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Network Working Group H. Haverinen (editor)
Internet Draft Nokia
J. Salowey (editor)
Cisco
Expires: 27 April, 2004 27 October, 2003
EAP SIM Authentication
draft-haverinen-pppext-eap-sim-12.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
Task Force (IETF), its areas, and its working groups. Note that
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Comments should be submitted to the eap@frascone.com 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.................................................6
4. Operation................................................8
4.1. Version Negotiation....................................8
4.2. Identity Management....................................9
4.3. Re-Authentication.....................................25
4.4. EAP/SIM Notifications.................................30
4.5. Error Cases...........................................31
4.6. Key Generation........................................33
5. Message Format and Protocol Extensibility...............35
5.1. Message Format........................................35
5.2. Protocol Extensibility................................37
6. Messages................................................37
6.1. EAP-Request/SIM/Start.................................37
6.2. EAP-Response/SIM/Start................................38
6.3. EAP-Request/SIM/Challenge.............................38
6.4. EAP-Response/SIM/Challenge............................39
6.5. EAP-Request/SIM/Re-authentication.....................40
6.6. EAP-Response/SIM/Re-authentication....................40
6.7. EAP-Response/SIM/Client-Error.........................40
6.8. EAP-Request/SIM/Notification..........................40
6.9. EAP-Response/SIM/Notification.........................41
7. Attributes..............................................41
7.1. Table of Attributes...................................41
7.2. AT_MAC................................................42
7.3. AT_IV, AT_ENCR_DATA and AT_PADDING....................43
7.4. AT_VERSION_LIST.......................................45
7.5. AT_SELECTED_VERSION...................................46
7.6. AT_NONCE_MT...........................................46
7.7. AT_PERMANENT_ID_REQ...................................46
7.8. AT_ANY_ID_REQ.........................................47
7.9. AT_FULLAUTH_ID_REQ....................................47
7.10. AT_IDENTITY..........................................47
7.11. AT_RAND..............................................48
7.12. AT_NEXT_PSEUDONYM....................................49
7.13. AT_NEXT_REAUTH_ID....................................49
7.14. AT_COUNTER...........................................50
7.15. AT_COUNTER_TOO_SMALL.................................50
7.16. AT_NONCE_S...........................................50
7.17. AT_NOTIFICATION......................................51
7.18. AT_CLIENT_ERROR_CODE.................................52
8. IANA Considerations.....................................52
9. Security Considerations.................................54
9.1. Identity Protection...................................54
9.2. Mutual Authentication and Triplet Exposure............54
9.3. Key Derivation........................................55
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9.4. Dictionary Attacks....................................56
9.5. Credentials Reuse.....................................56
9.6. Integrity and Replay Protection, and Confidentiality..57
9.7. Negotiation Attacks...................................57
9.8. Fast Reconnect........................................58
9.9. Acknowledged Result Indications.......................58
9.10. Man-in-the-middle Attacks............................58
9.11. Generating Random Numbers............................59
10. Security Claims........................................59
11. Intellectual Property Right Notice.....................59
12. Acknowledgements and Contributions.....................59
12.1. Contributors.........................................59
12.2. Acknowledgements.....................................60
Normative References.......................................60
Informative References.....................................61
Editors' and Contributors' Contact Information.............63
Annex A. Test Vectors......................................64
Annex B. Pseudo-Random Number Generator....................72
1. Introduction
This document specifies an Extensible Authentication Protocol (EAP)
[EAP] 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. Hence the secrecy of Kc is critical to the security
of this protocol. Please find more information about GSM
authentication in [GSM 03.20].
The lack of mutual authentication is a weakness in GSM
authentication. The 64 bit cipher key (Kc) that is derived is not
strong enough for data networks where stronger and longer keys are
required. Hence in EAP/SIM, several RAND challenges are used for
generating several 64-bit Kc keys, which are combined to constitute
stronger keying material. In EAP/SIM the client issues a random
number NONCE_MT to the network, in order to contribute to key
derivation, and to prevent replays of EAP/SIM requests from previous
exchanges. The NONCE_MT can be conceived as the client's challenge
to the network. EAP/SIM also extends the combined RAND challenges
and other messages with a message authentication code in order to
provide message integrity protection along with mutual
authentication.
EAP/SIM specifies optional support for protecting the privacy of
subscriber identity using the same concept as GSM, which is using
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pseudonyms/temporary identifiers. It also specifies 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 9 of this
document. Implementers and users of EAP/SIM are advised to carefully
study the security considerations in Section 9 in order to determine
whether the security properties are sufficient for the environment
in question, especially as the secrecy of Kc keys is key to the
security of EAP/SIM. 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. Many of the security features of EAP_SIM rely upon the
secrecy of the Kc values in the SIM triplets, so protecting these
values is key to the security of the EAP-SIM protocol. 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 [EAP AKA] specifies an
EAP method that is based on UMTS AKA. EAP AKA, which is a more
secure protocol, may be used instead of EAP/SIM, if USIMs and 3G
network infrastructure are available.
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].
The terms and abbreviations "authenticator", "backend authentication
server", "EAP server", "Silently Discard", "Master Session Key
(MSK)", and "Extended Master Session Key (EMSK)" in this document
are to be interpreted as described in [EAP].
This document frequently uses the following terms and abbreviations:
AAA protocol
Authentication, Authorization and Accounting protocol
AuC
Authentication Centre. The GSM network element that provides the
authentication triplets for authenticating the subscriber.
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Authentication vector
GSM triplets can be alternatively called authentication vectors.
EAP
Extensible Authentication Protocol.
GSM
Global System for Mobile communications.
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
Permanent Identity
The permanent identity of the peer, including an NAI realm
portion in environments where a realm is used. The permanent
identity is usually based on the IMSI. Used on full
authentication only.
Permanent Username
The username portion of permanent identity, ie. not including any
realm portions.
Pseudonym Identity
A pseudonym identity of the peer, including an NAI realm portion
in environments where a real is used. Used on full authentication
only.
Pseudonym Username
The username portion of pseudonym identity, ie. not including any
realm portions.
