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Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 2617 (Obsoleted by RFC 7235, RFC 7615, RFC 7616, RFC 7617) ** Obsolete normative reference: RFC 2831 (Obsoleted by RFC 6331) -- Obsolete informational reference (is this intentional?): RFC 822 (Obsoleted by RFC 2822) -- Obsolete informational reference (is this intentional?): RFC 4013 (Obsoleted by RFC 7613) Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Kitten Working Group A. Melnikov 3 Internet-Draft Isode Limited 4 Intended status: Informational June 24, 2010 5 Expires: December 26, 2010 7 Moving DIGEST-MD5 to Historic 8 draft-ietf-kitten-digest-to-historic-00 10 Abstract 12 This memo describes problems with the DIGEST-MD5 Simple 13 Authentication and Security Layer (SASL) mechanism as specified in 14 RFC 2831. It recommends that DIGEST-MD5 to be marked as OBSOLETE in 15 the IANA Registry of SASL mechanisms, and that RFC 2831 be moved to 16 Historic status. 18 Note 20 A revised version of this draft document will be submitted to the RFC 21 editor as a Proposed Standard for the Internet Community. Discussion 22 and suggestions for improvement are requested, and should be sent to 23 ietf-sasl@imc.org. 25 Status of this Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at http://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on December 26, 2010. 42 Copyright Notice 44 Copyright (c) 2010 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 This document may contain material from IETF Documents or IETF 58 Contributions published or made publicly available before November 59 10, 2008. The person(s) controlling the copyright in some of this 60 material may not have granted the IETF Trust the right to allow 61 modifications of such material outside the IETF Standards Process. 62 Without obtaining an adequate license from the person(s) controlling 63 the copyright in such materials, this document may not be modified 64 outside the IETF Standards Process, and derivative works of it may 65 not be created outside the IETF Standards Process, except to format 66 it for publication as an RFC or to translate it into languages other 67 than English. 69 Table of Contents 71 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4 73 2. Security Considerations . . . . . . . . . . . . . . . . . . . 6 75 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 77 4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 79 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 80 5.1. Normative References . . . . . . . . . . . . . . . . . . . . 6 81 5.2. Informative References . . . . . . . . . . . . . . . . . . . 7 83 Author's Address . . . . . . . . . . . . . . . . . . . . . . 7 85 1. Overview 87 [RFC2831] defined how HTTP Digest Authentication [RFC2617] can be 88 used as a Simple Authentication and Security Layer (SASL) [RFC4422] 89 mechanism for any protocol that has a SASL profile. It was intended 90 both as an improvement over CRAM-MD5 [RFC2195] and as a convenient 91 way to support a single authentication mechanism for web, mail, LDAP, 92 and other protocols. While it can be argued that it was an 93 improvement over CRAM-MD5, many implementors commented that the 94 additional complexity of DIGEST-MD5 made it difficult to implement 95 fully and securely. 97 Below is an incomplete list of problems with DIGEST-MD5 mechanism as 98 specified in RFC 2831: 100 1. The mechanism had too many options and modes. Some of them were 101 not well described and were not widely implemented. For example, 102 DIGEST-MD5 allowed the "qop" directive to contain multiple 103 values, but it also allowed for multiple qop directives to be 104 specified. The handling of multiple options was not specified, 105 which resulted in minor interoperability problems. Some 106 implementations amalgamated multiple qop values into one, while 107 others treated multiple qops as an error. Another example is the 108 use of an empty authorization identity. In SASL an empty 109 authorization identity means that the client is willing to 110 authorize as the authentication identity. The document was not 111 clear on whether the authzid must be omitted or can be specified 112 with the empty value to convey this. The requirement for 113 backward compatibility with HTTP Digest meant that the situation 114 was even worse. For example DIGEST-MD5 required all usernames/ 115 passwords which can be entirely represented in ISO-8859-1 charset 116 to be down converted from UTF-8 to ISO-8859-1. Another example 117 is use of quoted strings. Handling of characters that needed 118 escaping was not properly described and the DIGEST-MD5 document 119 had no examples to demonstrate correct behavior. 121 2. The document used ABNF from RFC 822 [RFC0822], which allows an 122 extra construct and allows for "implied folding whitespace" to be 123 inserted in many places. The difference from ABNF [RFC5234] was 124 confusing for some implementors. As a result, many 125 implementations didn't accept folding whitespace in many places 126 where it was allowed. 