Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter
Expires: August 24, 2015 Kirei AB
February 20, 2015
Using Secure DNS to Associate Certificates with Domain Names For S/MIME
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.
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Hoffman & Schlyter Expires August 24, 2015 [Page 1]Internet-Draft DNS-Based Authentication for S/MIME February 2015Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. The SMIMEA Resource Record . . . . . . . . . . . . . . . . . 3
3. Email Addresses in Domain Names for S/MIME Certificate
Associations . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Mandatory-to-Implement Features . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5.1. SMIMEA RRtype . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
S/MIME [RFC5751] messages often contain a certificate (some messages
contain more than one certificate). These certificates assist 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 who
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. There are other requirements on the MUA, such as
associating the identity in the certificate with that of the message,
that are out of scope for this document.
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
This document describes a mechanism for associating a user's
certificate with the domain that is similar to that described in DANE
itself [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
Hoffman & Schlyter Expires August 24, 2015 [Page 2]Internet-Draft DNS-Based Authentication for S/MIME February 2015
described here. Thus, the reader must be familiar with RFC 6698
before reading this document.
NOTE FOR FUTURE DRAFTS OF THIS DOCUMENT: The DANE WG needs to have a
serious discussion about what the DANE set of specifications covering
TLS for HTTP, TLS for SMTP, S/MIME, OpenPGP, and so on are meant for.
They could be used for acquisition of key assocation material, for
discovering services that use the keying material, for having
assurance that a service that uses the keying material should be
available, or some combination of these.
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 PKIX [RFC5280], DNSSEC [RFC4033], [RFC4034],
[RFC4035], and SMIME [RFC5751] 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.
Note that the information returned in the SMIMEA record might be for
the end entity certificate, or it might be for the trust anchor or an
The type value for the SMIMEA RRtype is defined in Section 5.1. The
SMIMEA resource record is class independent. The SMIMEA resource
record has no special TTL requirements.
The SMIMEA wire format and presentation format are the same as for
the TLSA record as described in section 2.1 of RFC 6698. The
certificate usage field, the selector field, and the matching type
field have the same format; the semantics are also the same except
where RFC 6698 talks about TLS at the target protocol for the
3. Email Addresses in Domain Names for S/MIME Certificate Associations
SMIMEA records are stored in the DNS on a per-user basis, based on
the email address domain name. The general form of the lookup name
is formulated from the user's email address:
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The algorithm for formulating the domain name for the record is:
1. The user name (the "left-hand side" of the email address, called
the "local-part" in the mail message format definition [RFC2822]
and the "local part" in the specification for internationalized
email [RFC6530]), is hashed using the SHA2-224 [RFC5754]
algorithm (with the hash being represented in its hexadecimal
representation, 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. The
string that is used for the local part is a Unicode string
encoded in UTF-8.
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 RFC 2822) 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 "email@example.com", calculate the SHA-224 of "chris",
which is 0x3f51f4663b2b798560c5b9e16d6069a28727f62518c3a1b33f7f5214.
The request is thus:
The corresponding resource record in the example.com zone might look
IN SMIMEA (
0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
Design note: Hashing the user name with SHA-224 and using the
hexidecimal encoding of that hash allows local parts that have
characters that would prevent their use in domain names in typical
applications. Even though the DNS protocol itself can use any octet
value in a label, most applications that use DNS names are limited to
a much smaller set of allowed characters. For example, a period
(".") is a valid character in a local part, but would wreak havoc in
a domain name unless the application using the name somehow quoted
it. Similarly, RFC 6530 allows non-ASCII characters in local parts,
and encoding a local part with non-ASCII characters as the hex of the
SHA-224 renders the name usable in applications that use the DNS.
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Wildcards can be more useful for SMIMEA than they are for TLSA. If a
site publishes a trust anchor certificate for all users on the site
(certificate usage 0 or 2), it could make sense to use a wildcard
resource record such as "*._smimecert.example.com".
4. 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
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).
5. IANA Considerations5.1. SMIMEA RRtype
This document uses a new DNS RRtype, SMIMEA, whose value will be
allocated by IANA from the Resource Record (RR) TYPEs subregistry of
the Domain Name System (DNS) Parameters registry.
TODO: there needs to be new registries for certificate usages,
selectors, and maching types, pre-populated with the values from
6. 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.
Client treatment of any information included in the trust anchor is a
matter of local policy. This specification does not mandate that
such information be inspected or validated by the domain name
Hoffman & Schlyter Expires August 24, 2015 [Page 5]Internet-Draft DNS-Based Authentication for S/MIME February 20157. Acknowledgements
Brian Dickson, Miek Gieben, and Martin Pels contributed technical
ideas and support to this document.
8. References8.1. 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.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[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.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, 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.
8.2. Informative References
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
Hoffman & Schlyter Expires August 24, 2015 [Page 6]Internet-Draft DNS-Based Authentication for S/MIME February 2015Authors' Addresses
Hoffman & Schlyter Expires August 24, 2015 [Page 7]