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Re-authentication Identity
A re-authentication identity of the peer, including an NAI realm
portion in environments where a real is used. Used on re-
authentication only.
Re-authentication Username
The username portion of re-authentication identity, ie. not
including any realm portions.
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. The authenticator typically communicates with an EAP
server that is located on a backend authentication server using an
AAA protocol. The AAA communications is not shown in the figure.
The first EAP Request issued by the network is EAP-Request/Identity.
On full authentication, the peer's response includes either the
user's International Mobile Subscriber Identity (IMSI) or a
temporary identity (pseudonym) if identity privacy is in effect, as
specified in Section 4.2.
Following the peer's EAP-Response/Identity packet, the peer receives
EAP Requests of type 18 (SIM) from the EAP server 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 5.
The EAP-Request/SIM/Start packet contains the list of EAP/SIM
version supported by the EAP server in the AT_VERSION_LIST
attribute. This packet may also include attributes for requesting
the subscriber identity, as specified in Section 4.2.
The peer 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 peer, and the
AT_SELECTED_VERSION attribute that contains the version number
selected by the peer. The version negotiation is protected by
including the version list and the selected version in the
calculation of keying material (Section 4.6).
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After receiving the EAP Response/SIM/Start, the EAP server obtains n
GSM triplets for use in authenticating the subscriber, where n = 2
or n = 3. From the triplets, the EAP server derives the keying
material, as specified in Section 4.6. The triplets may be obtained
by contacting an Authentication Centre (AuC) on the GSM network; per
GSM specifications, between 1 and 5 triplets may be obtained at a
time.
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.
On receipt of the EAP-Request/SIM/Challenge message, the peer runs
the GSM authentication algorithm and calculates a copy of the
message authentication code. The peer then verifies that the
calculated MAC equals the received MAC. If the MAC's do not match,
then the peer sends the EAP-Response/SIM/Client-Error packet and the
authentication exchange terminates.
Since the RAND's given to a peer are accompanied with the message
authentication code AT_MAC, and since the peer's NONCE_MT value
contributes to AT_MAC, the peer 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 peer responds with the EAP-
Response/SIM/Challenge, containing the AT_MAC attribute that covers
the peer's SRES response values (Section 6.4). 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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Peer 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) |
|<---------------------------------------------------------|
| |
+-------------------------------------+ |
| Peer 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.
If the peer has maintained state information for re-authentication
and wants to use re-authentication, then the peer indicates this by
using a specific re-authentication identity instead of the permanent
identity or a pseudonym identity. The re-authentication procedure is
described in Section 4.3.
4. Operation
4.1. 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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which the server always includes in EAP-Request/SIM/Start, and
AT_SELECTED_VERSION, which the peer includes in EAP-
Response/SIM/Start on full authentication.
AT_VERSION_LIST includes the EAP/SIM versions supported by the
server. If AT_VERSION_LIST does not include a version that is
implemented by the peer and allowed in the peer's security policy,
then the peer MUST send the EAP-Response/SIM/Client-Error packet
(Section 6.7) to the server with the error code "unsupported
version". If a suitable version is included, then the peer includes
the AT_SELECTED_VERSION attribute, containing the selected version,
in the EAP-Response/SIM/Start packet. The peer MUST only indicate a
version that is included in AT_VERSION_LIST. If several versions are
acceptable, then the peer 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 4.6). If an attacker modifies either one of these
attributes, then the peer and the server derive different keying
material. Because K_aut keys are different, the server and peer
calculate different AT_MAC values. Hence, the peer detects that
AT_MAC included in EAP-Request/SIM/Challenge is incorrect and sends
the EAP-Response/SIM/Client-Error packet. The authentication
procedure terminates.
4.2. Identity Management
4.2.1 Format, Generation and Usage of Peer Identities
General
In the beginning of EAP authentication, the Authenticator or the EAP
server usually issues the EAP-Request/Identity packet to the peer.
The peer responds with EAP-Response/Identity, which contains the
user's identity. The formats of these packets are specified in
[EAP].
GSM subscribers are identified with the International Mobile
Subscriber Identity (IMSI) [GSM 03.03]. 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 and help identify the AuC from which the authentication
vectors need to be retrieved for this subscriber.
Internet AAA protocols identify users with the Network Access
Identifier (NAI) [RFC 2486]. 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.
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This section specifies the peer identity format used in EAP/SIM. In
this document, the term identity or peer identity refers to the
whole identity string that is used to identify the peer. The peer
identity may include a realm portion. "Username" refers to the
portion of the peer identity that identifies the user, i.e. the
username does not include the realm portion.
Identity Privacy Support
EAP/SIM includes optional identity privacy (anonymity) support that
can be used to hide the cleartext permanent identity and thereby to
make the subscriber's EAP exchanges untraceable to eavesdroppers.
Because the permanent identity never changes, revealing it would
help observers to track the user. The permanent identity is usually
based on the IMSI, which may further help the tracking, because the
same identifier may used in other contexts as well. Identity privacy
is based on temporary identities, or pseudonyms, which are
equivalent to but separate from the Temporary Mobile Subscriber
Identities (TMSI) that are used on cellular networks. Please see
Section 9.1 for security considerations regarding identity privacy.
Username Types in EAP/SIM identities
There are three types of usernames in EAP/SIM peer identities:
(1) Permanent usernames. For example,
1123456789098765@myoperator.com might be a valid permanent identity.
In this example, 1123456789098765 is the permanent username.
(2) Pseudonym usernames. For example, 3s7ah6n9q@myoperator.com might
be a valid pseudonym identity. In this example, 3s7ah6n9q is the
pseudonym username.
(3) Re-authentication usernames. For example,
53953754a@myoperator.com might be a valid re-authentication
identity. In this case, 53953754 is the re-authentication username.
The first two types of identities 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 identity is used on full authentication. The re-
authentication exchange is specified in Section 4.3.
