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strongswan/doc/standards/rfc4186.txt

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Network Working Group H. Haverinen, Ed.
Request for Comments: 4186 Nokia
Category: Informational J. Salowey, Ed.
Cisco Systems
January 2006
Extensible Authentication Protocol Method for
Global System for Mobile Communications (GSM)
Subscriber Identity Modules (EAP-SIM)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
IESG Note
The EAP-SIM protocol was developed by 3GPP. The documentation of
EAP-SIM is provided as information to the Internet community. While
the EAP WG has verified that EAP-SIM is compatible with EAP, as
defined in RFC 3748, no other review has been done, including
validation of the security claims. The IETF has also not reviewed
the security of the cryptographic algorithms.
Abstract
This document specifies an Extensible Authentication Protocol (EAP)
mechanism for authentication and session key distribution using the
Global System for Mobile Communications (GSM) Subscriber Identity
Module (SIM). GSM is a second generation mobile network standard.
The EAP-SIM 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, result
indications, and a fast re-authentication procedure.
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RFC 4186 EAP-SIM Authentication January 2006
Table of Contents
1. Introduction ....................................................4
2. Terms ...........................................................5
3. Overview ........................................................8
4. Operation ......................................................10
4.1. Version Negotiation .......................................10
4.2. Identity Management .......................................11
4.2.1. Format, Generation and Usage of Peer Identities ....11
4.2.2. Communicating the Peer Identity to the Server ......17
4.2.3. Choice of Identity for the EAP-Response/Identity ...19
4.2.4. Server Operation in the Beginning of
EAP-SIM Exchange ...................................19
4.2.5. Processing of EAP-Request/SIM/Start by the Peer ....20
4.2.6. Attacks Against Identity Privacy ...................21
4.2.7. Processing of AT_IDENTITY by the Server ............22
4.3. Message Sequence Examples (Informative) ...................23
4.3.1. Full Authentication ................................24
4.3.2. Fast Re-authentication .............................25
4.3.3. Fall Back to Full Authentication ...................26
4.3.4. Requesting the Permanent Identity 1 ................27
4.3.5. Requesting the Permanent Identity 2 ................28
4.3.6. Three EAP-SIM/Start Roundtrips .....................28
5. Fast Re-Authentication .........................................30
5.1. General ...................................................30
5.2. Comparison to UMTS AKA ....................................31
5.3. Fast Re-authentication Identity ...........................31
5.4. Fast Re-authentication Procedure ..........................33
5.5. Fast Re-authentication Procedure when Counter Is
Too Small .................................................36
6. EAP-SIM Notifications ..........................................37
6.1. General ...................................................37
6.2. Result Indications ........................................39
6.3. Error Cases ...............................................40
6.3.1. Peer Operation .....................................40
6.3.2. Server Operation ...................................41
6.3.3. EAP-Failure ........................................42
6.3.4. EAP-Success ........................................42
7. Key Generation .................................................43
8. Message Format and Protocol Extensibility ......................45
8.1. Message Format ............................................45
8.2. Protocol Extensibility ....................................47
9. Messages .......................................................48
9.1. EAP-Request/SIM/Start .....................................48
9.2. EAP-Response/SIM/Start ....................................49
9.3. EAP-Request/SIM/Challenge .................................49
9.4. EAP-Response/SIM/Challenge ................................50
9.5. EAP-Request/SIM/Re-authentication .........................51
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9.6. EAP-Response/SIM/Re-authentication ........................51
9.7. EAP-Response/SIM/Client-Error .............................52
9.8. EAP-Request/SIM/Notification ..............................52
9.9. EAP-Response/SIM/Notification .............................53
10. Attributes ....................................................53
10.1. Table of Attributes ......................................53
10.2. AT_VERSION_LIST ..........................................54
10.3. AT_SELECTED_VERSION ......................................55
10.4. AT_NONCE_MT ..............................................55
10.5. AT_PERMANENT_ID_REQ ......................................56
10.6. AT_ANY_ID_REQ ............................................56
10.7. AT_FULLAUTH_ID_REQ .......................................57
10.8. AT_IDENTITY ..............................................57
10.9. AT_RAND ..................................................58
10.10. AT_NEXT_PSEUDONYM .......................................59
10.11. AT_NEXT_REAUTH_ID .......................................59
10.12. AT_IV, AT_ENCR_DATA, and AT_PADDING .....................60
10.13. AT_RESULT_IND ...........................................62
10.14. AT_MAC ..................................................62
10.15. AT_COUNTER ..............................................63
10.16. AT_COUNTER_TOO_SMALL ....................................63
10.17. AT_NONCE_S ..............................................64
10.18. AT_NOTIFICATION .........................................64
10.19. AT_CLIENT_ERROR_CODE ....................................65
11. IANA Considerations ...........................................66
12. Security Considerations .......................................66
12.1. A3 and A8 Algorithms .....................................66
12.2. Identity Protection ......................................66
12.3. Mutual Authentication and Triplet Exposure ...............67
12.4. Flooding the Authentication Centre .......................69
12.5. Key Derivation ...........................................69
12.6. Cryptographic Separation of Keys and Session
Independence .............................................70
12.7. Dictionary Attacks .......................................71
12.8. Credentials Re-use .......................................71
12.9. Integrity and Replay Protection, and Confidentiality .....72
12.10. Negotiation Attacks .....................................73
12.11. Protected Result Indications ............................73
12.12. Man-in-the-Middle Attacks ...............................74
12.13. Generating Random Numbers ...............................74
13. Security Claims ...............................................74
14. Acknowledgements and Contributions ............................75
14.1. Contributors .............................................75
14.2. Acknowledgements .........................................75
14.2.1. Contributors' Addresses ...........................77
15. References ....................................................78
15.1. Normative References .....................................78
15.2. Informative References ...................................79
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Appendix A. Test Vectors .........................................81
A.1. EAP-Request/Identity .....................................81
A.2. EAP-Response/Identity ....................................81
A.3. EAP-Request/SIM/Start ....................................82
A.4. EAP-Response/SIM/Start ...................................82
A.5. EAP-Request/SIM/Challenge ................................83
A.6. EAP-Response/SIM/Challenge ...............................86
A.7. EAP-Success ..............................................86
A.8. Fast Re-authentication ...................................86
A.9. EAP-Request/SIM/Re-authentication ........................87
A.10. EAP-Response/SIM/Re-authentication ......................89
Appendix B. Pseudo-Random Number Generator .......................90
1. Introduction
This document specifies an Extensible Authentication Protocol (EAP)
[RFC3748] mechanism for authentication and session key distribution
using the Global System for Mobile Communications (GSM) Subscriber
Identity Module (SIM).
GSM is a second generation mobile network standard. Second
generation mobile networks and third generation mobile networks use
different authentication and key agreement mechanisms. EAP-AKA
[EAP-AKA] specifies an EAP method that is based on the Authentication
and Key Agreement (AKA) mechanism used in 3rd generation mobile
networks.
