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Versions: 00 01 02 03 04 05 06 07 08 09 10 11
12 13 14 15 16 RFC 4186
Network Working Group H. Haverinen, Ed.
Internet-Draft Nokia
Expires: May 25, 2005 J. Salowey, Ed.
Cisco Systems
November 24, 2004
Extensible Authentication Protocol Method for GSM Subscriber Identity
Modules (EAP-SIM)
draft-haverinen-pppext-eap-sim-15.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
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which he or she become aware will be disclosed, in accordance with
RFC 3668.
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Copyright Notice
Copyright (C) The Internet Society (2004).
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
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1 Version Negotiation . . . . . . . . . . . . . . . . . . . 11
4.2 Identity Management . . . . . . . . . . . . . . . . . . . 12
4.2.1 Format, Generation and Usage of Peer Identities . . . 12
4.2.2 Communicating the Peer Identity to the Server . . . . 18
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 . . . . . . . . . . . . . . . . . 23
4.3.2 Fast Re-authentication . . . . . . . . . . . . . . . . 24
4.3.3 Fall Back to Full Authentication . . . . . . . . . . . 25
4.3.4 Requesting the Permanent Identity 1 . . . . . . . . . 26
4.3.5 Requesting the Permanent Identity 2 . . . . . . . . . 27
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 . . . . . . . . . . . . . 32
5.4 Fast Re-authentication Procedure . . . . . . . . . . . . . 33
5.5 Fast Re-authentication Procedure when Counter is Too
Small . . . . . . . . . . . . . . . . . . . . . . . . . . 36
6. EAP-SIM Notifications . . . . . . . . . . . . . . . . . . . 38
6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . 38
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
6.4 Key Generation . . . . . . . . . . . . . . . . . . . . . . 43
7. Message Format and Protocol Extensibility . . . . . . . . . 46
7.1 Message Format . . . . . . . . . . . . . . . . . . . . . . 46
7.2 Protocol Extensibility . . . . . . . . . . . . . . . . . . 47
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8. Messages . . . . . . . . . . . . . . . . . . . . . . . . . . 48
8.1 EAP-Request/SIM/Start . . . . . . . . . . . . . . . . . . 48
8.2 EAP-Response/SIM/Start . . . . . . . . . . . . . . . . . . 49
8.3 EAP-Request/SIM/Challenge . . . . . . . . . . . . . . . . 49
8.4 EAP-Response/SIM/Challenge . . . . . . . . . . . . . . . . 50
8.5 EAP-Request/SIM/Re-authentication . . . . . . . . . . . . 51
8.6 EAP-Response/SIM/Re-authentication . . . . . . . . . . . . 51
8.7 EAP-Response/SIM/Client-Error . . . . . . . . . . . . . . 52
8.8 EAP-Request/SIM/Notification . . . . . . . . . . . . . . . 52
8.9 EAP-Response/SIM/Notification . . . . . . . . . . . . . . 53
9. Attributes . . . . . . . . . . . . . . . . . . . . . . . . . 53
9.1 Table of Attributes . . . . . . . . . . . . . . . . . . . 53
9.2 AT_VERSION_LIST . . . . . . . . . . . . . . . . . . . . . 54
9.3 AT_SELECTED_VERSION . . . . . . . . . . . . . . . . . . . 55
9.4 AT_NONCE_MT . . . . . . . . . . . . . . . . . . . . . . . 56
9.5 AT_PERMANENT_ID_REQ . . . . . . . . . . . . . . . . . . . 56
9.6 AT_ANY_ID_REQ . . . . . . . . . . . . . . . . . . . . . . 56
9.7 AT_FULLAUTH_ID_REQ . . . . . . . . . . . . . . . . . . . . 57
9.8 AT_IDENTITY . . . . . . . . . . . . . . . . . . . . . . . 57
9.9 AT_RAND . . . . . . . . . . . . . . . . . . . . . . . . . 58
9.10 AT_NEXT_PSEUDONYM . . . . . . . . . . . . . . . . . . . 58
