EAP Working Group Bernard Aboba
INTERNET-DRAFT Dan Simon
Category: Standards Track Microsoft
<draft-ietf-eap-keying-12.txt> J. Arkko
13 April 2006
Ericsson
P. Eronen
Nokia
H. Levkowetz, Ed.
ipUnplugged
Extensible Authentication Protocol (EAP) Key Management Framework
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Copyright Notice
Copyright (C) The Internet Society 2006.
Abstract
The Extensible Authentication Protocol (EAP), defined in [RFC3748],
enables extensible network access authentication. This document
provides a framework for the transport and usage of keying material
generated by EAP authentication algorithms, known as "methods". It
also specifies the EAP key hierarchy.
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Table of Contents
1. Introduction .......................................... 3
1.1 Requirements Language ........................... 3
1.2 Terminology ..................................... 3
1.3 Overview ........................................ 5
1.4 EAP Key Hierarchy ............................... 8
1.5 Security Goals .................................. 12
1.6 EAP Invariants .................................. 12
2. Lower Layer Operation ................................. 16
2.1 Transient Session Keys .......................... 17
2.2 Authenticator Architecture ...................... 18
3. Key Management ........................................ 22
3.1 Secure Association Protocol ..................... 23
3.2 Key Scope ....................................... 25
3.3 Parent-Child Relationships ...................... 26
3.4 Local Key Lifetimes ............................. 26
3.5 Exported and Calculated Key Lifetimes ........... 27
3.6 Key Cache Synchronization ....................... 29
3.7 Key Strength .................................... 29
3.8 Key Wrap ........................................ 30
4. Handoff Vulnerabilities ............................... 30
4.1 Authorization ................................... 31
4.2 Correctness ..................................... 32
5. Security Considerations .............................. 35
5.1 Authenticator Compromise ........................ 36
5.2 Spoofing ........................................ 37
5.3 Downgrade Attacks ............................... 37
5.4 Unauthorized Disclosure ......................... 38
5.5 Replay Protection ............................... 40
5.6 Key Freshness ................................... 40
5.7 Elevation of Privilege .......................... 41
5.8 Man-in-the-Middle Attacks ....................... 42
5.9 Denial of Service Attacks ....................... 43
5.10 Impersonation ................................... 43
5.11 Channel Binding ................................. 44
6. IANA Considerations ................................... 45
7. References ............................................ 46
7.1 Normative References ............................ 46
7.2 Informative References .......................... 46
Acknowledgments .............................................. 50
Author's Addresses ........................................... 50
Appendix A - Exported Parameters in Existing Methods ......... 52
Intellectual Property Statement .............................. 53
Disclaimer of Validity ....................................... 54
Copyright Statement .......................................... 54
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1. Introduction
The Extensible Authentication Protocol (EAP), defined in [RFC3748],
was designed to enable extensible authentication for network access
in situations in which the IP protocol is not available. Originally
developed for use with PPP [RFC1661], it has subsequently also been
applied to IEEE 802 wired networks [IEEE-802.1X], wireless networks
such as [IEEE-802.11i] and [IEEE-802.16e], and IKEv2 [RFC4306].
This document provides a framework for the transport and usage of
keying material generated by EAP authentication algorithms, known as
"methods". In EAP, keying material is generated by EAP methods.
Part of this keying material may be used by EAP methods themselves
and part of this material may be exported. The exported keying
material may be transported by AAA protocols and used by Secure
Association Protocols in the generation or transport of session keys
which are used by lower layer ciphersuites. This document describes
each of these elements and provides a system-level security analysis.
It also specifies the EAP key hierarchy.
1.1. Requirements Language
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].
1.2. Terminology
The terms "Cryptographic binding", "Cryptographic separation", "Key
strength" and "Mutual authentication" are defined in [RFC3748] and
are used with the same meaning in this document, which also
frequently uses the following terms:
AAA Authentication, Authorization and Accounting. AAA protocols with
EAP support include RADIUS [RFC3579] and Diameter [RFC4072]. In
this document, the terms "AAA server" and "backend authentication
server" are used interchangeably.
authenticator
The end of the link initiating EAP authentication. The term
Authenticator is used in [IEEE-802.1X], and authenticator has the
same meaning in this document.
peer The end of the link that responds to the authenticator.
backend authentication server
A backend authentication server is an entity that provides an
authentication service to an authenticator. When used, this server
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typically executes EAP methods for the authenticator. This
terminology is also used in [IEEE-802.1X].
Channel Binding
The communication within an EAP method of integrity-protected
channel properties such as endpoint identifiers which can be
compared to values communicated via out of band mechanisms (such as
via a AAA or lower layer protocol).
EAP server
The entity that terminates the EAP authentication method with the
peer. In the case where no backend authentication server is used,
the EAP server is part of the authenticator. In the case where the
authenticator operates in pass-through mode, the EAP server is
located on the backend authentication server.
Lower Layer
The lower layer is responsible for carrying EAP frames between the
peer and authenticator.
Long Term Credential
EAP methods frequently make use of long term secrets in order to
enable authentication between the peer and server. In the case of
a method based on pre-shared key authentication, the long term
credential is the pre-shared key. In the case of a public-key
based method, the long term credential is the corresponding private
key.
Master Session Key (MSK)
Keying material that is derived between the EAP peer and server and
exported by the EAP method. The MSK is at least 64 octets in
length.
AAA-Key
The term AAA-Key is synonymous with MSK.
Extended Master Session Key (EMSK)
Additional keying material derived between the peer and server that
is exported by the EAP method. The EMSK is at least 64 octets in
length, and is never shared with a third party.
Initialization Vector (IV)
A quantity of at least 64 octets, suitable for use in an
initialization vector field, that is derived between the peer and
EAP server. Since the IV is a known value in methods such as EAP-
TLS [RFC2716], it cannot be used by itself for computation of any
quantity that needs to remain secret. As a result, its use has
been deprecated and EAP methods are not required to generate it.
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However, when it is generated it MUST be unpredictable.
Pairwise Master Key (PMK)
Lower layers use the MSK in lower-layer dependent manner. For
instance, in [IEEE-802.11i] Octets 0-31 of the MSK are known as the
Pairwise Master Key (PMK). In [IEEE-802.11i] the TKIP and AES CCMP
ciphersuites derive their Transient Session Keys (TSKs) solely from
the PMK, whereas the WEP ciphersuite as noted in [RFC3580], derives
its TSKs from both halves of the MSK. In [802.16e], the MSK is
truncated to 20 octets for PMK and 20 octets for PMK2.
security association
A set of policies and cryptographic state used to protect
information. Elements of a security association may include
cryptographic keys, negotiated ciphersuites and other parameters,
counters, sequence spaces, authorization attributes, etc.
Secure Association Protocol
An exchange that occurs between the EAP peer and authenticator in
order to manage the creation and deletion of unicast and multicast
security associations.
Transient EAP Keys (TEKs)
Session keys which are used to establish a protected channel
between the EAP peer and server during the EAP authentication
exchange. The TEKs are appropriate for use with the ciphersuite
negotiated between EAP peer and server for use in protecting the
EAP conversation. The TEKs are stored locally by the EAP method
and are not exported. Note that the ciphersuite used to set up the
protected channel between the EAP peer and server during EAP
authentication is unrelated to the ciphersuite used to subsequently
protect data sent between the EAP peer and authenticator.
Transient Session Keys (TSKs)
Session keys used to protect data exchanged after EAP
authentication has successfully completed, using the ciphersuite
negotiated between the EAP peer and authenticator.
1.3. Overview
Where EAP key derivation is supported, the conversation typically
takes place in three phases:
Phase 0: Discovery
Phase 1: Authentication
1a: EAP authentication
1b: AAA Key Transport (optional)
Phase 2: Secure Association Protocol
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2a: Unicast Secure Association
2b: Multicast Secure Association (optional)
Of these phases, Phase 0, 1b and Phase 2 are handled external to EAP.
Phases 0 and 2 are handled by the lower layer protocol and phase 1b
is typically handled by a AAA protocol.
In the discovery phase (phase 0), peers locate authenticators and
discover their capabilities. A peer may locate an authenticator
providing access to a particular network, or a peer may locate an
authenticator behind a bridge with which it desires to establish a
Secure Association. Discovery can occur manually or automatically,
depending on the lower layer over which EAP runs.
EAP peer Authenticator Auth. Server
-------- ------------- ------------
|<----------------------------->| |
| Discovery (phase 0) | |
|<----------------------------->|<----------------------------->|
| EAP auth (phase 1a) | AAA pass-through (optional) |
| | |
| |<----------------------------->|
| | AAA Key transport |
| | (optional; phase 1b) |
|<----------------------------->| |
| Unicast Secure association | |
| (phase 2a) | |
| | |
|<----------------------------->| |
| Multicast Secure association | |
| (optional; phase 2b) | |
| | |
Figure 1: Conversation Overview
The authentication phase (phase 1) may begin once the peer and
authenticator discover each other. This phase, if it occurs, always
includes EAP authentication (phase 1a). Where the chosen EAP method
supports key derivation, in phase 1a EAP keying material is derived
on both the peer and the EAP server.
An additional step (phase 1b) is required in deployments which
include a backend authentication server, in order to transport keying
material from the backend authentication server to the authenticator.
In order to obey the principle of Mode Independence (see Section
1.6.1), where a backend server is present, all keying material which
is required by the lower layer needs to be transported from the EAP
server to the authenticator. Since existing TSK derivation and
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transport techniques depend solely on the MSK, in existing
implementations, this is the only keying material replicated in the
AAA key transport phase 1b.
Successful completion of EAP authentication and key derivation by a
peer and EAP server does not necessarily imply that the peer is
committed to joining the network associated with an EAP server.
Rather, this commitment is implied by the creation of a security
association between the EAP peer and authenticator, as part of the
Secure Association Protocol (phase 2). The Secure Association
Protocol exchange (phase 2) occurs between the peer and authenticator
in order to manage the creation and deletion of unicast (phase 2a)
and multicast (phase 2b) security associations between the peer and
authenticator. The conversation between the parties is shown in
Figure 1.
Existing EAP lower layers implement phase 0, 2a and 2b in different
ways:
PPP PPP, defined in [RFC1661] does not support discovery, nor does it
include a Secure Association Protocol.
PPPoE
PPPoE, defined in [RFC2516], includes support for a Discovery stage
(phase 0). In this step, the EAP peer sends a PPPoE Active
Discovery Initiation (PADI) packet to the broadcast address,
indicating the service it is requesting. The Access Concentrator
replies with a PPPoE Active Discovery Offer (PADO) packet
containing its name, the service name and an indication of the
services offered by the concentrator. The discovery phase is not
secured. PPPoE, like PPP, does not include a Secure Association
Protocol.
IKEv2
IKEv2, defined in [RFC4306], includes support for EAP and handles
the establishment of unicast security associations (phase 2a).
However, the establishment of multicast security associations
(phase 2b) typically does not involve EAP and needs to be handled
by a group key management protocol such as GDOI [RFC3547], GSAKMP
[GSAKMP], MIKEY [RFC3830], or GKDP [GKDP]. Several mechanisms have
been proposed for discovery of IPsec security gateways. [RFC2230]
discusses the use of KX Resource Records (RRs) for IPsec gateway
discovery; while KX RRs are supported by many DNS server
implementations, they have not yet been widely deployed.
Alternatively, DNS SRV [RFC2782] can be used for this purpose.
Where DNS is used for gateway location, DNS security mechanisms
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such as DNSSEC ([RFC2535], [RFC2931]), TSIG [RFC2845], and Simple
Secure Dynamic Update [RFC3007] are available.
IEEE 802.11i
IEEE 802.11, defined in [IEEE-802.11], handles discovery via the
Beacon and Probe Request/Response mechanisms. IEEE 802.11 access
points periodically announce their Service Set Identifiers (SSIDs)
as well as capabilities using Beacon frames. Stations can query
for access points by sending a Probe Request to the broadcast
address. Neither Beacon nor Probe Request/Response frames are
secured. The 4-way handshake defined in [IEEE-802.11i] enables the
derivation of unicast (phase 2a) and multicast/broadcast (phase 2b)
secure associations. Since the group key exchange transports a
group key from the access point to the station, two 4-way
handshakes may be required in order to support peer-to-peer
communications. A proof of the security of the IEEE 802.11i 4-way
handshake when used with EAP-TLS [RFC2716], is provided in [He].
