view Side-By-Side changes
J. Arkko
Internet Draft Ericsson
Document: draft-arkko-pppext-eap-aka-03.txt draft-arkko-pppext-eap-aka-04.txt H. Haverinen
Expires: August December 2002 Nokia
February
June 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-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
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
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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. Obtaining Subscriber Identity Privacy Support.......................................10 via EAP AKA Messages.............10
5. Message Format.................................................12 Identity Privacy Support.......................................11
Arkko and Haverinen Expires in six months [Page 1]
EAP AKA Authentication February June 2002
6. Message Format.................................................14
7. Message Integrity and Privacy Protection.......................13
6.1. Protection.......................16
7.1. AT_MAC Attribute.............................................13
6.2. Attribute.............................................16
7.2. 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 Attributes............................16
8. Messages.......................................................17
8.1. EAP-Response/Identity........................................18
8.2. EAP-Request/AKA-Challenge....................................19
8.3. EAP-Response/AKA-Challenge...................................22
8.4. EAP-Response/AKA-Authentication-Reject.......................24
8.5. EAP-Response/AKA-Synchronization-Failure.....................24
8.6. EAP-Request/AKA-Identity.....................................25
8.7. EAP-Response/AKA-Identity....................................26
9. Key Derivation.................................................28
10. Interoperability with GSM......................................24
9. GSM.....................................29
11. IANA and Protocol Numbering Considerations.....................25
10. Considerations....................30
12. Security Considerations.......................................25
11. Considerations.......................................31
13. Intellectual Property Right Notices...........................26
Acknowledgements..................................................26 Notices...........................31
Acknowledgements and Contributions................................31
Authors' Addresses................................................26 Addresses................................................31
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. 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:
Arkko and Haverinen Expires in six months [Page 2]
EAP AKA Authentication February June 2002
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
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:
Arkko and Haverinen Expires in six months [Page 3]
EAP AKA Authentication February June 2002
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
AuC
Authentication Centre. The mobile network element that can
authorize
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 [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].
Arkko and Haverinen Expires in six months [Page 4]
EAP AKA Authentication June 2002
AUTS
A value generated by the client upon experiencing a
synchronization failure, 112 bits.
Arkko and Haverinen Expires in six months [Page 4]
EAP AKA Authentication February 2002
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].
SIM
Subscriber Identity Module. The SIM cards are is an application
traditionally resident on smart cards distributed by GSM operators.
SRES
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 USIM is an application that is
resident e.g. on 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. Alternatively, the EAP server 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 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. (As
Arkko and Haverinen Expires in six months [Page 5]
EAP AKA Authentication June 2002
specified in [6], the initial identity request is not required, and
MAY be bypassed in cases where the authenticator can presume the
identity, such as when using leased lines, dedicated dial-ups, etc.)
Arkko and Haverinen Expires in six months [Page 5] etc.
Please see also Section 4 for specification how to obtain the
identity via EAP AKA Authentication February 2002 messages.)
Next, the EAP server starts the actual AKA protocol by sending an
EAP-Request/AKA-Challenge message. This message contains a random
number (RAND) and an authorization vector (AUTN). The EAP-
Request/AKA-Challenge message MAY optionally contain encrypted data,
which is used for IMSI privacy support, as described in Section 4. 5.
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/AKA-
Challenge. This message contains a result parameter that allows the
EAP server in turn to verify that the client is a legitimate one.
Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and AUTN. |
| +------------------------------+
| |
| EAP-Request/AKA-Challenge |
| (RAND, AUTN) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM,| |
| verifies AUTN, derives RES | |
| and session key | |
+-------------------------------------+ |
| |
| EAP-Response/AKA-Challenge |
| (RES) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server checks the given RES, |
| | and finds it correct. |
| +------------------------------+
| |
| EAP-Success |
|<------------------------------------------------------|
Arkko and Haverinen Expires in six months [Page 6]
EAP AKA Authentication June 2002
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/AKA-Challenge packet.
Arkko and Haverinen Expires in six months [Page 6]
EAP AKA Authentication February 2002
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/AKA-Challenge packet.
Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and AUTN. |
| +------------------------------+
| |
| EAP-Request/AKA-Challenge |
| (RAND, AUTN) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM,| |
| possibly verifies AUTN, and sends an| |
| invalid response | |
+-------------------------------------+ |
| |
| 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
Arkko and Haverinen Expires in six months [Page 7]
EAP AKA Authentication June 2002
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.
