One document matched: draft-ietf-eap-keying-06.txt
Differences from draft-ietf-eap-keying-05.txt
EAP Working Group Bernard Aboba
INTERNET-DRAFT Dan Simon
Category: Standards Track Microsoft
<draft-ietf-eap-keying-06.txt> J. Arkko
1 April 2005 Ericsson
P. Eronen
Nokia
H. Levkowetz, Ed.
ipUnplugged
Extensible Authentication Protocol (EAP) Key Management Framework
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patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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This Internet-Draft will expire on November 22, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
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.
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Table of Contents
1. Introduction .......................................... 4
1.1 Requirements Language ........................... 4
1.2 Terminology ..................................... 4
1.3 Overview ........................................ 5
1.4 EAP Invariants .................................. 11
2. Key Derivation ........................................ 13
2.1 Key Terminology ................................. 13
2.2 Key Hierarchy ................................... 15
2.3 AAA-Key Derivation .............................. 19
2.4 Key Naming ...................................... 20
3. Security associations ................................. 22
3.1 EAP Method SA ................................... 23
3.2 EAP-Key SA ...................................... 24
3.3 AAA SA(s) ....................................... 24
3.4 Service SA(s) ................................... 24
4. Key Management ........................................ 27
4.1 Key Caching ..................................... 28
4.2 Parent-Child Relationships ...................... 29
4.3 Local Key Lifetimes ............................. 29
4.4 Exported and Calculated Key Lifetimes ........... 30
4.5 Key Cache Synchronization ....................... 31
4.6 Key Scope ....................................... 32
4.7 Key Strength .................................... 33
4.8 Key Wrap ........................................ 34
5. Handoff Vulnerabilities ............................... 35
5.1 Authorization ................................... 35
5.2 Correctness ..................................... 36
6. Security Considerations .............................. 39
6.1 Security Terminology ............................ 39
6.2 Threat Model .................................... 39
6.3 Security Analysis ............................... 41
6.4 Man-in-the-middle Attacks ....................... 44
6.5 Denial of Service Attacks ....................... 45
6.6 Impersonation ................................... 45
6.7 Channel Binding ................................. 46
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7. Security Requirements ................................. 47
7.1 EAP Method Requirements ......................... 47
7.2 AAA Protocol Requirements ....................... 50
7.3 Secure Association Protocol Requirements ........ 51
7.4 Ciphersuite Requirements ........................ 53
8. IANA Considerations ................................... 54
9. References ............................................ 54
9.1 Normative References ............................ 54
9.2 Informative References .......................... 54
Acknowledgments .............................................. 58
Author's Addresses ........................................... 58
Appendix A - Ciphersuite Keying Requirements ................. 60
Appendix B - Example Transient EAP Key (TEK) Hierarchy ....... 61
Appendix C - EAP-TLS Key Hierarchy ........................... 62
Appendix D - Example Transient Session Key (TSK) Derivation .. 64
Appendix E - Key Names and Scope in Existing Methods ......... 65
Appendix F - Security Association Examples ................... 66
Intellectual Property Statement .............................. 69
Disclaimer of Validity ....................................... 70
Copyright Statement .......................................... 70
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1. Introduction
The Extensible Authentication Protocol (EAP), defined in [RFC3748],
was designed to enable extensible authentication for network access
in situations in which the IP protocol is not available. Originally
developed for use with PPP [RFC1661], it has subsequently also been
applied to IEEE 802 wired networks [IEEE-802.1X].
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].
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AAA Authentication, Authorization and Accounting. AAA protocols with
EAP support include RADIUS [RFC3579] and Diameter [I-D.ietf-aaa-
eap]. 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.
1.3. Overview
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. Since both the peer and
authenticator may have more than one physical or logical port, a
given peer may simultaneously access the network via multiple
authenticators, or via multiple physical or logical ports on a given
authenticator. Similarly, an authenticator may offer network access
to multiple peers, each via a separate physical or logical port. The
situation is illustrated in Figure 1.
Where authenticators are deployed standalone, the EAP conversation
occurs between the peer and authenticator, and the authenticator must
locally implement an EAP method acceptable to the peer. However, 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 [I-
D.ietf-aaa-eap].
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+-+-+-+-+
| |
| EAP |
| Peer |
| |
+-+-+-+-+
| | | Peer Ports
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
| | | | | | | | | Authenticator Ports
+-+-+-+-+ +-+-+-+-+ +-+-+-+-+
| | | | | |
| Auth. | | Auth. | | Auth. |
| | | | | |
+-+-+-+-+ +-+-+-+-+ +-+-+-+-+
\ | /
\ | /
\ | /
EAP over AAA \ | /
(optional) \ | /
\ | /
\ | /
\ | /
+-+-+-+-+
| |
| AAA |
|Server |
| |
+-+-+-+-+
Figure 1: Relationship between peer, authenticator and backend server
Where EAP key derivation is supported, the conversation between the
peer and the authenticator 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)
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In the discovery phase (phase 0), peers locate authenticators and
discover their capabilities. For example, 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.
The authentication phase (phase 1) may begin once the peer and
authenticator discover each other. This phase always includes EAP
authentication (phase 1a). Where the chosen EAP method supports key
derivation, in phase 1a keying material is derived on both the peer
and the EAP server. This keying material may be used for multiple
purposes, including protection of the EAP conversation and subsequent
data exchanges.
An additional step (phase 1b) is required in deployments which
include a backend authentication server, in order to transport keying
material (known as the AAA-Key) from the backend authentication
server to the authenticator.
A Secure Association exchange (phase 2) then occurs between the peer
and authenticator in order to manage the creation and deletion of
unicast (phase 2a) and multicast (phase 2b) security associations
between the peer and authenticator.
The conversation phases and relationship 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
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1.3.1. Discovery Phase
In the discovery phase (phase 0), the EAP peer and authenticator
locate each other and discover each other's capabilities. Discovery
can occur manually or automatically, depending on the lower layer
over which EAP runs. Since authenticator discovery is handled
outside of EAP, there is no need to provide this functionality within
EAP.
For example, where EAP runs 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. In contrast, PPPoE
[RFC2516] provides 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.
IEEE 802.11 [IEEE-802.11] also provides integrated discovery support
utilizing Beacon and/or Probe Request/Response frames, allowing the
peer (known as the station or STA) to determine the MAC address and
capabilities of one or more authenticators (known as Access Point or
APs).
1.3.2. Authentication Phase
Once the peer and authenticator discover each other, they exchange
EAP packets. Typically, the peer desires access to the network, and
the authenticators provide 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
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association suitable for protecting data traffic.
Some EAP methods exist which only support one-way authentication;
however, EAP methods deriving keys are required to support mutual
authentication. 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 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). 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
The Secure Association phase (phase 2), if it occurs, begins after
the completion of EAP authentication (phase 1a) and key transport
(phase 1b). A Secure Association Protocol used with EAP typically
supports the following features:
[1] Generation of fresh transient session keys (TSKs). Where AAA-Key
caching is supported, the EAP peer may initiate a new session using
a AAA-Key that was used in a previous session. Were the TSKs to be
derived from a portion of the AAA-Key, this would result in reuse
of the session keys which could expose the underlying ciphersuite
to attack.
As a result, where AAA-Key caching is supported, the Secure
Association Protocol phase is REQUIRED, and MUST provide for
freshness of the TSKs. This is typically handled via the exchange
of nonces or counters, which are then mixed with the AAA-Key in
order to generate fresh unicast (phase 2a) and possibly multicast
(phase 2b) session keys. By not using the AAA-Key directly to
protect data, the Secure Association Protocol protects against
compromise of the AAA-Key.
