Internet Engineering Task Force (IETF) D. Nelson, Ed.
Request for Comments: 6421 Elbrys Networks, Inc.
Category: Informational November 2011
ISSN: 2070-1721
Crypto-Agility Requirements
for Remote Authentication Dial-In User Service (RADIUS)
Abstract
This memo describes the requirements for a crypto-agility solution
for Remote Authentication Dial-In User Service (RADIUS).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6421.
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RFC 6421 Crypto-Agility Requirements for RADIUS November 2011
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Table of Contents
1. Introduction ....................................................2
1.1. General ....................................................2
1.2. Requirements Language ......................................3
1.3. Publication Process ........................................3
2. A Working Definition of Crypto-Agility ..........................4
3. The Current State of RADIUS Security ............................5
4. The Requirements ................................................5
4.1. Overall Solution Approach ..................................5
4.2. Security Services ..........................................6
4.3. Backwards Compatibility ....................................7
4.4. Interoperability and Change Control ........................9
4.5. Scope of Work ..............................................9
4.6. Applicability of Automated Key Management Requirements .....9
5. Security Considerations ........................................10
6. Acknowledgments ................................................10
7. References .....................................................10
7.1. Normative References ......................................10
7.2. Informative References ....................................11
1. Introduction
1.1. General
At the IETF 66 meeting, the RADIUS Extensions (RADEXT) Working Group
(WG) was asked by members of the Security Area Directorate to prepare
a formal description of a crypto-agility work item and corresponding
charter milestones. After consultation with one of the Security Area
Directors (Russ Housley), text was initially proposed on the RADEXT
WG mailing list on October 26, 2006. The following summarizes that
proposal:
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RFC 6421 Crypto-Agility Requirements for RADIUS November 2011
The RADEXT WG will review the security requirements for crypto-
agility in IETF protocols, and identify the deficiencies of the
existing RADIUS protocol specifications against these
requirements. Specific attention will be paid to RFC 4962
[RFC4962].
The RADEXT WG will propose one or more specifications to remediate
any identified deficiencies in the crypto-agility properties of
the RADIUS protocol. The known deficiencies include the issue of
negotiation of substitute algorithms for the message digest
functions, the key-wrap functions, and the password-hiding
function. Additionally, at least one mandatory to implement
cryptographic algorithm will be defined in each of these areas, as
required.
This document describes the features, properties, and limitations of
RADIUS crypto-agility solutions; defines the term "crypto-agility" as
used in this context; and provides the motivations for this work.
The requirements defined in this memo have been developed based on
email messages posted to the RADEXT WG mailing list, which may be
found in the archives of that list. The purpose of framing the
requirements in this memo is to formalize and archive them for future
reference and to bring them explicitly to the attention of the IESG
and the IETF community as we proceed with this work.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
A RADIUS crypto-agility solution is not compliant with this
specification if it fails to satisfy one or more of the MUST or MUST
NOT statements. A solution 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".
1.3. Publication Process
RADIUS [RFC2865] is a widely deployed protocol that has attained
Draft Standard status based on multiple independent interoperable
implementations. Therefore, it is desirable that a high level of
interoperability be maintained for crypto-agility solutions.
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RFC 6421 Crypto-Agility Requirements for RADIUS November 2011
To ensure that crypto-agility solutions published on the standards
track are well specified and interoperable, the RADEXT WG has adopted
a two phase process for standards-track publication of crypto-agility
solutions.
In the initial phase, crypto-agility solutions adopted by the working
group will be published as Experimental. These documents should
contain a description of the implementations and experimental
deployments in progress as well as an evaluation of the proposal
against the requirements described in this document.
The working group will then select proposals to advance on the
standards track. Criteria to be used include evaluation of the
proposal against the requirements, summary of the experimental
deployment experience, and evidence of multiple interoperable
implementations.
2. A Working Definition of Crypto-Agility
Crypto-agility is the ability of a protocol to adapt to evolving
cryptography and security requirements. This may include the
provision of a modular mechanism to allow cryptographic algorithms to
be updated without substantial disruption to fielded implementations.
It may provide for the dynamic negotiation and installation of
cryptographic algorithms within protocol implementations (think of
Dynamic-Link Libraries (DLL)).
In the specific context of the RADIUS protocol and RADIUS
implementations, crypto-agility may be better defined as the ability
of RADIUS implementations to automatically negotiate cryptographic
algorithms for use in RADIUS exchanges, including the algorithms used
to integrity protect and authenticate RADIUS packets and to hide
RADIUS attributes. This capability covers all RADIUS message types:
Access-Request/Response, Accounting-Request/Response, CoA/Disconnect-
Request/Response, and Status-Server. Negotiation of cryptographic
algorithms MAY occur within the RADIUS protocol, or within a lower
layer such as the transport layer.
