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draft-arkko-pppext-eap-aka-01.txt --> draft-arkko-pppext-eap-aka-03.txt

J. Arkko
Internet Draft Ericsson
Document: draft-arkko-pppext-eap-aka-01.txt draft-arkko-pppext-eap-aka-03.txt H. Haverinen
Expires: December 2001 August 2002 Nokia
November 2001
February 2002

EAP AKA Authentication

Status of this Memo

This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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reference material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
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Abstract

This document specifies an Extensible Authentication Protocol (EAP)
mechanism for authentication and session key distribution using the
UMTS AKA authentication mechanism. AKA is based on symmetric keys,
and runs typically in a UMTS Subscriber Identity Module, a smart
card like device. AKA provides also backward compatibility to GSM
authentication, making it possible to use EAP AKA for authenticating
both GSM and UMTS subscribers.

Table of Contents

Status of this Memo................................................1
Abstract...........................................................1
1. Introduction and Motivation.....................................2
2. Conventions used in this document...............................3
3. Protocol Overview...............................................5
4. IMSI Identity Privacy Support...........................................10 Support.......................................10
5. Message Format.................................................12

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6. Messages.......................................................13 Message Integrity and Privacy Protection.......................13
6.1. EAP-Response/Identity........................................13 AT_MAC Attribute.............................................13
6.2. EAP-Request/USIM-Challenge...................................14
6.3. EAP-Response/USIM-Challenge..................................18
6.4. EAP-Response/USIM-Authentication-Reject......................20
6.5. EAP-Response/USIM-Synchronization-Failure....................20
6.6. EAP-Request/USIM-IMSI........................................21
6.7. EAP-Response/USIM-IMSI.......................................22 AT_IV and AT_ENCR_DATA Attributes............................14
7. Messages.......................................................15
7.1. EAP-Response/Identity........................................15
7.2. EAP-Request/AKA-Challenge....................................16
7.3. EAP-Response/AKA-Challenge...................................19
7.4. EAP-Response/AKA-Authentication-Reject.......................21
7.5. EAP-Response/AKA-Synchronization-Failure.....................21
7.6. EAP-Request/AKA-Cleartext-Identity...........................22
7.7. EAP-Response/AKA-Cleartext-Identity..........................23
8. Interoperability with GSM......................................23
8. GSM......................................24
9. IANA and Protocol Numbering Considerations.....................24
9. Security Considerations........................................24 Considerations.....................25
10. Security Considerations.......................................25
11. Intellectual Property Right Notices...........................24
Acknowledgements..................................................25 Notices...........................26
Acknowledgements..................................................26
Authors' Addresses................................................25 Addresses................................................26

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]. The Universal Mobile
Telecommunications System (UMTS) is a global third generation mobile
network standard.

AKA is based on challenge-response mechanisms and symmetric
cryptography. AKA typically runs in a UMTS Subscriber Identity
Module (USIM), a smart card like device. However, the applicability
of AKA is not limited to client devices with smart cards, but the
AKA mechanisms could also be implemented in host software, for
example
example. AKA also provides backward compatibility to the GSM
authentication mechanism [2]. Compared to the GSM mechanism, AKA
provides substantially longer key lengths and the authentication of
the server side as well as the client side.

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 801.1x
technology through EAP over Wireless [3, 4].

- 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:

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- The USIM and the home environment have agreed on a secret key
beforehand.

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- 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
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].

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

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) [5].

2. Conventions used in this document

The following terms will be used through this document:

AAA protocol

Authentication, Authorization and Accounting protocol

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AAA protocol

Authentication, Authorization and Accounting protocol

AAA server

In this document,

The AAA server refers to the network element that
resides on is responsible for storing shared secrets and
other credential information necessary for the border authentication of Internet AAA network and GSM network.
users. Cf. EAP server

AKA

Authentication and Key Agreement

AuC

Authentication Centre. The mobile network element that can
authorize 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 [6].

EAP server

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

GSM

Global System for Mobile communications.

NAI

Network Access Identifier [5].

AUTN

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

AUTS

A value generated by the client upon experiencing a
synchronization failure, 112 bits.

