One document matched: draft-ietf-eap-keying-08.txt
Differences from draft-ietf-eap-keying-07.txt
EAP Working Group Bernard Aboba
INTERNET-DRAFT Dan Simon
Category: Standards Track Microsoft
<draft-ietf-eap-keying-08.txt> J. Arkko
23 October 2005 Ericsson
P. Eronen
Nokia
H. Levkowetz, Ed.
ipUnplugged
Extensible Authentication Protocol (EAP) Key Management Framework
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 22, 2006.
Copyright Notice
Copyright (C) The Internet Society 2005.
Abstract
The Extensible Authentication Protocol (EAP), defined in [RFC3748],
enables extensible network access authentication. This document
provides a framework for the generation, transport and usage of
keying material generated by EAP authentication algorithms, known as
"methods". It also specifies the EAP key hierarchy.
Aboba, et al. Standards Track [Page 1]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
Table of Contents
1. Introduction .......................................... 3
1.1 Requirements Language ........................... 3
1.2 Terminology ..................................... 3
1.3 Overview ........................................ 5
1.4 EAP Invariants .................................. 9
2. Lower Layer Operation ................................. 13
2.1 Overview ........................................ 13
2.2 Layering ........................................ 14
2.3 Caching ......................................... 17
2.4 Key Scope ....................................... 18
3. Key Management ........................................ 21
3.1 Secure Association Protocol ..................... 22
3.2 Parent-Child Relationships ...................... 24
3.3 Local Key Lifetimes ............................. 25
3.4 Exported and Calculated Key Lifetimes ........... 25
3.5 Key Cache Synchronization ....................... 27
3.6 Key Strength .................................... 27
3.7 Key Wrap ........................................ 28
4. Handoff Vulnerabilities ............................... 29
4.1 Authorization ................................... 29
4.2 Correctness ..................................... 30
5. Security Considerations .............................. 33
5.1 Security Terminology ............................ 34
5.2 Threat Model .................................... 34
5.3 Authenticator Compromise ........................ 35
5.4 Spoofing ........................................ 36
5.5 Downgrade Attacks ............................... 36
5.6 Unauthorized Disclosure ......................... 37
5.7 Replay Protection ............................... 38
5.8 Key Freshness ................................... 39
5.9 Elevation of Privilege .......................... 40
5.10 Man-in-the-Middle Attacks ....................... 41
5.11 Denial of Service Attacks ....................... 41
5.12 Impersonation ................................... 42
5.13 Channel Binding ................................. 43
6. IANA Considerations ................................... 44
7. References ............................................ 44
7.1 Normative References ............................ 44
7.2 Informative References .......................... 45
Acknowledgments .............................................. 49
Author's Addresses ........................................... 49
Appendix A - EAP-TLS Key Hierarchy ........................... 51
Appendix B - Exported Parameters in Existing Methods ......... 53
Intellectual Property Statement .............................. 54
Disclaimer of Validity ....................................... 55
Copyright Statement .......................................... 55
Aboba, et al. Standards Track [Page 2]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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].
This document provides a framework for the generation, 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 or transformed by
Secure Association Protocols into 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 BCP 14 [RFC2119].
1.2. Terminology
This document frequently uses the following terms:
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. In
[IEEE-802.1X], this end is known as the Supplicant.
Supplicant
The end of the link that responds to the authenticator in
[IEEE-802.1X]. In this document, this end of the link is called
the peer.
backend authentication server
A backend authentication server is an entity that provides an
authentication service to an authenticator. When used, this server
typically executes EAP methods for the authenticator. This
terminology is also used in [IEEE-802.1X].
Aboba, et al. Standards Track [Page 3]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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.
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.
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.
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.
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.
However, when it is generated it MUST be unpredictable.
Pairwise Master Key (PMK)
The MSK is divided into two halves, the "Peer to Authenticator
Encryption Key" (Enc-RECV-Key) and "Authenticator to Peer
Aboba, et al. Standards Track [Page 4]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
Encryption Key" (Enc-SEND-Key) (reception is defined from the point
of view of the authenticator). Within [IEEE-802.11i] Octets 0-31
of the MSK (Enc-RECV-Key) 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.
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. An
example TEK key hierarchy is described in Appendix A.
Transient Session Keys (TSKs)
Session keys used to protect data exchanged in a session between
the peer and authenticator after the EAP authentication has
successfully completed. TSKs are appropriate for the lower layer
ciphersuite negotiated between the ports of the EAP peer and
authenticator. Examples of TSK derivation are provided in Appendix
B.
AAA-Key
A key derived by the peer and EAP server, used by the peer and
authenticator in the derivation of Transient Session Keys (TSKs).
Where a backend authentication server is present, the AAA-Key is
transported from the backend authentication server to the
authenticator. In existing usage, the AAA-Key is always derived
from the MSK and so can be referred to using the MSK name. AAA-Key
= MSK(0,63).