Username Decoration
In some environments, the peer may need to decorate the identity by
prepending or appending the username with a string, in order to
indicate supplementary AAA routing information in addition to the
NAI realm. (The usage of a NAI realm portion is not considered to be
decoration.) Username decoration is out of the scope of this
document. However, it should be noted that username decoration might
prevent the server from recognizing a valid username. Hence,
although the peer MAY use username decoration in the identities the
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peer includes in EAP-Response/Identity, and the EAP server MAY
accept a decorated peer username in this message, the peer or the
EAP server MUST NOT decorate any other peer identities that are used
in various EAP/SIM attributes. Only the identity used in EAP-
Response/Identity may be decorated.
NAI Realm Portion
The peer MAY include a realm portion in the peer identity, as per
the NAI format. The use of a realm portion is not mandatory.
If a realm is used, the realm MAY be chosen by the operator and it
MAY a configurable parameter in the EAP/SIM peer implementation. In
this case, the peer 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. When the peer is using a pseudonym username
instead of the permanent username, the peer selects the realm name
portion similarly as it select the realm portion when using the
permanent username.
If no configured realm name is available, the peer MAY derive the
realm name from the MCC and MNC portions of the IMSI. A recommended
way to derive the realm from the IMSI using the realm
3gppnetwork.org will be specified in [Draft 3GPP TS 23.234].
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".
The IMSI is a string of digits without any explicit structure, so
the peer may not be able to determine the length of the MNC portion.
If the peer is not able to determine whether the MNC is two or three
digits long, the peer MAY use a 3-digit MNC. If the correct length
of the MNC is two, then the MNC used in the realm name includes 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.
Format of the Permanent Username
The non-pseudonym permanent username SHOULD be derived from the
IMSI. In this case, the permanent username MUST be of the format "1"
| IMSI, where the character "|" denotes concatenation. 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 [GSM 03.03].
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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 out of the scope of this
document. In this case, the peer implementation MUST NOT prepend any
leading characters to the username.
Generating Pseudonyms and Re-authentication Identities by the Server
Pseudonym usernames and re-authentication identities are generated
by the EAP server. The EAP server produces pseudonym usernames and
re-authentication identities in an implementation-dependent manner.
Only the EAP server needs to be able to map the pseudonym username
to the permanent identity, or to recognize a re-authentication
identity. Regardless of construction method, the pseudonym username
MUST conform to the grammar specified for the username portion of an
NAI. The re-authentication identity also MUST conform to the NAI
grammar. The EAP servers that the subscribers of an operator can use
MUST ensure that the pseudonym usernames and the username portions
used in re-authentication identities they generate are unique.
In any case, it is necessary that permanent usernames, pseudonym
usernames and re-authentication usernames 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.
Pseudonym usernames and re-authentication usernames are both
produced and used by the EAP server. The EAP server MUST compose
pseudonym usernames and re-authentication usernames so that it can
recognize if a NAI username is an EAP SIM pseudonym username or an
EAP SIM re-authentication username. For instance, when the usernames
have been derived from the IMSI, the server could use different
leading characters in the pseudonym usernames and re-authentication
usernames (e.g. the pseudonym could begin with a leading "3"
character). When mapping a re-authentication identity to a permanent
identity, the server SHOULD only examine the username portion of the
re-authentication identity and ignore the realm portion of the
identity.
Because the peer may fail to save a pseudonym username sent to in an
EAP-Request/SIM/Challenge, for example due to malfunction, the EAP
server SHOULD maintain at least one old pseudonym username in
addition to the most recent pseudonym username.
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Transmitting Pseudonyms and Re-authentication Identities to the Peer
The server transmits pseudonym usernames and re-authentication
identities to the peer in cipher, using the AT_ENCR_DATA attribute.
The EAP-Request/SIM/Challenge message MAY include an encrypted
pseudonym username and/or an encrypted re-authentication identity in
the value field of the AT_ENCR_DATA attribute. Because identity
privacy support and re-authentication are optional to implement, the
peer MAY ignore the AT_ENCR_DATA attribute and always use the
permanent identity. On re-authentication (discussed in Section 4.3),
the server MAY include a new encrypted re-authentication identity in
the EAP-Request/SIM/Re-authentication message.
On receipt of the EAP-Request/SIM/Challenge, the peer MAY decrypt
the encrypted data in AT_ENCR_DATA and if a pseudonym username is
included, the peer may use the obtained pseudonym username on the
next full authentication. If a re-authentication identity is
included, then the peer MAY save it and other re-authentication
state information, as discussed in Section 4.3, for the next re-
authentication.
If the peer does not receive a new pseudonym username in the EAP-
Request/SIM/Challenge message, the peer MAY use an old pseudonym
username instead of the permanent username on next full
authentication. The username portions of re-authentication
identities are one-time usernames, which the peer MUST NOT re-use.
Usage of the Pseudonym by the Peer
When the optional identity privacy support is used on full
authentication, the peer MAY use the pseudonym username received as
part of the previous full authentication sequence as the username
portion of the NAI. The peer MUST NOT modify the pseudonym username
received in AT_NEXT_PSEUDONYM. However, as discussed above, the peer
MAY need to decorate the username in some environments by appending
or prepending the username with a string that indicates
supplementary AAA routing information.
When using a pseudonym username in an environment where a realm
portion is used, the peer concatenates the received pseudonym
username with the "@" character and a NAI realm portion. The
selection of the NAI realm is discussed above.
Usage of the Re-authentication Identity by the Peer
On re-authentication, the peer uses the re-authentication identity,
received as part of the previous authentication sequence. A new re-
authentication identity may be delivered as part of both full
authentication and re-authentication. The peer MUST NOT modify the
username part of the re-authentication identity received in
AT_NEXT_REAUTH_ID, except in cases when username decoration is
required. Even in these cases, the "root" re-authentication username
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must not be modified, but it may be appended or prepended with
another string.
4.2.2 Communicating the Peer Identity to the Server
General
The peer identity MAY be communicated to the server with the EAP-
Response/Identity message. This message MAY contain the permanent
identity, a pseudonym identity, or a re-authentication identity. If
the peer uses the permanent identity or a pseudonym identity, which
the server is able to map to the permanent identity, then the
authentication proceeds as discussed in the overview of Section 3.
If the peer uses a re-authentication identity, and the server
recognized the identity and agrees on using re-authentication, then
a re-authentication exchange is performed, as described in 4.3.
