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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 VPN Consortium 4 Intended status: Standards Track J. Schlyter 5 Expires: March 27, 2013 Kirei AB 6 September 23, 2012 8 Using Secure DNS to Associate Certificates with Domain Names For S/MIME 9 draft-ietf-dane-smime-00 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 March 27, 2013. 34 Copyright Notice 36 Copyright (c) 2012 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 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 . . . . . . . . . . . . . . . . 4 56 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 57 5.1. TLSA RRtype . . . . . . . . . . . . . . . . . . . . . . . . 5 58 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 59 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 60 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 61 8.1. Normative References . . . . . . . . . . . . . . . . . . . 5 62 8.2. Informative References . . . . . . . . . . . . . . . . . . 6 63 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 65 1. Introduction 67 S/MIME [RFC5751] messages often contain a certificate. This 68 certificate assists in authenticating the sender of the message and 69 can be used for encrypting messages that will be sent in reply. In 70 order for the S/MIME receiver to authenticate that a message is from 71 the sender whom is identified in the message, the receiver's mail 72 user agent (MUA) must validate that this certificate is associated 73 with the purported sender. Currently, the MUA must trust a trust 74 anchor upon which the sender's certificate is rooted, and must 75 successfully validate the certificate. 77 Some people want to authenticate the association of the sender's 78 certificate with the sender without trusting a configured trust 79 anchor. Given that the DNS administrator for a domain name is 80 authorized to give identifying information about the zone, it makes 81 sense to allow that administrator to also make an authoritative 82 binding between email messages purporting to come from the domain 83 name and a certificate that might be used by someone authorized to 84 send mail from those servers. The easiest way to do this is to use 85 the DNS. 87 This document describes a mechanism for associating a user's 88 certificate with the domain that is similar to that described in 89 [RFC6698]. Most of the operational and security considerations for 90 using the mechanism in this document are described in RFC 6698, and 91 are not described here at all. Only the major differences between 92 this mechanism and those used in RFC 6698 are described here. Thus, 93 the reader must be familiar with RFC 6698 before reading this 94 document. 96 1.1. Terminology 98 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 99 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 100 document are to be interpreted as described in RFC 2119 [RFC2119]. 102 This document also makes use of standard PKIX, DNSSEC, and S/MIME 103 terminology. See [RFC5280], [RFC4033], [RFC4034], [RFC4035], and 104 [RFC5751] respectively, for these terms. 106 2. The SMIMEA Resource Record 108 The SMIMEA DNS resource record (RR) is used to associate an end 109 entity certificate or public key with the associated email address, 110 thus forming a "SMIMEA certificate association". The semantics of 111 how the SMIMEA RR is interpreted are given later in this document. 113 The type value for the SMIMEA RR type is defined in Section 5.1. The 114 SMIMEA RR is class independent. The SMIMEA RR has no special TTL 115 requirements. The SMIMEA wire format and presentation format are the 116 same as for the TLSA record. 118 3. Domain Names for S/MIME Certificate Associations 120 Domain names are prepared for requests in the following manner. 122 1. The user name (the "left-hand side" of the email address, called 123 the "local-part" in [RFC2822] and the "local part" in [RFC6530]), 124 is encoded with Base32 [RFC4648], to become the left-most label 125 in the prepared domain name. This does not include the "@" 126 character that separates the left and right sides of the email 127 address. 129 2. The string "_smimecert" becomes the second left-most label in the 130 prepared domain name. 132 3. The domain name (the "right-hand side" of the email address, 133 called the "domain" in [RFC2822]) is appended to the result of 134 step 2 to complete the prepared domain name. 136 For example, to request a SMIMEA resource record for a user whose 137 address is "chris@example.com", you would use 138 "MNUHE2LT._smimecert.example.com" in the request. The corresponding 139 RR in the exmple.com zone might look like: 141 MNUHE2LT._smimecert.example.com. IN SMIMEA ( 142 0 0 1 d2abde240d7cd3ee6b4b28c54df034b9 143 7983a1d16e8a410e4561cb106618e971 ) 145 Design note: Encoding the user name with Base32 allows local parts 146 that have characters that would prevent their use in domain names. 147 For example, a period (".") is a valid character in a local part, but 148 would wreak havoc in a domain name. Similarly, [RFC6530] allows non- 149 ASCII characters in local parts, and encoding a local part with non- 150 ASCII characters with Base32 renders the name usable in the DNS. 152 4. Mandatory-to-Implement Features 154 S/MIME MUAs conforming to this specification MUST be able to 155 correctly interpret SMIMEA records with certificate usages 0, 1, 2, 156 and 3. S/MIME MUAs conforming to this specification MUST be able to 157 compare a certificate association with a certificate offered by 158 another S/MIME MUA using selector types 0 and 1, and matching type 0 159 (no hash used) and matching type 1 (SHA-256), and SHOULD be able to 160 make such comparisons with matching type 2 (SHA-512). 162 5. IANA Considerations 164 5.1. TLSA RRtype 166 This document uses a new DNS RR type, SMIMEA, whose value will be 167 allocated by IANA from the Resource Record (RR) TYPEs subregistry of 168 the Domain Name System (DNS) Parameters registry. 170 6. Security Considerations 172 DNS zones that are signed with DNSSEC using NSEC for denial of 173 existence are susceptible to zone-walking, a mechanism that allow 174 someone to enumerate all the names in the zone. Someone who wanted 175 to collect email addresses from a zone that uses SMIMEA might use 176 such a mechanism. DNSSEC-signed zones using NSEC3 for denial of 177 existence are significantly less susceptible to zone-walking. 178 Someone could still attempt a dictionary attack on the zone to find 179 SMIMEA records, just as they can use dictionary attacks on an SMTP 180 server to see which addresses are valid. 182 Client treatment of any information included in the trust anchor is a 183 matter of local policy. This specification does not mandate that 184 such information be inspected or validated by the domain name 185 administrator. 187 7. Acknowledgements 189 Miek Gieben and Martin Pels contributed technical ideas and support 190 to this document. 192 8. References 194 8.1. Normative References 196 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 197 Requirement Levels", BCP 14, RFC 2119, March 1997. 199 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 200 Rose, "DNS Security Introduction and Requirements", 201 RFC 4033, March 2005. 203 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 204 Rose, "Resource Records for the DNS Security Extensions", 205 RFC 4034, March 2005. 207 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 208 Rose, "Protocol Modifications for the DNS Security 209 Extensions", RFC 4035, March 2005. 211 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 212 Encodings", RFC 4648, October 2006. 214 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 215 Housley, R., and W. Polk, "Internet X.509 Public Key 216 Infrastructure Certificate and Certificate Revocation List 217 (CRL) Profile", RFC 5280, May 2008. 219 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 220 Mail Extensions (S/MIME) Version 3.2 Message 221 Specification", RFC 5751, January 2010. 223 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 224 of Named Entities (DANE) Transport Layer Security (TLS) 225 Protocol: TLSA", RFC 6698, August 2012. 227 8.2. Informative References 229 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, 230 April 2001. 232 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 233 Internationalized Email", RFC 6530, February 2012. 235 Authors' Addresses 237 Paul Hoffman 238 VPN Consortium 240 Email: paul.hoffman@vpnc.org 242 Jakob Schlyter 243 Kirei AB 245 Email: jakob@kirei.se