9.11 AT_NEXT_REAUTH_ID . . . . . . . . . . . . . . . . . . . 59
9.12 AT_IV, AT_ENCR_DATA and AT_PADDING . . . . . . . . . . . 60
9.13 AT_RESULT_IND . . . . . . . . . . . . . . . . . . . . . 62
9.14 AT_MAC . . . . . . . . . . . . . . . . . . . . . . . . . 62
9.15 AT_COUNTER . . . . . . . . . . . . . . . . . . . . . . . 63
9.16 AT_COUNTER_TOO_SMALL . . . . . . . . . . . . . . . . . . 63
9.17 AT_NONCE_S . . . . . . . . . . . . . . . . . . . . . . . 63
9.18 AT_NOTIFICATION . . . . . . . . . . . . . . . . . . . . 64
9.19 AT_CLIENT_ERROR_CODE . . . . . . . . . . . . . . . . . . 65
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 65
11. Security Considerations . . . . . . . . . . . . . . . . . . 66
11.1 A3 and A8 Algorithms . . . . . . . . . . . . . . . . . . 66
11.2 Identity Protection . . . . . . . . . . . . . . . . . . 66
11.3 Mutual Authentication and Triplet Exposure . . . . . . . 67
11.4 Flooding the Authentication Centre . . . . . . . . . . . 68
11.5 Key Derivation . . . . . . . . . . . . . . . . . . . . . 68
11.6 Cryptographic Separation of Keys and Session
Independence . . . . . . . . . . . . . . . . . . . . . . 69
11.7 Dictionary Attacks . . . . . . . . . . . . . . . . . . . 70
11.8 Credentials Reuse . . . . . . . . . . . . . . . . . . . 70
11.9 Integrity and Replay Protection, and Confidentiality . . 71
11.10 Negotiation Attacks . . . . . . . . . . . . . . . . . . 72
11.11 Protected Result Indications . . . . . . . . . . . . . . 73
11.12 Man-in-the-middle Attacks . . . . . . . . . . . . . . . 73
11.13 Generating Random Numbers . . . . . . . . . . . . . . . 73
12. Security Claims . . . . . . . . . . . . . . . . . . . . . . 74
13. Acknowledgements and Contributions . . . . . . . . . . . . . 75
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13.1 Contributors . . . . . . . . . . . . . . . . . . . . . . 75
13.2 Acknowledgements . . . . . . . . . . . . . . . . . . . . 75
13.2.1 Contributors' Addresses . . . . . . . . . . . . . . 76
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 76
14.1 Normative References . . . . . . . . . . . . . . . . . . . 76
14.2 Informative References . . . . . . . . . . . . . . . . . . 78
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 79
A. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 79
A.1 EAP-Request/Identity . . . . . . . . . . . . . . . . . . . 79
A.2 EAP-Response/Identity . . . . . . . . . . . . . . . . . . 80
A.3 EAP-Request/SIM/Start . . . . . . . . . . . . . . . . . . 80
A.4 EAP-Response/SIM/Start . . . . . . . . . . . . . . . . . . 80
A.5 EAP-Request/SIM/Challenge . . . . . . . . . . . . . . . . 81
A.6 EAP-Response/SIM/Challenge . . . . . . . . . . . . . . . . 83
A.7 EAP-Success . . . . . . . . . . . . . . . . . . . . . . . 84
A.8 Fast Re-authentication . . . . . . . . . . . . . . . . . . 84
A.9 EAP-Request/SIM/Re-authentication . . . . . . . . . . . . 84
A.10 EAP-Response/SIM/Re-authentication . . . . . . . . . . . 87
B. Pseudo-Random Number Generator . . . . . . . . . . . . . . . 88
Intellectual Property and Copyright Statements . . . . . . . 89
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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 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 not directly used. Hence the secrecy of Kc is critical
to the security of this protocol. Please find more information about
GSM authentication in [GSM 03.20]. Please see Section 11.1 for more
discussion about the GSM algorithms used in EAP-SIM.
