One document matched: draft-aboba-pppext-key-problem-07.txt
Differences from draft-aboba-pppext-key-problem-06.txt
EAP Working Group Bernard Aboba
INTERNET-DRAFT Dan Simon
Category: Informational Microsoft
<draft-aboba-pppext-key-problem-07.txt>
9 August 2003
EAP Key Management Framework
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.
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.
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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.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document provides a framework for EAP key management, including a
statement of applicability and guidelines for generation, transport and
usage of EAP keying material. Algorithms for key derivation or
mechanisms for key transport are not specified in this document.
Rather, this document provides a framework within which algorithms and
transport mechanisms can be discussed and evaluated.
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Table of Contents
1. Introduction .......................................... 3
1.1 Requirements Language ........................... 3
1.2 Terminology ..................................... 3
1.3 Conversation Overview ........................... 5
2. EAP key hierarchy ..................................... 9
2.1 EAP Invariants .................................. 9
2.2 Key Hierarchy ................................... 11
2.3 Exchanges ....................................... 14
2.4 Security Relationships .......................... 18
3. Security associations ................................. 19
3.1 EAP SA .......................................... 19
3.2 AAA-Key SA ...................................... 20
3.3 Unicast Secure Association SA ................... 21
3.4 Multicast Secure Association SA ................. 22
3.5 Key Naming ...................................... 23
4. Threat model .......................................... 25
4.1 Security Assumptions ............................ 25
4.2 Security Requirements ........................... 26
5. IANA Considerations ................................... 32
6. Security Considerations ............................... 32
6.1 Key Strength .................................... 32
6.2 Key Wrap ........................................ 32
6.3 Man-in-the-middle Attacks ....................... 33
6.4 Impersonation ................................... 33
7. References ............................................ 34
7.1 Normative References ............................ 34
7.2 Informative References .......................... 35
Appendix A - Ciphersuite Keying Requirements ................. 39
Appendix B - TEK Hierarchy ................................... 40
Appendix C - MSK and EMSK Hierarchy .......................... 41
Appendix D - TSK Derivation .................................. 43
Appendix E - AAA-Key Key Derivation .......................... 44
Acknowledgments .............................................. 44
Author's Addresses ........................................... 44
Intellectual Property Statement .............................. 45
Full Copyright Statement ..................................... 45
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1. Introduction
The Extensible Authentication Protocol (EAP), defined in [RFC2284bis],
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 [IEEE8021X].
This document provides a framework for the generation, transport and
usage of keying material generated by EAP authentication algorithms,
known as "methods". Since in EAP keying material is generated by EAP
methods, transported by AAA protocols, transformed into session keys by
secure association protocols and used by link layer ciphersuites, it is
necessary to describe each of these elements and provide a system-level
security analysis.
The document is organized as follows:
Section 1 provides an introduction and defines terminology.
Section 2 describes the EAP key hierarchy and exchanges.
Section 3 describes EAP security associations and key naming.
Section 4 describes the threat model and security requirements.
Section 5 describes the IANA considerations.
Section 6 describes security considerations.
Section 7 provides references.
Appendix A summarizes the keying requirements for link layer ciphersuites.
Appendix B provides an example transient EAP key (TEK) hierarchy.
Appendix C provides an example MSK and EMSK hierarchy.
Appendix D provides an example Transient Session Key (TSK) derivation.
Appendix E describes AAA-Key derivation, including fast handoff.
1.1. Requirements Language
In this document, several words are used to signify the requirements of
the specification. These words are often capitalized. 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. Where no
backend authentication server is present, the authenticator acts as
the EAP server, terminating the EAP conversation with the peer.
Where a backend authentication server is present, the authenticator
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may act as a pass-through for one or more authentication methods
and for non-local users. This terminology is also used in
[IEEE8021X], and has the same meaning in this document.
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 [IEEE8021X].
AAA-Token
The package within which keying material and one or more attributes
is transported between the backend authentication server and the
authenticator. The attributes provide the authenticator with usage
context and key names suitable to bind the key to the appropriate
context. For example, attributes might include the peer layer 2
address (Calling-Station-Id), the authenticator layer 2 address
(Called-Station-Id) and IP address (NAS-IP-Address), the key name,
etc. The format and wrapping of the AAA-Token, which is intended
to be accessible only to the backend authentication server and
authenticator, is defined by the AAA protocol.
Cryptographic binding
The demonstration of the EAP peer to the EAP server that a single
entity has acted as the EAP peer for all methods executed within a
sequence or tunnel. Binding MAY also imply that the EAP server
demonstrates to the peer that a single entity has acted as the EAP
server for all methods executed within a sequence or tunnel. If
executed correctly, binding serves to mitigate man-in-the-middle
vulnerabilities.
Cryptographic separation
Two keys (x and y) are "cryptographically separate" if an adversary
that knows all messages exchanged in the protocol cannot compute x
from y or y from x without "breaking" some cryptographic
assumption. In particular, this definition allows that the
adversary has the knowledge of all nonces sent in cleartext as well
as all predictable counter values used in the protocol. Breaking a
cryptographic assumption would typically require inverting a one-
way function or predicting the outcome of a cryptographic pseudo-
random number generator without knowledge of the secret state. In
other words, if the keys are cryptographically separate, there is
no shortcut to compute x from y or y from x, but the work an
adversary must do to perform this computation is equivalent to
performing exhaustive search for the secret state value.
EAP server
The entity which terminates EAP authentication with the peer is
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known as the EAP server. Where pass-through is supported, the
backend authentication server functions as the EAP server; where
authentication occurs locally, the EAP server is the authenticator.
Key derivation
This refers to the ability of the EAP method to export a
ciphersuite-independent Master Session Key (MSK), Extended Master
Session Key (EMSK), and (optionally) an Initialization Vector (IV).
Key strength
If the effective key strength is N bits, the best currently known
methods to recover the key (with non-negligible probability)
require an effort comparable to 2^N operations of a typical block
cipher.
Mutual authentication
This refers to an EAP method in which, within an interlocked
exchange, the authenticator authenticates the peer and the peer
authenticates the authenticator. Two one-way conversations,
running in opposite directions do not provide mutual authentication
as defined here.
peer The end of the link that responds to the authenticator. In
[IEEE8021X], this end is known as the Supplicant.
1.3. Conversation Overview
Where EAP key derivation is supported, EAP authentication is typically a
component of a three phase exchange:
Discovery phase (phase 0)
EAP authentication, key derivation and transport (phase 1)
Unicast and multicast secure association establishment (phase 2)
In the discovery phase (phase 0), the EAP peers locate each other and
discover their capabilities. This can include an EAP peer locating
an authenticator suitable for access to a particular network, or
it could involve an EAP peer locating an authenticator behind
a bridge with which it desires to establish a secure association. Typically
the discovery phase takes place between the EAP peer and authenticator.
Once the EAP peer and authenticator discover each other, they
authenticate using EAP (phase 1a). EAP enables deployment of new
authentication methods without requiring development of new code on the
authenticator. While the authenticator may implement some EAP methods
locally and use those methods to authenticate local users, it may at the
same time act as a pass-through for other users and methods, forwarding
EAP packets back and forth between the backend authentication server and
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the peer.
As described in Section 2, in addition to supporting authentication,
EAP methods may also support derivation of keying material for purposes
including protection of the EAP conversation and subsequent data
exchanges. EAP key derivation takes place between the EAP peer and EAP
server, and methods supporting key derivation MUST also support mutual
authentication. Where an authenticator server is present, it acts as
the EAP server and transports derived keying material (known as the AAA-
Key) to the authenticator (phase 1b). In 802.11 terminology, the first
32 octets of the AAA-Key is known as the Pairwise Master Key (PMK).
While EAP methods may be based on key management protocols, EAP itself
is not a key management protocol. Thus, while EAP may provide for
mutual authentication and derivation of keying material, it does not
provide for the derivation or naming of transient session keys, the
selection of traffic modes such as transport or tunnel mode, the secure
negotiation of capabilities such as ciphersuites or filters, or support
for key activation. As a result, where EAP is used for key derivation,
a secure association protocol (phase 2) should be provided, supporting
the creation and deletion of unicast (phase 2a) and multicast (phase 2b)
security associations used for the protection of data.
The phases and the relationship between the parties is illustrated
below.
