Network Working Group R. Housley
Request for Comments: 4962 Vigil Security
BCP: 132 B. Aboba
Category: Best Current Practice Microsoft
July 2007
Guidance for Authentication, Authorization, and Accounting (AAA)
Key Management
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document provides guidance to designers of Authentication,
Authorization, and Accounting (AAA) key management protocols. The
guidance is also useful to designers of systems and solutions that
include AAA key management protocols. Given the complexity and
difficulty in designing secure, long-lasting key management
algorithms and protocols by experts in the field, it is almost
certainly inappropriate for IETF working groups without deep
expertise in the area to be designing their own key management
algorithms and protocols based on Authentication, Authorization, and
Accounting (AAA) protocols. The guidelines in this document apply to
documents requesting publication as IETF RFCs. Further, these
guidelines will be useful to other standards development
organizations (SDOs) that specify AAA key management.
Table of Contents
1. Introduction ....................................................2
1.1. Requirements Specification .................................3
1.2. Mandatory to Implement .....................................3
1.3. Terminology ................................................3
2. AAA Environment Concerns ........................................5
3. AAA Key Management Requirements .................................7
4. AAA Key Management Recommendations .............................13
5. Security Considerations ........................................14
6. Normative References ...........................................15
7. Informative References .........................................15
Appendix: AAA Key Management History ..............................20
Acknowledgments ...................................................22
1. Introduction
This document provides architectural guidance to designers of AAA key
management protocols. The guidance is also useful to designers of
systems and solutions that include AAA key management protocols.
AAA key management often includes a collection of protocols, one of
which is the AAA protocol. Other protocols are used in conjunction
with the AAA protocol to provide an overall solution. These other
protocols often provide authentication and security association
establishment.
Given the complexity and difficulty in designing secure, long-lasting
key management algorithms and protocols by experts in the field, it
is almost certainly inappropriate for IETF working groups without
deep expertise in the area to be designing their own key management
algorithms and protocols based on Authentication, Authorization and
Accounting (AAA) protocols. These guidelines apply to documents
requesting publication as IETF RFCs. Further, these guidelines will
be useful to other standards development organizations (SDOs) that
specify AAA key management that depends on IETF specifications for
protocols such as Extensible Authentication Protocol (EAP) [RFC3748],
Remote Authentication Dial-In User Service (RADIUS) [RFC2865], and
Diameter [RFC3588].
In March 2003, at the IETF 56 AAA Working Group Session, Russ Housley
gave a presentation on "Key Management in AAA" [H]. That
presentation established the vast majority of the requirements
contained in this document. Over the last three years, this
collection of requirements have become known as the "Housley
Criteria".
1.1. Requirements Specification
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [RFC2119].
An AAA key management proposal is not compliant with this
specification if it fails to satisfy one or more of the MUST or MUST
NOT statements. An AAA key management proposal that satisfies all
the MUST, MUST NOT, SHOULD, and SHOULD NOT statements is said to be
"unconditionally compliant"; one that satisfies all the MUST and MUST
NOT statements but not all the SHOULD or SHOULD NOT requirements is
said to be "conditionally compliant".
1.2. Mandatory to Implement
The guidance provided in this document is mandatory to implement.
However, it is not mandatory to use. That is, configuration at the
time of deployment may result in a deployed implementation that does
not conform with all of these requirements.
For example, [RFC4072] enables EAP keying material to be delivered
from a AAA server to an AAA client without disclosure to third
parties. Thus, key confidentiality is mandatory to implement in
Diameter [RFC3588]. However, key confidentiality is not mandatory to
use.
1.3. Terminology
This section defines terms that are used in this document.
AAA
Authentication, Authorization, and Accounting (AAA). AAA
protocols include RADIUS [RFC2865] and Diameter [RFC3588].
Authenticator
The party initiating EAP authentication. The term
authenticator is used in [802.1X], and authenticator has the
same meaning in this document.
Backend authentication server
A backend authentication server is an entity that provides an
authentication service to an authenticator. This terminology
is also used in [802.1X].
CHAP
Challenge Handshake Authentication Protocol; a one-way
challenge/response authentication protocol defined in
[RFC1994].
EAP
Extensible Authentication Protocol, defined in [RFC3748].
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.
Key Wrap
The encryption of one symmetric cryptographic key in another.
