J. Arkko
Internet Draft Ericsson
Document: draft-arkko-pppext-eap-aka-05.txt H. Haverinen
Expires: March 2003 Nokia
October 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.
EAP AKA includes optional identity privacy support and an optional
re-authentication procedure.
Table of Contents
Status of this Memo................................................1
Abstract...........................................................1
1. Introduction and Motivation.....................................2
2. Conventions used in this document...............................4
3. Protocol Overview...............................................5
Arkko and Haverinen Expires in six months [Page 1]
EAP AKA Authentication October 2002
4. User identity in EAP-Response/Identity.........................10
5. Obtaining Subscriber Identity via EAP AKA Messages.............12
6. Identity Privacy Support.......................................14
7. Re-authentication..............................................20
8. Message Format.................................................25
9. Message Authentication and Encryption..........................26
9.1. AT_MAC Attribute.............................................26
9.2. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes................27
10. Messages......................................................28
10.1. EAP-Request/AKA-Challenge...................................28
10.2. EAP-Response/AKA-Challenge..................................32
10.3. EAP-Response/AKA-Authentication-Reject......................33
10.4. EAP-Response/AKA-Synchronization-Failure....................34
10.5. EAP-Request/AKA-Identity....................................35
10.6. EAP-Response/AKA-Identity...................................36
10.7. EAP-Request/AKA-Reauthentication............................37
10.8. EAP-Response/AKA-Reauthentication...........................40
11. Unsuccessful Cases............................................42
12. Key Derivation................................................42
13. Interoperability with GSM.....................................44
14. IANA and Protocol Numbering Considerations....................45
15. Security Considerations.......................................45
16. Intellectual Property Right Notices...........................46
Acknowledgements and Contributions................................46
Authors' Addresses................................................46
Annex A. Key Derivation for IEEE 802.11...........................47
Annex B. Pseudo-Random Number Generator...........................48
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:
Arkko and Haverinen Expires in six months [Page 2]
EAP AKA Authentication October 2002
- 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:
- 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.
Arkko and Haverinen Expires in six months [Page 3]
EAP AKA Authentication October 2002
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
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
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.
Arkko and Haverinen Expires in six months [Page 4]
EAP AKA Authentication October 2002
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.
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 is an application
traditionally resident on smart cards distributed by GSM
operators.SRES
The authentication result parameter in GSM, corresponds to the
RES parameter in UMTS aka, 32 bits.
USIM
UMTS Subscriber Identity Module. USIM is an application that is
resident e.g. on smart cards distributed by UMTS operators.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119 [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
Arkko and Haverinen Expires in six months [Page 5]
EAP AKA Authentication October 2002
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 full
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 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. Please see also Section 5 for specification how to
obtain the identity via EAP AKA messages.)
Next, the EAP server starts the actual AKA protocol by sending an
EAP-Request/AKA-Challenge message. EAP AKA packets encapsulate
parameters in attributes, encoded in a Type, Length, Value format.
The packet format and the use of attributes are specified in Section
8. The EAP-Request/AKA-Challenge message contains a random number
(AT_RAND) and an authorization vector (AT_AUTN), and a message
authentication code AT_MAC. The EAP-Request/AKA-Challenge message
MAY optionally contain encrypted data, which is used for IMSI
privacy support, as described in Section 6. The AT_MAC attribute
contains a message authentication code covering the EAP packet. The
encrypted data is not shown in the figures of this section.
The client 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,
and the AT_MAC attribute to integrity protect the EAP message.
Arkko and Haverinen Expires in six months [Page 6]
EAP AKA Authentication October 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 |
| (AT_RAND, AT_AUTN, AT_MAC) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM,| |
| verifies AUTN and MAC, derives RES | |
| and session key | |
+-------------------------------------+ |
| |
| EAP-Response/AKA-Challenge |
| (AT_RES, AT_MAC) |
|------------------------------------------------------>|
| |
| +--------------------------------+
| | Server checks the given RES, |
| | and MAC and finds them correct.|
| +--------------------------------+
| |
| EAP-Success |
|<------------------------------------------------------|
When EAP AKA is run in the GSM compatible mode, the message flow is
otherwise identical to the message flow below except that the
AT_AUTN attribute is not included in EAP-Request/AKA-Challenge
packet and AT_MAC attribute is not included in any attribute.
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 AT_AUTN attribute and AT_MAC
attribute are not used in the messages.
Arkko and Haverinen Expires in six months [Page 7]
EAP AKA Authentication October 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 |
| (AT_RAND, AT_AUTN, AT_MAC) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM,| |
| possibly verifies AUTN, and sends an| |
| invalid response | |
+-------------------------------------+ |
| |
| EAP-Response/AKA-Challenge |
| (AT_RES, AT_MAC) |
|------------------------------------------------------>|
| |
| +------------------------------------------+
| | Server checks the given RES and the MAC, |
| | and finds one of them incorrct. |
| +------------------------------------------+
| |
| EAP-Failure |
|<------------------------------------------------------|
The next message flow shows the 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.
Arkko and Haverinen Expires in six months [Page 8]
EAP AKA Authentication October 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 |
| (AT_RAND, AT_AUTN, AT_MAC) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| 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 by
silently ignoring any EAP-Request/AKA-Challenge packets that do not
include the AUTN parameter.
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. HereÆs what happens then:
Arkko and Haverinen Expires in six months [Page 9]
EAP AKA Authentication October 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 |
| (AT_RAND, AT_AUTN, AT_MAC) |
|<------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs UMTS algorithms on USIM | |
| and discovers AUTN that contains an | |
| inappropriate sequence number | |
+-------------------------------------+ |
| |
| EAP-Response/AKA-Synchronization-Failure |
| (AT_AUTS) |
|------------------------------------------------------>|
| |
| +---------------------------+
| | Perform resynchronization |
| | Using AUTS and |
| | the sent RAND |
| +---------------------------+
| |
After the resynchronization process takes place in the server and
AAA side, the process continues by the server side sending a new
EAP-Request/AKA-Challenge message.
In addition to the full authentication scenarios described above,
EAP AKA includes a re-authentication procedure, which is specified
in Section 7.
4. User identity in 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].
UMTS and GSM subscribers are identified with the International
Mobile Subscriber Identity (IMSI) [12]. 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 10]
EAP AKA Authentication October 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.