EAP peer Authenticator Auth. Server
-------- ------------- ------------
<----------------------------->
Discovery (phase 0)
<-----------------------------><------------------------------->
EAP auth (phase 1a) AAA pass-through (optional)
<--------------------------------
AAA-Key transport (phase 1b)
<----------------------------->
Unicast Secure association (phase 2a)
<----------------------------->
Multicast Secure association (phase 2b)
Figure 1 - Conversation Overview
1.3.1. Discovery Phase
In the peer discovery exchange (phase 0), the EAP peer and
authenticator locate each other and discover each other's capabilities.
For example, PPPoE [RFC2516] includes support for a Discovery Stage to
allow a peer to identify the Ethernet MAC address of one or more
authenticators and establish a PPPoE SESSION_ID. In [IEEE 80211], the
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EAP peer (also known as the Station or STA) discovers the authenticator
(Access Point or AP) and determines its capabilities using Beacon or
Probe Request/Response frames. Since device discovery is handled
outside of EAP, there is no need to provide this functionality within
EAP.
Device discovery can occur manually or automatically. In EAP
implementations running over PPP, the EAP peer might be configured with
a phone book providing phone numbers of authenticators and associated
capabilities such as supported rates, authentication protocols or
ciphersuites.
Since device discovery can occur prior to authentication and key
derivation, it may not be possible for the discovery phase to be
protected using keying material derived during an authentication
exchange. As a result, device discovery protocols may be insecure,
leaving them vulnerable to spoofing unless the discovered parameters can
subsequently be securely verified.
1.3.2. Authentication Phase
Once the EAP peer and authenticator discover each other, they
authenticate using EAP (phase 1a). Typically, the peer desires access
to the network, and the authenticators are Network Access Servers
(NASes) providing that access. In such a situation, access to the
network can be provided by any authenticator attaching to the desired
network, and the EAP peer is typically willing to send data traffic
through any authenticator that can demonstrate that it is authorized to
provide access to the desired network.
An EAP authenticator may handle the authentication locally, or it may
act as a pass-through to a backend authentication server. In the latter
case the EAP exchange occurs between the EAP peer and a backend
authenticator server, with the authenticator forwarding EAP packets
between the two. The entity which terminates EAP authentication with
the peer is known as the EAP server. Where pass-through is supported,
the backend authentication server functions as the EAP server; where
authentication occurs locally, the EAP server is the authenticator.
Where a backend authentication server is present, at the successful
completion of an authentication exchange, the AAA-Key is transported to
the authenticator (phase 1b).
EAP may also be used when it is desired for two network devices (e.g.
two switches or routers) to authenticate each other, or where two peers
desire to authenticate each other and set up a secure association
suitable for protecting data traffic.
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EAP supports either one-way authentication (in which the peer
authenticates to the EAP server), or mutual authentication (in which the
peer and EAP server mutually authenticate). In either case, it can be
assumed that the parties do not utilize the link to exchange data
traffic unless their authentication requirements have been met. For
example, a peer completing mutual authentication with an EAP server will
not send data traffic over the link until the EAP server has
authenticated successfully to the peer, and a secure association has
been negotiated.
Since EAP is a peer-to-peer protocol, an independent and simultaneous
authentication may take place in the reverse direction. Both peers may
act as authenticators and authenticatees at the same time.
Successful completion of EAP authentication and key derivation by an EAP
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). As a result, EAP may be used for "pre-
authentication" in situations where it is necessary to pre-establish EAP
security associations in order to decrease handoff or roaming latency.
1.3.3. Secure Association Phase
While EAP methods may be based on key management protocols, EAP itself
is not a protocol for negotiation of security associations. While EAP
methods supporting key derivation provide for mutual authentication and
creation of EAP (phase 1) security associations, in order to preserve
media independence, they typically do not support generation of
transient session keys or negotiation of ciphersuites used in the
protection of data. As a result, where EAP is used for key derivation,
a secure association protocol (phase 2) should be provided, supporting
the creation and deletion of phase 2 security associations used for the
protection of data.
The secure association phase (phase 2) always occurs after the
completion of EAP authentication (phase 1a) and key transport (phase
1b), and typically supports the following features:
[1] The secure negotiation of capabilities. This includes usage modes,
session parameters and ciphersuites, and required filters,
including confirmation of the capabilities discovered during phase
0. By securely negotiating session parameters, the secure
association protocol protects against spoofing during the discovery
phase and ensures that the peer and authenticator are in agreement
about how data is to be secured.
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[2] Generation of fresh transient session keys. This is typically
accomplished via the exchange of nonces within the secure
association protocol, and includes generation of both unicast
(phase 2a) and multicast (phase 2b) session keys. While multicast
traffic may only pass in one direction in certain cases (such as in
IEEE 802.11 infrastructure mode, where only the Access Point sends
multicast traffic), in other cases (such as IEEE 802.11 adhoc
mode), both endpoints may send multicast traffic. By not using the
AAA-Key directly to protect data, the secure association protocol
protects against compromise of the AAA-Key, and by guaranteeing the
freshness of transient session key, assures that session keys are
not reused.
[3] Key activation and deletion.
[4] Mutual proof of possession of the keying material generated during
EAP authentication (phase 1). By requiring a mutual proof of
possession of the AAA-Key, the secure association protocol
demonstrates that both the EAP peer and authenticator have been
authenticated and authorized by the AAA server. Note that mutual
proof of possession is not the same thing as mutual authentication.
For example, as a result of a secure association protocol exchange,
the EAP peer may not be able to confirm the identity of the
authenticator.
2. EAP Key Hierarchy
2.1. EAP Invariants
The EAP key management framework assumes that certain basic
characteristics, known as the "EAP Invariants" hold true for all
implementations of EAP. These include:
Media independence
Method independence
Ciphersuite independence
2.1.1. Media Independence
As described in [RFC2284bis], EAP authentication can run over multiple
lower layers, including PPP [RFC1661] and IEEE 802 wired networks
[IEEE8021X]. Use with IEEE 802.11 wireless LANs is also contemplated
[IEEE80211i]. Since EAP methods cannot be assumed to have knowledge of
the lower layer over which they are transported, EAP methods can
function on any lower layer meeting the criteria outlined in
[RFC2284bis], Section 3.1.
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2.1.2. Method Independence
Supporting pass-through of authentication to the backend authentication
server enables the authenticator to support any authentication method
implemented on the backend authentication server and peer, not just
locally implemented methods.
This implies that the authenticator need not implement code for each EAP
method required by authenticating peers. In fact, the authenticator is
not required to implement any EAP methods at all, nor cannot it be
assumed to implement code specific to 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 [RFC2284bis] 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 authentication.
2.1.3. Ciphersuite Independence
While EAP methods may negotiate the ciphersuite used in protection of
the EAP conversation, the ciphersuite used for the protection of data is
negotiated within the secure association protocol, out-of-band of EAP.
The backend authentication server is not a party to this negotiation nor
is it an intermediary in the data flow between the EAP peer and
authenticator. The backend authentication server may not even have
knowledge of the ciphersuites implemented by the peer and authenticator,
or be aware of the ciphersuite negotiated between them, and therefore
does not implement ciphersuite-specific code.
Since ciphersuite negotiation occurs in the secure association protocol,
not in EAP, ciphersuite-specific key generation, if implemented within
an EAP method, would potentially conflict with the transient session key
derivation occurring in the secure association protocol. As a result,
EAP methods generate keying material that is ciphersuite-independent.
Additional advantages of ciphersuite-independence include:
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.
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EAP method complexity
Requiring each EAP method to include ciphersuite-specific code for
transient session key derivation would increase the complexity of
each EAP method and would result in duplicated effort.
Ciphersuite negotiation
In practice, an EAP method may not have knowledge of the
ciphersuite that has been negotiated between the peer and
authenticator. In PPP, ciphersuite negotiation occurs in the
Encryption Control Protocol (ECP) [RFC1968]. Since ECP negotiation
occurs after authentication, unless an EAP method is utilized that
supports ciphersuite negotiation, the peer, authenticator and
backend authentication server may not be able to anticipate the
negotiated ciphersuite and therefore this information cannot be
provided to the EAP method. Since ciphersuite negotiation is
assumed to occur out-of-band, there is no need for ciphersuite
negotiation within EAP.
2.2. Key Hierarchy
The EAP keying hierarchy, illustrated in Figure 2, makes use of the
following types of keys:
EAP Master key (MK)
A key derived between the EAP client and server during the EAP
authentication process, and that is kept local to the EAP method
and not exported or made available to a third party. Since the MK
is a residue of a successful EAP authentication exchange, it is
possible to shorten future EAP exchanges between an EAP peer and
server by providing proof of MK possesion, a technique known as
"fast resume".