GSM authentication is based on a challenge-response mechanism. The
A3/A8 authentication and key derivation algorithms that run on the
SIM can be given a 128-bit random number (RAND) as a challenge. The
SIM runs operator-specific algorithms, which take the RAND and a
secret key Ki (stored on the SIM) as input, and produce 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 is not directly used. Hence, the secrecy of Kc is
critical to the security of this protocol. For more information
about GSM authentication, see [GSM-03.20]. See Section 12.1 for more
discussion about the GSM algorithms used in EAP-SIM.
The lack of mutual authentication is a weakness in GSM
authentication. The derived 64-bit cipher key (Kc) is not strong
enough for data networks in which 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
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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 the GSM, which uses
pseudonyms/temporary identifiers. It also specifies an optional fast
re-authentication procedure.
The security of EAP-SIM builds on underlying GSM mechanisms. The
security properties of EAP-SIM are documented in Section 11 of this
document. Implementers and users of EAP-SIM are advised to carefully
study the security considerations in Section 11 in order to determine
whether the security properties are sufficient for the environment in
question, especially as the secrecy of Kc keys is essential 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 are 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.
The 3rd Generation Partnership Project (3GPP) has specified an
enhanced Authentication and Key Agreement (AKA) architecture for the
Universal Mobile Telecommunications System (UMTS). The 3rd
generation 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 the 3rd generation AKA.
EAP-AKA, which is a more secure protocol, may be used instead of
EAP-SIM, if 3rd generation identity modules and 3G network
infrastructures 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 [RFC2119].
The terms and abbreviations "authenticator", "backend authentication
server", "EAP server", "peer", "Silently Discard", "Master Session
Key (MSK)", and "Extended Master Session Key (EMSK)" in this document
are to be interpreted as described in [RFC3748].
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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.
Authentication vector
GSM triplets can be alternatively called authentication
vectors.
EAP
Extensible Authentication Protocol
Fast re-authentication
An EAP-SIM authentication exchange that is based on keys
derived upon a preceding full authentication exchange.
The GSM authentication and key exchange algorithms are not
used in the fast re-authentication procedure.
Fast Re-authentication Identity
A fast re-authentication identity of the peer, including an NAI
realm portion in environments where a realm is used. Used on
fast re-authentication only.
Fast Re-authentication Username
The username portion of fast re-authentication identity,
i.e., not including any realm portions.
Full authentication
An EAP-SIM authentication exchange based on the GSM
authentication and key agreement algorithms.
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
Nonce
A value that is used at most once or that is never repeated
within the same cryptographic context. In general, a nonce can
be predictable (e.g., a counter) or unpredictable (e.g., a
random value). Since some cryptographic properties may depend
on the randomness of the nonce, attention should be paid to
whether a nonce is required to be random or not. In this
document, the term nonce is only used to denote random nonces,
and it is not used to denote counters.
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, i.e., not including
any realm portions.
Pseudonym Identity
A pseudonym identity of the peer, including an NAI realm
portion in environments where a realm is used. Used on
full authentication only.
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Pseudonym Username
The username portion of pseudonym identity, i.e., not including
any realm portions.
SIM
Subscriber Identity Module. The SIM is traditionally a smart
card distributed by a GSM operator.
3. Overview
Figure 1 shows an overview of the EAP-SIM full authentication
procedure, wherein optional protected success indications are not
used. The authenticator typically communicates with an EAP server
that is located on a backend authentication server using an AAA
protocol. The authenticator shown in the figure is often simply
relaying EAP messages to and from the EAP server, but these backend
AAA communications are not shown.
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
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The first EAP Request issued by the authenticator 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 8.
The EAP-Request/SIM/Start packet contains the list of EAP-SIM
versions 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 a 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 7).
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 7. 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.
Triplets may be stored in the EAP server for use at a later time, but
triplets MUST NOT be re-used, except in some error cases that are
specified in Section 10.9.
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 AT_MAC attribute is a general message authentication code
attribute that is used in many EAP-SIM messages.
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
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sends the EAP-Response/SIM/Client-Error packet and the authentication
exchange terminates.
Since the RANDs given to a peer are accompanied by 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 (i.e., 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 9.4). The EAP server
verifies that the MAC is correct. Because protected success
indications are not used in this example, the EAP server sends the
EAP-Success packet, indicating that the authentication was
successful. (Protected success indications are discussed in
Section 6.2.) The EAP server may also include derived keying
material in the message it sends to the authenticator. The peer has
derived the same keying material, so the authenticator does not
forward the keying material to the peer along with EAP-Success.
EAP-SIM also includes a separate fast re-authentication procedure
that does not make use of the A3/A8 algorithms or the GSM
infrastructure. Fast re-authentication is based on keys derived on
full authentication. If the peer has maintained state information
for fast re-authentication and wants to use fast re-authentication,
then the peer indicates this by using a specific fast
re-authentication identity instead of the permanent identity or a
pseudonym identity. The fast re-authentication procedure is
described in Section 5.
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,
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 9.7) to the server with the error code "unsupported
version". If a suitable version is included, then the peer includes
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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 the 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 7). 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
4.2.1.1. 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 the EAP-Response/Identity, which contains the
user's identity. The formats of these packets are specified in
[RFC3748].
GSM subscribers are identified with the International Mobile
Subscriber Identity (IMSI) [GSM-03.03]. The IMSI is a string of not
more than 15 digits. It is composed of a three digit Mobile Country
Code (MCC), a two or three digit Mobile Network Code (MNC), and a
Mobile Subscriber Identification Number (MSIN) of no more than 10
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) [RFC4282]. 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.
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
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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.
4.2.1.2. Identity Privacy Support
EAP-SIM includes optional identity privacy (anonymity) support that
can be used to hide the cleartext permanent identity and thereby 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 be 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 12.2
for security considerations regarding identity privacy.
4.2.1.3. 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) Fast re-authentication usernames. For example,
53953754@myoperator.com might be a valid fast re-authentication
identity. In this case, 53953754 is the fast re-authentication
username. Unlike permanent usernames and pseudonym usernames, fast
re-authentication usernames are one-time identifiers, which are not
re-used across EAP exchanges.
The first two types of identities are used only on full
authentication and the last one only on fast re-authentication. When
the optional identity privacy support is not used, the non-pseudonym
permanent identity is used on full authentication. The fast
re-authentication exchange is specified in Section 5.
4.2.1.4. 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
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realm. (The usage of an NAI realm portion is not considered
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 that
the peer includes in EAP-Response/Identity, and although 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
the EAP-Response/Identity may be decorated.
4.2.1.5. 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 subscriber's home
operator and it MAY be 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 a reserved
NAI realm may be a useful 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
is specified in [3GPP-TS-23.003].
Some old implementations derive the realm name from the IMSI by
concatenating "mnc", the MNC digits of IMSI, ".mcc", the MCC digits
of IMSI, and ".owlan.org". For example, if the IMSI is
123456789098765, and the MNC is three digits long, then the derived
realm name is "mnc456.mcc123.owlan.org". As there are no DNS servers
running at owlan.org, these realm names can only be used with
manually configured AAA routing. New implementations SHOULD use the
mechanism specified in [3GPP-TS-23.003] instead of 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 the MSIN. Hence, when configuring AAA networks for
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operators that have 2-digit MNCs, the network SHOULD also be prepared
for realm names with incorrect, 3-digit MNCs.