Internet AAA protocols identify users with the Network Access
Identifier (NAI) [5]. When used in a roaming environment, the NAI is
composed of a username and a realm, separated with "@"
(username@realm). The username portion identifies the subscriber
within the realm. The AAA nodes use the realm portion of the NAI to
route AAA requests to the correct AAA server. The realm name used in
this protocol MAY be chosen by the operator and it MAY a
configurable parameter in the EAP/AKA client implementation. In this
case, the client is typically configured with the NAI realm of the
home operator. Operators MAY reserve a specific realm name for
EAP/AKA users. This convention makes it easy to recognize that the
NAI identifies a subscriber that uses EAP/AKA. Such reserved NAI
realm may be useful as a hint as to the first authentication method
to use during method negotiation.
There are three types of NAI username portions in EAP/AKA: non-
pseudonym permanent usernames that are based on the IMSI, pseudonym
usernames and re-authentication usernames. The first two are only
used on full authentication and the last one only on re-
authentication. When the optional IMSI privacy support is not used,
the non-pseudonym permanent username is used. The non-pseudonym
permanent username is of the format "0imsi". In other words, the
first character of the username is the digit zero (ASCII value
0x30), followed by the IMSI. The IMSI is an ASCII string that
consists of not more than 15 decimal digits (ASCII values between
0x30 and 0x39) as specified in [12]
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 on full
authentication, the client MAY use the pseudonym received as part of
the previous full authentication sequence as the username portion of
the NAI, as specified in Section 6. The client MUST NOT modify the
pseudonym received in AT_NEXT_PSEUDONYM. For example, the client
MUST NOT append any leading characters in the pseudonym.
On re-authentication, the client uses the re-authentication identity
received as part of the previous authentication sequence as the NAI.
A new re-authentication identity may be delivered as part of both
full authentication and re-authentication. The client MUST NOT
modify the re-authentication identity received in AT_NEXT_REAUTH_ID.
For example, the client MUST NOT append any leading characters in
the re-authentication identity.
If no configured realm name is available, the client MAY derive the
realm name from the MCC and MNC portions of the IMSI. In this case,
the realm name is obtained by concatenating "mnc", the MNC digits of
Arkko and Haverinen Expires in six months [Page 11]
EAP AKA Authentication October 2002
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 for operators that have 2-digit MNC's, the network SHOULD
also be prepared for realm names with incorrect 3-digit MNC's.
5. Obtaining Subscriber Identity via EAP AKA Messages
It may be useful to obtain the identity of the subscriber through
means other than EAP Request/Identity. This can eliminate the need
for an identity request when using EAP method negotiation. If this
was not possible then it might not be possible to negotiate EAP/AKA
as the second method since it is not specified how to deal with a
new EAP Request/Identity.
If the EAP server does not have any identity (IMSI, pseudonym or re-
authentication username) available when sending the first EAP/AKA
request, then the EAP server may issue the EAP-Request/AKA-Identity
packet and includes the AT_ANY_ID_REQ attribute (specified in
Section 10.5). This attribute does not contain any data.
The AT_ANY_ID_REQ attribute requests the client to include the
AT_IDENTITY attribute (specified in Section 10.6) in the EAP-
Response/AKA-Identity packet. The identity format in the AT_IDENTITY
attribute is the same as in the EAP-Response/Identity packet. The
AT_IDENTITY attribute contains an IMSI-based permanent identity, a
pseudonym identity or a re-authentication identity. If the server
does not support re-authentication, it uses the AT_FULLAUTH_ID_REQ
attribute instead of the AT_ANY_ID_REQ attribute to directly request
for a full authentication identity (either the permanent identity or
a pseudonym identity). If the server uses the AT_FULLAUTH_ID_REQ
attribute, the client MUST NOT use a re-authentication identity in
the AT_IDENTITY attribute.
The use of pseudonyms for anonymity is specified in Section 6. The
use of re-authentication identities is specified in Section 7.
This case for full authentication is illustrated in the figure
below. In this case, AT_IDENTITY contains either the permanent
identity or a pseudonym identity. The same sequence is also used in
case the server uses the AT_FULLAUTH_ID_REQ in EAP-Request/AKA-
Identity
Arkko and Haverinen Expires in six months [Page 12]
EAP AKA Authentication October 2002
Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/AKA |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_ANY_ID_REQ) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/AKA-Identity |
| (AT_IDENTITY) |
|------------------------------------------------------>|
| |
If the client wants to perform full authentication, it includes the
permanent identity or a pseudonym identity in the AT_IDENTITY
attribute. The client may use these identities in response to either
AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ. If the server uses the
AT_ANY_ID_REQ and the client wants to perform re-authentication,
then the client includes a re-authentication identity in the
AT_IDENTITY attribute.
If the client uses its full authentication identity and the
AT_IDENTITY attribute contains a valid permanent identity or a valid
pseudonym identity that the EAP server is able to decode to the
permanent identity, then the full authentication sequence proceeds
as usual with the EAP Server issuing the EAP-Request/AKA-Challenge
message.
On re-authentication, if the AT_IDENTITY attribute contains a valid
re-authentication identity and the server agrees on using re-
authentication, then the server proceeds with the re-authentication
sequence and issues the EAP-Request/AKA-Reauthentication packet, as
specified in Section 7. If the server does not recognize the re-
authentication identity, then the server issues a second EAP-
Request/AKA-Identity message and includes the AT_FULLAUTH_ID_REQ
attribute. In this case, a second EAP/AKA-Identity round trip is
required. The messages used on the first roundtrip are ignored. This
is illustrated below.
Arkko and Haverinen Expires in six months [Page 13]
EAP AKA Authentication October 2002
Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/AKA |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_ANY_ID_REQ) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/AKA-Identity |
| (AT_IDENTITY containing a re-authentication identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server does not recognize |
| | The re-authentication |
| | Identity |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_FULLAUTH_ID_REQ) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/AKA-Identity |
| (AT_IDENTITY with a full-auth. Identity) |
|------------------------------------------------------>|
| |
If the server recognizes the re-authentication identity, but still
wants to fall back on full authentication, the server may issue the
EAP-Request/AKA-Challenge packet. In this case, the full
authentication procedure proceeds as usual.
An extra EAP/AKA-Identity round trip is also required in cases when
the AT_IDENTITY attribute contains a pseudonym identity that the EAP
server fails to decode. The operation in this case is specified in
Section 6.
6. Identity Privacy Support
EAP/AKA includes optional identity privacy (anonymity) support that
can be used to hide the cleartext IMSI and to make the subscriber's
connections unlinkable to eavesdroppers. Identity privacy is based
on temporary identities, or pseudonyms, which are equivalent to but
separate from the Temporary Mobile Subscriber Identities (TMSI) that
are used on cellular networks.
If identity privacy is not used or if the client does not have any
pseudonyms or re-authentication identities are available, the client
Arkko and Haverinen Expires in six months [Page 14]
EAP AKA Authentication October 2002
transmits the permanent identity (based on IMSI) in the EAP-
Response/Identity packet or in the AT_IDENTITY attribute.