IEEE 802.1X
IEEE 802.1X-2004, defined in [IEEE-802.1X] does not support
discovery (phase 0), nor does it provide for derivation of unicast
or multicast secure associations.
1.4. EAP Key Hierarchy
EAP, defined in [RFC3748], is a two-party protocol spoken between the
EAP peer and server. Within EAP, keying material is generated by EAP
methods. Part of this keying material may be used by EAP methods
themselves and part of this material may be exported. In addition to
export of keying material, EAP methods may also export associated
parameters, and may import and export Channel Bindings from the lower
layer.
As illustrated in Figure 2, the EAP method key derivation has at the
root the long term credential utilized by the selected EAP method.
If authentication is based on a pre-shared key, the parties store the
EAP method to be used and the pre-shared key. The EAP server also
stores the peer's identity as well as other information associated
with it. This information may be used to determine whether access to
some service should be granted. The peer stores information necessary
to choose which secret to use for which service.
If authentication is based on proof of possession of the private key
corresponding to the public key contained within a certificate, the
parties store the EAP method to be used and the trust anchors used to
validate the certificates. The EAP server also stores the peer's
identity and the peer stores information necessary to choose which
certificate to use for which service.
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Based on the long term credential established between the peer and
the server, EAP methods derive two types of keys:
[1] Keys calculated locally by the EAP method but not exported
by the EAP method, such as the TEKs.
[2] Keying material exported by the EAP method: MSK, EMSK, IV.
As noted in [RFC3748] Section 7.10, EAP methods generating keys are
required to calculate and export the MSK and EMSK, which must be at
least 64 octets in length. EAP methods also may export the IV;
however, the use of the IV is deprecated.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | ^
| EAP Method | |
| | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+ | |
| | | | | | |
| | EAP Method Key |<->| Long-Term | | |
| | Derivation | | Credential | | |
| | | | | | |
| | | +-+-+-+-+-+-+-+ | Local to |
| | | | EAP |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Method |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| | +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+-+ | |
| | | TEK | |MSK, EMSK | |IV | | |
| | |Derivation | |Derivation | |Derivation | | |
| | | | | | |(Deprecated) | | |
| | +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+-+ | |
| | ^ | | | |
| | | | | | V
+-+-|-+-+-+-+-+-+-+-+-|-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+ ---+
| | | | ^
| Peer-Id, | | | Exported |
| Server-Id, | Channel | MSK (64+B) | IV (64B) by |
| Session-Id, | Bindings | EMSK (64+B) | (Optional) EAP |
| Key-Lifetime | & Result | | Method |
V V V V V
Figure 2: EAP Method Parameter Import/Export
EAP methods also MAY export method-specific peer and server
identifiers (Peer-Id and Server-Id), a method-specific EAP
conversation identifier known as the Session-Id, and the lifetime of
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the exported keys, known as the Key-Lifetime. EAP methods MAY also
support the import and export of Channel Bindings. New EAP method
specifications MUST define the Peer-Id, Server-Id and Method-Id. The
combination of the Peer-Id and Server-Id uniquely specifies the
endpoints of the EAP method exchange when they are provided. The
Peer-Id, Server-Id, and Method-Id for existing EAP methods is defined
in Appendix A.
Peer-Id
As described in [RFC3748] Section 7.3, the identity provided in the
EAP-Response/Identity, may be different from the peer identity
authenticated by the EAP method. Where the EAP method authenticates
the peer identity, that identity is exported by the method as the
Peer-Id. A suitable EAP peer name may not always be available.
Where an EAP method does not define a method-specific peer identity,
the Peer-Id is the null string.
Server-Id
Where the EAP method authenticates the server identity, that identity
is exported by the method as the Server-Id. A suitable EAP server
name may not always be available. Where an EAP method does not
define a method-specific peer identity, the Server-Id is the null
string.
Method-Id
EAP method specifications deriving keys MUST specify a temporally
unique method identifier known as the Method-Id. The EAP Method-Id
uniquely identifies an EAP session of a given Type between an EAP
peer and server. The Method-Id is typically constructed from nonces
or counters used within the EAP method exchange.
Session-Id
The Session-Id uniquely identifies an EAP session between an EAP peer
(as identified by the Peer-Id) and server (as identified by the
Server-Id). The EAP Session-Id consists of the concatenation of the
Expanded EAP Type Code (including the Type, Vendor-Id and Vendor-Type
fields defined in [RFC3748] Section 5.7) and the Method-Id. The
inclusion of the Expanded Type Code in the EAP Session-Id ensures
that each EAP method has a distinct Session-Id space. Since an EAP
session is not bound to a particular authenticator or specific ports
on the peer and authenticator, the authenticator port or identity are
not included in the Session-Id.
Key-Lifetime
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While EAP itself does not support key lifetime negotiation, it is
possible to specify methods that do. However, systems that rely on
such negotiation for exported keys would only function with these
methods. As a result, it is NOT RECOMMENDED to use this approach as
the sole way to determine key lifetimes.
Channel Bindings
Channel Bindings include lower layer parameters that are verified for
consistency between the EAP peer and server. In order to avoid
introducing media dependencies, EAP methods that transport Channel
Binding data MUST treat this data as opaque octets.
Typically the EAP method imports Channel Bindings from the lower
layer on the peer, and transmits them securely to the EAP server,
which exports them to the lower layer or AAA layer. However,
transport may occur from EAP server to peer, or may be bi-
directional. On the side of the exchange (peer or server) where
Channel Bindings are verified, the lower layer or AAA layer passes
the result of the verification (TRUE or FALSE) up to the EAP method.
While the verification can be done either by the peer or the server,
typically only the server has the knowledge to determine the
correctness of the values, as opposed to merely verifying their
equality. See Section 5.11 for further discussion.
1.4.1. Key Naming
Each key created within the EAP key management framework has a name
(a unique identifier), as well as a scope (the parties to whom the
key is available). The scope of exported parameters is defined by
the EAP peer name (if securely exchanged within the method) and the
EAP server name (also only if securely exchanged). Where a peer or
server name is missing the null string is used.
MSK and EMSK Names
These parameters are exported by the EAP peer and EAP server, and
can be referred to using the EAP Session-Id and a binary or textual
indication of the parameter being referred to.
PMK Name
This document does not specify a naming scheme for the PMK. The
PMK is only identified by the key from which it is derived.
Note: IEEE 802.11i names the PMKID for the purposes of being able
to refer to it in the Secure Association protocol; this naming is
based on a hash of the PMK itself as well as some other parameters
(see Section 8.5.1.2 [IEEE-802.11i]).
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TEK Name
The TEKs may or may not be named. Their naming is specified in the
EAP method.
TSK Name
The TSKs are typically named. Their naming is specified in the
lower layer so that the correct set of transient session keys can
be identified for processing a given packet.
1.5. Security Goals
The goal of the EAP conversation is to derive fresh session keys
between the EAP peer and authenticator that are known only to those
parties, and for both the EAP peer and authenticator to demonstrate
that they are authorized to perform their roles either by each other
or by a trusted third party (the backend authentication server).
Completion of an EAP method exchange (Phase 1a) supporting key
derivation results in the derivation of EAP keying material (MSK,
EMSK, TEKs) known only to the EAP peer (identified by the Peer-Id)
and server (identified by the Server-Id). Both the EAP peer and EAP
server know the exported keying material to be fresh. Key freshness
is discussed in Sections 3.4, 3.5 and 5.6.
Completion of the AAA exchange (Phase 1b) results in the transport of
EAP keying material from the EAP server (identified by the Server-Id)
to the EAP authenticator (identified by the NAS-Identifier) without
disclosure to any other party. Both the EAP server and EAP
authenticator know this keying material to be fresh. Disclosure
issues are discussed in Section 5.4; security properties of AAA
protocols are discussed in Sections 5.1-5.7, and 5.10.
Completion of the Secure Association Protocol (Phase 2) results in
the derivation or transport of Transient Session Keys (TSKs) known
only to the EAP peer (identified by the Peer-Id) and authenticator
(identified by the NAS-Identifier). Both the EAP peer and
authenticator know the TSKs to be fresh. Both the EAP peer and
authenticator demonstrate that they are authorized to perform their
roles. Authorization issues are discussed in Section 5.7 and 5.8;
security properties of Secure Association Protocols are discussed in
Section 3.1.
1.6. EAP Invariants
Certain basic characteristics, known as "EAP Invariants", hold true
for EAP implementations on all media:
Mode independence
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Media independence
Method independence
Ciphersuite independence
1.6.1. Mode Independence
EAP is typically deployed to support extensible network access
authentication in situations where a peer desires network access via
one or more authenticators. Where authenticators are deployed
standalone, the EAP conversation occurs between the peer and
authenticator, and the authenticator must locally implement an EAP
method acceptable to the peer. However, when utilized in "pass-
through" mode, EAP enables deployment of new authentication methods
without requiring development of new code on the authenticator.
While the authenticator may implement some EAP methods locally and
use those methods to authenticate local users, it may at the same
time act as a pass-through for other users and methods, forwarding
EAP packets back and forth between the backend authentication server
and the peer. This is accomplished by encapsulating EAP packets
within the Authentication, Authorization and Accounting (AAA)
protocol, spoken between the authenticator and backend authentication
server. AAA protocols supporting EAP include RADIUS [RFC3579] and
Diameter [RFC4072].
It is a fundamental property of EAP that at the EAP method layer, the
conversation between the EAP peer and server is unaffected by whether
the EAP authenticator is operating in "pass-through" mode. EAP
methods operate identically in all aspects, including key derivation
and parameter import/export, regardless of whether the authenticator
is operating as a pass-through or not.
The successful completion of an EAP method that supports key
derivation results in the export of keying material and parameters on
the EAP peer and server. Even though the EAP peer or server may
import Channel-Bindings that may include the identity of the EAP
authenticator, this information is treated as opaque octets. As a
result, within EAP the only relevant identities are the Peer-Id and
Server-Id. Channel Bindings are only interpreted by the lower layer.
Within EAP, the primary function of the AAA protocol is to maintain
the principle of Mode Independence, so that as far as the EAP peer is
concerned, its conversation with the EAP authenticator, and all
consequences of that conversation, are identical, regardless of the
authenticator mode of operation.
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1.6.2. Media Independence
One of the goals of EAP is to allow EAP methods to function on any
lower layer meeting the criteria outlined in [RFC3748], Section 3.1.
For example, as described in [RFC3748], EAP authentication can be run
over PPP [RFC1661], IEEE 802 wired networks [IEEE-802.1X], and
wireless networks such as 802.11 [IEEE-802.11i] and 802.16
[IEEE-802.16e].
In order to maintain media independence, it is necessary for EAP to
avoid consideration of media-specific elements. For example, EAP
methods cannot be assumed to have knowledge of the lower layer over
which they are transported, and cannot be restricted to identifiers
associated with a particular usage environment (e.g. MAC addresses).
Note that media independence may be retained within EAP methods that
support Channel-Bindings or method-specific identification. An EAP
method need not be aware of the content of an identifier in order to
use it. This enables an EAP method to use media-specific identifiers
such as MAC addresses without compromising media independence.
Channel-Bindings are treated as opaque octets by EAP methods, so that
handling them does not require media-specific knowledge.
1.6.3. Method Independence
By enabling pass-through, authenticators can support any method
implemented on the peer and server, not just locally implemented
methods. This allows the authenticator to avoid implementing code
for each EAP method required by peers. In fact, since a pass-through
authenticator is not required to implement any EAP methods at all, it
cannot be assumed to support any EAP method-specific code.
