Point-to-Point Extensions Working Group H. Haverinen (editor)
Internet Draft Nokia
J. Salowey (editor)
Cisco
June 2003
EAP SIM Authentication
draft-haverinen-pppext-eap-sim-11.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
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Abstract
This document specifies an Extensible Authentication Protocol (EAP)
mechanism for authentication and session key distribution using the
GSM Subscriber Identity Module (SIM). The mechanism specifies
enhancements to GSM authentication and key agreement whereby
multiple authentication triplets can be combined to create
authentication responses and session keys of greater strength than
the individual GSM triplets. The mechanism also includes network
authentication, user anonymity support and a re-authentication
procedure.
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Table of Contents
Status of this Memo.........................................1
Abstract....................................................1
Table of Contents...........................................2
1. Introduction.............................................3
2. Terms....................................................4
3. Overview.................................................5
4. Version Negotiation......................................7
5. Identity Management......................................8
5.1. User identity in EAP-Response/Identity.................8
5.2. Obtaining Subscriber Identity via EAP/SIM Messages....10
5.3. Identity Privacy Support..............................13
6. Re-Authentication.......................................20
7. Message Format..........................................25
8. Message Authentication and Encryption...................26
8.1. AT_MAC Attribute......................................26
8.2. AT_CHECKCODE Attribute................................27
8.3. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes.........29
9. EAP-Request/SIM/Start...................................30
10. EAP-Response/SIM/Start.................................32
11. EAP-Request/SIM/Challenge..............................34
12. EAP-Response/SIM/Challenge.............................38
13. EAP-Request/SIM/Re-authentication......................39
14. EAP-Response/SIM/Re-authentication.....................43
15. Error Cases and the Usage of EAP-Failure and EAP-Success45
15.1. Processing Erroneous Packets.........................45
15.2. EAP-Failure..........................................46
15.3. EAP-Success..........................................46
16. EAP/SIM Notifications..................................46
17. Key Generation.........................................50
18. IANA Considerations....................................52
19. Security Considerations................................53
19.1. Identity Protection..................................53
19.2. Mutual Authentication and Triplet Exposure...........53
19.3. Key Derivation.......................................54
19.4. Dictionary Attacks...................................56
19.5. Credentials Reuse....................................56
19.6. Integrity Protection, Replay Protection and Confidentiality
56
19.7. Negotiation Attacks..................................57
19.8. Fast Reconnect.......................................57
19.9. Acknowledged Result Indications......................58
19.10. Man-in-the-middle Attacks...........................58
19.11. Generating Random Numbers...........................58
20. Security Claims........................................58
21. Intellectual Property Right Notice.....................59
22. Acknowledgements and Contributions.....................59
References.................................................60
Editors' and Contributors' Contact Information.............62
Annex A. Test Vectors......................................63
Annex B. Pseudo-Random Number Generator....................64
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1. Introduction
This document specifies an Extensible Authentication Protocol (EAP)
[1] mechanism for authentication and session key distribution using
the GSM Subscriber Identity Module (SIM).
GSM authentication is based on a challenge-response mechanism. The
A3/A8 authentication algorithms that run on the SIM can be given a
128-bit random number (RAND) as a challenge. The SIM runs an
operator-specific algorithm, which takes the RAND and a secret key
Ki stored on the SIM as input, and produces a 32-bit response (SRES)
and a 64-bit long key Kc as output. The Kc key is originally
intended to be used as an encryption key over the air interface, but
in this protocol it is used for deriving keying material and not
directly used. Please find more information about GSM authentication
in [2].
In EAP/SIM, several RAND challenges are used for generating several
64-bit Kc keys, which are combined to constitute stronger keying
material. EAP/SIM also enhances the basic GSM authentication
mechanism by accompanying the RAND challenges and other messages
with a message authentication code in order to provide mutual
authentication.
EAP/SIM specifies optional support for protecting the privacy of
subscriber identity and an optional re-authentication procedure.
The security of EAP/SIM builds on underlying GSM mechanisms. The
security properties of EAP/SIM are documented in Section 19 of this
document. Implementers and users of EAP/SIM are advised to carefully
study the security considerations in Section 19 in order to
determine whether the security properties are sufficient for the
environment in question. In brief, EAP/SIM is in no sense weaker
than the GSM mechanisms. In some cases EAP/SIM provides better
security properties than the underlying GSM mechanisms, particularly
if the SIM credentials are only used for EAP/SIM and not re-used
from GSM/GPRS. In any case, if the GSM authentication mechanisms are
considered to be sufficient for use on the cellular networks, then
EAP/SIM is expected to be sufficiently secure for other networks.
The 3rd Generation Partnership Project (3GPP) has specified an
enhanced Authentication and Key Exchange (AKA) architecture for the
Universal Mobile Telecommunications System (UMTS). The UMTS AKA
mechanism includes mutual authentication, replay protection and
derivation of longer session keys. EAP AKA [21] specifies an EAP
method that is based on UMTS AKA. EAP AKA may be used instead of
EAP/SIM if the security properties of EAP/SIM are not considered
sufficient.
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2. Terms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
This document frequently uses the following terms and abbreviations:
AAA protocol
Authentication, Authorization and Accounting protocol
AAA server
In this document, AAA server refers to the network element that
resides on the border of Internet AAA network and GSM network.
Cf. EAP server
AuC
Authentication Centre. The GSM network element that provides the
authentication triplets for authenticating the subscriber.
Authentication vector
GSM triplets can be alternatively called authentication vectors.
Client
The entity that processes the EAP protocol on the supplicant.
Typically, it is the end that needs to be authenticated by the
authenticator. The Client includes a SIM that provides the
subscriber credentials and securely executes sensible
cryptographic calculations.
EAP
Extensible Authentication Protocol.
EAP Server
The network element that terminates the EAP protocol and performs
the authentication of the EAP/SIM client. In this document, we
assume that the EAP server functionality is implemented in a AAA
server.
GSM
Global System for Mobile communications.
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GSM Triplet
The tuple formed by the three GSM authentication values RAND, Kc
and SRES
IMSI
International Mobile Subscriber Identifier, used in GSM to
identify subscribers.
MAC
Message Authentication Code
NAI
Network Access Identifier
SIM
Subscriber Identity Module. The SIM is an application
traditionally resident on smart cards distributed by GSM
operators.
3. Overview
Figure 1 shows an overview of the EAP/SIM full authentication
procedure. This version of EAP/SIM exchange uses three roundtrips to
authenticate the user and generate keying material. In this
document, the term EAP Server refers to the network element that
terminates the EAP protocol. The Authenticator typically
communicates with the user's EAP server using an AAA protocol. The
AAA communications is not shown in the figure.
The first EAP Request issued by the Authenticator is EAP-
Request/Identity. The client's response includes either the user's
International Mobile Subscriber Identity (IMSI) or a temporary
identity (pseudonym), as specified in Section 5.3.
Following the client's EAP-Response/Identity packet, the client
receives EAP Requests of type 18 (SIM) from the Authenticator and
sends the corresponding EAP Responses. The EAP packets that are of
the Type SIM also have a Subtype field. On full authentication, the
first EAP-Request/SIM packet is of the Subtype 10 (Start). EAP SIM
packets encapsulate parameters in attributes, encoded in a Type,
Length, Value format. The packet format and the use of attributes
are specified in Section 7.
The EAP-Request/SIM/Start packet contains the list of EAP/SIM
version supported by the Authenticator in the AT_VERSION_LIST
attribute. This packet may also include attributes for requesting
the subscriber identity, as specified in Section 5.3.
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The client responds to EAP-Request/SIM/Start with the EAP-
Response/SIM/Start packet, which includes the AT_NONCE_MT attribute
that contains a random number NONCE_MT, chosen by the client, and
the AT_SELECTED_VERSION attribute that contains the version number
selected by the client. The version negotiation is protected by
including the version list and the selected version in the
calculation of keying material (Section 17). The client MUST NOT
reuse the NONCE_MT value from previous sessions but the client MUST
choose it freshly for each EAP/SIM authentication exchange. The
client SHOULD use a good source of randomness to generate NONCE_MT.
In this document, we assume that the EAP server is implemented on
the AAA server and has an interface to the GSM network, so it
operates as a gateway between the Internet AAA network and the GSM
authentication infrastructure. After receiving the EAP
Response/SIM/Start, the EAP server obtains n GSM triplets from the
user's home operator's Authentication Centre (AuC) on the GSM
network, where n = 1, n = 2 or n = 3. From the triplets, the EAP
server derives the keying material, as specified in Section 17.
The next EAP Request the EAP Server issues is of the type SIM and
subtype Challenge (11). It contains the RAND challenges and a
message authentication code attribute AT_MAC to cover the
challenges.
The EAP server MUST NOT reuse the RAND values (triplets) from
previous successful sessions but the server MUST obtain fresh RANDs
for each EAP/SIM authentication exchange. However, if client
authentication fails, the server MAY reuse the RANDs in the next
authentication attempt.
On receipt of the EAP-Request/SIM/Challenge message, the client runs
the GSM authentication algorithm and calculates a copy of the
message authentication code. The client then verifies that the
calculated MAC equals the received MAC. If the MAC's do not match,
then the client silently ignores the EAP packet and does not send
any authentication values to the network. Eventually, if another
EAP-Request/SIM/Challenge packet with a valid AT_MAC is not
received, the connection establishment will time out.