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[2] Entity Naming. A basic feature of a Secure Association Protocol is
the explicit naming of the parties engaged in the exchange.
Explicit identification of the parties is critical, since without
this the parties engaged in the exchange are not identified and the
scope of the transient session keys (TSKs) generated during the
exchange is undefined. As illustrated in Figure 1, both the peer
and NAS may have more than one physical or virtual port, so that
port identifiers are NOT RECOMMENDED as a naming mechanism.
[3] Secure capabilities negotiation. This includes the secure
negotiation of usage modes, session parameters (such as key
lifetimes), ciphersuites and required filters, including
confirmation of the capabilities discovered during phase 0. It is
RECOMMENDED that the Secure Association Protocol support secure
capabilities negotiation, in order to protect against spoofing
during the discovery phase, and to ensure agreement between the
peer and authenticator about how data is to be secured.
[4] Key management. EAP as defined in [RFC3748] supports key
derivation, but not key management. While EAP methods may derive
keying material, EAP does 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
rekey. Although EAP methods may support "fast reconnect" as
defined in [RFC3748] Section 7.2.1, rekey of exported keys cannot
occur without reauthentication. In order to provide method
independence, key management of exported or derived keys SHOULD NOT
be provided within EAP methods.
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 key lifetime
management (such as via explicit key lifetime negotiation, or
seamless rekey), as well synchronization of the installation and
deletion of keys so as to enable recovery from partial or complete
loss of key state by the peer or authenticator. Since key
management requires a key naming scheme, Secure Association
Protocols supporting key management support MUST also support key
naming.
[5] Mutual proof of possession of the AAA-Key. The Secure Association
Protocol MUST demonstrate mutual proof of posession of the AAA-Key,
in order to show that both the peer and authenticator have been
authenticated and authorized by the backend authentication server.
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.
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1.4. EAP Invariants
Certain basic characteristics, known as the "EAP Invariants" hold
true for EAP implementations on all media:
Media independence
Method independence
Ciphersuite independence
1.4.1. 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 inclusion 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 utilize identifiers associated with
a particular usage environment (e.g. MAC addresses).
The need for media independence has also motivated the development of
the three phase exchange. Since discovery is typically media-
specific, this function is handled outside of EAP, rather than being
incorporated within it. Similarly, the Secure Association Protocol
often contains media dependencies such as negotiation of media-
specific ciphersuites or session parameters, and as a result this
functionality also cannot be incorporated within EAP.
Note that media independence may be retained within EAP methods that
support channel binding 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. To
support channel binding, an EAP method can pass binding parameters to
the AAA server in the form of an opaque blob, and receive
confirmation of whether the parameters match, without requiring
media-specific knowledge.
1.4.2. 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
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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. Since the Discovery and Secure
Association exchanges are also method independent, an authenticator
can carry out the three phase exchange without having an EAP method
in common with the peer.
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.
1.4.3. Ciphersuite 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 out-of-band of EAP. Since
ciphersuite negotiation is assumed to occur out-of-band, there is no
need for ciphersuite negotiation within EAP. Since ciphersuite
negotiation occurs outside of EAP, EAP methods generate keying
material that is ciphersuite-independent.
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 after EAP authentication has completed.
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
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transient session key derivation would increase method complexity
and result in duplicated effort.
Simplified configuration
The ciphersuite is negotiated between the peer and authenticator
out-of-band of EAP. The backend authentication server is neither 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 have knowledge of the ciphersuites
and negotiation policies implemented by the peer and authenticator,
or be aware of the ciphersuite negotiated between them. This
simplifies the configuration of the backend authentication server.
For example, since ECP negotiation occurs after authentication,
when run over PPP, the EAP peer, authenticator and backend
authentication server may not anticipate the negotiated ciphersuite
and therefore this information cannot be provided to the EAP
method.
2. Key Derivation
2.1. Key Terminology
The EAP Key Hierarchy makes use of the following types of keys:
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.
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, wrapped within the AAA-Token; it is therefore known
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by the peer, authenticator and backend authentication server.
Despite the name, the AAA-Key is computed regardless of whether a
backend authentication server is present. AAA-Key derivation is
discussed in Section 2.3; in existing implementations the MSK is
used as the AAA-Key.
AAA-Token
Where a backend server is present, the AAA-Key and one or more
attributes is transported between the backend authentication server
and the authenticator within a package known as the AAA-Token. 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. Examples include
RADIUS [RFC2548] and Diameter [I-D.ietf-aaa-eap].
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 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 [IEEE-802.11i] Octets 0-31
of the AAA-Key (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 AAA-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.
Transient Session Keys (TSKs)
Session keys used to protect data exchanged between the peer and
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the authenticator after the EAP authentication has successfully
completed. TSKs are appropriate for the lower layer ciphersuite
negotiated between the EAP peer and authenticator. Examples of TSK
derivation are provided in Appendix D.
2.2. Key Hierarchy
The EAP Key Hierarchy, illustrated in Figure 3, has at the root the
long term credential utilized by the selected EAP method. If
authentication is based on a pre-shared key, the parties store the
EAP method to be used and the pre-shared key. The EAP server also
stores the peer's identity 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 derives two types of keys:
[1] Keys calculated locally by the EAP method but not exported
by the EAP method, such as the TEKs.
[2] Keys exported by the EAP method: MSK, EMSK, IV
From the keys exported by the EAP method, two other types of keys may
be derived:
[3] Keys calculated from exported quantities: AAA-Key.
[4] Keys calculated by the Secure Association Protocol from the
AAA-Key: TSKs.
In order to protect the EAP conversation, methods supporting key
derivation typically negotiate a ciphersuite and derive Transient EAP
Keys (TEKs) for use with that ciphersuite. The TEKs are stored
locally by the EAP method and are not exported.
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. On both the peer and EAP
server, the exported MSK is utilized in order to calculate the AAA-
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Key, as described in Section 2.3. Where a backend authentication
server is present, the AAA-Key is transported from the backend
authentication server to the authenticator within the AAA-Token,
using the AAA protocol.
Once EAP authentication completes and is successful, the peer and
authenticator obtain the AAA-Key and the Secure Association Protocol
is run between the peer and authenticator in order to securely
negotiate the ciphersuite, derive fresh TSKs used to protect data,
and provide mutual proof of possession of the AAA-Key.
When 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. In this case, the MSK
and EMSK are known only to the peer and authenticator and no other
parties. The TEKs and TSKs also reside solely on the peer and
authenticator. This is illustrated in Figure 4. As demonstrated in
[I-D.ietf-roamops-cert], in this case it is still possible to support
roaming between providers, using certificate-based authentication.
Where a backend authentication server is utilized, the situation is
illustrated in Figure 5. Here 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). In this case, the authenticator
encapsulates EAP packet with a AAA protocol such as RADIUS [RFC3579]
or Diameter [I-D.ietf-aaa-eap], 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 reside only on
the peer and EAP server As a result, the TEKs, MSK and EMSK are
derived on the peer and EAP server.
On completion of EAP authentication, EAP methods on the peer and EAP
server export the Master Session Key (MSK) and Extended Master
Session Key (EMSK). The peer and EAP server then calculate the AAA-
Key from the MSK and EMSK, and the backend authentication server
sends an Access-Accept to the authenticator, providing the AAA-Key
within a protected package known as the AAA-Token.
