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Network Working Group                                        B. Haberman
Internet-Draft                                                   JHU APL
Intended status: Standards Track                               J. Levine
Expires: January 17, 2019                           Taughannock Networks
                                                           July 16, 2018


      Using a DNS SRV Record to Locate an X.509 Certificate Store
                         draft-bhjl-x509-srv-04

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 17, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Service Record Format . . . . . . . . . . . . . . . . . . . .   2
   3.  Certificate Store Queries . . . . . . . . . . . . . . . . . .   3
   4.  Name Matching . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Certificate Validation  . . . . . . . . . . . . . . . . . . .   4
   6.  Certificate use and cacheing  . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Certificates service  . . . . . . . . . . . . . . . . . .   6
     8.2.  Smimeca service . . . . . . . . . . . . . . . . . . . . .   6
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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 X.509-based Public Key Infrastructure (PKI) [RFC5280] 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 or PGP 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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC8174] when, and only when, they appear in all capitals, as
   shown here.

2.  Service Record Format

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






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


   To support the advertisement of an X.509 certificate store, service
   providers publish an SRV record for the certificates service with the
   appropriate 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 SHOULD 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
   SHOULD 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 bob@example.com, the URI
   would be:


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


   X.509 certificate stores MUST support the uri attribute and 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 bob@example.com, the URI
   would be:


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


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





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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 document.

   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 the two addresses to deliver
   mail to the same person or entity.

5.  Certificate Validation

   The certificate is returned as a blob of binary data.  If multiple
   certificates are returned, the response is encoded as multipart/mixed
   as described in [RFC4387] section 2.

   X509 S/MIME certificates 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 _smimeca._tcp.  The TLSA records MUST have PKIX-
   TA or DANE-TA usage[RFC7218].  A validation certificate published by
   a domain MUST NOT be used to validate certificates other than those
   with e-mail addresses in that domain.

   Since the relationship between a domain and its mailbox users is in
   general unknown to correspondents, a client applies 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 can
   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 applies 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.








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6.  Certificate use and cacheing

   Clients SHOULD cache responses to queries as advised by http cache
   headers.  This includes both returned certificates, and 404 failures
   saying that an address (or other search key) has no certificate.

   S/MIME keys retrieved from the certificate store SHOULD NOT be used
   for validation of signatures on incoming mail without further
   validation of the certificate.  S/MIME signed mail includes a copy of
   the signing certificate which, if it can be validated, typically
   would be used instead.

7.  Security Considerations

   Certificate queries could be used to try to validate lists of e-mail
   addresses.  This is essentially the same problem that mail servers
   face with VRFY, EXPN, and RCPT TO probes, and the same
   countermeasures would apply, such as rate limiting, blacklisting
   abusive clients, and returning fake results for non-existent
   addresses.

   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 correspondents often do not know the relationship between a
   domain and its mailbox users, 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 without further
   information about the certificates.

   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' correspondence.  In other cases, it
   could be intrusive or 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.

8.  IANA Considerations

   IANA is requested to update two entries in the Service Name and
   Transport Protocol Port Number Registry.







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8.1.  Certificates service

   Service Name: certificates

   Transport Protocol(s): tcp

   Assignee: IESG

   Contact: <chair@ietf.org>

   Description: Server for S/MIME and PGP certificates

   Reference: [this document]

   Port Number: none

   Service Code: none

   Known Unauthorized Uses: none

8.2.  Smimeca service

   Service Name: simeca

   Transport Protocol(s): tcp

   Assignee: IESG

   Contact: <chair@ietf.org>

   Description: Per-domain authority certificate for S/MIME certificates

   Reference: [this document]

   Port Number: none

   Service Code: none

   Known Unauthorized Uses: none

9.  Acknowledgements

   We thank Wei Chuang, Nicolas Lidzborski, and Andreas Schulze for
   comments and suggestions.







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10.  Normative References

   [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,
              <https://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,
              <https://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,
              <https://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, <https://www.rfc-editor.org/info/rfc4387>.

   [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, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://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, <https://www.rfc-editor.org/info/rfc7218>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Authors' Addresses

   Brian Haberman
   Johns Hopkins University Applied Physics Lab

   Email: brian@innovationslab.net




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   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

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












































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