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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: 1 error (**), 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 ICANN 4 Intended status: Standards Track J. Schlyter 5 Expires: August 27, 2016 Kirei AB 6 February 24, 2016 8 Using Secure DNS to Associate Certificates with Domain Names For S/MIME 9 draft-ietf-dane-smime-10 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 August 27, 2016. 34 Copyright Notice 36 Copyright (c) 2016 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. Email Addresses in Domain Names for S/MIME Certificate 55 Associations . . . . . . . . . . . . . . . . . . . . . . . . 3 56 4. Mandatory-to-Implement Features . . . . . . . . . . . . . . . 4 57 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 58 5.1. SMIMEA RRtype . . . . . . . . . . . . . . . . . . . . . . 5 59 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 60 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 61 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 62 8.1. Normative References . . . . . . . . . . . . . . . . . . 5 63 8.2. Informative References . . . . . . . . . . . . . . . . . 6 64 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 66 1. Introduction 68 S/MIME [RFC5751] messages often contain a certificate (some messages 69 contain more than one certificate). These certificates assist in 70 authenticating the sender of the message and can be used for 71 encrypting messages that will be sent in reply. In order for the S/ 72 MIME receiver to authenticate that a message is from the sender who 73 is identified in the message, the receiver's mail user agent (MUA) 74 must validate that this certificate is associated with the purported 75 sender. Currently, the MUA must trust a trust anchor upon which the 76 sender's certificate is rooted, and must successfully validate the 77 certificate. There are other requirements on the MUA, such as 78 associating the identity in the certificate with that of the message, 79 that are out of scope for this document. 81 Some people want to authenticate the association of the sender's 82 certificate with the sender without trusting a configured trust 83 anchor. Given that the DNS administrator for a domain name is 84 authorized to give identifying information about the zone, it makes 85 sense to allow that administrator to also make an authoritative 86 binding between email messages purporting to come from the domain 87 name and a certificate that might be used by someone authorized to 88 send mail from those servers. The easiest way to do this is to use 89 the DNS. 91 This document describes a mechanism for associating a user's 92 certificate with the domain that is similar to that described in DANE 93 itself [RFC6698]. Most of the operational and security 94 considerations for using the mechanism in this document are described 95 in RFC 6698, and are not described here at all. Only the major 96 differences between this mechanism and those used in RFC 6698 are 97 described here. Thus, the reader must be familiar with RFC 6698 98 before reading this document. 100 NOTE FOR FUTURE DRAFTS OF THIS DOCUMENT: The DANE WG needs to have a 101 serious discussion about what the DANE set of specifications covering 102 TLS for HTTP, TLS for SMTP, S/MIME, OpenPGP, and so on are meant for. 103 They could be used for acquisition of key assocation material, for 104 discovering services that use the keying material, for having 105 assurance that a service that uses the keying material should be 106 available, or some combination of these. 108 1.1. Terminology 110 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 111 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 112 document are to be interpreted as described in RFC 2119 [RFC2119]. 114 This document also makes use of standard PKIX, DNSSEC, and S/MIME 115 terminology. See PKIX [RFC5280], DNSSEC [RFC4033], [RFC4034], 116 [RFC4035], and SMIME [RFC5751] for these terms. 118 2. The SMIMEA Resource Record 120 The SMIMEA DNS resource record (RR) is used to associate an end 121 entity certificate or public key with the associated email address, 122 thus forming a "SMIMEA certificate association". The semantics of 123 how the SMIMEA RR is interpreted are given later in this document. 124 Note that the information returned in the SMIMEA record might be for 125 the end entity certificate, or it might be for the trust anchor or an 126 intermediate certificate. 128 The type value for the SMIMEA RRtype is defined in Section 5.1. The 129 SMIMEA resource record is class independent. The SMIMEA resource 130 record has no special TTL requirements. 132 The SMIMEA wire format and presentation format are the same as for 133 the TLSA record as described in section 2.1 of RFC 6698. The 134 certificate usage field, the selector field, and the matching type 135 field have the same format; the semantics are also the same except 136 where RFC 6698 talks about TLS at the target protocol for the 137 certificate information. 139 3. Email Addresses in Domain Names for S/MIME Certificate Associations 141 SMIMEA records are stored in the DNS on a per-user basis, based on 142 the email address domain name. The general form of the lookup name 143 is formulated from the user's email address: 145 ._smimecert. 