The AAA-Key is then used by the peer and authenticator within the
Secure Association Protocol to derive Transient Session Keys (TSKs)
required for the negotiated ciphersuite. The TSKs are known only to
the peer and authenticator.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | ^
| EAP Method | |
| | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+ | |
| | | | | | |
| | EAP Method Key |<->| Long-Term | | |
| | Derivation | | Credential | | |
| | | | | | |
| | | +-+-+-+-+-+-+-+ | Local to |
| | | | EAP |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Method |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| V | | | |
| +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ | |
| | TEK | | MSK | |EMSK | |IV | | |
| |Derivation | |Derivation | |Derivation | |Derivation | | |
| +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ | |
| | | | | |
| | | | | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | | ^
| | | |
| MSK (64B) | EMSK (64B) | IV (64B) |
| | | Exported|
| | | by |
| V V EAP v
| ---+
| AAA-Key Transported |
| by AAA |
| Protocol |
V V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | ^
| TSK Derivation | Lower layer |
| [AAA-Key Cache] | Specific |
| | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
Figure 3: EAP Key Hierarchy
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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 |
| | | |
| (TEKs) | | (TEKs) |
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
Figure 4: 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 |
| | | |AAA-Key/| |
| | Secure Assoc. | | Name | |
| 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 |
| | | |
| (TEKs) | | (TEKs) |
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
Figure 5: Pass-through relationship between EAP peer, authenticator
and backend authentication server.
2.3. AAA-Key Derivation
In existing usage, where a AAA-Key is generated as the result of a
successful EAP authentication with the authenticator, the AAA-Key is
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based on the MSK: AAA-Key = MSK(0,63).
2.4. Key Naming
Each key created within the EAP key management framework has a name
(the identifier by which the key can be identified), as well as a
scope (the parties to whom the key is available). This section
describes how keys are named, and the scope within which that name
applies.
Session-Id
EAP methods supporting key naming MUST specify a temporally unique
method identifier known as the EAP Method-Id, which is typically
constructed from nonces or counters used within the exchange. Since
multiple EAP sessions may exist between an EAP peer and EAP server,
the Method-Id allows MSKs to be differentiated.
The concatenation of the EAP Type (expressed in ASCII text), ":" and
the Method-Id (also expressed in ASCII text) is known as the EAP
Session-Id. The inclusion of the Type in the EAP Session-Id ensures
that each EAP method has a distinct name space.
The EAP Session-Id uniquely identifies the EAP session to the EAP
peer and server terminating the EAP conversation. However, suitable
EAP peer and server names may not always be available. As described
in [RFC3748] Section 7.3, the identity provided in the EAP-
Response/Identity, may be different from the identity authenticated
by the EAP method, and as a result the EAP-Response/Identity is
unsuitable for determination of the peer identity. As a result, the
Session-Id scope is defined by the EAP peer name (if securely
exchanged within the method) concatenated with the EAP server name
(also only if securely exchanged). Where a peer or server name is
missing the null string is used. Since an EAP session is not bound
to a particular authentication or specific ports on the peer and
authenticator, the authenticator port or identity are not included in
the Session-Id scope.
The EAP Session-Id is exported by the EAP method along with the
Session-Id scope, if available, and is used to construct names for
other EAP keys. Note that the EAP Session-Id and scope are only
known by the EAP method. As a result, the format of the EAP Session-
Id and the definition of the Session-Id scope needs to be specified
within the method. Appendix E defines the EAP Session-Id and scope
provided by existing methods.
MSK Name
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This key is created between the EAP peer and EAP server, and can be
referred to using the string "MSK:", concatenated with the EAP
Session-Id. As with the EAP Session-Id, the MSK scope 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.
EMSK Name
The EMSK can be referred to using the string "EMSK:", concatenated
with the EAP Session-Id.
As with the EAP Session-Id, the EMSK scope 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.
AAA-Key Name
In existing usage, the AAA-Key is always derived from the MSK so can
be referred to using the MSK name.
The AAA-Key scope is provided by the concatenation of the EAP peer
name (if securely provided to the authenticator), and the
authenticator name (if securely provided to the peer).
For the purpose of identifying the authenticator to the peer, the
value of the NAS-Identifier attribute is recommended. The
authenticator may include the NAS-Identifier attribute to the AAA
server in an Access-Request, and the authenticator may provide the
NAS-Identifier to the EAP peer. Mechanisms for this include use of
the EAP-Request/Identity (unsecured) or a lower layer mechanism (such
as the 802.11 Beacon/Probe Response). Where the NAS-Identifier is
provided by the authenticator to the peer a secure mechanism is
RECOMMENDED.
For the purpose of identifying the peer to the authenticator, the EAP
peer identifier provided within the EAP method is recommended. It
cannot be assumed that the authenticator is aware of the EAP peer
name used within the method. Therefore alternatives mechanisms need
to be used to provide the EAP peer name to the authenticator. For
example, the AAA server may include the EAP peer name in the User-
Name attribute of the Access-Accept or the peer may provide the
authenticator with its name via a lower layer mechanism.
Absent an explicit binding step within the Secure Association
Protocol, the AAA-Key is not bound to a specific peer or
authenticator port. As a result, the peer or authenticator port over
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which the EAP conversation takes place is not included in the AAA-Key
scope.
PMK Name
This document does not specify a naming scheme for the PMK. The PMK
is only identified by the AAA-Key from which it is derived.
Similarly, the PMK scope is the same as the AAA-Key scope.
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]).
TEKs
The TEKs may or may not be named. Their naming is specified in the
EAP method. Since the TEKs are only known by the EAP peer and
server, the TEK scope is the same as the Session-Id scope.
TSKs
The TSKs are typically named. Their naming is specified in the Secure
Association (phase 2) protocol, so that the correct set of transient
session keys can be identified for processing a given packet. The
scope of the TSKs is negotiated within the Secure Association
Protocol.
TSK creation and deletion operations are typically supported so that
establishment and re-establishment of TSKs can be synchronized
between the parties.
In order to avoid confusion in the case where an EAP peer has more
than one AAA-Key (phase 1b) applicable to establishment of a phase 2
security association, the secure Association protocol needs to
utilize the AAA-Key name so that the appropriate phase 1b keying
material can be identified for use in the Secure Association Protocol
exchange.
3. Security Associations
During EAP authentication and subsequent exchanges, four types of
security associations (SAs) are created:
[1] EAP method SA. This SA is between the peer and EAP server. It
stores state that can be used for "fast reconnect" or other
functionality in some EAP methods. Not all EAP methods create such
an SA.
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[2] EAP-Key SA. This is an SA between the peer and EAP server, which
is used to store the keying material exported by the EAP method.
Current EAP server implementations do not retain this SA after the
EAP conversation completes.
[3] AAA SA(s). These SAs are between the authenticator and the backend
authentication server. They permit the parties to mutually
authenticate each other and protect the communications between
them.
[4] Service SA(s). These SAs are between the peer and authenticator,
and they are created as a result of phases 1-2 of the conversation
(see Section 1.3).
Examples of security associations are provided in Appendix F.
3.1. EAP Method SA (peer - EAP server)
An EAP method may store some state on the peer and EAP server even
after phase 1a has completed.
Typically, this is used for "fast reconnect": the peer and EAP server
can confirm that they are still talking to the same party, perhaps
using fewer round-trips or less computational power. In this case,
the EAP method SA is essentially a cache for performance
optimization, and either party may remove the SA from its cache at
any point.
An EAP method may also keep state in order to support pseudonym-based
identity protection. This is typically a cache as well (the
information can be recreated if the original EAP method SA is lost),
but may be stored for longer periods of time.
The EAP method SA is not restricted to a particular service or
authenticator and is most useful when the peer accesses many
different authenticators. An EAP method is responsible for
specifying how the parties select if an existing EAP method SA should
be used, and if so, which one. Where multiple backend authentication
servers are used, EAP method SAs are not typically synchronized
between them.