Since the RAND's given to a client are accompanied with the message
authentication code AT_MAC, and since the client's NONCE_MT value
contributes to AT_MAC, the client is able to verify that the EAP SIM
message is fresh (not a replay) and that the sender possesses valid
GSM triplets for the subscriber.
If all checks out, the client responds with the EAP-
Response/SIM/Challenge, containing the AT_MAC attribute that covers
the client's SRES response values (Section 12). The EAP server
verifies that the MAC is correct and sends the EAP-Success packet,
indicating that the authentication was successful. The EAP server
may also include derived keying material in the message it sends to
the Authenticator.
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The EAP-Request/SIM/Challenge, EAP-Response/SIM/Challenge, or the
packets used on re-authentication may optionally include the
AT_CHECKCODE attribute, which enables the protocol peers to ensure
the integrity of the EAP-Request/SIM/Start and EAP-
Response/SIM/Start packets. AT_CHECKCODE is specified in Section
8.2.
Client Authenticator
| |
| EAP-Request/Identity |
|<---------------------------------------------------------|
| |
| EAP-Response/Identity |
|--------------------------------------------------------->|
| |
| EAP-Request/SIM/Start |
| (AT_VERSION_LIST) |
|<---------------------------------------------------------|
| |
| EAP-Response/SIM/Start |
| (AT_NONCE_MT, AT_SELECTED_VERSION) |
|--------------------------------------------------------->|
| |
| EAP-Request/SIM/Challenge |
| (AT_RAND, AT_MAC) |
|<---------------------------------------------------------|
| |
+-------------------------------------+ |
| Client runs GSM algorithms, | |
| verifies AT_MAC and derives | |
| session keys | |
+-------------------------------------+ |
| |
| EAP-Response/SIM/Challenge |
| (AT_MAC) |
|--------------------------------------------------------->|
| |
| |
| EAP-Success |
|<---------------------------------------------------------|
| |
Figure 1 EAP/SIM full authentication procedure
EAP SIM also includes a separate re-authentication procedure, which
does not make use of the A3/A8 algorithms or the GSM infrastructure.
Re-authentication is based on keys derived on full authentication.
4. Version Negotiation
EAP/SIM includes version negotiation so as to allow future
developments in the protocol. The version negotiation is performed
on full authentication and it uses two attributes, AT_VERSION_LIST
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(Section 9), which the server includes in EAP-Request/SIM/Start, and
AT_SELECTED_VERSION (Section 10), which the client includes in EAP-
Response/SIM/Start.
AT_VERSION_LIST includes the EAP/SIM versions supported by the
server. The server MUST only include versions that it implements and
that are allowed in its security policy. The versions are listed in
the order of preference, most preferred versions first. At least one
version number MUST be included. The version number for the protocol
described in this document is one (0x0001).
If AT_VERSION_LIST does not include a version that is implemented by
the client and allowed in the client's security policy, then the
client MUST silently ignore the EAP-Request/SIM/Start packet. If a
suitable version is included, then the client includes the
AT_SELECTED_VERSION attribute, containing the selected version, in
the EAP-Response/SIM/Start packet. The client MUST only indicate a
version that is included in AT_VERSION_LIST. If several versions are
acceptable, then the client SHOULD choose the version that occurs
first in the version list.
The version number list of AT_VERSION_LIST and the selected version
of AT_SELECTED_VERSION are included in the key derivation procedure
(Section 17). If an attacker modifies either one of these
attributes, then the client and the server will derive different
keying material. Because K_aut keys are different, the server and
client will calculate different AT_MAC values. Hence, the client
will detect that AT_MAC is incorrect and discard the EAP-
Request/SIM/Challenge packet. The authentication procedure will time
out.
5. Identity Management
5.1. 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 [1].
GSM subscribers are identified with the International Mobile
Subscriber Identity (IMSI) [4]. The IMSI is composed of a three
digit Mobile Country Code (MCC), a two or three digit Mobile Network
Code (MNC) and a not more than 10 digit Mobile Subscriber
Identification Number (MSIN). In other words, the IMSI is a string
of not more than 15 digits. MCC and MNC uniquely identify the GSM
operator.
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
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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/SIM 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/SIM users. This convention makes it easy to recognize that the
NAI identifies a GSM subscriber. 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/SIM: non-
pseudonym permanent usernames, 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 identity privacy support is not used, the non-pseudonym
permanent username is used.
The non-pseudonym permanent username MAY be derived from the IMSI.
In this case, the permanent username MUST be of the format "1imsi".
In other words, the first character of the username is the digit one
(ASCII value 0x31), 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 [4].
The EAP server MAY use the leading "1" as a hint to try EAP/SIM as
the first authentication method during method negotiation, rather
than for example EAP/AKA. The EAP/SIM server MAY propose EAP/SIM
even if the leading character was not "1".
Alternatively, an implementation MAY choose a permanent username
that is not based on the IMSI. In this case the selection of the
username, its format, and its processing is a local matter. In this
case, the client implementation MUST NOT prepend any leading
characters to the username.
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 5.3. The client MUST NOT modify the
pseudonym received in AT_NEXT_PSEUDONYM. For example, the client
MUST NOT prepend 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 prepend 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. A recommended
way to derive the realm from the IMSI will be specified in [6].
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Alternatively, the realm name may be obtained by concatenating
"mnc", the MNC digits of IMSI, ".mcc", the MCC digits of IMSI and
".owlan.org". For example, if the IMSI is 123456789098765, and the
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.
5.2. Obtaining Subscriber Identity via EAP/SIM 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/SIM
as the second method since it is not specified how to deal with a
new EAP Request/Identity.
If the EAP server has not received any identity (permanent identity,
pseudonym or re-authentication identity) from the client when
sending the first EAP/SIM request, then the EAP server SHOULD issue
the EAP-Request/SIM/Start packet and includes the AT_ANY_ID_REQ
attribute (specified in Section 9). This attribute does not contain
any data.
If the EAP server has received an EAP-Response/Identity packet but
the contents do not appear to be a valid permanent identity,
pseudonym or a re-authentication identity, the EAP server SHOULD
issue an EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ
attribute.
In some environments the intermediate entities or software layers in
the client may modify the identity string in the EAP-
Response/Identity packet. For example, some EAP layer
implementations may cache the identity string from the first
authentication and do not obtain a new identity string from the EAP
method implementation on subsequent authentication exchanges.
Because the identity string is used in key derivation, such
modifications will result in failed authentication unless the EAP
server uses the AT_ANY_ID_REQ attribute to obtain an unmodified copy
of the identity string. Therefore, in cases when there is a
possibility that an intermediate element or software layer may
modify the EAP-Response/Identity packet, the EAP server SHOULD
always use the AT_ANY_ID_REQ attribute, even if the identity
received in EAP-Response/Identity was valid.
The AT_ANY_ID_REQ attribute requests the client to include the
AT_IDENTITY attribute (specified in Section 10) in the EAP-
Response/SIM/Start packet. The identity format in the AT_IDENTITY
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attribute is the same as in the EAP-Response/Identity packet. The
AT_IDENTITY attribute contains a permanent identity, a pseudonym
identity or a re-authentication identity. If the server does not
support re-authentication, it uses the AT_FULLAUTH_ID_REQ attribute
instead of the AT_ANY_ID_REQ attribute to directly request for a
full authentication identity (either the permanent identity or a
pseudonym identity). If the server uses the AT_FULLAUTH_ID_REQ
attribute, the client MUST NOT use a re-authentication identity in
the AT_IDENTITY attribute.
The use of pseudonyms for anonymity is specified in Section 5.3. The
use of re-authentication identities is specified in Section 6.
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/SIM/Start.
Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/SIM |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY, AT_NONCE_MT, |
| AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| |
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. In this case, the client MUST
include AT_NONCE_MT and AT_SELECTED_VERSION attributes in EAP-
Response/SIM/Start message, as required on full authentication.
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. On re-authentication, the
client MUST NOT include AT_NONCE_MT or AT_SELECTED_VERSION
attributes. This case is illustrated below.
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Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/SIM |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY containing a re-authentication identity) |
|------------------------------------------------------>|
| |
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/SIM/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/SIM/Re-authentication packet, as
specified in Section 6. If the server does not recognize the re-
authentication identity, then the server issues a second EAP-
Request/SIM/Start message and includes the AT_FULLAUTH_ID_REQ
attribute. In this case, a second EAP/SIM/Start round trip is
required. The messages used on the first roundtrip are ignored.
(However, all EAP/SIM/Start messages are taken into account when
calculating the checkcode for AT_CHECKCODE. AT_CHECKCODE is
specified in Section 8.2.) This is illustrated below.
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Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/SIM |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY containing a re-authentication identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server does not recognize |
| | The re-authentication |
| | Identity |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_FULLAUTH_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY with a full-auth. identity, AT_NONCE_MT, |
| AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| |
If the server recognizes the re-authentication identity, but still
wants to fall back on full authentication, the server may issue the
EAP-Request/SIM/Start packet without any identity request attributes
(AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ). In this case, the
server only includes the AT_VERSION_LIST attribute, and full
authentication proceeds as usual. The client does not include any
identity attributes in the EAP-Response/SIM/Start packet.
An extra EAP/SIM/Start 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 5.3.
5.3. Identity Privacy Support
EAP/SIM includes optional identity privacy (anonymity) support that
can be used to hide the cleartext permanent identity and to make the
subscriber's connections unlinkable to eavesdroppers. Identity
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privacy is based on temporary identities, or pseudonyms, which are
equivalent to but separate from the Temporary Mobile Subscriber
Identities (TMSI) that are used on cellular networks. Please see
Section 19.1 for security considerations regarding identity privacy.
