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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (January 7, 2014) is 3762 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: 1 error (**), 0 flaws (~~), 1 warning (==), 3 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 Updates: 6698 (if approved) J. Schlyter 5 Intended status: Standards Track Kirei AB 6 Expires: July 11, 2014 January 7, 2014 8 Using Secure DNS to Associate Certificates with Domain Names For S/MIME 9 draft-ietf-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 July 11, 2014. 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 . . . . . . 3 55 4. Mandatory-to-Implement Features . . . . . . . . . . . . . . . 4 56 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 57 5.1. SMIMEA 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 who is identified in the message, the receiver's mail user 72 agent (MUA) must validate that this certificate is associated with 73 the purported sender. Currently, the MUA must trust a trust anchor 74 upon which the sender's certificate is rooted, and must successfully 75 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 DANE 89 itself [RFC6698]. Most of the operational and security 90 considerations for using the mechanism in this document are described 91 in RFC 6698, and are not described here at all. Only the major 92 differences between this mechanism and those used in RFC 6698 are 93 described here. Thus, the reader must be familiar with RFC 6698 94 before reading this 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 PKIX [RFC5280], DNSSEC [RFC4033], [RFC4034], 104 [RFC4035], and SMIME [RFC5751] 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 RRtype is defined in Section 5.1. The 114 SMIMEA resource record is class independent. The SMIMEA resource 115 record has no special TTL requirements. The SMIMEA wire format and 116 presentation format are the 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 the mail message format definition [RFC2822] 124 and the "local part" in the specification for internationalized 125 email [RFC6530]), is encoded with Base32 [RFC4648], to become the 126 left-most label in the prepared domain name. This does not 127 include the "@" character that separates the left and right sides 128 of the email 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 RFC 2822) 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. The corresponding 140 resource record in the example.com zone might look like: 142 MNUHE2LT._smimecert.example.com. IN SMIMEA ( 143 0 0 1 d2abde240d7cd3ee6b4b28c54df034b9 144 7983a1d16e8a410e4561cb106618e971 ) 146 Design note: Encoding the user name with Base32 allows local parts 147 that have characters that would prevent their use in domain names in 148 typical applications. Even though the DNS protocol itself can use 149 any octet value in a label, most applications that use DNS names are 150 limited to a much smaller set of allowed characters. For example, a 151 period (".") is a valid character in a local part, but would wreak 152 havoc in a domain name unless the application using the name somehow 153 quoted it. Similarly, RFC 6530 allows non-ASCII characters in local 154 parts, and encoding a local part with non-ASCII characters with 155 Base32 renders the name usable in applications that use the DNS. 157 Note that Base32 strings can end in one or more "=" characters. For 158 example, if the user name is "sal", the Base32 encoding is 159 "ONQWY===", so the request would be for 160 "ONQWY===._smimecert.example.com". 162 Also note that user names can be any length, and labels are limited 163 to 63 octets. Also note that user names that are encoded with Base32 164 are longer than the original user name. Any user name that would 165 cause a label of longer than 63 octets is expressly prohibited by 166 this specification. 168 Wildcards can be more useful for SMIMEA than they are for TLSA. If a 169 site publishes a trust anchor certificate for all users on the site 170 (certificate usage 0 or 2), it could make sense to use a wildcard 171 resource record such as "*._smimecert.example.com". 173 4. Mandatory-to-Implement Features 175 S/MIME MUAs conforming to this specification MUST be able to 176 correctly interpret SMIMEA records with certificate usages 0, 1, 2, 177 and 3. S/MIME MUAs conforming to this specification MUST be able to 178 compare a certificate association with a certificate offered by 179 another S/MIME MUA using selector types 0 and 1, and matching type 0 180 (no hash used) and matching type 1 (SHA-256), and SHOULD be able to 181 make such comparisons with matching type 2 (SHA-512). 183 5. IANA Considerations 184 5.1. SMIMEA RRtype 186 This document uses a new DNS RRtype, SMIMEA, whose value will be 187 allocated by IANA from the Resource Record (RR) TYPEs subregistry of 188 the Domain Name System (DNS) Parameters registry. 190 TODO: there needs to be new registries for certificate usages, 191 selectors, and maching types, pre-populated with the values from 192 TLSA. 194 6. Security Considerations 196 DNS zones that are signed with DNSSEC using NSEC for denial of 197 existence are susceptible to zone-walking, a mechanism that allow 198 someone to enumerate all the names in the zone. Someone who wanted 199 to collect email addresses from a zone that uses SMIMEA might use 200 such a mechanism. DNSSEC-signed zones using NSEC3 for denial of 201 existence are significantly less susceptible to zone-walking. 202 Someone could still attempt a dictionary attack on the zone to find 203 SMIMEA records, just as they can use dictionary attacks on an SMTP 204 server to see which addresses are valid. 206 Client treatment of any information included in the trust anchor is a 207 matter of local policy. This specification does not mandate that 208 such information be inspected or validated by the domain name 209 administrator. 211 7. Acknowledgements 213 Miek Gieben and Martin Pels contributed technical ideas and support 214 to this document. 216 8. References 218 8.1. Normative References 220 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 221 Requirement Levels", BCP 14, RFC 2119, March 1997. 223 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 224 Rose, "DNS Security Introduction and Requirements", RFC 225 4033, March 2005. 227 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 228 Rose, "Resource Records for the DNS Security Extensions", 229 RFC 4034, March 2005. 231 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 232 Rose, "Protocol Modifications for the DNS Security 233 Extensions", RFC 4035, March 2005. 235 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 236 Encodings", RFC 4648, October 2006. 238 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 239 Housley, R., and W. Polk, "Internet X.509 Public Key 240 Infrastructure Certificate and Certificate Revocation List 241 (CRL) Profile", RFC 5280, May 2008. 243 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 244 Mail Extensions (S/MIME) Version 3.2 Message 245 Specification", RFC 5751, January 2010. 247 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 248 of Named Entities (DANE) Transport Layer Security (TLS) 249 Protocol: TLSA", RFC 6698, August 2012. 251 8.2. Informative References 253 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 254 2001. 256 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 257 Internationalized Email", RFC 6530, February 2012. 259 Authors' Addresses 261 Paul Hoffman 262 VPN Consortium 264 Email: paul.hoffman@vpnc.org 266 Jakob Schlyter 267 Kirei AB 269 Email: jakob@kirei.se