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 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 11, 2002. 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 Schlyter, et al. Expires May 11, 2002 [Page 1] Internet-Draft Certificates in DNS for email November 2001 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 Schlyter, et al. Expires May 11, 2002 [Page 2] Internet-Draft Certificates in DNS for email November 2001 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 translated to label "leslie.example.example.com" collides with the translated mailbox as this would translate to the equal label. Another example is 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 is stored in DNS the owner name depends on what the certificate Schlyter, et al. Expires May 11, 2002 [Page 3] Internet-Draft Certificates in DNS for email November 2001 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 is stored at leslie._mail.example.com. A certificate for is stored at firstname.lastname._mail.example.com. Schlyter, et al. Expires May 11, 2002 [Page 4] Internet-Draft Certificates in DNS for email November 2001 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. Schlyter, et al. Expires May 11, 2002 [Page 5] Internet-Draft Certificates in DNS for email November 2001 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 Schlyter, et al. Expires May 11, 2002 [Page 6] Internet-Draft Certificates in DNS for email November 2001 Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. 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