If identity privacy is not used or if the client does not have any
pseudonyms or re-authentication identities are available, the client
transmits the permanent identity in the EAP-Response/Identity packet
or in the AT_IDENTITY attribute.
The EAP-Request/SIM/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 11. Because the identity
privacy support is optional to implement, the client MAY ignore the
AT_IV and AT_ENCR_DATA attributes and always transmit the permanent
identity in the EAP-Response/Identity packet and in the AT_IDENTITY
attribute.
On receipt of the EAP-Request/SIM/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/SIM/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 [7] 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.
In any case, it is necessary that permanent usernames and pseudonyms
are separate and recognizable from each other. It is also desirable
that EAP SIM and EAP AKA user names be recognizable from each other
as an aid for the server to which method to offer.
In general, it is the task of the EAP server and the policies of its
administrator to ensure sufficient separation in the usernames.
Pseudonyms, for instance, are both produced and used by the EAP
server. The EAP server MUST compose pseudonyms so that it can
recognize if a NAI username is an EAP SIM pseudonym. For instance,
when the usernames have been derived from the IMSI, the pseudonym
could begin with a leading "3" character.
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 "@"
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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 permanent identity, the authentication proceeds with
the EAP-Request/SIM/Start message as usual.
Because the client may fail to save a pseudonym sent to in an EAP-
Request/SIM/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/SIM/Start packet, as specified in Section 5.2, 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/SIM/Challenge packet as usual.
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 9)
in the EAP-Request/SIM/Start message. Because another EAP server may
have generated the pseudonym using a different coding scheme, the
EAP server SHOULD use AT_PERMANENT_ID_REQ also in cases when it does
not recognize the format of the client identity.
The EAP server issues the EAP-Request/SIM/Start message also in the
case when it received the undecodable pseudonym in AT_IDENTITY
included the EAP-Response/SIM/Start packet. In this case, an extra
EAP/SIM/Start 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/SIM/Start packet with an AT_PERMANENT_ID_REQ attribute to
the client, in an effort to find out the true identity of the user.
The client MAY silently discard any EAP-Request/SIM/Start messages
that include AT_PERMANENT_ID_REQ for a while in order to wait for an
EAP-Request/SIM/Start packet without AT_PERMANENT_ID_REQ. If the
valid network sent the message, the message will be retransmitted,
so the client can reconsider replying to the message when it
receives a retransmission.
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.
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On the other hand, a "liberal" client always accepts the
AT_PERMANENT_ID_REQ and responds with the permanent identity. The
benefit of this policy is that it works even if the valid network
sometimes loses pseudonyms and is not able to decode them to the
permanent identity.
Regardless how the identity is communicated to the server, the full
authentication message sequence and the attributes are the same in
all cases. For example, AT_NONCE_MT and AT_SELECTED_VERSION are
always included in the EAP-Response/SIM/Start packet on full
authentication, even if they were already transmitted in the
previous EAP-Response/SIM/Start. AT_VERSION_LIST is also included in
every EAP-Request/SIM/Start message. The values used on the last
EAP/SIM/Start round trip are used and the previous EAP/SIM/Start
round trips is ignored. (However, all EAP/SIM/Start rounds are taken
into account when calculating the checkcode for AT_CHECKCODE.
AT_CHECKCODE is specified in Section 8.2). The NONCE_MT value and
the version negotiation attributes included in the last EAP-
Response/SIM/Start packet are used in all calculations. The EAP/SIM
client MAY use the same NONCE_MT value in both EAP-
Response/SIM/Start packets.
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) with the permanent authentication
identity in the EAP-Response/SIM/Start message. In this case, the
AT_IDENTITY attribute contains the client's permanent identity in
the clear.
Please note that the EAP/SIM client and the EAP/SIM 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.
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Internet Draft EAP SIM Authentication June 2003
Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a pseudonym) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server fails to decode the |
| | Pseudonym. |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY with permanent identity, AT_NONCE_MT, |
| AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| |
If the server recognizes the permanent identity, then the
authentication sequence proceeds as usual with the EAP Server
issuing the EAP-Request/SIM/Challenge message.
If the server does not recognize the permanent identity, or if the
server is not able to continue the authentication exchange with the
client after receiving the permanent identity, then the server
issues the EAP Failure packet and the authentication exchange
terminates.
The figure below illustrates the case when the EAP server fails to
decode the pseudonym included in the AT_IDENTITY attribute.
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Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/SIM |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
|EAP-Response/SIM/Start |
|(AT_IDENTITY with a pseudonym identity, AT_NONCE_MT, |
| AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| |
| |
| +------------------------------+
| | Server fails to decode the |
| | Pseudonym in AT_IDENTITY |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY with permanent identity, |
| AT_NONCE_MT, AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| |
In the worst case, there are three EAP/SIM/Start 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/SIM/Start. 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/SIM/Start, including
AT_PERMANENT_ID_REQ. Finally, the server accepts the client's EAP-
Response/SIM/Start with the AT_IDENTITY attribute and proceeds with
full authentication. This is illustrated in the figure below.
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Client Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP/SIM |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (Includes AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY with re-authentication identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server does not accept |
| | The re-authentication |
| | Identity |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_FULLAUTH_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
|EAP-Response/SIM/Start |
|(AT_IDENTITY with a pseudonym identity, AT_NONCE_MT, |
| AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server fails to decode the |
| | Pseudonym in AT_IDENTITY |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY with permanent identity, AT_NONCE_MT, |
| AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| |
After the last EAP-Response/SIM/Start message, the full
authentication sequence proceeds as usual. If the EAP Server
recognizes the permanent identity and is able to proceed, the server
issues the EAP-Request/SIM/Challenge message. If the server does not
recognize the permanent identity, or if the server is not able to
continue the authentication exchange with the client after receiving
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the permanent identity, then the server issues the EAP Failure
packet and the authentication exchange terminates.
6. Re-Authentication
In some environments, EAP authentication may be performed
frequently. Because the EAP SIM full authentication procedure makes
use of the GSM SIM A3/A8 algorithms, and it therefore requires 2 or
3 fresh triplets from the Authentication Centre, the full
authentication procedure is not very well suitable for frequent use.
Therefore, EAP SIM includes a more inexpensive re-authentication
procedure that does not make use of the SIM A3/A8 algorithms and
does not need new triplets from the Authentication Centre. Re-
authentication can be performed in fewer roundtrips than the full
authentication.
Re-authentication is optional to implement for both the EAP SIM
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 SIM packets and attributes,
and the original Master Key from full authentication is used to
generate a fresh Master Session Key, as specified in Section 17.
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
Master Session Key.
As discussed in Section 5.3, 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
identity are reserved for full authentication only. The network does
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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.
If the EAP server supports re-authentication, it MAY include the
skippable AT_NEXT_REAUTH_ID attribute in the encrypted data of EAP-
Request/SIM/Challenge message (Section 11). 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 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/SIM/Start 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. In this case, the server starts the
full authentication procedure by issuing an EAP-Request/SIM/Start
packet. This packet always starts a full authentication sequence if
it does not include the AT_ANY_ID_REQ attribute. If the server was
not able to recover the client's identity from the re-authentication
identity, the server includes either the AT_FULLAUTH_ID_REQ or the
AT_PERMANENT_ID_REQ attribute in this EAP request. (As specified in
Sections 5.2 and 5.3, the server MAY use AT_ANY_ID_REQ,
AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ attributes if it does not
know the client's identity.)
Both the client and the server SHOULD have an upper limit for the
number of subsequent re-authentications allowed before a full
authentication needs to be performed. Because a 16-bit counter is
used in re-authentication, the theoretical maximum number of re-
authentications is reached when the counter value reaches 0xFFFF.
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In order to use re-authentication, the client and the server need to
store the following values: Master Key, 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-
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/SIM/Start 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/SIM/Start
packet.
If the server recognizes the re-authentication identity and agrees
on using re-authentication, then the server sends the EAP-
Request/SIM/Re-authentication 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/SIM/Re-
authentication 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/SIM/Re-
authentication packet. If these checks are successful, the re-
authentication has succeeded and the server sends the EAP-Success
packet to the client.
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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/SIM/Re-authentication |
| (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/SIM/Re-authentication |
| (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/SIM/Re-authentication, it indicates the counter
synchronization problem by including the encrypted
AT_COUNTER_TOO_SMALL in EAP-Response/SIM/Re-authentication. The
server responds with EAP-Request/SIM/Start to initiate a normal full
authentication procedure. This is illustrated in the following
figure. Encrypted attributes are denoted with '*'.
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Client Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a re-authentication identity) |
|------------------------------------------------------>|
| |
| EAP-Request/SIM/Re-authentication |
| (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/SIM/Re-authentication |
| (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/SIM/Start |
| (AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
+---------------------------------------------------------------+
| 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/SIM/Re-authentication. 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/SIM/Re-authentication packet. If not, the server silently
discards the EAP-Response/SIM/Re-authentication packet. If all
checks on the packet are successful, the server transmits a new EAP-
Request/SIM/Start packet and the full authentication procedure is
performed as usual. Since the server already knows the subscriber
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identity, it MUST NOT include AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or
AT_PERMANENT_ID_REQ in the EAP-Request/SIM/Start.
7. Message Format
The Type-Data of the EAP/SIM 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute Type
Indicates the particular type of attribute. The attribute type
values are listed in Section 18.
Length
Indicates the length of this attribute in multiples of four
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/SIM 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.