The algorithm used for the encryption is called a key wrap
algorithm or a key encryption algorithm. The key used in the
encryption process is called a key-encryption key (KEK).
PAP
Password Authentication Protocol; a deprecated cleartext
password PPP authentication protocol, originally defined in
[RFC1334].
Party
A party is a processing entity that can be identified as a
single role in a protocol.
Peer
The end of the link that responds to the authenticator. In
[802.1X], this end is known as the supplicant.
PPP
Point-to-Point Protocol, defined in [RFC1661], provides support
for multiprotocol serial datalinks. PPP is the primary IP
datalink used for dial-in NAS connection service.
Secure Association Protocol
A protocol for managing security associations derived from EAP
and/or AAA exchanges. The protocol establishes a security
association, which includes symmetric keys and a context for
the use of the keys. An example of a Secure Association
Protocol is the 4-way handshake defined within [802.11i].
Session Keys
Keying material used to protect data exchanged after
authentication has successfully completed, using the negotiated
ciphersuite.
Network Access Server (NAS)
A device that provides an access service for a user to a
network. The service may be a network connection, or a value
added service such as terminal emulation, as described in
[RFC2881].
4-Way Handshake
A Secure Association Protocol, defined in [802.11i], which
confirms mutual possession of a Pairwise Master Key by two
parties and distributes a Group Key.
2. AAA Environment Concerns
Examples of serious flaws plague the history of key management
protocol development, starting with the very first attempt to define
a key management protocol in the open literature, which was published
in 1978 [NS]. A flaw and a fix were published in 1981 [DS], and the
fix was broken in 1994 [AN]. In 1995 [L], a new flaw was found in
the original 1978 version, in an area not affected by the 1981/1994
issue. All of these flaws were blindingly obvious once described,
yet no one spotted them earlier. Note that the original protocol, if
it were revised to employ certificates, which of course had yet to be
invented, was only three messages. Many proposed AAA key management
schemes are significantly more complicated.
This bit of history shows that key management protocols are subtle.
Experts can easily miss a flaw. As a result, peer review by multiple
experts is essential, especially since many proposed AAA key
management schemes are significantly more complicated. In addition,
formal methods can help uncover problems [M].
AAA-based key management is being incorporated into standards
developed by the IETF and other standards development organizations
(SDOs), such as IEEE 802. However, due to ad hoc development of
AAA-based key management, AAA-based key distribution schemes have
poorly understood security properties, even when well-studied
cryptographic algorithms are employed. More academic research is
needed to fully understand the security properties of AAA-based key
management in the diverse protocol environments where it is being
employed today. In the absence of such research results, pragmatic
guidance based on sound security engineering principles is needed.
In addition to the need for interoperability, cryptographic algorithm
independent solutions are greatly preferable. Without algorithm
independence, the AAA-based key management protocol must be changed
whenever a problem is discovered with any of the selected algorithms.
As AAA history shows, problems are inevitable. Problems can surface
due to age or design failure.
DES [FIPS46] was a well-designed encryption algorithm, and it
provided protection for many years. Yet, the 56-bit key size was
eventually overcome by Moore's Law. No significant cryptographic
deficiencies have been discovered in DES.
The history of AAA underlines the importance of algorithm
independence as flaws have been found in authentication mechanisms
such as CHAP, MS-CHAPv1 [SM1], MS-CHAPv2 [SM2], Kerberos
[W][BM][DLS], and LEAP [B]. Unfortunately, RADIUS [RFC2865] mandates
use of the MD5 algorithm for integrity protection, which has known
deficiencies, and RADIUS has no provisions to negotiate substitute
algorithms. Similarly, the vendor-specific key wrap mechanism
defined in [RFC2548] has no provisions to negotiate substitute
algorithms.
The principle of least privilege is an important design guideline.
This principle requires that a party be given no more privilege than
necessary to perform the task assigned to them. Ensuring least
privilege requires clear identification of the tasks assigned to each
party, and explicit determination of the minimum set of privileges
required to perform those tasks. Only those privileges necessary to
perform the tasks are granted. By denying to parties unneeded
privileges, those denied privileges cannot be used to circumvent
security policy or enable attackers. With this principle in mind,
AAA key management schemes need to be designed in a manner where each
party has only the privileges necessary to perform their role. That
is, no party should have access to any keying material that is not
needed to perform their own role. A party has access to a particular
key if it has access to all of the secret information needed to
derive it.