EAP method implementations should consider the appropriate lifetime
for the EAP method SA. "Fast reconnect" assumes that the information
required (primarily the keys in the EAP method SA) hasn't been
compromised. In case the original authentication was carried out
using, for instance, a smart card, it may be easier to compromise the
EAP method SA (stored on the PC, for instance), so typically the EAP
method SAs have a limited lifetime.
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Contents:
o Implicitly, the EAP method this SA refers to
o Internal (non-exported) cryptographic state
o EAP method SA name
o SA lifetime
3.2. EAP-Key SA
This is an SA between the peer and EAP server, which is used to store
the keying material exported by the EAP method. Current EAP server
implementations do not retain this SA after the EAP conversation
completes. As a result, all keys exported by the EAP method
(including the MSK, EMSK and IV) on the AAA server are discarded and
are not cached. Calculated keys (such as the AAA-Key) are also
discarded and not cached.
3.3. AAA SA(s) (authenticator - backend authentication server)
In order for the authenticator and backend authentication server to
authenticate each other, they need to store some information.
In case the authenticator and backend authentication server are
colocated, and they communicate using local procedure calls or shared
memory, this SA need not necessarily contain any information.
3.4. Service SA(s) (peer - authenticator)
The service SAs store information about the service being provided.
These include the Root service SA and derived unicast and multicast
service SAs.
The Root service SA is established as the result of the completion of
EAP authentication (phase 1a) and AAA-Key derivation or transport
(phase 1b). It includes:
o Service parameters (or at least those parameters
that are still needed)
o On the authenticator, service authorization
information received from the backend authentication
server (or necessary parts of it)
o On the peer, usually locally configured service
authorization information.
o The AAA-Key, if it can be needed again (to refresh
and/or resynchronize other keys or for another reason)
o AAA-Key lifetime
Unicast and (optionally) multicast service SAs are derived from the
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Root service SA, via the Secure Association Protocol. In order for
unicast and multicast service SAs and associated TSKs to be
established, it is not necessary for EAP authentication (phase 1a) to
be rerun each time. Instead, the Secure Association Protocol can be
used to mutually prove possession of the AAA-Key and create
associated unicast (phase 2a) and multicast (phase 2b) service SAs
and TSKs, enabling the EAP exchange to be bypassed. Unicast and
multicast service SAs include:
o Service parameters negotiated by the Secure Association Protocol.
o Endpoint identifiers.
o Transient Session Keys used to protect the communication.
o Transient Session Key lifetime.
One function of the Secure Association Protocol is to bind the the
unicast and multicast service SAs and TSKs to endpoint identifiers.
For example, within [IEEE802.11i], the 4-way handshake binds the TSKs
to the MAC addresses of the endpoints; in [IKEv2], the TSKs are bound
to the IP addresses of the endpoints and the negotiated SPI.
It is possible for more than one unicast or multicast service SA to
be derived from a single Root service SA. However, a unicast or
multicast service SA is always descended from only one Root service
SA. Unicast or multicast service SAs descended from the same Root
service SA may utilize the same security parameters (e.g. mode,
ciphersuite, etc.) or they may utilize different parameters.
An EAP peer may be able to negotiate multiple service SAs with a
given authenticator, or may be able to maintain one or more service
SAs with multiple authenticators, depending on the properties of the
media.
Except where explicitly specified by the Secure Association Protocol,
it should not be assumed that the installation of new service SAs
implies deletion of old service SAs. It is possible for multicast
Root service SAs to between the same EAP peer and authenticator;
during a re-key of a unicast or multicast service SA it is possible
for two service SAs to exist during the period between when the new
service SA and corresponding TSKs are calculated and when they are
installed.
Similarly, deletion or creation of a unicast or multicast service SA
does not necessarily imply deletion or creation of related unicast or
multicast service SAs, unless specified by the Secure Association
protocol. For example, a unicast service SA may be rekeyed without
implying a rekey of the multicast service SA.
The deletion of the Root service SA does not necessarily imply the
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deletion of the derived unicast and multicast service SAs and
associated TSKs. Failure to mutually prove possession of the AAA-Key
during the Secure Association Protocol exchange need not be grounds
for deletion of the AAA-Key by both parties; the action to be taken
is defined by the Secure Association Protocol.
3.4.1. Sharing service SAs
A single service may be provided by multiple logical or physical
service elements. Each service is responsible for specifying how
changing service elements is handled. Some approaches include:
Transparent sharing
If the service parameters visible to the other party (either peer
or authenticator) do not change, the service can be moved without
requiring cooperation from the other party.
Whether such a move should be supported or used depends on
implementation and administrative considerations. For instance, an
administrator may decide to configure a group of IKEv2/IPsec
gateways in a cluster for high-availability purposes, if the
implementation used supports this. The peer does not necessarily
have any way of knowing when the change occurs.
No sharing
If the service parameters require changing, some changes may
require terminating the old service, and starting a new
conversation from phase 0. This approach is used by all services
for at least some parameters, and it doesn't require any protocol
for transferring the service SA between the service elements.
The service may support keeping the old service element active
while the new conversation takes phase, to decrease the time the
service is not available.
Some sharing
The service may allow changing some parameters by simply agreeing
about the new values. This may involve a similar exchange as in
phase 2, or perhaps a shorter conversation.
This option usually requires some protocol for transferring the
service SA between the elements. An administrator may decide not to
enable this feature at all, and typically the sharing is restricted
to some particular service elements (defined either by a service
parameter, or simple administrative decision). If the old and new
service element do not support such "context transfer", this
approach falls back to the previous option (no transfer).
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Services supporting this feature should also consider what changes
require new authorization from the backend authentication server
(see Section 4.2).
Note that these considerations are not limited to service
parameters related to the authenticator--they apply to peer
parameters as well.
4. Key Management
EAP supports key derivation, but not key management. As a result,
key management functionality needs to be provided by the Secure
Association Protocol. This includes:
[a] Generation of fresh transient session keys (TSKs). Where AAA-Key
caching is supported, the EAP peer may initiate a new session using
a AAA-Key that was used in a previous session. Were the TSKs to be
derived from a portion of the AAA-Key, this would result in reuse
of the session keys which could expose the underlying ciphersuite
to attack. As a result, where AAA-Key caching is supported, the
Secure Association Protocol phase is REQUIRED, and MUST provide for
freshness of the TSKs.
[b] Key lifetime determination. EAP does not support negotiation of
key lifetimes, nor does it support rekey without reauthentication.
As a result, the Secure Association Protocol may handle rekey and
determination of the key lifetime. Where key caching is supported,
secure negotiation of key lifetimes is RECOMMENDED. Lower layers
that support rekey, 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 rekeying the SA
when necessary.
[c] 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
may not be able to determine before attempting to use a AAA-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.
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[d] Key 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 needs to
support key selection, using the EAP Key Naming scheme described in
this document.
[e] Key scope determination. 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 AAA-Key caching is supported,
the EAP peer may not be able to determine the scope of validity of
a AAA-Key. 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 AAA-Key. To allow key scope
determination, the lower layer SHOULD provide a mechanism by which
the peer can determine the scope of the AAA-Key cache on each
authenticator, and by which the authenticator can determine the
scope of the AAA-Key cache on a peer.
4.1. Key Caching
In existing implementations, key caching may be supported on the EAP
peer and authenticator but not on the backend server. Where
explicitly supported by the lower layer, the EAP peer and
authenticator MAY cache the AAA-Key and/or TSKs. The structure of
the key cache on the peer and authenticator is defined by the lower
layer. Unless specified by the lower layer, the EAP peer and
authenticator MUST assume that peers and authenticators do not cache
the AAA-Key or TSKs.