4.2.1.6. 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 31
hexadecimal), followed by the IMSI. The IMSI is encoded as an ASCII
string that consists of not more than 15 decimal digits (ASCII values
between 30 and 39 hexadecimal), one character per IMSI digit, in the
order specified in [GSM-03.03]. For example, a permanent username
derived from the IMSI 295023820005424 would be encoded as the ASCII
string "1295023820005424" (byte values in hexadecimal notation: 31 32
39 35 30 32 33 38 32 30 30 30 35 34 32 34).
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.
4.2.1.7. Generating Pseudonyms and Fast Re-authentication Identities by
the Server
Pseudonym usernames and fast re-authentication identities are
generated by the EAP server. The EAP server produces pseudonym
usernames and fast 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 fast re-authentication identity.
EAP-SIM includes no provisions to ensure that the same EAP server
that generated a pseudonym username will be used on the
authentication exchange when the pseudonym username is used. It is
recommended that the EAP servers implement some centralized mechanism
to allow all EAP servers of the home operator to map pseudonyms
generated by other severs to the permanent identity. If no such
mechanism is available, then the EAP server failing to understand a
pseudonym issued by another server can request the that peer send the
permanent identity.
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When issuing a fast re-authentication identity, the EAP server may
include a realm name in the identity to make the fast
re-authentication request be forwarded to the same EAP server.
When generating fast re-authentication identities, the server SHOULD
choose a fresh, new fast re-authentication identity that is different
from the previous ones that were used after the same full
authentication exchange. A full authentication exchange and the
associated fast re-authentication exchanges are referred to here as
the same "full authentication context". The fast re-authentication
identity SHOULD include a random component. This random component
works as a full authentication context identifier. A
context-specific fast re-authentication identity can help the server
to detect whether its fast re-authentication state information
matches that of its peer (in other words, whether the state
information is from the same full authentication exchange). The
random component also makes the fast re-authentication identities
unpredictable, so an attacker cannot initiate a fast
re-authentication exchange to get the server's EAP-Request/SIM/
Re-authentication packet.
Transmitting pseudonyms and fast re-authentication identities from
the server to the peer is discussed in Section 4.2.1.8. The
pseudonym is transmitted as a username, without an NAI realm, and the
fast re-authentication identity is transmitted as a complete NAI,
including a realm portion if a realm is required. The realm is
included in the fast re-authentication identity to allow the server
to include a server-specific realm.
Regardless of the construction method, the pseudonym username MUST
conform to the grammar specified for the username portion of an NAI.
The fast 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 fast re-authentication identities they generate are unique.
In any case, it is necessary that permanent usernames, pseudonym
usernames, and fast re-authentication usernames are separate and
recognizable from each other. It is also desirable that EAP-SIM and
EAP-AKA [EAP-AKA] usernames be distinguishable from each other as an
aid for the server on which method to offer.
In general, it is the task of the EAP server and the policies of its
administrator to ensure sufficient separation of the usernames.
Pseudonym usernames and fast re-authentication usernames are both
produced and used by the EAP server. The EAP server MUST compose
pseudonym usernames and fast re-authentication usernames so that it
can determine if an NAI username is an EAP-SIM pseudonym username or
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an EAP-SIM fast 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 fast
re-authentication usernames (e.g., the pseudonym could begin with a
leading "3" character). When mapping a fast re-authentication
identity to a permanent identity, the server SHOULD only examine the
username portion of the fast re-authentication identity and ignore
the realm portion of the identity.
Because the peer may fail to save a pseudonym username sent in an
EAP-Request/SIM/Challenge, for example due to malfunction, the EAP
server SHOULD maintain at least the most recently used pseudonym
username in addition to the most recently issued pseudonym username.
If the authentication exchange is not completed successfully, then
the server SHOULD NOT overwrite the pseudonym username that was
issued during the most recent successful authentication exchange.
4.2.1.8. Transmitting Pseudonyms and Fast Re-authentication Identities
to the Peer
The server transmits pseudonym usernames and fast 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 fast re-authentication
identity in the value field of the AT_ENCR_DATA attribute. Because
identity privacy support and fast re-authentication are optional
implementations, the peer MAY ignore the AT_ENCR_DATA attribute and
always use the permanent identity. On fast re-authentication
(discussed in Section 5), the server MAY include a new, encrypted
fast 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. If the authentication exchange is
successful, and the encrypted data includes a pseudonym username,
then the peer may use the obtained pseudonym username on the next
full authentication. If a fast re-authentication identity is
included, then the peer MAY save it together with other fast
re-authentication state information, as discussed in Section 5, for
the next fast re-authentication. If the authentication exchange does
not complete successfully, the peer MUST ignore the received
pseudonym username and the fast re-authentication identity.
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 the next full
authentication. The username portions of fast re-authentication
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identities are one-time usernames, which the peer MUST NOT re-use.
When the peer uses a fast re-authentication identity in an EAP
exchange, the peer MUST discard the fast re-authentication identity
and not re-use it in another EAP authentication exchange, even if the
authentication exchange was not completed.
4.2.1.9. Usage of the Pseudonym by the Peer
When the optional identity privacy support is used on full
authentication, the peer MAY use a pseudonym username received as
part of a 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 an NAI realm portion. The
selection of the NAI realm is discussed above. The peer can select
the realm portion similarly, regardless of whether it uses the
permanent username or a pseudonym username.
4.2.1.10. Usage of the Fast Re-authentication Identity by the Peer
On fast re-authentication, the peer uses the fast re-authentication
identity that was received as part of the previous authentication
sequence. A new re-authentication identity may be delivered as part
of both full authentication and fast re-authentication. The peer
MUST NOT modify the username part of the fast re-authentication
identity received in AT_NEXT_REAUTH_ID, except in cases when username
decoration is required. Even in these cases, the "root" fast
re-authentication username must not be modified, but it may be
appended or prepended with another string.
4.2.2. Communicating the Peer Identity to the Server
4.2.2.1. 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 fast 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 fast re-authentication identity, and
if the fast re-authentication identity matches with a valid fast
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re-authentication identity maintained by the server, and if the
server agrees to use fast re-authentication, then a fast
re-authentication exchange is performed, as described in Section 5.
The peer identity can also be transmitted from the peer to the server
using EAP-SIM messages instead of the 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 that the
peer 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 fast re-authentication identity.
Please note that the EAP-SIM peer and the EAP-SIM server only process
the AT_IDENTITY attribute; entities that only pass through EAP
packets do not process this attribute. Hence, 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 unless the identity authenticated in
the AT_IDENTITY attribute is communicated to them in another way
within the AAA protocol.