1.3. Overview
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 1, the EAP method key derivation has at the
Aboba, et al. Standards Track [Page 5]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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 and/or other information necessary to
decide 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/or other information necessary to decide whether access
to some service should be granted. The peer stores information
necessary to choose which certificate to use for which service.
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 methods also MAY export method-specific peer and server
identifiers (peer-ID and server-ID), a method-specific EAP
conversation identifier known as the Method-ID, and the lifetime of
the exported keys, known 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.
Aboba, et al. Standards Track [Page 6]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | ^
| EAP Method | |
| | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+ | |
| | | | | | |
| | EAP Method Key |<->| Long-Term | | |
| | Derivation | | Credential | | |
| | | | | | |
| | | +-+-+-+-+-+-+-+ | Local to |
| | | | EAP |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Method |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| | +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ | |
| | | TEK | |MSK, EMSK | |IV | | |
| | |Derivation | |Derivation | |Derivation | | |
| | +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ | |
| | | | | |
| | ^ | | | V
+-+-|-+-+-+-+-+-+-+-+-|-+-+-+-+-+-|-+-+-+-+-+-+-+-+-|-+-+-+ ---+
| | | | ^
| Peer-ID, | | | Exported|
| Server-ID, | Channel | MSK (64+B) | IV (64B) by |
| Method-ID, | Bindings | EMSK (64+B) | EAP |
| Key-Lifetime | & Result | | Method |
V V V V V
Figure 1: EAP Method Parameter Import/Export
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. The Peer-ID for existing EAP
methods is defined in Appendix B.
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
Aboba, et al. Standards Track [Page 7]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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. The Server-ID for existing EAP methods is defined in
Appendix B.
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. The Method-ID for
existing EAP methods is defined in Appendix B.
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
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. 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 passes
the result of the verification (TRUE or FALSE) up to the EAP method.
Aboba, et al. Standards Track [Page 8]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
1.3.1. 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, EMSK and IV 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]).
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.
1.4. EAP Invariants
Certain basic characteristics, known as "EAP Invariants", hold true
for EAP implementations on all media:
Mode independence
Media independence
Method independence
Ciphersuite independence
1.4.1. Mode Independence
EAP is typically deployed in order 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
Aboba, et al. Standards Track [Page 9]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
authenticator, and the authenticator must locally implement an EAP
method acceptable to the peer. However, one of the advantages of EAP
is that it 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 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.
1.4.2. 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 IEEE
802.11 wireless LANs [IEEE-802.11i].
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
Aboba, et al. Standards Track [Page 10]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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.
1.4.3. 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.
1.4.4. Ciphersuite Independence
Ciphersuite Independence is a consequence of the principles of Mode
Independence and Media Independence.
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.
Aboba, et al. Standards Track [Page 11]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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 ciphersuite negotiation within EAP, and EAP methods generate
keying material that is ciphersuite-independent.
Algorithms for deriving TSKs MUST NOT depend on the EAP method,
although algorithms for TEK derivation MAY be specific to the EAP
method.
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.
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"
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.
Aboba, et al. Standards Track [Page 12]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
2. Lower Layer Operation
2.1. 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 Establishment
2a: Unicast Secure Association
2b: Multicast Secure Association (optional)
Of these phases, Phase 0, 1b and Phase 2 are handled by a lower
layer. 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.
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, where a backend
server is present AAA Key transport needs to provide the exported EAP
keying material and/or derived keys required for derivation of the
TSKs. Since existing TSK derivation techniques depend solely on the
MSK, in existing AAA 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 exchange (phase 2) occurs between the peer and
authenticator in order to manage the creation and deletion of unicast
Aboba, et al. Standards Track [Page 13]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
(phase 2a) and multicast (phase 2b) security associations between the
peer and authenticator. The conversation between the parties is
shown in Figure 2.
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 2: Conversation Overview
2.2. Layering
In completion of EAP authentication, EAP methods on the peer and EAP
server export the Master Session Key (MSK), Extended Master Session
Key (EMSK), Initialization Vector (IV), Peer-ID, Server-ID, Session-
ID and Key-Lifetime. As illustrated in Figure 3, EAP methods export
keying material and parameters to the EAP peer or authenticator
layers.
The EAP 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 EAP layer. The EAP
layer also MUST NOT cache keying material or parameters (including
Channel Bindings) passed to it by the EAP peer/authenticator layer or
the lower layer.
Based on the Method-ID exported by the EAP method, the EAP layer
forms the EAP Session-ID by concatenating the EAP Expanded Type with
the Method-ID. Together with the MSK, IV (deprecated), Peer-ID,
Server-ID, and Key-Lifetime, the EAP layer passes the Session-ID down
to the lower layer.
The EMSK MUST NOT be provided to the lower layer, nor is it permitted
Aboba, et al. Standards Track [Page 14]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
to pass any quantity to the lower layer from which the EMSK could be
computed without breaking some cryptographic assumption, such as
inverting a one-way function. As noted in [RFC3748] Section 7.10:
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. (This restriction will be relaxed in a
future document that specifies how the EMSK can be used.)