Unless otherwise specified, the order of the attributes in an
EAP/SIM message is insignificant, and an EAP/SIM implementation
should not assume a certain order to be used.
Haverinen and Salowey Expires in six months [Page 25]
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Attributes can be encapsulated within other attributes. In other
words, the value field of an attribute type can be specified to
contain other attributes.
8. Message Authentication and Encryption
This section specifies EAP/SIM attributes for attribute encryption
and EAP/SIM message authentication.
Because the K_encr and K_aut keys derived from the RAND challenges
(as specified in Section 17) are required to process the integrity
protection and encryption attributes, these attributes can only be
used in the EAP-Request/SIM/Challenge message and any EAP/SIM
messages sent after EAP-Requets/SIM/Challenge. For example, these
attributes cannot be used in EAP-Request/SIM/Start.
8.1. AT_MAC Attribute
The AT_MAC attribute is used for EAP/SIM message authentication. The
AT_MAC attribute MUST be included in all EAP/SIM packets whenever
key material (K_aut) has been derived. However, AT_MAC MUST NOT be
included in packets with the EAP/SIM message Subtype Start, even if
key material from the previous authentication was available.
Messages that do not meet these conditions 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/SIM 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 |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The MAC algorithm is HMAC-SHA1-128 [8] 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 derivation of the authentication key (K_aut) used in
the calculation of the MAC is specified in Section 17.
8.2. AT_CHECKCODE Attribute
The AT_MAC attribute is not used in the very first EAP/SIM messages,
because keying material has not been derived yet. The client and the
server exchange one or more pairs of EAP/SIM messages of the Subtype
Start before keys are derived and before the AT_MAC attribute can be
applied. The EAP/SIM/Start messages may also be used upon re-
authentication.
The AT_CHECKCODE attribute MAY be used to protect the EAP/SIM Start
messages. AT_CHECKCODE is included in EAP-Request/SIM/Challenge
and/or EAP-Response/SIM/Challenge upon full authentication. In re-
authentication, AT_CHECKCODE can be included in EAP-Request/SIM/Re-
authentication and/or EAP-Response/SIM/Re-authentication. Because
the AT_MAC attribute is used in these messages, AT_CHECKCODE will be
integrity protected with AT_MAC.
The format of the AT_CHECKCODE attribute is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_CHECKCODE | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Checkcode (0 or 20 bytes) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value field of AT_CHECKCODE begins with two reserved bytes,
which may be followed by a 20-byte checkcode. If the checkcode is
not included in AT_CHECKCODE, then the attribute indicates that no
EAP/SIM/Start messages were exchanged. This may occur in re-
authentication only, so upon full authentication, AT_CHECKCODE
always contains the 20-byte checkcode. The reserved bytes are set to
zero when sending and ignored on reception.
The checkcode is a hash value, calculated with SHA1 [11], over the
EAP-Request/SIM/Start and EAP-Response/SIM/Start packets exchanged
in this authentication exchange. The packets are included in the
order that they were transmitted, that is, starting with the first
EAP-Request/SIM/Start message, followed by the corresponding EAP-
Response/SIM/Start, followed by the second EAP-Request/SIM/Start (if
used) etc. EAP packets are included in the hash calculation "as-is",
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as they were transmitted or received. All reserved bytes, padding
bytes etc. that are specified for various attributes are included as
such, and the receiver must not reset them to zero. No delimiter
bytes, padding or any other framing are included between the EAP
packets when calculating the checkcode.
Messages are included in request/response pairs; in other words only
full "round trips" are included. Packets that are silently discarded
are not included. The EAP server must only include an EAP-
Request/SIM/Start in the calculation once it has received a
corresponding response, with the same Identifier value.
Retransmissions or requests to which the server does not receive
response are not included.
The client must include the EAP-Request/SIM/Start and the
corresponding response in the calculation only if the client
receives a subsequent EAP-Request/SIM/Challenge, or a follow-up EAP-
Request/SIM/Start with different attributes (attribute types) than
in the first EAP-Request/SIM/Start. After sending EAP-
Response/SIM/Start, if the client receives another EAP-
Request/SIM/Start with the same attributes as in the previous
request, then the client's response to the first request must have
been lost. In this case the client must not include the first
request and its response in the calculation of the checkcode.
The AT_CHECKCODE attribute is optional to implement. It is specified
in order to allow protecting the EAP/SIM/Start messages and any
future extensions to them. The implementation of AT_CHECKCODE is
recommended.
If the receiver of AT_CHECKCODE implements this attribute, then the
receiver MUST check that the checkcode is correct. If the checkcode
is invalid, the receiver must terminate the authentication exchange.
If the EAP_/SIM/Start messages are extended with new attributes then
AT_CHECKCODE must be implemented and used. More specifically, if the
server includes any other attributes than AT_PERMANENT_ID_REQ,
AT_FULLAUTH_ID_REQ, AT_ANY_ID_REQ or AT_VERSION_LIST in the EAP-
Request/SIM/Start packet, then the server MUST include AT_CHECKCODE
in EAP-Request/SIM/Challenge or EAP-Request/SIM/Re-authentication.
If the client includes any other attributes than AT_NONCE_MT,
AT_IDENTITY or AT_SELECTED_VERSION in the EAP-Response/SIM/Start
message, then the client MUST include AT_CHECKCODE in EAP-
Response/SIM/Challenge or EAP-Response/SIM/Re-authentication.
If the server implements the processing of any other attribute than
AT_NONCE_MT, AT_IDENTITY or AT_SELECTED_VERSION in the EAP-
Response/SIM/Start message, then the server MUST implement
AT_CHECKCODE. In this case, if the server receives any other
attribute than AT_NONCE_MT, AT_IDENTITY or AT_SELECTED_VERSION in
the EAP-Response/SIM/Start message, then the server MUST check that
AT_CHECKCODE is present in EAP-Response/SIM/Challenge or EAP-
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Response/SIM/Re-authentication. If AT_CHECKCODE is not included, the
server must terminate the authentication exchange.
Similarly, if the client implements the processing of any other
attribute than AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ,
AT_ANY_ID_REQ or AT_VERSION_LIST in the EAP-Request/SIM/Start
packet, then the client MUST implement AT_CHECKCODE. In this case,
if the client receives any other attribute than AT_PERMANENT_ID_REQ,
AT_FULLAUTH_ID_REQ, AT_ANY_ID_REQ or AT_VERSION_LIST in the EAP-
Request/SIM/Start packet, then the client MUST check that
AT_CHECKCODE is present in EAP-Request/SIM/Challenge or EAP-
Request/SIM/Re-authentication. If the attribute was not included,
the client must terminate the authentication exchange.
8.3. AT_IV, AT_ENCR_DATA and AT_PADDING Attributes
AT_IV and AT_ENCR_DATA attributes can be used to transmit encrypted
information between the EAP/SIM 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 SIM 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) [9] 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 [10] for a description of the CBC
mode. The format of the AT_ENCR_DATA attribute is shown below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_ENCR_DATA | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Encrypted Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The derivation of the encryption key (K_encr) is specified in
Section 17.
The plaintext consists of nested EAP/SIM 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... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9. EAP-Request/SIM/Start
The first SIM specific EAP Request is of subtype Start. The
EAP/SIM/Start roundtrip is used for two purposes. On full
authentication, this packet is used to request the client to send
the AT_NONCE_MT attribute to the server. In addition, as specified
in Section 5.2, the Start round trip may be used for obtaining the
client identity to the server. The format of the EAP
Request/SIM/Start packet is shown below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_VERSION_L..| Length | Actual Version List Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Supported Version 1 | Supported Version 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Supported Version N | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 for Request
Identifier
See [1].
Length
The length of the EAP packet.
Type
18
Subtype
10
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 5.3. It MUST NOT be
included if AT_ANY_ID_REQ or AT_FULLAUTH_ID_REQ is included. The
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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 5.3. 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 5.2. It MUST NOT be included if
AT_PERMANENT_ID_REQ or AT_FULLAUTH_ID_REQ is included. The value
field only contains two reserved bytes, which are set to zero on
sending and ignored on reception.
AT_VERSION_LIST
The AT_VERSION_LIST attribute MUST be included. This attribute is
used in version negotiation, as specified in Section 4. The value
field of this attribute begins with 2-byte Actual Version List
Length, which specifies the length of the Version List in bytes,
not including the Actual Version List Length bytes. This field is
followed by the list of supported version, each 2 bytes. For
example, if there is only one supported version, then the Actual
Version List Length is 2. Because the length of the attribute
must be a multiple of 4 bytes, the sender pads the value field
with zero bytes when necessary.
10. EAP-Response/SIM/Start
The format of the EAP Response/SIM/Start packet is shown below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_NONCE_MT | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NONCE_MT |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IDENTITY | Length | Actual Identity Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Identity (optional) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_SELECTED...| Length = 1 | Selected Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2 for Response
Identifier
See [1].
Length
The length of the EAP packet.
Type
18
Subtype
10
Reserved
Set to zero when sending, ignored on reception.
AT_NONCE_MT
The AT_NONCE_MT attribute MUST NOT be included on re-
authentication, that is, if the AT_IDENTITY with a re-
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authentication identity is included. AT_NONCE_MT MUST be included
in all other cases (full authentication). The value field
contains two reserved bytes followed by a random number generated
by the client (16 bytes) freshly for this EAP/SIM authentication.
The random number is used as a seed value for the new keying
material. The reserved bytes are set to zero upon sending and
ignored upon reception.