4.2.2.2. Relying on EAP-Response/Identity Discouraged
The EAP-Response/Identity packet is not method-specific, so in many
implementations it may be handled by an EAP Framework. This
introduces an additional layer of processing between the EAP peer and
EAP server. The extra layer of processing may cache identity
responses or add decorations to the identity. A modification of the
identity response will cause the EAP peer and EAP server to use
different identities in the key derivation, which will cause the
protocol to fail.
For this reason, it is RECOMMENDED that the EAP peer and server use
the method-specific identity attributes in EAP-SIM, and the server is
strongly discouraged from relying upon the EAP-Response/Identity.
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In particular, if the EAP server receives a decorated identity in
EAP-Response/Identity, then the EAP server MUST use the
identity-requesting attributes to request that the peer send an
unmodified and undecorated copy of the identity in AT_IDENTITY.
4.2.3. Choice of Identity for the EAP-Response/Identity
If EAP-SIM peer is started upon receiving an EAP-Request/Identity
message, then the peer MAY use an EAP-SIM identity in the EAP-
Response/Identity packet. In this case, the peer performs the
following steps.
If the peer has maintained fast re-authentication state information
and wants to use fast re-authentication, then the peer transmits the
fast 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.
4.2.4. Server Operation in the Beginning of EAP-SIM Exchange
As discussed in Section 4.2.2.2, the server SHOULD NOT rely on an
identity string received in EAP-Response/Identity. Therefore, the
RECOMMENDED way to start an EAP-SIM exchange is to ignore any
received identity strings. The server SHOULD begin the EAP-SIM
exchange by issuing the EAP-Request/SIM/Start packet with an
identity-requesting 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. Three methods to request an identity
from the peer are discussed below.
If the server chooses not to ignore the contents of EAP-
Response/Identity, then the server may have already received an EAP-
SIM identity in this packet. However, if the EAP server has not
received any EAP-SIM peer identity (permanent identity, pseudonym
identity, or fast 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 attribute to indicate that the server wants the
peer to include the permanent identity in the AT_IDENTITY attribute
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of the EAP-Response/SIM/Start message. This is done in the following
cases:
o The server does not support fast re-authentication or identity
privacy.
o The server decided to process a received identity, and the server
recognizes the received identity 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
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:
o The server does not support fast re-authentication and the server
supports identity privacy.
o The server decided to process a received identity, and the server
recognizes the received identity 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 ignores a received EAP-Response/Identity, the
server does not have any identity, or the server does not recognize
the format of a received identity.
4.2.5. 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 an 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, and if the
peer does not have a pseudonym available, then the peer MUST respond
with EAP-Response/SIM/Start and include the permanent identity in
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AT_IDENTITY. If the peer has a pseudonym available, then the peer
MAY refuse to send the permanent identity; hence, in this case 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 the 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 include the pseudonym identity in
AT_IDENTITY. If the peer does not have a pseudonym when it receives
this message, then the peer MUST respond with EAP-Response/SIM/Start
and include the permanent identity in AT_IDENTITY. 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 fast re-authentication state information and the peer
wants to use fast re-authentication, then the peer responds with
EAP-Response/SIM/Start and includes the fast 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-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 that the peer used in the previous AT_IDENTITY
attribute. At most, three EAP/SIM/Start rounds can be used, so the
peer MUST NOT respond to more than three EAP-Request/SIM/Start
messages within an EAP exchange. The peer MUST verify that the
sequence of EAP-Request/SIM/Start packets that the peer receives
comply with the sequencing rules defined in this document. That is,
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 or
an unexpected message, is specified in Section 6.3.1.
4.2.6. 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
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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.
4.2.7. 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-Request/SIM/Notification with AT_NOTIFICATION code "General
failure" (16384), and the EAP exchange terminates. If the server
recognizes 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.
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If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid
fast re-authentication identity and the server agrees on using
re-authentication, then the server proceeds with fast
re-authentication by sending EAP-Request/SIM/Re-authentication
(Section 5).
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
fast re-authentication identity that the server is able to map to a
permanent identity, and if the server does not want to use fast
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.
4.3. 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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4.3.1. Full Authentication
This case for full authentication is illustrated below in Figure 2.
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 a |
| | 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) |
|------------------------------------------------------>|
| |
Figure 2: Requesting any identity, full authentication
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.
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4.3.2. Fast Re-authentication
The case when the server uses the AT_ANY_ID_REQ and the peer wants to
perform fast re-authentication is illustrated below in Figure 3.
Peer Authenticator
| |
| +------------------------------+
| | Server does not have a |
| | 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 fast re-auth. identity) |
|------------------------------------------------------>|
| |
Figure 3: Requesting any identity, fast re-authentication
On fast re-authentication, if the AT_IDENTITY attribute contains a
valid fast re-authentication identity and the server agrees on using
fast re-authentication, then the server proceeds with the fast
re-authentication sequence and issues the EAP-Request/SIM/
Re-authentication packet, as specified in Section 5.
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4.3.3. Fall Back to Full Authentication
Figure 4 illustrates cases in which the server does not recognize the
fast re-authentication identity the peer used in AT_IDENTITY, and
issues a second EAP-Request/SIM/Start message.
Peer Authenticator
| |
| +------------------------------+
| | Server does not have a |
| | 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 fast re-auth. identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server does not recognize |
| | The fast re-auth. |
| | 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) |
|------------------------------------------------------>|
| |
Figure 4: Fall back to full authentication
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4.3.4. Requesting the Permanent Identity 1
Figure 5 illustrates the case in which the EAP server fails to map
the pseudonym identity included in the EAP-Response/Identity packet
to a valid permanent identity.
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) |
|------------------------------------------------------>|
| |
Figure 5: Requesting the permanent identity
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.
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4.3.5. Requesting the Permanent Identity 2
Figure 6 illustrates the case in which the EAP server fails to map
the pseudonym included in the AT_IDENTITY attribute to a valid
permanent identity.
Peer Authenticator
| |
| +------------------------------+
| | Server does not have a |
| | 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) |
|------------------------------------------------------>|
| |
Figure 6: Requesting a permanent identity (two EAP-SIM Start rounds)
4.3.6. Three EAP-SIM/Start Roundtrips
In the worst case, there are three EAP/SIM/Start round trips before
the server obtains an acceptable identity. This case is illustrated
in Figure 7.
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Peer Authenticator
| |
| +------------------------------+
| | Server does not have a |
| | 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 fast re-auth. identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server does not accept |
| | The fast re-auth. |
| | 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) |
|------------------------------------------------------>|
| |
Figure 7: Three EAP-SIM Start rounds
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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.
5. Fast Re-Authentication
5.1. 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 therefore requires 2 or 3 fresh
triplets from the Authentication Centre, the full authentication
procedure is not very well suited for frequent use. Therefore,
EAP-SIM includes a more inexpensive fast 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.
Fast 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 it does not
want to use fast re-authentication.
Fast re-authentication is based on the keys derived on the preceding
full authentication. The same K_aut and K_encr keys that were used
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
7.
