Using a DNS SRV Record to Locate an X.509 Certificate Store
draft-bhjl-x509-srv-00

Network Working Group                                        B. Haberman
Internet-Draft                                                   JHU APL
Intended status: Standards Track                               J. Levine
Expires: September 1, 2016                          Taughannock Networks
                                                       February 29, 2016

      Using a DNS SRV Record to Locate an X.509 Certificate Store
                       draft-bhjl-x509-srv-00.txt

Abstract

   This document describes a method to allow parties to locate X.509
   certificate stores with Domain Name System Service records in order
   to retrieve certificates and certificate revocation lists.  The
   primary purpose of such retrievals is to facilitate the association
   of X.509 and PGP public keys with e-mail addresses to allow for
   encrypted e-mail exchanges.

Status of This Memo

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   This Internet-Draft will expire on September 1, 2016.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Service Record Format . . . . . . . . . . . . . . . . . . . .   2
   3.  Certificate Store Queries . . . . . . . . . . . . . . . . . .   3
   4.  Name Matching . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Certificate Validation  . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   X.509 and PGP public keys can be used to encrypt or sign e-mail
   messages.  In order to verify a sender's signature or encrypt an
   e-mail, the e-mail client needs to locate the appropriate public key.
   The Internet Public Key Infrastructure (PKI) provides the necessary
   services to allow for the retrieval of certificates and certificate
   revocation lists, but lacks the discovery mechanism needed to
   associate e-mail domains with specific PKI servers.

   This document specifies an approach that uses a Domain Name System
   (DNS) Service Record (SRV) that allows mail service providers to
   advertise the X.509 certificate store [RFC4387] that contains
   certificates and certificate revocation lists for their e-mail users.
   Additionally, this document specifies the appropriate query strings
   to use when accessing the certificate store.

   The capitalized 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
   [RFC2119].

2.  Service Record Format

   The general format of a DNS SRV record is documented in [RFC2782] as:

        _Service._Proto.Name TTL Class SRV Priority Weight Port Target

   To support the advertisement of an X.509 certificate store, service
   providers will construct an SRV record with the appropriate

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   parameters, as described in [RFC4387], section 3.2.  An example of
   such an SRV record is:

        _certificates._tcp 86400 IN SRV 0 0 443 certs.example.com

   The parameters of the DNS SRV record are set based on the operational
   needs of the service provider.  The DNS SRV record MUST be signed via
   DNSSEC [RFC4033][RFC4034].  The server MUST be an https server and
   will typically use port 443.  The certificate of the https server
   MUST be validated by a DNSSEC signed TLSA record, and MAY also be
   validated by a certificate authority.

3.  Certificate Store Queries

   To retrieve an X.509 S/MIME certificate, the attribute type is "uri",
   and the URI is constructed using the path described in [RFC4387],
   Section 3.3, specifically "/certificates/search.cgi".  Using the SRV
   record above to look up a certificate for someuser@example.com, the
   URI would be:

https://certs.example.com/certificates/search.cgi?uri=someuser%40example.com

   X.509 certificate stores MUST support the uri attribute MAY support
   other attributes.

   To retrieve a PGP certificate, the attribute type is "email", and the
   URI is constructed using the path described in [RFC4387],
   Section 3.3, specifically "/pgpkeys/search.cgi".  Using the SRV
   record above to look up a certificate for someuser@example.com, the
   URI would be:

https://certs.example.com/pgpkeys/search.cgi?email=someuser%40example.com

   PGP certificate stores MUST support the email attribute MAY support
   other attributes.

4.  Name Matching

   SMTP [RFC5321] specifies that the local part of a mailbox is
   interpreted only by the mailbox domain itself.  This document does
   not update or modify that RFC.

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   If a certificate store has no certificate with an e-mail address that
   matches the uri or email attribute in a retrieval request, but it
   does have a certificate with an e-mail address that the mailbox
   domain treats similarly to the requested address, the server MAY
   return that certificate.  The definition of what is sufficiently
   similar is a matter of local policy, but the intention is that a
   human correspondent would consider the same person or entity to
   receive mail at the two addresses.

5.  Certificate Validation

   The certificate is returned as a blob of binary data.  If multiple
   certificates are returned, the response MUST be encoded as multipart/
   mixed.

   X509 S/MIME certificates have are validated by checking for a
   signature by a Certificate Authority (CA) that is acceptable to the
   validating party.  This specification defines an additional
   validation technique.  The domain MAY publish validation certificates
   using TLSA records at the name _smime._tcp.  The TLSA records MUST
   have PKIX-TA or DANE-TA usage[RFC7218].

   Since the relationship between a domain and its mailbox users is in
   general unknown to correspondents, a client MUST apply a local policy
   to decide whether to use a S/MIME certificate validated only by a
   signing certificate published by the domain.

   PGP certificates are validated by the PGP web of trust.  A domain may
   endorse the certificates it publishes by signing them with a
   signature of postmaster@<domain>.  Since the relationship between a
   domain and its mailbox users is in general unknown to correspondents.
   a client MUST apply a local policy to decide whether to use a PGP
   certificate retrieved from a certificate server.  This policy would
   typically be the same one used to decide whether to use a certificate
   retrieved from a traditional PGP key server.

6.  Security Considerations

   DNSSEC signatures on the SRV record and the https server certificate
   ensure that any keys retrieved by the technique described in this
   document are the ones published by the domain's management.  But
   since the relationship between a domain and its mailbox users is in
   general unknown to correspondents, it would be imprudent to assume
   that such certificates are in fact ones issued to or used by mailbox
   recipients or to assume that mail encrypted using the certificates
   will be readable only by the intended recipient.

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   A domain could publish man-in-the-middle certificates that allowed it
   to decode and read mail, and perhaps re-encrypt it using different
   certificates used by the recipients.  In some cases this would be
   entirely legitimate, e.g., a financial institution that is required
   to log all of its employees's correspondence.  In other cases, it
   could be improper surveillance of the contents of users' mail.
   Identifying or describing the relationship between a domain and its
   mail users is beyond the scope of this document.

7.  IANA Considerations

   [Note to IANA, to be removed prior to publication.]

   IANA is requested to add one entry to the Service Name and Transport
   Protocol Port Number Registry.  The service name is "smime"", with no
   associated port numbers. (more here)

8.  Acknowledgements

9.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <http://www.rfc-editor.org/info/rfc2782>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <http://www.rfc-editor.org/info/rfc4033>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <http://www.rfc-editor.org/info/rfc4034>.

   [RFC4387]  Gutmann, P., Ed., "Internet X.509 Public Key
              Infrastructure Operational Protocols: Certificate Store
              Access via HTTP", RFC 4387, DOI 10.17487/RFC4387, February
              2006, <http://www.rfc-editor.org/info/rfc4387>.

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   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <http://www.rfc-editor.org/info/rfc5321>.

   [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify
              Conversations about DNS-Based Authentication of Named
              Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218, April
              2014, <http://www.rfc-editor.org/info/rfc7218>.

Authors' Addresses

   Brian Haberman
   Johns Hopkins University Applied Physics Lab

   Email: brian@innovationslab.net

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +1 831 480 2300
   Email: standards@taugh.com

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