idnits 2.17.1 draft-ietf-dane-smime-07.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** There are 2 instances of too long lines in the document, the longest one being 11 characters in excess of 72. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (August 22, 2014) is 3535 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 5751 (Obsoleted by RFC 8551) -- Obsolete informational reference (is this intentional?): RFC 2822 (Obsoleted by RFC 5322) Summary: 2 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Hoffman 3 Internet-Draft VPN Consortium 4 Intended status: Standards Track J. Schlyter 5 Expires: February 23, 2015 Kirei AB 6 August 22, 2014 8 Using Secure DNS to Associate Certificates with Domain Names For S/MIME 9 draft-ietf-dane-smime-07 11 Abstract 13 This document describes how to use secure DNS to associate an S/MIME 14 user's certificate with the intended domain name, similar to the way 15 that DANE (RFC 6698) does for TLS. 17 Status of This Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at http://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on February 23, 2015. 34 Copyright Notice 36 Copyright (c) 2014 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (http://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 52 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 53 2. The SMIMEA Resource Record . . . . . . . . . . . . . . . . . 3 54 3. Domain Names for S/MIME Certificate Associations . . . . . . 4 55 4. Mandatory-to-Implement Features . . . . . . . . . . . . . . . 5 56 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 57 5.1. SMIMEA RRtype . . . . . . . . . . . . . . . . . . . . . . 5 58 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 59 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 60 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 61 8.1. Normative References . . . . . . . . . . . . . . . . . . 6 62 8.2. Informative References . . . . . . . . . . . . . . . . . 6 63 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 65 1. Introduction 67 S/MIME [RFC5751] messages often contain a certificate (some messages 68 contain more than one certificate). These certificates assist in 69 authenticating the sender of the message and can be used for 70 encrypting messages that will be sent in reply. In order for the S/ 71 MIME receiver to authenticate that a message is from the sender who 72 is identified in the message, the receiver's mail user agent (MUA) 73 must validate that this certificate is associated with the purported 74 sender. Currently, the MUA must trust a trust anchor upon which the 75 sender's certificate is rooted, and must successfully validate the 76 certificate. There are other requirements on the MUA, such as 77 associating the identity in the certificate with that of the message, 78 that are out of scope for this document. 80 Some people want to authenticate the association of the sender's 81 certificate with the sender without trusting a configured trust 82 anchor. Given that the DNS administrator for a domain name is 83 authorized to give identifying information about the zone, it makes 84 sense to allow that administrator to also make an authoritative 85 binding between email messages purporting to come from the domain 86 name and a certificate that might be used by someone authorized to 87 send mail from those servers. The easiest way to do this is to use 88 the DNS. 90 This document describes a mechanism for associating a user's 91 certificate with the domain that is similar to that described in DANE 92 itself [RFC6698]. Most of the operational and security 93 considerations for using the mechanism in this document are described 94 in RFC 6698, and are not described here at all. Only the major 95 differences between this mechanism and those used in RFC 6698 are 96 described here. Thus, the reader must be familiar with RFC 6698 97 before reading this document. 99 NOTE FOR FUTURE DRAFTS OF THIS DOCUMENT: The DANE WG needs to have a 100 serious discussion about what the DANE set of specifications covering 101 TLS for HTTP, TLS for SMTP, S/MIME, OpenPGP, and so on are meant for. 102 They could be used for acquisition of key assocation material, for 103 discovering services that use the keying material, for having 104 assurance that a service that uses the keying material should be 105 available, or some combination of these. 107 1.1. Terminology 109 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 110 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 111 document are to be interpreted as described in RFC 2119 [RFC2119]. 113 This document also makes use of standard PKIX, DNSSEC, and S/MIME 114 terminology. See PKIX [RFC5280], DNSSEC [RFC4033], [RFC4034], 115 [RFC4035], and SMIME [RFC5751] for these terms. 117 2. The SMIMEA Resource Record 119 The SMIMEA DNS resource record (RR) is used to associate an end 120 entity certificate or public key with the associated email address, 121 thus forming a "SMIMEA certificate association". The semantics of 122 how the SMIMEA RR is interpreted are given later in this document. 123 Note that the information returned in the SMIMEA record might be for 124 the end entity certificate, or it might be for the trust anchor or an 125 intermediate certificate. 127 The type value for the SMIMEA RRtype is defined in Section 5.1. The 128 SMIMEA resource record is class independent. The SMIMEA resource 129 record has no special TTL requirements. 131 The SMIMEA wire format and presentation format are the same as for 132 the TLSA record as described in section 2.1 of RFC 6698. The 133 certificate usage field, the selector field, and the matching type 134 field have the same format; the semantics are also the same except 135 where RFC 6698 talks about TLS at the target protocol for the 136 certificate information. 138 3. Domain Names for S/MIME Certificate Associations 140 Domain names are prepared for requests in the following manner. 