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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 (~~), 2 warnings (==), 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: September 10, 2012 Kirei AB 6 March 9, 2012 8 Using Secure DNS to Associate Certificates with Domain Names For S/MIME 9 draft-hoffman-dane-smime-03 11 Abstract 13 S/MIME uses certificates for authenticating and encrypting messages. 14 Users want their mail user agents to securely associate a certificate 15 with the sender of an encrypted and/or signed message. DNSSEC 16 provides a mechanism for a zone operator to sign DNS information 17 directly. This way, bindings of certificates to users within a 18 domain are asserted not by external entities, but by the entities 19 that operate the DNS. This document describes how to use secure DNS 20 to associate an S/MIME user's certificate with the intended domain 21 name. 23 IMPORTANT NOTE: This draft is intentionally sketchy. It is meant as 24 a possible starting point for the DANE WG if it wants to consider 25 making a protocol similar to TLSA, as described in 26 draft-ietf-dane-protocol, but that applies to S/MIME. The WG may or 27 may not want to adopt such work, or if it does, may want to use a 28 very different scheme from the one described here. 30 Status of this Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on September 10, 2012. 47 Copyright Notice 48 Copyright (c) 2012 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 64 1.1. Certificate Associations . . . . . . . . . . . . . . . . . 3 65 1.2. Securing Certificate Associations . . . . . . . . . . . . . 3 66 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 67 2. The SMIMEA Resource Record . . . . . . . . . . . . . . . . . . 3 68 3. TLSA RDATA Wire Format . . . . . . . . . . . . . . . . . . . . 4 69 4. TLSA RR Presentation Format . . . . . . . . . . . . . . . . . . 4 70 5. TLSA RR Examples . . . . . . . . . . . . . . . . . . . . . . . 4 71 6. Domain Names for S/MIME Certificate Associations . . . . . . . 4 72 7. Use of S/MIME Certificate Associations in S/MIME . . . . . . . 5 73 8. Mandatory-to-Implement Features . . . . . . . . . . . . . . . . 5 74 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 75 10. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 76 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 77 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 78 12.1. Normative References . . . . . . . . . . . . . . . . . . . 5 79 12.2. Informative References . . . . . . . . . . . . . . . . . . 6 80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 82 1. Introduction 84 S/MIME [RFC5751] messages often contain a certificate. This 85 certificate assists in authenticating the sender of the message and 86 can be used for encrypting messages that will be sent in reply. In 87 order for the S/MIME receiver to authenticate that a message is from 88 the sender whom is identified in the message, the receiver's mail 89 user agent (MUA) must validate that this certificate is associated 90 with the purported sender. Currently, the MUA must trust a trust 91 anchor upon which the sender's certificate is rooted, and must 92 successfully validate the certificate. 94 Some people want a different way to authenticate the association of 95 the sender's certificate with the sender without trusting a CA. 96 Given that the DNS administrator for a domain name is authorized to 97 give identifying information about the zone, it makes sense to allow 98 that administrator to also make an authoritative binding between 99 email messages purporting to come from the domain name and a 100 certificate that might be used by someone authorized to send mail 101 from those servers. The easiest way to do this is to use the DNS. 103 [[ More here about additional uses, such as CMS that is not S/MIME 104 where the certificates have email addresses for the subject name. ]] 106 1.1. Certificate Associations 108 [[ Will mostly duplicate the text from Section 1.1 of 109 draft-ietf-dane-protocol ]] 111 1.2. Securing Certificate Associations 113 [[ Will mostly duplicate the text from Section 1.2 of 114 draft-ietf-dane-protocol ]] 116 1.3. Terminology 118 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 119 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 120 document are to be interpreted as described in RFC 2119 [RFC2119]. 122 This document also makes use of standard PKIX, DNSSEC, and S/MIME 123 terminology. See [RFC5280], [RFC4033], [RFC4034], [RFC4035], and 124 [RFC5751] respectively, for these terms. 