The lack of mutual authentication is a weakness in GSM
authentication. The 64 bit cipher key (Kc) that is derived is not
strong enough for data networks where stronger and longer keys are
required. Hence in EAP-SIM, several RAND challenges are used for
generating several 64-bit Kc keys, which are combined to constitute
stronger keying material. In EAP-SIM the client issues a random
number NONCE_MT to the network, in order to contribute to key
derivation, and to prevent replays of EAP-SIM requests from previous
exchanges. The NONCE_MT can be conceived as the client's challenge
to the network. EAP-SIM also extends the combined RAND challenges
and other messages with a message authentication code in order to
provide message integrity protection along with mutual
authentication.
EAP-SIM specifies optional support for protecting the privacy of
subscriber identity using the same concept as GSM, which is using
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
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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 key to the security
of EAP-SIM. In brief, EAP-SIM is in no sense weaker than the GSM
mechanisms. In some cases EAP-SIM provides better security
properties than the underlying GSM mechanisms, particularly if the
SIM credentials are only used for EAP-SIM and not re-used from
GSM/GPRS. Many of the security features of EAP-SIM rely upon the
secrecy of the Kc values in the SIM triplets, so protecting these
values is key to the security of the EAP-SIM protocol.
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
infrastructure are available.
2. Terms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [ Internet-Draft EAP-SIM Authentication November 2004 Requirement Levels"'>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].
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
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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,
ie. 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.
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
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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, ie. 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.
Pseudonym Username
The username portion of pseudonym identity, ie. 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, when 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 back end AAA
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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
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 7.
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The EAP-Request/SIM/Start packet contains the list of EAP-SIM version
supported by the EAP server in the AT_VERSION_LIST attribute. This
packet may also include attributes for requesting the subscriber
identity, as specified in Section 4.2.
The peer responds to EAP-Request/SIM/Start with the
EAP-Response/SIM/Start packet, which includes the AT_NONCE_MT
attribute that contains a random number NONCE_MT, chosen by the peer,
and the AT_SELECTED_VERSION attribute that contains the version
number selected by the peer. The version negotiation is protected by
including the version list and the selected version in the
calculation of keying material (Section 6.4).
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 6.4. 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 reused, except in some error cases
that are specified in Section 9.9
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
sends the EAP-Response/SIM/Client-Error packet and the authentication
exchange terminates.
Since the RAND's given to a peer are accompanied with the message
authentication code AT_MAC, and since the peer's NONCE_MT value
contributes to AT_MAC, the peer is able to verify that the EAP-SIM
message is fresh (not a replay) and that the sender possesses valid
GSM triplets for the subscriber.
If all checks out, the peer responds with the
EAP-Response/SIM/Challenge, containing the AT_MAC attribute that
covers the peer's SRES response values (Section 8.4). 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
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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,
which 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 8.7) to the server with the error code "unsupported
version". If a suitable version is included, then the peer includes
the AT_SELECTED_VERSION attribute, containing the selected version,
in the EAP-Response/SIM/Start packet. The peer MUST only indicate a
version that is included in AT_VERSION_LIST. If several versions are
acceptable, then the peer SHOULD choose the version that occurs first
in the version list.
The version number list of AT_VERSION_LIST and the selected version
of AT_SELECTED_VERSION are included in the key derivation procedure
(Section 6.4). 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.
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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 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 composed of a
three digit Mobile Country Code (MCC), a two or three digit Mobile
Network Code (MNC) and a not more than 10 digit Mobile Subscriber
Identification Number (MSIN). In other words, the IMSI is a string
of not more than 15 digits. MCC and MNC uniquely identify the GSM
operator and help identify the AuC from which the authentication
vectors need to be retrieved for this subscriber.
Internet AAA protocols identify users with the Network Access
Identifier (NAI) [RFC2486]. 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 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 to
make the subscriber's EAP exchanges untraceable to eavesdroppers.
Because the permanent identity never changes, revealing it would help
observers to track the user. The permanent identity is usually based
on the IMSI, which may further help the tracking, because the same
identifier may 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 11.2
for security considerations regarding identity privacy.
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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 only used 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
realm. (The usage of a NAI realm portion is not considered to be
decoration.) Username decoration is out of the scope of this
document. However, it should be noted that username decoration might
prevent the server from recognizing a valid username. Hence,
although the peer MAY use username decoration in the identities the
peer includes in EAP-Response/Identity, and the EAP server MAY accept
a decorated peer username in this message, the peer or the EAP server
MUST NOT decorate any other peer identities that are used in various
EAP-SIM attributes. Only the identity used in EAP-Response/Identity
may be decorated.