142 1. The user name (the "left-hand side" of the email address, called 143 the "local-part" in the mail message format definition [RFC2822] 144 and the "local part" in the specification for internationalized 145 email [RFC6530]), is hashed using the SHA2-224 [RFC5754] 146 algorithm (with the hash being represented in its hexadecimal 147 representation, to become the left-most label in the prepared 148 domain name. This does not include the "@" character that 149 separates the left and right sides of the email address. The 150 string that is used for the local part is a Unicode string 151 encoded in UTF-8. 153 2. The string "_smimecert" becomes the second left-most label in the 154 prepared domain name. 156 3. The domain name (the "right-hand side" of the email address, 157 called the "domain" in RFC 2822) is appended to the result of 158 step 2 to complete the prepared domain name. 160 For example, to request a SMIMEA resource record for a user whose 161 address is "chris@example.com", calculate the SHA-224 of "chris", 162 which is 0x3f51f4663b2b798560c5b9e16d6069a28727f62518c3a1b33f7f5214. 163 The request is thus: 165 3f51f4663b2b798560c5b9e16d6069a28727f62518c3a1b33f7f5214._smimecert.example.com 167 The corresponding resource record in the example.com zone might look 168 like: 170 3f51f4663b2b798560c5b9e16d6069a28727f62518c3a1b33f7f5214._smimecert.example.com. 171 IN SMIMEA ( 172 0 0 1 d2abde240d7cd3ee6b4b28c54df034b9 173 7983a1d16e8a410e4561cb106618e971 ) 175 Design note: Hashing the user name with SHA-224 and using the 176 hexidecimal encoding of that hash allows local parts that have 177 characters that would prevent their use in domain names in typical 178 applications. Even though the DNS protocol itself can use any octet 179 value in a label, most applications that use DNS names are limited to 180 a much smaller set of allowed characters. For example, a period 181 (".") is a valid character in a local part, but would wreak havoc in 182 a domain name unless the application using the name somehow quoted 183 it. Similarly, RFC 6530 allows non-ASCII characters in local parts, 184 and encoding a local part with non-ASCII characters as the hex of the 185 SHA-224 renders the name usable in applications that use the DNS. 187 Wildcards can be more useful for SMIMEA than they are for TLSA. If a 188 site publishes a trust anchor certificate for all users on the site 189 (certificate usage 0 or 2), it could make sense to use a wildcard 190 resource record such as "*._smimecert.example.com". 192 4. Mandatory-to-Implement Features 194 S/MIME MUAs conforming to this specification MUST be able to 195 correctly interpret SMIMEA records with certificate usages 0, 1, 2, 196 and 3. S/MIME MUAs conforming to this specification MUST be able to 197 compare a certificate association with a certificate offered by 198 another S/MIME MUA using selector types 0 and 1, and matching type 0 199 (no hash used) and matching type 1 (SHA-256), and SHOULD be able to 200 make such comparisons with matching type 2 (SHA-512). 202 5. IANA Considerations 204 5.1. SMIMEA RRtype 206 This document uses a new DNS RRtype, SMIMEA, whose value will be 207 allocated by IANA from the Resource Record (RR) TYPEs subregistry of 208 the Domain Name System (DNS) Parameters registry. 210 TODO: there needs to be new registries for certificate usages, 211 selectors, and maching types, pre-populated with the values from 212 TLSA. 214 6. Security Considerations 216 DNS zones that are signed with DNSSEC using NSEC for denial of 217 existence are susceptible to zone-walking, a mechanism that allow 218 someone to enumerate all the names in the zone. Someone who wanted 219 to collect email addresses from a zone that uses SMIMEA might use 220 such a mechanism. DNSSEC-signed zones using NSEC3 for denial of 221 existence are significantly less susceptible to zone-walking. 222 Someone could still attempt a dictionary attack on the zone to find 223 SMIMEA records, just as they can use dictionary attacks on an SMTP 224 server to see which addresses are valid. 226 Client treatment of any information included in the trust anchor is a 227 matter of local policy. This specification does not mandate that 228 such information be inspected or validated by the domain name 229 administrator. 231 7. Acknowledgements 233 Brian Dickson, Miek Gieben, and Martin Pels contributed technical 234 ideas and support to this document. 236 8. References 238 8.1. Normative References 240 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 241 Requirement Levels", BCP 14, RFC 2119, March 1997. 243 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 244 Rose, "DNS Security Introduction and Requirements", RFC 245 4033, March 2005. 247 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 248 Rose, "Resource Records for the DNS Security Extensions", 249 RFC 4034, March 2005. 251 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 252 Rose, "Protocol Modifications for the DNS Security 253 Extensions", RFC 4035, March 2005. 255 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 256 Housley, R., and W. Polk, "Internet X.509 Public Key 257 Infrastructure Certificate and Certificate Revocation List 258 (CRL) Profile", RFC 5280, May 2008. 260 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 261 Mail Extensions (S/MIME) Version 3.2 Message 262 Specification", RFC 5751, January 2010. 264 [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic 265 Message Syntax", RFC 5754, January 2010. 267 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 268 of Named Entities (DANE) Transport Layer Security (TLS) 269 Protocol: TLSA", RFC 6698, August 2012. 271 8.2. Informative References 273 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 274 2001. 276 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 277 Internationalized Email", RFC 6530, February 2012. 279 Authors' Addresses 281 Paul Hoffman 282 VPN Consortium 284 Email: paul.hoffman@vpnc.org 286 Jakob Schlyter 287 Kirei AB 289 Email: jakob@kirei.se