The Method-ID is exported by EAP methods rather than the Session-ID
so as to prevent EAP methods from writing into each other's Session-
ID space.
In order to preserve the security of keys derived within EAP methods,
lower layers other than AAA 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 other
than AAA 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.
The exception to the "no sharing" rule is the AAA layer. On 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 may provide the replicated keying
material to the lower layer over which the EAP authentication
conversation took place. This enables "mode independence" to be
maintained.
As illustrated in Figure 4, a AAA client receiving transported EAP
keying material and parameters passes them to the EAP authenticator
and EAP layers, which then provide them to the authenticator lower
layer using the same mechanisms that would be used if the EAP peer
and authenticator were conducting a stand-alone conversation. The
resulting key state in the lower layer is indistinguishable between
the standalone and pass-through cases, as required by the principle
of mode independence. In order to prevent the compromise of
transported EAP keying material and parameters, the AAA client and
EAP authenticator MUST be co-resident.
Aboba, et al. Standards Track [Page 15]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| EAP method |
| |
| MSK, EMSK, IV, Channel |
| Peer-ID, Server-ID, Bindings |
| Method-ID, |
| Key-Lifetime |
| |
| V ^ ^ |
+-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+-!-+-+-+-+-!-+-+
| ! ! ! |
| EAP ! Peer or Authenticator ! ! |
| ! layer ! ! |
| ! ! ! |
+-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+-!-+-+-+-+-!-+-+
| ! ! ! |
| EAP ! layer ! ! |
| ! ! ! |
| ! Session-ID = ! ! |
| ! Expanded-Type || ! ! |
| ! Method-ID ! ! |
| ! ! ! |
+-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+-!-+-+-+-+-!-+-+
| ! ! ! |
| Lower ! layer ! ! |
| ! ! ! |
| V V ^ |
| MSK, IV, Peer-ID, Channel Result |
| Server-ID, Bindings |
| Session-ID, |
| Key-Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Flow of EAP parameters
Aboba, et al. Standards Track [Page 16]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
Peer Pass-through Authenticator Authentication
Server
+-+-+-+-+-+-+ +-+-+-+-+-+-+
| | | |
|EAP method | |EAP method |
| V | | V |
+-+-+-!-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-!-+-+-+
| ! | |EAP | EAP | | | ! |
| ! | |Peer | Auth.| EAP Auth. | | ! |
|EAP ! peer| | | +-----------+ | |EAP !Auth.|
| ! | | | ! | ! | | ! |
+-+-+-!-+-+-+ +-+-+-+-!-+-+-+-+-+-!-+-+-+-+ +-+-+-!-+-+-+
| ! | | ! | ! | | ! |
|EAP !layer| | EAP !layer| EAP !layer | |EAP !layer|
| ! | | ! | ! | | ! |
+-+-+-!-+-+-+ +-+-+-+-!-+-+-+-+-+-!-+-+-+-+ +-+-+-!-+-+-+
| V | | V | ! | | ! |
|Lower layer| | Lower layer| AAA ! /IP | | AAA ! /IP |
| | | | ! | | ! |
+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-!-+-+-+-+ +-+-+-!-+-+-+
! !
! !
+---------<-------+
Figure 4: Flow of EAP Keying Material and Parameters
2.3. Caching
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.
The caching behavior of existing EAP lower layers is as follows:
PPP PPP, defined in [RFC1661] does not support caching of EAP keying
material or parameters. Since PPP ciphersuites derive their TSKs
directly from the MSK as described in [RFC2716], were PPP to
support caching, this could result in stale TSKs. Therefore once
the PPP session is terminated, it is assumed that EAP keying
material and parameters are discarded.
Aboba, et al. Standards Track [Page 17]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
IKEv2
IKEv2, defined in [IKEv2] only uses EAP keying material for
authentication purposes and not key derivation. As a result, IKEv2
does not cache EAP keying material or parameters, nor does it
utilize the Key-Lifetime to determine the lifetime of IPsec SAs.
As result, once IKEv2 authentication completes it is assumed that
EAP keying material and parameters are discarded.
IEEE 802.11i
IEEE 802.11i enables caching of the MSK, but not the EMSK, IV,
Peer-ID, Server-ID, Session-ID, or Key-Lifetime. More details are
about the structure of the cache are available in [IEEE-802.11i].
IEEE 802.1X-2004
IEEE 802.1X-2004, defined in [IEEE-802.1X-2004] does not support
caching of EAP keying material or parameters. Therefore once EAP
authentication completes, it is assumed that EAP keying material
and parameters are discarded.
AAA Existing AAA servers supporting RADIUS/EAP [RFC3579] or Diameter
EAP [RFC4207] do not support caching of EAP keying material or
parameters. In existing AAA 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 to the authenticator. 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.
2.4. Key Scope
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.
The issues that arise from this are discussed below.