The EAP-Request/AKA-Challenge message MAY include an encrypted
pseudonym in the value field of the AT_ENCR_DATA attribute. The
AT_IV and AT_MAC attributes are also used to transport the pseudonym
to the client, as described in Section 10.1. Because the identity
privacy support is optional to implement, the client MAY ignore the
AT_IV and AT_ENCR_DATA attributes and always transmit the IMSI-based
permanent identity in the EAP-Response/Identity packet and in the
AT_IDENTITY attribute.
On receipt of the EAP-Request/AKA-Challenge, the client verifies the
AT_MAC attribute before looking at the AT_ENCR_DATA attribute. If
the AT_MAC is invalid, then the client MUST silently discard the EAP
packet. If the AT_MAC attribute is valid, then the client MAY
decrypt the encrypted data in AT_ENCR_DATA and use the obtained
pseudonym on the next full authentication.
If the client does not receive a new pseudonym in the EAP-
Request/AKA-Challenge message, the client MAY use an old pseudonym
instead of the permanent identity on next full authentication.
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
permanent 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 full authentication with 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 selects the realm
name portion similarly as it select the realm name portion when
using the permanent identity. If the EAP server successfully decodes
the pseudonym received in the EAP-Response/Identity packet to a
known client identity (IMSI), the authentication proceeds with the
EAP-Request/AKA-Challenge message as usual.
Because the client may fail to save a pseudonym sent to in an EAP-
Request/AKA-Challenge, for example due to malfunction, the EAP
server SHOULD maintain at least one old pseudonym in addition to the
most recent pseudonym.
If the EAP server requests the client to include its identity in the
EAP-Response/AKA-Identity packet, as specified in Section 5, the
client MAY transmit the received pseudonym in the AT_IDENTITY
attribute. 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.
Arkko and Haverinen Expires in six months [Page 15]
EAP AKA Authentication October 2002
If the EAP server fails to decode the pseudonym to a known identity,
then the EAP server requests the permanent identity (non-pseudonym
identity) by including the AT_PERMANENT_ID_REQ attribute (Section
10.5) in the EAP-Request/AKA-Challenge message.
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 packet. In this case, a
second EAP/AKA-Identity round trip is required.
A received AT_PERMANENT_ID_REQ does not necessarily originate from
the valid network, but an active attacker may transmit an EAP-
Request/AKA-Identity packet with an AT_PERMANENT_ID_REQ attribute to
the client, in an effort to find out the true identity of the user.
On receipt of EAP-Request/AKA-Identity that includes
AT_PERMANENT_ID_REQ, the client MAY delay the processing of the
message for a while in order to wait for another EAP AKA message
that does not include the AT_PERMANENT_ID_REQ attribute.
Basically, there are two different policies that the client can
employ with regard to AT_PERMANENT_ID_REQ. A "conservative" client
assumes that the network is able to maintain pseudonyms robustly.
Therefore, if a conservative client has a pseudonym, the client
silently ignores the EAP packet with AT_PERMANENT_ID_REQ, because
the client believes that the valid network is able to decode the
pseudonym. (Alternatively, the conservative client may respond to
AT_PERMANENT_ID_REQ in certain circumstances, for example if the
pseudonym was received a long time ago.) The benefit of this policy
is that it protects the client against active attacks on anonymity.
On the other hand, a "liberal" client always accepts the
AT_PERMANENT_ID_REQ and responds with the IMSI-based permanent
identity. The benefit of this policy is that it works even if the
valid network sometimes loses pseudonyms and is not able to decode
them to the permanent identity.
The value field of the AT_PERMANENT_ID_REQ does not contain any data
but the attribute is included to request the client to include the
AT_IDENTITY attribute (Section 10.6) with the permanent
authentication identity in the EAP-Response/AKA-Identity message. In
this case, the AT_IDENTITY attribute contains the client's permanent
identity in the clear.
Please note that the EAP/AKA client and the EAP/AKA server only
process the AT_IDENTITY attribute and entities that only pass
through EAP packets do not process this attribute. Hence, if the EAP
server is not co-located in the authenticator, then the
authenticator and other intermediate AAA elements (such as possible
AAA proxy servers) will continue to refer to the client with the
original identity from the EAP-Response/Identity packet regardless
if the decoding fails in the EAP server.
The figure below illustrates the case when the EAP server fails to
decode the pseudonym included in the EAP-Response/Identity packet.
Arkko and Haverinen Expires in six months [Page 16]
EAP AKA Authentication October 2002
Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a pseudonym) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server fails to decode the |
| | Pseudonym. |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_PERMANENT_ID_REQ) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/AKA-Identity |
| (AT_IDENTITY with permanent identity) |
|------------------------------------------------------>|
| |
After the EAP-Response/AKA-Identity message, the authentication
sequence proceeds as usual with the EAP Server issuing the EAP-
Request/AKA-Challenge message.
The figure below illustrates the case when the EAP server fails to
decode the pseudonym included in the AT_IDENTITY attribute.
Arkko and Haverinen Expires in six months [Page 17]
EAP AKA Authentication October 2002
Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/AKA |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_ANY_ID_REQ) |
|<------------------------------------------------------|
| |
| |
|EAP-Response/AKA-Identity |
|(AT_IDENTITY with a pseudonym identity) |
|------------------------------------------------------>|
| |
| |
| +------------------------------+
| | Server fails to decode the |
| | Pseudonym in AT_IDENTITY |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_PERMANENT_ID_REQ) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/AKA-Identity |
| (AT_IDENTITY with permanent identity) |
|------------------------------------------------------>|
| |
In the worst case, there are three EAP/AKA-Identity round trips
before the server has obtained an acceptable identity: on the first
round, the client sends its re-authentication identity in
AT_IDENTITY. The server fails to accept it and request a full
authentication identity with a second EAP-Request/AKA-Identity. The
client responds with a pseudonym identity in AT_IDENTITY. The server
fails to decode the pseudonym and has to issue a third EAP-
Request/AKA-Identity, including AT_PERMANENT_ID_REQ. Finally, the
server accepts the client's EAP-Response/AKA-Identity with the
AT_IDENTITY attribute and proceeds with full authentication. This is
illustrated in the figure below.