128 3. The DIGEST-MD5 document uses the concept of a "realm" to define a 129 collection of accounts. A DIGEST-MD5 server can support one or 130 more realms. The DIGEST-MD5 document didn't provide any guidance 131 on how realms should be named, and, more importantly, how they 132 can be entered in User Interfaces (UIs). As the result many 133 DIGEST-MD5 clients had confusing UIs, didn't allow users to enter 134 a realm and/or didn't allow users to pick one of the server 135 supported realms. 137 4. Use of username in the inner hash. The inner hash of DIGEST-MD5 138 is an MD5 hash of colon separated username, realm and password. 139 Implementations may choose to store inner hashes instead of clear 140 text passwords. While this has some useful properties, such as 141 protection from compromise of authentication databases containing 142 the same username and password on other servers, if a server with 143 the username and password is compromised, however this was rarely 144 done in practice. Firstly, the inner hash is not compatible with 145 widely deployed Unix password databases, and second, changing the 146 username would invalidate the inner hash. 148 5. Description of DES/3DES and RC4 security layers are inadequate to 149 produce independently-developed interoperable implementations. 150 In the DES/3DES case this was partly a problem with existing DES 151 APIs. 153 6. DIGEST-MD5 outer hash (the value of the "response" directive) 154 didn't protect the whole authentication exchange, which made the 155 mechanism vulnerable to "man in the middle" (MITM) attacks, such 156 as modification of the list of supported qops or ciphers. 158 7. The following features are missing from DIGEST-MD5, which make it 159 insecure or unsuitable for use in protocols: 161 A. Lack of channel bindings. 163 B. Lack of hash agility. 165 C. Lack of SASLPrep [RFC4013] support. The original DIGEST-MD5 166 document predates SASLPrep and doesn't recommend any Unicode 167 character normalization. 169 8. The cryptographic primitives in DIGEST-MD5 are not up to today's 170 standards, in particular: 172 A. The MD5 hash is sufficiently weak to make a brute force 173 attack on DIGEST-MD5 easy with common hardware. 175 B. Using the RC4 algorithm for the security layer without 176 discarding the initial key stream output is prone to attack. 178 Note that most of the problems listed above are already present in 179 the HTTP Digest authentication mechanism. 181 Because DIGEST-MD5 was defined as an extensible mechanism, it would 182 be possible to fix most of the problems listed above. However this 183 would increase implementation complexity of an already complex 184 mechanism even further, so the effort would not be worth the cost. 185 In addition, an implementation of a "fixed" DIGEST-MD5 specification 186 would likely either not interoperate with any existing implementation 187 of RFC 2831, or would be vulnerable to various downgrade attacks. 189 Note that despite DIGEST-MD5 seeing some deployment on the Internet, 190 this specification recommends obsoleting DIGEST-MD5 because DIGEST- 191 MD5, as implemented, is not a reasonable candidate for further 192 standardization and should be deprecated in favor of one or more new 193 password-based mechanisms currently being designed. 195 2. Security Considerations 197 Security issues are discussed through out this document. 199 3. IANA Considerations 201 IANA is requested to change the "Intended usage" of the DIGEST-MD5 202 mechanism registration in the SASL mechanism registry to OBSOLETE. 203 The SASL mechanism registry is specified in [RFC4422] and is 204 currently available at: 206 http://www.iana.org/assignments/sasl-mechanisms 208 4. Acknowledgements 210 The author gratefully acknowledges the feedback provided by Chris 211 Newman, Simon Josefsson, Kurt Zeilenga and Abhijit Menon-Sen. 212 [[anchor3: Various text was copied from other RFCs.]] 214 5. References 216 5.1. Normative References 218 [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., 219 Leach, P., Luotonen, A., and L. Stewart, "HTTP 220 Authentication: Basic and Digest Access Authentication", 221 RFC 2617, June 1999. 223 [RFC2831] Leach, P. and C. Newman, "Using Digest Authentication as a 224 SASL Mechanism", RFC 2831, May 2000. 226 5.2. Informative References 228 [RFC0822] Crocker, D., "Standard for the format of ARPA Internet 229 text messages", STD 11, RFC 822, August 1982. 231 [RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP 232 AUTHorize Extension for Simple Challenge/Response", 233 RFC 2195, September 1997. 235 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 236 and Passwords", RFC 4013, February 2005. 238 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and 239 Security Layer (SASL)", RFC 4422, June 2006. 241 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 242 Specifications: ABNF", STD 68, RFC 5234, January 2008. 244 Author's Address 246 Alexey Melnikov 247 Isode Limited 248 5 Castle Business Village 249 36 Station Road 250 Hampton, Middlesex TW12 2BX 251 UK 253 Email: Alexey.Melnikov@isode.com 254 URI: http://www.melnikov.ca/