2.4.1. Multiple Ports
Both the EAP peer and authenticator may have more than one physical
or logical port. A peer may simultaneously access the network via
Aboba, et al. Standards Track [Page 18]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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. The situation is illustrated in Figure 5.
+-+-+-+-+
| EAP |
| Peer |
+-+-+-+-+
| | | Peer Ports
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
| | | | | | | | | Authenticator Ports
+-+-+-+-+ +-+-+-+-+ +-+-+-+-+
| | | | | |
| Auth. | | Auth. | | Auth. |
| | | | | |
+-+-+-+-+ +-+-+-+-+ +-+-+-+-+
\ | /
\ | /
\ | /
EAP over AAA \ | /
(optional) \ | /
\ | /
\ | /
\ | /
+-+-+-+-+
| EAP |
|Server |
+-+-+-+-+
Figure 5: Relationship between EAP peer, authenticator and server
Absent explicit specification within the lower layer, EAP keying
material and parameters are not bound to a specific peer or
authenticator port. Where the peer and authenticator identify
themselves within the lower layer using a port identifier such as a
link layer address, this creates a problem, because it may not be
obvious to the peer which authenticator ports are associated with
which authenticators. Similarly, it may not be obvious to the
authenticator which peer ports are associated with which peers. As a
result, the peer and authenticator may not be able to determine the
Aboba, et al. Standards Track [Page 19]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
scope of the EAP keying material. This is particularly problematic
for lower layers where key caching is supported.
For example, where the EAP peer cannot identify the EAP
authenticator, it will be unable to determine whether 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.
The solution to this problem is for lower layers to identify EAP
peers and authenticators unambiguously, without incorporating
implicit assumptions about peer and authenticator architectures. Use
of port identifiers is NOT RECOMMENDED where peers and authenticators
may support multiple ports.
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
can be applied to the identification of EAP authenticators.
RADIUS 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 associated with it, the
NAS-Identifier attribute is RECOMMENDED for the unambiguous
identification of the EAP authenticator.
From the point of view of the AAA server, EAP keying material and
parameters are transported to the NAS identified by the NAS-
Identifier attribute. Since the NAS/ EAP authenticator MUST NOT
share EAP keying material or parameters with another party, if the
EAP peer or AAA 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 further limit the key scope the following measures are
suggested:
[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 AAA server and client/authenticator MAY implement additional
attributes in order to further restrict the scope of EAP keying
material. For example, in 802.11, the AAA 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.
Aboba, et al. Standards Track [Page 20]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
[c] Where the AAA 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.
2.4.2. Virtual Authenticators
When a single physical authenticator advertises itself as multiple
"virtual authenticators", the EAP peer and authenticator also may not
be able to agree on the scope of the EAP keying material, creating a
security vulnerability. 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.
Several measures are recommended to address these issues:
[d] 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".
[e] It is RECOMMENDED that physical authenticators maintain separate
key caches for each "virtual authenticator".
[f] It is RECOMMENDED that each "virtual authenticator" identify itself
distinctly to the AAA server, such as by utilizing a distinct NAS-
Identifier attribute. This enables the AAA server to utilize a
separate credential to authenticate each "virtual authenticator".
3. Key Management
EAP as defined in [RFC3748] supports key derivation, but not key
management. While EAP methods may derive keying material, EAP does
not provide for the management of exported or derived keys. For
example, EAP does not support negotiation of the key lifetime of
exported or derived keys, nor does it support re-key. Although EAP
methods may support "fast reconnect" as defined in [RFC3748] Section
7.2.1, re-key of exported keys cannot occur without re-
Aboba, et al. Standards Track [Page 21]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
authentication. In order to provide 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. As
shown in Figure 5, both the peer and authenticator may have more
than one physical or virtual port, and as a result SHOULD identify
themselves in a manner that is independent of their attached ports.
[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 and
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.
[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. As part of secure capabilities
negotiation, the Secure Association Protocol MUST support integrity
and replay protection of all 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 this
Aboba, et al. Standards Track [Page 22]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
document is RECOMMENDED.
In order to support the correct processing of phase 2 security
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 negotiation
of key lifetimes, nor does it support re-key without re-
authentication. 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. To take an
example from IKE, the difference between IKEv1 and IKEv2 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 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
Aboba, et al. Standards Track [Page 23]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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
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. Since the Discovery phase is handled
out-of-band, EAP does not provide a mechanism by which the peer can
determine the authenticator identity. As a result, where the
authenticator has multiple ports and key caching is supported, the
EAP peer may not be able to determine the scope of validity of the
exported EAP keying material. Similarly, where the EAP peer has
multiple ports, the authenticator may not be able to determine
whether a peer has authorization to use a particular key. To allow
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.
3.2. 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
Aboba, et al. Standards Track [Page 24]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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, 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.
3.3. 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 a conversation.
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.
3.4. 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 support the negotiation of lifetimes for exported
keying material such as the MSK, EMSK and IV.