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RAND

Random number generated by the AuC, 128 bits [1].

RES

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

SQN

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

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SIM

Subscriber Identity Module. SIM cards are 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. These cards are smart cards
Similar to SIMs and are distributed by UMTS operators.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119 [7]

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. Typically, the authenticator does not contain Alternatively, the EAP server functionality, but functionality may be
co-located in the authenticator although typically, the the EAP
server functionality is implemented on a separate AAA server with
whom the authenticator communicates using an AAA protocol. (The
exact AAA communications is are outside the scope of this document,
however.)

The below message flow shows the basic successful authentication
case with the EAP AKA. The 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. (For this
particular EAP protocol, (As
specified in [6], the initial identity request is defined to not required, and
MAY be
optional, to shorten bypassed in cases where the authentication process to a minimal one.) authenticator can presume the
identity, such as when using leased lines, dedicated dial-ups, etc.)

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Next, the EAP server starts the actual AKA protocol by sending an
EAP-Request/USIM-Challenge
EAP-Request/AKA-Challenge message. This message contains a random
number (RAND) and an authorization vector (AUTN). The EAP-
Request/USIM-Challenge
Request/AKA-Challenge message MAY optionally contain encrypted data,
which is used for IMSI privacy support, as described in Section 4.
The encrypted data is not shown in the figures of this section. The
client runs the AKA algorithm (perhaps inside an USIM) and verifies
the AUTN. If this is successful, the client is talking to a
legitimate EAP server and proceeds to send the EAP-
Response/USIM-Challenge. EAP-Response/AKA-
Challenge. This message contains a result parameter that allows the
EAP server in turn to verify that the client is a legitimate one.

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Client Authenticator
| |
| EAP-Request/Identity (optional) |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and AUTN. |
| +------------------------------+
| |
| EAP-Request/USIM-Challenge EAP-Request/AKA-Challenge |
| (RAND, AUTN) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM,| |
| verifies AUTN, derives RES | |
| and session key | |
+-------------------------------------+ |
| |
| EAP-Response/USIM-Challenge EAP-Response/AKA-Challenge |
| (RES) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server checks the given RES, |
| | and finds it correct. |
| +------------------------------+
| |
| EAP-Success |
|<------------------------------------------------------|

When EAP AKA is run in the GSM compatible mode, the message flow is
otherwise identical to the message flow below except that the AUTN
attribute is not included in EAP-Request/USIM-Challenge EAP-Request/AKA-Challenge packet.

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The second message flow shows how the EAP server rejects the Client
due to failed authentication. The same flow is also used in the GSM
compatible mode, except that the AUTN parameter is not included in
the EAP-Request/USIM-Challenge EAP-Request/AKA-Challenge packet.

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Client Authenticator
| |
| EAP-Request/Identity (optional) |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and AUTN. |
| +------------------------------+
| |
| EAP-Request/USIM-Challenge EAP-Request/AKA-Challenge |
| (RAND, AUTN) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM,| |
| possibly verifies AUTN, and sends an| |
| invalid response | |
+-------------------------------------+ |
| |
| EAP-Response/USIM-Challenge EAP-Response/AKA-Challenge |
| (RES) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server checks the given RES, |
| | and finds it incorrect. |
| +------------------------------+
| |
| EAP-Failure |
|<------------------------------------------------------|

The next message flow shows the client rejecting the AUTN of the EAP
server. This flow is not used in the GSM compatible mode.

The client sends an explicit error message (EAP-Response/AKA-
Authentication-Reject) to the Authenticator, 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, when the client
silently discards the EAP/AKA message.