Arkko and Haverinen Expires in six months [Page 7]
EAP AKA Authentication February 2002
Client 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 |
| (RAND, AUTN) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM | |
| and discovers AUTN that can not be | |
| verified | |
+-------------------------------------+ |
| |
| 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/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:
Arkko and Haverinen Expires in six months [Page 8]
EAP AKA Authentication February June 2002
Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs GSM algorithms, |
| | generates RAND |
| +------------------------------+
| |
| 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. HereËs HereÆs what happens then:
Arkko and Haverinen Expires in six months [Page 9]
EAP AKA Authentication February June 2002
Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server runs UMTS algorithms, |
| | generates RAND and AUTN. |
| +------------------------------+
| |
| EAP-Request/AKA-Challenge |
| (RAND, AUTN) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM | |
| and discovers AUTN that contains an | |
| inappropriate sequence number | |
+-------------------------------------+ |
| |
| EAP-Response/AKA-Synchronization-Failure |
| (AUTS) |
|------------------------------------------------------>|
| |
| +---------------------------+
| | Perform resynchronization |
| | Using 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/AKA-Challenge message.
4. Obtaining Subscriber Identity Privacy Support
In the very first connection to an via EAP server, the client always
transmits AKA Messages
It may be useful to obtain the cleartext identity (IMSI) in the EAP-Response/Identity
packet. In subsequent connections, of the optional identity privacy
support subscriber through
means other than EAP Request/Identity. This can be used to hide eliminate the need
for an identity and request when using EAP method negotiation. If this
was not possible then it might not be possible to make negotiate EAP/AKA
as the connections
unlinkable second method since it is not specified how to deal with a passive eavesdropper.
The EAP-Request/AKA-Challenge message MAY include an encrypted
pseudonym in
new EAP Request/Identity.
If the value field of EAP server does not have any identity (IMSI or pseudonym)
available when sending the AT_ENCR_DATA attribute. The
AT_IV and AT_MAC attributes are also used to transport first EAP/AKA request (usually EAP-
Request/AKA-Challenge), then the pseudonym
to EAP server issues the client, EAP-
Request/AKA-Identity as described in Section 7.2. Because the identity
privacy support is optional to implement, first message and includes the client MAY ignore
AT_IDENTITY_REQ attribute (Section 8.6). This attribute does not
contain any data. It requests the
AT_IV, AT_ENCR_DATA, and AT_MAC attributes and always transmit client to include the
cleartext identity in AT_IDENTITY
Arkko and Haverinen Expires in six months [Page 10]
EAP AKA Authentication February June 2002
attribute (specified in Section 8.7) in the EAP-Response/Identity packet.
On receipt EAP-Response/AKA-
Identity. The AT_IDENTITY attribute contains the current identity of
the EAP-Request/AKA-Challenge, subscriber (IMSI or pseudonym). The use of pseudonyms for
anonymity is specified in Section 5.
This case is illustrated in the client verifies figure below.
Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/AKA |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (Includes AT_IDENTITY_REQ) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/AKA-Idenity |
| (Includes AT_IDENTITY) |
|------------------------------------------------------>|
| |
If the
AT_AUTN AT_IDENTITY attribute before looking at the AT_ENCR_DATA contains a valid cleartext identity or AT_MAC
attributes. If
a pseudonym identity that the AUTN EAP server is invalid, then the client MUST ignore able to decode to the
AT_IV, AT_ENCR_DATA and AT_MAC attributes. If AUTN is valid,
cleartext identity, then the client MAY derive the K_encr and K_int keys authentication sequence proceeds as described in
Section 7.2 and verify
usual with the AT_MAC attribute. EAP Server issuing the EAP-Request/AKA-Challenge
message. The operation in the case when the AT_IDENTITY attribute
contains a pseudonym that the EAP server fails to decode is
specified in Section 5.
5. Identity Privacy Support
In the very first connection to an EAP server, the client always
transmits the cleartext identity (IMSI) in the EAP-Response/Identity
packet or in the AT_IDENTITY attribute. In subsequent connections,
the optional identity privacy support can be used to hide the
identity and to make the connections unlinkable to a passive
eavesdropper.
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 8.2. Because the 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
cleartext 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_AUTN attribute before looking at the AT_ENCR_DATA or AT_MAC
attributes. If the AUTN is invalid, then the client MUST ignore the
Arkko and Haverinen Expires in six months [Page 11]
EAP AKA Authentication June 2002
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 9 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.