As a result, as noted in [RFC3748], authenticators must by default be
capable of supporting any EAP method. This is useful where there is
no single EAP method that is both mandatory-to-implement and offers
acceptable security for the media in use. For example, the [RFC3748]
mandatory-to-implement EAP method (MD5-Challenge) does not provide
dictionary attack resistance, mutual authentication or key
derivation, and as a result is not appropriate for use in wireless
LAN authentication [RFC4017]. However, despite this it is possible
for the peer and authenticator to interoperate as long as a suitable
EAP method is supported on the EAP server.
1.6.4. Ciphersuite Independence
Ciphersuite Independence is a requirement for Media Independence.
Since lower layer ciphersuites vary between media, media independence
requires that EAP keying material needs to be large enough (with
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sufficient entropy) to handle any ciphersuite.
While EAP methods may negotiate the ciphersuite used in protection of
the EAP conversation, the ciphersuite used for the protection of the
data exchanged after EAP authentication has completed is negotiated
between the peer and authenticator within the lower layer, outside of
EAP.
For example, within PPP, the ciphersuite is negotiated within the
Encryption Control Protocol (ECP) defined in [RFC1968], after EAP
authentication is completed. Within [IEEE-802.11i], the AP
ciphersuites are advertised in the Beacon and Probe Responses prior
to EAP authentication, and are securely verified during a 4-way
handshake exchange.
Since the ciphersuites used to protect data depend on the lower
layer, requiring EAP methods have knowledge of lower layer
ciphersuites would compromise the principle of Media Independence.
Since ciphersuite negotiation occurs in the lower layer, there is no
need for lower layer ciphersuite negotiation within EAP, and EAP
methods generate keying material that is ciphersuite-independent.
In order to allow a ciphersuite to be usable within the EAP keying
framework, a specification MUST be provided describing how TSKs
suitable for use with the ciphersuite are derived from exported EAP
keying parameters. To maintain Method Independence, algorithms for
deriving TSKs MUST NOT depend on the EAP method, although algorithms
for TEK derivation MAY be specific to the EAP method.
Advantages of ciphersuite-independence include:
Reduced update requirements
If EAP methods were to specify how to derive transient session keys
for each ciphersuite, they would need to be updated each time a new
ciphersuite is developed. In addition, backend authentication
servers might not be usable with all EAP-capable authenticators,
since the backend authentication server would also need to be
updated each time support for a new ciphersuite is added to the
authenticator.
Reduced EAP method complexity
Requiring each EAP method to include ciphersuite-specific code for
transient session key derivation would increase method complexity
and result in duplicated effort.
Simplified configuration
The ciphersuite is negotiated between the peer and authenticator
outside of EAP. Where the authenticator operates in "pass-through"
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mode, the EAP server is not a party to this negotiation, nor is it
involved in the data flow between the EAP peer and authenticator.
As a result, the EAP server may not have knowledge of the
ciphersuites and negotiation policies implemented by the peer and
authenticator, or be aware of the ciphersuite negotiated between
them. For example, since ECP negotiation occurs after
authentication, when run over PPP, the EAP peer and server may not
anticipate the negotiated ciphersuite and therefore this
information cannot be provided to the EAP method.
2. Lower Layer Operation
On completion of EAP authentication, keying material and material and
parameters exported by the EAP method are provided to the lower layer
and AAA layer (if present). These include the Master Session Key
(MSK), Extended Master Session Key (EMSK), Peer-Id, Server-Id,
Session-Id and Key-Lifetime. The Initialization Vector (IV) is
deprecated.
In order to preserve the security of keys derived within EAP methods,
lower layers MUST NOT export keys passed down by EAP methods. This
implies that EAP keying material or parameters passed down to a lower
layer are for the exclusive use of that lower layer and MUST NOT be
used within another lower layer. This prevents compromise of one
lower layer from compromising other applications using EAP keying
parameters.
EAP keying material and parameters provided to a lower layer MUST NOT
be transported to another entity. For example, EAP keying material
and parameters passed down to the EAP peer lower layer MUST NOT leave
the peer; EAP keying material and parameters passed down or
transported to the EAP authenticator lower layer MUST NOT leave the
authenticator.
On the EAP server, keying material requested by and passed down to
the AAA layer may be replicated to the AAA layer on the
authenticator. On the authenticator, the AAA layer provides the
replicated keying material to the lower layer over which the EAP
authentication conversation took place. This enables "mode
independence" to be maintained.
The EMSK MUST NOT be provided to an entity outside the EAP server or
peer, nor is it permitted to pass any quantity to an entity outside
the EAP server or peer from which the EMSK could be computed without
breaking some cryptographic assumption, such as inverting a one-way
function. The EMSK MUST NOT be transported by the AAA layer. As
noted in [RFC3748] Section 7.10:
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The EMSK is reserved for future use and MUST remain on the EAP
peer and EAP server where it is derived; it MUST NOT be
transported to, or shared with, additional parties, or used to
derive any other keys.
The EAP layer as well as the peer and authenticator layers MUST NOT
modify or cache keying material or parameters (including Channel
Bindings) passing in either direction between the EAP method layer
and the lower layer or AAA layer.
2.1. Transient Session Keys
Where explicitly supported by the lower layer, lower layers MAY cache
the exported EAP keying material and parameters and/or TSKs. The
structure of this key cache is defined by the lower layer. So as to
enable interoperability, new lower layer specifications MUST describe
EAP key caching behavior. Unless explicitly specified by the lower
layer, the EAP peer, server and authenticator MUST assume that peers
and authenticators do not cache exported EAP keying parameters or
TSKs. Existing EAP lower layers and AAA layers handle the caching of
EAP keying material and the generation of transient session keys in
different ways:
IEEE 802.1X-2004
IEEE 802.1X-2004, defined in [IEEE-802.1X] does not support caching
of EAP keying material or parameters. Once EAP authentication
completes, it is assumed that EAP keying material and parameters
are discarded.
PPP PPP, defined in [RFC1661] does not support caching of EAP keying
material or parameters. PPP ciphersuites derive their TSKs
directly from the MSK, as described in [RFC2716]. This method is
NOT RECOMMENDED, since if PPP were to support caching, this could
result in TSK reuse. As a result, once the PPP session is
terminated, EAP keying material and parameters MUST be discarded.
Since caching of EAP keying material is not permitted, within PPP
there is no way to handle TSK rekey without EAP re-authentication.
Perfect Forward Secrecy (PFS) is only possible if the negotiated
EAP method supports this.
IKEv2
IKEv2, defined in [RFC4306] only uses the MSK for authentication
purposes and not key derivation. The EMSK, IV, Peer-Id, Server-Id
or Session-Id are not used. As a result, the keying material
derived within IKEv2 is independent of the EAP keying material and
rekey of IPsec SAs can be handled without requiring EAP re-
authentication. Since generation of keying material is independent
of EAP, within IKEv2 it is possible to negotiate PFS, regardless of
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the EAP method that is used. IKEv2 does not cache EAP keying
material or parameters; once IKEv2 authentication completes it is
assumed that EAP keying material and parameters are discarded. The
Session-Timeout attribute is therefore interpreted as a limit on
the VPN session time, rather than an indication of the MSK key
lifetime.
IEEE 802.11i
IEEE 802.11i enables caching of the MSK, but not the EMSK, IV,
Peer-Id, Server-Id, or Session-Id. More details about the
structure of the cache are available in [IEEE-802.11i]. In IEEE
802.11i, TSKs are derived from the MSK using the 4-way handshake,
which includes a nonce exchange. This guarantees TSK freshness
even if the MSK is reused. The 4-way handshake also enables TSK
rekey without EAP re-authentication. PFS is only possible within
IEEE 802.11i if caching is not enabled and the negotiated EAP
method supports PFS.
IEEE 802.16e
IEEE 802.16e, defined in [IEEE-802.16e] supports caching of the
MSK, but not the EMSK, IV, Peer-Id, Server-Id or Session-Id. In
IEEE 802.16e, TSKs are generated by the authenticator without any
contribution by the peer. The TSKs are encrypted, authenticated
and integrity protected using the MSK. As a result, TSK rekey is
possible without EAP re-authentication. PFS is not possible even
if the negotiated EAP method supports it.
AAA Existing implementations of RADIUS/EAP [RFC3579] or Diameter EAP
[RFC4072] do not support caching of EAP keying material or
parameters. In existing AAA client, proxy and server
implementations, exported EAP keying material (MSK, EMSK and IV) as
well as parameters and derived keys are not cached and MUST be
presumed lost after the AAA exchange completes.
In order to avoid key reuse, the AAA layer MUST delete transported
keys once they are sent. The AAA layer MUST NOT retain keys that
it has previously sent. For example, a AAA layer that has
transported the MSK MUST delete it, and keys MUST NOT be derived
from the MSK from that point forward.
2.2. Authenticator Architecture
This specification does not impose constraints on the architecture of
the EAP authenticator or peer. Any of the authenticator
architectures described in [RFC4118] can be used. As a result, lower
layers need to identify EAP peers and authenticators unambiguously,
without incorporating implicit assumptions about peer and
authenticator architectures.
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For example, it is possible for multiple base stations and a
"controller" (e.g. WLAN switch) to comprise a single EAP
authenticator. In such a situation, the "base station identity" is
irrelevant to the EAP method conversation, except perhaps as an
opaque blob to be used in Channel Bindings. Many base stations can
share the same authenticator identity. It should be understood that
an EAP authenticator or peer:
[a] may contain one or more physical or logical ports;
[b] may advertise itself as one or more "virtual"
authenticators or peers;
[c] may utilize multiple CPUs;
[d] may support clustering services for load balancing or failover.
Both the EAP peer and authenticator may have more than one physical
or logical port. A peer may simultaneously access the network via
multiple authenticators, or via multiple physical or logical ports on
a given authenticator. Similarly, an authenticator may offer network
access to multiple peers, each via a separate physical or logical
port. When a single physical authenticator advertises itself as
multiple "virtual authenticators", it is possible for a single
physical port to belong to multiple "virtual authenticators". The
situation is illustrated in Figure 3.
2.2.1. Authenticator Identification
The EAP method conversation is between the EAP peer and server, as
identified by the Peer-Id and Server-Id. The authenticator identity,
if considered at all by the EAP method, is treated as an opaque blob
for the purposes of Channel Bindings (see Section 5.12). However,
the Secure Association Protocol conversation is between the peer and
the authenticator, and therefore the authenticator and peer
identities are relevant to that exchange, and define the scope of use
of the EAP keying material passed down to the lower layer.
Where the EAP peer and authenticator cannot unambiguously identify
each other they may not be able to determine the scope of transported
EAP keying material. This is particularly problematic for lower
layers where key caching is supported.
For example, if the EAP peer cannot identify the EAP authenticator,
it will be unable to determine whether transported EAP keying
material has been shared outside of its authorized scope, and
therefore needs to be considered compromised. There is also a
practical problem because the EAP peer will be unable to utilize the
EAP authenticator key cache in an efficient way.
Where the peer and authenticator identify themselves within the lower
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layer using a port identifier such as a link layer address, this
creates a number of problems:
[1] It may not be obvious to the peer which authenticator ports are
associated with which authenticators.
[2] It may not be obvious to the authenticator which peer ports are
associated with which peers.
[3] It may not be obvious to the peer which "virtual authenticator" it
is communicating with.
[4] It may not be obvious to the authenticator which "virtual peer" it
is communicating with.
+-+-+-+-+
| EAP |
| Peer |
+-+-+-+-+
| | | Peer Ports
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
| | | | | | | | | Authenticator Ports
+-+-+-+-+ +-+-+-+-+ +-+-+-+-+
| | | | | |
| Auth. | | Auth. | | Auth. |
| | | | | |
+-+-+-+-+ +-+-+-+-+ +-+-+-+-+
\ | /
\ | /
\ | /
EAP over AAA \ | /
(optional) \ | /
\ | /
\ | /
\ | /
+-+-+-+-+
| EAP |
|Server |
+-+-+-+-+
Figure 3: Relationship between EAP peer, authenticator and server
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Since an authenticator may have multiple ports, the authenticator
identifier used within the Secure Association Protocol exchange
SHOULD be distinct from any port identifier (e.g. MAC address).