AT_IDENTITY
The AT_IDENTITY attribute is optional to include and it is
included in cases defined in Section 5.2 and 5.3. 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 identity, a pseudonym identity or a
re-authentication identity. The identity format is specified in
Section 5.1. 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.
AT_SELECTED_VERSION
The AT_SELECTED_VERSION attribute MUST NOT be included on re-
authentication, that is, if the AT_IDENTITY attribute with a re-
authentication identity is included. In all other cases,
AT_SELECTED_VERSION MUST be included (full authentication). This
attribute is used in version negotiation, as specified in Section
4. The value field of this attribute contains a two-byte version
number, which indicates the EAP/SIM version that the client wants
to use.
11. EAP-Request/SIM/Challenge
The format of the EAP-Request/SIM/Challenge packet is shown below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_RAND | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. n*RAND .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IV | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Initialization Vector (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_ENCR_DATA | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Encrypted Data (optional) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_CHECKCODE | Length = 6 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Checkcode (optional) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 for Request
Identifier
See [1]
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Length
The length of the EAP packet.
Type
18
Subtype
11
Reserved
Set to zero when sending, ignored on reception.
AT_RAND
The AT_RAND attribute MUST be included. The value field of this
attribute contains two reserved bytes followed by n GSM RANDs
(each 16 bytes long). The reserved bytes are set to zero upon
sending and ignored upon reception.
The number of RAND challenges MUST be one, two or three. The
client MUST silently ignore the EAP-Request/SIM/Challenge
message, if the number of RAND challenges is smaller than what is
required by client's local policy.
If two or three RAND challenges are used, the server MUST use
different RAND values. In other words, a RAND value can only be
included once in AT_RAND. The client MUST check that the RANDs
are different. If the client detects that the server has repeated
a RAND value, the client MUST terminate the EAP exchange.
AT_IV
The AT_IV attribute is optional to include. See section 8.3.
AT_ENCR_DATA
The AT_ENCR_DATA attribute is optional to include. See section
8.3. The plaintext consists of nested attributes as described
below.
AT_CHECKCODE
The AT_CHECKCODE attribute is optional to include. See section
8.2.
AT_MAC
AT_MAC MUST be included. For EAP-Request/SIM/Challenge, the MAC
code is calculated over the following data:
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EAP packet| NONCE_MT
The EAP packet is represented as specified in Section 8.1. It is
followed by the 16-byte NONCE_MT value from the client's
AT_NONCE_MT attribute.
The AT_IV, AT_ENCR_DATA and AT_MAC attributes are used for identity
privacy and for communicating the next re-authentication identity.
The plaintext of the AT_ENCR_DATA value field consists of nested
attributes, which are shown below.
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_PSEU..| 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
The AT_NEXT_PSEUDONYM attribute is optional to include. 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 username, of the indicated
actual length, that the client can use in the next
authentication, as described in Section 5.3. The username does
not include any terminating null characters. Because the length
of the attribute must be a multiple of 4 bytes, the sender pads
the 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
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re-authentication identity, of the indicated actual length, that
the client can use in the next re-authentication, as described in
Section 6. 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
The AT_PADDING attribute is optional. See section 8.3
12. EAP-Response/SIM/Challenge
The format of the EAP-Response/SIM/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_ENCR_DATA and AT_IV attributes are not shown in the
figure below. If present, they are processed as in EAP-
Request/SIM/Challenge packet. The EAP server MUST process EAP-
Response/SIM/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_CHECKCODE | Length = 6 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Checkcode (optional) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2 for Response
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Identifier
See [1].
Length
The length of the EAP packet.
Type
18
Subtype
11
Reserved
Set to zero when sending, ignored on reception.
AT_CHECKCODE
The AT_CHECKCODE attribute is optional to include. See section
8.2.
AT_MAC
AT_MAC MUST be included. For EAP-Response/SIM/Challenge, the MAC
code is calculated over the following data:
EAP packet| n*SRES
The EAP packet is represented as specified in Section 8.1. The
EAP packet bytes are immediately followed by the one, two or
three SRES values concatenated, denoted above with the notation
n*SRES. The SRES values are used in the same order as the
corresponding RAND challenges in AT_RAND attribute.
13. EAP-Request/SIM/Re-authentication
The format of the EAP-Request/SIM/Re-authentication packet is shown
below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IV | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Initialization Vector |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_ENCR_DATA | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Encrypted Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_CHECKCODE | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Checkcode (optional) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 for Request
Identifier
See [1].
Length
The length of the EAP packet.
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Type
18
Subtype
13
Reserved
Set to zero when sending, ignored on reception.
AT_IV
The AT_IV attribute is MUST be included. See section 8.3.
AT_ENCR_DATA
The AT_ENCR_DATA attribute MUST be included. See section 8.3. The
plaintext consists of nested attributes as described below.
AT_CHECKCODE
The AT_CHECKCODE attribute is optional to include. See section
8.2.
AT_MAC
AT_MAC MUST be included. No message-specific data is included in
the MAC calculation. See Section 8.1.
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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_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
by the server (16 bytes) freshly for this EAP/SIM 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 11.
AT_PADDING
The AT_PADDING attribute is optional to include. See section 8.3
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14. EAP-Response/SIM/Re-authentication
The format of the EAP-Response/SIM/Re-authentication 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_CHECKCODE | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Checkcode (optional) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2 for Response
Identifier
See [1].
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Length
The length of the EAP packet.
Type
18
Subtype
13
Reserved
Set to zero when sending, ignored on reception.
AT_IV
The AT_IV attribute is MUST be included. See section 8.3.
AT_ENCR_DATA
The AT_ENCR_DATA attribute MUST be included. See section 8.3. The
plaintext consists of nested attributes as described below.
AT_CHECKCODE
The AT_CHECKCODE attribute is optional to include. See section
8.2.
AT_MAC
For EAP-Response/SIM/Re-authentication, the MAC code is
calculated over the following data:
EAP packet| NONCE_S
The EAP packet is represented as specified in Section 8.1. It is
followed by the 16-byte NONCE_S value from the server'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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_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 13.
AT_COUNTER_TOO_SMALL
The AT_COUNTER_TOO_SMALL attribute is optional to include, and it
is included in cases specified in Section 6.
AT_PADDING
The AT_PADDING attribute is optional to include. See section 8.3
15. Error Cases and the Usage of EAP-Failure and EAP-Success
15.1. Processing Erroneous Packets
In general, if an EAP/SIM client or server implementation detects an
error in a received EAP/SIM packet, the EAP/SIM 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, insufficient number of RAND challenges included in
AT_RAND, no acceptable version included in AT_VERSION_LIST, a
mandatory attribute is missing, illegal attributes included and an
unrecognized non-skippable attribute.
If the EAP/SIM client receives an EAP/SIM Request of an unrecognized
subtype, the EAP/SIM client MUST silently discard the EAP request.
The rationale for this error case behavior is that an active
attacker may have sent the erroneous packet. As the EAP/SIM client
or server does not process the packet and does not change its state,
it is possible to successfully process a valid packet if such packet
is received later. If no valid packets are received, the
authentication exchange will eventually time out.
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15.2. EAP-Failure
As normally in EAP, the EAP server sends the EAP-Failure packet to
the client when the authentication procedure fails on the EAP
Server. In EAP/SIM, this may occur for example if the EAP server
does not recognize the user identity, or if the EAP server is not
able to obtain the GSM triplets for the subscriber or the
authentication exchange times out.
The server can send EAP-Failure at any time in the EAP exchange. The
client MUST process EAP-Failure.
15.3. EAP-Success
On full authentication, the server can only send EAP-Success after
the EAP/SIM/Challenge round. The client MUST silently discard any
EAP-Success packets if they are received before the client has
successfully authenticated the server and sent the EAP-
Response/SIM/Challenge packet.
On re-authentication, EAP-Success can only be sent after the
EAP/SIM/Re-authentication round. The client MUST silently discard
any EAP-Success packets if they are received before the client has
successfully authenticated the server and sent the EAP-
Response/SIM/Re-authentication packet.
If the client receives an EAP/SIM notification (section 16) that
indicates failure, then the client MUST no longer accept the EAP-
Success packet even if the server authentication was successfully
completed.
16. EAP/SIM Notifications
The EAP-Request/Notification, specified in [1], can be used to
convey a displayable message from the authenticator to the client.
Because these messages are textual messages, it may be hard for the
client to present them in the user's preferred language. Therefore,
EAP/SIM uses a separate EAP/SIM message subtype to transmit
localizable notification codes instead of the EAP-
Request/Notification packet.
The EAP server MAY issue an EAP-Request/SIM/Notification packet to
the client. The client MAY show a notification message to the user
and the client MUST respond to the EAP server with an EAP-
Response/SIM/Notification packet, even if the client did not
recognize the notification code.
The notification code is a 16-bit number. The most significant bit
is called the Failure bit (F bit). The F bit specifies whether the
notification implies failure. The code values with the F bit set to
zero (code values 0...32767) are used on unsuccessful cases. The
receipt of a notification code from this range implies failed
authentication, so the client can use the notification as a failure
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indication. After receiving the EAP-Response/SIM/Notification for
these notification codes, the server MUST send the EAP-Failure
packet.
The receipt of a notification code with the F bit set to one (values
32768...65536) does not imply failure, so the client MUST NOT change
its state when it receives such a notification.