Arkko and Haverinen Expires in six months [Page 18]
EAP AKA Authentication October 2002
Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/AKA |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_ANY_ID_REQ) |
|<------------------------------------------------------|
| |
| EAP-Response/AKA-Identity |
| (AT_IDENTITY with re-authentication identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server does not accept |
| | The re-authentication |
| | Identity |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_FULLAUTH_ID_REQ) |
|<------------------------------------------------------|
| |
|EAP-Response/AKA-Identity |
|(AT_IDENTITY with a pseudonym identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server fails to decode the |
| | Pseudonym in AT_IDENTITY |
| +------------------------------+
| |
| EAP-Request/AKA-Identity |
| (AT_PERMANENT_ID_REQ) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/AKA-Identity |
| (AT_IDENTITY with permanent identity) |
|------------------------------------------------------>|
| |
After the last EAP-Response/AKA-Identity message, the full
authentication sequence proceeds as usual with the EAP Server
issuing the EAP-Request/AKA-Challenge message.
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.
Arkko and Haverinen Expires in six months [Page 19]
EAP AKA Authentication October 2002
7. Re-authentication
In some environments, EAP authentication may be performed
frequently. Because the EAP AKA full authentication procedure makes
use of the UMTS AKA algorithms, and it therefore requires fresh
authentication vectors from the Authentication Centre, the full
authentication procedure is not very well suitable for frequent use.
Therefore, EAP AKA includes a more inexpensive re-authentication
procedure that does not make use of the UMTS AKA algorithms and does
not need new vectors from the Authentication Centre.
Re-authentication is optional to implement for both the EAP AKA
server and client. On each EAP authentication, either one of the
entities may also fall back on full authentication if they do not
want to use re-authentication.
Re-authentication is based on the keys derived on the preceding full
authentication. The same K_aut and K_encr keys as in full
authentication are used to protect EAP AKA packets and attributes,
and the original XKEY seed value from full authentication is used to
generate fresh application specific keys, as specified in Section
12.
On re-authentication, the client protects against replays with an
unsigned 16-bit counter, included in the AT_COUNTER attribute. On
full authentication, both the server and the client initialize the
counter to one. The counter value of at least one is used on the
first re-authentication. On subsequent re-authentications, the
counter MUST be greater than on any of the previous re-
authentications. For example, on the second re-authentication,
counter value is two or greater etc. The AT_COUNTER attribute is
encrypted.
The server includes an encrypted server nonce (AT_NONCE_S) in the
re-authentication request. The AT_MAC attribute in the client's
response is calculated over NONCE_S to provide a challenge/response
authentication scheme. The NONCE_S also contributes to the new
application specific keys.
As discussed in Section 6, in some environments the client may
assume that the network can reliably store pseudonyms and therefore
the client may fail to respond to the AT_PERMANENT_ID_REQ attribute.
The network SHOULD store pseudonyms on a reliable database. Because
one of the objectives of the re-authentication procedure is to
reduce load on the network, the re-authentication procedure does not
require the EAP server to contact a reliable database. Therefore,
the re-authentication procedure makes use of separate re-
authentication user identities. Pseudonyms and the permanent IMSI-
based identity are reserved for full authentication only. The
network does not need to store re-authentication identities as
carefully as pseudonyms. If a re-authentication identity is lost and
the network does not recognize it, the EAP server can fall back on
full authentication.
Arkko and Haverinen Expires in six months [Page 20]
EAP AKA Authentication October 2002
If the EAP server supports re-authentication, it MAY include the
skippable AT_NEXT_REAUTH_ID attribute in the encrypted data of EAP-
Request/AKA-Challenge message. This attribute contains a new re-
authentication identity for the next re-authentication. The client
MAY ignore this attribute, in which case it will use full
authentication next time. If the client wants to use re-
authentication, it uses this re-authentication identity on next
authentication. Even if the client has a re-authentication identity,
the client MAY discard the re-authentication identity and use a
pseudonym or the IMSI-based permanent identity instead, in which
case full authentication will be performed.
The re-authentication identity received in AT_NEXT_REAUTH_ID
contains both the username portion and the realm portion of the
Network Access Identifier. The EAP Server can choose an appropriate
realm part in order to have the AAA infrastructure route subsequent
re-authentication related requests to the same AAA server. For
example, the realm part MAY include a portion that is specific to
the AAA server. Hence, it is sufficient to store the context
required for re-authentication in the AAA server that performed the
full authentication.
The client MAY use the re-authentication identity in the EAP-
Response/Identity packet or, in response to server's AT_ANY_ID_REQ
attribute, the client MAY use the re-authentication identity in the
AT_IDENTITY attribute of the EAP-Response/AKA-Identity packet.
Even if the client uses a re-authentication identity, the server may
want to fall back on full authentication, for example because the
server does not recognize the re-authentication identity or does not
want to use re-authentication. If the server was able to decode the
re-authentication identity to the permanent identity, the server
issues the EAP-Request/AKA-Challenge packet to initiate full
authentication. If the server was not able to recover the client's
identity from the re-authentication identity, the server starts the
full authentication procedure by issuing an EAP-Request/AKA-Identity
packet. This packet always starts a full authentication sequence if
it does not include the AT_ANY_ID_REQ attribute. (As specified in
Sections 5 and 6, the server MAY use AT_ANY_ID_REQ,
AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ attributes if it does not
know the client's identity.)
Both the client and the server SHOULD have an upper limit for the
number of subsequent re-authentications allowed before a full
authentication needs to be performed. Because a 16-bit counter is
used in re-authentication, the theoretical maximum number of re-
authentications is reached when the counter value reaches 0xFFFF.
In order to use re-authentication, the client and the server need to
store the following values: original XKEY, K_aut, K_encr, latest
counter value and the next re-authentication identity.
The following figure illustrates the re-authentication procedure.
Encrypted attributes are denoted with '*'. The client uses its re-
Arkko and Haverinen Expires in six months [Page 21]
EAP AKA Authentication October 2002
authentication identity in the EAP-Response/Identity packet. As
discussed above, an alternative way to communicate the re-
authentication identity to the server is for the client to use the
AT_IDENTITY attribute in the EAP-Response/AKA-Identity message. This
latter case is not illustrated in the figure below, and it is only
possible when the server requests the client to send its identity by
including the AT_ANY_ID_REQ attribute in the EAP-Request/AKA-
Identity packet.
If the server recognizes the re-authentication identity and agrees
on using re-authentication, then the server sends the EAP-
Request/AKA-Reauthentication packet to the client. This packet MUST
include the encrypted AT_COUNTER attribute, with a fresh counter
value, the encrypted AT_NONCE_S attribute that contains a random
number chosen by the server, the AT_ENCR_DATA and the AT_IV
attributes used for encryption, and the AT_MAC attribute that
contains a message authentication code over the packet. The packet
MAY also include an encrypted AT_NEXT_REAUTH_ID attribute that
contains the next re-authentication identity.