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Client Authenticator
| |
| EAP-Request/Identity (optional) |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and a bad AUTN|
| +------------------------------+
| |
| EAP-Request/USIM-Challenge EAP-Request/AKA-Challenge |
| (RAND, AUTN) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM | |
| and discovers AUTN that can not be | |
| verified | |
+-------------------------------------+ |
| |
| EAP-Response/USIM-Authentication-Reject EAP-Response/AKA-Authentication-Reject |
|------------------------------------------------------>|
| |
| |
| EAP-Failure |
|<------------------------------------------------------|

Networks that are not UMTS aware use the GSM compatible version of
this protocol even for UMTS subscribers. In this case, the AUTN
parameter is not included in the EAP-Request/USIM-Challenge EAP-Request/AKA-Challenge packet.
If a UMTS capable client does not want to accept the use of the GSM
compatible mode, the client can reject the authentication with the
EAP-Response/Nak message [6], as shown in the following figure:

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Client Authenticator
| |
| EAP-Request/Identity (optional) |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs GSM algorithms, |
| | generates RAND |
| +------------------------------+
| |
| EAP-Request/USIM-Challenge EAP-Request/AKA-Challenge |
| (RAND) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client does not accept the GSM | |
| compatible version of this protocol.| |
+-------------------------------------+ |
| |
| EAP-Response/Nak |
|------------------------------------------------------>|
| |
| |
| EAP-Failure |
|<------------------------------------------------------|

The AKA uses shared secrets between the Client and the Client'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 Here�s what happens then:

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Client Authenticator
| |
| EAP-Request/Identity (optional) |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and AUTN. |
| +------------------------------+
| |
| EAP-Request/USIM-Challenge EAP-Request/AKA-Challenge |
| (RAND, AUTN) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM | |
| and discovers AUTN that contains an | |
| inappropriate sequence number | |
+-------------------------------------+ |
| |
| EAP-Response/USIM-Synchronization-Failure EAP-Response/AKA-Synchronization-Failure |
| (AUTS) |
|------------------------------------------------------>|
| |
| +---------------------------+
| | Perform resynchronization |
| | towards the AAA using Using AUTS and |
| | AUTS and the sent RAND |
| +---------------------------+
| |

After the resynchronization process takes place in the server and
AAA side, the process continues by the server side sending a new
EAP-Request/USIM-Challenge
EAP-Request/AKA-Challenge message.

4. IMSI Identity Privacy Support

In the very first connection to an EAP server, the client always
transmits the cleartext IMSI identity (IMSI) in the EAP-Response/Identity
packet. In subsequent connections, the optional IMSI identity privacy
support can be used to hide the IMSI identity and to make the connections
unlinkable to a passive eavesdropper.

The EAP-Request/USIM-Challenge 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 6.2. 7.2. Because the IMSI identity
privacy support is optional to implement, the client MAY ignore the
AT_IV, AT_ENCR_DATA, and AT_MAC attributes and always transmit the IMSI
cleartext identity in
the EAP-Response/Identity packet.

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the EAP-Response/Identity packet.

On receipt of the EAP-Request/USIM-Challenge, EAP-Request/AKA-Challenge, the client verifies the
AT_AUTN attribute before looking at the AT_ENCR_DATA or AT_MAC
attributes. If the AUTN is invalid, then the client MUST ignore the
AT_IV, AT_ENCR_DATA and AT_MAC attributes. If AUTN is valid, then
the client MAY derive the K_encr and K_int keys as described in
Section 6.2 7.2 and verify the AT_MAC attribute. If the AT_MAC attribute
is valid, then the client MAY decrypt the encrypted data and use the
pseudonym in the next authentication. If the MAC is invalid, then
the encrypted data MUST be ignored and the whole EAP packet MAY be
silently ignored.

The EAP server produces pseudonyms in an implementation-dependent
manner. Please see [8] for examples on how to produce pseudonyms.
The pseudonyms need
Only the EAP server needs to be reversible able to map the IMSI only on pseudonym to the EAP
server.
cleartext identity. Regardless of construction method, the pseudonym
MUST conform to the grammar specified for the username portion of an
NAI.

On the next connection to the EAP server, the client MAY transmit
the received pseudonym in the first EAP-Response/Identity packet.
The client concatenates the received pseudonym with the "@"
character and the NAI realm portion. The client MUST use the same
realm portion that it used in the connection when it received the
pseudonym.