Master Session Key (MSK)
Keying material (at least 64 octets) that is derived between the
EAP client and server and exported by the EAP method. Whenever a
full EAP authentication is performed (not fast handoff), the MSK is
chosen as the AAA-Key (see Appendix E for details).
AAA-Key
Where a backend authentication server is present, acting as an EAP
server, keying material known as the AAA-Key is transported from
the authentication server to the authenticator wrapped within the
AAA-Token. The AAA-Key is used by the EAP peer and authenticator
in the derivation of Transient Session Keys (TSKs) for the
ciphersuite negotiated between the EAP peer and authenticator. As
a result, the AAA-Key is typically known by all parties in the EAP
exchange: the peer, authenticator and the authentication server (if
present). AAA-Key derivation is discussed in Appendix E.
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Extended Master Session Key (EMSK)
Additional keying material (64 octets) derived between the EAP
client and server that is exported by the EAP method. Unlike the
MSK which is transported from the authentication server to the
authenticator, the EMSK is known only to the EAP peer and server
and is not provided to a third party. The EMSK therefore is not
transported by the backend authentication server to the
authenticator, although quantities derived from it may be used as
the AAA-Key in situations in which EAP authentication is bypassed
(e.g. fast handoff).
Currently the EMSK is reserved for future uses that are not defined
yet. For example, it could be used to derive additional keying
material for purposes such as fast handoff, man-in-the-middle
vulnerability protection, etc.
Initialization Vector (IV)
A quantity of at least 64 octets, suitable for use in an
initialization vector field, that is derived between the EAP client
and server. Since the IV is a known value in methods such as EAP-
TLS [RFC2716] it cannot be used in computation of any quantity that
needs to remain secret, and is not used with any known ciphersuite.
As a result, its use has been deprecated and EAP methods are not
required to generate it.
Pairwise Master Key (PMK)
The AAA-Key is divided into two halves, the "Peer to Authenticator
Encryption Key" (Enc-RECV-Key) and "Authenticator to Peer
Encryption Key" (Enc-SEND-Key) (reception is defined from the point
of view of the authenticator). Within [IEEE80211i] Octets 0-31 of
the AAA-Key (Enc-RECV-Key) are known as the Pairwise Master Key
(PMK). [IEEE80211i] ciphersuites derive their Transient Session
Keys (TSKs) solely from the PMK, whereas the WEP ciphersuite, when
used with [IEEE8021X], as noted in [RFC3580], derives its TSKs from
both halves of the AAA-Key, the Enc-RECV-Key and the Enc-SEND-Key.
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. 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 C.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | ^
| EAP Method | |
| | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | | | |
| | EAP Method Key | | |
| | Derivation | | |
| | | | Local to |
| | | | EAP |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Method |
| | | | | |
| | |
| | | | | |
| | |
| V | | | |
| +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ | |
| | TEK | | MSK | |EMSK | |IV | | |
| |Derivation | |Derivation | |Derivation | |Derivation | | |
| +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ | |
| | | | | |
| | | | | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | | ^
| | | |
| MSK (64B) | EMSK (64B) | IV (64B) |
| | | Exported|
| | | by |
V V V EAP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+ Method|
| AAA Key Derivation, | | Known | |
| Naming & Binding | |(Not Secret) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+ V
| ---+
| Transported |
| AAA-Key by AAA |
| Protocol |
V V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | ^
| TSK | Ciphersuite |
| Derivation | Specific |
| | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
Figure 2 - EAP Key Hierarchy
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Transient Session Keys (TSKs)
Session keys used to protect data which are appropriate for the
ciphersuite negotiated between the EAP peer and authenticator. The
TSKs are derived from the keying material included in the AAA-Token
via the secure association protocol. In the case of IEEE 802.11,
the role of the secure association protocol is handled by the 4-way
handshake and group key derivation. An example TSK derivation is
provided in Appendix D.
2.3. Exchanges
EAP supports both a two party exchange between an EAP peer and an
authenticator, as well as a three party exchange between an EAP peer, an
authenticator and an EAP server.
Figure 3 illustrates the two party exchange. Here EAP is spoken between
the peer and authenticator, encapsulated within a lower layer protocol,
such as PPP, defined in [RFC1661] or IEEE 802, defined in [IEEE802].
Since the authenticator acts as an endpoint of the EAP conversation
rather than a pass-through, EAP methods are implemented on the
authenticator as well as the peer. If the EAP method negotiated between
the EAP peer and authenticator supports mutual authentication and key
derivation, the EAP Master Session Key (MSK) and Extended Master Session
Key (EMSK) are derived on the EAP peer and authenticator and exported by
the EAP method.
Where no backend authentication server is present, the MSK and EMSK are
known only to the peer and authenticator and neither is transported to a
third party. As demonstrated in [RoamCERT], despite the absence of a
backend authentication server, such exchanges can support roaming
between providers; it is even possible to support fast handoff without
re-authentication. However, this is typically only possible where both
the EAP peer and authenticator support certificate-based authentication,
or where the user base is sufficiently small that EAP authentication can
occur locally.
In order to protect the EAP conversation, the EAP method may negotiate a
ciphersuite and derive Transient EAP Keys (TEKs) to provide keys for
that ciphersuite in order to protect some or all of the EAP exchange.
The TEKs are stored locally within the EAP method and are not exported.
Once EAP mutual authentication completes and is successful, the secure
association protocol is run between the peer and authenticator. This
derives fresh transient session keys (TSKs), provides for the secure
negotiation of the ciphersuite used to protect data, and supports mutual
proof of possession of the AAA-Key.
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+-+-+-+-+-+ +-+-+-+-+-+
| | | |
| | | |
| Cipher- | | Cipher- |
| Suite | | Suite |
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
^ ^
| |
| |
| |
V V
+-+-+-+-+-+ +-+-+-+-+-+
| | | |
| |===============| |
| |EAP, TEK Deriv.|Authenti-|
| |<------------->| cator |
| | | |
| | Secure Assoc. | |
| peer |<------------->| (EAP |
| |===============| server) |
| | Link layer | |
| | (PPP,IEEE802) | |
| | | |
|MSK,EMSK | |MSK,EMSK |
| (TSKs) | | (TSKs) |
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
^ ^
| |
| MSK, EMSK | MSK, EMSK
| |
| |
+-+-+-+-+-+ +-+-+-+-+-+
| | | |
| EAP | | EAP |
| Method | | Method |
| | | |
|(MK,TEKs)| |(MK,TEKs)|
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
Figure 3 - Relationship between EAP peer and authenticator
(acting as an EAP server), where no backend
authentication server is present.
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+-+-+-+-+-+ +-+-+-+-+-+
| | | |
| | | |
| Cipher- | | Cipher- |
| Suite | | Suite |
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
^ ^
| |
| |
| |
V V
+-+-+-+-+-+ +-+-+-+-+-+ +-+-+-+-+-+
| |===============| |========| |
| |EAP, TEK Deriv.| | | |
| |<-------------------------------->| backend |
| | | | | |
| | Secure Assoc. | | AAA-Key| |
| peer |<------------->|Authenti-|<-------| auth |
| |===============| cator |========| server |
| | Link Layer | | AAA | (EAP |
| | (PPP,IEEE 802)| |Protocol| server) |
| | | | | |
|MSK,EMSK | | MSK | |MSK,EMSK |
| (TSKs) | | (TSKs) | | |
| | | | | |
+-+-+-+-+-+ +-+-+-+-+-+ +-+-+-+-+-+
^ ^
| |
| MSK, EMSK | MSK, EMSK
| |
| |
+-+-+-+-+-+ +-+-+-+-+-+
| | | |
| EAP | | EAP |
| Method | | Method |
| | | |
|(MK,TEKs)| |(MK,TEKs)|
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
Figure 4 - Pass-through relationship between EAP peer,
authenticator and backend authentication server.
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Where these conditions cannot be met, a backend authentication server is
typically required. In this exchange, as described in [RFC3579], the
authenticator acts as a pass-through between the EAP peer and a backend
authentication server. In this model, the authenticator delegates the
access control decision to the backend authentication server, which acts
as a Key Distribution Center (KDC), supplying keying material to both
the EAP peer and authenticator.
Figure 4 illustrates the case where the authenticator acts as a pass-
through. Here EAP is spoken between the peer and authenticator as
before. The authenticator then encapsulates EAP packets within a AAA
protocol such as RADIUS [RFC3579] or Diameter [DiamEAP], and forwards
packets to and from the backend authentication server, which acts as the
EAP server. Since the authenticator acts as a pass-through, EAP methods
(as well as the derived EAP Master Key, and TEKs) reside only on the
peer and backend authentication server.