The peer identity can also be transmitted from the peer to the
server using EAP/SIM messages instead of EAP-Response/Identity. In
this case, the server includes an identity requesting attribute
(AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ) in the
EAP-Request/SIM/Start message, and the peer includes the AT_IDENTITY
attribute, which contains the peer's identity, in the EAP-
Response/SIM/Start message. The AT_ANY_ID_REQ attribute is a general
identity requesting attribute, which the server uses if it does not
specify which kind of an identity the peer should return in
AT_IDENTITY. The server uses the AT_FULLAUTH_ID_REQ attribute to
request either the permanent identity or a pseudonym identity. The
server uses the AT_PERMANENT_ID_REQ attribute to request the peer to
send its permanent identity.
The identity format in the AT_IDENTITY attribute is the same as in
the EAP-Response/Identity packet (except that identity decoration is
not allowed). The AT_IDENTITY attribute contains a permanent
identity, a pseudonym identity or a re-authentication identity.
Obtaining the subscriber identity via EAP/SIM messages is useful if
the server does not have any EAP/SIM peer identity at the beginning
of the EAP/SIM exchange or does not recognize the identity the peer
used in EAP-Response/Identity. This may happen if, for example, the
EAP-Response/Identity has been issued by some EAP method other than
EAP/SIM or if intermediate entities or software layers in the peer
have modified the identity string in the EAP-Response/Identity
packet. Also, some EAP layer implementations may cache the identity
string from the first EAP authentication and do not obtain a new
identity string from the EAP method implementation on subsequent
authentication exchanges.
As the identity string is used in key derivation, any of these cases
will result in failed authentication unless the EAP server uses
EAP/SIM attributes to obtain an unmodified copy of the identity
string. Therefore, unless the EAP server can be certain that no
intermediate element or software layer has modified the EAP-
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Response/Identity packet, the EAP server SHOULD always use the
EAP/SIM attributes to obtain the identity, even if the identity
received in EAP-Response/Identity was valid.
Please note that the EAP/SIM peer 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 peer with the
original identity from the EAP-Response/Identity packet regardless
of whether the AT_IDENTITY attribute is used in EAP/SIM to transmit
another identity.
Choice of Identity for the EAP-Response/Identity
If EAP/SIM peer is started upon receiving an EAP-Request/Identity
message, then the peer performs the following steps.
If the peer has maintained re-authentication state information and
if the peer wants to use re-authentication, then the peer transmits
the re-authentication identity in EAP-Response/Identity.
Else, if the peer has a pseudonym username available, then the peer
transmits the pseudonym identity in EAP-Response/Identity.
In other cases, the peer transmits the permanent identity in EAP-
Response/Identity.
Server Operation in the Beginning of EAP/SIM Exchange
If the EAP server has not received any identity (permanent identity,
pseudonym identity or re-authentication identity) from the peer when
sending the first EAP/SIM request, or if the EAP server has received
an EAP-Response/Identity packet but the contents do not appear to be
a valid permanent identity, pseudonym identity or a re-
authentication identity, then the server MUST request an identity
from the peer using one of the methods below.
The server sends the EAP-Request/SIM/Start message with the
AT_PERMANENT_ID_REQ message to indicate that the server wants the
peer to include the permanent identity in the AT_IDENTITY attribute
of the EAP-Response/SIM/Start message. This is done in the following
cases:
- The server does not support re-authentication or identity privacy.
- The server received an identity that it recognizes as a pseudonym
identity but the server is not able to map the pseudonym identity to
a permanent identity.
The server issues the EAP-Request/SIM/Start packet with the
AT_FULLAUTH_ID_REQ attribute to indicate that the server wants the
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peer to include a full authentication identity (pseudonym identity
or permanent identity) in the AT_IDENTITY attribute of the EAP-
Response/SIM/Start message. This is done in the following cases:
- The server does not support re-authentication and the server
supports identity privacy
- The server received an identity that it recognizes as a re-
authentication identity but the server is not able to map the re-
authentication identity to a permanent identity
The server issues the EAP-Request/SIM/Start packet with the
AT_ANY_ID_REQ attribute to indicate that the server wants the peer
to include an identity in the AT_IDENTITY attribute of the EAP-
Response/SIM/Start message, and the server does not indicate any
preferred type for the identity. This is done in other cases, such
as when the server does not have any identity, or the server does
not recognize the format of a received identity.
Processing of EAP-Request/SIM/Start by the Peer
Upon receipt of an EAP-Request/SIM/Start message, the peer MUST
perform the following steps.
If the EAP-Request/SIM/Start does not include any identity request
attribute, then the peer responds with EAP-Response/SIM/Start
without AT_IDENTITY. The peer includes the AT_SELECTED_VERSION and
AT_NONCE_MT attributes, because the exchange is a full
authentication exchange.
If the EAP-Request/SIM/Start includes AT_PERMANENT_ID_REQ the peer
MUST either respond with EAP-Response/SIM/Start and include the
permanent identity in AT_IDENTITY or respond with EAP-
Response/SIM/Client-Error packet with code "unable to process
packet".
If the EAP-Request/SIM/Start includes AT_FULL_AUTH_ID_REQ, and if
the peer has a pseudonym available, then the peer SHOULD respond
with EAP-Response/SIM/Start and includes the pseudonym identity in
AT_IDENTITY. If the peer does not have a pseudonym when it receives
this message, then the peer MUST either respond with EAP-
Response/SIM/Start and include the permanent identity in AT_IDENTITY
or respond with EAP-Response/SIM/Client-Error packet with code
"unable to process packet." The Peer MUST NOT use a re-
authentication identity in the AT_IDENTITY attribute.
If the EAP-Request/SIM/Start includes AT_ANY_ID_REQ, and if the peer
has maintained re-authentication state information and the peer
wants to use re-authentication, then the peer responds with EAP-
Response/SIM/Start and includes the re-authentication identity in
AT_IDENTITY. Else, if the peer has a pseudonym identity available,
then the peer responds with EAP-Response/SIM/Start and includes the
pseudonym identity in AT_IDENTITY. Else, the peer responds with EAP-
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Response/SIM/Start and includes the permanent identity in
AT_IDENTITY.
