zero when sending, ignored on reception.

AT_MAC

AT_MAC is included in cases described above. No message-specific
data is included in the MAC calculation. See Section 7.1.

9. Error Cases and the Usage of EAP-Failure and EAP-Success

9.1. Processing Erroneous Packets

In general, if an EAP/AKA client or server implementation detects an
error in a received EAP/AKA packet, the EAP/AKA implementation
silently ignores the EAP packet, does not change its state and does
not send any EAP messages to its peer. Examples of such errors,
specified in detail elsewhere in this document, are an invalid
AT_MAC value, a mandatory attribute is missing, illegal attributes
included and an unrecognized non-skippable attribute. If no valid
packets are received, the authentication exchange will eventually
time out.

If the EAP/AKA client receives an EAP/AKA Request value field of an unrecognized
subtype, the EAP/AKA client MUST silently discard the EAP request.

9.2. EAP-Failure

As normally in EAP, the EAP server sends the EAP-Failure packet to
the client 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 user identity, or if the EAP server is not

EAP AKA Authentication June 2003

able to obtain authentication vectors for the subscriber or the
authentication exchange times out.

The server can send EAP-Failure at any time in the EAP exchange. The
client MUST process EAP-Failure.

9.3. EAP-Success

On full authentication, the server can only send EAP-Success after
the EAP/AKA-Challenge round. The client MUST silently discard any
EAP-Success packets if they are received before the client 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 client MUST silently discard any
EAP-Success packets if they are received before the client has
successfully authenticated the server and sent the EAP-Response/AKA-
Reauthentication packet.

If the client receives an EAP/AKA notification (section 8.9) that
indicates failure, then the client MUST no longer accept the EAP-
Success packet even if the server authentication was successfully
completed.

10. Key Derivation

This section specifies how EAP AKA keying material is derived.

On EAP AKA full authentication, a Master Key (MK) is derived from
the underlying UMTS AKA values (IK and CK keys) and the Identity as
follows.

MK = SHA1(Identity|IK|CK)

The hash function SHA1 is specified in [10]. In the formula above,
the "|" character denotes concatenation. Identity denotes the user
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 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 will be used for
protecting EAP packets, but a new MSK and a new EMSK will be derived
from the original MK and new values exchanged in the re-
authentication.

EAP AKA requires two TEKs for its own purposes, a message
authentication key K_aut and an encryption key K_encr, to be used

EAP AKA Authentication June 2003

with the AT_MAC and AT_ENCR_DATA attributes. The same K_aut and
K_encr keys are used in full authentication and subsequent re-
authentications.

Key derivation is based on the pseudo-random number generator
specified in NIST Federal Information Processing Standards
Publication 186-2 [14]. The pseudo-random number generator is
specified in the change notice 1 (2001 October 5)of [14] (Algorithm
1). As specified in the change notice (page 74), when Algorithm 1 is
used as contains a general-purpose random number generator, the "mod q" term
in step 3.2 is omitted. The function G used in the algorithm is
constructed via Secure Hash Standard as specified in Appendix 3.3 of
the standard. For convenience, the pseudo-random number algorithm
with the correct modification is cited in Annex A.

160-bit XKEY and XVAL values are used, so b = 160. On full
authentication, the Master Key is used as the initial secret seed
value XKEY

The optional user input values (XSEED_j) in Step 3.1 are set to
zero. two-byte client error
code. 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 error code values have been reserved.

0 "unable to generate a new Master Session Key and process packet": 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 re-authentication
user 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 from 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 Master Session Key and the new Extended Master
Session Key.

The first 32 bytes of the MSK can be used as the Pairwise Master Key
(PMK) for IEEE 802.11i.

EAP AKA Authentication June 2003

When the RADIUS attributes specified in [16] 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 are used.

11. general error code

8. IANA and Protocol Numbering Considerations

The realm name "owlan.org" has been reserved for NAI realm names
generated from the IMSI.

IANA has assigned the number 23 for EAP AKA authentication.