On completion of a successful authentication, EAP methods on the EAP
peer and EAP server export the Master Session Key (MSK) and Extended
Master Session Key (EMSK). The backend authentication server then sends
a message to the authenticator indicating that authentication has been
successful, providing the AAA-Key within a protected package known as
the AAA-Token. Along with the keying material, the AAA-Token contains
attributes naming the enclosed keys and providing context.
The MSK and EMSK are used to derive the AAA-Key and key name which are
enclosed within the AAA-Token, transported to the NAS by the AAA
server, and used within the secure association protocol for derivation
of Transient Session Keys (TSKs) required for the negotiated
ciphersuite. The TSKs are known only to the peer and authenticator.
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2.4. Security Relationships
Figure 5 illustrates the relationship between the peer, authenticator
and backend authentication server. As noted in the figure, each party
in the exchange mutually authenticates with each of the other parties,
and derives a unique key. All parties in the diagram have access to the
AAA-Key.
EAP peer
/\
/ \
Protocol: EAP / \ Protocol: Secure Association
Auth: Mutual / \ Auth: Mutual
Unique keys: MK, / \ Unique keys: TSKs
TEKs,EMSK / \
/ \
Auth. server +--------------+ Authenticator
Protocol: AAA
Auth: Mutual
Unique key: AAA session key
Figure 5: Three-party EAP key distribution
The EAP peer and backend authentication server mutually authenticate via
the EAP method, and derive the MK, TEKs and EMSK which are known only to
them. The TEKs are used to protect some or all of the EAP conversation
between the peer and authenticator, so as to guard against modification
or insertion of EAP packets by an attacker. The degree of protection
afforded by the TEKs is determined by the EAP method; some methods may
protect the entire EAP packet, including the EAP header, while other
methods may only protect the contents of the Type-Data field, defined in
[RFC2284bis].
Since EAP is spoken only between the EAP peer and server, if a backend
authentication server is present then the EAP conversation does not
provide mutual authentication between the peer and authenticator, only
between the EAP peer and EAP server (backend authentication server). As
a result, mutual authentication between the peer and authenticator only
occurs where a secure association protocol is used, such the unicast and
group key derivation handshake supported in [IEEE80211i]. This means
that absent use of a secure association protocol, from the point of view
of the peer, EAP mutual authentication only proves that the
authenticator is trusted by the backend authentication server; the
identity of the authenticator is not confirmed.
Utilizing the AAA protocol, the authenticator and backend authentication
server mutually authenticate and derive session keys known only to them,
used to provide per-packet integrity and replay protection,
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authentication and confidentiality. The MSK is distributed by the
backend authentication server to the authenticator over this channel,
bound to attributes constraining its usage, as part of the AAA-Token.
The binding of attributes to the MSK within a protected package is
important so the authenticator receiving the AAA-Token can determine
that it has not been compromised, and that the keying material has not
been replayed, or mis-directed in some way.
3. Security Associations
The EAP model has four types of security associations (SAs):
[1] An EAP SA. This is an SA between the EAP peer and the EAP server,
created as the result of an EAP authentication exchange (phase 1a).
This is a bi-directional SA; that is, both parties use the
information in the SA for both sending and receiving.
[2] A AAA-Key SA, known in [IEEE80211i] as a PMK SA. This is a bi-
directional SA between the EAP peer and authenticator. The keying
material for the AAA-Key SA (known as the AAA-Key) is derived on
the EAP peer and server, and transported by the EAP server to the
authenticator (phase 1b). The choice of keying material is
proposed by the EAP peer and confirmed by the EAP authenticator
during the unicast secure association protocol (phase 2a).
[3] A unicast secure association SA. This is a bi-directional SA
created as the result of a successful unicast secure association
exchange (phase 2a). A unicast secure association SA is bound to a
single EAP SA and a single AAA-Key SA.
[4] A multicast secure association SA (phase 2b). This SA is created
as the result of a successful multicast secure association
exchange. This SA may be uni-directional (e.g. 802.11 group-key
exchange) or bi-directional depending on the design of the
multicast secure association protocol, and can be created either
from the unicast secure association SA (phase 2a) or directly as
the result of a multicast secure association exchange (phase 2b).
3.1. EAP SA
An EAP SA includes:
the EAP peer and EAP server identities
the EAP method type
the EAP peer and server nonces
the Transient EAP Keys (TEKs)
the Master Session Key (MSK)
the Extended Master Session Key (EMSK)
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The EAP SA is not explicitly bound to a particular port on the EAP peer.
An EAP peer with multiple ports may create an EAP SA on one port and
then choose to use that SA to subsequently create a phase 2 SA on
another port.
It cannot be assumed that the EAP SA expires after the EAP
authentication and key derivation is complete. Some methods may be
support "fast resume" by caching EAP SA state on the EAP peer and
server.
EAP does not support SA lifetime negotiation or an SA "delete"
operation, although some EAP methods may support this. Either the EAP
peer or EAP server may delete an EAP SA at any time, and methods which
allow an EAP SA to persist need to permit the EAP peer and server to
recognize when they have gotten out of sync with respect to the EAP SA
state.
For example, EAP TLS [RFC 2716] supports "fast resume" (TLS session
resumption), which assumes that both the EAP peer and server cache EAP
master keys (the TLS master secret). An EAP peer attempting a fast
resume provides the session-id identifying the session that it wishes to
resume. If the EAP server retains the master key corresponding to this
session in its cache, then the "fast resume" can proceed; otherwise a
full TLS exchange ensues.
An EAP peer may negotiate EAP SAs with one or more EAP servers as the
result of pre-authentication or AAA load balancing and failover effects.
For example, an EAP peer may pre-authenticate to one or more EAP
servers, or may be directed to more than one EAP server as the result of
an authentication server becoming unreachable. In general, EAP servers
cannot be assumed to be synchronized with respect to EAP SA state,
particularly since they may not exist within the same administrative
domain. Since an EAP SA is typically created prior to secure
association, the EAP SA is not bound to a particular target network.
3.2. AAA-Key SA
An AAA-Key SA includes:
the EAP peer (Calling-Station-Id) identifier
the EAP authenticator (Called-Station-Id) identifier
the AAA-Key
the AAA-Key maximum lifetime (if known)
the advertised peer and authenticator capabilities
The AAA-Key SA is created as the result of the transport of the AAA-
Token from the authentication server to the NAS/authenticator. The AAA-
Key SA is distinct from the EAP SA in that it is bound to the EAP peer
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and authenticator ports. This binding occurs during the unicast secure
association protocol (phase 2a) when the EAP peer and authenticator
prove possession of the AAA-Key, which is transported from the
authentication server to the NAS/EAP authenticator after completion of
EAP authentication.
Since the AAA-Key SA is bound to a particular authenticator port, a
NAS/authenticator that operates on a shared use network will share the
AAA-Key SA between multiple virtual NAS devices unless a separate port
(as identified by the Called-Station-Id) is used for each virtual NAS.
This would represent a security vulnerability.
In fast handoff, a single EAP SA may be used to establish multiple AAA-
Key SAs (see Appendix E for details). Although a AAA-Key SA may not
persist longer than the maximum SA lifetime negotiated for an EAP SA
(for methods that support such a negotiation), if an EAP SA is deleted
by an EAP peer or authenticator, this does not necessarily imply
deletion of the child AAA-Key SA. For example, fast handoff keying
material provided by an authentication server may continue to be cached
by NASes/authenticators after the corresponding EAP SA has been deleted
by the authentication server and/or peer.
3.3. Unicast Secure Association SA
The unicast secure association SA includes:
the EAP peer (Calling-Station-Id) identifier
the EAP authenticator (Called-Station-Id) identifier
the unicast Transient Session Keys (TSKs)
the unicast secure association peer nonce
the unicast secure association authenticator nonce
the negotiated unicast capabilities and unicast ciphersuite.
During the phase 2a exchange, the EAP peer and authenticator demonstrate
mutual possession of the AAA-Key derived and transported in phase 1;
securely negotiate the session capabilities (including unicast
ciphersuites), and derive fresh unicast transient session keys. The
AAA-Key SA (phase 1b) is therefore used to create the unicast secure
association SA (phase 2a), and in the process the phase 2a unicast
secure association SA is bound to ports on the EAP peer and
authenticator. However in order for a phase 2a security association to
be established, it is not necessary for the phase 1a exchange to be
rerun each time. This enables the EAP exchange to be bypassed when fast
handoff support is desired.