EAP is being used in new ways. The inclusion of support for EAP
within Internet Key Exchange Protocol version 2 (IKEv2) and the
standardization of robust Wireless LAN security [802.11i] based on
EAP are two examples. EAP has also been proposed within IEEE 802.16e
[802.16e] and by the IETF PANA Working Group. AAA-based key
management is being incorporated into standards developed by the IETF
and other standards development organizations (SDOs), such as IEEE
802. However, due to ad hoc development of AAA-based key management,
AAA-based key distribution schemes have poorly understood security
properties, even when well-studied cryptographic algorithms are
employed. More academic research is needed to fully understand the
security properties of AAA-based key management in the diverse
protocol environments where it is being employed today. In the
absence of research results, pragmatic guidance based on sound
security engineering principles is needed.
EAP selects one end-to-end authentication mechanism. The mechanisms
defined in [RFC3748] only support unilateral authentication, and they
do not support mutual authentication or key derivation. As a result,
these mechanisms do not fulfill the security requirements for many
deployment scenarios, including Wireless LAN authentication
[RFC4017].
To ensure adequate security and interoperability, EAP applications
need to specify mandatory-to-implement algorithms. As described in
[RFC3748], EAP methods seeking publication as an IETF RFC need to
document their security claims. However, some EAP methods are not
based on well-studied models, which makes the validity of these
security claims difficult to determine.
In the context of EAP, the EAP peer and server are the parties
involved in the EAP method conversation, and they gain access to key
material when the conversation completes successfully. However, the
lower-layer needs keying material to provide the desired protection
through the use of cryptographic mechanisms. As a result, a "pass-
through" mode is used to provide the keying material, and the lower-
layer keying material is replicated from the AAA server to the
authenticator. The only parties authorized to obtain all of the
keying material are the EAP peer and server; the authenticator
obtains only the keying material necessary for its specific role. No
other party can obtain direct access to any of the keying material;
however, other parties may receive keys that are derived from this
keying material for a specific purpose as long as the requirements
defined in the next section are met.
3. AAA Key Management Requirements
The overall goal of AAA key management is to provide cryptographic
keying material in situations where key derivation cannot be used by
the peer and authenticator. It may not be possible because the
authenticator lacks computational power, because it lacks the
resources necessary to implement the various authentication
mechanisms that might be required, or because it is undesirable for
each authenticator to engage in a separate key management
conversation.
This section provides guidance to AAA protocol designers, EAP method
designers, and security association protocol designers. Acceptable
solutions MUST meet all of these requirements.
Cryptographic algorithm independent
The AAA key management protocol MUST be cryptographic algorithm
independent. However, an EAP method MAY depend on a specific
cryptographic algorithm. The ability to negotiate the use of a
particular cryptographic algorithm provides resilience against
compromise of a particular cryptographic algorithm. Algorithm
independence is also REQUIRED with a Secure Association
Protocol if one is defined. This is usually accomplished by
including an algorithm identifier and parameters in the
protocol, and by specifying the algorithm requirements in the
protocol specification. While highly desirable, the ability to
negotiate key derivation functions (KDFs) is not required. For
interoperability, at least one suite of mandatory-to-implement
algorithms MUST be selected. Note that without protection by
IPsec as described in [RFC3579] Section 4.2, RADIUS [RFC2865]
does not meet this requirement, since the integrity protection
algorithm cannot be negotiated.
This requirement does not mean that a protocol must support
both public-key and symmetric-key cryptographic algorithms. It
means that the protocol needs to be structured in such a way
that multiple public-key algorithms can be used whenever a
public-key algorithm is employed. Likewise, it means that the
protocol needs to be structured in such a way that multiple
symmetric-key algorithms can be used whenever a symmetric-key
algorithm is employed.
Strong, fresh session keys
While preserving algorithm independence, session keys MUST be
strong and fresh. Each session deserves an independent session
key. Fresh keys are required even when a long replay counter
(that is, one that "will never wrap") is used to ensure that
loss of state does not cause the same counter value to be used
more than once with the same session key.
Some EAP methods are capable of deriving keys of varying
strength, and these EAP methods MUST permit the generation of
keys meeting a minimum equivalent key strength. BCP 86
[RFC3766] offers advice on appropriate key sizes. The National
Institute for Standards and Technology (NIST) also offers
advice on appropriate key sizes in [SP800-57].