EAP AKA messages include a Subtype field. The following Subtypes are
specified:

AKA-Challenge...................................1
AKA-Authentication-Reject.......................2
AKA-Synchronization-Failure.....................4
AKA-Identity....................................5
AKA-Notification...............................12
AKA-Reauthentication...........................13
AKA-Client-Error...............................14

EAP AKA Authentication 27 October, 2003

The Subtype-specific data is composed of attributes, which have
attribute type numbers. The following attribute types are specified:

AT_RAND.........................................1
AT_AUTN.........................................2
AT_RES..........................................3
AT_AUTS.........................................4
AT_PADDING......................................6
AT_PERMANENT_ID_REQ............................10
AT_MAC.........................................11
AT_NOTIFICATION................................12
AT_ANY_ID_REQ..................................13
AT_IDENTITY....................................14
AT_FULLAUTH_ID_REQ.............................17
AT_COUNTER.....................................19
AT_COUNTER_TOO_SMALL...........................20
AT_NONCE_S.....................................21
AT_CLIENT_ERROR_CODE...........................22

AT_IV.........................................129
AT_ENCR_DATA..................................130
AT_NEXT_PSEUDONYM.............................132
AT_NEXT_REAUTH_ID.............................133
AT_CHECKCODE..................................134

The AT_NOTIFICATION attribute contains a notification code value.
Values 1024, 1026 and 1031 have been specified in Section 7.18 of
this document.

The AT_CLIENT_ERROR_CODE attribute contains a client error code.
Value 0 has been specified in Section 7.19 of this document.

All requests for value assignment from the various number spaces
described in this document require proper documentation, according
to the "Specification Required" policy described in [17]. [RFC 2434].
Requests must be specified in sufficient detail so that
interoperability between independent implementations is possible.
Possible forms of documentation include, but are not limited to,
RFCs, the products of

EAP AKA Authentication June 2003 another standards body (e.g. 3GPP), or
permanently and readily available vendor design notes.

12.

EAP AKA and EAP SIM [EAP SIM] are "sister" protocols with similar
message structure and protocol numbering spaces. Many attributes and
message Subtypes have the same protocol numbers in these two
protocols. Hence, it is recommended that the same protocol number
value SHOULD NOT be allocated for two different purposes in EAP AKA
and EAP SIM.

9. Security Considerations

The revised EAP base protocol [18] specification [EAP] highlights several attacks
that are possible against the EAP protocol. This section discusses

EAP AKA Authentication 27 October, 2003

the claimed security properties of EAP AKA as well as
vulnerabilities and security recommendations.

12.1.

9.1. Identity Protection

EAP/AKA includes optional Identity privacy support that protects the
privacy of the subscriber identity against passive eavesdropping.
The mechanism cannot be used on the first connection exchange with a given
server, when the IMSI will have to be sent in the clear. The
terminal SHOULD store the pseudonym in a non-volatile memory so that
it can be maintained across reboots. An active attacker that
impersonates the network may use the AT_PERMANENT_ID_REQ attribute
(Section 4.3) 1.1) to learn the subscriber's IMSI. However, as discussed
in Section 4.3, 1.1, the terminal can refuse to send the cleartext IMSI
if it believes that the network should be able to recognize the
pseudonym.

If the client peer and server cannot guarantee that the pseudonym will be
maintained reliably and Identity privacy is required then additional
protection from an external security mechanism such as Protected
Extensible Authentication Protocol (PEAP) [19] [PEAP] may be used. The
benefits and the security considerations of using an external
security mechanism with EAP/AKA are beyond the scope of this
document.

12.2.

9.2. Mutual Authentication

EAP/AKA provides mutual authentication via the UMTS AKA mechanisms.

12.3.

9.3. Key Derivation

EAP/AKA supports key derivation with 128-bit effective key strength.
The key hierarchy is specified in Section 10. 0.

The Transient EAP Keys used to protect EAP AKA packets (K_encr,
K_aut) and the Master Session Keys are cryptographically separate.
An attacker cannot derive any non-trivial information from K_encr or
K_aut based on the Master Session Key or vice versa. An attacker
also cannot calculate the pre-shared secret from the UMTS AKA IK,
UMTS AKA CK, EAP AKA K_encr, EAP AKA K_aut or from the Master
Session Key.

12.4.

9.4. Brute-Force and Dictionary Attacks

The effective strength of EAP/AKA values is 128 bits, and there are
no known computationally feasible brute-force attacks. Because UMTS

EAP AKA Authentication June 2003
AKA is not a password protocol (the pre-shared secret must not be a
weak password), EAP/AKA is not vulnerable to dictionary attacks.

12.5.

