Kitten Working Group A. Melnikov Internet-Draft Isode Limited Obsoletes: RFC 2831 (if approved) April 22, 2011 (if approved) Intended status: Informational Expires: October 24, 2011 Moving DIGEST-MD5 to Historic draft-ietf-kitten-digest-to-historic-04 Abstract This memo describes problems with the DIGEST-MD5 Simple Authentication and Security Layer (SASL) mechanism as specified in RFC 2831. It marks DIGEST-MD5 as OBSOLETE in the IANA Registry of SASL mechanisms, and moves RFC 2831 to Historic. status. 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). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on October 24, 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 Melnikov Expires October 24, 2011 [Page 1]
Internet-Draft Moving DIGEST-MD5 to Historic April 2011 described in the Simplified 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. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Security Considerations . . . . . . . . . . . . . . . . . . . 5 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1. Normative References . . . . . . . . . . . . . . . . . . . . 6 5.2. Informative References . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . 7 Melnikov Expires October 24, 2011 [Page 2]
Internet-Draft Moving DIGEST-MD5 to Historic April 2011 1. Overview [RFC2831] defined how HTTP Digest Authentication [RFC2617] can be used as a Simple Authentication and Security Layer (SASL) [RFC4422] mechanism for any protocol that has a SASL profile. It was intended both as an improvement over CRAM-MD5 [RFC2195] and as a convenient way to support a single authentication mechanism for web, email, LDAP, and other protocols. While it can be argued that it was an improvement over CRAM-MD5, many implementors commented that the additional complexity of DIGEST-MD5 made it difficult to implement fully and securely. Below is an incomplete list of problems with DIGEST-MD5 mechanism as specified in RFC 2831: 1. The mechanism had too many options and modes. Some of them were not well described and were not widely implemented. For example, DIGEST-MD5 allowed the "qop" directive to contain multiple values, but it also allowed for multiple qop directives to be specified. The handling of multiple options was not specified, which resulted in minor interoperability problems. Some implementations amalgamated multiple qop values into one, while others treated multiple qops as an error. Another example is the use of an empty authorization identity. In SASL an empty authorization identity means that the client is willing to authorize as the authentication identity. The document was not clear on whether the authzid must be omitted or can be specified with the empty value to convey this. The requirement for backward compatibility with HTTP Digest meant that the situation was even worse. For example DIGEST-MD5 required all usernames/ passwords which can be entirely represented in ISO-8859-1 charset to be down converted from UTF-8 to ISO-8859-1. Another example is use of quoted strings. Handling of characters that needed escaping was not properly described and the DIGEST-MD5 document had no examples to demonstrate correct behavior. 2. The document used ABNF from RFC 822 [RFC0822], which allows an extra construct and allows for "implied folding whitespace" to be inserted in many places. The difference from ABNF [RFC5234] was confusing for some implementors. As a result, many implementations didn't accept folding whitespace in many places where it was allowed. 3. The DIGEST-MD5 document uses the concept of a "realm" to define a collection of accounts. A DIGEST-MD5 server can support one or more realms. The DIGEST-MD5 document didn't provide any guidance on how realms should be named, and, more importantly, how they can be entered in User Interfaces (UIs). As the result many Melnikov Expires October 24, 2011 [Page 3]
Internet-Draft Moving DIGEST-MD5 to Historic April 2011 DIGEST-MD5 clients had confusing UIs, didn't allow users to enter a realm and/or didn't allow users to pick one of the server supported realms. 4. Use of username in the inner hash. The inner hash of DIGEST-MD5 is an MD5 hash of colon separated username, realm and password. Implementations may choose to store inner hashes instead of clear text passwords. While this has some useful properties, such as protection from compromise of authentication databases containing the same username and password on other servers, if a server with the username and password is compromised, however this was rarely done in practice. Firstly, the inner hash is not compatible with widely deployed Unix password databases, and second, changing the username would invalidate the inner hash. 5. Description of DES/3DES [DES] and RC4 security layers are inadequate to produce independently-developed interoperable implementations. In the DES/3DES case this was partly a problem with existing DES APIs. 6. DIGEST-MD5 outer hash (the value of the "response" directive) didn't protect the whole authentication exchange, which made the mechanism vulnerable to "man in the middle" (MITM) attacks, such as modification of the list of supported qops or ciphers. 