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Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: February 25, 2011                                      Kirei AB
                                                               W. Kumari
                                                              A. Langley
                                                                  Google
                                                         August 24, 2010


  Using Secure DNS to Associate Certificates with Domain Names For TLS
                 draft-hoffman-keys-linkage-from-dns-01

Abstract

   TLS and DTLS uses certificates for authenticating the server.  Users
   want their applications to verify that the certificate provided by
   the TLS server is in fact associated with the domain name they
   expect.  Instead of trusting a certificate authority to have made
   this association correctly, the user might instead trust the
   authoritative DNS server for the domain name to make that
   association.  This document describes how to use secure DNS to
   associate the TLS server's certificate with the the intended domain
   name.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on February 25, 2011.

Copyright Notice

   Copyright (c) 2010 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



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   Provisions Relating to IETF Documents
   (http://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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


1.  Introduction

   The first response from the server in TLS may contain a certificate.
   In order for the TLS client to authenticate that it is talking to the
   expected TLS server, the client must validate that this certificate
   is associated with the domain name used by the client to get to the
   server.  Currently, the client must extract the domain name from one
   of many places in the certificate, must trust the trust anchor upon
   which the server's certificate is rooted, and must perform correct
   validation on the certificate.

   This document applies to both TLS [RFC5246] and DTLS [4347bis].  In
   order to make the document more readable, it mostly only talks about
   "TLS", but in all cases, it means "TLS or DTLS".

   Some people want a different way to authenticate the association of
   the server's certificate with the intended domain name without
   trusting the CA.  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 the domain name and a certificate that might be used
   by a host at that domain name.  The easiest way to do this is to use
   the DNS.

   A certificate association is a cryptographic hash of a certificate
   (sometimes called a "fingerprint").  That is, a hash is taken of the
   certificate, and that hash is the certificate association.  The type
   of hash function used can be chosen by the DNS administrator.

   DNSSEC, which is defined in RFCs 4033, 4034, and 4035 ([RFC4033],
   [RFC4034], and [RFC4035]), uses cryptographic keys and digital
   signatures to provide authentication of DNS data.  Information
   retrieved from the DNS and that is validated using DNSSEC is thereby
   proved to be the authoritative data.

   This document defines a secure method to associate the certificate
   that is obtained from the TLS server with a domain name using DNS
   protected by DNSSEC.  Because the certificate association was



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   retrieved based on a DNS query, the domain name in the query is by
   definition associated with the certificate.

   This document only relates to securely getting the DNS information
   for the certificate association using DNSSEC; other secure DNS
   mechanisms are out of scope.  The DNSSEC signature MUST be validated
   on all responses in order to assure the proof of origin of the data.

   This document only relates to securely associating certificates for
   TLS and DTLS; other security protocols are handled in other
   documents.  For example, keys for IPsec are covered in [RFC4025] and
   keys for SSH are covered in [RFC4255].

   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 [RFC2119].

   This document is being discussed on the "keyassure" mailing list; see
   <https://www.ietf.org/mailman/listinfo/keyassure>.


2.  Getting TLS Certificate Associations from the DNS with the CERT RR

   This section describes the TLSFP Certificate Type of the CERT RR.
   The CERT RR [RFC4398] allows expansion by defining new certificate
   types.  The new TLSFP certificate type is defined here.  A query on a
   domain name for the CERT RR can return one or more records of the
   type CERT, and zero or more of those CERT responses can be of type
   TLSFP.

   o  The TLSFP certificate type is TBD.

   o  The key tag and algorithm fields are both set to zero.

   The format of the TLSFP certificate is a binary record, which MUST be
   in the order defined here, is:

   o  A one-octet value, called "hash type", specifying the type of hash
      algorithm used for the certificate association.  This value has
      the same values as those of the DS RR, as defined in [RFC4034] and
      its successors.

   o  A one-octet value, called "validation preference", specifying the
      preferences for further validation of the certificate in TLS.  A
      certificate association that contains a validation preference
      whose value is 1 indicates that the TLS administrator believes
      that it is sufficient to match the certificate association with
      the hash of certificate received in the TLS negotiation, and that



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      no further certificate validation is necessary.  A certificate
      association that contains a validation preference whose value is 0
      indicates that the TLS administrator believes that it is not
      sufficient to match the certificate association with the hash of
      certificate received in the TLS negotiation, and that normal
      certificate validation is necessary.  Note that the validation
      preference is an advisory, and it is not mandatory for a TLS
      client to follow the advice.

   o  A variable-length set of bytes containing the hash of the
      associated certificate.

   For example:

      www.example.com. IN CERT TLSFP 0 0 ( AQGa+FZd8sAeS03ca8xDigDQ
                                           ePgJnQvgMe/kKyf8rzluiQ== )


3.  Use of TLS Certificate Associations from the DNS in TLS

   In order to use one or more TLS certificate associations obtained
   from the DNS, an application MUST assure that the certificates were
   obtained using DNS protected by DNSSEC.  There may be other methods
   to securely obtain certificates in DNS, but those methods are not
   covered by this document.

   An application that requests TLS certificate associations using the
   method described in the previous section obtains zero or more
   certificate associations.  If the application receives zero
   certificate associations, it process TLS in the normal fashion.

   If a certificate association contains a hash type that is not
   understood by the TLS client, that certificate association MUST be
   completely ignored.

   If a match between the certificate association(s) and the server's
   end entity certificate in TLS is not found, the TLS client MUST abort
   the handshake with an "access_denied" error.