Similarly, where a peer may have multiple ports, and sharing of EAP
keying material and parameters between peer ports of the same link
type is allowed, the peer identifier used within the Secure
Association Protocol exchange SHOULD also be distinct from any port
identifier.
AAA protocols such as RADIUS [RFC3579] and Diameter [RFC4072] provide
a mechanism for the identification of AAA clients; since the EAP
authenticator and AAA client are always co-resident, this mechanism
is applicable to the identification of EAP authenticators.
RADIUS [RFC2865] requires that an Access-Request packet contain one
or more of the NAS-Identifier, NAS-IP-Address and NAS-IPv6-Address
attributes. Since a NAS may have more than one IP address, the NAS-
Identifier attribute is RECOMMENDED for the unambiguous
identification of the EAP authenticator.
From the point of view of the backend authentication server, EAP
keying material and parameters are transported to the EAP
authenticator identified by the NAS-Identifier attribute. Since an
EAP authenticator MUST NOT share EAP keying material or parameters
with another party, if the EAP peer or backend authentication server
detects use of EAP keying material and parameters outside the scope
defined by the NAS-Identifier, the keying material MUST be considered
compromised.
In order to ensure that lower layer identities are securely verified
by all parties, it is RECOMMENDED that the parties use a set of
identities that are consistent between the conversation phases. This
can be achieved by:
[a] Specifying the lower layer parameters used to identify the
authenticator and peer;
[b] Communicating the lower layer identities between the peer and
authenticator within phase 0;
[c] Communicating the lower layer authenticator identity between the
authenticator and backend server within the NAS-Identifier
attribute;
[d] Including the lower layer identities within Channel Bindings (if
supported) in phase 1a, ensuring that they are communicated between
the EAP peer and server;
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[e] Supporting the integrity-protected exchange of identities within
phase 2a;
[f] Utilizing the advertised lower layer identities to enable the peer
and authenticator to verify that keys are maintained within the
advertised scope;
2.2.2. Virtual Authenticators
When a single physical authenticator advertises itself as multiple
"virtual authenticators", a number of security vulnerabilities may
arise if the peer and authenticator are not correctly identified.
For example, the peer may assume that the "virtual authenticators"
are distinct and do not share a key cache, whereas, depending on the
architecture of the physical authenticator, a shared key cache may or
may not be implemented.
Where EAP keying material is shared between "virtual authenticators"
an attacker acting as a peer could authenticate with the "Guest"
"virtual authenticator" and derive EAP keying material. If the
virtual authenticators share a key cache, then the peer can utilize
the EAP keying material derived for the "Guest" network to obtain
access to the "Corporate Intranet" virtual authenticator.
In order to address these issues:
[g] Authenticators are REQUIRED to cache associated authorizations
along with EAP keying material and parameters and to apply
authorizations consistently. This ensures that an attacker cannot
obtain elevated privileges even where the key cache is shared
between "virtual authenticators".
[h] It is RECOMMENDED that physical authenticators maintain separate
key caches for each "virtual authenticator".
[i] It is RECOMMENDED that each "virtual authenticator" identify itself
distinctly to the backend authentication server, such as by
utilizing a distinct NAS-Identifier attribute. This enables the
backend authentication server to utilize a separate credential to
authenticate each "virtual authenticator".
3. Key Management
EAP as defined in [RFC3748] supports key derivation, but does not
provide for the management of exported or derived keys. Although EAP
methods may support "fast reconnect" as defined in [RFC3748] Section
7.2.1, EAP does not support re-key of exported keys without re-
authentication. Existing EAP methods do not export the Key-Lifetime
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parameter; in the interest of method independence, key management of
exported or derived keys SHOULD NOT be provided within EAP methods.
3.1. Secure Association Protocol
Since neither EAP nor EAP methods provide key management support, it
is RECOMMENDED that key management facilities be provided within the
Secure Association Protocol. This includes:
[a] Entity Naming. A basic feature of a Secure Association Protocol is
the explicit naming of the parties engaged in the exchange.
Without explicit identification, the parties engaged in the
exchange are not identified and the scope of the EAP keying
parameters negotiated during the EAP exchange is undefined.
[b] Mutual proof of possession of EAP keying material. During the
Secure Association Protocol the EAP peer and authenticator MUST
demonstrate possession of the keying material transported between
the backend authentication server and authenticator (e.g. MSK), in
order to demonstrate that the peer and authenticator have been
authorized. Since mutual proof of possession is not the same as
mutual authentication, the peer cannot verify authenticator
assertions (including the authenticator identity) as a result of
this exchange. Identity verification is discussed in Section
2.2.1.
[c] Secure capabilities negotiation. In order to protect against
spoofing during the discovery phase, ensure selection of the "best"
ciphersuite, and protect against forging of negotiated security
parameters, the Secure Association Protocol MUST support secure
capabilities negotiation. This includes the secure negotiation of
usage modes, session parameters (such as security association
identifiers (SAIDs) and key lifetimes), ciphersuites and required
filters, including confirmation of security-relevant capabilities
discovered during phase 0. The Secure Association Protocol MUST
support integrity and replay protection of all capability
negotiation messages.
[d] Key naming and selection. Where key caching is supported, it may
be possible for the EAP peer and authenticator to share more than
one key of a given type. As a result, the Secure Association
Protocol MUST explicitly name the keys used in the proof of
possession exchange, so as to prevent confusion when more than one
set of keying material could potentially be used as the basis for
the exchange. Use of the key naming mechanism described in Section
1.4.1 is RECOMMENDED.
In order to support the correct processing of phase 2 security
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associations, the Secure Association (phase 2) protocol MUST
support the naming of phase 2 security associations and associated
transient session keys, so that the correct set of transient
session keys can be identified for processing a given packet. The
phase 2 Secure Association Protocol also MUST support transient
session key activation and SHOULD support deletion, so that
establishment and re-establishment of transient session keys can be
synchronized between the parties.
[e] Generation of fresh transient session keys (TSKs). Where the lower
layer supports caching of exported EAP keying material, the EAP
peer lower layer may initiate a new session using keying material
that was derived in a previous session. Were the TSKs to be
derived from a portion of the exported EAP keying material, this
would result in reuse of the session keys which could expose the
underlying ciphersuite to attack.
In lower layers where caching of EAP keying material is supported,
the Secure Association Protocol phase is REQUIRED, and MUST support
the derivation of fresh unicast and multicast TSKs, even when the
keying material provided by the backend authentication server is
not fresh. This is typically supported via the exchange of nonces
or counters, which are then mixed with the exported keying material
in order to generate fresh unicast (phase 2a) and possibly
multicast (phase 2b) session keys. By not using EAP keying
material directly to protect data, the Secure Association Protocol
protects it against compromise.
[f] Key lifetime management. This includes explicit key lifetime
negotiation or seamless re-key. EAP does not support re-key
without re-authentication and existing EAP methods do not support
key lifetime negotiation. As a result, the Secure Association
Protocol may handle re-key and determination of the key lifetime.
Where key caching is supported, secure negotiation of key lifetimes
is RECOMMENDED. Lower layers that support re-key, but not key
caching, may not require key lifetime negotiation. For example, a
difference between IKEv1 [RFC2409] and IKEv2 [RFC4306] is that in
IKEv1 SA lifetimes were negotiated; in IKEv2, each end of the SA is
responsible for enforcing its own lifetime policy on the SA and re-
keying the SA when necessary.
[g] Key state resynchronization. It is possible for the peer or
authenticator to reboot or reclaim resources, clearing portions or
all of the key cache. Therefore, key lifetime negotiation cannot
guarantee that the key cache will remain synchronized, and the peer
may not be able to determine before attempting to use a key whether
it exists within the authenticator cache. It is therefore
RECOMMENDED for the Secure Association Protocol to provide a
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mechanism for key state resynchronization. Since in this situation
one or more of the parties initially do not possess a key with
which to protect the resynchronization exchange, securing this
mechanism may be difficult.
[h] Key scope synchronization. To support key scope determination, the
Secure Association Protocol SHOULD provide a mechanism by which the
peer can determine the scope of the key cache on each
authenticator, and by which the authenticator can determine the
scope of the key cache on a peer. This includes negotiation of
restrictions on key usage.
[i] Direct operation. Since the phase 2 Secure Association Protocol is
concerned with the establishment of security associations between
the EAP peer and authenticator, including the derivation of
transient session keys, only those parties have "a need to know"
the transient session keys. The Secure Association Protocol MUST
operate directly between the peer and authenticator, and MUST NOT
be passed-through to the backend authentication server, or include
additional parties.
[j] Bi-directional operation. While some ciphersuites only require a
single set of transient session keys to protect traffic in both
directions, other ciphersuites require a unique set of transient
session keys in each direction. The phase 2 Secure Association
Protocol SHOULD provide for the derivation of unicast and multicast
keys in each direction, so as not to require two separate phase 2
exchanges in order to create a bi-directional phase 2 security
association. See [RFC3748] Section 2.4 for more discussion.
3.2. Key Scope
Absent explicit specification within the lower layer, after the
completion of phase 1b, EAP keying material and parameters are bound
to the EAP peer and authenticator, but are not bound to a specific
peer or authenticator port.
While EAP Keying Material passed down to the lower layer is not
intrinsically bound to particular authenticator and peer ports,
Transient Session Keys MAY be bound to particular authenticator and
peer ports by the Secure Association Protocol. However, a lower
layer MAY also permit TSKs to be used on multiple peer and/or
authenticator ports, providing that TSK freshness is guaranteed (such
as by keeping replay counter state within the authenticator).
In order to further limit the key scope the following measures are
suggested:
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[a] The lower layer MAY specify additional restrictions on key usage,
such as limiting the use of EAP keying material and parameters on
the EAP peer to the port over which on the EAP conversation was
conducted.
[b] The backend authentication server and authenticator MAY implement
additional attributes in order to further restrict the scope of EAP
keying material. For example, in 802.11, the backend
authentication server may provide the authenticator with a list of
authorized Called or Calling-Station-Ids and/or SSIDs for which EAP
keying material is valid.
[c] Where the backend authentication server provides attributes
restricting the key scope, it is RECOMMENDED that restrictions be
securely communicated by the authenticator to the peer. This can
be accomplished using the Secure Association Protocol, but also
can be accomplished via the EAP method or the lower layer.
3.3. Parent-Child Relationships
When keying material exported by EAP methods expires, all keying
material derived from the exported keying material expires, including
the TSKs.
When an EAP re-authentication takes place, new keying material is
derived and exported by the EAP method, which eventually results in
replacement of calculated keys, including the TSKs.
As a result, while the lifetime of calculated keys can be less than
or equal that of the exported keys they are derived from, it cannot
be greater. For example, when EAP re-authentication occurs, TSK re-
key will also occur. However, this does not prohibit TSK re-key from
occurring prior to expiration of the lifetime of exported keys. For
example, TSK re-key may occur prior to EAP re-authentication.
Failure to mutually prove possession of keying material during the
Secure Association Protocol exchange need not be grounds for deletion
of the keying material by both parties; rate-limiting Secure
Association Protocol exchanges could be used to prevent a brute force
attack.
3.4. Local Key Lifetimes
The Transient EAP Keys (TEKs) are session keys used to protect the
EAP conversation. The TEKs are internal to the EAP method and are
not exported. TEKs are typically created during an EAP conversation,
used until the end of the conversation and then discarded. However,
methods may re-key TEKs during an EAP conversation.
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When using TEKs within an EAP conversation or across conversations,
it is necessary to ensure that replay protection and key separation
requirements are fulfilled. For instance, if a replay counter is
used, TEK re-key MUST occur prior to wrapping of the counter.
Similarly, TSKs MUST remain cryptographically separate from TEKs
despite TEK re-keying or caching. This prevents TEK compromise from
leading directly to compromise of the TSKs and vice versa.
EAP methods may cache local keying material which may persist for
multiple EAP conversations when fast reconnect is used [RFC 3748].