Only the EAP server needs to be able to map the pseudonym to the
cleartext identity. Regardless of construction method, the pseudonym
MUST conform to the grammar specified for the username portion of an
NAI. The EAP AKA server MAY produce pseudonyms that begin with a
leading "0" character in order to be able to use the leading
character as a hint in EAP method negotiation during next
authentication.
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 issues the EAP-Request/AKA-Identity packet and
requests the client to include the AT_IDENTITY attribute in the EAP-
Response/AKA-Identity packet, as specified in Section 4, the client
MAY transmit a pseudonym in the AT_IDENTITY packet. If the EAP
server successfully decodes the pseudonym to a known identity, then
the authentication proceeds with the EAP-Request/AKA-Challenge
packet as usual.
If the EAP server fails to decode the pseudonym to a known client
name, then the EAP server requests the cleartext identity (non-
pseudonym identity) by issuing the EAP-Request/AKA-Cleartext-
Identity EAP-Request/AKA-Identity packet
to the client. This In this case, the EAP request packet includes no attributes.
AT_PERMANENT_IDENTITY_REQ to request the client to send its non-
pseudonym identity. The client responds with the EAP-Response/AKA-Cleartext-Identity, EAP-Response/AKA-
Identity, which includes the client's identity in the clear. clear in the
AT_PERMANENT_IDENTITY attribute.
The EAP server issues the EAP-Request/AKA-Identity message also in
the case when it received the undecodable pseudonym in AT_IDENTITY
included the EAP-Response/AKA-Identity. In this case, there are two
EAP/AKA-Identity round trips. The authentication sequence proceeds
similarly in both cases.
Please note that the EAP/AKA client and the EAP/AKA server only
process the AKA-
Cleartext-Identity AKA-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
Arkko and Haverinen Expires in six months [Page 12]
EAP AKA Authentication June 2002
identity from the EAP-Response/Identity packet regardless if the
decoding fails in the EAP server. This
The figure below illustrates the case when an undecodable pseudonym
is illustrated in the
figure below.
Arkko and Haverinen Expires received in six months [Page 11]
EAP AKA Authentication February 2002 EAP-Response/Identity.
Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a pseudonym) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server fails to decode the |
| | Pseudonym. |
| +------------------------------+
| |
| EAP-Request/AKA-Cleartext-Identity EAP-Request/AKA-Identity |
|<------------------------------------------------------|
| (Includes AT_PERMANENT_IDENTITY_REQ) |
|<------------------------------------------------------|
| |
| | EAP-Response/AKA-Cleartext-Identity
| EAP-Response/AKA-Identity | (Cleartext identity)
| (Includes cleartext identity in AT_PERMANENT_IDENTITY)|
|------------------------------------------------------>|
| |
After receiving the EAP-Response/AKA-Cleartext-Identity EAP-Response/AKA-Identity packet, the EAP server
issues the EAP-Request/AKA-Challenge and the authentication proceeds
as usual.
Because
The figure below illustrates the keys that are used case when the EAP server fails to protect
decode the pseudonym are derived
from included in the AKA cipher key (CK) AT_IDENTITY attribute.
Arkko and the AKA integrity key (IK), the
identity privacy support is not available when EAP AKA is used Haverinen Expires in
the GSM compatible mode.
5. Message Format
The Type-Data of the six months [Page 13]
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 Authentication June 2002
Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/AKA |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (Includes AT_IDENTITY_REQ) |
|<------------------------------------------------------|
| |
| |
|EAP-Response/AKA-Identity |
|(Includes a pseudonym AT_IDENTITY) |
|------------------------------------------------------>|
| |
| |
| +------------------------------+
| | Server fails to decode the |
| | Pseudonym in AT_IDENTITY |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (Includes AT_PERMANENT_IDENTITY_REQ) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/AKA-Identity |
| (Includes AT_PERMANENT_IDENTITY) |
|------------------------------------------------------>|
| |
After the latter EAP-Response/AKA-Identity message, the
authentication sequence proceeds as usual with the EAP Server
issuing the EAP-Request/AKA-Challenge message.
If the client believes that the server should be able to decode the
pseudonym identity, the client MAY refuse to send a clear text
identity. In this case, the client silently ignores the EAP-
Request/AKA-Identity packet that contains AT_PERMANENT_IDENTITY_REQ.