Since both peer and authenticator nonces are used in the creation of the
unicast secure association SA, the transient session keys (TSKs) are
guaranteed to be fresh, even if the AAA-Key is not. As a result one or
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more unicast secure association SAs (phase 2a) may be derived from a
single AAA-Key SA (phase 1b). The phase 2a security associations may
utilize the same security parameters (e.g. mode, ciphersuite, etc.) or
they may utilize different parameters.
A unicast secure association SA (phase 2a) may not persist longer than
the maximum lifetime of its parent AAA-Key SA (if known). However, the
deletion of a parent EAP or AAA-Key SA does not necessarily imply
deletion of the corresponding unicast secure association SA. Similarly,
the deletion of a unicast secure association protocol SA does not imply
the deletion of the parent AAA-key SA or EAP SA. However, the failure
to mutually prove possession of the AAA-Key during the unicast secure
association protocol exchange (phase 2a) is grounds for removal of a
AAA-Key SA by both parties.
An EAP peer may be able to negotiate multiple phase 2a SAs with a single
EAP authenticator, or may be able to maintain multiple phase 2a SAs with
multiple authenticators, based on a single EAP SA derived in phase 1a.
For example, during a re-key of the secure association protocol SA, it
is possible for two phase 2a SAs to exist during the period between when
the new phase 2a SA parameters (such as the TSKs) are calculated and
when they are installed. Except where explicitly specified by the
semantics of the unicast secure association protocol, it should not be
assumed that the installation of a new phase 2a SA necessarily implies
deletion of the old phase 2a SA.
On some media (e.g. 802.11) a port on an EAP peer may only establish
phase 2a and 2b SAs with a single port of an authenticator within a
given Local Area Network (LAN). This implies that the successful
negotiation of phase 2a and/or 2b SAs between an EAP peer port and a new
authentiator port within a given LAN implies the deletion of existing
phase 2a and 2b SAs with authenticators offering access to that Local
Area Network (LAN). However, since a given IEEE 820.11 SSID may be
comprised of multiple LANs, this does not imply an implicit binding of
phase 2a and 2b SAs to an SSID.
3.4. Multicast Secure Association SA
The multicast secure association SA includes:
the multicast Transient Session Keys
the direction vector (for a uni-directional SA)
the negotiated multicast capabilities and multicast ciphersuite
It is possible for more than one multicast secure association SA to be
derived from a single unicast secure association SA. However, a
multicast secure association SA is bound to a single EAP SA and a single
AAA-Key SA.
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During a re-key of the multicast secure association protocol SA, it is
possible for two phase 2b SAs to exist during the period between when
the new phase 2b SA parameters (such as the multicast TSKs) are
calculated and when they are installed. Except where explicitly
specified by the semantics of the multicast secure association protocol,
it should not be assumed that the installation of a new phase 2b SA
necessarily implies deletion of the old phase 2b SA.
A multicast secure association SA (phase 2b) may not persist longer than
the maximum lifetime of its parent AAA-Key or unicast secure association
SA. However, the deletion of a parent EAP, AAA-Key or unicast secure
association SA does not necessarily imply deletion of the corresponding
multicast secure association SA. For example, a unicast secure
association SA may be rekeyed without implying a rekey of the multicast
secure association SA.
Similarly, the deletion of a multicast secure association protocol SA
does not imply the deletion of the parent EAP, AAA-Key or unicast secure
association SA. However, the failure to mutually prove possession of
the AAA-Key during the unicast secure association protocol exchange
(phase 2a) is grounds for removal of the AAA-Key, unicast secure
association and multicast secure association SAs.
3.5. Key Naming
In order to support the correct processing of phase 2 security
associations, the secure association (phase 2) protocol supports 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. Explicit creation and
deletion operations are also typically supported so that establishment
and re-establishment of transient session keys can be synchronized
between the parties.
In order to securely bind the EAP security association (phase 1) to its
child phase 2 security association, the phase 2 secure association
protocol allows the EAP peer and authenticator to mutually prove
possession of the EAP (phase 1) keying material derived during the EAP
exchange (phase 1). In order to avoid confusion in the case where an
EAP peer has more than one EAP security association (phase 1) applicable
to establishment of a given phase 2 security association, the secure
association protocol (phase 2) supports key naming so that the
appropriate phase 1 keying material can be utilized by both parties in
the secure association protocol exchange.
As noted earlier, the discovery phase (phase 0) may be insecure so that
in order to prevent spoofing of discovery packets, the secure
association (phase 2) protocol should support the secure verification of
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discovered capabilities, including ciphersuites and other security
parameters. This is more scalable than attempting to configure the
supported capabilities on each peer and authenticator and more secure
than unprotected capabilities negotiation.
For example, a peer might be pre-configured with policy indicating the
ciphersuite to be used in communicating with a given authenticator.
Within PPP, the ciphersuite is negotiated within the Encryption Control
Protocol (ECP), after EAP authentication is completed. Within
[IEEE80211i], the AP ciphersuites are advertised in the Beacon and Probe
Responses, and are securely verified during a 4-way exchange after EAP
authentication has completed.
As part of the secure association protocol (phase 2), it is necessary to
bind the Transient Session Keys (TSKs) to the keying material provided
in the AAA-Token. This ensures that the EAP peer and authenticator are
both clear about what key to use to provide mutual proof of possession.
Keys within the EAP key hierarchy are named as follows:
EAP SA name
The EAP security association is negotiated between the EAP peer and
EAP server, and is uniquely named as follows <EAP peer name, EAP
server name, EAP Method Type, EAP peer nonce, EAP server nonce>.
Here the EAP peer name and EAP server name are the identifiers
securely exchanged within the EAP method. Since multiple EAP SAs
may exist between an EAP peer and EAP server, the EAP peer nonce
and EAP server nonce allow EAP SAs to be differentiated. The
inclusion of the Method Type in the EAP SA name ensures that each
EAP method has a distinct EAP SA space.
MK Name
The EAP Master Key, if supported by an EAP method, is named by the
concatenation of the EAP SA name and a method-specific session-id.
MSK Name
The MSK is named by the concatenation of the EAP SA name, "MSK" and
the authenticator name, since the MSK may only be bound to a single
authenticator. While the AAA server has several potentially unique
authenticator identifiers (including the NAS-Identifier, NAS-IP-
Address, and NAS-IPv6-Address attributes), only the Called-Station-
Id is guaranteed to be known by the EAP peer, EAP server and
authenticator and as a result, this is used as the NAS name.
EMSK Name
The EMSK is named by the concatenation of the EAP SA name and
"EMSK".
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4. Threat Model
4.1. Security Assumptions
The security properties of the EAP exchange are dependent on each leg of
the triangle: the selected EAP method, AAA protocol and the secure
association protocol.
Assuming that the AAA protocol provides protection against rogue
authenticators forging their identity, then the AAA-Token can be assumed
to be sent to the correct authenticator, and where it is wrapped
appropriately, it can be assumed to be immune to compromise by a
snooping attacker.
Where an untrusted AAA intermediary is present, the AAA-Token must not
be provided to the intermediary so as to avoid compromise of the AAA-
Token. This can be avoided by use of re-direct as defined in
[DiamBase].
When EAP is used for authentication on PPP or wired IEEE 802 networks,
it is typically assumed that the link is physically secure, so that an
attacker cannot gain access to the link, or insert a rogue device. EAP
methods defined in [RFC2284bis] reflect this usage model. These include
EAP MD5, as well as One-Time Password (OTP) and Generic Token Card.
These methods support one-way authentication (from EAP peer to
authenticator) but not mutual authentication or key derivation. As a
result, these methods do not bind the initial authentication and
subsequent data traffic, even when the the ciphersuite used to protect
data supports per-packet authentication and integrity protection. As a
result, EAP methods not supporting mutual authentication are vulnerable
to session hijacking as well as attacks by rogue devices.
On wireless networks such as IEEE 802.11 [IEEE80211], these attacks
become easy to mount, since any attacker within range can access the
wireless medium, or act as an access point. As a result, new
ciphersuites have been proposed for use with wireless LANs [IEEE80211i]
which provide per-packet authentication, integrity and replay
protection. In addition, mutual authentication and key derivation,
provided by methods such as EAP TLS [RFC2716] are required [IEEE80211i],
so as to address the threat of rogue devices, and provide keying
material to bind the initial authentication to subsequent data traffic.