If the EAP server fails to decode the pseudonym to a known client
name, then the EAP server requests the regular IMSI (non-pseudonym cleartext identity (non-
pseudonym identity) by issuing the EAP-Request/USIM-IMSI EAP-Request/AKA-Cleartext-
Identity packet to the client. This packet includes no attributes.
The client responds with the
EAP-Response/USIM-IMSI, EAP-Response/AKA-Cleartext-Identity,
which includes the client's IMSI identity in the clear. Please note that
the EAP/AKA client and the EAP/AKA server only process the AKA-
Cleartext-Identity packets and entities that only pass through EAP
packets do not process these packets. 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 with the original pseudonym
identity from the EAP-Response/Identity packet regardless if the
decoding fails in the EAP server. This case is illustrated in the
figure below.

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Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a pseudonym) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server fails to decode the |
| | Pseudonym. |
| +------------------------------+
| |
| EAP-Request/USIM-IMSI EAP-Request/AKA-Cleartext-Identity |
|<------------------------------------------------------|
| |
| |
| EAP-Response/USIM-IMSI EAP-Response/AKA-Cleartext-Identity |
| (IMSI) (Cleartext identity) |
|------------------------------------------------------>|
| |

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After receiving the EAP-Response/USIM-IMSI EAP-Response/AKA-Cleartext-Identity packet, the
EAP server issues the EAP-Request/USIM-Challenge EAP-Request/AKA-Challenge and the
authentication proceeds as usual.

Because the keys that are used to protect the pseudonym are derived
from the AKA cipher key (CK) and the AKA integrity key (IK), the
IMSI
identity privacy support is not available when EAP AKA is used in
the GSM compatible mode.

5. Message Format

The Type-Data of the EAP AKA packets begins with a 1-octet Subtype
field, which is followed by a 2-octet reserved field. The rest of
the Type-Data consists of attributes that are encoded in Type,
Length, Value format. The figure below shows the generic format of
an attribute.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Attribute Type | Length | Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Attribute Type

Indicates the particular type of attribute. The attribute type
values are listed in Section 8. 9.

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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 may be two or more bytes in length. The
type and length fields determine the format and length of the
value field.

When an attribute numbered within the range 0 through 127 is
encountered but not recognized, the EAP/USIM EAP/AKA message containing that
attribute MUST be silently discarded. These attributes are called
non-skippable attributes.

When an attribute numbered in the range 128 through 255 is
encountered but not recognized that particular attribute is ignored,
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 in searching for the next attribute. These
attributes are called skippable attributes.

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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.

6. Messages

6.1. EAP-Response/Identity

In the beginning of EAP authentication, the Message Integrity and Privacy Protection

This section specifies EAP/AKA attributes for attribute encryption
and EAP/AKA message integrity protection.

Because the CK and IK keys derived from the RAND challenge are
required to process the integrity protection and encryption
attributes, these attributes can only be used in the EAP-
Request/AKA-Challenge message and any EAP/AKA messages sent after
EAP-Requets/AKA-Challenge. For example, these attributes cannot be
used in EAP-Request/AKA-Cleartext-Identity.

6.1. AT_MAC Attribute

The AT_MAC attribute can optionally be used for EAP/AKA message
integrity protection. Whenever AT_ENCR_DATA (Section 6.2) is
included in an EAP message, it MUST be followed (not necessarily
immediately) by an AT_MAC attribute. Messages that do not meet this
condition MUST be silently discarded.

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The value field of the AT_MAC attribute contains two reserved bytes
followed by a message authentication code (MAC). The MAC is
calculated over the whole EAP packet with the exception that the
value field of the MAC attribute is set to zero when calculating the
MAC. The reserved bytes are set to zero when sending and ignored on
reception. The format of the AT_MAC 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_MAC | Length = 6 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The MAC algorithm is HMAC-SHA1 [9] keyed hash value, so the length
of the MAC is 20 bytes. The integrity protection key (K_int) used in
the calculation of the MAC is derived from the AKA integrity key
(IK) with the following formula. The notation A|0 denotes A
concatenated with the byte zero 0x00.