An EAP/SIM exchange may include several EAP/SIM/Start rounds. The
server may issue a second EAP-Request/SIM/Start, if it was not able
to recognize the identity the peer used in the previous AT_IDENTITY
attribute. At most three EAP/SIM/Start rounds can be used.
AT_ANY_ID_REQ can only be used in the first EAP-Request/SIM/Start,
in other words AT_ANY_ID_REQ MUST NOT be used in the second or third
EAP-Request/SIM/Start. AT_FULLAUTH_ID_REQ MUST NOT be used if the
previous EAP-Request/SIM/Start included AT_PERMANENT_ID_REQ. The
peer operation in cases when it receives an unexpected attribute is
specified in Section 4.5.1.
Attacks against Identity Privacy
The section above specifies two possible ways the peer can operate
upon receipt of AT_PERMANENT_ID_REQ. This is because 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 peer, in an effort to find out the true identity of the user. If
the peer does not want to reveal its permanent identity, then the
peer sends the EAP-Response/SIM/Client-Error packet with the error
code "unable to process packet", and the authentication exchange
terminates.
Basically, there are two different policies that the peer can employ
with regard to AT_PERMANENT_ID_REQ. A "conservative" peer assumes
that the network is able to maintain pseudonyms robustly. Therefore,
if a conservative peer has a pseudonym username, the peer responds
with EAP-Response/SIM/Client-Error to the EAP packet with
AT_PERMANENT_ID_REQ, because the peer believes that the valid
network is able to map the pseudonym identity to the peer's
permanent identity. (Alternatively, the conservative peer may accept
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 peer against active attacks on anonymity. On
the other hand, a "liberal" peer 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 map them to the
permanent identity.
Processing of AT_IDENTITY by the Server
When the server receives an EAP-Response/SIM/Start message with the
AT_IDENTITY (in response to the server's identity requesting
attribute), the server MUST operate as follows.
If the server used AT_PERMANENT_ID_REQ, and if the AT_IDENTITY does
not contain a valid permanent identity, then the server sends EAP
Failure and the EAP exchange terminates. If the server recognizes
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the permanent identity and is able to continue, then the server
proceeds with full authentication by sending EAP-
Request/SIM/Challenge.
If the server used AT_FULLAUTH_ID_REQ, and if AT_IDENTITY contains a
valid permanent identity or a pseudonym identity that the server can
map to a valid permanent identity, then the server proceeds with
full authentication by sending EAP-Request/SIM/Challenge. If
AT_IDENTITY contains a pseudonym identity that the server is not
able to map to a valid permanent identity, or an identity that the
server is not able to recognize or classify, then the server sends
EAP-Request/SIM/Start with AT_PERMANENT_ID_REQ.
If the server used AT_ANY_ID_REQ, and if the AT_IDENTITY contains a
valid permanent identity or a pseudonym identity that the server can
map to a valid permanent identity, then the server proceeds with
full authentication by sending EAP-Request/SIM/Challenge.
If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a
valid re-authentication identity and the server agrees on using re-
authentication, then the server proceeds with re-authentication by
sending EAP-Request/SIM/Re-authentication (Section 4.3).
If the server used AT_ANY_ID_REQ, and if the peer sent an EAP-
Response/SIM/Start with only AT_IDENTITY (indicating re-
authentication), but the server is not able to map the identity to a
permanent identity, then the server sends EAP-Request/SIM/Start with
AT_FULLAUTH_ID_REQ.
If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a
valid re-authentication identity, which the server is able to map to
a permanent identity, and if the server does not want to use re-
authentication, then the server sends EAP-Request/SIM/Start without
any identity requesting attributes.
If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an
identity that the server recognizes as a pseudonym identity but the
server is not able to map the pseudonym identity to a permanent
identity, then the server sends EAP-Request/SIM/Start with
AT_PERMANENT_ID_REQ.
If the server used AT_ANY_ID_REQ, and AT_IDENTITY contains an
identity that the server is not able to recognize or classify, then
the server sends EAP-Request/SIM/Start with AT_FULLAUTH_ID_REQ.
4.2.3 Message Sequence Examples (Informative)
This section contains non-normative message sequence examples to
illustrate how the peer identity can be communicated to the server.
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Full Authentication
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.
Peer 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 peer 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 map to the
permanent identity, then the full authentication sequence proceeds
as usual with the EAP Server issuing the EAP-Request/SIM/Challenge
message.
Re-authentication
The case when the server uses the AT_ANY_ID_REQ and the peer wants
to perform re-authentication is illustrated below.
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Peer 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) |
|------------------------------------------------------>|
| |
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 4.3.
Fall Back to Full Authentication
The case when the server does not recognize the re-authentication
identity the peer used in AT_IDENTITY, and issues a second EAP-
Request/SIM/Start message is illustrated below.
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Peer 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) |
|------------------------------------------------------>|
| |
Requesting the Permanent Identity 1
The figure below illustrates the case when the EAP server fails to
map the pseudonym identity included in the EAP-Response/Identity
packet to a valid permanent identity.
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Peer Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a pseudonym) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server fails to map the |
| | Pseudonym to a permanent id. |
| +------------------------------+
| |
| 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.
Requesting the Permanent Identity 2
The figure below illustrates the case when the EAP server fails to
map the pseudonym included in the AT_IDENTITY attribute to a valid
permanent identity.
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Peer 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 map the |
| | Pseudonym in AT_IDENTITY |
| | to a valid permanent 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) |
|------------------------------------------------------>|
| |
Three EAP/SIM/Start Roundtrips
In the worst case, there are three EAP/SIM/Start round trips before
the server has obtained an acceptable identity. This case is
illustrated below.
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Peer 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 map the |
| | Pseudonym in AT_IDENTITY |
| | to a valid permanent 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.
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4.3. Re-Authentication
4.3.1 General
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 peer. 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 4.6.
On re-authentication, the peer protects against replays with an
unsigned 16-bit counter, included in the AT_COUNTER attribute. On
full authentication, both the server and the peer 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 peer'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.
