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Internet Engineering Task Force (IETF)                    D. Nelson, Ed.Request for Comments: 6421                         Elbrys Networks, Inc.Category: Informational                                    November 2011ISSN: 2070-1721Crypto-Agility Requirementsfor 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 Contents1. Introduction ....................................................21.1. General ....................................................21.2. Requirements Language ......................................31.3. Publication Process ........................................32. A Working Definition of Crypto-Agility ..........................43. The Current State of RADIUS Security ............................54. The Requirements ................................................54.1. Overall Solution Approach ..................................54.2. Security Services ..........................................64.3. Backwards Compatibility ....................................74.4. Interoperability and Change Control ........................94.5. Scope of Work ..............................................94.6. Applicability of Automated Key Management Requirements .....95. Security Considerations ........................................106. Acknowledgments ................................................107. References .....................................................107.1. Normative References ......................................107.2. Informative References ....................................111.  Introduction1.1.  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 toRFC 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 andAppendix 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 inSection 4.3.Nelson                        Informational                     [Page 4]

RFC 6421         Crypto-Agility Requirements for RADIUS    November 20113.  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 Requirements4.1.  Overall Solution Approach   RADIUS crypto-agility solutions are not restricted to utilizing   technology described in existing RFCs.  Since RADIUS over IPsec is   already described inSection 5 of "RADIUS and IPv6" [RFC3162] andSection 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 20114.2.  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-agilityNelson                        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 servicesNelson                        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 20114.4.  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 notRFC 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:   oRFC 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   inSection 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.  References7.1.  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)",RFC2865, 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",BCP132,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",RFC3162, 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 2011Author's Address   David B. Nelson (editor)   Elbrys Networks, Inc.   282 Corporate Drive, Unit 1   Portsmouth, NH  03801   USA   EMail: d.b.nelson@comcast.netNelson                        Informational                    [Page 12]