K_int = SHA1(IK|0)

6.2. AT_IV and AT_ENCR_DATA 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 when sending and
ignored on reception. The AT_IV 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 format of AT_IV 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_IV | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Initialization Vector |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The value field of the AT_ENCR_DATA attribute consists of two
reserved bytes followed by bytes encrypted using the Advanced
Encryption Standard (AES) [10] in the Cipher Block Chaining (CBC)
mode of operation, using the initialization vector from the AT_IV

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attribute. The reserved bytes are set to zero when sending and
ignored on reception. Please see [11] 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 encryption key (K_encr) is derived from the AKA Cipher Key (CK)
with the following formula. The result of the SHA-1 hash value [12]
is truncated to 128 bits by ignoring the 32 rightmost bits. The
notation A|0 denotes A concatenated with the byte zero 0x00.

K_encr = 128 leftmost bits of SHA1(CK|0)

The plaintext consists of nested EAP/AKA attributes.

7. Messages

7.1. EAP-Response/Identity

In the beginning of EAP authentication, the Authenticator issues the
EAP-Request/Identity packet to the client. The client responds with
EAP-Response/Identity, which contains the user's identity. The
formats of these packets are specified in [6].

The EAP AKA mechanism uses the NAI format [5] as the identity.
In order to facilitate the use of the existing cellular roaming
infrastructure, the subscriber's IMSI is used as the client
identifier. When IMSI identity privacy is not used, the EAP AKA client
transmits the user's IMSI within the NAI in the EAP
Response/Identity packet. The NAI is of the format "0imsi@realm". In
other words, the first character is the digit zero (ASCII value
0x30), followed by the IMSI, followed by the @ character and the
realm. The IMSI is an ASCII string that consists of not more than 15
decimal digits (ASCII values between 0x30 and 0x39) as specified in
[9].
[13].

When the optional IMSI identity privacy support is used, the client MAY
use the pseudonym received as part of the previous authentication
sequence as the user name portion of the NAI, as specified in
Section 4.

The AAA network routes AAA requests to the correct AAA server using
the realm part of the NAI. The realm part MAY be a configurable

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parameter in the EAP/AKA client implementation. In this case, the
client is typically configured with the NAI realm of the home
operator. Other ways to obtain the realm may later be specified but
they are not in the scope of this document.

Because cellular roaming can be used with EAP AKA, the AAA request
can be routed to an AAA server in the visited network instead of the
server indicated in the NAI realm. The operators need to agree on
this special AAA routing in advance. It is recommended that
operators should reserve the realm portion of NAI for EAP AKA users
exclusively, so that exactly the same realm is not used with other
authentication methods. This convention makes it easy to recognize
that the NAI identifies a UMTS or GSM subscriber of this operator,
which may be useful when configuring the routing rules in the
visited AAA networks.

In the EAP AKA protocol, the EAP-Request/Identity message is
optional when applicable. If the client can positively determine
that it has to authenticate, it MAY send an unsolicited EAP-
Response/Identity to the authenticator with an EAP Identifier value
it has picked up itself. The client MUST NOT send an unsolicited
EAP-Response/Identity if it has already received an EAP-
Request/Identity packet. The client MUST send an EAP-
Response/Identity to all received EAP-Request/Identity packets,

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using the Identifier value in the EAP-Request/Identity. If the
authenticator receives an unsolicited EAP-Response/Identity, it
SHOULD process the packet as if it had requested it. If the
authenticator receives an EAP-Response/Identity with an incorrect
Identifier value in response to the first EAP-Request/Identity it
has sent to the client, then the authenticator SHOULD still accept
the EAP-Response/Identity packet.

6.2. EAP-Request/USIM-Challenge

7.2. EAP-Request/AKA-Challenge

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

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The semantics of the fields is described below:

Code

1 for Request

Identifier

See [6]

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Length

The length of the 17]

EAP Request packet.

Type

TBD

Subtype

1 for USIM-Challenge

Reserved

Set to zero when sending, ignored on reception.

AT_RAND

The value field of this attribute contains two reserved bytes
followed by the AKA RAND parameter, 16 bytes (128 bits). The
reserved bytes are set to zero when sending and ignored on
reception. The AT_RAND attribute MUST be present in EAP-
Request/USIM-Challenge.