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
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implementation. In this case, the peer is typically configured with
the NAI realm of the home operator. Operators MAY reserve a specific
realm name for EAP-SIM users. This convention makes it easy to
recognize that the NAI identifies a GSM subscriber. Such reserved
NAI realm may be useful as a hint as to the first authentication
method to use during method negotiation. When the peer is using a
pseudonym username instead of the permanent username, the peer
selects the realm name portion similarly as it select the realm
portion when using the permanent username.
If no configured realm name is available, the peer MAY derive the
realm name from the MCC and MNC portions of the IMSI. A RECOMMENDED
way to derive the realm from the IMSI using the realm 3gppnetwork.org
will be specified in [Draft 3GPP TS 23.003].
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 [Draft 3GPP TS 23.003] instead of owlan.org as
soon as the 3GPP specification is finalized.
The IMSI is a string of digits without any explicit structure, so the
peer may not be able to determine the length of the MNC portion. If
the peer is not able to determine whether the MNC is two or three
digits long, the peer MAY use a 3-digit MNC. If the correct length
of the MNC is two, then the MNC used in the realm name includes the
first digit of MSIN. Hence, when configuring AAA networks for
operators that have 2-digit MNC's, the network SHOULD also be
prepared for realm names with incorrect 3-digit MNC's.
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 as 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)
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The EAP server MAY use the leading "1" as a hint to try EAP-SIM as
the first authentication method during method negotiation, rather
than for example EAP/AKA. The EAP-SIM server MAY propose EAP-SIM
even if the leading character was not "1".
Alternatively, an implementation MAY choose a permanent username that
is not based on the IMSI. In this case the selection of the
username, its format, and its processing is out of the scope of this
document. In this case, the peer implementation MUST NOT prepend any
leading characters to the username.
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 peer to send the
permanent identity.
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 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. The 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 the peer's fast
re-authentication state information (in other words whether the state
information is from the same full authentication exchange). The
random component also makes the fast re-authentication identities
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unpredictable, so an attacker cannot initiate a fast
re-authentication exchange to get the server's
EAP-Request/SIM/Re-authentication packet.
Regardless of 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] user names be recognizable from each other as an
aid for the server to which method to offer.
In general, it is the task of the EAP server and the policies of its
administrator to ensure sufficient separation in the usernames.
Pseudonym usernames and 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 recognize if a NAI username is an EAP-SIM pseudonym username or
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 to 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
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identity in the value field of the AT_ENCR_DATA attribute. Because
identity privacy support and fast re-authentication are optional to
implement, 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 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 next full
authentication. The username portions of fast re-authentication
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 a 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.
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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, 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
re-authentication identity maintained by the server, and if the
server agrees on using 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 EAP-Response/Identity. In this
case, the server includes an identity requesting attribute
(AT_ANY_ID_REQ, AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ) in the
EAP-Request/SIM/Start message, and the peer includes the AT_IDENTITY
attribute, which contains the peer's identity, in the
EAP-Response/SIM/Start message. The AT_ANY_ID_REQ attribute is a
general identity requesting attribute, which the server uses if it
does not specify which kind of an identity the peer should return in
AT_IDENTITY. The server uses the AT_FULLAUTH_ID_REQ attribute to
request either the permanent identity or a pseudonym identity. The
server uses the AT_PERMANENT_ID_REQ attribute to request the peer to
send its permanent identity.
The identity format in the AT_IDENTITY attribute is the same as in
the EAP-Response/Identity packet (except that identity decoration is
not allowed). The AT_IDENTITY attribute contains a permanent
identity, a pseudonym identity or a fast re-authentication identity.
Please note that the EAP-SIM peer and the EAP-SIM server only process
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the AT_IDENTITY attribute and 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.
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 the peer to 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 performs the following steps.
