RADEXT Working Group                                  D. Nelson (Editor)
INTERNET-DRAFT                                     Elbrys Networks, Inc.
Category: Informational
Expires: October 16, November 2, 2011
16 April
1 May 2011

  Crypto-Agility Requirements for Remote Dial-In User Service (RADIUS)


   This memo describes the requirements for a crypto-agility solution
   for Remote Authentication Dial-In User Service (RADIUS).

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  General . . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.2   Requirements Language . . . . . . . . . . . . . . . . . . . 3
     1.3.  The Charge  . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.4   Publication Process . . . . . . . . . . . . . . . . . . . . 4
   2.  A Working Definition of Crypto-Agility  . . . . . . . . . . . . 4
   3.  The Current State of RADIUS Security  . . . . . . . . . . . . . 5
   4.  The Requirements  . . . . . . . . . . . . . . . . . . . . . . . 6
     4.1.  Overall Solution Approach . . . . . . . . . . . . . . . . . 6
     4.2.  Security Services . . . . . . . . . . . . . . . . . . . . . 6
     4.3.  Backwards Compatibility . . . . . . . . . . . . . . . . . . 8
     4.4.  Interoperability and Change Control . . . . . . . . . . . . 9
     4.5.  Scope of Work . . . . . . . . . . . . . . . . . . . . . . . 9
     4.6.  Applicability of Automated Key Management Requirements  . . 9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgments   . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Informative  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11  12

1.  Introduction

1.1.  General

   This memo describes the requirements for a crypto-agility solution
   for Remote Authentication Dial-In User Service (RADIUS).  This memo,
   when approved, reflects the consensus of the RADIUS Extensions
   (RADEXT) Working Group of the IETF as to the features, properties and
   limitations of the crypto-agility work item for RADIUS.  It also
   defines the term "crypto-agility" as used in this context, and
   provides the motivations for undertaking and completing this work.

   The requirements defined in this memo have been developed based on e-
   mail 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 memorialize 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",
   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.  The Charge

   At the IETF-66 meeting, the RADEXT WG was asked by members of the
   Security Area Directorate to undertake the action item to prepare a
   formal description of a crypto-agility work item, and corresponding
   milestones in the RADEXT Charter.  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.  That text reads as

      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

      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

1.4.  Publication Process

   RADIUS [RFC2865] is a widely deployed protocol that has attained
   Draft Standard status based on multiple independent interoperable
   implementations.  It  Therefore it is therefore highly desirable that a high level of
   interoperability and security be maintained for crypto-agility solutions.

   To ensure that crypto-agility solutions published on the standards
   track are well specified, secure specified and interoperable, the RADEXT WG has adopted
   a two phase process for standards-track publication of crypto-agility

   In the initial phase, crypto-agility solutions adopted by the working
   group will be published on the Experimental Track.  Experimental
   Track as Experimental.  These documents should
   contain a description of the implementations and experimental
   deployments and implementations 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 evaluations, implementation and deployment
   experience, and against the results requirements, summary of interoperability tests, initial
   proposals will be evaluated for publication on the standards track. experimental
   deployment experience and evidence of multiple interoperable

2.  A Working Definition of Crypto-Agility

   A generalized definition of crypto-agility was offered up at the
   RADEXT WG session during IETF-68.  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.

   Since RADIUS is a request/response protocol, the ability to negotiate
   cryptographic algorithms within a single RADIUS exchange is
   inherently limited.  While a RADIUS request can provide a list of
   supported cryptographic algorithms which can selected for use within
   a response, prior to the receipt of a response, the cryptographic
   algorithms utilized to provide security services within the request
   will need to be determined a-priori.

   Proposals MUST NOT introduce new capabilities negotiation features
   into the RADIUS protocol and crypto-agility solutions SHOULD NOT
   require changes to the RADIUS operational model as defined in "RADIUS
   Design Guidelines" [RFC6158] Section 3.1 and Appendix A.4.
   Similarly, 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.

   Acceptable solutions for determining the mechanisms to be used within
   a request include manual configuration as well as backward compatible
   negotiation techniques such as those described in Section 4.3.
   Solutions for determining the mechanisms to be used in a response
   include manual configuration and "advertise and select" mechanisms
   (e.g. selection within the response from mechanisms advertised in a

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

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 "RADIUS and IPv6" [RFC3162] Section 5 and
   [RFC3579] Section 4.2, this technique is already available to those
   who wish to use it.  Therefore, it is expected that proposals will
   utilize other techniques.