This is necessary in some environments to prevent Man-in-the-Middle
attackers from claiming to be servers that do not recognize the
pseudonym, in an effort to find out the true identity of the user.
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
identity privacy support is not available when EAP AKA is used in
the GSM compatible mode.
6. 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
Arkko and Haverinen Expires in six months [Page 14]
EAP AKA Authentication June 2002
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 9.
Arkko and Haverinen Expires in six months [Page 12]
EAP AKA Authentication February 2002 11.
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/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.
EAP/AKA packets do not include a version field. However, should
there be 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, 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.
Arkko and Haverinen Expires in six months [Page 15]
EAP AKA Authentication June 2002
7. Message Integrity and Privacy Protection
This section specifies EAP/AKA attributes for attribute encryption
and EAP/AKA message integrity protection.
Encryption and integrity protection are based on the AKA session
keys CK and IK. Because the CK and IK keys are derived from the RAND challenge are
required to process the integrity protection and encryption
attributes,
challenge, these attributes can only be used in the EAP-
Request/AKA-Challenge EAP-Request/AKA-
Challenge message and any EAP/AKA messages sent after
EAP-Requets/AKA-Challenge. EAP-
Requets/AKA-Challenge. For example, these attributes cannot be used
in EAP-Request/AKA-Cleartext-Identity.
6.1. EAP-Request/AKA-Identity, because the RAND challenge has not yet
been transmitted at that point. As there is no key derivation
specification for the GSM mode, attribute encryption and message
integrity protection are not available in the GSM mode.
7.1. AT_MAC Attribute
The AT_MAC attribute can optionally be used for EAP/AKA message
integrity protection. Whenever AT_ENCR_DATA (Section 6.2) 7.2) 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.
Arkko and Haverinen Expires in six months [Page 13]
EAP AKA Authentication February 2002
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 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The MAC algorithm is HMAC-SHA1 HMAC-SHA1-128 [9] keyed hash value, so value. (The HMAC-
SHA1-128 value is obtained from the 20-byte HMAC-SHA1 value by
truncating the output to 16 bytes. Hence, the length of the MAC is 20 bytes.
16 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.
and cipher key (CK), as specified in Section 9.
7.2. 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.
Arkko and Haverinen Expires in six months [Page 16]
EAP AKA Authentication June 2002
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 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 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
Arkko and Haverinen Expires in six months [Page 14]
EAP AKA Authentication February 2002
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 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)
integrity key (IK) and cipher key (CK), as specified in Section 9.
The plaintext consists of nested EAP/AKA attributes.
7.
8. Messages
7.1.
Arkko and Haverinen Expires in six months [Page 17]
EAP AKA Authentication June 2002
8.1. 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 identity privacy is not used, used in a roaming environment, the EAP AKA client
transmits NAI is composed
of a username and a realm, separated with "@" (username@realm). The
username portion identifies the user's IMSI subscriber within the realm.
There are two types of NAI username portions in the EAP
Response/Identity packet. AKA: non-
pseudonym permanent usernames and pseudonym usernames. When identity
privacy is not used, the non-pseudonym permanent username is used.
The NAI non-pseudonym permanent username is of the format "0imsi@realm". "0imsi". In
other words, the first character of the username is the digit zero
(ASCII value 0x30), followed by the IMSI, followed by the @ character and the
realm. 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 [13].
The EAP server MAY use the leading "0" as a hint to try EAP/AKA as
the first authentication method during method negotiation, rather
than for example EAP/SIM. The EAP/AKA server MAY propose EAP/AKA
even if the leading character was not "0".
When the optional identity privacy support is used, the client MAY
use the pseudonym received as part of the previous authentication
sequence as the user name username portion of the NAI, as specified in Section 4.
5. The client MUST NOT modify the pseudonym received in
AT_PSEUDONYM. For example, the client MUST NOT append any leading
characters in the pseudonym.
The AAA network routes AAA requests to the correct AAA server using
the realm part of the NAI. The realm part MAY be decided by the
operator and it MAY be a configurable
Arkko and Haverinen Expires in six months [Page 15]
EAP AKA Authentication February 2002 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 Network operators need that wish to agree on
apply this approach must make the necessary arrangements before this
special AAA routing in advance. It is recommended that
operators should can be enabled. Operators MAY reserve the a specific
realm portion of NAI for EAP AKA users
exclusively, so that exactly the same realm is not used with other
authentication methods. users. This convention makes it
easy to recognize that the NAI identifies a UMTS or GSM subscriber of this operator,
which subscriber.