If the selected EAP method does not support mutual authentication, then
the peer will be vulnerable to attack by rogue authenticators and
backend authentication servers. If the EAP method does not derive keys,
then TSKs will not be available for use with a negotiated ciphersuite,
and there will be no binding between the initial EAP authentication and
subsequent data traffic, leaving the session vulnerable to hijack.
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If the authenticator and backend authentication server do not mutually
authenticate, then the peer will be vulnerable to rogue backend
authentication servers, authenticators, or both. If there is no per-
packet authentication, integrity and replay protection between the
authenticator and backend authentication server, then an attacker can
spoof or modify packets in transit. If the backend authentication
server does not protect against authenticator masquerade, or provide the
proper binding of the AAA-Key to the session within the AAA-Token, then
one or more AAA-Keys may be sent to an unauthorized party, and an
attacker may be able to gain access to the network. If the AAA-Token is
provided to an untrusted AAA intermediary, then that intermediary may be
able to modify the AAA-Key, or the attributes associated with it, as
described in [RFC2607].
If the secure association protocol does not provide mutual proof of
possession of the AAA-Key material, then the peer will not have
assurance that it is connected to the correct authenticator, only that
the authenticator and backend authentication server share a trust
relationship (since AAA protocols support mutual authentication). This
distinction can become important when multiple authenticators receive
AAA-Keys from the backend authentication server, such as where fast
handoff is supported. If the TSK derivation does not provide for
protected ciphersuite and capabilities negotiation, then downgrade
attacks are possible.
4.2. Security Requirements
This section describes the security requirements for EAP methods, AAA
protocols, secure association protocols and Ciphersuites. These
requirements MUST be met by specifications requesting publication as an
RFC. Based on these requirements, the security properties of EAP
exchanges are analyzed.
4.2.1. EAP method requirements
Key usage
Key material exported by EAP methods MUST NOT be used directly to
protect data.
Mutual authentication
EAP Methods deriving keys MUST provide for mutual authentication
between the EAP peer and EAP Server.
State synchronization
EAP peers, authenticators and authentication servers MUST be
prepared for situations in which one or more of the parties have
discarded an EAP SA (phase 1), which is still valid on another
party.
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TEK derivation
Methods deriving keys MUST specify how Transient EAP Keys (TEKs)
are derived. TEKs MUST remain local to the EAP method and MUST NOT
be provided to third parties.
MSK and EMSK
EAP methods supporting key derivation MUST export two quantities of
at least 64 octets each, known as the Master Session Key (MSK), and
the Extended Master Session Key (EMSK).
Cryptographic separation
Methods supporting key derivation MUST demonstrate cryptographic
separation between the TEK, MSK and EMSK branches of the EAP key
hierarchy. Without violating a fundamental cryptographic
assumption (such as the non-invertibility of a one-way function) an
attacker recovering the TEKs, MSK or EMSK MUST NOT be able to
recover the other quantities with a level of effort less than brute
force. Since Transient Session Keys (TSKs) are derived from the
MSK, if branch independence holds, then it is also true that the
TSKs are cryptographically separate from the EMSK and TEKs.
EMSK reservation
While the EMSK is exported by the EAP method, its use is reserved,
and as a result it MUST remain known only to the EAP peer and
server and MUST NOT be provided to third parties. Since the EMSK is
the only keying material exported by an EAP method that is neither
provided to a third party nor a known quantity, it is attractive
for use in future applications such as fast handoff or man-in-the-
middle detection. Given its potential future uses, damage due to
EMSK compromise is second only in effect to compromise of the MK,
yielding an attacker the ability to access the network at will, and
to decrypt past and future data traffic.
TEK derivation
In order to establish a protected channel between the EAP peer and
server as part of the EAP exchange, a ciphersuite needs to be
negotiated and suitable keys need to be provided (known as the
transient EAP keys). The ciphersuite used to protect the EAP
exchange between the peer and server is distinct from the
ciphersuite negotiated between the peer and authenticator, used to
protect data. Where a protected channel is established within the
EAP method, the method specification MUST specify the mechanism by
which the EAP ciphersuite is negotiated, as well as the algorithms
for derivation of TEKs.
Ciphersuite Independence
Keying material exported by EAP methods MUST be independent of the
ciphersuite negotiated to protect data.
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Key Strength
The strength of Transient Session Keys (TSKs) and Transient EAP
Keys (TEKs) used to protect data is ultimately dependent on the
strength of keys generated by the EAP method. If EAP method does
not produce keying material of sufficient strength, then the TSKs
and TEKs may be subject to brute force attack. EAP methods
supporting key derivation MUST be capable of generating an MSK and
EMSK, each with an effective key strength of at least 128 bits.
More details on key strength are provided in Section 6.1.
Resilience against compromise
In order to protect against compromise of the AAA-Token, EAP
methods MUST demonstrate that compromise of the MSK or EMSK of a
given EAP security association does not enable compromise of
subsequent or prior MSKs or EMSKs derived between the EAP peer and
EAP server.
Freshness
In order to support key naming and assure freshness of TSKs even in
cases where one party may not have a high quality random number
generator, EAP methods generating keys MUST support a two-nonce
exchange in the derivation of the MSK and EMSK, using nonces of at
least 128-bits.
Known-good algorithms
The development and validation of key derivation algorithms is
difficult, and as a result EAP methods SHOULD reuse existing key
derivation algorithms, rather than inventing new ones. EAP methods
requesting publication as an RFC MUST provide citations to
literature justifying the security of the chosen algorithms. EAP
methods SHOULD utilize well established and analyzed mechanisms for
MSK, EMSK, TSK and TEK derivation.
4.2.2. AAA Protocol Requirements
AAA protocols suitable for use in transporting EAP MUST provide the
following facilities:
Security services
AAA protocols used for transport of EAP keying material MUST
implement and SHOULD use per-packet integrity and authentication,
replay protection and confidentiality. These requirements are met
by Diameter EAP [DiamEAP], as well as RADIUS over IPsec [RFC3579].
Session Keys
AAA protocols used for transport of EAP keying material MUST
implement and SHOULD use session keys, as in Diameter EAP
[DiamEAP] and RADIUS over IPsec [RFC3579], rather than using a
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static key, as originally defined in RADIUS [RFC2865].
Mutual authentication
AAA protocols used for transport of EAP keying material MUST
provide for mutual authentication between the authenticator and
backend authentication server. These requirements are met by
Diameter EAP [DiamEAP] as well as by RADIUS/EAP [RFC3579].
Forgery protection
AAA protocols used for transport of EAP keying material SHOULD
provide protection against rogue authenticators masquerading as
other authenticators. This can be accomplished, for example, by
requiring that AAA agents check the source address of packets
against the origin attributes (Origin-Host AVP in Diameter, NAS-IP-
Address, NAS-IPv6-Address, NAS-Identifier in RADIUS). For details,
see Section 4.3.7 of [RFC3579].
Key transport
Since EAP methods do not export Transient Session Keys (TSKs) in
order to maintain media and ciphersuite independence, the AAA
server MUST NOT transport TSKs from the backend authentication
server to authenticator.
Key transport specification
In order to enable backend authentication servers to provide keying
material to the authenticator in a well defined format, AAA
protocols suitable for use with EAP MUST define the format and
wrapping of the AAA-Token.
EMSK transport
Since the EMSK is a secret known only to the backend authentication
server and peer, the AAA-Token MUST NOT transport the EMSK from the
backend authentication server to the authenticator.
AAA-Token protection
To ensure against compromise, the AAA-Token MUST be integrity
protected, authenticated, replay protected and encrypted in
transit, using well-established cryptographic algorithms.
Session Keys
The AAA-Token SHOULD be protected with session keys as in Diameter
[DiamBASE] or RADIUS over IPsec [RFC3579] rather than static keys,
as in [RFC2548].
Key naming
In order to ensure against confusion between the appropriate keying
material to be used in a given secure association protocol
exchange, the AAA-Token SHOULD include explicit key names and
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context appropriate for informing the authenticator how the keying
material is to be used.
Key Compromise
Where untrusted intermediaries are present, the AAA-Token SHOULD
NOT be provided to the intermediaries. In Diameter, handling of
keys by intermediaries can be avoided using Redirect functionality
[DiamBASE].
4.2.3. Secure Association Protocol Requirements
The Secure Association Protocol supports the following:
Mutual proof of possession
The peer and authenticator MUST each demonstrate possession of the
keying material transported between the AAA server and
authenticator (AAA-Key).
Key Naming
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.
Creation and Deletion
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.