9.5. Integrity Protection, Replay Protection and Confidentiality

AT_MAC, AT_IV and AT_ENCR_DATA attributes are used to provide
integrity, replay and confidentiality protection for EAP/AKA
Requests and Responses. Integrity protection includes the EAP

EAP AKA Authentication 27 October, 2003

header. Integrity protection (AT_MAC) is based on a keyed message
authentication code. Confidentiality (AT_ENCR_DATA and AT_IV) is
based on a block cipher.

Because keys are not available in the beginning of the EAP methods,
the AT_MAC attribute cannot be used for protecting EAP/AKA-Identity
messages. However, the AT_CHECKCODE attribute can optionally be used
to protect the integrity of the EAP/AKA-Identity roundtrip.

On full authentication, replay protection is provided by RAND and
AUTN values from the underlying UMTS AKA scheme, which makes use of the RAND and AUTN
values. scheme. On re-authentication, re-
authentication, a counter and a server nonce is used to provide
replay protection.
The contents of the EAP-Response/Identity packet are implicitly
integrity protected by including them in key derivation.

Because EAP/AKA is not a tunneling method, EAP Notification, EAP
Success or EAP Failure packets are not confidential, integrity
protected or replay protected. On physically insecure networks, this
may enable an attacker to mount denial of service attacks by sending
false EAP Notification, EAP Success or EAP Failure packets. However,
the attacker cannot force the peers to believe successful
authentication has occurred when mutual authentication failed or has
not happened yet.

An eavesdropper will see the EAP Notification, EAP Success and EAP
Failure packets sent in the clear. With EAP AKA, confidential
information MUST NOT be transmitted in EAP Notification packets.

12.6.

9.6. Negotiation Attacks

EAP/AKA does not protect the EAP-Response/Nak packet. Because
EAP/AKA does not protect the EAP method negotiation, EAP method
downgrading attacks may be possible, especially if the user uses the
same identity with EAP/AKA and other EAP methods.

As described in Section 6, 5, EAP/AKA allows the protocol to be
extended by defining new attribute types. When defining such
attributes, it should be noted that any extra attributes included in
EAP-Request/AKA-Identity or EAP-Response/AKA-Identity packets are
not included in the MACs later on, and thus some other precautions
must be taken to avoid modifications to them.

EAP/AKA does not support ciphersuite negotiation or EAP/AKA protocol
version negotiation.

EAP AKA Authentication June 2003

12.7.

9.7. Fast Reconnect

EAP/AKA includes an optional re-authentication ("fast reconnect")
procedure, as recommended in [18] [EAP] for EAP types that are intended
for physically insecure networks.

12.8.

9.8. Acknowledged Result Indications

EAP AKA Authentication 27 October, 2003

EAP/AKA does not provide acknowledged or integrity protected Success
or Failure indications.

If an EAP Success or an EAP Failure packet is lost when using
EAP/AKA over an unreliable medium, and if the protocol over which
EAP/AKA is transported does not address the possible loss of Success
or Failure, then the peer and authenticator EAP server may end up having a
different interpretation of the state of the authentication
conversation.

On physically insecure networks, an attacker may mount denial of
service attacks by sending false EAP Success or EAP Failure
indications. However, the attacker cannot force the client peer or the
authenticator EAP
server to believe successful authentication has occurred when mutual
authentication failed or has not happened yet.

12.9.

9.9. Man-in-the-middle Attacks

In order to avoid man-in-the-middle attacks and session hijacking,
user data SHOULD be integrity protected on physically insecure
networks. The EAP/AKA Master Session Key or keys derived from it MAY
be used as the integrity protection keys, or, if an external
security mechanism such as PEAP is used, then the link integrity
protection keys MAY be derived by the external security mechanism.

There are man-in-the-middle attacks associated with the use of any
EAP method within a tunneled protocol such as PEAP, or within a
sequence of EAP methods followed by each other. This specification
does not address these attacks. If EAP/AKA is used with a tunneling
protocol or as part of a sequence of methods, there should be
cryptographic binding provided between the protocols and EAP/AKA to
prevent man-in-the-middle attacks through rogue authenticators being
able to setup one-way authenticated tunnels. EAP/AKA Master Session
Key MAY be used to provide the cryptographic binding. However the
mechanism how the binding is provided depends on the tunneling or
sequencing protocol, and it is beyond the scope of this document.

12.10.

9.10. Generating Random Numbers

An EAP/AKA implementation SHOULD use a good source of randomness to
generate the random numbers required in the protocol. Please see
[20]
[RFC 1750] for more information on generating random numbers for
security applications.

