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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   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:

      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.

   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 4.3">Section 4.3.

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.

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

      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

   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.

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.

   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.

   [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.

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