The second most significant bit of the notification code is called
the Phase bit (P bit). It specifies at which phase of the EAP/SIM
exchange the notification can be used. If the P bit is set to zero,
the notification can only be used after the EAP/SIM/Challenge round
in full authentication or the EAP/SIM/Reauthentication round in
reautentication. For these notifications, the AT_MAC attribute MUST
be included in both EAP-Request/SIM/Notification and EAP-
Response/SIM/Notification.
If the P bit is set to one, the notification can only by used before
the EAP/SIM/Challenge round in full authentication or the
EAP/SIM/Reauthentication round in reauthentication. For these
notifications, the AT_MAC attribute MUST NOT be included in either
EAP-Request/SIM/Notification or EAP-Response/SIM/Notification.
Some of the notification codes are authorization related and hence
not usually considered as part of the responsibility of an EAP
method. However, they are included as part of EAP/SIM because there
are currently no other ways to convey this information to the user
in a localizable way, and the information is potentially useful for
the user. An EAP/SIM server implementation may decide never to send
these EAP/SIM notifications.
The format of the EAP-Request/SIM/Notification packet is shown
below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_NOTIFICATION| Length = 1 |F|P| Notification Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 for Request
Identifier
See [1].
Length
The length of the EAP packet.
Type
18
Subtype
12
Reserved
Set to zero when sending, ignored on reception.
AT_NOTIFICATION
The AT_NOTIFICATION attribute MUST be included. The value field
of this attribute contains a two-byte notification code. The
first and second bit (F and P) of the notification code are
interpreted as described above.
The following notification code values have been reserved. The
descriptions below illustrate the semantics of the notifications.
The client implementation MAY use different wordings when
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presenting the notifications to the user. The "requested service"
depends on the environment where EAP/SIM is applied.
1026 - User has been temporarily denied access to the requested
service. (Implies failure, used after the challenge round)
1031 - User has not subscribed to the requested service (implies
failure, used after the challenge round)
AT_MAC
AT_MAC is included in cases described above. No message-specific
data is included in the MAC calculation. See Section 8.1.
The format of the EAP-Response/SIM/Notification packet is shown
below. Because this packet is only an acknowledgement of EAP-
Request/SIM/Notification, it does not contain any mandatory
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_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2 for Response
Identifier
See [1].
Length
The length of the EAP packet.
Type
18
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Subtype
12
Reserved
Set to zero when sending, ignored on reception.
AT_MAC
AT_MAC is included in cases described above. No message-specific
data is included in the MAC calculation. See Section 8.1.
17. Key Generation
This section specifies how keying material is generated.
On EAP SIM full authentication, a Master Key (MK) is derived from
the underlying GSM authentication values (Kc keys), the NONCE_MT and
other relevant context as follows.
MK = SHA1(Identity|n*Kc| NONCE_MT| Version List| Selected Version)
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/SIM/Start packet, or, if AT_IDENTITY was
not used, the identity from the EAP-Response/Identity packet. The
notation n*Kc denotes the n Kc values concatenated. The Kc keys are
used in the same order as the RAND challenges in AT_RAND attribute.
NONCE_MT denotes the NONCE_MT value (not the AT_NONCE_MT attribute
but just the nonce value). The Version List includes the 2-byte
supported version numbers from AT_VERSION_LIST, in the same order as
in the attribute. The Selected Version is the 2-byte selected
version from AT_SELECTED_VERSION. Network byte order is used, just
as in the attributes. The hash function SHA1 is specified in [11].
The Master Key is fed into a Pseudo-Random number Function (PRF)
which generates separate Transient EAP Keys (TEKs) for protecting
EAP SIM packets, as well as a Master Session Key (MSK) for link
layer security and an Extended Master Session Key (EMSK) for other
purposes. On re-authentication, the same TEKs will be used for
protecting EAP packets, but a new MSK and a new EMSK will be derived
from the original MK and new values exchanged in the re-
authentication.
EAP SIM requires two TEKs for its own purposes, the authentication
key K_aut to be used with the AT_MAC attribute, and the encryption
key K_encr, to be used with the AT_ENCR_DATA attribute. The same
K_aut and K_encr keys are used in full authentication and subsequent
re-authentications.
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Key derivation is based on the random number generation specified in
NIST Federal Information Processing Standards (FIPS) Publication
186-2 [12]. The pseudo-random number generator is specified in the
change notice 1 (2001 October 5) of [12] (Algorithm 1). As specified
in the change notice (page 74), when Algorithm 1 is used as a
general-purpose pseudo-random number generator, the "mod q" term in
step 3.3 is omitted. The function G used in the algorithm is
constructed via Secure Hash Standard as specified in Appendix 3.3 of
the standard. For convenience, the random number algorithm with the
correct modification is cited in Annex B.
160-bit XKEY and XVAL values are used, so b = 160. On each full
authentication, the Master Key is used as the initial secret seed-
key XKEY. The optional user input values (XSEED_j) in step 3.1 are
set to zero.
The resulting 320-bit random numbers x_0, x_1, ..., x_m-1 are
concatenated and partitioned into suitable-sized chunks and used as
keys in the following order: K_encr (128 bits), K_aut (128 bits),
Master Session Key (64 bytes), Extended Master Session Key (64
bytes).
On re-authentication, the same pseudo-random number generator can be
used to generate a new Master Session Key and new Initialization
Vectors. The seed value XKEY' is calculated as follows:
XKEY' = SHA1(Identity|counter|NONCE_S| MK)
In the formula above, the Identity denotes the re-authentication
user identity, without any terminating null characters, from the
AT_IDENTITY attribute of the EAP-Response/SIM/Start packet, or, if
EAP-Response/SIM/Start was not used on re-authentication, the
identity string from the EAP-Response/Identity packet. The counter
denotes the counter value from AT_COUNTER attribute used in the EAP-
Response/SIM/Re-authentication 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/SIM/Re-
authentication packet. The MK is the Master Key derived on the
preceding full authentication. The pseudo-random number generator is
run with the new seed value XKEY', and the resulting 320-bit random
numbers x_0, x_1, ..., x_m-1 are concatenated and partitioned into
64-byte chunks and used as the new 64-byte Master Session Key and
the new 64-byte Extended Master Session Key.
The first 32 bytes of the MSK can be used as the Pairwise Master Key
(PMK) for IEEE 802.11i.
When the RADIUS attributes specified in [14] are used to transport
keying material, then the first 32 bytes of the MSK correspond to
MS-MPPE-RECV-KEY and the second 32 bytes to MS-MPPE-SEND-KEY. In
this case, only 64 bytes of keying material (the MSK) are used.
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When generating the initial Master Key, the hash function is used as
a mixing function to combine several session keys (Kc's) generated
by the GSM authentication procedure and the random number NONCE_MT
into a single session key. There are several reasons for this. The
current GSM session keys are at most 64 bits, so two or more of them
are needed to generate a longer key. By using a one-way function to
combine the keys, we are assured that even if an attacker managed to
learn one of the EAP/SIM session keys, it wouldn't help him in
learning the original GSM Kc's. In addition, since we include the
random number NONCE_MT in the calculation, the client is able to
verify that the EAP SIM packets it receives from the network are
fresh and not a replay. (Please see also Section 19.)
18. IANA Considerations
The realm name "owlan.org" has been reserved for NAI realm names
generated from the IMSI.
IANA has assigned the EAP type number 18 for this protocol.
EAP/SIM messages include a Subtype field. The following Subtypes are
specified:
Start..........................................10
Challenge......................................11
Notification...................................12
Re-authentication..............................13
The Subtype-specific data is composed of attributes, which have
attribute type numbers. The following attribute types are specified:
AT_RAND.........................................1
AT_PADDING......................................6
AT_NONCE_MT.....................................7
AT_PERMANENT_ID_REQ............................10
AT_MAC.........................................11
AT_NOTIFICATION................................12
AT_ANY_ID_REQ..................................13
AT_IDENTITY....................................14
AT_VERSION_LIST................................15
AT_SELECTED_VERSION............................16
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
AT_CHECKCODE..................................134
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The AT_NOTIFICATION attribute contains a notification code value.
Values 1024, 1026 and 1031 have been specified in Section 16 of this
document.
The AT_VERSION_LIST and AT_SELECTED_VERSION attributes contain
version numbers. Version 1 has been specified in Section 4 of this
document.
All requests for value assignment from the various number spaces
described in this document require proper documentation, according
to the "Specification Required" policy described in [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.
19. Security Considerations
The revised EAP base protocol [16] highlights several attacks that
are possible against the EAP protocol as there is no inherent
security mechanisms provided. This section discusses the claimed
security properties of EAP SIM as well as vulnerabilities and
security recommendations.
19.1. Identity Protection
EAP/SIM includes optional identity privacy support that protects the
privacy of the subscriber identity against passive eavesdropping.
The mechanism cannot be used on the first connection with a given
server, when the permanent identity will have to be sent in the
clear. The terminal SHOULD store the pseudonym in a non-volatile
memory so that it can be maintained across reboots. An active
attacker that impersonates the network may use the
AT_PERMANENT_ID_REQ attribute (Section 5.3) to learn the
subscriber's permanent identity. However, as discussed in Section
5.3, the terminal can refuse to send the cleartext permanent
identity if it believes that the network should be able to recognize
the pseudonym.
If the client and server cannot guarantee that the pseudonym will be
maintained reliably and identity privacy is required then additional
protection from an external security mechanism such as Protected
Extensible Authentication Protocol (PEAP) [17] may be used. If an
external security mechanism is in use identity Privacy features of
EAP-SIM may not be useful. The security considerations of using an
external security mechanism with EAP-SIM are beyond the scope of
this document.