The fast re-authentication exchange makes use of an unsigned 16-bit
counter, included in the AT_COUNTER attribute. The counter has three
goals: 1) it can be used to limit the number of successive
reauthentication exchanges without full authentication 2) it
contributes to the keying material, and 3) it protects the peer and
the server from replays. 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 fast re-authentication. On subsequent
fast re-authentications, the counter MUST be greater than on any of
the previous re-authentications. For example, on the second fast
re-authentication, the counter value is two or greater. The
AT_COUNTER attribute is encrypted.
Both the peer and the EAP server maintain a copy of the counter. The
EAP server sends its counter value to the peer in the fast
re-authentication request. The peer MUST verify that its counter
value is less than or equal to the value sent by the EAP server.
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The server includes an encrypted server random nonce (AT_NONCE_S) in
the fast 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 fast re-authentications allowed before a full
authentication needs to be performed. Because a 16-bit counter is
used in fast re-authentication, the theoretical maximum number of
re-authentications is reached when the counter value reaches FFFF
hexadecimal.
In order to use fast re-authentication, the peer and the EAP server
need to store the following values: Master Key, latest counter value
and the next fast 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.
5.2. Comparison to UMTS AKA
When analyzing the fast re-authentication exchange, it may be helpful
to compare it with the UMTS Authentication and Key Agreement (AKA)
exchange, which it resembles closely. The counter corresponds to the
UMTS AKA sequence number, NONCE_S corresponds to RAND, AT_MAC in
EAP-Request/SIM/Re-authentication corresponds to AUTN, the AT_MAC in
EAP-Response/SIM/Re-authentication corresponds to RES,
AT_COUNTER_TOO_SMALL corresponds to AUTS, and encrypting the counter
corresponds to the usage of the Anonymity Key. Also, the key
generation on fast re-authentication, with regard to random or fresh
material, is similar to UMTS AKA -- the server generates the NONCE_S
and counter values, and the peer only verifies that the counter value
is fresh.
It should also be noted that encrypting the AT_NONCE_S, AT_COUNTER,
or AT_COUNTER_TOO_SMALL attributes is not important to the security
of the fast re-authentication exchange.
5.3. Fast Re-authentication Identity
The fast 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.
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RFC 4186 EAP-SIM Authentication January 2006
If the EAP server supports fast re-authentication, it MAY include the
skippable AT_NEXT_REAUTH_ID attribute in the encrypted data of
EAP-Request/SIM/Challenge message (Section 9.3). This attribute
contains a new fast re-authentication identity for the next fast
re-authentication. The attribute also works as a capability flag
that, indicating that the server supports fast re-authentication, and
that the server wants to continue using fast re-authentication within
the current context. 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 fast re-authentication identity
on next authentication. Even if the peer has a fast
re-authentication identity, the peer MAY discard the fast
re-authentication identity and use a pseudonym or the permanent
identity instead, in which case full authentication MUST be
performed. If the EAP server does not include the AT_NEXT_REAUTH_ID
in the encrypted data of EAP-Request/SIM/Challenge or
EAP-Request/SIM/ Re-authentication, then the peer MUST discard its
current fast re-authentication state information and perform a full
authentication next time.
In environments where a realm portion is needed in the peer identity,
the fast 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 fast
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 fast re-authentication in the AAA server that performed the full
authentication.
The peer MAY use the fast re-authentication identity in the
EAP-Response/Identity packet or, in response to the server's
AT_ANY_ID_REQ attribute, the peer MAY use the fast 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 fast
re-authentication identity, but the peer MAY modify the realm portion
or replace it with another realm portion. The peer might need to
modify the realm in order to influence the AAA routing, for example,
to make sure that the correct server is reached. It should be noted
that sharing the same fast re-authentication key among several
servers may have security risks, so changing the realm portion of the
NAI in order to change the EAP server is not desirable.
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Even if the peer uses a fast re-authentication identity, the server
may want to fall back on full authentication, for example because the
server does not recognize the fast re-authentication identity or does
not want to use fast 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 fast re-authentication identity, the server includes either
the AT_FULLAUTH_ID_REQ or the AT_PERMANENT_ID_REQ attribute in this
EAP request.
5.4. Fast Re-authentication Procedure
Figure 8 illustrates the fast re-authentication procedure. In this
example, the optional protected success indication is not used.
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 that the peer send its identity by
including the AT_ANY_ID_REQ attribute in the EAP-Request/SIM/Start
packet.
If the server recognizes the identity as a valid fast
re-authentication identity, and if the server agrees to use fast
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 fast
re-authentication identity.
Fast re-authentication identities are one-time identities. If the
peer does not receive a new fast 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 AT_MAC is correct, and that the counter value
is fresh (greater than any previously used value). The peer MAY save
the next fast 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,
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RFC 4186 EAP-SIM Authentication January 2006
including the AT_COUNTER attribute with the same counter value and
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.
If protected success indications (Section 6.2) were used, the
EAP-Success packet would be preceded by an EAP-SIM notification
round.
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Peer Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a fast re-authentication identity) |
|------------------------------------------------------>|
| |
| +--------------------------------+
| | Server recognizes the identity |
| | and agrees to use 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 fast 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 |
|<------------------------------------------------------|
| |
Figure 8: Fast Re-authentication
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RFC 4186 EAP-SIM Authentication January 2006
5.5. Fast 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 Figure 9. Encrypted attributes are denoted with '*'.
Peer Authenticator
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY) |
| (Includes a fast 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. |
+---------------------------------------------------------------+
| |
Figure 9: Fast Re-authentication, counter is not fresh
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In the figure above, the first three messages are similar to the
basic fast 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 counter value as in the
EAP-Request/SIM/Re-authentication packet. If not, the server
terminates the authentication exchange by sending the
EAP-Request/SIM/Notification with AT_NOTIFICATION code "General
failure" (16384). 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 identity, it MUST NOT include
AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ, or AT_PERMANENT_ID_REQ in the
EAP-Request/SIM/Start.
It should be noted that in this case, peer identity is only
transmitted in the AT_IDENTITY attribute at the beginning of the
whole EAP exchange. The fast re-authentication identity used in this
AT_IDENTITY attribute will be used in key derivation (see Section 7).
6. EAP-SIM Notifications
6.1. General
EAP-SIM does not prohibit the use of the EAP Notifications as
specified in [RFC3748]. EAP Notifications can be used at any time in
the EAP-SIM exchange. It should be noted that EAP-SIM does not
protect EAP Notifications. EAP-SIM also specifies method-specific
EAP-SIM notifications that are protected in some cases.
The EAP server can use EAP-SIM notifications to convey notifications
and result indications (Section 6.2) to the peer.
The server MUST use notifications in cases discussed in
Section 6.3.2. When the EAP server issues an
EAP-Request/SIM/Notification packet to the peer, the peer MUST
process the notification packet. 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.
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An EAP-SIM full authentication exchange or a fast re-authentication
exchange MUST NOT include more than one EAP-SIM notification round.