For example, EAP methods based on TLS (such as EAP-TLS [RFC2716])
derive and cache the TLS Master Secret, typically for substantial
time periods. The lifetime of other local keying material calculated
within the EAP method is defined by the method. Note that in
general, when using fast reconnect, there is no guarantee to that the
original long-term credentials are still in the possession of the
peer. For instance, a card hold holding the private key for EAP-TLS
may have been removed. EAP servers SHOULD also verify that the long-
term credentials are still valid, such as by checking that
certificate used in the original authentication has not yet expired.
3.5. Exported and Calculated Key Lifetimes
All EAP methods generating keys are required to generate the MSK and
EMSK, and may optionally generate the IV. However, EAP, defined in
[RFC3748], does not itself support the negotiation of lifetimes for
exported keying material such as the MSK, EMSK and IV.
Several mechanisms exist for managing key lifetimes:
[a] AAA attributes. AAA protocols such as RADIUS [RFC2865] and
Diameter [RFC4072] support the Session-Timeout attribute. The
Session-Timeout attribute represents the maximum lifetime of the
exported keys, and all keys calculated from it, on the
authenticator. Since existing backend authentication servers do
not cache keys exported by EAP methods, or keys calculated from
exported keys, the value of the Session-Timeout attribute has no
bearing on the key lifetime within the backend authentication
server.
On the authenticator, where EAP is used for authentication, the
Session-Timeout attribute represents the maximum session time prior
to re-authentication. As described in [RFC3580] Section 3.17, when
sent in an Access-Accept along with a Termination-Action value of
RADIUS-Request, the Session-Timeout attribute specifies the maximum
number of seconds of service provided prior to re-authentication.
Where EAP is used for pre-authentication, the session may not start
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until some future time, or may never occur. Nevertheless, the
Session-Timeout value represents the maximum time after which
transported EAP keying material, and all keys calculated from it,
will have expired on the authenticator. If the session
subsequently starts, re-authentication will be initiated once the
Session-Time has expired. If the session never started, or started
and ended, by default keys transported by AAA and all keys
calculated from them will be expired by the authenticator prior to
the future time indicated by Session-Timeout; this feature is
utilized by [IEEE-802.11i]. Note that in future additional
attributes may be specified to control the lifetime of cached keys;
these attributes may modify the meaning of the Session-Timeout
attribute in specific circumstances.
Since the TSK lifetime is often determined by authenticator
resources, the backend authentication server has no insight into
the TSK derivation process, and by the principle of ciphersuite
independence, it is not appropriate for the backend authentication
server to manage any aspect of the TSK derivation process,
including the TSK lifetime.
[b] Lower layer mechanisms. While AAA attributes can communicate the
maximum exported key lifetime, this only serves to synchronize the
key lifetime between the backend authentication server and the
authenticator. Lower layer mechanisms such as the Secure
Association Protocol can then be used to enable the lifetime of
exported and calculated keys to be negotiated between the peer and
authenticator.
Where TSKs are established as the result of a Secure Association
Protocol exchange, it is RECOMMENDED that the Secure Association
Protocol include support for TSK rekey. Where the TSK is taken
directly from the MSK, there is no need to manage the TSK lifetime
as a separate parameter, since the TSK lifetime and MSK lifetime
are identical.
[c] System defaults. Where the EAP method does not support the
negotiation of the exported key lifetime, and a key lifetime
negotiation mechanism is not provided by the lower lower, there may
be no way for the peer to learn the exported key lifetime. In this
case it is RECOMMENDED that the peer assume a default value of the
exported key lifetime; 8 hours is recommended. Similarly, the
lifetime of calculated keys can also be managed as a system
parameter on the authenticator.
[d] Method specific negotiation within EAP. While EAP itself does not
support lifetime negotiation, it would be possible to specify
methods that do. However, systems that rely on such negotiation
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for exported keys would only function with these methods. As a
result, it is NOT RECOMMENDED to use this approach as the sole way
to determine key lifetimes.
3.6. Key cache synchronization
Issues arise when attempting to synchronize the key cache on the peer
and authenticator.
While the AAA protocol can enable the backend authentication server
to provide guidance on the lifetime of transported EAP keying
material to the authenticator, this does not address the problem of
key lifetime synchronization between the peer and authenticator.
Where the EAP method does not export the Key-Lifetime parameter, the
lifetime of the EAP keying material may not be defined until
completion of the Secure Association Protocol, if ever. This can
leave the peer uncertain how long the authenticator will maintain EAP
keying material within the key cache.
However, key lifetime negotiation alone cannot guarantee key cache
synchronization. Even where the Secure Association Protocol is run
immediately after EAP and determines the lifetime of EAP keying
material, it is still possible for the authenticator to reclaim
resources.
The lower layer may utilize the Discovery phase 0 to improve key
cache synchronization. For example, if the authenticator manages the
key cache by deleting the oldest key first (LIFO), the relative
creation time of the last key to be deleted could be advertised
within the Discovery phase, enabling the peer to determine whether
keying material had been prematurely expired from the authenticator
key cache.
3.7. Key Strength
In order to guard against brute force attacks, EAP methods deriving
keys need to be capable of generating keys with an appropriate
effective symmetric key strength. In order to ensure that key
generation is not the weakest link, it is RECOMMENDED that EAP
methods utilizing public key cryptography choose a public key that
has a cryptographic strength meeting the symmetric key strength
requirement.
As noted in [RFC3766] Section 5, this results in the following
required RSA or DH module and DSA subgroup size in bits, for a given
level of attack resistance in bits:
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Attack Resistance RSA or DH Modulus DSA subgroup
(bits) size (bits) size (bits)
----------------- ----------------- ------------
70 947 128
80 1228 145
90 1553 153
100 1926 184
150 4575 279
200 8719 373
250 14596 475
3.8. Key Wrap
The key wrap specified in [RFC2548], which is based on an MD5-based
stream cipher, has known problems, as described in [RFC3579] Section
4.3. RADIUS uses the shared secret for multiple purposes, including
per-packet authentication and attribute hiding, considerable
information is exposed about the shared secret with each packet.
This exposes the shared secret to dictionary attacks. MD5 is used
both to compute the RADIUS Response Authenticator and the Message-
Authenticator attribute, and concerns exist relating to the security
of this hash [MD5Collision].
As discussed in [RFC3579] Section 4.3, the security vulnerabilities
of RADIUS are extensive, and therefore development of an alternative
key wrap technique based on the RADIUS shared secret would not
substantially improve security. As a result, [RFC3759] Section 4.2
recommends running RADIUS over IPsec. The same approach is taken in
Diameter EAP [RFC4072], which defines cleartext key attributes, to be
protected by IPsec or TLS.
4. Handoff Vulnerabilities
With EAP, several mechanisms are available to reduce the latency in
handoff between authenticators:
[1] EAP pre-authentication. This utilizes EAP to pre-establish EAP
keying material on an authenticator prior to arrival of the peer.
Use of pre-authentication within IEEE 802.11 is described in
[8021XHandoff] and [IEEE-802.11i].
[2] Key caching. This mechanism enables an EAP peer to re-attach to an
authenticator without requiring EAP re-authentication.
[3] Context transfer, such as is defined in [IEEE-802.11F] (now
deprecated) and [RFC4067]. Use of context transfer for handoff
latency improvement is described in [IEEE-02-758].
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[4] Proactive key distribution, such as is described in [IEEE-02-758]
and [I-D.irtf-aaaarch-handoff].
The sections that follow discuss the security vulnerabilities
introduced by the above mechanisms.
4.1. Authorization
In a typical network access scenario (dial-in, wireless LAN, etc.)
access control mechanisms are typically applied. These mechanisms
include user authentication as well as authorization for the offered
service.
As a part of the authentication process, the backend authentication
server determines the user's authorization profile. The user
authorizations are transmitted by the backend authentication server
to the EAP authenticator (also known as the Network Access Server or
authenticator) along with the transported EAP keying material, in
Phase 1b of the EAP conversation. Typically, the profile is
determined based on the user identity, but a certificate presented by
the user may also provide authorization information.
The backend authentication server is responsible for making a user
authorization decision, which requires answering the following
questions:
[a] Is this a legitimate user for this particular network?
[b] Is the user allowed the type of access he or she is requesting?
[c] Are there any specific parameters (mandatory tunneling, bandwidth,
filters, and so on) that the access network should be aware of for
this user?
[d] Is the user operating within the time of day subscription rules?
[e] Is the user within his limits for concurrent sessions?
[f] Are there any fraud, credit limit, or other concerns that indicate
that access should be denied?
While the authorization decision is in principle simple, the process
is complicated by the distributed nature of the decision making.
Where brokering entities or proxies are involved, all of the AAA
entities in the chain from the authenticator to the home backend
authentication server are involved in the decision. For instance, a
broker can disallow access even if the home backend authentication
server would allow it, or a proxy can add authorizations (e.g.,
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bandwidth limits).
Decisions can be based on static policy definitions and profiles as
well as dynamic state (e.g. time of day or limits on the number of
concurrent sessions). In addition to the Accept/Reject decision made
by the AAA chain, parameters or constraints can be communicated to
the authenticator.
The criteria for Accept/Reject decisions or the reasons for choosing
particular authorizations are typically not communicated to the
authenticator, only the final result. As a result, the authenticator
has no way to know what the decision was based on. Was a set of
authorization parameters sent because this service is always provided
to the user, or was the decision based on the time/day and the
capabilities of the requesting authenticator device?
4.2. Correctness
When the AAA exchange is bypassed via use of techniques such as key
caching, it can be challenging to ensure that authorization is
properly handled. Challenges include:
[a] Consistent application of session time limits. Bypassing AAA
should not automatically increase the available session time,
allowing a user to endlessly extend their network access by
changing the point of attachment.
[b] Avoidance of privilege elevation. Bypassing AAA should not result
in a user being granted access to services which they are not
entitled to.
[c] Consideration of dynamic state. In situations in which dynamic
state is involved in the access decision (day/time, simultaneous
session limit) it should be possible to take this state into
account either before or after access is granted. Note that
consideration of network-wide state such as simultaneous session
limits can typically only be taken into account by the backend
authentication server.
[d] Encoding of restrictions. Since a authenticator may not be aware
of the criteria considered by a backend authentication server when
allowing access, in order to ensure consistent authorization during
a fast handoff it may be necessary to explicitly encode the
restrictions within the authorizations provided by the backend
authentication server.
[e] State validity. The introduction of fast handoff should not render
the authentication server incapable of keeping track of network-
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wide state.
A handoff mechanism capable of addressing these concerns is said to
be "correct". One condition for correctness is as follows:
For a handoff to be "correct" it MUST establish on the new device
the same context as would have been created had the new device
completed a AAA conversation with the backend authentication
server.
A properly designed handoff scheme will only succeed if it is
"correct" in this way. If a successful handoff would establish
"incorrect" state, it is preferable for it to fail, in order to avoid
creation of incorrect context.
Some authenticator and backend authentication server configurations
are incapable of meeting this definition of "correctness". For
example, if the old and new device differ in their capabilities, a
handoff mechanism that bypasses AAA may find it difficult to meet
this definition of correctness. Backend authentication servers often
perform conditional evaluation, in which the authorizations returned
in an Access-Accept message are contingent on the authenticator or on
dynamic state such as the time of day or number of simultaneous
sessions. For example, in a heterogeneous deployment, the backend
authentication server might return different authorizations depending
on the authenticator making the request, in order to make sure that
the requested service is consistent with the authenticator
capabilities.
If differences between the new and old device would result in the
backend authentication server sending a different set of messages to
the new device than were sent to the old device, then if the handoff
mechanism bypasses AAA, the handoff cannot be carried out correctly.
For example, if some authenticators support dynamic Virtual LANs
(VLANs) while others do not, then attributes present in the Access-
Request (such as the NAS-IP-Address, NAS-IPv6-Address, NAS-
Identifier, etc.) could be examined to determine when VLAN attributes
will be returned, as described in [RFC3580]. VLAN support is
defined in [IEEE-802.1Q]. If a handoff bypassing the backend
authentication server were to occur between a authenticator
supporting dynamic VLANs and another authenticator which does not,
then a guest user with access restricted to a guest VLAN could be
given unrestricted access to the network.