In existing AAA server implementations, all keys exported by EAP
methods (including the MSK, EMSK and IV) and calculated keys (e.g.
AAA-Key) are not cached and are lost after EAP authentication
completes:
[1] In order to avoid key reuse, on the EAP server, transported keys
are deleted once they are sent. An EAP server MUST NOT retain keys
that it has previously sent to the authenticator. For example, an
EAP server that has transported a AAA-Key based on the MSK MUST
delete the MSK, and no keys may be derived from the MSK from that
point forward by the server.
[2] Keys which are not transported, such as the EMSK, are also deleted
by existing implementations.
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4.2. Parent-Child Relationships
When keying material exported by EAP methods expires, all keying
material derived from the exported keying material expires, including
the AAA-Key and TSKs.
When an EAP reauthentication takes place, new keying material is
derived and exported by the EAP method, which eventually results in
replacement of calculated keys, including the AAA-Key and 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 rekey may occur prior to EAP
reauthentication.
Failure to mutually prove possession of the AAA-Key during the Secure
Association Protocol exchange need not be grounds for deletion of the
AAA-Key by both parties; rate-limiting Secure Association Protocol
exchanges could be used to prevent a brute force attack.
4.3. 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 rekey 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 rekey MUST occur prior to wrapping of the counter.
Similarly, TSKs MUST remain cryptographically separate from TEKs
despite TEK rekeying 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.
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4.4. 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.
Several mechanisms exist for managing key lifetimes:
[a] AAA attributes. AAA protocols such as RADIUS [RFC2865] and
Diameter [I-D.ietf-aaa-eap] 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 the AAA-Key, 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, the AAA-Key and all keys calculated from it
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
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is taken from the AAA-Key, there is no need to manage the TSK
lifetime as a separate parameter, since the TSK lifetime and AAA-
Key lifetime are identical.
[c] System defaults. Where the EAP method does not support the
negotiation of the exported key lifetime, and a key lifetime
negotiation mechanism is not provided by the lower lower, there may
be no way for the peer to learn the exported key lifetime. In this
case it is RECOMMENDED that the peer assume a default value of the
exported key lifetime; 8 hours is recommended. Similarly, the
lifetime of calculated keys can also be managed as a system
parameter on the authenticator.
[d] Method specific negotiation within EAP. While EAP itself does not
support lifetime negotiation, it would be possible to specify
methods that do. However, systems that rely on such negotiation
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.
4.5. 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 AAA-Key cache by deleting
the oldest AAA-Key first (LIFO), the relative creation time of the
last AAA-Key to be deleted could be advertised with the Discovery
phase, enabling the peer to determine whether a given AAA-Key had
been expired from the authenticator key cache prematurely.
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4.6. Key Scope
As described in Section 2.3, in existing applications the AAA-Key is
derived from the MSK by the EAP peer and server, and is used as the
root of the ciphersuite-specific key hierarchy. Where a backend
authentication server is present, the AAA-Key is transported from the
EAP server to the authenticator; where it is not present, the AAA-Key
is calculated on the authenticator.
Regardless of how many sessions are initiated using it, the AAA-Key
scope is between the EAP peer that calculates it, and the
authenticator that either calculates it (where no backend
authenticator is present) or receives it from the server (where a
backend authenticator server is present).
It should be understood that an authenticator or peer:
[a] may contain multiple physical ports;
[b] may advertise itself as multiple "virtual" authenticators
or peers;
[c] may utilize multiple CPUs;
[d] may support clustering services for load balancing or failover.
As illustrated in Figure 1, an EAP peer with multiple ports may be
attached to one or more authenticators, each with multiple ports.
Where the peer and authenticator identify themselves using a port
identifier such as a link layer address, 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
scope of the AAA-Key.
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 AAA-Key, 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 AP, a shared key cache
may or may not be implemented.
Where the AAA-Key is shared between "virtual authenticators" an
attacker acting as a peer could authenticate with the "Guest"
"virtual authenticator" and derive a AAA-Key. If the virtual
authenticators share a key cache, then the peer can utilize the AAA-
Key derived for the "Guest" network to obtain access to the
"Corporate Intranet" virtual authenticator.
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Several measures are recommended to address these issues:
[a] Authenticators are REQUIRED to cache associated authorizations
along with the AAA-Key and apply authorizations consistently. This
ensures that an attacker cannot obtain elevated privileges even
where the AAA-Key cache is shared between "virtual authenticators".
[b] It is RECOMMENDED that physical authenticators maintain separate
AAA-Key caches for each "virtual authenticator".
[c] 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".
[d] It is RECOMMENDED that Secure Association Protocols identify peers
and authenticators unambiguously, without incorporating implicit
assumptions about peer and authenticator architectures. Using
port-specific MAC addresses as identifiers is NOT RECOMMENDED where
peers and authenticators may support multiple ports.
[e] The AAA server and authenticator MAY implement additional
attributes in order to further restrict the AAA-Key scope. 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 the AAA-Key is valid.
[f] 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.
4.7. Key Strength
In order to guard against brute force attacks, EAP methods deriving
keys need to be capable of generating keys with an appropriate
effective symmetric key strength. In order to ensure that key
generation is not the weakest link, it is RECOMMENDED that EAP
methods utilizing public key cryptography choose a public key that
has a cryptographic strength meeting the symmetric key strength
requirement.
As noted in [RFC3766] Section 5, this results in the following
required RSA or DH module and DSA subgroup size in bits, for a given
level of attack resistance in bits:
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Attack Resistance RSA or DH Modulus DSA subgroup
(bits) size (bits) size (bits)
----------------- ----------------- ------------
70 947 128
80 1228 145
90 1553 153
100 1926 184
150 4575 279
200 8719 373
250 14596 475
4.8. 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 [I-D.ietf-aaa-eap], 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, the
AAA-Key may be recovered by an attacker in control of the untrusted
intermediary. Possession of the AAA-Key enables decryption of data
traffic sent between the peer and a specific authenticator. However,
as long as a AAA-Key or keys derived from it is only utilized by a
single authenticator, compromise of the AAA-Key does not enable an
attacker to impersonate the peer to another authenticator.
Vulnerability to an untrusted AAA intermediary can be mitigated by
implementation of redirect functionality, as described in [RFC3588]
and [I-D.ietf-aaa-eap].
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5. 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 a
AAA-Key on an authenticator prior to arrival of the peer. Another
such mechanism is AAA-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.
5.1. 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
AAA-Key, 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?
[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.
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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?
5.2. Correctness
When the AAA exchange is bypassed via use of techniques such as AAA-
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.
[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.
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[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
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
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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
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
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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].
6. Security Considerations
6.1. Security Terminology
"Cryptographic binding", "Cryptographic separation", "Key strength"
and "Mutual authentication" are defined in [RFC3748] and are used
with the same meaning here.
6.2. Threat Model
The EAP threat model is described in [RFC3748] Section 7.1. In order
to address these threats, EAP relies on the security properties of
EAP methods (known as "security claims", described in [RFC3784]
Section 7.2.1). EAP method requirements for application 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. Among other things, [RFC3579] Section 4.2 recommends the
use of IPsec ESP with non-null transform to provide per-packet
authentication and confidentiality, integrity and replay protection
for RADIUS/EAP.
Given the existing documentation of EAP and AAA threat models and
responses, there is no need to duplicate that material here.
However, there are many other system-level threats no covered in
these document which have not been described or analyzed elsewhere.
These include:
[1] An attacker may try to modify or spoof Secure Association Protocol
packets.
[2] An attacker compromising an 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.