The notification code is a 16-bit number. The most significant bit
is called the Success bit (S bit). The S bit specifies whether the
notification implies failure. The code values with the S bit set to
zero (code values 0...32767) are used on unsuccessful cases. The
receipt of a notification code from this range implies a failed EAP
exchange, 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 S bit set to one (values
32768...65536) does not imply failure. Notification code "Success"
(32768) has been reserved as a general notification code to indicate
successful authentication.
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 a successful
EAP/SIM/Challenge round in full authentication or a successful
EAP/SIM/Re-authentication round in reauthentication. A
re-authentication round is considered successful only if the peer has
successfully verified AT_MAC and AT_COUNTER attributes, and does not
include the AT_COUNTER_TOO_SMALL attribute in
EAP-Response/SIM/Re-authentication.
If the P bit is set to one, the notification can only by used before
the EAP/SIM/Challenge round in full authentication, or before the
EAP/SIM/Re-authentication round in reauthentication. These
notifications can only be used to indicate various failure cases. In
other words, if the P bit is set to one, then the S bit MUST be set
to zero.
Section 9.8 and Section 9.9 specify what other attributes must be
included in the notification packets.
Some of the notification codes are authorization related and, hence,
are not usually considered 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.
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6.2. Result Indications
As discussed in Section 6.3, the server and the peer use explicit
error messages in all error cases. If the server detects an error
after successful authentication, the server uses an EAP-SIM
notification to indicate failure to the peer. In this case, the
result indication is integrity and replay protected.
By sending an EAP-Response/SIM/Challenge packet or an
EAP-Response/SIM/Re-authentication packet (without
AT_COUNTER_TOO_SMALL), the peer indicates that it has successfully
authenticated the server and that the peer's local policy accepts the
EAP exchange. In other words, these packets are implicit success
indications from the peer to the server.
EAP-SIM also supports optional protected success indications from the
server to the peer. If the EAP server wants to use protected success
indications, it includes the AT_RESULT_IND attribute in the
EAP-Request/SIM/Challenge or the EAP-Request/SIM/Re-authentication
packet. This attribute indicates that the EAP server would like to
use result indications in both successful and unsuccessful cases. If
the peer also wants this, the peer includes AT_RESULT_IND in
EAP-Response/SIM/Challenge or EAP-Response/SIM/Re-authentication.
The peer MUST NOT include AT_RESULT_IND if it did not receive
AT_RESULT_IND from the server. If both the peer and the server used
AT_RESULT_IND, then the EAP exchange is not complete yet, but an
EAP-SIM notification round will follow. The following EAP-SIM
notification may indicate either failure or success.
Success indications with the AT_NOTIFICATION code "Success" (32768)
can only be used if both the server and the peer indicate they want
to use them with AT_RESULT_IND. If the server did not include
AT_RESULT_IND in the EAP-Request/SIM/Challenge or
EAP-Request/SIM/Re-authentication packet, or if the peer did not
include AT_RESULT_IND in the corresponding response packet, then the
server MUST NOT use protected success indications.
Because the server uses the AT_NOTIFICATION code "Success" (32768) to
indicate that the EAP exchange has completed successfully, the EAP
exchange cannot fail when the server processes the EAP-SIM response
to this notification. Hence, the server MUST ignore the contents of
the EAP-SIM response it receives from the
EAP-Request/SIM/Notification with this code. Regardless of the
contents of the EAP-SIM response, the server MUST send EAP-Success as
the next packet.
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6.3. 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 from
the peer or EAP-Request/SIM/Notification from the server) 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.
6.3.1. 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:
o EAP exchange is not acceptable according to the peer's local
policy.
o the peer is not able to parse the EAP request, i.e., the EAP
request is malformed.
o the peer encountered a malformed attribute.
o wrong attribute types or duplicate attributes have been included
in the EAP request.
o a mandatory attribute is missing.
o unrecognized, non-skippable attribute.
o unrecognized or unexpected EAP-SIM Subtype in the EAP request.
o A RAND challenge repeated in AT_RAND.
o invalid AT_MAC. The peer SHOULD log this event.
o invalid pad bytes in AT_PADDING.
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o the peer does not want to process AT_PERMANENT_ID_REQ.
Separate error codes have been defined for the following error cases
in Section 10.19:
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".
If the number of RAND challenges is smaller than what is required by
peer's local policy 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 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".
6.3.2. Server Operation
If an EAP-SIM server detects an error in a received EAP-SIM response,
the server MUST issue the EAP-Request/SIM/Notification packet with an
AT_NOTIFICATION code that implies failure. By default, the server
uses one of the general failure codes ("General failure after
authentication" (0), or "General failure" (16384)). The choice
between these two codes depends on the phase of the EAP-SIM exchange,
see Section 6. When the server issues an EAP-
Request/SIM/Notification that implies failure, the error cases
include the following:
o the server is not able to parse the peer's EAP response
o the server encounters a malformed attribute, a non-recognized
non-skippable attribute, or a duplicate attribute
o a mandatory attribute is missing or an invalid attribute was
included
o unrecognized or unexpected EAP-SIM Subtype in the EAP Response
o invalid AT_MAC. The server SHOULD log this event.
o invalid AT_COUNTER
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6.3.3. EAP-Failure
The EAP-SIM server sends EAP-Failure in two cases:
1) In response to an EAP-Response/SIM/Client-Error packet the server
has received from the peer, or
2) Following an EAP-SIM notification round, when the AT_NOTIFICATION
code implies failure.
The EAP-SIM server MUST NOT send EAP-Failure in cases other than
these two. However, it should be noted that even though the EAP-SIM
server would not send an EAP-Failure, an authorization decision that
happens outside EAP-SIM, such as in the AAA server or in an
intermediate AAA proxy, may result in a failed exchange.
The peer MUST accept the EAP-Failure packet in case 1) and case 2),
above. The peer SHOULD silently discard the EAP-Failure packet in
other cases.
6.3.4. 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.
If the peer did not indicate that it wants to use protected success
indications with AT_RESULT_IND (as discussed in Section 6.2) on full
authentication, then the peer MUST accept EAP-Success after a
successful EAP/SIM/Challenge round.
If the peer indicated that it wants to use protected success
indications with AT_RESULT_IND (as discussed in Section 6.2), then
the peer MUST NOT accept EAP-Success after a successful
EAP/SIM/Challenge round. In this case, the peer MUST only accept
EAP-Success after receiving an EAP-SIM Notification with the
AT_NOTIFICATION code "Success" (32768).
On fast 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 did not indicate that it wants to use protected success
indications with AT_RESULT_IND (as discussed in Section 6.2) on fast
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re-authentication, then the peer MUST accept EAP-Success after a
successful EAP/SIM/Re-authentication round.
If the peer indicated that it wants to use protected success
indications with AT_RESULT_IND (as discussed in Section 6.2), then
the peer MUST NOT accept EAP-Success after a successful EAP/SIM/Re-
authentication round. In this case, the peer MUST only accept
EAP-Success after receiving an EAP-SIM Notification with the
AT_NOTIFICATION code "Success" (32768).