A fresh cryptographic key is one that is generated specifically
for the intended use. In this situation, a secure association
protocol is used to establish session keys. The AAA protocol
and EAP method MUST ensure that the keying material supplied as
an input to session key derivation is fresh, and the secure
association protocol MUST generate a separate session key for
each session, even if the keying material provided by EAP is
cached. A cached key persists after the authentication
exchange has completed. For the AAA/EAP server, key caching
can happen when state is kept on the server. For the NAS or
client, key caching can happen when the NAS or client does not
destroy keying material immediately following the derivation of
session keys.
Session keys MUST NOT be dependent on one another. Multiple
session keys may be derived from a higher-level shared secret
as long as a one-time value, usually called a nonce, is used to
ensure that each session key is fresh. The mechanism used to
generate session keys MUST ensure that the disclosure of one
session key does not aid the attacker in discovering any other
session keys.
Limit key scope
Following the principle of least privilege, parties MUST NOT
have access to keying material that is not needed to perform
their role. A party has access to a particular key if it has
access to all of the secret information needed to derive it.
Any protocol that is used to establish session keys MUST
specify the scope for session keys, clearly identifying the
parties to whom the session key is available.
Replay detection mechanism
The AAA key management protocol exchanges MUST be replay
protected, including AAA, EAP, and Secure Association Protocol
exchanges. Replay protection allows a protocol message
recipient to discard any message that was recorded during a
previous legitimate dialogue and presented as though it
belonged to the current dialogue.
Authenticate all parties
Each party in the AAA key management protocol MUST be
authenticated to the other parties with whom they communicate.
Authentication mechanisms MUST maintain the confidentiality of
any secret values used in the authentication process.
When a secure association protocol is used to establish session
keys, the parties involved in the secure association protocol
MUST identify themselves using identities that are meaningful
in the lower-layer protocol environment that will employ the
session keys. In this situation, the authenticator and peer
may be known by different identifiers in the AAA protocol
environment and the lower-layer protocol environment, making
authorization decisions difficult without a clear key scope.
If the lower-layer identifier of the peer will be used to make
authorization decisions, then the pair of identifiers
associated with the peer MUST be authorized by the
authenticator and/or the AAA server.
AAA protocols, such as RADIUS [RFC2865] and Diameter [RFC3588],
provide a mechanism for the identification of AAA clients;
since the EAP authenticator and AAA client are always co-
resident, this mechanism is applicable to the identification of
EAP authenticators.
When multiple base stations and a "controller" (such as a WLAN
switch) comprise a single EAP authenticator, the "base station
identity" is not relevant; the EAP method conversation takes
place between the EAP peer and the EAP server. Also, many base
stations can share the same authenticator identity. The
authenticator identity is important in the AAA protocol
exchange and the secure association protocol conversation.
Authentication mechanisms MUST NOT employ plaintext passwords.
Passwords may be used provided that they are not sent to
another party without confidentiality protection.
Peer and authenticator authorization
Peer and authenticator authorization MUST be performed. These
entities MUST demonstrate possession of the appropriate keying
material, without disclosing it. Authorization is REQUIRED
whenever a peer associates with a new authenticator. The
authorization checking prevents an elevation of privilege
attack, and it ensures that an unauthorized authenticator is
detected.
Authorizations SHOULD be synchronized between the peer, NAS,
and backend authentication server. Once the AAA key management
protocol exchanges are complete, all of these parties should
hold a common view of the authorizations associated with the
other parties.
In addition to authenticating all parties, key management
protocols need to demonstrate that the parties are authorized
to possess keying material. Note that proof of possession of
keying material does not necessarily prove authorization to
hold that keying material. For example, within an IEEE
802.11i, the 4-way handshake demonstrates that both the peer
and authenticator possess the same EAP keying material.
However, by itself, this possession proof does not demonstrate
that the authenticator was authorized by the backend
authentication server to possess that keying material. As
noted in RFC 3579 in Section 4.3.7, where AAA proxies are
present, it is possible for one authenticator to impersonate
another, unless each link in the AAA chain implements checks
against impersonation. Even with these checks in place, an
authenticator may still claim different identities to the peer
and the backend authentication server. As described in RFC
3748 in Section 7.15, channel binding is required to enable the
peer to verify that the authenticator claim of identity is both
consistent and correct.