Both the peer and the server SHOULD have an upper limit for the
number of subsequent re-authentications allowed before a full
authentication needs to be performed. Because a 16-bit counter is
used in re-authentication, the theoretical maximum number of re-
authentications is reached when the counter value reaches 0xFFFF.
In order to use re-authentication, the peer and the EAP server need
to store the following values: Master Key, latest counter value and
the next re-authentication identity. K_aut, K_encr may either be
stored or derived again from MK. The server may also need to store
the permanent identity of the user.
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4.3.2 Re-authentication Identity
The re-authentication procedure makes use of separate re-
authentication user identities. Pseudonyms and the permanent
identity are reserved for full authentication only. 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 6.3). This attribute contains
a new re-authentication identity for the next re-authentication. The
peer MAY ignore this attribute, in which case it MUST use full
authentication next time. If the peer wants to use re-
authentication, it uses this re-authentication identity on next
authentication. Even if the peer has a re-authentication identity,
the peer MAY discard the re-authentication identity and use a
pseudonym or the permanent identity instead, in which case full
authentication MUST be performed.
In environments where a realm portion is needed in the peer
identity, the re-authentication identity received in
AT_NEXT_REAUTH_ID MUST contain both a username portion and a realm
portion, as per the NAI format. 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 peer MAY use the re-authentication identity in the EAP-
Response/Identity packet or, in response to server's AT_ANY_ID_REQ
attribute, the peer MAY use the re-authentication identity in the
AT_IDENTITY attribute of the EAP-Response/SIM/Start packet. The peer
MUST NOT modify the username portion of the re-authentication
identity, but the peer MAY modify the realm portion or replace it
with another realm portion.
Even if the peer 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 peer'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.
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4.3.3 Re-authentication Procedure
The following figure illustrates the re-authentication procedure.
Encrypted attributes are denoted with '*'. The peer 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 peer 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 peer 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 peer. 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 peer
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 peer verifies that the counter value is fresh (greater than any
previously used value). The peer also verifies that AT_MAC is
correct. The peer MAY save the next re-authentication identity from
the encrypted AT_NEXT_REAUTH_ID for next time. If all checks are
successful, the peer 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 peer.
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Peer 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) |
|<------------------------------------------------------|
| |
| |
+-----------------------------------------------+ |
| Peer verifies AT_MAC and the freshness of | |
| the counter. Peer 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 |
|<------------------------------------------------------|
| |
4.3.4 Re-authentication Procedure when Counter is Too Small
If the peer 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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Peer 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 peer 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 peer 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 peer 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 terminates
the authentication exchange and sends the EAP Failure 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.
4.4. EAP/SIM Notifications
The EAP-Request/Notification, specified in [EAP], can be used to
convey a displayable message from the EAP server to the peer.
Because these messages are textual messages, it may be hard for the
peer 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 peer. The peer MAY show a notification message to the user and
the peer MUST respond to the EAP server with an EAP-
Response/SIM/Notification packet, even if the peer 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 peer can use the notification as a failure
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 peer MUST NOT change
its state when it receives such a notification. (This version of the
protocol does not specify any notification codes with the F bit set
to one.)
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/Re-authentication 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/Re-authentication 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. (This
version of the protocol does not specify any notification codes with
the P bit set to one.)
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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.
4.5. Error Cases
This section specifies the operation of the peer and the server in
error cases. The subsections below require the EAP/SIM peer and
server to send an error packet (EAP-Response/SIM/Client-Error or EAP
Failure) in error cases. However, implementations SHOULD NOT rely
upon the correct error reporting behavior of the peer,
authenticator, or the server. It is possible for error and other
messages to be lost in transit or for a malicious participant to
attempt to consume resources by not issuing error messages. Both
the peer and the EAP server SHOULD have a mechanism to clean up
state even if an error message or EAP Success is not received after
a timeout period.
4.5.1 Peer Operation
In general, if an EAP/SIM peer detects an error in a received
EAP/SIM packet, the EAP/SIM implementation responds with the EAP-
Response/SIM/Client-Error packet. In response to the EAP-
Response/SIM/Client-Error, the EAP server MUST issue the EAP Failure
packet and the authentication exchange terminates.
By default, the peer uses the client error code 0, "unable to
process packet". This error code is used in the following cases:
- the peer is not able to parse the EAP request, i.e. the EAP
request is malformed
- the peer encountered a malformed attribute
- wrong attribute types or duplicate attributes have been included
in the EAP request
- a mandatory attribute is missing
- unrecognized non-skippable attribute
- unrecognized or unexpected EAP/SIM Subtype in the EAP request
- A RAND challenge repeated in AT_RAND
- invalid AT_MAC
- invalid pad bytes in AT_PADDING
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- the peer does not want to process AT_PERMANENT_ID_REQ
Separate error codes have been defined for the following error cases
in Section 7.18:
As specified in Section 4.1, when processing the AT_VERSION_LIST
attribute, which lists the EAP/SIM versions supported by the server,
if the attribute does not include a version that is implemented by
the peer and allowed in the peer's security policy, then the peer
MUST send the EAP-Response/SIM/Client-Error packet with the error
code "unsupported version".
When processing the AT_RAND attribute, the peer MUST send the EAP-
Response/SIM/Client-Error packet with the error code "insufficient
number of challenges", if the number of RAND challenges is smaller
than what is required by peer's local policy.
If the peer believes that the RAND challenges included in AT_RAND
are not fresh e.g. because it is capable of remembering some
previously used RANDs, the peer MUST send the EAP-
Response/SIM/Client-Error packet with the error code "RANDs are not
fresh".
4.5.2 Server Operation
If an EAP/SIM server detects an error in a received EAP/SIM
response, the server MUST issue the EAP Failure packet and the
authentication exchange terminates. The errors cases when the server
issues an EAP Failure include the following:
- the server is not able to parse the peer's EAP response
- the server encounters a malformed attribute, a non-recognized non-
skippable attribute, or a duplicate attribute
- a mandatory attribute is missing or an invalid attribute was
included
- unrecognized or unexpected EAP/SIM Subtype in the EAP Response
- invalid AT_MAC
- invalid AT_COUNTER
4.5.3 EAP Failure
As normally in EAP, the EAP server sends the EAP-Failure packet to
the peer 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 peer 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 may also send EAP Failure if there is
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an error in the received EAP/SIM response, as discussed in Section
4.5.2.