If the peer has maintained fast re-authentication state information
and if the peer 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
If the EAP server has not received any EAP-SIM peer identity
(permanent identity, pseudonym identity or fast re-authentication
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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
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 received an identity that it recognizes as a pseudonym
identity but the server is not able to map the pseudonym identity
to a permanent identity.
The server issues the EAP-Request/SIM/Start packet with the
AT_FULLAUTH_ID_REQ attribute to indicate that the server wants the
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 received an identity that it recognizes as a
re-authentication identity but the server is not able to map the
re-authentication identity to a permanent identity
The server issues the EAP-Request/SIM/Start packet with the
AT_ANY_ID_REQ attribute to indicate that the server wants the peer to
include an identity in the AT_IDENTITY attribute of the
EAP-Response/SIM/Start message, and the server does not indicate any
preferred type for the identity. This is done in other cases, such
as when the server does not have any identity, or the server does not
recognize the format of a received identity.
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 any identity request
attribute, then the peer responds with EAP-Response/SIM/Start without
AT_IDENTITY. The peer includes the AT_SELECTED_VERSION and
AT_NONCE_MT attributes, because the exchange is a full authentication
exchange.
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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
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 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 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 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
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network, but an active attacker may transmit an EAP-
Request/SIM/Start packet with an AT_PERMANENT_ID_REQ attribute to the
peer, in an effort to find out the true identity of the user. If the
peer does not want to reveal its permanent identity, then the peer
sends the EAP-Response/SIM/Client-Error packet with the error code
"unable to process packet", and the authentication exchange
terminates.
Basically, there are two different policies that the peer can employ
with regard to AT_PERMANENT_ID_REQ. A "conservative" peer assumes
that the network is able to maintain pseudonyms robustly. Therefore,
if a conservative peer has a pseudonym username, the peer responds
with EAP-Response/SIM/Client-Error to the EAP packet with
AT_PERMANENT_ID_REQ, because the peer believes that the valid network
is able to map the pseudonym identity to the peer's permanent
identity. (Alternatively, the conservative peer may accept
AT_PERMANENT_ID_REQ in certain circumstances, for example if the
pseudonym was received a long time ago.) The benefit of this policy
is that it protects the peer against active attacks on anonymity. On
the other hand, a "liberal" peer always accepts the
AT_PERMANENT_ID_REQ and responds with the permanent identity. The
benefit of this policy is that it works even if the valid network
sometimes loses pseudonyms and is not able to map them to the
permanent identity.
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 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
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valid permanent identity or a pseudonym identity that the server can
map to a valid permanent identity, then the server proceeds with full
authentication by sending EAP-Request/SIM/Challenge.
If the server used AT_ANY_ID_REQ, and if AT_IDENTITY contains a valid
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, which 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.
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.
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Peer Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP-SIM |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY, 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.
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.
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Peer Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP-SIM |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY containing a 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.
4.3.3 Fall Back to Full Authentication
The case when 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 is illustrated in Figure 4.
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Peer Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP-SIM |
| +------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY containing a 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
4.3.4 Requesting the Permanent Identity 1
Figure 5 illustrates the case when the EAP server fails to map the
pseudonym identity included in the EAP-Response/Identity packet to a
valid permanent identity.
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Peer Authenticator
| |
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes a pseudonym) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server fails to map the |
| | Pseudonym to a permanent id. |
| +------------------------------+
| EAP-Request/SIM/Start |
| (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY with permanent identity, AT_NONCE_MT, |
| AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| |
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.
4.3.5 Requesting the Permanent Identity 2
Figure 6 illustrates the case when the EAP server fails to map the
pseudonym included in the AT_IDENTITY attribute to a valid permanent
identity.
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Peer Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP-SIM |
| +------------------------------+
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
|EAP-Response/SIM/Start |
|(AT_IDENTITY with a pseudonym identity, AT_NONCE_MT, |
| AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| +-------------------------------+
| | Server fails to map the |
| | Pseudonym in AT_IDENTITY |
| | to a valid permanent identity |
| +-------------------------------+
| |
| EAP-Request/SIM/Start |
| (AT_PERMANENT_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY with permanent identity, |
| AT_NONCE_MT, AT_SELECTED_VERSION) |
|------------------------------------------------------>|
| |
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 has obtained an acceptable identity. This case is
illustrated in Figure 7.