126 2. The SMIMEA Resource Record 128 [[ Mostly copied from draft-ietf-dane-protocol, but will define 129 "SMIMEA" instead of "TLSA". ]] 131 3. TLSA RDATA Wire Format 133 [[ Mostly copied from draft-ietf-dane-protocol. ]] 135 4. TLSA RR Presentation Format 137 [[ Mostly copied from draft-ietf-dane-protocol]] 139 5. TLSA RR Examples 141 [[ Similar in format to draft-ietf-dane-protocol, but with very 142 different examples, of course. ]] 144 6. Domain Names for S/MIME Certificate Associations 146 Domain names are prepared for requests in the following manner. 148 1. The user name (the "left-hand side" of the email address, called 149 the "local-part" in [RFC2822] and the "local part" in [RFC6530]), 150 is encoded with Base32 [RFC4648], to become the left-most label 151 in the prepared domain name. This does not include the "@" 152 character that separates the left and right sides of the email 153 address. 155 2. The string "_smimecert" becomes the second left-most label in the 156 prepared domain name. 158 3. The domain name (the "right-hand side" of the email address, 159 called the "domain" in [RFC2822]) is appended to the result of 160 step 2 to complete the prepared domain name. 162 For example, to request a SMIMEA resource record for a user whose 163 address is "chris@example.com", you would use 164 "MNUHE2LT._smimecert.example.com" in the request. 166 Design note: Encoding the user name with Base32 allows local parts 167 that have characters that would prevent their use in domain names. 168 For example, a period (".") is a valid character in a local part, but 169 would wreak havoc in a domain name. Similarly, [RFC6530] allows non- 170 ASCII characters in local parts, and encoding a local part with non- 171 ASCII characters with Base32 renders the name usable in the DNS. 173 7. Use of S/MIME Certificate Associations in S/MIME 175 [[ Stuff here that sounds like TLSA but is actually about S/MIME 176 senders and receivers. Lots of text lifted from 177 draft-ietf-dane-protocol. ]] 179 8. Mandatory-to-Implement Features 181 [[ Mostly copied from draft-ietf-dane-protocol. ]] 183 9. IANA Considerations 185 [[ Mostly copied from draft-ietf-dane-protocol but using "SMIMEA" 186 instead. ]] 188 10. Security Considerations 190 DNS zones that are signed with DNSSEC using NSEC for denial of 191 existence are susceptible to zone-walking, a mechanism that allow 192 someone to enumerate all the names in the zone. Someone who wanted 193 to collect email addresses from a zone that uses SMIMEA might use 194 such a mechanism. DNSSEC-signed zones using NSEC3 for denial of 195 existence are significantly less susceptible to zone-walking. 196 Someone could still attempt a dictionary attack on the zone to find 197 SMIMEA records, just as they can use dictionary attacks on an SMTP 198 server to see which addresses are valid. 200 [[ More copied from draft-ietf-dane-protocol but is actually about 201 S/MIME senders and receivers. ]] 203 11. Acknowledgements 205 Miek Gieben and Martin Pels contributed technical ideas and support 206 to this document. 208 12. References 210 12.1. Normative References 212 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 213 Requirement Levels", BCP 14, RFC 2119, March 1997. 215 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 217 Rose, "DNS Security Introduction and Requirements", 218 RFC 4033, March 2005. 220 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 221 Rose, "Resource Records for the DNS Security Extensions", 222 RFC 4034, March 2005. 224 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 225 Rose, "Protocol Modifications for the DNS Security 226 Extensions", RFC 4035, March 2005. 228 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 229 Encodings", RFC 4648, October 2006. 231 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 232 Housley, R., and W. Polk, "Internet X.509 Public Key 233 Infrastructure Certificate and Certificate Revocation List 234 (CRL) Profile", RFC 5280, May 2008. 236 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 237 Mail Extensions (S/MIME) Version 3.2 Message 238 Specification", RFC 5751, January 2010. 240 12.2. Informative References 242 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, 243 April 2001. 245 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 246 Internationalized Email", RFC 6530, February 2012. 248 Authors' Addresses 250 Paul Hoffman 251 VPN Consortium 253 Email: paul.hoffman@vpnc.org 255 Jakob Schlyter 256 Kirei AB 258 Email: jakob@kirei.se