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The disclaimer is usually necessary only for documents that revise or obsolete older RFCs, and that take significant amounts of text from those RFCs. If you can contact all authors of the source material and they are willing to grant the BCP78 rights to the IETF Trust, you can and should remove the disclaimer. Otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (March 28, 2011) is 4772 days in the past. Is this intentional? 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 March 28, 2011 5 Expires: September 29, 2011 7 Moving DIGEST-MD5 to Historic 8 draft-ietf-kitten-digest-to-historic-03 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 Informational document for the Internet Community. 22 Discussion and suggestions for improvement are requested, and should 23 be sent to kitten@ietf.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 September 29, 2011. 42 Copyright Notice 44 Copyright (c) 2011 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 . . . . . . . . . . . . . . . . . . . . . . . . . 7 80 5.1. Normative References . . . . . . . . . . . . . . . . . . . . 7 81 5.2. Informative References . . . . . . . . . . . . . . . . . . . 7 83 Author's Address . . . . . . . . . . . . . . . . . . . . . . 8 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, email, 92 LDAP, 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 [DES] and RC4 security layers are 149 inadequate to produce independently-developed interoperable 150 implementations. In the DES/3DES case this was partly a problem 151 with existing DES 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 [RFC5056]. 163 B. Lack of hash agility. 165 C. Lack of support for SASLPrep [RFC4013] or any other type of 166 Unicode character normalization of usernames and passwords. 167 The original DIGEST-MD5 document predates SASLPrep and 168 doesn't recommend any Unicode character normalization. 170 8. The cryptographic primitives in DIGEST-MD5 are not up to today's 171 standards, in particular: 173 A. The MD5 hash is sufficiently weak to make a brute force 174 attack on DIGEST-MD5 easy with common hardware [RFC6151]. 176 B. Using the RC4 algorithm for the security layer without 177 discarding the initial key stream output is prone to attack 178 [RC4]. 180 C. The DES cipher for the security layer is considered insecure 181 due to its small key space [RFC3766]. 183 Note that most of the problems listed above are already present in 184 the HTTP Digest authentication mechanism. 186 Because DIGEST-MD5 was defined as an extensible mechanism, it would 187 be possible to fix most of the problems listed above. However this 188 would increase implementation complexity of an already complex 189 mechanism even further, so the effort would not be worth the cost. 190 In addition, an implementation of a "fixed" DIGEST-MD5 specification 191 would likely either not interoperate with any existing implementation 192 of RFC 2831, or would be vulnerable to various downgrade attacks. 194 Note that despite DIGEST-MD5 seeing some deployment on the Internet, 195 this specification recommends obsoleting DIGEST-MD5 because DIGEST- 196 MD5, as implemented, is not a reasonable candidate for further 197 standardization and should be deprecated in favor of one or more new 198 password-based mechanisms currently being designed. 200 The SCRAM family of SASL mechanisms [RFC5802] has been developed to 201 provide similar features as DIGEST-MD5 but with a better design. 203 2. Security Considerations 205 Security issues are discussed through out this document. 207 3. IANA Considerations 209 IANA is requested to change the "Intended usage" of the DIGEST-MD5 210 mechanism registration in the SASL mechanism registry to OBSOLETE. 211 The SASL mechanism registry is specified in [RFC4422] and is 212 currently available at: 214 http://www.iana.org/assignments/sasl-mechanisms 216 4. Acknowledgements 218 The author gratefully acknowledges the feedback provided by Chris 219 Newman, Simon Josefsson, Kurt Zeilenga, Sean Turner and Abhijit 220 Menon-Sen. Various text was copied from other RFCs, in particular 221 from RFC 2831. 223 5. References 225 5.1. Normative References 227 [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., 228 Leach, P., Luotonen, A., and L. Stewart, "HTTP 229 Authentication: Basic and Digest Access Authentication", 230 RFC 2617, June 1999. 232 [RFC2831] Leach, P. and C. Newman, "Using Digest Authentication as a 233 SASL Mechanism", RFC 2831, May 2000. 235 5.2. Informative References 237 [DES] National Institute of Standards and Technology, "Data 238 Encryption Standard (DES)", FIPS PUB 46-3, October 1999. 240 [RC4] Strombergson, J. and S. Josefsson, "Test vectors for the 241 stream cipher RC4", 242 draft-josefsson-rc4-test-vectors-02.txt (work in 243 progress), June 2010. 245 [RFC0822] Crocker, D., "Standard for the format of ARPA Internet 246 text messages", STD 11, RFC 822, August 1982. 248 [RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP 249 AUTHorize Extension for Simple Challenge/Response", 250 RFC 2195, September 1997. 252 [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For 253 Public Keys Used For Exchanging Symmetric Keys", BCP 86, 254 RFC 3766, April 2004. 256 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 257 and Passwords", RFC 4013, February 2005. 259 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and 260 Security Layer (SASL)", RFC 4422, June 2006. 262 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 263 Channels", RFC 5056, November 2007. 265 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 266 Specifications: ABNF", STD 68, RFC 5234, January 2008. 268 [RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, 269 "Salted Challenge Response Authentication Mechanism 270 (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010. 272 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 273 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 274 RFC 6151, March 2011. 276 Author's Address 278 Alexey Melnikov 279 Isode Limited 280 5 Castle Business Village 281 36 Station Road 282 Hampton, Middlesex TW12 2BX 283 UK 285 Email: Alexey.Melnikov@isode.com 286 URI: http://www.melnikov.ca/