Keying material confidentiality and integrity
While preserving algorithm independence, confidentiality and
integrity of all keying material MUST be maintained.
Confirm ciphersuite selection
The selection of the "best" ciphersuite SHOULD be securely
confirmed. The mechanism SHOULD detect attempted roll-back
attacks.
Uniquely named keys
AAA key management proposals require a robust key naming
scheme, particularly where key caching is supported. The key
name provides a way to refer to a key in a protocol so that it
is clear to all parties which key is being referenced. Objects
that cannot be named cannot be managed. All keys MUST be
uniquely named, and the key name MUST NOT directly or
indirectly disclose the keying material. If the key name is
not based on the keying material, then one can be sure that it
cannot be used to assist in a search for the key value.
Prevent the Domino effect
Compromise of a single peer MUST NOT compromise keying material
held by any other peer within the system, including session
keys and long-term keys. Likewise, compromise of a single
authenticator MUST NOT compromise keying material held by any
other authenticator within the system. In the context of a key
hierarchy, this means that the compromise of one node in the
key hierarchy must not disclose the information necessary to
compromise other branches in the key hierarchy. Obviously, the
compromise of the root of the key hierarchy will compromise all
of the keys; however, a compromise in one branch MUST NOT
result in the compromise of other branches. There are many
implications of this requirement; however, two implications
deserve highlighting. First, the scope of the keying material
must be defined and understood by all parties that communicate
with a party that holds that keying material. Second, a party
that holds keying material in a key hierarchy must not share
that keying material with parties that are associated with
other branches in the key hierarchy.
Group keys are an obvious exception. Since all members of the
group have a copy of the same key, compromise of any one of the
group members will result in the disclosure of the group key.
Bind key to its context
Keying material MUST be bound to the appropriate context. The
context includes the following.
o The manner in which the keying material is expected to be
used.
o The other parties that are expected to have access to the
keying material.
o The expected lifetime of the keying material. Lifetime
of a child key SHOULD NOT be greater than the lifetime of
its parent in the key hierarchy.
Any party with legitimate access to keying material can
determine its context. In addition, the protocol MUST ensure
that all parties with legitimate access to keying material have
the same context for the keying material. This requires that
the parties are properly identified and authenticated, so that
all of the parties that have access to the keying material can
be determined.
The context will include the peer and NAS identities in more
than one form. One (or more) name form is needed to identify
these parties in the authentication exchange and the AAA
protocol. Another name form may be needed to identify these
parties within the lower layer that will employ the session
key.
4. AAA Key Management Recommendations
Acceptable solutions SHOULD meet all of these requirements.
Confidentiality of identity
In many environments, it is important to provide
confidentiality protection for identities. However, this is
not important in other environments. For this reason, EAP
methods are encouraged to provide a mechanism for identity
protection of EAP peers, but such protection is not a
requirement.
Authorization restriction
If peer authorization is restricted, then the peer SHOULD be
made aware of the restriction. Otherwise, the peer may
inadvertently attempt to circumvent the restriction. For
example, authorization restrictions in an IEEE 802.11
environment include:
o Key lifetimes, where the keying material can only be used
for a certain period of time;
o SSID restrictions, where the keying material can only be
used with a specific IEEE 802.11 SSID;
o Called-Station-ID restrictions, where the keying material
can only be used with a single IEEE 802.11 BSSID; and
o Calling-Station-ID restrictions, where the keying
material can only be used with a single peer IEEE 802 MAC
address.
5. Security Considerations
This document provides architectural guidance to designers of AAA key
management protocols. The guidance is also useful to designers of
systems and solutions that include AAA key management protocols.
In some deployment scenarios, more than one party in the AAA key
management protocol can reside on the same host. For example, the
EAP authenticator and AAA client are expected to reside on the same
entity. Colocation enables a single unique authenticator identity to
be sent by the authenticator to the AAA server as well as by the
authenticator to the EAP peer. Use of the same identity in both
conversations enables the peer and AAA server to confirm that the
authenticator is consistent in its identification, avoiding potential
impersonation attacks. If the authenticator and AAA client are not
colocated, then the authenticator and AAA client identities will
differ, and the key scope will not be synchronized between the EAP
peer, authenticator, and server. Lack of key scope synchronization
enables a number of security vulnerabilities, including
impersonation. For this reason, a design needs to include mechanisms
to ensure that the key scope and key naming are unambiguous.