147 The algorithm for formulating the domain name for the record is: 149 1. The user name (the "left-hand side" of the email address, called 150 the "local-part" in the mail message format definition [RFC2822] 151 and the "local part" in the specification for internationalized 152 email [RFC6530]) should already be encoded in UTF-8 (or its 153 subset ASCII). If it is written in another encoding it should be 154 converted to UTF-8. Next, it is hashed using the SHA2-256 155 [RFC5754] algorithm, with the hash truncated to 28 octets and 156 represented in its hexadecimal representation, to become the 157 left-most label in the prepared domain name. Truncation comes 158 from the right-most octets. This does not include the at symbol 159 ("@") that separates the left and right sides of the email 160 address. 162 2. The string "_smimecert" becomes the second left-most label in the 163 prepared domain name. 165 3. The domain name (the "right-hand side" of the email address, 166 called the "domain" in RFC 2822) is appended to the result of 167 step 2 to complete the prepared domain name. 169 For example, to request an SMIMEA resource record for a user whose 170 email address is "hugh@example.com", an SMIMEA query would be placed 171 for the following QNAME: "c93f1e400f26708f98cb19d936620da35eec8f72e57 172 f9eec01c1afd6._smimecert.example.com". The corresponding RR in the 173 example.com zone might look like (key shortened for formatting): 175 c9[..]d6._smimecert.example.com. IN SMIMEA ( 176 0 0 1 d2abde240d7cd3ee6b4b28c54df034b9 177 7983a1d16e8a410e4561cb106618e971 ) 179 Wildcards can be more useful for SMIMEA than they are for TLSA. If a 180 site publishes a trust anchor certificate for all users on the site 181 (certificate usage 0 or 2), it could make sense to use a wildcard 182 resource record such as "*._smimecert.example.com". 184 4. Mandatory-to-Implement Features 186 S/MIME MUAs conforming to this specification MUST be able to 187 correctly interpret SMIMEA records with certificate usages 0, 1, 2, 188 and 3. S/MIME MUAs conforming to this specification MUST be able to 189 compare a certificate association with a certificate offered by 190 another S/MIME MUA using selector types 0 and 1, and matching type 0 191 (no hash used) and matching type 1 (SHA-256), and SHOULD be able to 192 make such comparisons with matching type 2 (SHA-512). 194 5. IANA Considerations 196 5.1. SMIMEA RRtype 198 This document uses a new DNS RRtype, SMIMEA, whose value (53) was 199 allocated by IANA from the Resource Record (RR) TYPEs subregistry of 200 the Domain Name System (DNS) Parameters registry. 202 6. Security Considerations 204 DNS zones that are signed with DNSSEC using NSEC for denial of 205 existence are susceptible to zone-walking, a mechanism that allow 206 someone to enumerate all the names in the zone. Someone who wanted 207 to collect email addresses from a zone that uses SMIMEA might use 208 such a mechanism. DNSSEC-signed zones using NSEC3 for denial of 209 existence are significantly less susceptible to zone-walking. 210 Someone could still attempt a dictionary attack on the zone to find 211 SMIMEA records, just as they can use dictionary attacks on an SMTP 212 server to see which addresses are valid. 214 Client treatment of any information included in the trust anchor is a 215 matter of local policy. This specification does not mandate that 216 such information be inspected or validated by the domain name 217 administrator. 219 7. Acknowledgements 221 Brian Dickson, Miek Gieben, and Martin Pels contributed technical 222 ideas and support to this document. 224 8. References 226 8.1. Normative References 228 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 229 Requirement Levels", BCP 14, RFC 2119, 230 DOI 10.17487/RFC2119, March 1997, 231 . 233 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 234 Rose, "DNS Security Introduction and Requirements", 235 RFC 4033, DOI 10.17487/RFC4033, March 2005, 236 . 238 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 239 Rose, "Resource Records for the DNS Security Extensions", 240 RFC 4034, DOI 10.17487/RFC4034, March 2005, 241 . 243 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 244 Rose, "Protocol Modifications for the DNS Security 245 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 246 . 248 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 249 Housley, R., and W. Polk, "Internet X.509 Public Key 250 Infrastructure Certificate and Certificate Revocation List 251 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 252 . 254 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 255 Mail Extensions (S/MIME) Version 3.2 Message 256 Specification", RFC 5751, DOI 10.17487/RFC5751, January 257 2010, . 259 [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic 260 Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January 261 2010, . 263 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 264 of Named Entities (DANE) Transport Layer Security (TLS) 265 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 266 2012, . 268 8.2. Informative References 270 [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, 271 DOI 10.17487/RFC2822, April 2001, 272 . 274 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 275 Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, 276 February 2012, . 278 Authors' Addresses 280 Paul Hoffman 281 ICANN 283 Email: paul.hoffman@icann.org 285 Jakob Schlyter 286 Kirei AB 288 Email: jakob@kirei.se