Aboba, et al. Standards Track [Page 25]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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 value represents the maximum lifetime of the
exported keys, and all keys calculated from it, on the
authenticator. Since existing AAA 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 AAA server.
On the authenticator, where EAP is used for authentication, the
Session-Timeout value represents the maximum session time prior to
re-authentication, as described in [RFC3580]. Where EAP is used
for pre-authentication, the session may not start until some future
time, or may never occur. Nevertheless, the Session-Timeout value
represents the 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.
Since the TSK lifetime is often determined by authenticator
resources, the AAA server has no insight into the TSK derivation
process, and by the principle of ciphersuite independence, it is
not appropriate for the AAA 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 resynchronization. Where the TSK
is taken 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
Aboba, et al. Standards Track [Page 26]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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
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.
3.5. Key cache synchronization
Issues arise when attempting to synchronize the key cache on the peer
and authenticator. Lifetime negotiation alone cannot guarantee key
cache synchronization.
One problem is that the AAA protocol cannot guarantee synchronization
of key lifetimes between the peer and authenticator. Where the
Secure Association Protocol is not run immediately after EAP
authentication, the exported and calculated key lifetimes will not be
known by the peer during the hiatus. Where EAP pre-authentication
occurs, this can leave the peer uncertain whether a subsequent
attempt to use the exported keys will prove successful.
However, even where the Secure Association Protocol is run
immediately after EAP, it is still possible for the authenticator to
reclaim resources if the created key state is not immediately
utilized.
The lower layer may utilize Discovery mechanisms to assist in this.
For example, 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 with the Discovery phase, enabling
the peer to determine whether a given key had been expired from the
authenticator key cache prematurely.
3.6. 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
Aboba, et al. Standards Track [Page 27]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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:
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
3.7. Key Wrap
As described in [RFC3579] Section 4.3, known problems exist in the
key wrap specified in [RFC2548]. Where the same RADIUS shared secret
is used by a PAP authenticator and an EAP authenticator, there is a
vulnerability to known plaintext attack. Since RADIUS uses the
shared secret for multiple purposes, including per-packet
authentication, 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
some concerns exist relating to the security of this hash
[MD5Attack].
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.
Where an untrusted AAA intermediary is present (such as a RADIUS
proxy or a Diameter agent), and data object security is not used,
transported keying material may be recovered by an attacker in
control of the untrusted intermediary. 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 is only utilized by a single
authenticator, compromise of the transported keying material does not
enable an attacker to impersonate the peer to another authenticator.
Aboba, et al. Standards Track [Page 28]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
Vulnerability to an untrusted AAA intermediary can be mitigated by
implementation of redirect functionality, as described in [RFC3588]
and [RFC4072].
4. Handoff Vulnerabilities
With EAP, a number of mechanisms are be utilized in order to reduce
the latency of handoff between authenticators. One such mechanism is
EAP pre-authentication, in which EAP is utilized to pre-establish EAP
keying material on an authenticator prior to arrival of the peer.
Another such mechanism is key caching, in which an EAP peer can re-
attach to an authenticator without having to re-authenticate using
EAP. Yet another mechanism is context transfer, such as is defined
in [IEEE-802.11F] and [CTP]. These mechanisms introduce new security
vulnerabilities, as discussed in the sections that follow.
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 AAA network 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) included with the AAA-Token, which also contains 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, answering the following questions:
[a] Is this a legitimate user for this particular network?
[b] Is this 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 this user within the subscription rules regarding time of day?
[e] Is this user within his limits for concurrent sessions?
Aboba, et al. Standards Track [Page 29]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
[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 AAA decision making.
Where brokering entities or proxies are involved, all of the AAA
devices in the chain from the authenticator to the home AAA server
are involved in the decision. For instance, a broker can disallow
access even if the home AAA server would allow it, or a proxy can add
authorizations (e.g., 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, this creates challenges in ensuring that authorization is
properly handled. These 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.
Aboba, et al. Standards Track [Page 30]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
[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 in the AAA-Token.
[e] State validity. The introduction of fast handoff should not render
the authentication server incapable of keeping track of network-
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 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 backend authentication server and authenticator configurations
are incapable of meeting this definition of "correctness". For
example, if the old and new device differ in their capabilities, it
may be difficult to meet this definition of correctness in a handoff
mechanism that bypasses AAA. 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, then the handoff cannot be carried out
correctly.
For example, if some authenticator devices within a deployment
support dynamic VLANs while others do not, then attributes present in
the Access-Request (such as the authenticator-IP-Address,
authenticator-Identifier, Vendor-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
Aboba, et al. Standards Track [Page 31]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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
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 AAA server in order to receive the appropriate service
Aboba, et al. Standards Track [Page 32]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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
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
In order to analyze whether the EAP conversation achieves it security
goals, it is first necessary to state those goals as well as the
underlying security assumptions.
The overall 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 AAA server).
The principals of the authentication phase are the EAP peer and
server. Completion of an EAP method exchange 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.