Re-authentication identities are one-time identities. If the client
does not receive a new re-authentication identity, it MUST use
either the permanent identity or a pseudonym identity on the next
authentication to initiate full authentication.
The client verifies that the counter value is fresh (greater than
any previously used value). The client also verifies that AT_MAC is
correct. The client MAY save the next re-authentication identity
from the encrypted AT_NEXT_REAUTH_ID for next time. If all checks
are successful, the client responds with the EAP-Response/AKA-
Reauthentication packet, including the AT_COUNTER attribute with the
same counter value and the AT_MAC attribute.
The server verifies the AT_MAC attribute and also verifies that the
counter value is the same that it used in the EAP-Request/AKA-
Reauthentication packet. If these checks are successful, the re-
authentication has succeeded and the server sends the EAP-Success
packet to the client.
Arkko and Haverinen Expires in six months [Page 22]
EAP AKA Authentication October 2002
Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a re-authentication identity) |
|------------------------------------------------------>|
| |
| +--------------------------------+
| | Server recognizes the identity |
| | and agrees on using fast |
| | re-authentication |
| +--------------------------------+
| |
| EAP-Request/AKA-Reauthentication |
| (AT_IV, AT_ENCR_DATA, *AT_COUNTER, |
| *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC) |
|<------------------------------------------------------|
| |
| |
+-----------------------------------------------+ |
| Client verifies AT_MAC and the freshness of | |
| the counter. Client MAY store the new re- | |
| authentication identity for next re-auth. | |
+-----------------------------------------------+ |
| |
| EAP-Response/AKA-Reauthentication |
| (AT_IV, AT_ENCR_DATA, *AT_COUNTER with same value, |
| AT_MAC) |
|------------------------------------------------------>|
| |
| +--------------------------------+
| | Server verifies AT_MAC and |
| | the counter |
| +--------------------------------+
| |
| EAP-Success |
|<------------------------------------------------------|
| |
If the client does not accept the counter value of EAP-Request/AKA-
Reauthentication, it indicates the counter synchronization problem
by including the encrypted AT_COUNTER_TOO_SMALL in EAP-Response/AKA-
Reauthentication. The server responds with EAP-Request/AKA-Challenge
to initiate a normal full authentication procedure. This is
illustrated in the following figure. Encrypted attributes are
denoted with '*'.
Arkko and Haverinen Expires in six months [Page 23]
EAP AKA Authentication October 2002
Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a re-authentication identity) |
|------------------------------------------------------>|
| |
| EAP-Request/AKA-Reauthentication |
| (AT_IV, AT_ENCR_DATA, *AT_COUNTER, |
| *AT_NONCE_S, *AT_NEXT_REAUTH_ID, AT_MAC) |
|<------------------------------------------------------|
| |
+-----------------------------------------------+ |
| AT_MAC is valid but the counter is not fresh. | |
+-----------------------------------------------+ |
| |
| EAP-Response/AKA-Reauthentication |
| (AT_IV, AT_ENCR_DATA, *AT_COUNTER_TOO_SMALL, |
| *AT_COUNTER, AT_MAC) |
|------------------------------------------------------>|
| |
| +----------------------------------------------+
| | Server verifies AT_MAC but detects |
| | That client has included AT_COUNTER_TOO_SMALL|
| +----------------------------------------------+
| |
| EAP-Request/AKA-Challenge |
|<------------------------------------------------------|
| |
+---------------------------------------------------------------+
| Normal full authentication follows. |
+---------------------------------------------------------------+
| |
In the figure above, the first three messages are similar to the
basic re-authentication case. When the client detects that the
counter value is not fresh, it includes the AT_COUNTER_TOO_SMALL
attribute in EAP-Response/AKA-Reauthentication. This attribute
doesn't contain any data but it is a request for the server to
initiate full authentication. In this case, the client MUST ignore
the contents of the server's AT_NEXT_REAUTH_ID attribute.
On receipt of AT_COUNTER_TOO_SMALL, the server verifies AT_MAC and
verifies that AT_COUNTER contains the same as in the EAP-
Request/AKA-Reauthentication packet. If not, the server silently
discards the EAP-Response/AKA-Reauthentication packet. If all checks
on the packet are successful, the server transmits a EAP-
Request/AKA-Challenge packet and the full authentication procedure
is performed as usual. Since the server already knows the subscriber
identity, it MUST NOT use the EAP-Request/AKA-Identity packet to
request the subscriber identity.
Arkko and Haverinen Expires in six months [Page 24]
EAP AKA Authentication October 2002
8. 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 14.
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.
Arkko and Haverinen Expires in six months [Page 25]
EAP AKA Authentication October 2002
Attributes can be encapsulated within other attributes. In other
words, the value field of an attribute type can be specified to
contain other attributes.
9. Message Authentication and Encryption
This section specifies EAP/AKA attributes for attribute encryption
and EAP/AKA message authentication.
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, these attributes can only be used in the EAP-Request/AKA-
Challenge message and any EAP/AKA messages sent after EAP-
Request/AKA-Challenge. For example, these attributes cannot be used
in 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.
9.1. AT_MAC Attribute
The AT_MAC attribute can optionally be used for EAP/AKA message
integrity protection. Whenever AT_ENCR_DATA (Section 9.2) 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.
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, concatenated with optional
message-specific data, with the exception that the value field of
the MAC attribute is set to zero when calculating the MAC. The
reserved bytes are set to zero when sending and ignored on
reception.
The contents of the message-specific data, if present, are specified
separately for each EAP/AKA message. The message-specific data is
included in order to protect data that is not transmitted with the
EAP packet.
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 = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Arkko and Haverinen Expires in six months [Page 26]
EAP AKA Authentication October 2002
The MAC algorithm is HMAC-SHA1-128 [9] keyed hash 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 16 bytes.) The message authentication key (K_aut) used in
the calculation of the MAC is derived from the AKA integrity key
(IK) and cipher key (CK), as specified in Section Error!
Reference source not found..
9.2. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes
AT_IV and AT_ENCR_DATA attributes can be optionally used to transmit
encrypted information between the EAP/AKA client and server.
The value field of AT_IV contains two reserved bytes followed by a
16-byte initialization vector required by the AT_ENCR_DATA
attribute. The reserved bytes are set to zero 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
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.
Arkko and Haverinen Expires in six months [Page 27]
EAP AKA Authentication October 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_ENCR_DATA | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Encrypted Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The encryption key (K_encr) is derived is derived from the AKA
integrity key (IK) and cipher key (CK), as specified in Section
Error! Reference source not found..
The plaintext consists of nested EAP/AKA attributes.