Similarly, in a network where access is restricted based on the day
and time, Service Set Identifier (SSID), Calling-Station-Id or other
factors, unless the restrictions are encoded within the
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authorizations, or a partial AAA conversation is included, then a
handoff could result in the user bypassing the restrictions.
In practice, these considerations limit the situations in which fast
handoff mechanisms bypassing AAA can be expected to be successful.
Where the deployed devices implement the same set of services, it may
be possible to do successful handoffs within such mechanisms.
However, where the supported services differ between devices, the
handoff may not succeed. For example, [RFC2865] section 1.1 states:
"A authenticator that does not implement a given service MUST NOT
implement the RADIUS attributes for that service. For example, a
authenticator that is unable to offer ARAP service MUST NOT
implement the RADIUS attributes for ARAP. A authenticator MUST
treat a RADIUS access-accept authorizing an unavailable service as
an access-reject instead."
Note that this behavior only applies to attributes that are known,
but not implemented. For attributes that are unknown, [RFC2865]
Section 5 states:
"A RADIUS server MAY ignore Attributes with an unknown Type. A
RADIUS client MAY ignore Attributes with an unknown Type."
In order to perform a correct handoff, if a new device is provided
with RADIUS context for a known but unavailable service, then it MUST
process this context the same way it would handle a RADIUS Access-
Accept requesting an unavailable service. This MUST cause the
handoff to fail. However, if a new device is provided with RADIUS
context that indicates an unknown attribute, then this attribute MAY
be ignored.
Although it may seem somewhat counter-intuitive, failure is indeed
the "correct" result where a known but unsupported service is
requested. Presumably a correctly configured backend authentication
server would not request that a device carry out a service that it
does not implement. This implies that if the new device were to
complete a AAA conversation that it would be likely to receive
different service instructions. In such a case, failure of the
handoff is the desired result. This will cause the new device to go
back to the backend server in order to receive the appropriate
service definition.
In practice, this implies that handoff mechanisms which bypass AAA
are most likely to be successful within a homogeneous device
deployment within a single administrative domain. For example, it
would not be advisable to carry out a fast handoff bypassing AAA
between a authenticator providing confidentiality and another
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authenticator that does not support this service. The correct result
of such a handoff would be a failure, since if the handoff were
blindly carried out, then the user would be moved from a secure to an
insecure channel without permission from the backend authentication
server. Thus the definition of a "known but unsupported service"
MUST encompass requests for unavailable security services. This
includes vendor-specific attributes related to security, such as
those described in [RFC2548].
5. Security Considerations
The EAP threat model is described in [RFC3748] Section 7.1. The
security properties of EAP methods (known as "security claims") are
described in [RFC3784] Section 7.2.1. EAP method requirements for
applications such as Wireless LAN authentication are described in
[RFC4017]. The RADIUS threat model is described in [RFC3579] Section
4.1, and responses to these threats are described in [RFC3579]
Sections 4.2 and 4.3.
However, in addition to threats against EAP and AAA, there are other
system-level threats:
[1] An attacker may compromise or steal an EAP authenticator, in an
attempt to gain access to other EAP authenticators or obtain long-
term secrets.
[2] An attacker may try to modify or spoof packets, including Discovery
or Secure Association Protocol frames, EAP or AAA packets.
[3] An attacker may attempt a downgrade attack in order to exploit
known weaknesses in an authentication method or cryptographic
transform.
[4] An attacker may attempt to induce an EAP peer, authenticator or
server to disclose keying material to an unauthorized party, or
utilize keying material outside the context that it was intended
for.
[5] An attacker may replay packets.
[6] An attacker may cause an EAP peer, authenticator or server to reuse
an stale key. Use of stale keys may also occur unintentionally.
For example, a poorly implemented backend authentication server may
provide stale keying material to an authenticator, or a poorly
implemented authenticator may reuse nonces.
[7] An authenticated attacker may attempt to obtain elevated privilege
in order to access information that it does not have rights to.
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[8] An attacker may attempt a man-in-the-middle attack in order to gain
access to the network.
[9] An attacker may launch a denial of service attack against the EAP
peer, authenticator or backend authentication server.
[10] An attacker may compromise an EAP authenticator in an effort to
commit fraud. For example, a compromised authenticator may provide
incorrect information to the EAP peer and/or server via out-of-band
mechanisms (such as via a AAA or lower layer protocol). This
includes impersonating another authenticator, or providing
inconsistent information to the peer and EAP server.
In order to address these threats, [Housley] provides a description
of mandatory system security properties. Issues relating to system
security requirements are discussed in the sections that follow.
5.1. Authenticator Compromise
In the event that an authenticator is compromised or stolen, an
attacker may gain access to the network via that authenticator, or
may obtain the credentials required for that authenticator/AAA client
to communicate with one or more backend authentication servers.
However, this should not allow the attacker to compromise other
authenticators or the backend authentication server, or obtain long-
term user credentials.
The implications of this requirement are many, but some of the more
important are as follows:
No Key Sharing
An EAP authenticator MUST NOT share any keying material with
another EAP authenticator, since if one EAP authenticator were
compromised, this would enable the compromise of keying material on
another authenticator. In order to be able to determine whether
keying material has been shared, it is necessary for the identity
of the EAP authenticator to be defined and understood by all
parties that communicate with it.
No AAA Credential Sharing
AAA credentials (such as RADIUS shared secrets, IPsec pre-shared
keys or certificates) MUST NOT be shared between AAA clients, since
if one AAA client were compromised, this would enable an attacker
to impersonate other AAA clients to the backend authentication
server, or even to impersonate a backend authentication server to
other AAA clients.
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No Compromise of Long-Term Credentials
An attacker obtaining TSKs, TEKs or EAP keying material such as the
MSK MUST NOT be able to obtain long-term user credentials such as
pre-shared keys, passwords or private-keys without breaking a
fundamental cryptographic assumption.
5.2. Spoofing
The use of per-packet authentication and integrity protection
provides protection against spoofing attacks. Diameter [RFC3588]
provides support for per-packet authentication and integrity
protection via use of IPsec or TLS. RADIUS/EAP [RFC3579] provides
for per-packet authentication and integrity protection via use of the
Message-Authenticator attribute.
[RFC3748] Section 7.2.1 describes the "integrity protection" security
claim and [RFC4017] requires use of EAP methods supporting this
claim.
In order to prevent forgery of Secure Association Protocol frames,
per-frame authentication and integrity protection is RECOMMENDED on
all messages. [IEEE-802.11i] supports per-frame integrity protection
and authentication on all messages within the 4-way handshake except
the first message. An attack leveraging this ommission is described
in [Analysis].
5.3. Downgrade Attacks
The ability to negotiate the use of a particular cryptographic
algorithm provides resilience against compromise of a particular
cryptographic algorithm. This is usually accomplished by including
an algorithm identifier in the protocol, and by specifying the
algorithm requirements in the protocol specification. In order to
prevent downgrade attacks, secure confirmation of the "best"
ciphersuite is required.
[RFC3748] Section 7.2.1 describes the "protected ciphersuite
negotiation" security claim that refers to the ability of an EAP
method to negotiate the ciphersuite used to protect the EAP
conversation, as well as to integrity protect the negotiation.
[RFC4017] requires EAP methods satisfying this security claim.
Diameter [RFC3588] provides support for cryptographic algorithm
negotiation via use of IPsec or TLS. RADIUS [RFC3579] does not
support the negotiation of cryptographic algorithms, and relies on
MD5 for integrity protection, authentication and confidentiality,
despite known weaknesses in the algorithm [MD5Collision]. This issue
can be addressed via use of RADIUS over IPsec, as described in
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[RFC3579] Section 4.2.
As a result, EAP methods and AAA protocols are capable of addressing
downgrade attacks. To ensure against downgrade attacks within lower
layer protocols, algorithm independence is REQUIRED with lower layers
using EAP for key derivation. For interoperability, at least one
suite of mandatory-to-implement algorithm MUST be selected. Lower
layer protocols supporting EAP for key derivation SHOULD also support
secure ciphersuite negotiation. As described in [RFC1968], PPP ECP
does not provide support for secure ciphersuite negotiation.
However, [IEEE-802.11i] does support secure ciphersuite negotiation.
5.4. Unauthorized Disclosure
While preserving algorithm independence, confidentiality of all
keying material MUST be maintained. To prevent unauthorized disclose
of keys, each party in the EAP conversation MUST be authenticated to
the other parties with whom it communicates. Keying material MUST be
bound to the appropriate context.
[RFC3748] Section 7.2.1 describes the "mutual authentication" and
"dictionary attack resistance" claims, and [RFC4017] requires EAP
methods satisfying these claims. EAP methods complying with
[RFC4017] therefore provide for mutual authentication between the EAP
peer and server. Within EAP, binding of EAP keying material (MSK,
EMSK) to the appropriate context is provided by the Peer-Id and
Server-Id which are exported along with the keying material.
Diameter [RFC3588] provides for per-packet authentication and
integrity protection via IPsec or TLS, and RADIUS/EAP [RFC3579] also
provides for per-packet authentication and integrity protection.
Where the NAS/authenticator and backend authentication server
communicate directly and credible keywrap is used (see Section 3.8),
this ensures that the AAA Key Transport phase achieves its security
objectives: mutually authenticating the AAA client/authenticator and
backend authentication server and providing EAP keying material to
the EAP authenticator and to no other party. Within the AAA
protocol, the authorization attributes provide the information
binding the transported keying material to the appropriate context.
For example, transported keying material is destined for the EAP
authenticator identified by the NAS-Identifier attribute within the
request, and relates to the EAP peer identified by the Peer-Id, User-
Name [RFC2865] or CUI [RFC4372] attributes.
[RFC2607] Section 7 describes the security issues occurring when the
authenticator and backend authentication server do not communicate
directly. Where an untrusted AAA intermediary is present (such as a
RADIUS proxy or a Diameter agent), and data object security is not
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used, transported keying material may be recovered by an attacker in
control of the untrusted intermediary. As discussed in Section 2.1,
unless the TSKs are derived independently from EAP keying material
(as in IKEv2), possession of transported keying material enables
decryption of data traffic sent between the peer and a specific
authenticator. However, as long as EAP keying material or keys
derived from it are only utilized by a single authenticator,
compromise of the transported keying material does not enable an
attacker to impersonate the peer to another authenticator.
Vulnerability to an untrusted AAA intermediary can be mitigated by
implementation of redirect functionality, as described in [RFC3588]
and [RFC4072].
As noted in Section 3.1, the Secure Association Protocol does not by
itself provide for mutual authentication between the EAP peer and
authenticator, even if mutual possession of EAP keying material is
proven. Where the NAS/authenticator and backend authentication
server communicate directly, the backend authentication server can
verify the correspondence between NAS identification attributes, the
source address of packets sent by the NAS, and the AAA credentials.
As long as the NAS has not shared its AAA credentials with another
NAS, this allows the backend authentication server to authenticate
the NAS. Using Channel Bindings, the EAP peer can then determine
whether the NAS/authenticator has provided the same identifying
information to the EAP peer and backend authentication server.
Peer and authenticator authorization MUST be performed.
Authorization is REQUIRED whenever a peer associates with a new
authenticator. Authorization checking prevents an elevation of
privilege attack, and ensures that an unauthorized authenticator is
detected. Authorizations SHOULD be synchronized between the EAP
peer, server, authenticator. Once the EAP conversation exchanges are
complete, all of these parties should hold the same view of the
authorizations associated the other parties. If peer authorization
is restricted, then the peer SHOULD be made aware of the restriction.
The AAA exchange provides the EAP authenticator with authorizations
relating to the EAP peer. However, neither the EAP nor AAA exchanges
provides authorizations to the EAP peer. In order to ensure that all
parties hold the same view of the authorizations it is RECOMMENDED
that the Secure Association Protocol enable communication of
authorizations between the EAP authenticator and peer.
