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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
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This document is an individual submission for the Point-to-Point
Extensions Working Group of the Internet Engineering Task Force
(IETF). Comments should be submitted to the ietf-ppp@merit.edu
mailing list.
Distribution of this memo is unlimited.
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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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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Internet Draft EAP SIM Authentication June 2003
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_MA