The AAA server is a trusted entity. When keying material is present
at all, it establishes keying material with the peer and distributes
keying material to the authenticator using the AAA protocol. It is
trusted to only distribute keying material to the authenticator that
was established with the peer, and it is trusted to provide that
keying material to no other parties. In many systems, keying
material established by the EAP peer and EAP server are combined with
publicly available data to derive other keys. The AAA server is
trusted to refrain from deriving these same keys even though it has
access to the secret values that are needed to do so.
The authenticator is also a trusted party. The authenticator is
trusted not to distribute keying material provided by the AAA server
to any other parties. If the authenticator uses a key derivation
function to derive additional keying material, the authenticator is
trusted to distribute the derived keying material only to the
appropriate party that is known to the peer, and no other party.
When this approach is used, care must be taken to ensure that the
resulting key management system meets all of the principles in this
document, confirming that keys used to protect data are to be known
only by the peer and authenticator.
EAP is used to authenticate the peer to the AAA/EAP server.
Following successful authentication, the AAA/EAP server authorizes
the peer. In many situations, this is accomplished by sending keying
material to the authenticator and the peer in separate protocol
messages. The authenticator is not directly authenticated to the
peer. Rather, the peer determines that the authenticator has been
authorized by the AAA/EAP server by confirming that the authenticator
has the same AAA/EAP server-provided keying material. In some
systems, explicit authenticator and peer mutual authentication is
possible. This is desirable since it greatly improves
accountability.
When MIB modules are developed for AAA protocols or EAP methods,
these MIB modules might include managed objects for keying material.
The existence of managed objects associated with keying material
offers an additional avenue for key compromise if these objects
include the keying material itself. Therefore, these MIB modules
MUST NOT include objects for private keys or symmetric keys.
However, these MIB modules MAY include management objects that expose
names and context associated with keys, and they MAY provide a means
to delete keys.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7. Informative References
[802.1X] IEEE Standards for Local and Metropolitan Area Networks:
Port based Network Access Control, IEEE Std 802.1X-2004,
December 2004.
[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 LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications:
Specification for Enhanced Security", IEEE 802.11i, July
2004.
[802.16e] Institute of Electrical and Electronics Engineers,
"Supplement to Standard for Telecommunications and
Information Exchange Between Systems -- LAN/MAN Specific
Requirements - Part 16: Air Interface for Fixed and Mobile
Broadband Wireless Access Systems -- Amendment for
Physical and Medium Access Control Layers for Combined
Fixed and Mobile Operation in Licensed Bands", Draft, IEEE
802.16e/D8, May 2005.
[AN] M. Abadi and R. Needham, "Prudent Engineering Practice for
Cryptographic Protocols", Proc. IEEE Computer Society
Symposium on Research in Security and Privacy, May 1994.
[B] Brewin, B., "LEAP attack tool author says he wants to
alert users to risks", Computerworld, October 17, 2003.
[BM] Bellovin, S. and M. Merrit, "Limitations of the Kerberos
authentication system", Proceedings of the 1991 Winter
USENIX Conference, pp. 253-267, 1991.
[DDNN39.2] DCA DDN Program Management Office, "MILNET TAC Access
Control", Defense Data Network Newsletter, DDN News 39,
Special Issue, 26 Apr 1985, <http://www.isi.edu/
in-notes/museum/ddn-news/ddn-news.n39.2>.
[DLS] Dole, B., Lodin, S. and E. Spafford, "Misplaced trust:
Kerberos 4 session keys", Proceedings of the Internet
Society Network and Distributed System Security Symposium,
pp. 60-70, March 1997.
[DS] D. Denning and G. Sacco. "Timestamps in key distributed
protocols", Communication of the ACM, 24(8):533--535,
1981.
[FIPS46] Federal Information Processing Standards Publication (FIPS
PUB) 46, Data Encryption Standard, 1977 January 15.
[H] Housley, R., "Key Management in AAA", Presentation to the
AAA WG at IETF 56, March 2003, <http://www.ietf.org/
proceedings/03mar/slides/aaa-5/index.html>.
[L] G. Lowe. "An attack on the Needham-Schroeder public key
authentication protocol", Information Processing Letters,
56(3):131--136, November 1995.