The encryption algorithm requires the length of the plaintext to be
a multiple of 16 bytes. The sender may need to include the
AT_PADDING attribute as the last attribute within AT_ENCR_DATA. The
AT_PADDING attribute is not included if the total length of other
nested attributes within the AT_ENCR_DATA attribute is a multiple of
16 bytes. As usual, the Length of the Padding attribute includes the
Attribute Type and Attribute Length fields. The Length of the
Padding attribute is 4, 8 or 12 bytes. It is chosen so that the
length of the value field of the AT_ENCR_DATA attribute becomes a
multiple of 16 bytes. The actual pad bytes in the value field are
set to zero (0x00) on sending. The recipient of the message MUST
verify that the pad bytes are set to zero, and silently drop the
message if this verification fails. The format of the AT_PADDING
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_PADDING | Length | Padding... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
10. Messages
AT_NEXT_PSEUDONYMEAP-Request/AKA-Challenge
The format of the EAP-Request/AKA-Challenge packet is shown below.
Arkko and Haverinen Expires in six months [Page 28]
EAP AKA Authentication October 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 = 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 29]
EAP AKA Authentication October 2002
Length
The length of the EAP Request packet.
Type
23
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 9.2.
AT_ENCR_DATA
See Section 9.2. The nested attributes that are included in the
plaintext of AT_ENCR_DATA are described below.
AT_MAC
AT_MAC MUST NOT be included in GSM compatible mode; otherwise it
MUST be included. In EAP-Request/AKA-Challenge, there is no
message-specific data covered by the MAC. See Section 9.1.
In the EAP-Request/AKA-Challege message, the AT_IV, AT_ENCR_DATA and
AT_MAC attributes are used for IMSI privacy and for communicating
the next re-authentication identity. The plaintext of the
AT_ENCR_DATA value field consists of nested attributes, which are
shown below. Later versions of this protocol MAY specify additional
attributes to be included within the encrypted data.
Arkko and Haverinen Expires in six months [Page 30]
EAP AKA Authentication October 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_NEXT_PS... | Length | Actual Pseudonym Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Next Pseudonym .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_NEXT_REAU..| Length | Actual Re-Auth Identity Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Next Re-authentication Username .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_PADDING | Length | Padding... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
AT_NEXT_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 6. 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_NEXT_REAUTH_ID
The AT_NEXT_REAUTH_ID attribute is optional to include. The value
field of this attribute begins with 2-byte actual re-
authentication identity length, which specifies the length of the
re-authentication identity in bytes. This field is followed by a
re-authentication identity, of the indicated actual length, that
the client can use in the next re-authentication, as described in
Section 7. The re-authentication identity includes both a
username portion and a realm name portion. The re-authentication
identity does not include any terminating null characters.
Because the length of the attribute must be a multiple of 4
bytes, the sender pads the re-authentication identity with zero
bytes when necessary.
AT_PADDING
AT_PADDING is optional to include. See Section 9.2.
Arkko and Haverinen Expires in six months [Page 31]
EAP AKA Authentication October 2002
10.2. EAP-Response/AKA-Challenge
The format of the EAP-Response/AKA-Challenge packet is shown below.
Later versions of this protocol MAY make use of the AT_ENCR_DATA and
AT_IV attributes in this message to include encrypted (skippable)
attributes. AT_MAC, AT_ENCR_DATA and AT_IV attributes are not shown
in the figure below. If present, they are processed as in EAP-
Request/AKA-Challenge packet. The EAP server MUST process EAP-
Response/AKA-Challenge messages that include these attributes even
if the server did not implement these optional attributes.
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 |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MAC (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
2 for Response
Identifier
See [6]
Length
The length of the EAP Response packet.
Type
23
Arkko and Haverinen Expires in six months [Page 32]
EAP AKA Authentication October 2002
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 [13] the length of
the AKA RES can vary between 32 and 128 bits. The GSM SRES
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.
AT_MAC
AT_MAC MUST NOT be included in GSM compatible mode; otherwise it
MUST be included. In EAP-Response/AKA-Challenge, there is no
message-specific data covered by the MAC. See Section 9.1.
10.3. 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.
Arkko and Haverinen Expires in six months [Page 33]
EAP AKA Authentication October 2002
Type
23
Subtype
2 for AKA-Authentication-Reject
Reserved
Set to zero on sending, ignored on reception.
10.4. EAP-Response/AKA-Synchronization-Failure
The format of the EAP-Response/AKA-Synchronization-Failure 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_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
23
Subtype
4 for AKA-Synchronization-Failure
Arkko and Haverinen Expires in six months [Page 34]
EAP AKA Authentication October 2002
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).
10.5. EAP-Request/AKA-Identity
The format of the 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_PERM..._REQ | Length = 1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_FULL..._REQ | Length = 1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_ANY_ID_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
23
Subtype
5 for AKA-Identity
Reserved
Set to zero on sending, ignored on reception.
AT_PERMANENT_ID_REQ
The AT_PERMANENT_ID_REQ attribute is optional to include and it
is included in the cases defined in Section 6. It MUST NOT be
Arkko and Haverinen Expires in six months [Page 35]
EAP AKA Authentication October 2002
included if AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ is included. The
value field only contains two reserved bytes, which are set to
zero on sending and ignored on reception.
AT_FULLAUTH_ID_REQ
The AT_FULLAUTH_ID_REQ attribute is optional to include and it is
included in the cases defined in Section 5. It MUST NOT be
included if AT_ANY_ID_REQ or AT_PERMANENT_ID_REQ is included. The
value field only contains two reserved bytes, which are set to
zero on sending and ignored on reception.
AT_ANY_ID_REQ
The AT_ANY_ID_REQ attribute is optional and it is included in the
cases defined in Section 5. It MUST NOT be included if
AT_PERMANENT_ID_REQ or AT_FULLAUTH_ID_REQ is included. The value
field only contains two reserved bytes, which are set to zero on
sending and ignored on reception.
10.6. EAP-Response/AKA-Identity
The format of the EAP-Response/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_IDENTITY | Length | Actual Identity Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Current Identity .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
2 for Response
Identifier
See [6]
Length
The length of the EAP Response packet.
Arkko and Haverinen Expires in six months [Page 36]
EAP AKA Authentication October 2002
Type
23
Subtype
5 for AKA-Identity
Reserved
Set to zero on sending, ignored on reception.
AT_IDENTITY
The AT_IDENTITY attribute is optional to include and it is
included in cases defined in Section 5 and 6. 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 subscriber identity of the indicated actual
length, in the same Network Access Identifier format that is used
in EAP-Response/Identity, i.e. including the NAI realm portion.