   If the TLS server authenticates itself with a self-signed
   certificate, it SHOULD be sure that the validation preference in the
   CERT RR is set to 1.  If the TLS server administrator believes that
   there is information in its certificate that is relevant to the TLS
   client other than the public key (such as a extended value (EV)
   name), it SHOULD be sure that the validation preference in the CERT
   RR is set to 0.





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4.  IANA Considerations

   This document requests that IANA allocates one certificate type from
   the CERT RR certificate type registry.  The type is to be allocated
   from the 'IETF Consensus' range.

      Decimal type: TBD

      Type: TLSFP

      Meaning: TLS Certificate Associations


5.  Security Considerations

   [[ TBD.  This section will need to describe, at least, the "attack"
   where a DNS administrator goes rogue and changes both the A and CERT
   records for a domain name.  Also will discuss the need for secure
   DNS. ]]


6.  Acknowledgements

   Many of the ideas in this document have been discussed over many
   years.  More recently, the ideas have been discussed by the authors
   and others in a more focused fashion.  In particular, some of the
   ideas here originated with Paul Vixie, Dan Kaminsky, Jeff Hodges,
   Simon Josefsson, among others.


7.  References

7.1.  Normative References

   [4347bis]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security version 1.2", draft-ietf-tls-rfc4347-bis (work in
              progress), July 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.



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   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4398]  Josefsson, S., "Storing Certificates in the Domain Name
              System (DNS)", RFC 4398, March 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

7.2.  Informative References

   [RFC4025]  Richardson, M., "A Method for Storing IPsec Keying
              Material in DNS", RFC 4025, March 2005.

   [RFC4255]  Schlyter, J. and W. Griffin, "Using DNS to Securely
              Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
              January 2006.


Appendix A.  Ideas Considered But Not Necessarily Chosen

   This appendix will list some of the ideas that have been kicked
   around in this space and give a few paragraphs why they weren't
   chosen for the current version this proposal.  The following is a
   placeholder for the list that will be filled out more in future
   versions of this document:

   o  A flag that indicates that the certificate with the associated key
      must be signed by a trusted CA.  Briefly: this was not added
      because it is up to the TLS server to decide which type of
      certificate it wants to serve up.  Serving a self-signed
      certificate would effectively disable traditional PKIX validation,
      whereas serving a certificate signed by a trusted CA would require
      both validation by DNSSEC and the trusted CA.

   o  A flag that indicates that all connections to this server need to
      be TLS secured.  Briefly: this is a good idea but it is not
      related to the key of the certificate given in TLS, and thus
      should be indicated in a different method.

   o  Giving keys instead of hashes of keys.  Briefly: TLS requires that
      the server gives a certificate, and some systems use the metadata
      from a CA-signed certificate for display, so there is value to
      always looking in the certificate.

   o  Hashes of keys vs. hashes of certificates.  Briefly: we have
      changed our minds (at least once) on this.  Our original thinking



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      was that there are many reasons why someone might change their
      certificate while leaving the public key alone, and those changes
      should not have to force them to change the DNS record because
      they do not actually change what the TLS client cares about; thus,
      use hashes of keys.  Our new thinking is that there are
      certificate semantics that we want to pass (namely, should the
      client actually do the certificate validation), and attaching
      those semantics to keys is confusing; thus, use hashes of
      certificates.

   o  List TLS/DTLS ports or services for which the certificate is
      associated.  Briefly: we had this in an earlier version of this
      document but got rid of it when it was pointed out that this is an
      edge case, and most servers differentiate these services by domain
      names such as "mail.example.com" and "www.example.com".

   o  Different ways of encoding this information in the DNS.  Briefly:
      we considered a new RR type and coming up with an encoding of the
      TXT RR type, but didn't see any significant advantage of them over
      using the CERT RR, and there were disadvantages.  A disadvantage
      of a new RR type is getting DNS servers and clients to recognize
      it; a disadvantage of coming up with a new TXT format is that
      doing so prevents wildcards.  There is a lot more to discuss here,
      but the authors are now happy with a new sub-type for the CERT RR.

   o  Having the hash be over the TLS certificate structure instead of
      just the end-entity certificate.  Briefly: the TLS certificate
      structure currently allows a chain of PKIX certificates, and the
      semantics of what is being associated in a chain is not clear.
      Further, the structure might be changed in the future (such as to
      allow a group of web-of-trust OpenPGP certificates), and the
      semantics of what is being associated would become even less
      clear.


Appendix B.  Changes between -00 and -01

   Change the association from being a hash of the key of a PKIX
   certificate to being a hash of a certificate (PKIX or other).  This,
   of course, makes large changes throughout the document.

   Expanded the document to cover DTLS as well.

   Added a pointer to the keyassure mailing list.

   Removed the proposals for two alternate formats (the TLSFP Resource
   Record and the TXT record encoding).  Added a bit to Appendix A about
   this.



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   Got rid of the specification for ports within a single domain name.

   Made the hash type one octet and used the DS registry instead of
   defining our own.

   Added "Necessarily" chosen in the title of Appendix A to show that we
   might (continue to) change our minds after discussion.

   Added Simon Josefsson to the acknowledgements.


Authors' Addresses

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org


   Jakob Schlyter
   Kirei AB

   Email: jakob@kirei.se


   Warren Kumari
   Google

   Email: warren@kumari.net


   Adam Langley
   Google

   Email: agl@google.com
















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