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[3] An attacker may attempt to perform downgrading attacks on the
ciphersuite negotiation within the Secure Association Protocol in
order to ensure that a weaker ciphersuite is used to protect data.
Depending on the lower layer, these attacks may be carried out
without requiring physical proximity.
In order to address these threats, [Housley56] describes the
mandatory system security properties:
Algorithm independence
Wherever cryptographic algorithms are chosen, the algorithms must
be negotiable, in order to provide resilient against compromise of
a particular algorithm. Algorithm independence must be
demonstrated within all aspects of the system, including within
EAP, AAA and the Secure Association Protocol. However, for
interoperability, at least one suite of algorithms MUST be
implemented.
Strong, fresh session keys
Session keys must be demonstrated to be strong and fresh in all
circumstances, while at the same time retaining algorithm
independence.
Replay protection
All protocol exchanges must be replay protected. This includes
exchanges within EAP, AAA, and the Secure Association Protocol.
Authentication
All parties need to be authenticated. The confidentiality of the
authenticator must be maintained. No plaintext passwords are
allowed.
Authorization
EAP peer and authenticator authorization must be performed.
Session keys
Confidentiality of session keys must be maintained.
Ciphersuite negotiation
The selection of the "best" ciphersuite must be securely confirmed.
Unique naming
Session keys must be uniquely named.
Domino effect
Compromise of a single authenticator cannot compromise any other
part of the system, including session keys and long-term secrets.
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Key binding
The key must be bound to the appropriate context.
6.3. Security Analysis
Figure 6 illustrates the relationship between the peer, authenticator
and backend authentication server.
EAP peer
/\
/ \
Protocol: EAP / \ Protocol: Secure Association
Auth: Mutual / \ Auth: Mutual
Unique keys: / \ Unique keys: TSKs
TEKs,EMSK / \
/ \
EAP server +--------------+ Authenticator
Protocol: AAA
Auth: Mutual
Unique key: AAA session key
Figure 6: Relationship between peer, authenticator and auth. server
The peer and EAP server communicate using EAP [RFC3748]. The
security properties of this communication are largely determined by
the chosen EAP method. Method security claims are described in
[RFC3748] Section 7.2. These include the key strength, protected
ciphersuite negotiation, mutual authentication, integrity protection,
replay protection, confidentiality, key derivation, key strength,
dictionary attack resistance, fast reconnect, cryptographic binding,
session independence, fragmentation and channel binding claims. At a
minimum, methods claiming to support key derivation must also support
mutual authentication. As noted in [RFC3748] Section 7.10:
EAP Methods deriving keys MUST provide for mutual authentication
between the EAP peer and the EAP Server.
Ciphersuite independence is also required:
Keying material exported by EAP methods MUST be independent of the
ciphersuite negotiated to protect data.
In terms of key strength and freshness, [RFC3748] Section 10 says:
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.... In order to preserve algorithm independence, EAP
methods deriving keys SHOULD support (and document) the protected
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negotiation of the ciphersuite used to protect the EAP
conversation between the peer and server... In order to enable
deployments requiring strong keys, EAP methods supporting key
derivation SHOULD be capable of generating an MSK and EMSK, each
with an effective key strength of at least 128 bits.
The authenticator and backend authentication server communicate using
a AAA protocol such as RADIUS [RFC3579] or Diameter [I-D.ietf-aaa-
eap]. As noted in [RFC3588] Section 13, Diameter must be protected
by either IPsec ESP with non-null transform or TLS. As a result,
Diameter requires per-packet integrity and confidentiality. Replay
protection must be supported. For RADIUS, [RFC3579] Section 4.2
recommends that RADIUS be protected by IPsec ESP with a non-null
transform, and where IPsec is implemented replay protection must be
supported.
The peer and authenticator communicate using the Secure Association
Protocol.
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.
The EAP peer and backend authentication server mutually authenticate
via the EAP method, and derive the 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 [RFC3748].
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 [IEEE-802.11i]. 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
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known only to them, used to provide per-packet integrity and replay
protection, authentication and confidentiality. The AAA-Key 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 AAA-Key
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.
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
[RFC3588].
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 [RFC3748] 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 [IEEE-802.11], 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
[IEEE-802.11i] 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 [IEEE-802.11i], so as to address the threat of rogue
devices, and provide keying material to bind the initial
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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.
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.
6.4. 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
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derived from the EMSK, such as fast handoff keys, discussed in
Section 2.3.
6.5. Denial of Service Attacks
The caching of security associations may result in vulnerability to
denial of service attacks. Since an EAP peer may derive multiple EAP
SAs with a given EAP server, and creation of a new EAP SA does not
implicitly delete a previous EAP SA, EAP methods that result in
creation of persistent state may be vulnerable to denial of service
attacks by a rogue EAP peer.
As a result, EAP methods creating persistent state may wish to limit
the number of cached EAP SAs (Phase 1a) corresponding to an EAP peer.
For example, an EAP server may choose to only retain a few EAP SAs
for each peer. This prevents a rogue peer from denying access to
other peers.
Similarly, an authenticator may have multiple AAA-Key SAs
corresponding to a given EAP peer; to conserve resources an
authenticator may choose to limit the number of cached AAA-Key (Phase
1 b) SAs for each peer.
Depending on the media, creation of a new unicast Secure Association
SA may or may not imply deletion of a previous unicast secure
association SA. Where there is no implied deletion, the
authenticator may choose to limit Phase 2 (unicast and multicast)
Secure Association SAs for each peer.
6.6. 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
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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
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 [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].
6.7. 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 protocol).
Where EAP is used in pass-through mode, the EAP peer typically does
not verify the identity of the pass-through authenticator, it only
verifies that the pass-through authenticator is trusted by the EAP
server. This creates a potential security vulnerability, described in
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[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 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 a lower layer protocol.
For example, it is possible for a compromised authenticator to
utilize another authenticator's Called-Station-Id or NAS-Identifier
in communicating with the EAP peer via a lower layer protocol, 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 use of EAP methods that support a protected exchange of
channel properties such as endpoint identifiers, including (but not
limited to): Called-Station-Id [RFC2865][RFC3580], Calling-Station-Id
[RFC2865][RFC3580], NAS-Identifier [RFC2865], NAS-IP-Address
[RFC2865], and NAS-IPv6-Address [RFC3162].
Using such a protected exchange, it is possible to match the channel
properties provided by the authenticator via out-of-band mechanisms
against those exchanged within the EAP method. For example, see
[ServiceIdent].
7. Security Requirements
This section summarizes the security requirements that must be met by
EAP methods, AAA protocols, Secure Association Protocols and
Ciphersuites in order to address the security threats described in
this document. These requirements MUST be met by specifications
requesting publication as an RFC. Each requirement provides a
pointer to the sections of this document describing the threat that
it mitigates.
7.1. EAP Method Requirements
It is possible for the peer and EAP server to mutually authenticate
and derive keys. In order to provide keying material for use in a
subsequently negotiated ciphersuite, an EAP method supporting key
derivation MUST export a Master Session Key (MSK) of at least 64
octets, and an Extended Master Session Key (EMSK) of at least 64
octets. EAP Methods deriving keys MUST provide for mutual
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authentication between the EAP peer and the EAP Server.
The MSK and EMSK MUST NOT be used directly to protect data; however,
they are of sufficient size to enable derivation of a AAA-Key
subsequently used to derive Transient Session Keys (TSKs) for use
with the selected ciphersuite. Each ciphersuite is responsible for
specifying how to derive the TSKs from the AAA-Key.