19.2. Mutual Authentication and Triplet Exposure
EAP/SIM provides mutual authentication. The client believes that the
network is authentic because the network can calculate a correct
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AT_MAC value in the EAP-Request/SIM/Challenge packet. To calculate
AT_MAC, it is sufficient to know the RAND and Kc values from the GSM
triplets (RAND, SRES, Kc) used in the authentication. Because the
network selects the RAND challenges and hereby the triplets, an
attacker that knows n (1, 2 or 3) GSM triplets for the subscriber is
able to impersonate a valid network to the client. Given physical
access to the SIM card, it is easy to obtain any number of GSM
triplets. Another way to obtain triplets is to mount an attack on
the client platform via a virus or other malicious piece of
software. The client SHOULD be protected against triplet querying
attacks by malicious software.
If the same SIM credentials are also used for GSM traffic, the
triplets could be revealed in the GSM network; see Section 19.5.
Since the security of EAP/SIM is based on the secrecy of Kc keys
care should be taken not to expose these values to attackers when
they are transmitted between entities, stored or handled. Steps
should be taken to limit the transport, storage and handling of
these values outside a protected environment. These considerations
are important at both the client and authenticator implementations.
In GSM, the network is allowed to reuse the RAND challenge in
consecutive authentication exchanges. This is not allowed in EAP/SIM
but the EAP/SIM server is mandated to use fresh triplets (RAND
challenges) in consecutive authentication exchanges, as specified in
Section 3. However, EAP SIM does not include means for the client to
check if the RANDs are fresh, so the security of the scheme leans on
the secrecy of the triplets.
Preventing the re-use of authentication vectors has been taken into
account in the design of the UMTS Authentication and Key Agreement
(AKA), which is used in EAP AKA [21]. In cases when the triplet re-
use considerations of EAP SIM are not considered sufficient, it is
advised to use EAP AKA.
19.3. Key Derivation
EAP/SIM supports key derivation. The key hierarchy is specified in
Section 17. EAP/SIM combines several GSM triplets in order to
generate stronger keying material and stronger AT_MAC values. The
actual strength of the resulting keys depends, among other things,
on the operator-specific authentication algorithms, the strength of
the Ki key, and the quality of the RAND challenges, which is also
operator specific. For example, some SIM cards generate Kc keys with
10 bits set to zero. Such restrictions may prevent the concatenation
technique from yielding strong session keys. Because the strength of
the Ki key is 128 bits, the ultimate strength of any derived secret
key material is never more than 128 bits.
EAP SIM client is recommended to require at least two triplets to be
used in each EAP SIM exchange (n = 2 or n = 3). The server is
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recommended to always include at least two triplets in EAP-
Request/SIM/Challenge.
In the case where one triplet is used (n = 1), if the client is the
attacker, he needs to guess 96 bits (64 in k_int and 32 in SRES) if
the server is the attacker he needs to guess 64 bits, but given the
128 bit nonce he cannot make any precomputations. This provides a
reasonable level of security for the MAC exchange, but only provides
64 bits for the Master Session Key. Therefore a single triplet can
only be used in the case where a Master Session Key is not needed or
is provided by an external security mechanism such as PEAP. However,
the use of a single triplet is strongly discouraged in this case
too, because it may enable an adversary to attack on individual
triplets. It should also be noted that a security policy that allows
n=1 to be used may compromise the security of a future policy that
requires more than one triplet, because adversaries will be able to
exploit the triplets they have learnt when the weaker policy was
applied. Security considerations for the use of an external security
mechanism with EAP/SIM are beyond the scope of this document.
There is no known way to obtain complete GSM triplets by mounting an
attack against EAP/SIM. A passive eavesdropper can learn n*RAND and
AT_MAC and may be able to link this information to the subscriber
identity. An active attacker that impersonates a GSM subscriber can
easily obtain n*RAND and AT_MAC values from the EAP server for any
given subscriber identity. However, calculating the Kc and SRES
values from AT_MAC would require the attacker to reverse the keyed
message authentication code function HMAC-SHA1-128.
As EAP SIM does not expose any values calculated from an individual
GSM Kc keys when more than one triplet is used, it is not possible
to mount a brute force attack on just one of the Kc keys in EAP SIM.
Therefore, when considering brute force attacks on the values
exposed in EAP SIM, the effective length of EAP SIM session keys is
not compromised by the fact that they are combined from several
shorter keys, i.e the effective length of 128 bits may be achieved.
However, see Section 19.5.The EAP Transient Keys used to protect EAP
SIM packets (K_encr, K_aut) and the Master Session Key are
cryptographically separate. An attacker cannot derive any non-
trivial information from K_encr or K_aut based on the Master Session
Key or vice versa. An attacker also cannot calculate the pre-shared
secret from the GSM Kc keys used, EAP SIM K_encr, EAP SIM K_aut or
from the Master Session Key.
Each EAP/SIM exchange generates fresh keying material. The EAP SIM
client contributes to the keying material with the NONCE_MT
parameter, which must be chosen freshly for each exchange. Hence,
even if the RAND challenges were reused from a previous session, the
session keys will be different. Please see section 19.2 for more
information about RAND reuse.
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19.4. Dictionary Attacks
Because EAP/SIM is not a password protocol, it is not vulnerable to
dictionary attacks. (The pre-shared symmetric secret shall not be a
weak password.)
19.5. Credentials Reuse
EAP SIM cannot prevent attacks over the GSM or GPRS radio networks.
If the same SIM credentials are also used in GSM or GPRS, it is
possible to mount attacks over the cellular interface.
A passive attacker can eavesdrop GSM or GPRS traffic and obtain
RAND, SRES pairs. He can then use a brute force attack to obtain the
64-bit Kc keys used to encrypt the GSM or GPRS data. This makes it
possible to attack each 64-bit key separately. If the attacker can
obtain 1-3 Kc keys, he can then impersonate a valid network to an
EAP-SIM client.
An active attacker can mount a "rogue GSM/GPRS base station attack",
replaying previously seen RAND challenges to obtain SRES values. He
can then use a brute force attack to obtain the Kc keys. If
successful, the attacker can impersonate a valid network or decrypt
previously seen traffic, because EAP-SIM does not provide Perfect
Forward Secrecy (PFS).
Because this attack requires the attacker to build a rogue GSM base
station (or at least eavesdrop the GSM traffic), the cost of the
attack is not negligible; it is the same cost as usually in GSM.
However, due to several weaknesses in the GSM encryption algorithms,
the effective key strength of the Kc keys is much less than the
expected 64 bits (no more than 40 bits if the A5/1 GSM encryption
algorithm is used; an active attacker can force the client to use
the weaker A5/2 algorithm that can be broken in less than a second).
Because the A5 encryption algorithm is not used in EAP SIM, and
because EAP SIM does not expose any values calculated from
individual Kc keys, it should be noted that these attacks are not
possible if the SIM credentials used in EAP/SIM are not shared in
GSM/GPRS.
19.6. Integrity Protection, Replay Protection and Confidentiality
AT_MAC, AT_IV and AT_ENCR_DATA attributes are used to provide
integrity, replay and confidentiality protection for EAP/SIM
Requests and Responses. Integrity protection includes the EAP
header. These attributes cannot be used during the EAP/SIM/Start
roundtrip. However, the protocol values (identity, NONCE_MT and
version negotiation parameters) are protected by later EAP/SIM
messages. The AT_CHECKCODE attribute can optionally be used to
protect the integrity of the EAP/SIM/Start roundtrip.
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Integrity protection (AT_MAC) is based on a keyed message
authentication code. Confidentiality (AT_ENCR_DATA and AT_IV) is
based on a block cipher.
On full authentication, replay protection is provided by the
underlying GSM authentication scheme, which makes use of the RAND
value, and the use of the NONCE_MT value. On re-authentication, a
counter and a server nonce is used to provide replay protection.
Contents of the EAP-Response/Identity packet are implicitly
integrity protected by including them in key derivation.
Because EAP/SIM is not a tunneling method, EAP Notification, EAP
Success or EAP Failure packets are not confidential, integrity
protected or replay protected in EAP/SIM. On physically insecure
networks, this may enable an attacker to send false notifications to
the peer and to mount denial of service attacks by spoofing these
packets.
An eavesdropper will see the EAP Notification, EAP Success and EAP
Failure packets sent in the clear. With EAP SIM, confidential
information MUST NOT be transmitted in EAP Notification packets.
19.7. Negotiation Attacks
EAP/SIM does not protect the EAP-Response/Nak packet. Because
EAP/SIM does not protect the EAP method negotiation, EAP method
downgrading attacks may be possible, especially if the user uses the
same identity with EAP/SIM and other EAP methods.
EAP/SIM includes a version negotiation procedure. In EAP/SIM the
keying material derivation includes the version list and selected
version, ensuring the protocol cannot be downgraded and that the
client and server use the same version of EAP/SIM.
As described in Section 7, EAP/SIM allows the protocol to be
extended by defining new attribute types. When defining such
attributes, it should noted that any extra attributes included in
EAP-Request/SIM/Start or EAP-Response/SIM/Start packets are not
included in the MACs later on, and thus some other precautions must
be taken to avoid modifications to them.
EAP/SIM does not support ciphersuite negotiation.
19.8. Fast Reconnect
EAP/SIM includes an optional re-authentication ("fast reconnect")
procedure, as recommended in [16] for EAP types that are intended
for physically insecure networks.