The principals of the AAA Key transport exchange are the EAP
authenticator and the EAP server. Completion of the AAA exchange
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.
The principals of the Secure Association Protocol are the EAP peer
(identified by the Peer-ID) and authenticator (identified by the NAS-
Identifier). Completion of the Secure Association Protocol results
in the derivation of TSKs known only to the EAP peer and
authenticator. Both the EAP peer and authenticator know the TSKs to
Aboba, et al. Standards Track [Page 33]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
be fresh.
5.1. Terminology
"Cryptographic binding", "Cryptographic separation", "Key strength"
and "Mutual authentication" are defined in [RFC3748] and are used
with the same meaning here.
5.2. Threat Model
The EAP threat model is described in [RFC3748] Section 7.1. The
security properties of EAP methods (known as "security claims",
described in [RFC3784] Section 7.2.1), address these threats. 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 worth discussing. These include:
[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 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.
[3] An attacker may try to modify or spoof packets, including Discovery
or Secure Association Protocol frames, EAP or AAA packets.
[4] An attacker may attempt a downgrade attack in order to exploit
known weaknesses in an authentication method or cryptographic
transform.
[5] 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.
[6] An attacker may replay packets.
[7] An attacker may cause an EAP peer, authenticator or server to reuse
an stale key. Use of stale keys may also occur unintentionally.
Aboba, et al. Standards Track [Page 34]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
For example, a poorly implemented AAA server may provide stale
keying material to an authenticator, or a poorly implemented
authenticator may reuse nonces.
[8] An authenticated attacker may attempt to obtain elevated privilege
in order to access information that it does not have rights to.
In order to address these threats, [Housley] provides a description
of mandatory system security properties. Issues relating system
security requirements are discussed in the sections that follow.
5.3. 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 AAA servers. However, this should
not allow the attacker to compromise other authenticators or the AAA
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 AAA server, or even to
impersonate a AAA server to other AAA clients.
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.
Aboba, et al. Standards Track [Page 35]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
5.4. 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].
5.5. 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 [MD5Attack]. This issue
can be addressed via use of RADIUS over IPsec, as described in
[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
Aboba, et al. Standards Track [Page 36]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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.
5.6. 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. 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 AAA server communicate directly and
credible keywrap is used (see Section 3.7), this ensures that the AAA
Key Transport phase achieves its security objectives: mutually
authenticating the AAA client/authenticator and AAA server and
providing EAP keying material to the EAP authenticator and to no
other party.
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. However, where the NAS/authenticator and AAA server
communicate directly, the AAA 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 AAA
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 AAA 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
Aboba, et al. Standards Track [Page 37]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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.
In order to enable key binding and authorization of all parties, it
is RECOMMENDED that the parties use a set of identities that are
consistent between the conversation phases. RADIUS [RFC2865] and
Diameter NASREQ [RFC4005] require that the NAS/EAP authenticator
identify itself by including one or more identification attributes
within an Access-Request packet (NAS-Identifier, NAS-IP-Address, NAS-
IPv6-Address).
Since the AAA server provides EAP keying material for use by the EAP
authenticator as identified by these attributes, where an EAP
authenticator may have multiple ports, it is RECOMMENDED for the EAP
authenticator to identify itself using NAS identification attributes
during the Secure Association Protocol exchange with the EAP peer.
This enables the EAP peer to determine whether EAP keying material
has been shared between EAP authenticators as well as to confirm with
the AAA server that an EAP authenticator proving possession of EAP
keying material during the Secure Association Protocol was authorized
to obtain it. Typically, the NAS-Identifier attribute is most
convenient for this purpose, since a NAS/authenticator may have
multiple IP addresses.
Similarly, the AAA server authorizes the EAP authenticator to provide
access to the EAP peer identified by the Peer-ID, securely verified
during the EAP authentication exchange. In order to determine
whether EAP keying material has been shared between EAP peers, where
the EAP peer has multiple ports it is RECOMMENDED for the EAP peer to
identify itself using the Peer-ID during the Secure Association
Protocol exchange with the EAP authenticator.
5.7. 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.
Aboba, et al. Standards Track [Page 38]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
[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.
5.8. 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:
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.
Aboba, et al. Standards Track [Page 39]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
[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.
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.
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. [IEEE-802.11i]
also utilizes the Session-Timeout attribute to limit the maximum
time that EAP keying material may be cache. Therefore the use of
the Session-Timeout attribute enables the AAA server to limit the
exposure of EAP keying material.
Lower Layer
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 the peer or
authenticator lacks a high entropy random number generator. A
RECOMMENDED method is for the peer and authenticator to each
provide a nonce or counter of at least 128 bits, used in session
key derivation.
5.9. 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 post-EAP handshake is used to establish session keys, the post-EAP
handshake MUST specify the scope for session keys.
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
Aboba, et al. Standards Track [Page 40]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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. Since the TSKs can be determined from the transported
EAP keying material and the cleartext of the Secure Association
Protocol exchange, the AAA server will have access to the TSKs unless
it deletes the transported EAP keying material after sending it.