If the peer receives an EAP-SIM notification (Section 6) that
indicates failure, then the peer MUST no longer accept the
EAP-Success packet, even if the server authentication was
successfully completed.
7. 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 last AT_IDENTITY
attribute sent by the peer in this exchange, or, if AT_IDENTITY was
not used, it is the identity from the EAP-Response/Identity packet.
The identity string is included as-is, without any changes. As
discussed in Section 4.2.2.2, relying on EAP-Response/Identity for
conveying the EAP-SIM peer identity is discouraged, and the server
SHOULD use the EAP-SIM method-specific identity attributes.
The notation n*Kc in the formula above 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 only 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 fast 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 from new values exchanged in the fast
re-authentication.
EAP-SIM requires two TEKs for its own purposes; the authentication
key K_aut is to be used with the AT_MAC attribute, and the encryption
key K_encr is to be used with the AT_ENCR_DATA attribute. The same
K_aut and K_encr keys are used in full authentication and subsequent
fast 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 the 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 Appendix 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.
On full authentication, 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 fast re-authentication, the same pseudo-random number generator
can be used to generate a new Master Session Key and a new Extended
Master Session Key. The seed value XKEY' is calculated as follows:
XKEY' = SHA1(Identity|counter|NONCE_S| MK)
In the formula above, the Identity denotes the fast re-authentication
identity, without any terminating null characters, from the
AT_IDENTITY attribute of the EAP-Response/SIM/Start packet, or, if
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EAP-Response/SIM/Start was not used on fast re-authentication, it
denotes the identity string from the EAP-Response/Identity packet.
The counter denotes the counter value from the AT_COUNTER attribute
used in the EAP-Response/SIM/Re-authentication packet. The counter
is used in network byte order. NONCE_S denotes the 16-byte NONCE_S
value from the AT_NONCE_S attribute used in the
EAP-Request/SIM/Re-authentication packet. The MK is the Master Key
derived on the preceding full authentication.
On fast re-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
two 64-byte chunks and used as the new 64-byte Master Session Key and
the new 64-byte Extended Master Session Key. Note that because
K_encr and K_aut are not derived on fast re-authentication, the
Master Session Key and the Extended Master Session key are obtained
from the beginning of the key stream (x_0, x_1, ...).
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 [RFC2548] 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 replays (also see Section 11).
8. Message Format and Protocol Extensibility
8.1. Message Format
As specified in [RFC3748], 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
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Length and Identifier fields in the EAP header are also specified in
[RFC3748]. 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 the IANA considerations section
of the EAP-AKA specification [EAP-AKA]. The formats of the EAP
header and the EAP-SIM header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The rest of the Type-Data that immediately follows 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 the IANA considerations section of the
EAP-AKA specification [EAP-AKA].
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.
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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-Request/SIM/Notification packet with an
AT_NOTIFICATION code, which implies general failure ("General failure
after authentication" (0), or "General failure" (16384), depending on
the phase of the exchange), which terminates the authentication
exchange.
Attributes within the range of 128 through 255 are called skippable
attributes. When a skippable attribute is encountered and 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
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.
8.2. 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. If the list of supported EAP-SIM versions in the
AT_VERSION_LIST does not include versions other than 1, then the
server MUST NOT include attributes other than those specified in this
document in the EAP-Request/SIM/Start message. Note that future
versions of this protocol might specify new attributes for
EAP-Request/SIM/Start and still support version 1 of the protocol.
In this case, the server might send an EAP-Request/SIM/Start message
that includes new attributes and indicates support for protocol
version 1 and other versions in the AT_VERSION_LIST attribute. If
the peer selects version 1, then the peer MUST ignore any other
attributes included in EAP-Request/SIM/Start, other than those
specified in this document. If the selected EAP-SIM version in
peer's AT_SELECTED_VERSION is 1, then the peer MUST NOT include other
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attributes aside from those specified in this document in the
EAP-Response/SIM/Start message.
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 [RFC3748],
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.
9. 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 8.1.
9.1. 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 to obtain a valid peer identity.
The server MUST always include the AT_VERSION_LIST attribute.
The server MAY include one of the following identity-requesting
attributes: AT_PERMANENT_ID_REQ, AT_FULLAUTH_ID_REQ, or
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 previously issued
an EAP-Request/SIM/Start message either 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.
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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.
9.2. 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_IDENTITY is defined in
Section 4.2.
The AT_NONCE_MT attribute MUST NOT be included if the AT_IDENTITY
with a fast re-authentication identity is present for fast
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 fast re-authentication identity is
present for fast 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.
9.3. EAP-Request/SIM/Challenge
The server sends the EAP-Request/SIM/Challenge after receiving a
valid EAP-Response/SIM/Start that contains AT_NONCE_MT and
AT_SELECTED_VERSION, and after successfully obtaining the subscriber
identity.
The AT_RAND attribute MUST be included.
The AT_RESULT_IND attribute MAY be included. The usage of this
attribute is discussed in Section 6.2.
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
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The EAP packet is represented as specified in Section 8.1. 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 fast
re-authentication identity. In this case, the AT_IV and AT_ENCR_DATA
attributes are included (Section 10.12).
The plaintext of the AT_ENCR_DATA value field consists of nested
attributes. The nested attributes MAY include AT_PADDING (as
specified in Section 10.12). 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 fast re-authentication
identity to the peer, then the nested encrypted attributes include
the AT_NEXT_REAUTH_ID attribute.
When processing this message, the peer MUST process AT_RAND before
processing other attributes. Only if AT_RAND is verified to be
valid, the peer derives keys and verifies AT_MAC. The operation in
case an error occurs is specified in Section 6.3.1.
9.4. EAP-Response/SIM/Challenge
The peer sends EAP-Response/SIM/Challenge in response to a valid
EAP-Request/SIM/Challenge.
Sending this packet indicates that the peer has successfully
authenticated the server and that the EAP exchange will be accepted
by the peer's local policy. Hence, if these conditions are not met,
then the peer MUST NOT send EAP-Response/SIM/Challenge, but the peer
MUST send EAP-Response/SIM/Client-Error.
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 8.1. 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 the server's AT_RAND attribute.
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The AT_RESULT_IND attribute MAY be included if it was included in
EAP-Request/SIM/Challenge. The usage of this attribute is discussed
in Section 6.2.
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. The EAP server MUST process EAP-Response/SIM/Challenge
messages that include these attributes even if the server did not
implement these optional attributes.
9.5. EAP-Request/SIM/Re-authentication
The server sends the EAP-Request/SIM/Re-authentication message if it
wants to use fast re-authentication, and if it has received a valid
fast 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 10.14.
The AT_RESULT_IND attribute MAY be included. The usage of this
attribute is discussed in Section 6.2.
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.
9.6. 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 8.1. It is
followed by the 16-byte NONCE_S value from the server's AT_NONCE_S
attribute.
The AT_IV and AT_ENCR_DATA attributes 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
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encrypted attributes, and it is included in cases specified in
Section 5. The AT_PADDING attribute MAY be included.