The AAA-Key is derived from the keying material exported by the EAP
method (MSK and EMSK). This derivation occurs on the AAA server. In
many existing protocols that use EAP, the AAA-Key and MSK are
equivalent, but more complicated mechanisms are possible (see Section
2.3 for details).
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.
EAP methods export the MSK and EMSK and not Transient Session Keys so
as to allow EAP methods to be ciphersuite and media independent.
Keying material exported by EAP methods MUST be independent of the
ciphersuite negotiated to protect data.
Depending on the lower layer, EAP methods may run before or after
ciphersuite negotiation, so that the selected ciphersuite may not be
known to the EAP method. By providing keying material usable with
any ciphersuite, EAP methods can used with a wide range of
ciphersuites and media.
It is RECOMMENDED that methods providing integrity protection of EAP
packets include coverage of all the EAP header fields, including the
Code, Identifier, Length, Type and Type-Data fields.
In order to preserve algorithm independence, EAP methods deriving
keys SHOULD support (and document) the protected negotiation of the
ciphersuite used to protect the EAP conversation between the peer and
server. This is distinct from the ciphersuite negotiated between the
peer and authenticator, used to protect data.
The strength of Transient Session Keys (TSKs) used to protect data is
ultimately dependent on the strength of keys generated by the EAP
method. If an EAP method cannot produce keying material of
sufficient strength, then the TSKs may be subject to brute force
attack. In order to enable deployments requiring strong keys, EAP
methods supporting key derivation SHOULD be capable of generating an
MSK and EMSK, each with an effective key strength of at least 128
bits.
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Methods supporting key derivation MUST demonstrate cryptographic
separation between the 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 MSK or EMSK MUST NOT be able to recover the other
quantity with a level of effort less than brute force.
Non-overlapping substrings of the MSK MUST be cryptographically
separate from each other. That is, knowledge of one substring MUST
NOT help in recovering some other non-overlapping substring without
breaking some hard cryptographic assumption. This is required
because some existing ciphersuites form TSKs by simply splitting the
AAA-Key to pieces of appropriate length. Likewise, non-overlapping
substrings of the EMSK MUST be cryptographically separate from each
other, and from substrings of the MSK. The EMSK MUST NOT be
transported to, or shared with, additional parties.
Since EAP does not provide for explicit key lifetime negotiation, EAP
peers, authenticators and authentication servers MUST be prepared for
situations in which one of the parties discards key state which
remains valid on another party.
The development and validation of key derivation algorithms is
difficult, and as a result EAP methods SHOULD reuse well established
and analyzed mechanisms for MSK and EMSK key derivation (such as
those specified in IKE [RFC2409] or TLS [RFC2246]), rather than
inventing new ones.
7.1.1. Requirements for EAP methods
In order for an EAP method to meet the guidelines for EMSK usage it
must meet the following requirements:
o It MUST specify how to derive the EMSK
o The key material used for the EMSK MUST be
computationally independent of the MSK and TEKs.
o The EMSK MUST NOT be used for any other purpose than the key
derivation described in this document.
o The EMSK MUST be secret and not known to someone observing
the authentication mechanism protocol exchange.
o The EMSK MUST NOT be exported from the EAP server.
o The EMSK MUST be unique for each session.
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o The EAP mechanism SHOULD a unique identifier suitable for naming the EMSK.
7.1.2. Requirements for EAP applications
In order for an application to meet the guidelines for EMSK usage it
must meet the following requirements:
o New applications following this specification SHOULD NOT use the
MSK. If more than one application uses the MSK, then the
cryptographic separation is not achieved. Implementations SHOULD
prevent such combinations.
o A peer MUST NOT use the EMSK directly for cryptographic
protection of data.
7.2. 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 [I-D.ietf-aaa-eap], as well as RADIUS over IPsec
[RFC3579].
Session Keys
AAA protocols used for transport of EAP keying material MUST
implement and SHOULD use dynamic key management in order to derive
fresh session keys, as in Diameter EAP [I-D.ietf-aaa-eap] and
RADIUS over IPsec [RFC3579], rather than using a 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 [I-D.ietf-aaa-eap] as well as by RADIUS EAP [RFC3579].
Authorization
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 [RFC3579] Section 4.3.7.
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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
[RFC3588] 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
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
[RFC3588].
7.3. Secure Association Protocol Requirements
The Secure Association Protocol supports the following:
Entity Naming
The peer and authenticator SHOULD identify themselves in a manner
that is independent of their attached ports.
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Mutual proof of possession
The peer and authenticator MUST each demonstrate possession of the
keying material transported between the backend authentication
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.
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 backend
authentication server is not fresh. This is typically supported by
including an exchange of nonces within the Secure Association
Protocol.
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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, as well as
negotiation of the lifetime of the TSKs, AAA-Key and exported EAP
keys. Secure capabilities negotiation 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.
Key Scoping
The Secure Association Protocol MUST ensure the synchronization of
key scope between the peer and authenticator. This includes
negotiation of restrictions on key usage.
7.4. 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
cryptographically separate from each other. In addition, the TSKs
MUST be cryptographically separate from the TEKs.
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8. IANA Considerations
This document does not create any new name spaces nor does it
allocate any protocol parameters.
9. References
9.1. 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.
9.2. Informative References
[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.
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INTERNET-DRAFT EAP Key Management Framework 1 April 2005
[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.
[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.
[RFC4017] Stanley, D., Walker, J. and B. Aboba, "EAP Method Requirements
for Wireless LANs", RFC 4017, March 2005.
[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.
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[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.
[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.
[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.
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[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.ietf-aaa-eap]
Eronen, P., Hiller, T. and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application", draft-ietf-aaa-
eap-10 (work in progress), November 2004.
[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.
[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", draft-
ietf-ipsec-ikev2-17 (work in progress), September 2004.
[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, 1996.
Aboba, et al. Standards Track [Page 57]
INTERNET-DRAFT EAP Key Management Framework 1 April 2005
[Housley56]
Housley, R., "Key Management in AAA", Presentation to the AAA
WG at IETF 56,
http://www.ietf.org/proceedings/03mar/slides/aaa-5/index.html,
March 2003.
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
Aboba, et al. Standards Track [Page 58]
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EMail: pasi.eronen@nokia.com
Henrik Levkowetz (editor)
ipUnplugged AB
Arenavagen 27
Stockholm S-121 28
SWEDEN
Phone: +46 708 32 16 08
EMail: henrik@levkowetz.com
Aboba, et al. Standards Track [Page 59]
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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 [IEEE-802.11], 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
[IEEE-802.11], 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 [IEEE-802.11i]. These include TKIP,
which requires a single 128-bit encryption key and two 64-bit
authentication keys (one for each direction); and AES CCMP, which
requires a single 128-bit key (used in both directions) in order to
authenticate and encrypt data.
As with WEP, authentication and encryption keys are also required to
wrap the multicast encryption (and possibly, authentication) keys.
Aboba, et al. Standards Track [Page 60]
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Appendix B - Transient EAP Key (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.
| | |
| | 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 B-1 - TLS [RFC2246] Key Hierarchy
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Appendix C - EAP-TLS Key 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 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.
The key derivation scheme specified in RFC 2716 that was specified
prior to the introduction of the terminology MSK and EMSK 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)
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:
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AAA-Key(W,Z) = Octets W through Z includes of the AAA-Key.
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 C-1 describes the process by which the MSK,EMSK,IV and
ultimately the TSKs, are derived from the TLS Master Secret.
---+
| ^
| TLS Master Secret (TMS) |
| |
V |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | EAP |
| Master Session Key (MSK) | Method |
| Derivation | |
| | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ EAP ---+
| | | API ^
| MSK | EMSK | IV |
| | | |
V V V v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
| | |
| | |
| backend authentication 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
Aboba, et al. Standards Track [Page 63]
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Appendix D - Example 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 [IEEE-802.11i], 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 [IEEE-802.11i].