The server can send EAP-Failure at any time in the EAP exchange. The
peer MUST process EAP-Failure.
.5.4 EAP Success
On full authentication, the server can only send EAP-Success after
the EAP/SIM/Challenge round. The peer MUST silently discard any EAP-
Success packets if they are received before the peer 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 peer MUST silently discard any
EAP-Success packets if they are received before the peer has
successfully authenticated the server and sent the EAP-
Response/SIM/Re-authentication packet.
If the peer receives an EAP/SIM notification (section 4.4) that
indicates failure, then the peer MUST no longer accept the EAP-
Success packet even if the server authentication was successfully
completed.
4.6. 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 peer 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
identity string is included as-is, without any changes and including
the possible identity decoration. 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 SHA-1 is specified in [SHA-1]. If several EAP/SIM/Start
roundtrips are used in an EAP/SIM exchange, then the NONCE_MT,
Version List and Selected version from the last EAP/SIM/Start round
are used, and the previous EAP/SIM/Start rounds are ignored.
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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 MUST be used for
protecting EAP packets, but a new MSK and a new EMSK MUST 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.
Key derivation is based on the random number generation specified in
NIST Federal Information Processing Standards (FIPS) Publication
186-2 [PRF]. The pseudo-random number generator is specified in the
change notice 1 (2001 October 5) of [PRF] (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. It should be noted that the function G is very similar
to SHA-1, but the message padding is different. Please refer to
[PRF] for full details. 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
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
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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 [RFC 2548] 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.
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 peer is able to
verify that the EAP SIM packets it receives from the network are
fresh and not a replay. (Please see also Section 9.)
5. Message Format and Protocol Extensibility
5.1. Message Format
As specified in [EAP], EAP packets begin with the Code, Identifiers,
Length, and Type fields, which are followed by EAP method specific
Type-Data. The Code field in the EAP header is set to 1 for EAP
requests, and to 2 for EAP Responses. The usage of the Length and
Identifier fields in the EAP header are also specified in [EAP]. In
EAP/SIM, the Type field is set to 18.
In EAP/SIM, the Type-Data begins with an EAP/SIM header that
consists of a 1-octet Subtype field and a 2-octet reserved field.
The Subtype values used in EAP/SIM are defined in Section 8. The
formats of the EAP header and the EAP/SIM header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The rest of the Type-Data, immediately following the EAP/SIM header,
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 8.
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.
Attributes numbered within the range 0 through 127 are called non-
skippable attributes. When an EAP/SIM peer encounters a non-
skippable attribute that the peer does not recognize, the peer MUST
send the EAP-Response/SIM/Client-Error packet which terminates the
authentication exchange. If an EAP/SIM server encounters a non-
skippable attribute that the server does not recognize, then the
server sends the EAP Failure packet which terminates the
authentication exchange.
Attributes within the range of 128 through 255 are called skippable
attributes. When a skippable attribute is encountered that is not
recognized it is ignored. The rest of the attributes and message
data MUST still be processed. The Length field of the attribute is
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used to skip the attribute value in searching for the next
attribute.
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.
Attributes can be encapsulated within other attributes. In other
words, the value field of an attribute type can be specified to
contain other attributes.
5.2. Protocol Extensibility
EAP/SIM can be extended by specifying new attribute types. If
skippable attributes are used, it is possible to extend the protocol
without breaking old implementations. However, any new attributes
added to the EAP-Request/SIM/Start or EAP-Response/SIM/Start packets
would not be integrity protected. Therefore, these messages MUST NOT
be extended in the current version of EAP/SIM.
When specifying new attributes, it should be noted that EAP/SIM does
not support message fragmentation. Hence, the sizes of the new
extensions MUST be limited so that the maximum transfer unit (MTU)
of the underlying lower layer is not exceeded. According to [EAP],
lower layers must provide an EAP MTU of 1020 bytes or greater, so
any extensions to EAP/SIM SHOULD NOT exceed the EAP MTU of 1020
bytes.
Because EAP/SIM supports version negotiation, new versions of the
protocol can also be specified by using a new version number.
6. Messages
This section specifies the messages used in EAP/SIM. It specifies
when a message may be transmitted or accepted, which attributes are
allowed in a message, which attributes are required in a message,
and other message specific details. The general message format is
specified in Section 5.1.
6.1. EAP-Request/SIM/Start
In full authentication the first SIM specific EAP Request is EAP-
Request/SIM/Start. The EAP/SIM/Start roundtrip is used for two
purposes. In full authentication this packet is used to request the
peer to send the AT_NONCE_MT attribute to the server. In addition,
as specified in Section 4.2, the Start round trip may be used by the
server for obtaining the peer identity. As discussed in Section 4.2,
several Start rounds may be required in order to obtain a valid peer
identity.
The server MUST always include the AT_VERSION_LIST attribute.
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The server MAY include one of the following identity requesting
attributes: AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, and
AT_ANY_ID_REQ. These three attributes are mutually exclusive, so the
server MUST NOT include more than one of the attributes.
If the server has received a response from the peer, it MUST NOT
issue a new EAP-Request/SIM/Start packet if it has either previously
issued an EAP-Request/SIM/Start message without any identity
requesting attributes or with the AT_PERMANENT_ID_REQ attribute.
If the server has received a response from the peer, it MUST NOT
issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ or
AT_FULLAUTH_ID_REQ attributes if it has previously issued an EAP-
Request/SIM/Start message with the AT_FULLAUTH_ID_REQ attribute
If the server has received a response from the peer, it MUST NOT
issue a new EAP-Request/SIM/Start packet with the AT_ANY_ID_REQ
attribute if the server has previously issued an EAP-
Request/SIM/Start message with the AT_ANY_ID_REQ attribute.
This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.