The AT_RESULT_IND attribute MAY be included if it was included in
EAP-Request/SIM/Re-authentication. The usage of this attribute is
discussed in Section 6.2.
Sending this packet without AT_COUNTER_TOO_SMALL indicates that the
peer has successfully authenticated the server and that the EAP
exchange will be accepted by the peer's local policy. Hence, if
these conditions are not met, then the peer MUST NOT send
EAP-Response/SIM/Re-authentication, but the peer MUST send
EAP-Response/SIM/Client-Error.
9.7. EAP-Response/SIM/Client-Error
The peer sends EAP-Response/SIM/Client-Error in error cases, as
specified in Section 6.3.1.
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.
9.8. EAP-Request/SIM/Notification
The usage of this message is specified in Section 6. The
AT_NOTIFICATION attribute MUST be included.
The AT_MAC attribute MUST be included if the P bit of the
notification code in AT_NOTIFICATION is set to zero, and MUST NOT be
included in cases when the P bit is set to one. The P bit is
discussed in Section 6.
No message-specific data is included in the MAC calculation. See
Section 10.14.
If EAP-Request/SIM/Notification is used on a fast re-authentication
exchange, and if the P bit in AT_NOTIFICATION is set to zero, then
AT_COUNTER is used for replay protection. In this case, the
AT_ENCR_DATA and AT_IV attributes MUST be included, and the
encapsulated plaintext attributes MUST include the AT_COUNTER
attribute. The counter value included in AT_COUNTER MUST be the same
as in the EAP-Request/SIM/Re-authentication packet on the same fast
re-authentication exchange.
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9.9. EAP-Response/SIM/Notification
The usage of this message is specified in Section 6. This packet is
an acknowledgement of EAP-Request/SIM/Notification.
The AT_MAC attribute MUST be included in cases when the P bit of the
notification code in AT_NOTIFICATION of EAP-Request/SIM/Notification
is set to zero, and MUST NOT be included in cases when the P bit is
set to one. The P bit is discussed in Section 6.
No message-specific data is included in the MAC calculation, see
Section 10.14.
If EAP-Request/SIM/Notification is used on a fast re-authentication
exchange, and if the P bit in AT_NOTIFICATION is set to zero, then
AT_COUNTER is used for replay protection. In this case, the
AT_ENCR_DATA and AT_IV attributes MUST be included, and the
encapsulated plaintext attributes MUST include the AT_COUNTER
attribute. The counter value included in AT_COUNTER MUST be the same
as in the EAP-Request/SIM/Re-authentication packet on the same fast
re-authentication exchange.
10. Attributes
This section specifies the format of message attributes. The
attribute type numbers are specified in the IANA considerations
section of the EAP-AKA specification [EAP-AKA].
10.1. 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 is sometimes included in the
message, and "0*" indicates that the attribute is not included in the
message in cases specified in this document, but MAY be included in
future versions of the protocol.
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Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) Encr Skip
AT_VERSION_LIST 1 0 0 0 0 0 0 0 0 N N
AT_SELECTED_VERSION 0 0-1 0 0 0 0 0 0 0 N N
AT_NONCE_MT 0 0-1 0 0 0 0 0 0 0 N N
AT_PERMANENT_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N
AT_ANY_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N
AT_FULLAUTH_ID_REQ 0-1 0 0 0 0 0 0 0 0 N N
AT_IDENTITY 0 0-1 0 0 0 0 0 0 0 N N
AT_RAND 0 0 1 0 0 0 0 0 0 N N
AT_NEXT_PSEUDONYM 0 0 0-1 0 0 0 0 0 0 Y Y
AT_NEXT_REAUTH_ID 0 0 0-1 0 0 0 0 0-1 0 Y Y
AT_IV 0 0 0-1 0* 0-1 0-1 0 1 1 N Y
AT_ENCR_DATA 0 0 0-1 0* 0-1 0-1 0 1 1 N Y
AT_PADDING 0 0 0-1 0* 0-1 0-1 0 0-1 0-1 Y N
AT_RESULT_IND 0 0 0-1 0-1 0 0 0 0-1 0-1 N Y
AT_MAC 0 0 1 1 0-1 0-1 0 1 1 N N
AT_COUNTER 0 0 0 0 0-1 0-1 0 1 1 Y N
AT_COUNTER_TOO_SMALL 0 0 0 0 0 0 0 0 0-1 Y N
AT_NONCE_S 0 0 0 0 0 0 0 1 0 Y N
AT_NOTIFICATION 0 0 0 0 1 0 0 0 0 N N
AT_CLIENT_ERROR_CODE 0 0 0 0 0 0 1 0 0 N N
It should be noted that attributes AT_PERMANENT_ID_REQ,
AT_ANY_ID_REQ, and AT_FULLAUTH_ID_REQ are mutually exclusive; only
one of them can be included at the same time. If one of the
attributes AT_IV and AT_ENCR_DATA is included, then both of the
attributes MUST be included.
10.2. AT_VERSION_LIST
The format of the AT_VERSION_LIST 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_VERSION_L..| Length | Actual Version List Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Supported Version 1 | Supported Version 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Supported Version N | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This attribute is used in version negotiation, as specified in
Section 4.1. The attribute contains the version numbers supported by
the EAP-SIM server. The server MUST only include versions that it
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implements and that are allowed in its security policy. The server
SHOULD list the versions in the order of preference, with the most
preferred versions listed first. At least one version number MUST be
included. The version number for the protocol described in this
document is one (0001 hexadecimal).
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 attribute length. This
field is followed by the list of the versions supported by the
server, which each have a length of 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.
10.3. AT_SELECTED_VERSION
The format of the AT_SELECTED_VERSION 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_SELECTED...| Length = 1 | Selected Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This attribute is used in version negotiation, as specified in
Section 4.1. The value field of this attribute contains a two-byte
version number, which indicates the EAP-SIM version that the peer
wants to use.
10.4. AT_NONCE_MT
The format of the AT_NONCE_MT 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_NONCE_MT | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NONCE_MT |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The value field of the NONCE_MT attribute contains two reserved bytes
followed by a random number freshly generated by the peer (16 bytes
long) for this EAP-SIM authentication exchange. 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.
The peer MUST NOT re-use the NONCE_MT value from a previous EAP-SIM
authentication exchange. If an EAP-SIM exchange includes several
EAP/SIM/Start rounds, then the peer SHOULD use the same NONCE_MT
value in all EAP-Response/SIM/Start packets. The peer SHOULD use a
good source of randomness to generate NONCE_MT. Please see [RFC4086]
for more information about generating random numbers for security
applications.
10.5. AT_PERMANENT_ID_REQ
The format of the AT_PERMANENT_ID_REQ 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_PERM..._REQ | Length = 1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The use of the AT_PERMANENT_ID_REQ is defined in Section 4.2. The
value field contains only two reserved bytes, which are set to zero
on sending and ignored on reception.
10.6. AT_ANY_ID_REQ
The format of the AT_ANY_ID_REQ attribute is shown below.