13.

10. Security Claims

EAP AKA Authentication June 2003

This section provides the security claims required by [18]. [EAP].

[a] Intended use. EAP AKA is intended for use over both physically
insecure networks and physically or otherwise secure networks.
Applicable media include but are not limited to PPP, IEEE 802 wired
networks and IEEE 802.11.

EAP AKA Authentication 27 October, 2003

[b] Mechanism. EAP AKA is based on the UMTS AKA mechanism, which is
an authentication and key agreement mechanism based on a symmetric
128-bit pre-shared secret.

[c] Security claims. The security properties of the method are
discussed in Section 12. 9.

[d] Key strength. EAP/AKA supports key derivation with 128-bit
effective key strength.

[e] Description of key hierarchy. Please see Section 10. 0.

[f] Indication of vulnerabilities. Vulnerabilities are discussed in
Section 12.

14. 9.

11. Intellectual Property Right Notices

On IPR related issues, Nokia and Ericsson refer to the their
respective statements on patent licensing. Please see
http://www.ietf.org/ietf/IPR/NOKIA and
http://www.ietf.org/ietf/IPR/ERICSSON-General

Acknowledgements and Contributions

The authors wish to thank Rolf Blom of Ericsson, Bernard Aboba of
Microsoft, Arne Norefors of Ericsson, N.Asokan of Nokia, Valtteri
Niemi of Nokia, Kaisa Nyberg of Nokia, Jukka-Pekka Honkanen of
Nokia, Pasi Eronen of Nokia, Olivier Paridaens of Alcatel and Ilkka
Uusitalo of Ericsson for interesting discussions in this problem
space.

The attribute format is based on the extension format of Mobile IPv4
[21].
[RFC 3344].

Authors' Addresses

Jari Arkko
Ericsson
02420 Jorvas Phone: +358 40 5079256
Finland Email: jari.arkko@ericsson.com

Henry Haverinen
Nokia Mobile Phones
P.O. Box 88
33721 Tampere Phone: +358 50 594 4899
Finland E-mail: henry.haverinen@nokia.com

EAP AKA Authentication June 27 October, 2003

Annex A. Pseudo-Random Number Generator

The "|" character denotes concatenation, and "^" denotes involution.

Step 1: Choose a new, secret value for the seed-key, XKEY

Step 2: In hexadecimal notation let
t = 67452301 EFCDAB89 98BADCFE 10325476 C3D2E1F0
This is the initial value for H0|H1|H2|H3|H4
in the FIPS SHS [10] [SHA-1]

Step 3: For j = 0 to m - 1 do
3.1 XSEED_j = 0 /* no optional user input */
3.2 For i = 0 to 1 do
a. XVAL = (XKEY + XSEED_j) mod 2^b
b. w_i = G(t, XVAL)
c. XKEY = (1 + XKEY + w_i) mod 2^b
3.3 x_j = w_0|w_1

EAP AKA Authentication June 27 October, 2003

Normative References

[1]

[TS 33.102] 3GPP Technical Specification 3GPP TS 33.102 V5.1.0:
"Technical Specification Group Services and System Aspects; 3G
Security; Security Architecture (Release 5)", 3rd Generation
Partnership Project, December 2002. (NORMATIVE)

[2] IEEE P802.1X/D11, "Standards for Local Area and Metropolitan
Area Networks: Standard for Port Based Network Access
Control", March 2001. (INFORMATIVE)

[3] IEEE Draft 802.11eS/D1, "Draft Supplement to STANDARD FOR
Telecommunications and Information Exchange between Systems -
LAN/MAN Specific Requirements - Part 11: Wireless Medium
Access Control (MAC) and physical layer (PHY) specifications:
Specification for Enhanced Security", March 2001.
(INFORMATIVE)

[4]

[RFC 2486] Aboba, B. and M. Beadles, "The Network Access
Identifier", RFC 2486, January 1999. (NORMATIVE)

[5]

[EAP] L. Blunk, J. Vollbrecht, "PPP Extensible Blunk et al., "Extensible Authentication Protocol (EAP)", RFC 2284, March 1998. (NORMATIVE)

[6]
draft-ietf-eap-rfc2284bis-05.txt, work-in-progress, September 2003.