[M] Meadows, C., "Analysis of the Internet Key Exchange
Protocol using the NRL Protocol Analyser", Proceedings of
the 1999 IEEE Symposium on Security & Privacy, Oakland,
CA, USA, IEEE Computer Society, May 1999,
<http://chacs.nrl.navy.mil/publications/CHACS/1999/
1999meadows-IEEE99.pdf>.
[NS] R. Needham and M. Schroeder. "Using encryption for
authentication in large networks of computers",
Communications of the ACM, 21(12), December 1978.
[RFC0927] Anderson, B.A., "TACACS user identification Telnet
option", RFC 927, December 1984.
[RFC1334] Lloyd, B. and B. Simpson, "PPP Authentication Protocols",
RFC 1334, October 1992, Obsoleted by RFC 1994.
[RFC1492] Finseth, C., "An Access Control Protocol, Sometimes Called
TACACS", RFC 1492, July 1993.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC1968] Meyer, G., "The PPP Encryption Protocol (ECP)", RFC 1968,
June 1996.
[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2419] Sklower, K. and G. Meyer, "The PPP DES Encryption
Protocol, Version 2 (DESE-bis)", RFC 2419, September 1998.
[RFC2420] Hummert, K., "The PPP Triple-DES Encryption Protocol
(3DESE)", RFC 2420, September 1998.
[RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC
2433, October 1998.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
RFC 2548, March 1999.
[RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,
W., and G. Zorn, "Point-to-Point Tunneling Protocol
(PPTP)", RFC 2637, July 1999.
[RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC
2759, January 2000.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, June 2000.
[RFC2881] Mitton, D. and M. Beadles, "Network Access Server
Requirements Next Generation (NASREQNG) NAS Model", RFC
2881, July 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.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strength for Public
Keys Used For Exchanging Symmetric Keys", BCP 86, RFC
3766, April 2004.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
[RFC4072] Eronen, P., Ed., Hiller, T., and G. Zorn, "Diameter
Extensible Authentication Protocol (EAP) Application", RFC
4072, August 2005.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, December 2005.
[SM1] Schneier, B. and Mudge, "Cryptanalysis of Microsoft's
Point-to-Point Tunneling Protocol", Proceedings of the 5th
ACM Conference on Communications and Computer Security,
ACM Press, November 1998.
[SM2] Schneier, B. and Mudge, "Cryptanalysis of Microsoft's PPTP
Authentication Extensions (MS-CHAPv2)", CQRE 99,
Springer-Verlag, 1999, pp. 192-203.
[SP800-57] National Institute of Standards and Technology,
"Recommendation for Key Management", Special Publication
800-57, May 2006.
[W] Wu, T., "A Real-World Analysis of Kerberos Password
Security", Proceedings of the 1999 ISOC Network and
Distributed System Security Symposium,
<http://www.isoc.org/isoc/conferences/ndss/99/
proceedings/papers/wu.pdf>.
Appendix: AAA Key Management History
Protocols for Authentication, Authorization, and Accounting (AAA)
were originally developed to support deployments of Network Access
Servers (NASes). In the ARPAnet, the Terminal Access Controller
(TAC) provided a means for "dumb terminals" to access the network,
and the TACACS [RFC0927][RFC1492] AAA protocol was designed by BBN
under contract to the Defense Data Network Program Management Office
(DDN PMO) for this environment. [RFC1492] documents a later version
of TACACS, not the original version that was widely deployed in
ARPAnet and MILNET [DDNN39.2].
Later, additional AAA protocols were developed to support deployments
of NASes providing access to the Internet via PPP [RFC1661]. In
deployments supporting more than a modest number of users, it became
impractical for each NAS to contain its own list of users and
associated credentials. As a result, additional AAA protocols were
developed, including RADIUS [RFC2865] and Diameter [RFC3588]. These
protocols enabled a central AAA server to authenticate users
requesting network access, as well as providing authorization and
accounting.
While PPP [RFC1661] originally supported only PAP [RFC1334] and CHAP
[RFC1661] authentication, the limitations of these authentication
mechanisms became apparent. For example, both PAP and CHAP are
unilateral authentication schemes supporting only authentication of
the PPP peer to the NAS. Since PAP is a cleartext password scheme,
it is vulnerable to snooping by an attacker with access to the
conversation between the PPP peer and NAS. In addition, the use of
PAP creates vulnerabilities within RADIUS as described in Section 4.3
of [RFC3579]. As a result, use of PAP is deprecated. While CHAP, a
challenge-response scheme based on MD5, offers better security than
cleartext passwords, it does not provide for mutual authentication,
and CHAP is vulnerable to dictionary attack.