Consistently identifying the EAP authenticator enables the EAP peer
to determine whether EAP keying material has been shared between EAP
authenticators as well as to confirm with the backend authentication
server that an EAP authenticator proving possession of EAP keying
material during the Secure Association Protocol was authorized to
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obtain it. Identification issues are discussed in Section 2.2 and
key scope issues are discussed in Section 3.2.
5.5. Replay Protection
Replay protection allows a protocol message recipient to discard any
message that was recorded during a previous legitimate dialogue and
presented as though it belonged to the current dialogue.
[RFC3748] Section 7.2.1 describes the "replay protection" security
claim and [RFC4017] requires use of EAP methods supporting this
claim.
Diameter [RFC3588] provides support for replay protection via use of
IPsec or TLS. RADIUS/EAP [RFC3579] protects against replay of keying
material via the Request Authenticator. However, some RADIUS packets
are not replay protected. In Accounting, Disconnect and CoA-Request
packets the Request Authenticator contains a keyed MAC rather than a
Nonce. The Response Authenticator in Accounting, Disconnect and CoA
Response packets also contains a keyed MAC whose calculation does not
depend on a Nonce in either the Request or Response packets.
Therefore unless an Event-Timestamp attribute is included or IPsec is
used, the recipient may not be able to determine whether these
packets have been replayed.
In order to prevent replay of Secure Association Protocol frames,
replay protection is REQUIRED on all messages. [IEEE-802.11i]
supports replay protection on all messages within the 4-way
handshake.
5.6. Key Freshness
A session key should be considered compromised if it remains in use
too long. As noted in [Housley], session keys MUST be strong and
fresh, while preserving algorithm independence. A fresh
cryptographic key is one that is generated specifically for the
intended use. Each session deserves an independent session key;
disclosure of one session key MUST NOT aid the attacker in
discovering any other session keys.
Fresh keys are required even when a long replay counter (that is, one
that "will never wrap") is used to ensure that loss of state does not
cause the same counter value to be used more than once with the same
session key.
EAP, AAA and the lower layer each bear responsibility for ensuring
the use of fresh, strong session keys:
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EAP EAP methods need to ensure the freshness and strength of EAP keying
material provided as an input to session key derivation. [RFC3748]
Section 7.10 states that "EAP methods SHOULD ensure the freshness
of the MSK and EMSK, even in cases where one party may not have a
high quality random number generator. A RECOMMENDED method is for
each party to provide a nonce of at least 128 bits, used in the
derivation of the MSK and EMSK." The contribution of nonces
enables the EAP peer and server to ensure that exported EAP keying
material is fresh.
[RFC3748] Section 7.2.1 describes the "key strength" and "session
independence" security claims, and and [RFC4017] requires use of
EAP methods supporting these claims as well as being capable of
providing an equivalent key strength of 128 bits or greater.
AAA The AAA protocol needs to ensure that transported keying material
is fresh and is not utilized outside its recommended lifetime.
Replay protection is necessary for key freshness, but an attacker
can deliver a stale (and therefore potentially compromised) key in
a replay-protected message, so replay protection is not sufficient.
As discussed in Section 3.5, the Session-Timeout attribute enables
the backend authentication server to limit the exposure of
transported EAP keying material.
The EAP Session-Id, derived from the EAP Type and Method-Id (based
on the nonces contributed by the peer and server) enables the EAP
peer, authenticator and server to distinguish EAP conversations.
However, unless the authenticator keeps track of EAP Session-Ids,
the authenticator cannot use the Session-Id to guarantee the
freshness of EAP keying material.
Lower Layer
As described in Section 3.1, the lower layer Secure Association
Protocol MUST generate a fresh session key for each session, even
if the keying material and parameters provided by EAP methods are
cached, or either the peer or authenticator lack a high entropy
random number generator. A RECOMMENDED method is for the peer and
authenticator to each provide a nonce or counter used in session
key derivation. If a nonce is used, it is RECOMMENDED that it be
at least 128 bits.
5.7. Elevation of Privilege
Parties MUST NOT have access to keying material that is not needed to
perform their own role. A party has access to a particular key if it
has access to all of the secret information needed to derive it. If
a Secure Association Protocol is used to establish session keys, it
MUST specify the scope for session keys.
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Transported EAP keying material is permitted to be accessed by the
EAP peer, authenticator and server. The EAP peer and server derive
the transported keying material during the process of mutually
authenticating each other using the selected EAP method. During the
Secure Association Protocol, the EAP peer utilizes the transported
EAP keying material to demonstrate to the authenticator that it is
the same party that authenticated to the EAP server and was
authorized by it. The EAP authenticator utilizes the transported EAP
keying material to prove to the peer not only that the EAP
conversation was transported through it (this could be demonstrated
by a man-in-the-middle), but that it was uniquely authorized by the
EAP server to provide the peer with access to the network. Unique
authorization can only be demonstrated if the EAP authenticator does
not share the transported keying material with a party other than the
EAP peer and server.
TSKs are permitted to be accessed only by the EAP peer and
authenticator (see Section 1.5). As discussed in Section 2.1, PPP
and 802.11i derive the TSKs from transported EAP keying material;
802.16e utilizes transported EAP keying material for TSK keywrap;
IKEv2 utilizes transported EAP keying material only to authenticate
the derivation of TSKs.
Where demonstration of authorization depends entirely on possession
of transported EAP keying material (such as in PPP, 802.11i and
802.16e), this enables the backend server to masquerade as the
authenticator, and possibly to obtain the TSKs unless the backend
server deletes the transported EAP keying material after sending it.
5.8. Man-in-the-middle Attacks
As described in [I-D.puthenkulam-eap-binding], EAP method sequences
and compound authentication mechanisms may be subject to man-in-the-
middle attacks. When such attacks are successfully carried out, the
attacker acts as an intermediary between a victim and a legitimate
authenticator. This allows the attacker to authenticate successfully
to the authenticator, as well as to obtain access to the network.
In order to prevent these attacks, [I-D.puthenkulam-eap-binding]
recommends derivation of a compound key by which the EAP peer and
server can prove that they have participated in the entire EAP
exchange. Since the compound key must not be known to an attacker
posing as an authenticator, and yet must be derived from quantities
that are exported by EAP methods, it may be desirable to derive the
compound key from a portion of the EMSK. In order to provide proper
key hygiene, it is recommended that the compound key used for man-in-
the-middle protection be cryptographically separate from other keys
derived from the EMSK.
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5.9. Denial of Service Attacks
Key caching may result in vulnerability to denial of service attacks.
For example, EAP methods that create persistent state may be
vulnerable to denial of service attacks on the EAP server by a rogue
EAP peer.
To address this vulnerability, EAP methods creating persistent state
may wish to limit the persistent state created by an EAP peer. For
example, for each peer an EAP server may choose to limit persistent
state to a few EAP conversations, distinguished by the EAP Session-
Id. This prevents a rogue peer from denying access to other peers.
Similarly, to conserve resources an authenticator may choose to limit
the persistent state corresponding to each peer. This can be
accomplished by limiting each peer to persistent state corresponding
to a few EAP conversations, distinguished by the EAP Session-Id.
Depending on the media, creation of new TSKs may or may not imply
deletion of previously derived TSKs. Where there is no implied
deletion, the authenticator may choose to limit the number of TSKs
and associated state that can be stored for each peer.
5.10. Impersonation
Both the RADIUS [RFC2865] and Diameter [RFC3588] protocols are
potentially vulnerable to impersonation by a rogue authenticator.
While both protocols support mutual authentication between the
authenticator/AAA client and the backend authentication server, the
security mechanisms vary.
In RADIUS, the shared secret used for authentication is determined by
the source address of the RADIUS packet. As noted in [RFC3579]
Section 4.3.7, it is highly desirable that the source address be
checked against one or more NAS identification attributes so as to
detect and prevent impersonation attacks.
When RADIUS Access-Requests are forwarded by a proxy, the NAS-IP-
Address or NAS-IPv6-Address attributes may not correspond to the
source address. Since the NAS-Identifier attribute need not contain
an FQDN, it also may not correspond to the source address, even
indirectly. [RFC2865] Section 3 states:
A RADIUS server MUST use the source IP address of the RADIUS
UDP packet to decide which shared secret to use, so that
RADIUS requests can be proxied.
This implies that it is possible for a rogue authenticator to forge
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NAS-IP-Address, NAS-IPv6-Address or NAS-Identifier attributes within
a RADIUS Access-Request in order to impersonate another
authenticator. Among other things, this can result in messages (and
transported keying material) being sent to the wrong authenticator.
Since the rogue authenticator is authenticated by the RADIUS proxy or
server purely based on the source address, other mechanisms are
required to detect the forgery. In addition, it is possible for
attributes such as the Called-Station-Id and Calling-Station-Id to be
forged as well.
[RFC3579] Section 4.3.7 describes how an EAP pass-through
authenticator acting as a AAA client can be detected if it attempts
to impersonate another authenticator (such by sending incorrect
Called-Station-Id [RFC2865], NAS-Identifier [RFC2865], NAS-IP-Address
[RFC2865] or NAS-IPv6-Address [RFC3162] attributes via the AAA
protocol). This vulnerability can be mitigated by having RADIUS
proxies check NAS identification attributes against the source
address.
While [RFC3588] requires use of the Route-Record AVP, this utilizes
FQDNs, so that impersonation detection requires DNS A, AAAA and PTR
Resource Records (RRs) to be properly configured. As a result,
Diameter is as vulnerable to this attack as RADIUS, if not more so.
To address this vulnerability, it is necessary to allow the backend
authentication server to communicate with the authenticator directly,
such as via the redirect functionality supported in [RFC3588].
5.11. Channel Binding
It is possible for a compromised or poorly implemented EAP
authenticator to communicate incorrect information to the EAP peer
and/or server. This may enable an authenticator to impersonate
another authenticator or communicate incorrect information via out-
of-band mechanisms (such as via AAA or the lower layer).
Where EAP is used in pass-through mode, the EAP peer does not verify
the identity of the pass-through authenticator. Within the Secure
Association Protocol, the EAP peer and authenticator only demonstrate
mutual possession of the transported EAP keying material. This
creates a potential security vulnerability, described in [RFC3748]
Section 7.15.
As described in the previous section, it is possible for a proxy to
detect a AAA client attempting to impersonate another authenticator
(such by sending incorrect Called-Station-Id [RFC2865], NAS-
Identifier [RFC2865], NAS-IP-Address [RFC2865] or NAS-IPv6-Address
[RFC3162] attributes via the AAA protocol). However, it is possible
for a pass-through authenticator acting as a AAA client to provide
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correct information to the backend authentication server while
communicating misleading information to the EAP peer via the lower
layer.
For example, a compromised authenticator can utilize another
authenticator's Called-Station-Id or NAS-Identifier in communicating
with the EAP peer via the lower layer. Also, a pass-through
authenticator acting as a AAA client can provide an incorrect peer
Calling-Station-Id [RFC2865][RFC3580] to the backend authentication
server via the AAA protocol.
As noted in [RFC3748] Section 7.15, this vulnerability can be
addressed by EAP methods that support a protected exchange of channel
properties such as endpoint identifiers, including (but not limited
to): Called-Station-Id [RFC2865][RFC3580], Calling-Station-Id
[RFC2865][RFC3580], NAS-Identifier [RFC2865], NAS-IP-Address
[RFC2865], and NAS-IPv6-Address [RFC3162].
Using such a protected exchange, it is possible to match the channel
properties provided by the authenticator via out-of-band mechanisms
against those exchanged within the EAP method. For example, see the
discussion in Section 1.4 as well as [I-D.arkko-eap-service-identity-
auth].
It is also possible to achieve Channel Bindings without transporting
data over EAP. For example, see [I-D.draft-ohba-eap-aaakey-binding].
In this approach the authenticator informs the backend server about
the Channel Binding parameters using AAA, and the backend server
calculates transported keying material based on this parameter set,
making it impossible for the peer and authenticator to complete the
Secure Association Protocol if there was a mismatch in the
parameters.
