RADEXT Working Group D. Nelson (Editor) INTERNET-DRAFT Elbrys Networks, Inc. Category: Informational Expires: October 16, 2011 16 April 2011 Crypto-Agility Requirements for Remote Dial-In User Service (RADIUS) draft-ietf-radext-crypto-agility-requirements-05.txt Abstract This memo describes the requirements for a crypto-agility solution for Remote Authentication Dial-In User Service (RADIUS). Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 16, 2011. Nelson Informational [Page 1]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 2011 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 BSD License. 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 . . . . . . . . . . . . . . . . . . . . . . . . . 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 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11 Nelson Informational [Page 2]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 2011 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", "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. 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 follows: 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]. Nelson Informational [Page 3]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 2011 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. 1.4. Publication Process RADIUS [RFC2865] is a widely deployed protocol that has attained Draft Standard status based on multiple independent interoperable implementations. 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 and interoperable, the RADEXT WG has adopted a two phase process for publication of crypto-agility solutions. In the initial phase, crypto-agility solutions adopted by the working group will be published on the Experimental Track. Experimental Track documents should contain a description of experimental deployments and implementations in progress, as well as an evaluation of the proposal against the requirements described in this document. Based on the proposal evaluations, implementation and deployment experience, and the results of interoperability tests, initial proposals will be evaluated for publication on the standards track. 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 Nelson Informational [Page 4]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 2011 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 request). 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. Nelson Informational [Page 5]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 2011 "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 "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 Nelson Informational [Page 6]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 2011 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 enable a NAS and RADIUS server to exchange confidential information such as keying material without disclosure to third parties. In order to accomplish this, 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 for authentication between the NAS and RADIUS server. 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. Nelson Informational [Page 7]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 2011 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. Nelson Informational [Page 8]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 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. 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) 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]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 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 use cryptographic modes of operation such as a counter mode cipher that require frequent key changes. However, the same time, it is recognized that features recommended in Section 4.2 such as support for PFS 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. 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 security. 7. Acknowledgments Thanks to all the reviewers and contributors, including Bernard Aboba, Joe Salowey and Glen Zorn. 8. Informative 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. [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. Nelson Informational [Page 10]
INTERNET-DRAFT RADIUS Crypto-Agility Requirements 16 April 2011 [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 2000. [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 USA Email: d.b.nelson@comcast.net Nelson Informational [Page 11]