With the addition of the Encryption Control Protocol (ECP) to PPP
[RFC1968] as well as the definition of PPP ciphersuites in [RFC2419],
[RFC2420], and [RFC3078], the need arose to provide keying material
for use with link layer ciphersuites. As with user authentication,
provisioning of static keys on each NAS did not scale well.
Additional vendor-specific PPP authentication protocols such as
MS-CHAP [RFC2433] and MS-CHAPv2 [RFC2759] were developed to provide
mutual authentication as well as key derivation [RFC3079] for use
with negotiated ciphersuites, and they were subsequently adapted for
use with PPP-based VPNs [RFC2637]. As with PAP and CHAP, flaws were
subsequently found in these new mechanisms [SM1][SM2].
Even though PPP provided for negotiation of authentication
algorithms, addressing the vulnerabilities found in authentication
mechanisms still proved painful, since new code needed to be deployed
on PPP peers as well as on the AAA server. In order to enable more
rapid deployment of new authentication mechanisms, as well as fixes
for vulnerabilities found in existing methods, the Extensible
Authentication Protocol (EAP) [RFC3748] was developed, along with
support for centralized authentication via RADIUS/EAP [RFC3579].
By enabling "pass through" authentication on the NAS, EAP enabled
deployment of new authentication methods or updates to existing
methods by revising code only on the EAP peer and AAA server. The
initial authentication mechanisms defined in [RFC2284] (MD5-
Challenge, One-Time Password (OTP), and Generic Token Card (GTC))
only supported unilateral authentication, and these mechanisms do not
support key derivation. Subsequent authentication methods such as
EAP-TLS [RFC2716] supported mutual authentication and key derivation.
In order to support the provisioning of dynamic keying material for
link layer ciphersuites in an environment supporting centralized
authentication, a mechanism was needed for the transport of keying
material between the AAA server and NAS. Vendor-specific RADIUS
attributes were developed for this purpose [RFC2548].
Vulnerabilities were subsequently found in the key wrap technique, as
described in Section 4.3 of [RFC3579].
In theory, public key authentication mechanisms such as EAP-TLS are
capable of supporting mutual authentication and key derivation
between the EAP peer and NAS without requiring AAA key distribution.
However, in practice, such pure two-party schemes are rarely
deployed. Operation of a centralized AAA server significantly
reduces the effort required to deploy certificates to NASes, and even
though an AAA server may not be required for key derivation and
possibly authentication, its participation is required for service
authorization and accounting.
"Pass-through" authentication and AAA key distribution has retained
popularity even in the face of rapid improvements in processor and
memory capabilities. In addition to producing NAS devices of
increased capability for enterprise and carrier customers,
implementers have also produced low-cost/high-volume NAS devices such
as 802.11 Access Points, causing the resources available on an
average NAS to increase more slowly than Moore's law. Despite
widespread support for certificate handling and sophisticated key
derivation mechanisms such as IKEv1 [RFC2409] within host operating
systems, these security capabilities are rarely deployed on low-end
NASes and clients.
Even on more capable NASes, such as VPN servers, centralized
authentication and AAA key management has proven popular. For
example, one of the major limitations of IKEv1 [RFC2409] was the lack
of integration with EAP and AAA, requiring proprietary extensions to
enable use of IPsec VPNs by organizations deploying password or
authentication tokens. These limitations were addressed in IKEv2
[RFC4306], which while handling key derivation solely between the VPN
client and server, supports EAP methods for user authentication. In
order to enable cryptographic binding of EAP user authentication to
keys derived within the IKEv2 exchange, the transport of EAP-derived
keys within AAA is required where the selected EAP method supports
key derivation.
Acknowledgments
Many thanks to James Kempf, Sam Hartman, and Joe Salowey for their
quality review and encouragement.
Thanks to the IETF AAA Working Group and the IETF EAP Working Group
for their review and comment. The document is greatly improved by
their contribution.
Authors' Addresses
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
Phone: +1 703-435-1775
Fax: +1 703-435-1274
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: bernarda@microsoft.com
Phone: +1 425-706-6605
Fax: +1 425-936-7329
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