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 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.

   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.

   Solutions providing for encryption of entire RADIUS packets need not
   also provide support for encryption of individual RADIUS attributes.
   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 2
   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
     solutions are RECOMMENDED to support Perfect Forward Secrecy (PFS)
     with respect to session keys negotiated between the RADIUS client
     and server.

Limit key scope
     It is RECOMMENDED that solutions
     In order to enable a NAS and RADIUS server to exchange confidential
     information such as keying material without disclosure to third parties.  In order to accomplish this,
     parties, it is RECOMMENDED that a RADIUS crypto-agility solution be compatible
     with NAI-based Dynamic Peer Discovery [RADYN] as well as that it
     support the use of public key credentials X.509 certificates for authentication between the NAS and
     RADIUS server.  Manual configuration as well as 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 not

     For compatibility with existing operations, RADIUS crypto-agility
     solutions SHOULD also support pre-shared key credentials.  However,
     support for end-to-end confidentiality of attributes or direct communications between the NAS and RADIUS server
     is not required when pre-shared key credentials are used.

Prevent the Domino effect
     In order to prevent the domino effect, RADIUS crypto-agility
     solutions MUST enable each RADIUS client and server pair to
     authenticate utilizing unique credentials.

4.3.  Backwards Compatibility

   Solutions to the problem MUST demonstrate backward compatibility with existing
   RADIUS implementations.  That is, an implementation that supports
   both the crypto-agility solution 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.

   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 and in addition attributes providing security services
   using more secure algorithms can be included.  This approach allows a
   RADIUS packet to be processed by legacy implementations as well as by
   crypto-agile implementations, and does not result in additional
   response delays.

   In this approach to backward compatibility, legacy mechanisms are
   initially used between crypto-agile implementations.  However, if the
   responder indicates support for crypto-agility, future requests can
   omit use of legacy mechanisms.

   Probing techniques can be used avoid the use of legacy algorithms
   between crypto-agile implementations.  An initial request can omit
   use of legacy mechanisms, and 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.

   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 may be required to
   distinguish lack of support for crypto-agility from packet loss or
   other failure conditions.  As a result, 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.

4.4.  Interoperability and Change Control

   Proposals MUST indicate a willingness to cede change control to the

   Crypto-agility solutions MUST be interoperable between independent
   implementations based purely on the information provided in the

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

   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.  At the
   IETF-70 meeting, and leading up to that meeting, the RADEXT WG
   debated whether or not RFC 4107 would require a RADIUS Crypto-Agility
   solution to feature Automated Key Management (AKM).  The working
   group 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)

   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.

   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 use cryptographic modes of
      operation such as a counter mode cipher that require frequent key

   However, the same time, it is recognized that features recommended in
   Section 4.2 such as support for PFS 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 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.  IANA Considerations

   This document makes no request of IANA.

6.  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, the subject matter of this memo is all about

7.  Acknowledgments

   Thanks to all the reviewers and contributors, including Bernard
   Aboba, Pasi Eronen, Joe Salowey and Glen Zorn.

8.  Informative  References

8.1.  Normative References

          Barker, E. and A. Roginsky, "Transitions: Recommendation for
          Transitioning the Use of Cryptographic Algorithms and Key
          Lengths", NIST SP-800-131A, January 2011.

[RADYN]   Winter, S. and M. McCauley, "NAI-based Dynamic Peer Discovery
          for RADIUS over TLS and DTLS", Internet draft (work in
          progress), draft-ietf-radext-dynamic-discovery-02, March 2010.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC
          2548, March 1999.

[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote
          Authentication Dial In User Service (RADIUS)", RFC 2865, June

[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., "RADIUS Design Guidelines", BCP 158, RFC 6158,
          March 2011.

8.2.  Informative References

[RADYN]   Winter, S. and M. McCauley, "NAI-based Dynamic Peer Discovery
          for RADIUS over TLS and DTLS", Internet draft (work in
          progress), draft-ietf-radext-dynamic-discovery-02, March 2010.

[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.

[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.

[RFC5997] DeKok, A., "Use of Status-Server Packets in the Remote
          Authentication Dialin User Service (RADIUS) Protocol", RFC
          5997, August 2010.

[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., "RADIUS Design Guidelines", BCP 158, RFC 6158,
          March 2011.

Author's Address

   David B. Nelson
   Elbrys Networks, Inc.
   282 Corporate Drive, Unit 1
   Portsmouth, NH  03801

   Email: d.b.nelson@comcast.net