[RFC 2119] S. Bradner, "Key words for use in RFCs to indicate
Requirement Levels", RFC 2119, March 1997. (NORMATIVE)

[7]

[TS 23.003] 3GPP Technical Specification 3GPP TS 23.003 V5.5.1: "3rd
Generation Parnership Project; Technical Specification Group Core
Network; Numbering, addressing and identification (Release 5)", 3rd
Generation Parnership Project, January 2003
(NORMATIVE)

[8] Draft 3GPP Technical Specification 3GPP TS 23.234 V 1.4.0:
"Technical Specification Group Services and System Aspects;
3GPP system to Wireless Local Area Network (WLAN)
Interworking; System Description", 3rd Generation Partnership Project, work in progress, January 2003. (INFORMATIVE)

[9] 2003

[RFC 2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC2104, February 1997. (NORMATIVE)

[10]

[SHA-1] Federal Information Processing Standard (FIPS) Publication
180-1, "Secure Hash Standard," National Institute of Standards and
Technology, U.S. Department of Commerce, April 17, 1995.
(NORMATIVE)

[11]

[AES] Federal Information Processing Standard Standards (FIPS) draft standard, Publication
197, "Advanced Encryption Standard (AES)",

EAP AKA Authentication June 2003

http://csrc.nist.gov/publications/drafts/dfips-AES.pdf,
September 2001. (NORMATIVE)

[12] US National Bureau Institute of Standards, "DES
Standards and Technology, November 26, 2001.
http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf

[CBC] NIST Special Publication 800-38A, "Recommendation for Block
Cipher Modes of Operation",
Federal Information Processing Standard (FIPS) Publication 81, Operation - Methods and Techniques", National
Institute of Standards and Technology, December 1980. (NORMATIVE)

[13] 2001.
http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf

[TS 33.105] 3GPP Technical Specification 3GPP TS 33.105 4.1.0:
"Technical Specification Group Services and System Aspects; 3G
Security; Cryptographic Algorithm Requirements (Release 4)", 3rd
Generation Partnership Project, June 2001 (NORMATIVE)

[14]

[PRF] Federal Information Processing Standards (FIPS) Publication
186-2 (with change notice), "Digital Signature Standard (DSS)",
National Institute of Standards and Technology, January 27, 2000, (NORMATIVE) 2000
Available on-line at:
http://csrc.nist.gov/publications/fips/fips186-2/
fips186-2-change1.pdf

[15] B. Aboba, D. Simon, "PPP
http://csrc.nist.gov/publications/fips/fips186-2/fips186-2-
change1.pdf

EAP TLS AKA Authentication Protocol", RFC
2716, October 1999 (INFORMATIVE)

[16] G. Zorn, "Microsoft Vendor-specific RADIUS Attributes", RFC
2548, March 1999 (INFORMATIVE)

[17] 27 October, 2003

[RFC 2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
(NORMATIVE)

[18] L. Blunk, J. Vollbrecht, B. Aboba, "Extensible Authentication
Protocol (EAP)", draft-ietf-pppext-rfc2284bis-07.txt, work-in-
progress, October 2002. (NORMATIVE)

[19]

Informative References

[RFC 2548] G. Zorn, "Microsoft Vendor-specific RADIUS Attributes",
RFC 2548, March 1999

[PEAP] H. Andersson, S. Josefsson, G. Zorn, D. Simon, A. Palekar,
"Protected EAP Protocol (PEAP)", draft-josefsson-pppext-eap-
tls-eap-05.txt, draft-josefsson-pppext-eap-tls-eap-
05.txt, work-in-progress, September 2002.
(IMFORMATIVE)

[20]

[RFC 1750] D. Eastlake, 3rd, S. Crocker, J. Schiller, "Randomness
Recommendations for Security", RFC 1750 (Informational), December
1994. (INFORMATIVE)

[21]

[RFC 3344] C. Perkins (editor), "IP Mobility Support", RFC 3344,
August 2002. (INFORMATIVE)

[EAP SIM] H. Haverinen, J. Salowey, "EAP SIM Authentication", draft-
haverinen-pppext-eap-sim-12.txt, October 2003, work in progress

[TS 23.234] Draft 3GPP Technical Specification 3GPP TS 23.234 V
1.4.0: "Technical Specification Group Services and System Aspects;
3GPP system to Wireless Local Area Network (WLAN) Interworking;
System Description", 3rd Generation Partnership Project, work in
progress, January 2003.