Aboba, et al. Standards Track [Page 64]
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Appendix E - Key Names and Scope in Existing Methods
This appendix specifies the key names and scope in methods that have
been published prior to the publication of this RFC. What is needed
in addition to the rules in Section 2.4 is the definition of what EAP
peer and server names are used, what Method-Id is used, and how these
are encoded.
EAP-TLS
The EAP-TLS Method-Id is provided by the concatenation of the peer
and server nonces.
Where certificates are used, the Session-Id scope is determined via
the EAP peer and server names, deduced from the altSubjectName in the
peer and server certificates.
Issue: What happens if a pre-shaked key ciphersuite is negotiated?
How are the EAP peer and server names determined?
EAP-AKA
The EAP-AKA Method-Id is the contents of the RAND field from the
AT_RAND attribute, followed by the contents of the AUTN field in the
AT_AUTN attribute.
The EAP peer name 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 reauthentication identity. The EAP
server name is an empty string.
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 EAP peer name 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 reauthentication identity. The EAP
server name is an empty string.
Aboba, et al. Standards Track [Page 65]
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Appendix F - Security Association Examples
EAP Method SA Example: EAP-TLS
In EAP-TLS [RFC2716], after the EAP authentication the client (peer)
and server can store the following information:
o Implicitly, the EAP method this SA refers to (EAP-TLS)
o Session identifier (a value selected by the server)
o Certificate of the other party (server stores the client's
certificate and vice versa)
o Ciphersuite and compression method
o TLS Master secret (known as the EAP-TLS Master Key)
o SA lifetime (ensuring that the SA is not stored forever)
o If the client has multiple different credentials (certificates
and corresponding private keys), a pointer to those credentials
When the server initiates EAP-TLS, the client can look up the EAP-TLS
SA based on the credentials it was going to use (certificate and
private key), and the expected credentials (certificate or name) of
the server. If an EAP-TLS SA exists, and it is not too old, the
client informs the server about the existence of this SA by including
its Session-Id in the TLS ClientHello message. The server then looks
up the correct SA based on the Session-Id (or detects that it doesn't
yet have one).
EAP Method SA Example: EAP-AKA
In EAP-AKA [I-D.arkko-pppext-eap-aka], after EAP authentication the
client and server can store the following information:
o Implicitly, the EAP method this SA refers to (EAP-AKA)
o A re-authentication pseudonym
o The client's permanent identity (IMSI)
o Replay protection counter
o Authentication key (K_aut)
o Encryption key (K_encr)
o Original Master Key (MK)
o SA lifetime (ensuring that the SA is not stored forever)
When the server initiates EAP-AKA, the client can look up the EAP-AKA
SA based on the credentials it was going to use (permanent identity).
If an EAP-AKA SA exists, and it is not too old, the client informs
the server about the existence of this SA by sending its re-
authentication pseudonym as its identity in EAP Identity Response
message, instead of its permanent identity. The server then looks up
the correct SA based on this identity.
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AAA SA Example: RADIUS
In RADIUS, where shared secret authentication is used, the client and
server store each other's IP address and the shared secret, which is
used to calculate the Response Authenticator [RFC2865] and Message-
Authenticator [RFC3579] values, and to encrypt some attributes (such
as the AAA-Key, see [RFC3580] Section 3.16).
Where IPsec is used to protect RADIUS [RFC3579] and IKE is used for
key management, the parties store information necessary to
authenticate and authorize the other party (e.g. certificates, trust
anchors and names). The IKE exchange results in IKE Phase 1 and Phase
2 SAs containing information used to protect the conversation
(session keys, selected ciphersuite, etc.)
AAA SA Example: Diameter with TLS
When using Diameter protected by TLS, the parties store information
necessary to authenticate and authorize the other party (e.g.
certificates, trust anchors and names). The TLS handshake results in
a short-term TLS SA that contains information used to protect the
actual communications (session keys, selected TLS ciphersuite, etc.).
Service SA Example: 802.11i
[IEEE802.11i] Section 8.4.1.1 defines the security associations used
within IEEE 802.11. A summary follows; the standard should be
consulted for details.
o Pairwise Master Key Security Association (PMKSA)
The PMKSA is a bi-directional SA, used by both parties for sending
and receiving. The PMKSA is the Root Service SA. It is created
on the peer when EAP authentication completes successfully or a
pre-shared key is configured. The PMKSA is created on the
authenticator when the PMK is received or created on the
authenticator or a pre-shared key is configured. The PMKSA is
used to create the PTKSA. PMKSAs are cached for their lifetimes.
The PMKSA consists of the following elements:
- PMKID (security association identifier)
- Authenticator MAC address
- PMK
- Lifetime
- Authenticated Key Management Protocol (AKMP)
- Authorization parameters specified by the AAA server or
by local configuration. This can include
parameters such as the peer's authorized SSID.
Aboba, et al. Standards Track [Page 67]
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On the peer, this information can be locally
configured.
- Key replay counters (for EAPOL-Key messages)
- Reference to PTKSA (if any), needed to:
o delete it (e.g. AAA server-initiated disconnect)
o replace it when a new four-way handshake is done
- Reference to accounting context, the details of which depend
on the accounting protocol used, the implementation
and administrative details. In RADIUS, this could include
(e.g. packet and octet counters, and Acct-Multi-Session-Id).
o Pairwise Transient Key Security Association (PTKSA)
The PTKSA is a bi-directional SA created as the result of a
successful four-way handshake. The PTKSA is a unicast service SA.
There may only be one PTKSA between a pair of peer and
authenticator MAC addresses. PTKSAs are cached for the lifetime
of the PMKSA. Since the PTKSA is tied to the PMKSA, it only has
the additional information from the 4-way handshake. The PTKSA
consists of the following:
- Key (PTK)
- Selected ciphersuite
- MAC addresses of the parties
- Replay counters, and ciphersuite specific state
- Reference to PMKSA: This is needed when:
o A new four-way handshake is needed (lifetime, TKIP
countermeasures), and we need to know which PMKSA to use
o Group Transient Key Security Association (GTKSA)
The GTKSA is a uni-directional SA created based on the four-way
handshake or the group key handshake. The GTKSA is a multicast
service SA. A GTKSA consists of the following:
- Direction vector (whether the GTK is used for transmit or receive)
- Group cipher suite selector
- Key (GTK)
- Authenticator MAC address
- Via reference to PMKSA, or copied here:
o Authorization parameters
o Reference to accounting context
Service SA Example: IKEv2/IPsec
Note that this example is intended to be informative, and it does
not necessarily include all information stored.
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o IKEv2 SA
- Protocol version
- Identities of the parties
- IKEv2 SPIs
- Selected ciphersuite
- Replay protection counters (Message ID)
- Keys for protecting IKEv2 messages (SK_ai/SK_ar/SK_ei/SK_er)
- Key for deriving keys for IPsec SAs (SK_d)
- Lifetime information
- On the authenticator, service authorization information
received from the backend authentication server.
When processing an incoming message, the correct SA is looked up
based on the SPIs.
o IPsec SAs/SPD
- Traffic selectors
- Replay protection counters
- Selected ciphersuite
- IPsec SPI
- Keys
- Lifetime information
- Protocol mode (tunnel or transport)
The correct SA is looked up based on SPI (for inbound packets), or
SPD traffic selectors (for outbound traffic). A separate IPsec SA
exists for each direction.
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Open issues relating to this specification are tracked on the
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Aboba, et al. Standards Track [Page 70]
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