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19.9. Acknowledged Result Indications
EAP/SIM does not provide acknowledged or integrity protected Success
or Failure indications.
If an EAP Success or EAP Failure packet is lost when using EAP/SIM
over an unreliable medium, and if the protocol over which EAP/SIM is
transported does not address the possible loss of Success or
Failure, then the peer and authenticator may end up having a
different interpretation of the state of the authentication
conversation.
On physically insecure networks, an attacker may mount denial of
service attacks by sending false EAP Success or EAP Failure
indications. However, the attacker cannot force the client or the
authenticator to believe successful authentication has occurred when
mutual authentication failed or has not happened yet.
19.10. Man-in-the-middle Attacks
In order to avoid man-in-the-middle attacks and session hijacking,
user data SHOULD be integrity protected on physically insecure
networks. The EAP/SIM Master Session Key or keys derived from it MAY
be used as the integrity protection keys, or, if an external
security mechanism such as PEAP is used, then the link integrity
protection keys MAY be derived by the external security mechanism.
There are man-in-the-middle attacks associated with the use of any
EAP method within a tunneled protocol such as PEAP, or within a
sequence of EAP methods followed by each other. This specification
does not address these attacks. If EAP/SIM is used with a tunneling
protocol or as part of a sequence of methods, there should be
cryptographic binding provided between the protocols and EAP/SIM to
prevent man-in-the-middle attacks through rogue authenticators being
able to setup one-way authenticated tunnels. The EAP/SIM Master
Session Key MAY be used to provide the cryptographic binding.
However the mechanism how the binding is provided depends on the
tunneling or sequencing protocol, and it is beyond the scope of this
document.
19.11. Generating Random Numbers
An EAP/SIM implementation SHOULD use a good source of randomness to
generate the random numbers required in the protocol. Please see
[18] for more information on generating random numbers for security
applications.
20. Security Claims
This section provides the security claims required by [16].
[a] Intended use. EAP SIM is intended for use over both physically
insecure networks and physically or otherwise secure networks.
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Applicable media include but are not limited to PPP, IEEE 802 wired
networks and IEEE 802.11.
[b] Mechanism. EAP SIM is based on the GSM SIM mechanism, which is a
challenge/response authentication and key agreement mechanism based
on a symmetric 128-bit pre-shared secret. EAP SIM also makes use of
a client challenge to provide mutual authentication.
[c] Security claims. The security properties of the method are
discussed in Section 19.
[d] Key strength. EAP SIM supports key derivation with 128-bit
effective key strength if several triplets are used per
authentication exchange, and 64-bit effective key strength if only
one triplet is used. (The policies in the client and the server can
prevent authentication exchanges with just one triplet.) However, as
discussed in Section 19, if the same credentials are used in
GSM/GPRS and in EAP/SIM, then the key strength may be reduced
considerably, basically to the same level as in GSM, by mounting
attacks over GSM/GPRS. For example an active attack using a false
GSM/GPRS base station reduces the effective key strength to almost
zero.
[e] Description of key hierarchy. Please see Section 17.
[f] Indication of vulnerabilities. Vulnerabilities are discussed in
Section 19.
21. Intellectual Property Right Notice
On IPR related issues, Nokia refers to the Nokia Statement on Patent
licensing, see http://www.ietf.org/ietf/IPR/NOKIA.
22. Acknowledgements and Contributions
In addition to the editors, Nora Dabbous, Jose Puthenkulam, and
Prasanna Satarasinghe are significant contributors of this document.
Juha Ala-Laurila, N. Asokan, Jan-Erik Ekberg, Patrik Flykt, Jukka-
Pekka Honkanen, Antti Kuikka, Jukka Latva, Lassi Lehtinen, Jyri
Rinnemaa, Timo Takam„ki and Raimo Vuonnala have contributed many of
the original ideas and concepts of this protocol.
N. Asokan and Jukka-Pekka Honkanen have contributed and helped in
innumerable ways during the whole development of the protocol.
Valtteri Niemi and Kaisa Nyberg contributed substantially to the
design of the key derivation and the re-authentication procedure,
and have also provided their cryptographic expertise in many
discussions related to this protocol.
Simon Blake-Wilson provided most helpful comments on key derivation
and version negotiation.
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Thanks to Greg Rose for his most valuable comments [19].
Thanks to Bernard Aboba, Vladimir Alperovich, Jacques Caron, Gopal
Dommety, Pasi Eronen, Augustin Farrugia, Mark Grayson, Max de Groot,
Prakash Iyer, Nishi Kant, Victor Lortz, Sarvar Patel, Stefan
Schr÷der, Jesse Walker and Thomas Wieland for their contributions
and critiques. Special thanks to Max for proposing improvements to
the MAC calculation.
The identity privacy support is based on the identity privacy
support of [7]. The attribute format is based on the extension
format of Mobile IPv4 [20].
This protocol has been partly developed in parallel with EAP AKA
[21], and hence this specification incorporates many ideas from Jari
Arkko.
References
[1] L. Blunk, J. Vollbrecht, "PPP Extensible Authentication
Protocol (EAP)", RFC 2284, March 1998. (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] S. Bradner, "Key words for use in RFCs to indicate Requirement
Levels", RFC 2119, March 1997. (NORMATIVE)
[4] GSM Technical Specification GSM 03.03 (ETS 300 523): "Digital
cellular telecommunication system (Phase 2); Numbering,
addressing and identification", European Telecommunications
Standards Institute, April 1997. (NORMATIVE)
[5] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999. (NORMATIVE)
[6] Draft 3GPP Technical Specification 3GPP TS 23.234 V 1.4.0:
"Technical Specification Group Services and System Aspects;
3GPP system to Wireless Local Area Network (WLAN)
Interworking; System Description", 3rd Generation Partnership
Project, work in progress, January 2003. (INFORMATIVE)
[7] J. Carlson, B. Aboba, H. Haverinen, "EAP SRP-SHA1
Authentication Protocol", draft-ietf-pppext-eap-srp-03.txt,
July 2001 (work-in-progress). (INFORMATIVE)
[8] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for
Message Authentication", RFC 2104, February 1997. (NORMATIVE)
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[9] Federal Information Processing Standard (FIPS) draft standard,
"Advanced Encryption Standard (AES)",
http://csrc.nist.gov/publications/drafts/dfips-AES.pdf,
September 2001. (NORMATIVE)
[10] US National Bureau of Standards, "DES Modes of Operation",
Federal Information Processing Standard (FIPS) Publication 81,
December 1980. (NORMATIVE)
[11] Federal Information Processing Standard (FIPS) Publication
180-1, "Secure Hash Standard," National Institute of Standards
and Technology, U.S. Department of Commerce, April 17, 1995.
(NORMATIVE)
[12] 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
[13] B. Aboba, D. Simon, "PPP EAP TLS Authentication Protocol", RFC
2716, October 1999 (INFORMATIVE)
[14] G. Zorn, "Microsoft Vendor-specific RADIUS Attributes", RFC
2548, March 1999 (INFORMATIVE)
[15] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
(NORMATIVE)
[16] L. Blunk, J. Vollbrecht, B. Aboba, "Extensible Authentication
Protocol (EAP)", draft-ietf-pppext-rfc2284bis-07.txt, work-in-
progress, October 2002. (NORMATIVE)
[17] H. Andersson, S. Josefsson, G. Zorn, D. Simon, A. Palekar,
"Protected EAP Protocol (PEAP)", draft-josefsson-pppext-eap-
tls-eap-05.txt, work-in-progress, September 2002.
(IMFORMATIVE)
[18] D. Eastlake, 3rd, S. Crocker, J. Schiller, "Randomness
Recommendations for Security", RFC 1750 (Informational),
December 1994. (INFORMATIVE)
[19] Qualcomm, "Comments on draft EAP/SIM", 3rd Generation
Partnership Project document 3GPP TSG SA WG3 Security S3#22,
S3-020125, February 2002. (INFORMATIVE)
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[20] C. Perkins (editor), "IP Mobility Support", RFC 3344, August
2002. (INFORMATIVE)
[21] J. Arkko, H. Haverinen, "EAP AKA Authentication", draft-arkko-
pppext-eap-aka-10.txt, June 2003 (work in progress).
(INFORMATIVE)
Editors' and Contributors' Contact Information
Henry Haverinen
Nokia Mobile Phones
P.O. Box 88
FIN-33721 Tampere
Finland
E-mail: henry.haverinen@nokia.com
Phone: +358 50 594 4899
Joseph Salowey
Cisco Systems
2901 Third Avenue
Seattle, WA 98121
US
E-mail: jsalowey@cisco.com
Phone: +1 206 256 3380
Nora Dabbous
Gemplus
34 rue Guynemer
92447 Issy les Moulineaux France
E-mail: nora.dabbous@gemplus.com
Phone: +33 1 4648 2000
Jose Puthenkulam
Intel Corporation
2111 NE 25th Avenue, JF2-58
Hillsboro, OR 97124
US
E-mail: jose.p.puthenkulam@intel.com
Phone: +1 503 264 6121
Prasanna Satarasinghe
Transat Technologies
180 State Street, Suite 240
Southlake, TX 76092
US
E-mail: prasannas@transat-tech.com
Phone: + 1 817 4814412
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Annex A. Test Vectors
Test vectors for the NIST FIPS 186-2 pseudo-random number generator
[12] are available at the following URL:
http://csrc.nist.gov/encryption/dss/Examples-1024bit.pdf
TBD: Test vectors for EAP SIM values
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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 [11]
Step 3: For j = 0 to m - 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
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