5.10. 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.
5.11. 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
Aboba, et al. Standards Track [Page 41]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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 sttate corresponding
to a few EAP converations, 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.
5.12. Impersonation
Both the RADIUS and Diameter protocols are potentially vulnerable to
impersonation by a rogue authenticator.
While AAA protocols such as RADIUS [RFC2865] or Diameter [RFC3588]
support mutual authentication between the authenticator (known as the
AAA client) and the backend authentication server (known as the AAA
server), the security mechanisms vary according to the AAA protocol.
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 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
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
transorted 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
Aboba, et al. Standards Track [Page 42]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
forged as well.
As recommended in [RFC3579] Section 4.3.7, 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
RRs to be properly configured. As a result, it appears that 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].
5.13. 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.
[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). However, it is possible for a pass-through authenticator
acting as a AAA client to provide correct information to the AAA
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, or for a pass-through
authenticator acting as a AAA client to provide an incorrect peer
Calling-Station-Id [RFC2865][RFC3580] to the AAA 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
Aboba, et al. Standards Track [Page 43]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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 [I-
D.arkko-eap-service-identity-auth].
It is also possible to achieve Channel Bindings without transporting
data over EAP. For example, see [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 document does not create any new name spaces nor does it
allocate any protocol parameters.
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.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
Lefkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
Aboba, et al. Standards Track [Page 44]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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.
[CTP] Loughney, J., Nakhjiri, M., Perkins, C. and R. Koodli,
"Context Transfer Protocol", draft-ietf-seamoby-ctp-11.txt,
Internet draft (work in progress), August 2004.
[DESMODES]
National Institute of Standards and Technology, "DES Modes of
Operation", FIPS PUB 81, December 1980, <http://
www.itl.nist.gov/fipspubs/fip81.htm>.
[FIPSDES] National Institute of Standards and Technology, "Data
Encryption Standard", FIPS PUB 46, January 1977.
[Housley] Housley, R. and B. Aboba, "AAA Key Management", draft-housley-
aaa-key-mgmt-00.txt, Internet draft (work in progress), June
2005.
[IEEE-802]
Institute of Electrical and Electronics Engineers, "IEEE
Standards for Local and Metropolitan Area Networks: Overview
and Architecture", ANSI/IEEE Standard 802, 1990.
[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.
[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.
Aboba, et al. Standards Track [Page 45]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
[IEEE-802.11i]
Institute of Electrical and Electronics Engineers, "Supplement
to STANDARD FOR Telecommunications and Information Exchange
between Systems - LAN/MAN Specific Requirements - Part 11:
Wireless Medium Access Control (MAC) and physical layer (PHY)
specifications: Specification for Enhanced Security", IEEE
802.11i, December 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.
[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.
[IEEE-03-155]
Aboba, B., "Fast Handoff Issues", IEEE 802.11 Working Group,
IEEE-03-155r0-I, http://www.ieee802.org/11/
Documents/DocumentHolder/3-155.zip, March 2003.
[I-D.ietf-roamops-cert]
Aboba, B., "Certificate-Based Roaming", draft-ietf-roamops-
cert-02 (work in progress), April 1999.
[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-pppext-eap-aka]
Arkko, J. and H. Haverinen, "EAP AKA Authentication", draft-
arkko-pppext-eap-aka-15.txt (work in progress), December 2004.
[I-D.arkko-eap-service-identity-auth]
Arkko, J. and P. Eronen, "Authenticated Service Information
Aboba, et al. Standards Track [Page 46]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
for the Extensible Authentication Protocol (EAP)", draft-
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.
[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", draft-
ietf-ipsec-ikev2-17 (work in progress), September 2004.
[MD5Attack]
Dobbertin, H., "The Status of MD5 After a Recent Attack",
CryptoBytes, Vol.2 No.2, 1996.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[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.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[RFC2246] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A.
and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246,
January 1999.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[RFC2419] Sklower, K. and G. Meyer, "The PPP DES Encryption Protocol,
Version 2 (DESE-bis)", RFC 2419, September 1998.
[RFC2420] Kummert, H., "The PPP Triple-DES Encryption Protocol (3DESE)",
RFC 2420, September 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.
Aboba, et al. Standards Track [Page 47]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
[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.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[RFC3078] Pall, G. and G. Zorn, "Microsoft Point-To-Point Encryption
(MPPE) Protocol", RFC 3078, March 2001.
[RFC3079] Zorn, G., "Deriving Keys for use with Microsoft Point-to-Point
Encryption (MPPE)", RFC 3079, March 2001.
[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.
[RFC4005] Calhoun, P., Zorn, G., Spence, D. and D. Mitton, "Diameter
Network Access Server Application", RFC 4005, August 2005.
[RFC4017] Stanley, D., Walker, J. and B. Aboba, "EAP Method Requirements
for Wireless LANs", RFC 4017, March 2005.
