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rfc:rfc6421

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.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Nelson Informational [Page 1] RFC 6421 Crypto-Agility Requirements for RADIUS November 2011

 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

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:

Nelson Informational [Page 2] 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.

Nelson Informational [Page 3] 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.

Nelson Informational [Page 4] RFC 6421 Crypto-Agility Requirements for RADIUS November 2011

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.

Nelson Informational [Page 5] RFC 6421 Crypto-Agility Requirements for RADIUS November 2011

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

Nelson Informational [Page 6] RFC 6421 Crypto-Agility Requirements for RADIUS November 2011

    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

Nelson Informational [Page 7] RFC 6421 Crypto-Agility Requirements for RADIUS November 2011

 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.

Nelson Informational [Page 8] RFC 6421 Crypto-Agility Requirements for RADIUS November 2011

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.

Nelson Informational [Page 9] 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.

Nelson Informational [Page 10] 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.

Nelson Informational [Page 11] 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

Nelson Informational [Page 12]

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