The identity is the permanent IMSI-based identity, a pseudonym
identity or a re-authentication identity. The identity format is
specified in Section 4. The identity does not include any
terminating null characters. Because the length of the attribute
must be a multiple of 4 bytes, the sender pads the identity with
zero bytes when necessary.
10.7. EAP-Request/AKA-Reauthentication
The format of the EAP-Request/AKA-Reauthentication packet is shown
below.
Arkko and Haverinen Expires in six months [Page 37]
EAP AKA Authentication October 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_IV | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Initialization Vector |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_ENCR_DATA | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Encrypted Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 for Request
Identifier
See [6].
Length
The length of the EAP packet.
Type
23
Subtype
13
Reserved
Set to zero when sending, ignored on reception.
Arkko and Haverinen Expires in six months [Page 38]
EAP AKA Authentication October 2002
AT_IV
The AT_IV attribute is MUST be included. See Section 9.2.
AT_ENCR_DATA
The AT_ENCR_DATA attribute MUST be included. See Section 9.2. The
plaintext consists of nested attributes as described below.
AT_MAC
AT_MAC MUST be included. No message-specific data is included in
the MAC calculation. See Section 9.1.
The AT_IV and AT_ENCR_DATA attributes are used for communicating
encrypted attributes. The plaintext of the AT_ENCR_DATA value field
consists of nested attributes, which are shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_COUNTER | Length = 1 | Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_NONCE_S | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| NONCE_S |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_NEXT_REAU..| Length | Actual Re-Auth Identity Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Next Re-authentication Username .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_PADDING | Length | Padding... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
AT_COUNTER
The AT_COUNTER attribute MUST be included. The value field
consists of a 16-bit unsigned integer counter value, represented
in network byte order.
AT_NONCE_S
The AT_NONCE_S attribute MUST be included. The value field
contains two reserved bytes followed by a random number generated
Arkko and Haverinen Expires in six months [Page 39]
EAP AKA Authentication October 2002
by the server (16 bytes) freshly for this EAP/AKA re-
authentication. The random number is used as challenge for the
client and also a seed value for the new keying material. The
reserved bytes are set to zero upon sending and ignored upon
reception.
AT_NEXT_REAUTH_ID
The AT_NEXT_REAUTH_ID attribute is optional to include. The
attribute is described in Section 10.1.
AT_PADDING
The AT_PADDING attribute is optional to include. See section 9.2
10.8. EAP-Response/AKA-Reauthentication
The format of the EAP-Response/AKA-Reauthentication 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_IV | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Initialization Vector |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_ENCR_DATA | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Encrypted Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2 for Response
Arkko and Haverinen Expires in six months [Page 40]
EAP AKA Authentication October 2002
Identifier
See [6].
Length
The length of the EAP packet.
Type
23
Subtype
13
Reserved
Set to zero when sending, ignored on reception.
AT_IV
The AT_IV attribute is MUST be included. See Section 9.2.
AT_ENCR_DATA
The AT_ENCR_DATA attribute MUST be included. See Section 9.2. The
plaintext consists of nested attributes as described below.
AT_MAC
For EAP-Response/AKA-Reauthentication, the MAC code is calculated
over the following data:
EAP packet| NONCE_S
The EAP packet is represented as specified in Section 9.1. It is
followed by the 16-byte NONCE_S value from the client's
AT_NONCE_S attribute.
The AT_IV and AT_ENCR_DATA attributes are used for communicating
encrypted attributes. The plaintext of the AT_ENCR_DATA value field
consists of nested attributes, which are shown below.
Arkko and Haverinen Expires in six months [Page 41]
EAP AKA Authentication October 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_COUNTER | Length = 1 | Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_COUNTER...| Length = 1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_PADDING | Length | Padding... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
AT_COUNTER
The AT_COUNTER attribute MUST be included. The format of this
attribute is specified in Section 10.7.
AT_COUNTER_TOO_SMALL
The AT_COUNTER_TOO_SMALL attribute is optional to include, and it
is included in cases specified in Section 7.
AT_PADDING
The AT_PADDING attribute is optional to include. See section 9.2
11. Unsuccessful Cases
In general, if an EAP/AKA client or server implementation detects an
error in a received EAP/AKA packet, the EAP/AKA implementation
silently ignores the EAP packet, does not change its state and does
not send any EAP messages to its peer. Examples of such errors,
specified in detail elsewhere in this document, are an invalid
AT_MAC value, a mandatory attribute is missing, illegal attributes
included and an unrecognized non-skippable attribute. If no valid
packets are received, the authentication exchange will eventually
time out.
As normally in EAP, the EAP server sends the EAP-Failure packet to
the client when the authentication procedure fails on the EAP
Server. In EAP/AKA, this may occur for example if the EAP server is
not able to obtain authentication vectors for the subscriber or the
authentication exchange times out.
12. 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.
Arkko and Haverinen Expires in six months [Page 42]
EAP AKA Authentication October 2002
EAP AKA requires two keys for its own purposes, a message
authentication key K_aut and an encryption key K_encr, to be used
with the AT_MAC and AT_ENCR_DATA attributes. The same K_aut and
K_encr keys are used in full authentication and subsequent re-
authentications. In addition, it is possible to derive additional
application specific key material, such as a master key to be used
with IEEE 802.11i.
Key derivation is based on the pseudo-random number generator
specified in NIST Federal Information Processing Standards
Publication 186-2 [14]. The pseudo-random number generator is
specified in the change notice 1 (2001 October 5)of [14] (Algorithm
1). As specified in the change notice (page 74), when Algorithm 1 is
used as a general-purpose random number generator, the "mod q" term
in step 3.2 is omitted. The function G used in the algorithm is
constructed via Secure Hash Standard as specified in Appendix 3.3 of
the standard. For convenience, the pseudo-random number algorithm
with the correct modification is cited in Annex B.
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(Identity|IK|CK)
In the formula above, the "|" character denotes concatenation.
Identity denotes the user identity string without any terminating
null characters. It is the identity from the AT_IDENTITY attribute
from the last EAP-Response/AKA-Identity packet, or, if AT_IDENTITY
was not used, the identity from the EAP-Response/Identity packet.
The optional user input values (XSEED_j) in Step 3.1 are set to
zero.
The resulting 320-bit random numbers x_0, x_1, ..., x_m-1 are
concatenated and partitioned into suitable-sized chunks and used as
keys in the following order: K_encr (128 bits), K_aut (128 bits),
EAP application specific keys. The number of pseudo-random number
generator iterations (m) depends on the amount of required keying
material. The EAP application specific material immediately follows
K_aut.