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Peer Authenticator
| |
| +------------------------------+
| | Server does not have any |
| | Subscriber identity available|
| | When starting EAP-SIM |
| +------------------------------+
| EAP-Request/SIM/Start |
| (Includes AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY with fast re-auth. identity) |
|------------------------------------------------------>|
| |
| +------------------------------+
| | Server does not accept |
| | The fast re-auth. |
| | Identity |
| +------------------------------+
| EAP-Request/SIM/Start |
| (AT_FULLAUTH_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
: :
: :
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: :
: :
|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
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 it therefore requires 2 or 3 fresh
triplets from the Authentication Centre, the full authentication
procedure is not very well suitable for frequent use. Therefore,
EAP-SIM includes a more inexpensive 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 they do not
want to use fast re-authentication.
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Fast re-authentication is based on the keys derived on the preceding
full authentication. The same K_aut and K_encr keys as in full
authentication are used to protect EAP-SIM packets and attributes,
and the original Master Key from full authentication is used to
generate a fresh Master Session Key, as specified in Section 6.4.
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, counter value is two or greater etc. 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.
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
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UMTS AKA sequence number, NONCE_S corresponds to RAND, and 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.
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 8.3). This attribute
contains a new fast re-authentication identity for the next fast
re-authentication. The attribute also works as a capability flag
that indicates the fact 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
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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 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.
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 the peer to send its identity by
including the AT_ANY_ID_REQ attribute in the EAP-Request/SIM/Start
packet.
If the server recognizes the identity as a valid fast
re-authentication identity, and if the server agrees on using fast
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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,
including the AT_COUNTER attribute with the same counter value and
the AT_MAC attribute.
The server verifies the AT_MAC attribute and also verifies that the
counter value is the same that it used in the
EAP-Request/SIM/Re-authentication packet. If these checks are
successful, the re-authentication has succeeded and the server sends
the EAP-Success packet to the peer.
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 on using fast |
| | re-authentication |
| +--------------------------------+
| |
: :
: :
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: :
: :
| 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
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 '*'.
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Peer Authenticator
| EAP-Request/SIM/Start |
| (AT_ANY_ID_REQ, AT_VERSION_LIST) |
|<------------------------------------------------------|
| |
| EAP-Response/SIM/Start |
| (AT_IDENTITY, AT_NONCE_MT |
| AT_SELECTED_VERSION) |
| (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
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
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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 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
Section 6.4).
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, and as the contents of the notification is
a displayable string, these notifications are not easily localizable.
EAP-SIM also specifies method specific EAP-SIM notifications.
The EAP server can use EAP-SIM notifications to convey localizable
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.
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 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.
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The receipt of a notification code with the S bit set to one (values
32768...65536) does not imply failure. Notification code 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 reautentication. 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 8.8 and Section 8.9 specify what other attributes must be
included in the notification packets.
Some of the notification codes are authorization related and hence
not usually considered as part of the responsibility of an EAP
method. However, they are included as part of EAP-SIM because there
are currently no other ways to convey this information to the user in
a localizable way, and the information is potentially useful for the
user. An EAP-SIM server implementation may decide never to send
these EAP-SIM notifications.
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.
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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 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 AT_NOTIFICATION code 32768 is used to indicate success,
the server MUST ignore the contents of the EAP-SIM response it
receives to 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.
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
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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
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 9.19:
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".
When processing the AT_RAND attribute, the peer MUST send the EAP-
Response/SIM/Client-Error packet with the error code "insufficient
number of challenges", if the number of RAND challenges is smaller
than what is required by peer's local policy.
If the peer believes that the RAND challenges included in AT_RAND are
not fresh e.g. because it is capable of remembering some previously
used RANDs, the peer MUST send the EAP-Response/SIM/Client-Error
packet with the error code "RANDs are not fresh".
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 (0 or 16384). The choice
between these two codes depends on the phase of the EAP-SIM exchange,
see Section 6. The error cases when the server issues an
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EAP-Request/SIM/Notification that implies failure 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
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 other cases 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
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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