Network Working Group J. Schlyter
Internet-Draft Carlstedt Research &
Expires: May 11, 2002 Technology
S. Josefsson
RSA Security
R. Arends
Nominum
November 10, 2001
Storing certificates in DNS for email applications
draft-schlyter-mailcert-dns-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
The Domain Name System (DNS) can be used to store certificates used
to identify mail addresses. This document describes on how to name
these certificates when stored in DNS. This document updates RFC
2538.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in RFC 2119 [2].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problems with the current representation . . . . . . . . . . . 3
2.1 Name collisions . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 No automatic locating of PKI material of entities . . . . . . 3
2.3 Administrative boundaries . . . . . . . . . . . . . . . . . . 4
3. Proposed representation . . . . . . . . . . . . . . . . . . . 4
3.1 Algorithm to convert RFC 2822 address to domain name . . . . . 4
3.2 Case handling . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
RFC 2538 [5] section 3.1 describes how to translate X.509 subject and
issuer names and into a domain name. The translation used is a
fairly complicated set of recommendations to use in priority order
depending on what is available in the X.509 certificate.
RFC 2538 section 3.2 describes how to translate a general character
string PGP User ID, as defined in RFC 2440 [3], that includes a RFC
2822 [6] email address into a domain name. The translation used is
the standard translation of an email address into a domain name.
Using the translations described in RFC 2538 has several
disadvantages. We explore these disadvantages in section 2 and
propose a new representation in section 3.
2. Problems with the current representation
2.1 Name collisions
When the standard translation, as specified in RFC 2538 [5] 3.2 is
used, translated mailbox names, as specified in RFC 2822 [6], may
collide with hostnames and/or other mailboxes.
For example <Leslie.Example@example.com> translated to label
"leslie.example.example.com" collides with the translated mailbox
<Leslie@example.example.com> as this would translate to the equal
label. Another example is <hostmaster@example.com> that would
collide with the host called "hostmaster.example.com".
2.2 No automatic locating of PKI material of entities
The RFC 2538 [5] X.509 owner name guidelines is not adequate because
they focus on the content of a certificate to determine how it should
be stored. This imposes a dilemma for a third party that wishes to
locate a certificate for an remote entity (e.g. identified with an
mail address) - they need to know parts of the certificate they want
to retrieve. In email applications the parties can in general only
be assumed to know a limited set of information about the other
entity. Such as the mail address. They do not know apriori the
X.509 DN of the remote entity.
When the RFC 2538 owner names for X.509 certificates are used,
clients that only knows e.g. the email address of a certificate
owner cannot infer the DNS name where the certificate is used.
For example, when the certificate for <Leslie.Example@example.com> is
stored in DNS the owner name depends on what the certificate
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contains. For instance if the users's URI is present in the
certificate the owner name for the certificate should, according to
the RFC 2538 rules, be the domain name in the URI. A mail client
that only knows the email address but not the URI cannot infer the
domain name used.
2.3 Administrative boundaries
3. Proposed representation
As we have seen, the DNS "owner name" guidelines described in RFC
2538 has several flaws. They also do not make the owner name
guidelines mandatory, which would be a advantage for interoperable
secure email. Below we specify a scheme for applications that use
RFC 2822 addresses to identify identities, such as Internet Mail and
the UseNet News.
N.B., the RFC 2538 guideliness MAY still be used in addition to the
owner names specified here. One of the owner names MAY be CNAMEs to
the other.
3.1 Algorithm to convert RFC 2822 address to domain name
To encode a RFC 2822 "addr-spec" into the string used to a DNS domain
name as represented in zone files, the "local-part" is appended with
"._mail." and concatenated with the "domain" part.
;; INPUT (from RFC 2882 EBNF):
addr-spec = local-part "@" domain
;; OUTPUT (domain name for DNS zone file):
local-part._mail.domain.
3.2 Case handling
Even though the local-part of a mail address may be case sensitive in
theory, the address SHOULD be converted to lower case before use.
3.3 Examples
A certificate for <leslie@example.com> is stored at
leslie._mail.example.com.
A certificate for <firstname.lastname@example.com> is stored at
firstname.lastname._mail.example.com.
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4. Security Considerations
Since certificates are digitally signed, no additional integrity
service is necessary. Certificates do not need to be kept secret,
and anonymous access to certificates is generally acceptable, thus no
privacy service is necessary. However, clients that retrieve CRLs
without some way of verifying the server run the risk of being sent a
still current but superceded CRL.
Operators of DNS servers should authenticate end entities, CAs and
RAs who publish certificates. However, authentication is not
necessary to retrieve certificates.
When a zone is signed and published using the DNS security
extensions, it is feasible to traverse a zone by NXT-chaining to
collect mailboxes. This may not be desired. One solution might be
to store the certificates as unsigned RRsets [7] or use a hashed
alternative to the NXT chain [8].
References
[1] Crocker, D., "Standard for the format of ARPA Internet text
messages", STD 11, RFC 822, August 1982.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
Message Format", RFC 2440, November 1998.
[4] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[5] Eastlake, D. and O. Gudmundsson, "Storing Certificates in the
Domain Name System (DNS)", RFC 2538, March 1999.
[6] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[7] Arends, R., Kosters, M. and D. Blacka, "DNSSEC Opt-In", work in
progress draft-ietf-dnsext-dnssec-opt-in-01, November 2001.
[8] Josefsson, S., "Authenticating denial of existence in DNS with
minimum disclosure", work in progress draft-ietf-dnsext-not-
existing-rr-01, November 2000.
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Authors' Addresses
Jakob Schlyter
Carlstedt Research & Technology
Stora Badhusgatan 18-20
Goteborg SE-411 21
Sweden
EMail: jakob@crt.se
URI: http://www.crt.se/~jakob/
Simon Josefsson
RSA Security
Arenavagen 29
Stockholm SE-121 29
Sweden
Phone: +46 8 725 09 14
EMail: sjosefsson@rsasecurity.com
Roy Arends
Nominum
1e Atjehstraat 174-2
Amsterdam 1094 KX
The Netherlands
EMail: roy.arends@nominum.com
URI: http://www.nominum.com/
Appendix A. Acknowledgements
The authors gratefully acknowledges, in no particular order, the
contributions of the following persons:
Mats Dufberg
Olafur Gudmundsson
Dan Massey
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