Proposals MUST NOT introduce generic new capability negotiation
features into the RADIUS protocol or require changes to the RADIUS
operational model as defined in "RADIUS Design Guidelines" [RFC6158],
Section 3.1 and Appendix A.4. A proposal SHOULD focus on the crypto-
agility problem and nothing else. For example, proposals SHOULD NOT
require new attribute formats and SHOULD be compatible with the
guidance provided in [RFC6158], Section 2.3. Issues of backward
compatibility are described in more detail in Section 4.3.
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3. The Current State of RADIUS Security
RADIUS packets, as defined in [RFC2865], are protected by an MD5
message integrity check (MIC) within the Authenticator field of
RADIUS packets other than Access-Request [RFC2865] and Status-Server
[RFC5997]. The Message-Authenticator Attribute utilizes HMAC-MD5 to
authenticate and integrity protect RADIUS packets.
While RADIUS does not support confidentiality of entire packets,
various RADIUS attributes support encrypted (also known as "hidden")
values, including User-Password (defined in [RFC2865], Section 5.2),
Tunnel-Password (defined in [RFC2868], Section 3.5), and various
Vendor-Specific Attributes, such as the MS-MPPE-Send-Key and
MS-MPPE-Recv-Key attributes (defined in [RFC2548], Section 2.4).
Generally speaking, the hiding mechanism uses a stream cipher based
on a key stream from an MD5 digest. Attacks against this mechanism
are described in "RADIUS Support for EAP" [RFC3579], Section 4.3.4.
"Updated Security Considerations for the MD5 Message-Digest and the
HMAC-MD5 Algorithms" [RFC6151] discusses security considerations for
use of the MD5 and HMAC-MD5 algorithms. While the advances in MD5
collisions do not immediately compromise the use of MD5 or HMAC-MD5
for the purposes used within RADIUS absent knowledge of the
RADIUS shared secret, the progress toward compromise of MD5's basic
cryptographic assumptions has resulted in the deprecation of MD5
usage in a variety of applications. As noted in [RFC6151],
Section 2:
MD5 is no longer acceptable where collision resistance is required
such as digital signatures. It is not urgent to stop using MD5 in
other ways, such as HMAC-MD5; however, since MD5 must not be used
for digital signatures, new protocol designs should not employ
HMAC-MD5.
4. The Requirements
4.1. Overall Solution Approach
RADIUS crypto-agility solutions are not restricted to utilizing
technology described in existing RFCs. Since RADIUS over IPsec is
already described in Section 5 of "RADIUS and IPv6" [RFC3162] and
Section 4.2 of [RFC3579], this technique is already available to
those who wish to use it. Therefore, it is expected that proposals
will utilize other techniques.
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4.2. Security Services
Proposals MUST support the negotiation of cryptographic algorithms
for per-packet integrity/authentication protection. Proposals also
MUST support per-packet replay protection for all RADIUS message
types. Crypto-agility solutions MUST specify mandatory-to-implement
cryptographic algorithms for each defined mechanism.
Crypto-agility solutions MUST avoid security compromise, even in
situations where the existing cryptographic algorithms utilized by
RADIUS implementations are shown to be weak enough to provide little
or no security (e.g., in the event of compromise of the legacy RADIUS
shared secret). Included in this would be protection against
bidding-down attacks. In analyzing the resilience of a crypto-
agility solution, it can be assumed that RADIUS requesters and
responders can be configured to require the use of new secure
algorithms in the event of a compromise of existing cryptographic
algorithms or the legacy RADIUS shared secret.
Guidance on acceptable algorithms can be found in [NIST-SP800-131A].
It is RECOMMENDED that mandatory-to-implement cryptographic
algorithms be chosen from among those classified as "Acceptable" with
no known deprecation date from within this or successor documents.
It is RECOMMENDED that solutions provide support for confidentiality,
either by supporting encryption of entire RADIUS packets or by
encrypting individual RADIUS attributes. Proposals supporting
confidentiality MUST support the negotiation of cryptographic
algorithms for encryption.
Support for encryption of individual RADIUS attributes is OPTIONAL
for solutions that provide encryption of entire RADIUS packets.