Such reserved NAI realm may be useful when configuring as a hint as to the routing rules in first
authentication method to use during method negotiation.
If no configured realm name is available in the
visited client, the client
MAY derive the realm name from the IMSI. 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
Arkko and Haverinen Expires in six months [Page 18]
EAP AKA Authentication June 2002
Identification Number (MSIN). In other words, the IMSI is a string
of not more than 15 digits. MCC and MNC uniquely identify the
operator. A NAI realm name can be derived from the IMSI by
concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits
of IMSI and ".owlan.org". For example, if the IMSI is
123456789098765, and the MNC is three digits long, then the derived
realm name is "mnc456.mcc123.owlan.org".
If the client is not able to determine whether the MNC is two or
three digits long, the client 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 the first digit of MSIN. Hence, when configuring AAA networks.
7.2.
networks for operators that have 2-digit MNC's, the network SHOULD
also be prepared for realm names with incorrect 3-digit MNC's.
8.2. EAP-Request/AKA-Challenge
The format of the EAP-Request/AKA-Challenge packet is shown below.
Arkko and Haverinen Expires in six months [Page 16] 19]
EAP AKA Authentication February June 2002
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_RAND | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| RAND |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_AUTN | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| AUTN (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IV | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Initialization Vector (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_ENCR_DATA | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Encrypted Data (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 6 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MAC (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
1 for Request
Identifier
See [6]
Arkko and Haverinen Expires in six months [Page 17] 20]
EAP AKA Authentication February June 2002
Length
The length of the EAP Request packet.
Type
TBD
Subtype
1 for AKA-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/AKA-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
See Section 6.2. 7.2.
AT_ENCR_DATA
See Section 6.2. 7.2. The nested attributes that are included in the
plaintext of AT_ENCR_DATA are described below.
AT_MAC
See Section 6.1. 7.1.
In the EAP-Request/AKA-Challege message, 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.
Arkko and Haverinen Expires in six months [Page 18] 21]
EAP AKA Authentication February June 2002
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... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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, username, of the indicated actual length, that the
client can use in the next authentication, as described in
Section 4. 5. The user name username 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.
7.3.
8.3. EAP-Response/AKA-Challenge
The format of the EAP-Response/AKA-Challenge packet is shown below.
As specified in Section 6, 7, 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
Arkko and Haverinen Expires in six months [Page 19] 22]
EAP AKA Authentication February June 2002
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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_RES | Length | RES Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
| RES |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 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 (or SRES)
in bits. The RES length is followed by the UMTS AKA RES or GSM
SRES parameter. According to the specification [18] [14] the length of
the AKA RES can vary between 32 and 128 bits. The GSM SRES
Arkko and Haverinen Expires in six months [Page 20] 23]
EAP AKA Authentication February June 2002
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.
7.4.
8.4. EAP-Response/AKA-Authentication-Reject
The format of the EAP-Response/AKA-Authentication-Reject packet 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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 AKA-Authentication-Reject
Reserved
Set to zero on sending, ignored on reception.
7.5.
8.5. EAP-Response/AKA-Synchronization-Failure
The format of the EAP-Response/AKA-Synchronization-Failure packet is
shown below.
Arkko and Haverinen Expires in six months [Page 21] 24]
EAP AKA Authentication February June 2002
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
| AT_AUTS | Length = 4 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| AUTS |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 AKA-Synchronization-Failure
AT_AUTS
This attribute MUST be included in EAP-Response/AKA-
Synchronization-Failure. The value field of this attribute
contains the AKA AUTS parameter, 112 bits (14 bytes).
7.6. EAP-Request/AKA-Cleartext-Identity
8.6. EAP-Request/AKA-Identity
The format of the EAP-Request/AKA-Cleartext-Identity EAP-Request/AKA-Identity packet 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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_ID..._REQ | Length = 1 | Reserved |
Arkko and Haverinen Expires in six months [Page 22] 25]
EAP AKA Authentication February June 2002
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_PERM..._REQ | Length = 1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 AKA-Cleartext-Identity AKA-Identity
Reserved
Set to zero on sending, ignored on reception.
7.7. EAP-Response/AKA-Cleartext-Identity
AT_PERMANENT_IDENTITY_REQ
The format of AT_PERMANENT_IDENTITY_REQ attribute is optional and it is
included in the EAP-Response/AKA-Cleartext-Identity packet cases defined in Section 5. It MUST NOT be
included if AT_IDENTITY_REQ is
shown below.