Integrity and Replay Protection
The Secure Association Protocol MUST support integrity and replay
protection of all messages.
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.
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Derivation of transient session keys
The secure association protocol negotiation MUST support derivation
of unicast and multicast transient session keys suitable for use
with the negotiated ciphersuite.
TSK freshness
The secure association (phase 2) protocol MUST support the
derivation of fresh unicast and multicast transient session keys,
even when the keying material provided by the AAA server is not
fresh. This is typically supported by including an exchange of
nonces within the secure association protocol.
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.
Secure capabilities negotiation
The Secure Association Protocol MUST support secure capabilities
negotiation. This includes security parameters such as the security
association identifier (SAID) and ciphersuites. It also includes
confirmation of the capabilities discovered during the discovery
phase (phase 0), so as to ensure that the announced capabilities
have not been forged.
4.2.4. Ciphersuite Requirements
Ciphersuites suitable for keying by EAP methods MUST provide the
following facilities:
TSK derivation
In order to allow a ciphersuite to be usable within the EAP keying
framework, a specification MUST be provided describing how
transient session keys suitable for use with the ciphersuite are
derived from the AAA-Key.
EAP method independence
Algorithms for deriving transient session keys from the AAA-Key
MUST NOT depend on the EAP method. However, algorithms for
deriving TEKs MAY be specific to the EAP method.
Cryptographic separation
The TSKs derived from the AAA-Key MUST be cryptographically
separate from each other. Similarly, TEKs MUST be
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cryptographically separate from each other. In addition, the TSKs
MUST be cryptographically separate from the TEKs.
5. IANA Considerations
This specification does not create any new registries, or define any new
EAP codes or types.
6. Security Considerations
6.1. 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 necessary for EAP methods utilizing public key
cryptography to choose a public key that has a cryptographic strength
meeting the symmetric key strength requirement.
As noted in Section 5 of [KeyLen], 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
6.2. 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 4.2, these and other RADIUS vulnerabilities may be
addressed by running RADIUS over IPsec.
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Where an untrusted AAA intermediary is present (such as a RADIUS proxy
or a Diameter agent), and data object security is not used, the MSK may
be recovered by an attacker in control of the untrusted intermediary.
Possession of the MSK enables decryption of data traffic sent between
the peer and a specific authenticator; however where Perfect Forward
Secrecy (PFS) is implemented, compromise of the MSK does enable an
attacker to impersonate the peer to another authenticator, since that
requires possession of the MK or EMSK, which are not transported by the
AAA protocol. This vulnerability may be mitigated by implementation of
redirect functionality, as provided in [DiamBASE].
6.3. Man-in-the-middle Attacks
As described in [MiTM], 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, [MiTM] recommends derivation of a
compound key by which the EAP peer and authenticator 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, such as fast handoff keys,
discussed in Appendix E.
6.4. Impersonation
Both the RADIUS and Diameter protocols are potentially vulnerable to
impersonation by a rogue authenticator.
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
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RADIUS Access-Request in order to impersonate another authenticator.
Among other things, this can result in messages (and MSKs) being sent to
the wrong authenticator. Since the rogue authenticator is authenticated
by the RADIUS proxy or server purely based on the source address, other
mechanisms are required to detect the forgery. In addition, it is
possible for attributes such as the Called-Station-Id and Calling-
Station-Id to be forged as well.
As recommended in [RFC3579], this vulnerability can be mitigated by
having RADIUS proxies check authenticator identification attributes
against the source address.
To allow verification of session parameters such as the Called-Station-
Id and Calling-Station-Id, these can be sent by the EAP peer to the
server, protected by the TEKs. The RADIUS server can then check the
parameters sent by the EAP peer against those claimed by the
authenticator. If a discrepancy is found, an error can be logged.
While [DiamBASE] 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 [DiamBase].
7. References
7.1. Normative References
[RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2284bis] Blunk, L., et al. "Extensible Authentication Protocol
(EAP)", Internet draft (work in progress), draft-ietf-
eap-rfc2284bis-04.txt, June 2003.
[IEEE802] IEEE Standards for Local and Metropolitan Area Networks:
Overview and Architecture, ANSI/IEEE Std 802, 1990.
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7.2. Informative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC1968] Meyer, G., "The PPP Encryption Protocol (ECP)", RFC 1968,
June 1996.
[RFC2104] Krawczyk, et al., "HMAC: Keyed-Hashing for Message
Authentication", RFC 2104, February 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, 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] Hummert, K., "The PPP Triple-DES Encryption Protocol
(3DESE)", RFC 2420, September 1998.
[RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2516] Mamakos, L., "A Method for Transmitting PPP Over Ethernet
(PPPoE)", RFC 2516, February 1999.
[RFC2548] Zorn, G., "Microsoft Vendor-Specific RADIUS Attributes",
RFC 2548, March 1999.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
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INTERNET-DRAFT EAP Key Mgmt. Framework 9 August 2003
[RFC2960] R. Stewart et al., "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC3078] Pall, G. and G. Zorn, "Microsoft Point-to-Point
Encryption (MPPE) RFC 3078, March 2001.
[RFC3079] Zorn, G. "Deriving Keys for use with Microsoft Point-to-
Point Encryption (MPPE)," RFC 3079, March 2001.
[RFC3394] R. Housley, "Advance Encryption Standard (AES) Key Wrap
Algorithm", RFC 3394, September 2002.
[RFC3580] Congdon, P., et al., "IEEE 802.1X RADIUS Usage
Guidelines", RFC 3580, August 2003.
[FIPSDES] National Bureau of Standards, "Data Encryption Standard",
FIPS PUB 46 (January 1977).
[DESMODES] National Bureau of Standards, "DES Modes of Operation",
FIPS PUB 81 (December 1980).
[FIPS197] FIPS PUB 197, Advanced Encryption Standard (AES), 2001
November 26H.
[SHA] National Institute of Standards and Technology (NIST),
"Announcing the Secure Hash Standard," FIPS 180-1, U.S.
Department of Commerce, 04/1995
[IEEE80211] 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 Std.
802.11-1997, 1997.
[IEEE8021X] IEEE Standards for Local and Metropolitan Area Networks:
Port based Network Access Control, IEEE Std 802.1X-2001,
June 2002.
[IEEE80211f] IEEE 802.11F, "Recommended Practice for Multi-Vendor
Access Point Interoperability via an Inter-Access Point
Protocol Across Distribution Systems Supporting IEEE
802.11 Operation", July 2003.
[IEEE80211i] IEEE Draft 802.11I/D5.0, "Draft Supplement to STANDARD
FOR Telecommunications and Information Exchange between
Systems - LAN/MAN Specific Requirements - Part 11:
Wireless Medium Access Control (MAC) and physical layer
Aboba & Simon Informational [Page 36]
INTERNET-DRAFT EAP Key Mgmt. Framework 9 August 2003
(PHY) specifications: Specification for Enhanced
Security", August 2003.
[EAPAPI] Microsoft Developer Network, "Windows 2000 EAP API",
August 2000, http://msdn.microsoft.com/library/
default.asp?url=/library/en-us/eap/eapport_0fj9.asp
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS Support For Extensible
Authentication Protocol (EAP)", RFC 3579, August 2003.
[RoamCERT] Aboba, B., "Certificate-Based Roaming", Internet draft
(work in progress), draft-ietf-roamops-cert-02.txt, April
1999.
[DiamBASE] Calhoun, P., et al., "Diameter Base Protocol", Internet
draft (work in progress), draft-ietf-aaa-diameter-17.txt,
December 2002.
[DiamEAP] Eronen, P., et al., "Diameter Extensible Authentication
Protocol (EAP) Application", Internet draft (work in
progress), draft-ietf-aaa-eap-02.txt, June 2003.
[Handoff] Arbaugh, B. and B. Aboba, "Experimental Handoff Extension
to RADIUS", Internet draft (work in progress), draft-
irtf-aaaarch-handoff-02.txt, June 2003.
[IEEE-02-758] Mishra, A., Shin, M., Arbaugh, W., Lee, I., Jang, K.,
"Proactive Caching Strategies for IAPP Latency
Improvement during 802.11 Handoff", IEEE 802.11 Working
Group, IEEE-02-758r1-F, November 2002.
[IEEE-03-084] Mishra, A., Shin, M., Arbaugh, W., Lee, I., Jang, K.,
"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.
[KeyLen] Orman, H., and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", Internet
draft (work in progress), draft-orman-public-key-
lengths-05.txt, December 2001.