Solutions providing for encryption of individual RADIUS attributes
are REQUIRED to provide support for improving the confidentiality of
existing encrypted (sometimes referred to as "hidden") attributes as
well as encrypting attributes (such as location attributes) that are
currently transmitted in cleartext.
In addition to the goals referred to above, [RFC4962] Section 3
describes additional security requirements, which translate into the
following requirements for RADIUS crypto-agility solutions:
Strong, fresh session keys:
RADIUS crypto-agility solutions are REQUIRED to generate fresh
session keys for use between the RADIUS client and server. In
order to prevent the disclosure of one session key from aiding an
attacker in discovering other session keys, RADIUS crypto-agility
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solutions are RECOMMENDED to support Perfect Forward Secrecy (PFS)
with respect to session keys negotiated between the RADIUS client
and server.
Limit key scope:
In order to enable a Network Access Server (NAS) and RADIUS server
to exchange confidential information such as keying material
without disclosure to third parties, it is RECOMMENDED that a
RADIUS crypto-agility solution support X.509 certificates for
authentication between the NAS and RADIUS server. Manual
configuration or automated discovery mechanisms such as NAI-based
Dynamic Peer Discovery [RADYN] can be used to enable
direct NAS-RADIUS server communications. Support for end-to-end
confidentiality of RADIUS attributes is OPTIONAL.
For compatibility with existing operations, RADIUS crypto-agility
solutions SHOULD also support pre-shared key credentials.
However, support for direct communications between the NAS and
RADIUS server is OPTIONAL when pre-shared key credentials are
used.
4.3. Backwards Compatibility
Solutions MUST demonstrate backward compatibility with existing
RADIUS implementations. That is, an implementation that supports
both crypto-agility and legacy mechanisms MUST be able to talk with
legacy RADIUS clients and servers (using the legacy mechanisms).
While backward compatibility is needed to ease the transition between
legacy RADIUS and crypto-agile RADIUS, use of legacy mechanisms is
only appropriate prior to the compromise of those mechanisms. After
legacy mechanisms have been compromised, secure algorithms MUST be
used so that backward compatibility is no longer possible.
Since RADIUS is a request/response protocol, the ability to negotiate
cryptographic algorithms within a single RADIUS exchange is
inherently limited. Prior to receipt of a response, a requester will
not know what algorithms are supported by the responder. Therefore,
while a RADIUS request can provide a list of supported cryptographic
algorithms that can be selected for use within a response, prior to
the receipt of a response, the cryptographic algorithms utilized to
provide security services within an initial request will need to be
predetermined.
In order to enable a request to be handled both by legacy as well as
crypto-agile implementations, a request can be secured with legacy
algorithms was well as with attributes providing security services
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using more secure algorithms. This approach allows a RADIUS packet
to be processed by legacy implementations as well as by crypto-agile
implementations, and it does not result in additional response
delays. If this technique is used, credentials used with legacy
algorithms MUST be cryptographically independent of the credentials
used with the more secure algorithms, so that compromise of the
legacy credentials does not result in compromise of the credentials
used with more secure algorithms.
In this approach to backward compatibility, legacy mechanisms are
initially used in requests sent between crypto-agile implementations.
However, if the responder indicates support for crypto-agility,
future requests can use more secure mechanisms. Note that if a
responder is upgraded and then subsequently needs to be downgraded
(e.g., due to bugs), this could result in requesters being unable to
communicate with the downgraded responder unless a mechanism is
provided to configure the requester to re-enable use of legacy
algorithms.
Probing techniques can be used to avoid the use of legacy algorithms
in requests sent between crypto-agile implementations. For example,
an initial request can omit use of legacy mechanisms. If a response
is received, then the recipient can be assumed to be crypto-agile and
future requests to that recipient can utilize secure mechanisms.
Similarly, the responder can assume that the requester supports
crypto-agility and can prohibit use of legacy mechanisms in future
requests. Note that if a requester is upgraded and then subsequently
needs to be downgraded (e.g., due to bugs), this could result in the
requester being unable to interpret responses, unless a mechanism is
provided to configure the responder to re-enable use of legacy
algorithms.
If a response is not received, in the absence of information
indicating responder support for crypto-agility (such as pre-
configuration or previous receipt of a crypto-agile response), a new
request can be composed utilizing legacy mechanisms.
Since legacy implementations not supporting crypto-agility will
silently discard requests not protected by legacy algorithms rather
than returning an error, repeated requests can be required to
distinguish lack of support for crypto-agility from packet loss or
other failure conditions. Therefore, probing techniques can delay
initial communication between crypto-agile requesters and legacy
responders. This can be addressed by upgrading the responders (e.g.,
RADIUS servers) first.
