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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 10, 2013 Kirei AB 6 September 6, 2012 8 Using Secure DNS to Associate Certificates with Domain Names For S/MIME 9 draft-hoffman-dane-smime-04 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 10, 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. SMIMEA RR Examples . . . . . . . . . . . . . . . . . . . . . . 4 56 5. Mandatory-to-Implement Features . . . . . . . . . . . . . . . . 4 57 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 58 6.1. TLSA RRtype . . . . . . . . . . . . . . . . . . . . . . . . 5 59 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 60 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 61 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 62 9.1. Normative References . . . . . . . . . . . . . . . . . . . 5 63 9.2. Informative References . . . . . . . . . . . . . . . . . . 6 64 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 66 1. Introduction 68 S/MIME [RFC5751] messages often contain a certificate. This 69 certificate assists in authenticating the sender of the message and 70 can be used for encrypting messages that will be sent in reply. In 71 order for the S/MIME receiver to authenticate that a message is from 72 the sender whom is identified in the message, the receiver's mail 73 user agent (MUA) must validate that this certificate is associated 74 with the purported sender. Currently, the MUA must trust a trust 75 anchor upon which the sender's certificate is rooted, and must 76 successfully validate the certificate. 78 Some people want to authenticate the association of the sender's 79 certificate with the sender without trusting a configured trust 80 anchor. Given that the DNS administrator for a domain name is 81 authorized to give identifying information about the zone, it makes 82 sense to allow that administrator to also make an authoritative 83 binding between email messages purporting to come from the domain 84 name and a certificate that might be used by someone authorized to 85 send mail from those servers. The easiest way to do this is to use 86 the DNS. 88 This document describes a mechanism for associating a user's 89 certificate with the domain that is similar to that described in 90 [RFC6698]. Most of the operational and security considerations for 91 using the mechanism in this document are described in RFC 6698, and 92 are not described here at all. Only the major differences between 93 this mechanism and those used in RFC 6698 are described here. Thus, 94 the reader must be familiar with RFC 6698 before reading this 95 document. 97 1.1. Terminology 99 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 100 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 101 document are to be interpreted as described in RFC 2119 [RFC2119]. 103 This document also makes use of standard PKIX, DNSSEC, and S/MIME 104 terminology. See [RFC5280], [RFC4033], [RFC4034], [RFC4035], and 105 [RFC5751] respectively, for these terms. 107 2. The SMIMEA Resource Record 109 The SMIMEA DNS resource record (RR) is used to associate an end 110 entity certificate or public key with the associated email address, 111 thus forming a "SMIMEA certificate association". The semantics of 112 how the SMIMEA RR is interpreted are given later in this document. 114 The type value for the SMIMEA RR type is defined in Section 6.1. The 115 SMIMEA RR is class independent. The SMIMEA RR has no special TTL 116 requirements. The SMIMEA wire format and presentation format are the 117 same as for the TLSA record. 119 3. Domain Names for S/MIME Certificate Associations 121 Domain names are prepared for requests in the following manner. 123 1. The user name (the "left-hand side" of the email address, called 124 the "local-part" in [RFC2822] and the "local part" in [RFC6530]), 125 is encoded with Base32 [RFC4648], to become the left-most label 126 in the prepared domain name. This does not include the "@" 127 character that separates the left and right sides of the email 128 address. 130 2. The string "_smimecert" becomes the second left-most label in the 131 prepared domain name. 133 3. The domain name (the "right-hand side" of the email address, 134 called the "domain" in [RFC2822]) is appended to the result of 135 step 2 to complete the prepared domain name. 137 For example, to request a SMIMEA resource record for a user whose 138 address is "chris@example.com", you would use 139 "MNUHE2LT._smimecert.example.com" in the request. 141 Design note: Encoding the user name with Base32 allows local parts 142 that have characters that would prevent their use in domain names. 143 For example, a period (".") is a valid character in a local part, but 144 would wreak havoc in a domain name. Similarly, [RFC6530] allows non- 145 ASCII characters in local parts, and encoding a local part with non- 146 ASCII characters with Base32 renders the name usable in the DNS. 148 4. SMIMEA RR Examples 150 [[ Similar in format to draft-ietf-dane-protocol, but with very 151 different examples, of course. ]] 153 5. Mandatory-to-Implement Features 155 S/MIME MUAs conforming to this specification MUST be able to 156 correctly interpret SMIMEA records with certificate usages 0, 1, 2, 157 and 3. S/MIME MUAs conforming to this specification MUST be able to 158 compare a certificate association with a certificate offered by 159 another S/MIME MUA using selector types 0 and 1, and matching type 0 160 (no hash used) and matching type 1 (SHA-256), and SHOULD be able to 161 make such comparisons with matching type 2 (SHA-512). 163 6. IANA Considerations 165 6.1. TLSA RRtype 167 This document uses a new DNS RR type, SMIMEA, whose value will be 168 allocated by IANA from the Resource Record (RR) TYPEs subregistry of 169 the Domain Name System (DNS) Parameters registry. 171 7. Security Considerations 173 DNS zones that are signed with DNSSEC using NSEC for denial of 174 existence are susceptible to zone-walking, a mechanism that allow 175 someone to enumerate all the names in the zone. Someone who wanted 176 to collect email addresses from a zone that uses SMIMEA might use 177 such a mechanism. DNSSEC-signed zones using NSEC3 for denial of 178 existence are significantly less susceptible to zone-walking. 179 Someone could still attempt a dictionary attack on the zone to find 180 SMIMEA records, just as they can use dictionary attacks on an SMTP 181 server to see which addresses are valid. 183 8. Acknowledgements 185 Miek Gieben and Martin Pels contributed technical ideas and support 186 to this document. 188 9. References 190 9.1. Normative References 192 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 193 Requirement Levels", BCP 14, RFC 2119, March 1997. 195 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 196 Rose, "DNS Security Introduction and Requirements", 197 RFC 4033, March 2005. 199 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 200 Rose, "Resource Records for the DNS Security Extensions", 201 RFC 4034, March 2005. 203 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 204 Rose, "Protocol Modifications for the DNS Security 205 Extensions", RFC 4035, March 2005. 207 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 208 Encodings", RFC 4648, October 2006. 210 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 211 Housley, R., and W. Polk, "Internet X.509 Public Key 212 Infrastructure Certificate and Certificate Revocation List 213 (CRL) Profile", RFC 5280, May 2008. 215 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 216 Mail Extensions (S/MIME) Version 3.2 Message 217 Specification", RFC 5751, January 2010. 219 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 220 of Named Entities (DANE) Transport Layer Security (TLS) 221 Protocol: TLSA", RFC 6698, August 2012. 223 9.2. Informative References 225 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, 226 April 2001. 228 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 229 Internationalized Email", RFC 6530, February 2012. 231 Authors' Addresses 233 Paul Hoffman 234 VPN Consortium 236 Email: paul.hoffman@vpnc.org 238 Jakob Schlyter 239 Kirei AB 241 Email: jakob@kirei.se