0 1 2 3
0 1 2 3 4 5 6 7 included. The value field only
contains two reserved bytes, which are set to zero on sending and
ignored on reception.
AT_IDENTITY_REQ
The AT_IDENTITY_REQ attribute is optional and it is included in
the cases defined in Section 4. It MUST NOT be included if
AT_PERMANENT_IDENTITY_REQ is included. The value field only
contains two reserved bytes, which are set to zero on sending and
ignored on reception.
8.7. EAP-Response/AKA-Identity
The format of the EAP-Response/AKA-Identity packet is shown below.
Arkko and Haverinen Expires in six months [Page 26]
EAP AKA Authentication June 2002
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IDENTITY AT_PERM... | Length | Actual Identity Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Cleartext Identity (optional) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IDENTITY | Length | Actual Identity Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Current Identity (optional) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
2 for Response
Arkko and Haverinen Expires in six months [Page 23]
EAP AKA Authentication February 2002
Identifier
See [6]
Length
The length of the EAP Response packet.
Type
TBD
Subtype
5 for AKA-Cleartext-Identity AKA-Identity
Reserved
Set to zero on sending, ignored on reception.
AT_IDENTITY
AT_PERMANENT_IDENTITY
This attribute is optional and it is included in EAP-
Response/AKA-Identity in cases specified in Section 5. It MUST
NOT be included in EAP-Response/AKA-Cleartext-
Identity. if AT_IDENTITY is included. The value field of
this attribute begins with 2-byte actual identity length, which
Arkko and Haverinen Expires in six months [Page 27]
EAP AKA Authentication June 2002
specifies the length of the identity in bytes. This field is
followed by the cleartext non-pseudonym permanent Network Access Identitier Identifier
username portion of the indicated actual length. The EAP/AKA
username format is specified in Section 7.1. 8.1. The user name username 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.
AT_IDENTITY
The AT_IDENTITY attribute is optional and it is included in cases
defined in Section 4. It MUST NOT be included if
AT_PERMANENT_IDENTITY is included. The value field of this
attribute begins with 2-byte actual identity length, which
specifies the length of the identity in bytes. This field is
followed by the Network Access Identifier username portion of the
indicated actual length. The username format is specified in
Section 8.1. The username is either the non-pseudonym permanent
username or a pseudonym username. The username 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.
9. Key Derivation
This section specifies how EAP AKA keying material is derived from
the IK and CK keys. Because IK and CK are not available in the GSM
mode, this key derivation specification can only be applied in the
UMTS AKA mode.
EAP AKA requires two keys for its own purposes, an integrity
protection key K_int and an encryption key K_encr, to be used with
the AT_MAC and AT_ENCR_DATA attributes. In addition, it is possible
to derive additional key material, such as a master key to be used
with IEEE 802.11i.
Key derivation is based on the random number generation specified in
NIST Federal Information Processing Standards Publication 186-2
[15]. The random number generator is specified in the change notice
1 (2001 October 5)of [15] (Algorithm 1). As specified in the change
notice (page 74), when Algorithm 1 is used as a general-purpose
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.
160-bit XKEY and XVAL values are used, so b = 160. The initial
secret seed value XKEY is computed from the AKA integrity key IK and
cipher key CK with the following formula:
XKEY = SHA1(IK|CK)
The notation IK|CK denotes IK concatenated with CK.
The optional user input values (XSEED_j) are set to zero.
Arkko and Haverinen Expires in six months [Page 28]
EAP AKA Authentication June 2002
The resulting 160-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_int (128 bits),
EAP application specific keys. The number of random number generator
iterations (m) depends on the amount of required keying material.
Even if K_encr or K_int were not used in the particular
authentication sequence, they are derived and the EAP application
specific material begins after K_int.
For example, the EAP application specific material can be used for
packet security between the client and the authenticator. Because
the required keying material depends on the EAP application and the
EAP key derivation standardization has not been finalized yet, exact
rules of key derivation cannot be given here. As a guideline, the
EAP application specific keys resulting from the key expansion
scheme is used in the following order:
any master session keys required,
any encryption keys required,
any integrity protection keys required,
any initialization vectors required
If separate keys or IV's are required for each direction, then the
downlink material (to protect traffic to user) is taken before the
uplink material (to protect traffic from user).