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[8021XHandoff] Pack, S. and Y. Choi, "Pre-Authenticated Fast Handoff in
a Public Wireless LAN Based on IEEE 802.1X Model", School
of Computer Science and Engineering, Seoul National
University, Seoul, Korea, 2002.
[MD5Attack] Dobbertin, H., "The Status of MD5 After a Recent Attack",
CryptoBytes Vol.2 No.2, Summer 1996.
[MiTM] Puthenkulam, J., et al, "The Compound Authentication
Binding Problem", Internet draft (work in progress),
draft-puthekulam-eap-binding-03.txt, June 2003.
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Appendix A - Ciphersuite Keying Requirements
To date, PPP and IEEE 802.11 ciphersuites are suitable for keying by
EAP. This Appendix describes the keying requirements of common PPP and
802.11 ciphersuites.
PPP ciphersuites include DESEbis [RFC2419], 3DES [RFC2420], and MPPE
[RFC3078]. The DES algorithm is described in [FIPSDES], and DES modes
(such as CBC, used in [RFC2419] and DES-EDE3-CBC, used in [RFC2420]) are
described in [DESMODES]. For PPP DESEbis, a single 56-bit encryption
key is required, used in both directions. For PPP 3DES, a 168-bit
encryption key is needed, used in both directions. As described in
[RFC2419] for DESEbis and [RFC2420] for 3DES, the IV, which is different
in each direction, is "deduced from an explicit 64-bit nonce, which is
exchanged in the clear during the [ECP] negotiation phase." There is
therefore no need for the IV to be provided by EAP.
For MPPE, 40-bit, 56-bit or 128-bit encryption keys are required in each
direction, as described in [RFC3078]. No initialization vector is
required.
While these PPP ciphersuites provide encryption, they do not provide
per-packet authentication or integrity protection, so an authentication
key is not required in either direction.
Within [IEEE80211], Transient Session Keys (TSKs) are required both for
unicast traffic as well as for multicast traffic, and therefore separate
key hierarchies are required for unicast keys and multicast keys. IEEE
802.11 ciphersuites include WEP-40, described in [IEEE80211], which
requires a 40-bit encryption key, the same in either direction; and
WEP-128, which requires a 104-bit encryption key, the same in either
direction. These ciphersuites also do not support per-packet
authentication and integrity protection. In addition to these unicast
keys, authentication and encryption keys are required to wrap the
multicast encryption key.
Recently, new ciphersuites have been proposed for use with IEEE 802.11
that provide per-packet authentication and integrity protection as well
as encryption [IEEE80211i]. These include TKIP, which requires a single
128-bit encryption key and a 128-bit authentication key (used in both
directions); AES CCMP, which requires a single 128-bit key (used in both
directions) in order to authenticate and encrypt data; and WRAP, which
requires a single 128-bit key (used in both directions).
As with WEP, authentication and encryption keys are also required to
wrap the multicast encryption (and possibly, authentication) keys.
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Appendix B - TEK Hierarchy
Figure B-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.
master_secret = TLS term for the MK.
| | |
| | pre_master_secret |
server| | | client
Random| V | Random
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | | |
| | | |
+---->| master_secret |<------+
| | (MK) | |
| | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
| | |
| | |
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 B-1 - TLS [RFC2246] Key Hierarchy
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Appendix C - MSK and EMSK Hierarchy
In EAP-TLS [RFC2716], 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.
In EAP-TLS, the MSK, EMSK and IV are derived from the MK via a one-way
function. This ensures that the MK 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 MK, if the MK is compromised then the MSK is also
compromised.
As described in [RFC2716], the formula for the derivation of the MSK,
EMSK and IV from the MK is as follows:
MSK = TLS-PRF-64(MK, "client EAP encryption",
client.random || server.random)
EMSK = second 64 octets of:
TLS-PRF-128(MK, "client EAP encryption",
client.random || server.random)
IV = TLS-PRF-64("", "client EAP encryption",
client.random || server.random)
AAA-Key(0,31) = Peer to Authenticator Encryption Key (Enc-RECV-Key)
(MS-MPPE-Recv-Key in [RFC2548]). Also known as the
PMK.
AAA-Key(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)
Where:
AAA-Key(W,Z) = Octets W through Z includes of the AAA-Key.
IV(W,Z) = Octets W through Z inclusive of the IV.
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MSK(W,Z) = Octets W through Z inclusive of the MSK.
EMSK(W,Z) = Octets W through Z inclusive of the EMSK.
MK = 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 C-1 describes the process by which the MSK,EMSK,IV and ultimately
the TSKs, are derived from the MK. Note that in [RFC2716], the MK is
referred to as the "TLS Master Secret".
---+
| ^
| TLS Master Secret (MK) |
| |
V |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | EAP |
| Master Session Key (MSK) | Method |
| Derivation | |
| | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ EAP ---+
| | | API ^
| MSK | EMSK | IV |
| | | |
V V V v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | |
| | |
| AAA server | |
| | |
| | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | ^
| AAA-Key(0,31) | AAA-Key(32,63) |
| (PMK) | Transported |
| | via AAA |
| | |
V V V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | ^
| Ciphersuite-Specific Transient Session | Auth.|
| Key Derivation | |
| | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
Figure C-1 - EAP TLS [RFC2716] Key hierarchy
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Appendix D - Transient Session Key (TSK) Derivation
Within IEEE 802.11 RSN, the Pairwise Transient Key (PTK), a transient
session key used to protect unicast traffic, is derived from the PMK
(octets 0-31 of the MSK), known in [RFC2716] as the Peer to
Authenticator Encryption Key. In [IEEE80211i], the PTK is derived from
the PMK via the following formula:
PTK = EAPOL-PRF-X(PMK, "Pairwise key expansion", Min(AA,SA) ||
Max(AA, SA) || Min(ANonce,SNonce) || Max(ANonce,SNonce))
Where:
PMK = AAA-Key(0,31)
SA = Station MAC address (Calling-Station-Id)
AA = Access Point MAC address (Called-Station-Id)
ANonce = Access Point Nonce
SNonce = Station Nonce
EAPOL-PRF-X = Pseudo-Random Function based on HMAC-SHA1, generating
a PTK of size X octets.
TKIP uses X = 64, while CCMP, WRAP, and WEP use X = 48.
The EAPOL-Key Confirmation Key (KCK) is used to provide data origin
authenticity in the TSK derivation. It utilizes the first 128 bits (bits
0-127) of the PTK. The EAPOL-Key Encryption Key (KEK) provides
confidentiality in the TSK derivation. It utilizes bits 128-255 of the
PTK. Bits 256-383 of the PTK are used by Temporal Key 1, and Bits
384-511 are used by Temporal Key 2. Usage of TK1 and TK2 is ciphersuite
specific. Details are available in [IEEE80211i].
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Appendix E - AAA-Key Derivation
As discussed in [Handoff], [IEEE-02-758], [IEEE-03-084], and
[8021XHandoff], keying material may be required for use in fast handoff
between IEEE 802.11 authenticators. Where the backend authentication
server provides keying material to multiple authenticators in order to
fascilitate fast handoff, it is highly desirable for the keying material
used on different authenticators to be cryptographically separate, so
that if one authenticator is compromised, it does not lead to the
compromise of other authenticators. Where keying material is provided
by the backend authentication server, a key hierarchy derived from the
EMSK, as suggested in [IEEE-03-155] can be used to provide
cryptographically separate keying material for use in fast handoff:
AAA-Key-A = MSK(0,63)
AAA-Key-B = PRF(EMSK(0,63),AAA-Key-A,B-Called-Station-Id,Calling-Station-Id)
AAA-Key-E = PRF(EMSK(0,63),AAA-Key-A,E-Called-Station-Id,Calling-Station-Id)
Where:
Calling-Station-Id = STA MAC address
B-Called-Station-Id = AP B MAC address
E-Called-Station-Id = AP E MAC address
Here AAA-Key-A is the AAA-Key derived during the initial EAP
authentication between the peer and authenticator A. Based on this
initial EAP authentication, the EMSK is also derived, which can be used
to derive AAA-Keys for fast authentication between the EAP peer and
authenticators B and E. Since the EMSK is cryptographically separate
from the MSK, each of these AAA-Keys is cryptographically separate from
each other, and are guaranteed to be unique between the EAP peer (also
known as the STA) and the authenticator (also known as the AP).
Acknowledgments
Thanks to Arun Ayyagari, Ashwin Palekar, and Tim Moore of Microsoft,
Dorothy Stanley of Agere, Bob Moskowitz of TruSecure, 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
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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
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