On re-authentication, the same pseudo-random number generator can be
used to generate new application specific keys. The seed value XKEYÆ
is calculated as follows:
XKEYÆ = SHA1(Identity|counter|NONCE_S|original XKEY)
In the formula above, the Identity denotes the re-authentication
user identity, without any terminating null characters, from the
AT_IDENTITY attribute of the EAP-Response/AKA-Identity packet, or,
if EAP-Response/AKA-Identity was not used on re-authentication, the
identity string from the EAP-Response/Identity packet. The counter
denotes the counter value from AT_COUNTER attribute used in the EAP-
Arkko and Haverinen Expires in six months [Page 43]
EAP AKA Authentication October 2002
Response/AKA-Reauthentication packet. The counter is used in network
byte order. NONCE_S denotes the 16-byte NONCE_S value from the
AT_NONCE_S attribute used in the EAP-Request/AKA-Reauthentication
packet. The original XKEY is the XKEY value from the preceding full
authentication. The pseudo-random number generator is run with the
new seed value XKEYÆ, and the resulting 320-bit random numbers x_0,
x_1, ..., x_m-1 are concatenated and partitioned into suitable-sized
chunks and used as new application specific keys.
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, rules
of key derivation cannot be given here. ). However, please see Annex
A for a specification of how keys for IEEE 802.11 are derived.
13. 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
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
reject GSM authentication by silently ignoring the GSM mode EAP-
Request/AKA-Challenge 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
Arkko and Haverinen Expires in six months [Page 44]
EAP AKA Authentication October 2002
14. IANA and Protocol Numbering Considerations
The realm name "owlan.org" has been reserved for NAI realm names
generated from the IMSI.
IANA has assigned the number 23 for EAP AKA authentication.
EAP AKA messages include a Subtype field. The following Subtypes are
specified:
AKA-Challenge...................................1
AKA-Authentication-Reject.......................2
AKA-Synchronization-Failure.....................4
AKA-Identity....................................5
AKA-Reauthentication...........................13
The Subtype-specific data is composed of attributes, which have
attribute type numbers. The following attribute types are specified:
AT_RAND.........................................1
AT_AUTN.........................................2
AT_RES..........................................3
AT_AUTS.........................................4
AT_PADDING......................................6
AT_PERMANENT_ID_REQ............................10
AT_MAC.........................................11
AT_ANY_ID_REQ..................................13
AT_IDENTITY....................................14
AT_FULLAUTH_ID_REQ.............................17
AT_COUNTER.....................................19
AT_COUNTER_TOO_SMALL...........................20
AT_NONCE_S.....................................21
AT_IV.........................................129
AT_ENCR_DATA..................................130
AT_NEXT_PSEUDONYM.............................132
AT_NEXT_REAUTH_ID.............................133
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 [15]. 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
another standards body (e.g. 3GPP), or permanently and readily
available vendor design notes.
15. 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 45]
EAP AKA Authentication October 2002
16. Intellectual Property Right Notices
On IPR related issues, Nokia and Ericsson refer to the their
respective statements on patent licensing. Please see
http://www.ietf.org/ietf/IPR/NOKIA and
http://www.ietf.org/ietf/IPR/ERICSSON-General
Acknowledgements and Contributions
The authors wish to thank Rolf Blom of Ericsson, Bernard Aboba of
Microsoft, Arne Norefors of Ericsson, N.Asokan of Nokia, Valtteri
Niemi of Nokia, Kaisa Nyberg of Nokia, Jukka-Pekka Honkanen of Nokia
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 [16].
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
Arkko and Haverinen Expires in six months [Page 46]
EAP AKA Authentication October 2002
Annex A. Key Derivation for IEEE 802.11
As specified in Section 12, application specific keying material can
be derived with the pseudo-random function.
The key hierarchy in IEEE 802.11i currently assumes that EAP methods
produce a 256-bit Pairwise Master Key (PMK). When a Pairwise Master
Key is required, it is the first EAP application specific key that
is derived. On full authentication, the PMK immediately follows
K_aut in the key stream resulting from the key expansion scheme. On
re-authentication, the PMK is the first new application specific key
that is derived.
For pre 802.11i networks, the signature key used to authenticate
broadcast keys in IEEE 802.1x is selected as the first 256 bits of
the EAP application specific keys immediately after K_aut. (On re-
authentication, the first 256 application specific key bits are used
as the signature key.) The next 256 bits are used as the WEP
session key. The full 256-bit key is not usually used during WEP
encryption, unused bits at then end should be ignored by the
implementation. When the keys are transmitted from the authenticator
to the access point using the RADIUS protocol the session key is
placed in an MS-MPPE-RECV-KEY attribute and the signature key is
placed in an MS-MPPE-SEND-KEY attribute. These attributes are
defined in RFC 2548.
Arkko and Haverinen Expires in six months [Page 47]
EAP AKA Authentication October 2002
Annex B. Pseudo-Random Number Generator
The "|" character denotes concatenation, and "^" denotes involution.
Step 1: Choose a new, secret value for the seed-key, XKEY
Step 2: In hexadecimal notation let
t = 67452301 EFCDAB89 98BADCFE 10325476 C3D2E1F0
This is the initial value for H0|H1|H2|H3|H4
in the FIPS SHS [12]
Step 3: For j = 0 to m û 1 do
3.1 XSEED_j = optional user input
3.2 For i = 0 to 1 do
a. XVAL = (XKEY + XSEED_j) mod 2^b
b. w_i = G(t, XVAL)
c. XKEY = (1 + XKEY + w_i) mod 2^b
3.3 x_j = w_0|w_1
Arkko and Haverinen Expires in six months [Page 48]
EAP AKA Authentication October 2002
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)
[3] IEEE P802.1X/D11, "Standards for Local Area and Metropolitan
Area Networks: Standard for Port Based Network Access
Control", March 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.
(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). (INFORMATIVE)
[9] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for
Message Authentication", RFC2104, February 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. (NORMATIVE)
[11] US National Bureau of Standards, "DES Modes of Operation",
Federal Information Processing Standard (FIPS) Publication 81,
December 1980. (NORMATIVE)
[12] GSM Technical Specification GSM 03.03 (ETS 300 523): "Digital
cellular telecommunication system (Phase 2); Numbering,
Arkko and Haverinen Expires in six months [Page 49]
EAP AKA Authentication October 2002
addressing and identification", European Telecommunications
Standards Institute, April 1997. (NORMATIVE)
[13] 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)
[14] 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
[15] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
(NORMATIVE)
[16] C. Perkins (editor), "IP Mobility Support", RFC 2002, October
1996. (INFORMATIVE)
Arkko and Haverinen Expires in six months [Page 50]