7. The following features are missing from DIGEST-MD5, which make it insecure or unsuitable for use in protocols: A. Lack of channel bindings [RFC5056]. B. Lack of hash agility (i.e. no easy way to replace the MD5 hash function with another one). C. Lack of support for SASLPrep [RFC4013] or any other type of Unicode character normalization of usernames and passwords. The original DIGEST-MD5 document predates SASLPrep and doesn't recommend any Unicode character normalization. 8. The cryptographic primitives in DIGEST-MD5 are not up to today's standards, in particular: A. The MD5 hash is sufficiently weak to make a brute force attack on DIGEST-MD5 easy with common hardware [RFC6151]. B. Using the RC4 algorithm for the security layer without discarding the initial key stream output is prone to attack [RC4]. Melnikov Expires October 24, 2011 [Page 4]
Internet-Draft Moving DIGEST-MD5 to Historic April 2011 C. The DES cipher for the security layer is considered insecure due to its small key space [RFC3766]. Note that most of the problems listed above are already present in the HTTP Digest authentication mechanism. Because DIGEST-MD5 was defined as an extensible mechanism, it would be possible to fix most of the problems listed above. However this would increase implementation complexity of an already complex mechanism even further, so the effort would not be worth the cost. In addition, an implementation of a "fixed" DIGEST-MD5 specification would likely either not interoperate with any existing implementation of RFC 2831, or would be vulnerable to various downgrade attacks. Note that despite DIGEST-MD5 seeing some deployment on the Internet, this specification recommends obsoleting DIGEST-MD5 because DIGEST- MD5, as implemented, is not a reasonable candidate for further standardization and should be deprecated in favor of one or more new password-based mechanisms currently being designed. The SCRAM family of SASL mechanisms [RFC5802] has been developed to provide similar features as DIGEST-MD5 but with a better design. 2. Security Considerations Security issues are discussed through out this document. 3. IANA Considerations IANA is requested to change the "Intended usage" of the DIGEST-MD5 mechanism registration in the SASL mechanism registry to OBSOLETE. The SASL mechanism registry is specified in [RFC4422] and is currently available at: http://www.iana.org/assignments/sasl-mechanisms 4. Acknowledgements The author gratefully acknowledges the feedback provided by Chris Newman, Simon Josefsson, Kurt Zeilenga, Sean Turner and Abhijit Menon-Sen. Various text was copied from other RFCs, in particular from RFC 2831. Melnikov Expires October 24, 2011 [Page 5]
Internet-Draft Moving DIGEST-MD5 to Historic April 2011 5. References 5.1. Normative References [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication", RFC 2617, June 1999. [RFC2831] Leach, P. and C. Newman, "Using Digest Authentication as a SASL Mechanism", RFC 2831, May 2000. 5.2. Informative References [DES] National Institute of Standards and Technology, "Data Encryption Standard (DES)", FIPS PUB 46-3, October 1999. [RC4] Strombergson, J. and S. Josefsson, "Test vectors for the stream cipher RC4", draft-josefsson-rc4-test-vectors-02.txt (work in progress), June 2010. [RFC0822] Crocker, D., "Standard for the format of ARPA Internet text messages", STD 11, RFC 822, August 1982. [RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP AUTHorize Extension for Simple Challenge/Response", RFC 2195, September 1997. [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004. [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, February 2005. [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, June 2006. [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, November 2007. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, "Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010. Melnikov Expires October 24, 2011 [Page 6]
Internet-Draft Moving DIGEST-MD5 to Historic April 2011 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, March 2011. Author's Address Alexey Melnikov Isode Limited 5 Castle Business Village 36 Station Road Hampton, Middlesex TW12 2BX UK Email: Alexey.Melnikov@isode.com URI: http://www.melnikov.ca/ Melnikov Expires October 24, 2011 [Page 7]
