Internet DRAFT - draft-hoffman-dane-smime
draft-hoffman-dane-smime
Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter
Expires: March 10, 2013 Kirei AB
September 6, 2012
Using Secure DNS to Associate Certificates with Domain Names For S/MIME
draft-hoffman-dane-smime-04
Abstract
This document describes how to use secure DNS to associate an S/MIME
user's certificate with the intended domain name, similar to the way
that DANE (RFC 6698) does for TLS.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on March 10, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The SMIMEA Resource Record . . . . . . . . . . . . . . . . . . 3
3. Domain Names for S/MIME Certificate Associations . . . . . . . 4
4. SMIMEA RR Examples . . . . . . . . . . . . . . . . . . . . . . 4
5. Mandatory-to-Implement Features . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6.1. TLSA RRtype . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
S/MIME [RFC5751] messages often contain a certificate. This
certificate assists in authenticating the sender of the message and
can be used for encrypting messages that will be sent in reply. In
order for the S/MIME receiver to authenticate that a message is from
the sender whom is identified in the message, the receiver's mail
user agent (MUA) must validate that this certificate is associated
with the purported sender. Currently, the MUA must trust a trust
anchor upon which the sender's certificate is rooted, and must
successfully validate the certificate.
Some people want to authenticate the association of the sender's
certificate with the sender without trusting a configured trust
anchor. Given that the DNS administrator for a domain name is
authorized to give identifying information about the zone, it makes
sense to allow that administrator to also make an authoritative
binding between email messages purporting to come from the domain
name and a certificate that might be used by someone authorized to
send mail from those servers. The easiest way to do this is to use
the DNS.
This document describes a mechanism for associating a user's
certificate with the domain that is similar to that described in
[RFC6698]. Most of the operational and security considerations for
using the mechanism in this document are described in RFC 6698, and
are not described here at all. Only the major differences between
this mechanism and those used in RFC 6698 are described here. Thus,
the reader must be familiar with RFC 6698 before reading this
document.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This document also makes use of standard PKIX, DNSSEC, and S/MIME
terminology. See [RFC5280], [RFC4033], [RFC4034], [RFC4035], and
[RFC5751] respectively, for these terms.
2. The SMIMEA Resource Record
The SMIMEA DNS resource record (RR) is used to associate an end
entity certificate or public key with the associated email address,
thus forming a "SMIMEA certificate association". The semantics of
how the SMIMEA RR is interpreted are given later in this document.
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The type value for the SMIMEA RR type is defined in Section 6.1. The
SMIMEA RR is class independent. The SMIMEA RR has no special TTL
requirements. The SMIMEA wire format and presentation format are the
same as for the TLSA record.
3. Domain Names for S/MIME Certificate Associations
Domain names are prepared for requests in the following manner.
1. The user name (the "left-hand side" of the email address, called
the "local-part" in [RFC2822] and the "local part" in [RFC6530]),
is encoded with Base32 [RFC4648], to become the left-most label
in the prepared domain name. This does not include the "@"
character that separates the left and right sides of the email
address.
2. The string "_smimecert" becomes the second left-most label in the
prepared domain name.
3. The domain name (the "right-hand side" of the email address,
called the "domain" in [RFC2822]) is appended to the result of
step 2 to complete the prepared domain name.
For example, to request a SMIMEA resource record for a user whose
address is "chris@example.com", you would use
"MNUHE2LT._smimecert.example.com" in the request.
Design note: Encoding the user name with Base32 allows local parts
that have characters that would prevent their use in domain names.
For example, a period (".") is a valid character in a local part, but
would wreak havoc in a domain name. Similarly, [RFC6530] allows non-
ASCII characters in local parts, and encoding a local part with non-
ASCII characters with Base32 renders the name usable in the DNS.
4. SMIMEA RR Examples
[[ Similar in format to draft-ietf-dane-protocol, but with very
different examples, of course. ]]
5. Mandatory-to-Implement Features
S/MIME MUAs conforming to this specification MUST be able to
correctly interpret SMIMEA records with certificate usages 0, 1, 2,
and 3. S/MIME MUAs conforming to this specification MUST be able to
compare a certificate association with a certificate offered by
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another S/MIME MUA using selector types 0 and 1, and matching type 0
(no hash used) and matching type 1 (SHA-256), and SHOULD be able to
make such comparisons with matching type 2 (SHA-512).
6. IANA Considerations
6.1. TLSA RRtype
This document uses a new DNS RR type, SMIMEA, whose value will be
allocated by IANA from the Resource Record (RR) TYPEs subregistry of
the Domain Name System (DNS) Parameters registry.
7. Security Considerations
DNS zones that are signed with DNSSEC using NSEC for denial of
existence are susceptible to zone-walking, a mechanism that allow
someone to enumerate all the names in the zone. Someone who wanted
to collect email addresses from a zone that uses SMIMEA might use
such a mechanism. DNSSEC-signed zones using NSEC3 for denial of
existence are significantly less susceptible to zone-walking.
Someone could still attempt a dictionary attack on the zone to find
SMIMEA records, just as they can use dictionary attacks on an SMTP
server to see which addresses are valid.
8. Acknowledgements
Miek Gieben and Martin Pels contributed technical ideas and support
to this document.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
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[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
9.2. Informative References
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
Authors' Addresses
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
Jakob Schlyter
Kirei AB
Email: jakob@kirei.se
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