AT_AUTN

The value field of this attribute contains two reserved bytes
followed by the AKA AUTN parameter, 16 bytes (128 bits). The
reserved bytes are set to zero when sending and ignored on
reception. The AT_AUTN attribute MUST NOT be included in the GSM
compatible mode of this protocol; otherwise it MUST be included.

AT_IV Authentication February 2002

Length

The value field contains two reserved bytes followed by a 16-byte
initialization vector required by length of the AT_ENCR_DATA attribute. The
reserved bytes are set EAP Request packet.

Type

TBD

Subtype

1 for AKA-Challenge

Reserved

Set to zero when sending and sending, ignored on reception. This 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.

AT_ENCR_DATA

The AT_ENCR_DATA MAY is optional.

AT_RAND

The value field of this attribute consists of contains two reserved bytes
followed by bytes
encrypted using the Advanced Encryption Standard (AES) [10] in
the Cipher Block Chaining (CBC) mode of operation, using the
initialization vector from the AT_IV attribute. AKA RAND parameter, 16 bytes (128 bits). The
reserved bytes are set to zero when sending and ignored on
reception.
Please see [11] for a description of the CBC mode.

The encryption key (K_encr) is derived from the AKA Cipher Key
(CK) with the following formula. The result of the SHA-1 hash
value [12] is truncated to 128 bits by ignoring the 32 rightmost

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bits. The notation A|0 denotes A concatenated with the byte zero
0x00.

K_encr = 128 leftmost bits of SHA1(CK|0) The plaintext consists of nested attributes as described below.

AT_MAC

This attribute is optional, but it MUST be included whenever the
AT_ENCR_DATA AT_RAND attribute is included. Messages that do not meet
this condition MUST be silently discarded. present in EAP-
Request/AKA-Challenge.

AT_AUTN

The value field of the AT_MAC this attribute contains two reserved bytes
followed by a message authentication code (MAC). The MAC is
calculated over the whole EAP packet with the exception that the
value field of the MAC attribute is set to zero when calculating the MAC. AKA AUTN parameter, 16 bytes (128 bits). The
reserved bytes are set to zero when sending and ignored on
reception. The MAC algorithm is HMAC-SHA1 [13] keyed hash value, so AT_AUTN attribute MUST NOT be included in the
length GSM
compatible mode of the MAC is 20 bytes. this protocol; otherwise it MUST be included.

AT_IV

See Section 6.2.

AT_ENCR_DATA

See Section 6.2. The integrity protection key (K_int) used nested attributes that are included in the calculation
plaintext of AT_ENCR_DATA are described below.

AT_MAC

See Section 6.1.

In the MAC is derived from the AKA integrity key (IK) with the
following formula. The notation A|0 denotes A concatenated with EAP-Request/AKA-Challege message, the byte zero 0x00.

K_int = SHA1(IK|0)

The AT_IV, AT_ENCR_DATA and
AT_MAC attributes are used for IMSI privacy. 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.

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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_PSEUDONYM | Length | Actual Pseudonym Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
|
. Pseudonym |
| | .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_PADDING | Length | Padding... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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AT_PSEUDONYM

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

AT_PADDING

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, and silently drop the message if this verification
fails.

6.3. EAP-Response/USIM-Challenge

7.3. EAP-Response/AKA-Challenge

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

EAP-Response/USIM-Challenge

As specified in Section 6, EAP-Response/AKA-Challenge MAY include
the AT_MAC attribute to integrity protect the EAP packet. 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

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

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The semantics of the fields is described below:

Code

2 for Response

Identifier

See [6]

Length

The length of the EAP Response packet.

Type

TBD

Subtype

1 for USIM-Challenge AKA-Challenge

Reserved

Set to zero when sending, ignored on reception.

AT_RES

This attribute MUST be included in EAP-Response/USIM-Challenge. 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 (or SRES)
in bits. The RES length is followed by the UMTS AKA RES or GSM
SRES parameter. According to the specification [14] [18] the length of
the AKA RES can vary between 32 and 128 bits. The GSM SRES

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parameter is always 32 bits long. 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.