6.2. EAP-Response/SIM/Start
The peer sends EAP-Response/SIM/Start in response to a valid EAP-
Request/SIM/Start from the server.
If and only if the server's EAP-Request/SIM/Start includes one of
the identity requesting attributes, then the peer MUST include the
AT_IDENTITY attribute. The usage of AT_IDENITY is defined in Section
4.2.
The AT_NONCE_MT attribute MUST NOT be included if the AT_IDENTITY
with a re-authentication identity is present for re-authentication.
AT_NONCE_MT MUST be included in all other cases (full
authentication).
The AT_SELECTED_VERSION attribute MUST NOT be included if the
AT_IDENTITY attribute with a re-authentication identity is present
for re-authentication. In all other cases, AT_SELECTED_VERSION MUST
be included (full authentication). This attribute is used in version
negotiation, as specified in Section 4.1.
This message MUST NOT include AT_MAC, AT_IV, or AT_ENCR_DATA.
6.3. EAP-Request/SIM/Challenge
The server sends the EAP-Request/SIM/Challenge after receiving a
valid EAP-Response/SIM/Start, containing AT_NONCE_MT and
AT_SELECTED_VERSION, and after successfully obtaining the subscriber
identity.
The AT_RAND attribute MUST be included.
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The AT_MAC attribute MUST be included. For EAP-
Request/SIM/Challenge, the MAC code is calculated over the following
data:
EAP packet| NONCE_MT
The EAP packet is represented as specified in Section 5.1. It is
followed by the 16-byte NONCE_MT value from the peer's AT_NONCE_MT
attribute.
The EAP-Request/SIM/Challenge packet MAY include encrypted
attributes for identity privacy and for communicating the next re-
authentication identity. In this case, the AT_IV and AT_ENCR_DATA
attributes are included (Section 7.3).
The plaintext of the AT_ENCR_DATA value field consists of nested
attributes. The nested attributes MAY include AT_PADDING (as
specified in Section 7.3). If the server supports identity privacy
and wants to communicate a pseudonym to the peer for the next full
authentication, then the nested encrypted attributes include the
AT_NEXT_PSEUDONYM attribute. If the server supports re-
authentication and wants to communicate a re-authentication identity
to the peer, then the nested encrypted attributes include the
AT_NEXT_REAUTH_ID attribute.
6.4. EAP-Response/SIM/Challenge
The peer sends EAP-Response/SIM/Challenge in response to a valid
EAP-Request/SIM/Challenge.
The AT_MAC attribute 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 5.1. The EAP
packet bytes are immediately followed by the 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 server's AT_RAND attribute.
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. The EAP server MUST process EAP-Response/SIM/Challenge
messages that include these attributes even if the server did not
implement these optional attributes.
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6.5. EAP-Request/SIM/Re-authentication
The server sends the EAP-Request/SIM/Re-authentication message if it
wants to use re-authentication, and if it has received a valid re-
authentication identity in EAP-Response/Identity or EAP-
Response/SIM/Start.
AT_MAC MUST be included. No message-specific data is included in the
MAC calculation. See Section 7.2.
The AT_IV and AT_ENCR_DATA attributes MUST be included. The
plaintext consists of the following nested encrypted attributes,
which MUST be included: AT_COUNTER and AT_NONCE_S. In addition, the
nested encrypted attributes MAY include the following attributes:
AT_NEXT_REAUTH_ID and AT_PADDING.
6.6. EAP-Response/SIM/Re-authentication
The client sends the EAP-Response/SIM/Re-authentication packet in
response to a valid EAP-Request/SIM/Re-authentication.
The AT_MAC attribute MUST be included. 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 5.1. 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 MUST be included. The nested
encrypted attributes MUST include the AT_COUNTER attribute. The
AT_COUNTER_TOO_SMALL attribute MAY be included in the nested
encrypted attributes, and it is included in cases specified in
Section 4.3. The AT_PADDING attribute MAY be included.
6.7. EAP-Response/SIM/Client-Error
The peer sends EAP-Response/SIM/Client-Error in error cases, as
specified in Section 4.5.1.
The AT_CLIENT_ERROR_CODE attribute MUST be included.
The AT_MAC, AT_IV, or AT_ENCR_DATA attributes MUST NOT be used with
this packet.
6.8. EAP-Request/SIM/Notification
The usage of this message is specified in Section 4.4.
The AT_NOTIFICATION attribute MUST be included.
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The AT_MAC attribute is included in cases discussed in Section 4.4.
No message-specific data is included in the MAC calculation. See
Section 7.2.
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. These attributes MAY be included only if the P bit of
the notification code in AT_NOTIFICATION is set to zero.
6.9. EAP-Response/SIM/Notification
The usage of this message is specified in Section 4.4. Because this
packet is only an acknowledgement of EAP-Request/SIM/Notification,
it does not contain any mandatory attributes.
The AT_MAC attribute is included in cases described in Section 4.4.
No message-specific data is included in the MAC calculation, see
Section 7.2.
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. These attributes MAY be included only if the P bit of
the notification code in the AT_NOTIFICATION attribute of the
server's EAP-Request/SIM/Notification packet is set to zero.
7. Attributes
This section specifies the format of message attributes. The
attribute type numbers are specified in Section 8.
7.1. Table of Attributes
The following table provides a guide to which attributes may be
found in which kinds of messages, and in what quantity. Messages are
denoted with numbers in parentheses as follows: (1) EAP-
Request/SIM/Start, (2) EAP-Response/SIM/Start, (3) EAP-
Request/SIM/Challenge, (4) EAP-Response/SIM/Challenge, (5) EAP-
Request/SIM/Notification, (6) EAP-Response/SIM/Notification, (7)
EAP-Response/SIM/Client-Error (8) EAP-Request/SIM/Re-authentication,
and (9) EAP-Response/SIM/Re-authentication. The column denoted with
"Encr" indicates whether the attribute is a nested attribute that
MUST be included within AT_ENCR_DATA, and the column denoted with
"Skip" indicates whether the attribute is a skippable attribute.
"0" indicates that the attribute MUST NOT be included in the
message, "1" indicates that the attribute MUST be included in the
message, "0-1" indicates that the attribute