One document matched: draft-ietf-dane-smime-07.xml

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<rfc ipr="trust200902"
    docName="draft-ietf-dane-smime-07"
    category="std"
>

<front>
<title abbrev="DNS-Based Authentication for S/MIME">
Using Secure DNS to Associate Certificates with Domain Names For S/MIME</title>

<author initials='P.' surname="Hoffman" fullname='Paul Hoffman'>
<organization>VPN Consortium</organization>
<address>
<email>paul.hoffman@vpnc.org</email>
</address>
</author>
<author initials="J." surname="Schlyter" fullname="Jakob Schlyter">
<organization>Kirei AB</organization>
<address>
<email>jakob@kirei.se</email>
</address>
</author>

<date/>

<abstract>

<t>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.</t>

</abstract>

</front>

<middle>

<section title="Introduction">

<t>S/MIME <xref target="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.</t>

<t>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.</t>

<t>This document describes a mechanism for associating a user's certificate
with the domain that is similar to that described in DANE itself <xref target='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.</t>

<t>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.</t>

<section title="Terminology">

<t>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 <xref target='RFC2119'/>.</t>

<t>This document also makes use of standard PKIX, DNSSEC, and S/MIME
terminology. See PKIX <xref target='RFC5280'/>, DNSSEC <xref target='RFC4033'/>, 
<xref target='RFC4034'/>, <xref target='RFC4035'/>, and SMIME <xref
target='RFC5751'/> for these terms.</t>

</section>

</section>

<section title="The SMIMEA Resource Record" anchor="smimea_rr">

<t>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 intermediate certificate.</t>

<t>The type value for the SMIMEA RRtype is defined in <xref target="ianarrtype"/>.
The SMIMEA resource record is class independent.
The SMIMEA resource record has no special TTL requirements.</t>

<t>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 certificate information.</t>

</section>

<section title="Domain Names for S/MIME Certificate Associations">

<t>Domain names are prepared for requests in the following manner.

<list style="numbers">

<t>The user name (the "left-hand side" of the email address, called the "local-part" in the
mail message format definition <xref target="RFC2822"/> and the "local part" in the
specification for internationalized email <xref target="RFC6530"/>), is hashed using the
SHA2-224 <xref target="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.</t>

<t>The string "_smimecert" becomes the second
left-most label in the prepared domain name.</t>

<t>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.</t>

</list></t>

<t>For example, to request a SMIMEA resource record for a
user whose address is "chris@example.com", calculate
the SHA-224 of "chris", which is
0x3f51f4663b2b798560c5b9e16d6069a28727f62518c3a1b33f7f5214.
The request is thus:
<figure><artwork><![CDATA[
3f51f4663b2b798560c5b9e16d6069a28727f62518c3a1b33f7f5214._smimecert.example.com
]]></artwork></figure>
The corresponding resource record in the example.com zone might
look like:

<figure><artwork><![CDATA[
3f51f4663b2b798560c5b9e16d6069a28727f62518c3a1b33f7f5214._smimecert.example.com.
   IN SMIMEA (
   0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
         7983a1d16e8a410e4561cb106618e971 )
]]></artwork></figure>

</t>

<t>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.</t>

<t>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".</t>

</section>

<section title="Mandatory-to-Implement Features">

<t>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).</t>


</section>

<section title='IANA Considerations'>

<section title='SMIMEA RRtype' anchor="ianarrtype">

<t>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.</t>

<t>TODO: there needs to be new registries for certificate usages, selectors, and maching types,
pre-populated with the values from TLSA.</t>

</section>

</section>

<section title='Security Considerations'>

<t>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.</t>

<t>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
administrator.</t>

</section>

<section title='Acknowledgements'>

<t>Brian Dickson, Miek Gieben, and Martin Pels contributed technical ideas and support
to this document.</t>

</section>

</middle>

<back>

<references title="Normative References">

&rfc2119;
&rfc4033;
&rfc4034;
&rfc4035;
&rfc5280;
&rfc5751;
&rfc5754;
&rfc6698;

</references>

<references title="Informative References">

&rfc2822;
&rfc6530;

</references>

</back>

</rfc>