[RFC4072] Eronen, P., Hiller, T. and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application", RFC 4072, August
2005.
[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,
Aboba, et al. Standards Track [Page 48]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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.
Author 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
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
Aboba, et al. Standards Track [Page 49]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
Arenavagen 27
Stockholm S-121 28
SWEDEN
Phone: +46 708 32 16 08
EMail: henrik@levkowetz.com
Aboba, et al. Standards Track [Page 50]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
Appendix A - EAP-TLS Key Hierarchy
EAP-TLS [RFC 2716] was documented prior to the development of EAP key
management terminology [RFC3748], and therefore does not explicitly
define the MSK and EMSK.
In EAP-TLS, the MSK, EMSK and IV are derived from the TLS master
secret via a one-way function. This ensures that the TLS master
secret cannot be derived from the MSK, EMSK or IV unless the one-way
function (TLS PRF) is broken. Since the MSK is derived from the the
TLS master secret, if the TLS master secret is compromised then the
MSK is also compromised.
[RFC2716] specifies that the MSK is divided into two halves,
corresponding to the "Peer to Authenticator Encryption Key" (Enc-
RECV-Key, 32 octets, also known as the PMK) and "Authenticator to
Peer Encryption Key" (Enc-SEND-Key, 32 octets). In [RFC2548], the
Enc-RECV-Key (the PMK) is transported in the MS-MPPE-Recv-Key
attribute, and the Enc-SEND-Key is transported in the MS-MPPE-Send-
Key attribute.
The EMSK is also divided into two halves, corresponding to the "Peer
to Authenticator Authentication Key" (Auth-RECV-Key, 32 octets) and
"Authenticator to Peer Authentication Key" (Auth-SEND-Key, 32
octets). The IV is a 64 octet quantity that is a known value; octets
0-31 are known as the "Peer to Authenticator IV" or RECV-IV, and
Octets 32-63 are known as the "Authenticator to Peer IV", or SEND-IV.
The key derivation scheme MUST be interpreted as follows:
MSK = TLS-PRF-64(TMS, "client EAP encryption",
client.random || server.random)
EMSK = second 64 octets of:
TLS-PRF-128(TMS, "client EAP encryption",
client.random || server.random)
IV = TLS-PRF-64("", "client EAP encryption",
client.random || server.random)
MSK(0,31) = Peer to Authenticator Encryption Key (Enc-RECV-Key)
(MS-MPPE-Recv-Key in [RFC2548]). Also known as the
PMK.
MSK(32,63) = Authenticator to Peer Encryption Key (Enc-SEND-Key)
(MS-MPPE-Send-Key in [RFC2548])
EMSK(0,31) = Peer to Authenticator Authentication Key (Auth-RECV-Key)
EMSK(32,63) = Authenticator to Peer Authentication Key (Auth-Send-Key)
IV(0,31) = Peer to Authenticator Initialization Vector (RECV-IV)
IV(32,63) = Authenticator to Peer Initialization vector (SEND-IV)
Aboba, et al. Standards Track [Page 51]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
Where:
IV(W,Z) = Octets W through Z inclusive of the IV.
MSK(W,Z) = Octets W through Z inclusive of the MSK.
EMSK(W,Z) = Octets W through Z inclusive of the EMSK.
TMS = TLS master_secret
TLS-PRF-X = TLS PRF function defined in [RFC2246] computed to X octets
client.random = Nonce generated by the TLS client.
server.random = Nonce generated by the TLS server.
Figure A-1 illustrates the TEK key hierarchy for EAP-TLS [RFC2716],
which is based on the TLS key hierarchy described in [RFC2246]. The
TLS-negotiated ciphersuite is used to set up a protected channel for
use in protecting the EAP conversation, keyed by the derived TEKs.
The TEK derivation proceeds as follows:
master_secret = TLS-PRF-48(pre_master_secret, "master secret",
client.random || server.random)
TEK = TLS-PRF-X(master_secret, "key expansion",
server.random || client.random)
Where:
TLS-PRF-X = TLS pseudo-random function defined in [RFC2246],
computed to X octets.
| | pre_master_secret |
server| | | client
Random| V | Random
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | | |
+---->| master_secret |<------+
| | (TMS) | |
| | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
V V V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Key Block (TEKs) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |
| client | server | client | server | client | server
| MAC | MAC | write | write | IV | IV
| | | | | |
V V V V V V
Figure A-1 - TLS [RFC2246] Key Hierarchy
Aboba, et al. Standards Track [Page 52]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
Appendix B - 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 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 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 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 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
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
The EAP-AKA Method-Id is the contents of the RAND field from the
Aboba, et al. Standards Track [Page 53]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
AT_RAND attribute, followed by 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
The 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
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
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 54]
INTERNET-DRAFT EAP Key Management Framework 23 October 2005
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 (2005). 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 55]
| PAFTECH AB 2003-2026 | 2026-04-23 05:27:24 |