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6.4. EAP-Response/USIM-Authentication-Reject

7.4. EAP-Response/AKA-Authentication-Reject

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

The semantics of the fields is described below:

Code

2 for Response

Identifier

See [6]

Length

The length of the EAP Response packet.

Type

TBD

Subtype

2 for USIM-Authentication-Reject AKA-Authentication-Reject

Reserved

Set to zero on sending, ignored on reception.

6.5. EAP-Response/USIM-Synchronization-Failure

7.5. EAP-Response/AKA-Synchronization-Failure

The format of the EAP-Response/USIM-Synchronization-Failure EAP-Response/AKA-Synchronization-Failure packet is
shown below.

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The semantics of the fields is described below:

Code

2 for Response

Identifier

See [6]

Length

The length of the EAP Response packet, 20.

Type

TBD

Subtype

4 for USIM-Synchronization-Failure AKA-Synchronization-Failure

AT_AUTS

This attribute MUST be included in EAP-Response/USIM- EAP-Response/AKA-
Synchronization-Failure. The value field of this attribute
contains the AKA AUTS parameter, 112 bits (14 bytes).

6.6. EAP-Request/USIM-IMSI

7.6. EAP-Request/AKA-Cleartext-Identity

The format of the EAP-Request/USIM-IMSI EAP-Request/AKA-Cleartext-Identity packet is shown
below.

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The semantics of the fields is described below:

Code

1 for Request

Identifier

See [6]

Length

The length of the EAP Request packet.

Type

TBD

Subtype

5 for USIM-IMSI AKA-Cleartext-Identity

Reserved

Set to zero on sending, ignored on reception.

6.7. EAP-Response/USIM-IMSI

7.7. EAP-Response/AKA-Cleartext-Identity

The format of the EAP-Response/USIM-IMSI EAP-Response/AKA-Cleartext-Identity packet is
shown below.

The semantics of the fields is described below:

Code

2 for Response

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Identifier

See [6]

Length

The length of the EAP Response packet.

Type

TBD

Subtype

5 for USIM-IMSI AKA-Cleartext-Identity

Reserved

Set to zero on sending, ignored on reception.

AT_IMSI

AT_IDENTITY

This attribute MUST be included in EAP-Response/USIM-IMSI. The
value field of this attribute contains two reserved bytes
followed by the IMSI, represented as an ASCII string that
consists of not more than 15 decimal digits (ASCII values between
0x30 and 0x39) [9]. The reserved bytes are set to zero on sending
and ignored on reception. EAP-Response/AKA-Cleartext-
Identity. The IMSI characters are followed by one
or more "F" characters (ASCII value 0x46). They are included to
make field of this attribute begins with 2-byte
actual identity length, which specifies the length of the value field 16
identity in bytes.

7. This field is followed by the cleartext
Network Access Identitier username portion of the indicated
actual length. The EAP/AKA username format is specified in
Section 7.1. The user name does not include any terminating null
characters. Because the length of the attribute must be a
multiple of 4 bytes, the sender pads the identity with zero bytes
when necessary.

8. Interoperability with GSM

The EAP AKA protocol is able to authenticate both UMTS and GSM
users, if the subscriber's operator's network is UMTS aware. This is
because the home network will be able to determine from the
subscriber records whether the subscriber is equipped with a UMTS
USIM or a GSM SIM. A UMTS aware home network will hence always use
UMTS AKA with UMTS subscribers and GSM authentication with GSM
subscribers. With GSM subscribers, the EAP AKA protocol is always
used in the GSM compatible mode.

It is not possible to use a GSM AuC to authenticate UMTS
subscribers. (Note that if the home network doesn't support an
authentication method it should not distribute SIMs for that
method.)