The main difference between these approaches is that Channel Binding
support within an EAP method may require upgrading or changing the
EAP method, impacting both the peer and the server. Where Channel
Bindings are implemented in AAA, the peer, authenticator and the
backend server need to be upgraded, but the EAP method need not be
modified.
6. IANA Considerations
This specification does not request the creation of any new parameter
registries, nor does it require any other IANA assignments.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
Lefkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
7.2. Informative References
[Analysis] He, C. and J. Mitchell, "Analysis of the 802.11i 4-Way
Handshake", Proceedings of the 2004 ACM Workshop on
Wireless Security, pp. 43-50, ISBN: 1-58113-925-X.
[GKDP] Dondeti, L., Xiang, J. and S. Rowles, "GKDP: Group Key
Distribution Protocol", Internet draft (work in progress),
draft-ietf-msec-gkdp-01, March 2006.
[GSAKMP] Harney, H., Meth, U., Colegrove, A., and G. Gross, "GSAKMP:
Group Secure Association Group Management Protocol",
Internet draft (work in progress), draft-ietf-msec-gsakmp-
sec-10, May 2005.
[He] He, C., Sundararajan, M., Datta, A. Derek, A. and J. C.
Mitchell, "A Modular Correctness Proof of TLS and IEEE
802.11i", ACM Conference on Computer and Communications
Security (CCS '05), November, 2005.
[Housley] Housley, R. and B. Aboba, "AAA Key Management", draft-
housley-aaa-key-mgmt-01.txt, Internet draft (work in
progress), November 2005.
[IEEE-802.11]
Institute of Electrical and Electronics Engineers,
"Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific Requirements Part 11:
Wireless LAN Medium Access Control (MAC) and Physical Layer
(PHY) Specifications", IEEE IEEE Standard 802.11-2003,
2003.
[IEEE-802.1X]
Institute of Electrical and Electronics Engineers, "Local
and Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X-2004, December 2004.
Aboba, et al. Standards Track [Page 46]
INTERNET-DRAFT EAP Key Management Framework 13 April 2006
[IEEE-802.1Q]
Institute of Electrical and Electronics Engineers, "IEEE
Standards for Local and Metropolitan Area Networks: Draft
Standard for Virtual Bridged Local Area Networks", IEEE
Standard 802.1Q/D8, January 1998. [IEEE802.11i] Institute
of Electrical and Electronics Engineers, "Supplement to
Standard for Telecommunications and Information Exchange
Between Systems - LAN/MAN Specific Requirements - Part 11:
Wireless LAN Medium Access Control (MAC) and Physical Layer
(PHY) Specifications: Specification for Enhanced Security",
IEEE 802.11i, July 2004.
[IEEE-802.11F]
Institute of Electrical and Electronics Engineers,
"Recommended Practice for Multi-Vendor Access Point
Interoperability via an Inter-Access Point Protocol Across
Distribution Systems Supporting IEEE 802.11 Operation",
IEEE 802.11F, July 2003 (now deprecated).
[IEEE-802.16e]
Institute of Electrical and Electronics Engineers, "IEEE
Standard for Local and Metropolitan Area Networks: Part 16:
Air Interface for Fixed and Mobile Broadband Wireless
Access Systems: Amendment for Physical and Medium Access
Control Layers for Combined Fixed and Mobile Operations in
Licensed Bands" IEEE 802.16e, August 2005.
[IEEE-02-758]
Mishra, A., Shin, M., Arbaugh, W., Lee, I. and K. Jang,
"Proactive Caching Strategies for IAPP Latency Improvement
during 802.11 Handoff", IEEE 802.11 Working Group,
IEEE-02-758r1-F Draft 802.11I/D5.0, November 2002.
[IEEE-03-084]
Mishra, A., Shin, M., Arbaugh, W., Lee, I. and K. Jang,
"Proactive Key Distribution to support fast and secure
roaming", IEEE 802.11 Working Group, IEEE-03-084r1-I,
http://www.ieee802.org/11/Documents/DocumentHolder/
3-084.zip, January 2003.
[I-D.puthenkulam-eap-binding]
Puthenkulam, J., "The Compound Authentication Binding
Problem", draft-puthenkulam-eap-binding-04 (work in
progress), October 2003.
[I-D.arkko-eap-service-identity-auth]
Arkko, J. and P. Eronen, "Authenticated Service Information
for the Extensible Authentication Protocol (EAP)", draft-
Aboba, et al. Standards Track [Page 47]
INTERNET-DRAFT EAP Key Management Framework 13 April 2006
arkko-eap-service-identity-auth-02.txt (work in progress),
May 2005.
[I-D.ohba-eap-aaakey-binding]
Ohba, Y., "AAA-Key Derivation with Channel Binding", draft-
ohba-eap-aaakey-binding-00.txt (work in progress), May
2005.
[I-D.irtf-aaaarch-handoff]
Arbaugh, W. and B. Aboba, "Handoff Extension to RADIUS",
draft-irtf-aaaarch-handoff-04.txt (work in progress),
October 2003.
[MD5Collision]
Klima, V., "Tunnels in Hash Functions: MD5 Collisions
Within a Minute", Cryptology ePrint Archive, March 2006,
http://eprint.iacr.org/2006/105.pdf
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC1968] Meyer, G. and K. Fox, "The PPP Encryption Control Protocol
(ECP)", RFC 1968, June 1996.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.
and R. Wheeler, "A Method for Transmitting PPP Over
Ethernet (PPPoE)", RFC 2516, February 1999.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
RFC 2548, March 1999.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
Aboba, et al. Standards Track [Page 48]
INTERNET-DRAFT EAP Key Management Framework 13 April 2006
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D. and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865,
June 2000.
[RFC2931] Eastlake, D., "DNS Request and Transaction Signatures
(SIG(0)s )", RFC 2931, September 2000.
[RFC3007] Wellington, B., "Simple Secure Domain Name System (DNS)
Dynamic Update", RFC 3007, November 2000.
[RFC3547] Baugher, M., Weis, B., Hardjono, T. and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible Authentication
Protocol (EAP)", RFC 3579, September 2003.
[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G. and J. Roese,
"IEEE 802.1X Remote Authentication Dial In User Service
(RADIUS) Usage Guidelines", RFC 3580, September 2003.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public
Keys Used For Exchanging Symmetric Keys", RFC 3766, April
2004.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M. and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC4017] Stanley, D., Walker, J. and B. Aboba, "EAP Method
Requirements for Wireless LANs", RFC 4017, March 2005.
[RFC4046] Baugher, M., Canetti, R., Dondeti, L. and F. Lindholm,
"Multicast Security (MSEC) Group Key Management
Architecture", RFC 4046, April 2005.
[RFC4067] Loughney, J., Nakhjiri, M., Perkins, C. and R. Koodli,
"Context Transfer Protocol (CXTP)", RFC 4067, July 2005.
[RFC4072] Eronen, P., Hiller, T. and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application", RFC 4072,
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INTERNET-DRAFT EAP Key Management Framework 13 April 2006
August 2005.
[RFC4118] Yang, L., Zerfos, P. and E. Sadot, "Architecture Taxonomy
for Control and Provisioning of Wireless Access Points
(CAPWAP)", RFC 4118, June 2005.
[RFC4186] Haverinen, H. and J. Salowey, "Extensible Authentication
Protocol Method for Global System for Mobile Communications
(GSM) Subscriber Identity Modules (EAP-SIM)", RFC 4186,
January 2006.
[RFC4187] Arkko, J. and H. Haverinen, "Extensible Authentication
Protocol Method for 3rd Generation Authentication and Key
Agreement (EAP-AKA)", RFC 4187, January 2006.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, December 2005.
[RFC4372] Adrangi, F., Lior, A., Korhonen, J. and J. Loughney,
"Chargeable User Identity", RFC 4372, January 2006.
[8021XHandoff]
Pack, S. and Y. Choi, "Pre-Authenticated Fast Handoff in a
Public Wireless LAN Based on IEEE 802.1X Model", School of
Computer Science and Engineering, Seoul National
University, Seoul, Korea, 2002.
Acknowledgments
Thanks to Arun Ayyagari, Ashwin Palekar, and Tim Moore of Microsoft,
Dorothy Stanley of Agere, Bob Moskowitz of TruSecure, Jesse Walker of
Intel, Joe Salowey of Cisco and Russ Housley of Vigil Security for
useful feedback.
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: bernarda@microsoft.com
Phone: +1 425 706 6605
Fax: +1 425 936 7329
Dan Simon
Microsoft Research
Microsoft Corporation
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One Microsoft Way
Redmond, WA 98052
EMail: dansimon@microsoft.com
Phone: +1 425 706 6711
Fax: +1 425 936 7329
Jari Arkko
Ericsson
Jorvas 02420
Finland
Phone:
EMail: jari.arkko@ericsson.com
Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland
EMail: pasi.eronen@nokia.com
Henrik Levkowetz (editor)
ipUnplugged AB
Arenavagen 27
Stockholm S-121 28
SWEDEN
Phone: +46 708 32 16 08
EMail: henrik@levkowetz.com
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Appendix A - Exported Parameters in Existing Methods
This Appendix specifies Method-Id, Peer-Id, Server-Id and Key-
Lifetime for EAP methods that have been published prior to this
specification. Future EAP method specifications MUST include a
definition of the Method-Id, Peer-Id, and Server-Id (could be the
empty string) and MAY also define the Key-Lifetime (assumed to be
indeterminate if not described).
EAP-Identity
The EAP-Identity method is defined in [RC3748]. It does not
derive keys, and therefore does not define the Key-Lifetime or
Method-Id. The Peer-Id exported by the Identity method is
determined by the octets included within the EAP-
Response/Identity. The Server-Id is the empty string (zero
length).
EAP-Notification
The EAP-Notification method is defined in [RFC3748]. It does not
derive keys and therefore does not define the Key-Lifetime and
Method-Id. The Peer-Id and Server-Id are the empty string (zero
length).
EAP-GTC
The EAP-GTC method is defined in [RFC3748]. It does not derive
keys and therefore does not define the Key-Lifetime and Method-Id.
The Peer-Id and Server-Id are the empty string.
EAP-OTP
The EAP-OTP method is defined in [RFC3748]. It does not derive
keys and therefore does not define the Key-Lifetime and Method-Id.
The Peer-Id and Server-Id are the empty string.
EAP-TLS
EAP-TLS is defined in [RFC2716]. The EAP-TLS Method-Id is the
concatenation of the peer and server nonces. The Peer-Id and
Server-Id are the contents of the altSubjectName in the peer and
server certificates. EAP-TLS does not negotiate a Key-Lifetime.
EAP-AKA
EAP-AKA is defined in [RFC4187]. The EAP-AKA Method-Id is the
contents of the RAND field from the AT_RAND attribute, followed by
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the contents of the AUTN field in the AT_AUTN attribute.
The Peer-Id is the contents of the Identity field from the
AT_IDENTITY attribute, using only the Actual Identity Length
octets from the beginning, however. Note that the contents are
used as they are transmitted, regardless of whether the
transmitted identity was a permanent, pseudonym, or fast re-
authentication identity. The Server-Id is an empty string. EAP-
AKA does not negotiate a key lifetime.
EAP-SIM
EAP-SIM is defined in [RFC4186]. The EAP-SIM Method-Id is the
contents of the RAND field from the AT_RAND attribute, followed by
the contents of the NONCE_MT field in the AT_NONCE_MT attribute.
The Peer-Id is the contents of the Identity field from the
AT_IDENTITY attribute, using only the Actual Identity Length
octets from the beginning, however. Note that the contents are
used as they are transmitted, regardless of whether the
transmitted identity was a permanent, pseudonym, or fast re-
authentication identity. The Server-Id is an empty string. EAP-
SIM does not negotiate a key lifetime.
Intellectual Property Statement
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pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
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made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Aboba, et al. Standards Track [Page 53]
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Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Open Issues
Open issues relating to this specification are tracked on the
following web site:
http://www.drizzle.com/~aboba/EAP/eapissues.html
Aboba, et al. Standards Track [Page 54]