10. 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
Arkko and Haverinen Expires in six months [Page 24]
EAP AKA Authentication February 2002
achieved using the GSM compatible version of EAP AKA. This
authentication will be weaker, since the GSM compatible mode does
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
Arkko and Haverinen Expires in six months [Page 29]
EAP AKA Authentication June 2002
reject the authentication with the 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.
11. 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-Cleartext-Identity..........................5
AKA-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_IDENTITY.....................................5
AT_PERMANENT_IDENTITY...........................5
AT_PADDING......................................6
AT_PERMANENT_IDENTITY_REQ.......................7
AT_IDENTITY_REQ.................................8
AT_IDENTITY.....................................9
AT_IV.........................................129
AT_ENCR_DATA..................................130
AT_MAC........................................131
AT_PSEUDONYM..................................132
10.
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 [16]. 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
Arkko and Haverinen Expires in six months [Page 30]
EAP AKA Authentication June 2002
another standards body (e.g. 3GPP), or permanently and readily
available vendor design notes.
12. 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.
Arkko and Haverinen Expires in six months [Page 25]
EAP AKA Authentication February 2002
11.
13. 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 and Jukka-
Pekka Nokia, Valtteri
Niemi of Nokia, Kaisa Nyberg of Nokia, Jukka-Pekka Honkanen of Nokia
and Olivier Paridaens of Alcatel for interesting discussions in this
problem space.
The identiy privacy support is based on the identity privacy support
of [8]. The attribute format is based on the extension format of
Mobile IPv4 [19]. [17].
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. (NORMATIVE)
[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. (NORMATIVE)
Arkko and Haverinen Expires in six months [Page 31]
EAP AKA Authentication June 2002
[3] IEEE P802.1X/D11, "Standards for Local Area and Metropolitan
Area Networks: Standard for Port Based Network Access
Control", March 2001 2001. (INFORMATIVE)
[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
Arkko and Haverinen Expires in six months [Page 26]
EAP AKA Authentication February 2002 2001.
(INFORMATIVE)
[5] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999. (NORMATIVE)
[6] L. Blunk, J. Vollbrecht, "PPP Extensible Authentication
Protocol (EAP)", RFC 2284, March 1998. (NORMATIVE)
[7] S. Bradner, "Key words for use in RFCs to indicate Requirement
Levels", RFC 2119, March 1997. (NORMATIVE)
[8] J. Carlson, B. Aboba, H. Haverinen, "EAP SRP-SHA1
Authentication Protocol", draft-ietf-pppext-eap-srp-03.txt,
July 2001 (work-in-progress) (work-in-progress). (INFORMATIVE)
[9] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for
Message Authentication", RFC2104, February 1997 1997. (NORMATIVE)
[10] Federal Information Processing Standard (FIPS) draft standard,
"Advanced Encryption Standard (AES)",
http://csrc.nist.gov/publications/drafts/dfips-AES.pdf,
September 2001 2001. (NORMATIVE)
[11] US National Bureau of Standards, "DES Modes of Operation",
Federal Information Processing Standard (FIPS) Publication 81,
December 1980. (NORMATIVE)
[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.
(NORMATIVE)
[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. (NORMATIVE)
[14] Federal Information Processing Standard (FIPS) draft standard,
"Advanced Encryption Standard (AES)",
http://csrc.nist.gov/publications/drafts/dfips-AES.pdf,
September 2001
[15] US National Bureau of Standards, "DES Modes of Operation",
Federal Information Processing Standard (FIPS) Publication 81,
December 1980.
[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.
[17] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for
Message Authentication", RFC2104, February 1997
[18] 3GPP Technical Specification 3GPP TS 33.105 V3.5.0: "Technical
Specification Group Services and System Aspects; 3G Security;
Cryptographic Algorithm Requirements (Release 1999)",
3rdGeneration Partnership Project, October 2000 (NORMATIVE)
Arkko and Haverinen Expires in six months [Page 27] 32]
EAP AKA Authentication February June 2002
Cryptographic Algorithm Requirements (Release 1999)",
3rdGeneration Partnership Project,
[15] 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)
Available on-line at:
http://csrc.nist.gov/publications/fips/fips186-2/
fips186-2-change1.pdf
[16] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 2000
[19] 1998.
(NORMATIVE)
[17] C. Perkins (editor), "IP Mobility Support", RFC 2002, October
1996
1996. (INFORMATIVE)
Arkko and Haverinen Expires in six months [Page 28] 33]
----