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4.4. Interoperability and Change Control
Proposals MUST indicate a willingness to cede change control to the
IETF.
Crypto-agility solutions MUST be interoperable between independent
implementations based purely on the information provided in the
specification.
4.5. Scope of Work
Crypto-agility solutions MUST apply to all RADIUS packet types,
including Access-Request, Access-Challenge, Access-Reject,
Access-Accept, Accounting-Request, Accounting-Response, Status-Server
and CoA/Disconnect messages.
Since it is expected that the work will occur purely within RADIUS or
in the transport, message data exchanged with Diameter SHOULD NOT be
affected.
Proposals MUST discuss any inherent assumptions about, or limitations
on, client/server operations or deployment and SHOULD provide
recommendations for transition of deployments from legacy RADIUS to
crypto-agile RADIUS. Issues regarding cipher-suite negotiation,
legacy interoperability, and the potential for bidding-down attacks
SHOULD be among these discussions.
4.6. Applicability of Automated Key Management Requirements
"Guidelines for Cryptographic Key Management" [RFC4107] provides
guidelines for when automated key management is necessary.
Consideration was given as to whether or not RFC 4107 would require a
RADIUS crypto-agility solution to feature Automated Key Management
(AKM). It was determined that AKM was not inherently required for
RADIUS based on the following points:
o RFC 4107 requires AKM for protocols that involve O(n^2) keys.
This does not apply to RADIUS deployments, which require O(n)
keys.
o Requirements for session key freshness can be met without AKM, for
example, by utilizing a pre-shared key along with an exchange of
nonces.
o RADIUS does not require the encryption of large amounts of data in
a short time.
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RFC 6421 Crypto-Agility Requirements for RADIUS November 2011
o Organizations already have operational practices to manage
existing RADIUS shared secrets to address key changes required as
a result of personnel changes.
o The crypto-agility solution can avoid the use of cryptographic
modes of operation, such as a counter mode cipher, that require
frequent key changes.
However, at the same time, it is recognized that features recommended
in Section 4.2 such as support for perfect forward secrecy and direct
transport of keys between a NAS and RADIUS server can only be
provided by a solution supporting AKM. As a result, support for
Automated Key Management is RECOMMENDED within a RADIUS crypto-
agility solution.
Also, automated key management is REQUIRED for RADIUS crypto-agility
solutions that use cryptographic modes of operation that require
frequent key changes.
5. Security Considerations
Potential attacks against the RADIUS protocol are described in
[RFC3579], Section 4.1, and details of known exploits as well as
potential mitigations are discussed in [RFC3579], Section 4.3.
This specification describes the requirements for new cryptographic
protection mechanisms, including the modular selection of algorithms
and modes. Therefore, all the subject matter of this memo is related
to security.
6. Acknowledgments
Thanks to all the reviewers and contributors, including Bernard
Aboba, Mary Barnes, Pasi Eronen, Dan Romascanu, Joe Salowey, and Glen
Zorn.
7. References
7.1. Normative References
[NIST-SP800-131A]
Barker, E. and A. Roginsky, "Transitions: Recommendation
for Transitioning the Use of Cryptographic Algorithms and
Key Lengths", NIST SP-800-131A, January 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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RFC 6421 Crypto-Agility Requirements for RADIUS November 2011
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, June 2000.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
[RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication,
Authorization, and Accounting (AAA) Key Management", BCP
132, RFC 4962, July 2007.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, March 2011.
[RFC6158] DeKok, A., Ed., and G. Weber, "RADIUS Design Guidelines",
BCP 158, RFC 6158, March 2011.
7.2. Informative References
[RADYN] Winter, S. and M. McCauley, "NAI-based Dynamic Peer
Discovery for RADIUS/TLS and RADIUS/DTLS", Work in
Progress, July 2011.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
RFC 2548, March 1999.
[RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege,
M., and I. Goyret, "RADIUS Attributes for Tunnel Protocol
Support", RFC 2868, June 2000.
[RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC
3162, August 2001.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.
[RFC5997] DeKok, A., "Use of Status-Server Packets in the Remote
Authentication Dial In User Service (RADIUS) Protocol",
RFC 5997, August 2010.
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RFC 6421 Crypto-Agility Requirements for RADIUS November 2011
Author's Address
David B. Nelson (editor)
Elbrys Networks, Inc.
282 Corporate Drive, Unit 1
Portsmouth, NH 03801
USA
EMail: d.b.nelson@comcast.net
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