However, it is possible that the node actually terminating EAP and
the node that stores the authentication keys (AuC) are separate, and
support different authentication types. If the node terminating EAP
is GSM-only but AuC is UMTS-aware, then authentication can still be

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achieved using the GSM compatible version of EAP AKA. This
authentication will be weaker, since the GSM compatible mode does

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EAP AKA Authentication November 2001
not provide for mutual authentication. Section 6.8.1.1 in [1]
specifies how the GSM SRES parameter and the Kc key can be
calculated on the USIM and the AuC. If a UMTS terminal does not want
to accept the GSM compatible version of this protocol, then it can
reject the authentication with the EAP-Response/USIM-GSM- EAP-Response/AKA-GSM-
Authentication-Reject packet.

In conclusion, the following table shows which variant of the EAP
AKA protocol should be run under different conditions:

SIM EAP node AuC EAP AKA mode
----------------------------------------------------
GSM (any) (any) GSM
UMTS (any) GSM (illegal)
UMTS GSM GSM+UMTS GSM
UMTS GSM+UMTS GSM+UMTS UMTS

9. IANA and Protocol Numbering Considerations

IANA has assigned the number TBD 23 for EAP AKA authentication.

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

USIM-Challenge..................................1
USIM-Authentication-Reject......................2
USIM-Synchronization-Failure....................4
USIM-IMSI.......................................5

AKA-Challenge...................................1
AKA-Authentication-Reject.......................2
AKA-Synchronization-Failure.....................4
AKA-Cleartext-Identity..........................5

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_IMSI.........................................5
AT_IDENTITY.....................................5
AT_PADDING......................................6

AT_IV.........................................129
AT_ENCR_DATA..................................130
AT_MAC........................................131
AT_PSEUDONYM..................................132

10. Security Considerations

Implementations running the EAP AKA protocol will rely on the
security of the AKA scheme, and the secrecy of the symmetric keys
stored in the USIM and the AuC.

10. Intellectual Property Right Notices

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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

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

The IMSI identiy privacy support is based on the identity privacy support
of [8]. The attribute format is based on the extension format of
Mobile IPv4 [15]. [19].

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

References

[1] 3GPP Technical Specification 3GPP TS 33.102 V3.6.0: "Technical
Specification Group Services and System Aspects; 3G Security;
Security Architecture (Release 1999)", 3rd Generation
Partnership Project, November 2000.

[2] GSM Technical Specification GSM 03.20 (ETS 300 534): "Digital
cellular telecommunication system (Phase 2); Security related
network functions", European Telecommunications Standards,
Institute, August 1997.

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

[4] 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

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EAP AKA Authentication February 2002

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

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EAP AKA Authentication November 2001

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

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

[8] J. Carlson, B. Aboba, H. Haverinen, "EAP SRP-SHA1
Authentication Protocol", draft-ietf-pppext-eap-srp-03.txt,
July 2001 (work-in-progress)

[9] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for
Message Authentication", RFC2104, February 1997

[10] Federal Information Processing Standard (FIPS) draft standard,
"Advanced Encryption Standard (AES)",
http://csrc.nist.gov/publications/drafts/dfips-AES.pdf,
September 2001

[11] US National Bureau of Standards, "DES Modes of Operation",
Federal Information Processing Standard (FIPS) Publication 81,
December 1980.

[12] 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.

[13] GSM Technical Specification GSM 03.03 (ETS 300 523): "Digital
cellular telecommunication system (Phase 2); Numbering,
addressing and identification", European Telecommunications
Standards Institute, April 1997.

[10]

[14] Federal Information Processing Standard (FIPS) draft standard,
"Advanced Encryption Standard (AES)",
http://csrc.nist.gov/publications/drafts/dfips-AES.pdf,
September 2001

[11]

[15] US National Bureau of Standards, "DES Modes of Operation",
Federal Information Processing Standard (FIPS) Publication 81,
December 1980.

[12]

[16] 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.

[13]

[17] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for
Message Authentication", RFC2104, February 1997

[14]

[18] 3GPP Technical Specification 3GPP TS 33.105 V3.5.0: "Technical
Specification Group Services and System Aspects; 3G Security;

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Cryptographic Algorithm Requirements (Release 1999)",
3rdGeneration Partnership Project, October 2000

[15]

[19] C. Perkins (editor), "IP Mobility Support", RFC 2002, October
1996

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----