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Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: February 14, 2011                                      Kirei AB
                                                               W. Kumari
                                                              A. Langley
                                                                  Google
                                                         August 13, 2010


      Using Secure DNS to Associate Keys with Domain Names For TLS
                 draft-hoffman-keys-linkage-from-dns-00

Abstract

   TLS uses PKIX certificates for authenticating the server.  Users want
   their applications to verify that the key in 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 key that appears in a 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 14, 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 [RFC5246] may contain a
   PKIX certificate.  In order for the TLS client to authenticate that
   it is talking to the expected TLS server, the client must validate
   that the key in 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 PKIX
   certificate, must trust the trust anchor upon which the server's PKIX
   certificate is rooted, and must perform correct PKIX validation on
   the certificate.

   Some people want a different way to authenticate the association of
   the key in 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 public key that
   might be used by a host at that domain name.  The easiest way to do
   this is to use the DNS.

   A key association is a cryptographic hash of the public key in a PKIX
   certificate (sometimes called a "fingerprint").  That is, a hash is
   taken of the DER-encoded subjectPublicKeyInfo field of the PKIX
   certificate as defined in [RFC5280], and that hash is the key
   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 key in the
   PKIX certificate that is obtained from the TLS server with a domain
   name using DNS protected by DNSSEC.  Because the key association was
   retrieved based on a DNS query, the domain name in the query is by



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   definition associated with the key.

   This document only relates to securely getting the DNS information
   for the key 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 getting keys for TLS; 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].


2.  Getting TLS Key Associations from the DNS

   This section describes three equivalent methods for encoding TLS
   associations: a new certificate type of the existing CERT RR (defined
   in [RFC4398]), a new resource record (RR) called "TLSFP" and a TXT RR
   that can be emitted when the query has "_tlsfp" as the leftmost
   label.

   EXTREMELY IMPORTANT NOTE: Only one of these methods describe in this
   document should be selected for the final protocol.  We have listed
   them in our approximate order of preference, but look forward to
   discussion.  When that decision is made, the two methods not used
   will be moved to the appendix.

2.1.  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.

   The format of the TLSFP certificate type is binary.  In the record,
   all integers consist of two bytes in network byte order.  The record,
   which MUST be in the order defined here, is:

   o  An integer specifying how many port numbers are listed.  If this
      value is zero (0), the key association is valid for any port.

   o  An optional unordered set of two-byte integers, ranging from 1 to
      65535, specifying the TCP/UDP ports for which the key association



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      is valid.

   o  An integer specifying the type of hash algorithm used for the key
      association.

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

   For example:

      www.example.com. IN CERT TLSFP 0 0 ( AQG7ASCWCnpVpwaT
      wRsZLt3FmDw45y/8H/Ie9tyEWLd2nZF9 )

   Note that, unlike the following two format proposals, no version
   number is needed for the certificate type because a request for a
   CERT RR can yield multiple results.  If there is a later improvement
   to the TLSFP certificate type, it could be sent along with a TLSFP
   certificate type in a response.

2.2.  The TLSFP Resource Record

   The new RR TLSFP resource record is defined here.  A query on a
   domain name for the TLSFP type can return one or more records of the
   type TLSFP.

   The format of the TLSFP response is binary.  In the record, all
   integers consist of two bytes in network byte order.  The record,
   which MUST be in the order defined here, is:

   o  The version number.  This is useful if non-critical changes are
      made to this RR later.  The initial version number is 42.

   o  An integer specifying how many port numbers are listed.  If this
      value is zero (0), the key association is valid for any port.

   o  An optional unordered set of two-byte integers, ranging from 1 to
      65535, specifying the TCP/UDP ports for which the key association
      is valid.

   o  An integer specifying the type of hash algorithm used for the key
      association.

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

   For example:

      www.example.com. IN TLSFP 42 1 443 1 20960a7a55a706



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      93c11b192eddc5983c38e72ffc1ff21ef6dc8458b7769d917d

   [[ This will need a proper RRTYPE definition.  That will be added
   later if this option is chosen. ]]

2.3.  Using a TXT Resource Record with a _TLSFP Label Prefix

   A request for a TXT RR whose domain is the label _tlsfp prepended to
   a domain name can be used to get the KEY associated with the domain
   name.  A query of this can return one or more records of the type
   TXT.

   The format of the TXT response is ASCII text.  The record, which MUST
   be in the order defined here, is:

   o  One instance of "ver=", the version number, followed by ";",
      followed by ";".  This is useful if non-critical changes are made
      to this RR later.  The initial version number is 42.

   o  Zero or more instances of "port=" followed by an TCP/UDP port for
      which the key association is valid (expressed as an integer),
      followed by ";".  If a port is not specified, the key association
      is valid for all ports.

   o  The type of hash algorithm used for key association, specified as
      "type=nn;" where "nn" is an integer defined below.

   o  "hash=" followed by the set of bytes containing the hash of the
      associated key; the bytes are encoded as lower-case hexadecimal.

   For example:

      _tlsfp.www.example.com. IN TXT "ver=42; port=443; type=1;
      hash=20960a7a55a70693c11b192eddc5983c38e72ffc1ff21ef6dc84
      58b7769d917d

2.4.  Key Association Hash Algorithms

   The initial list of key association hash algorithms is:

   o  0 - reserved

   o  1 - SHA2-256 [RFC4634]

   o  2 - SHA2-384 [RFC4634]

   o  3 - SHA2-512 [RFC4634]




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   Defining other key association hash types requires IETF consensus as
   defined in [RFC2434].

   For interoperability reasons, as few hash algorithm as possible
   should be reserved.  The only reason to reserve additional types is
   to increase security.


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

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

   An application that requests TLS keys using the method described in
   the previous section obtains zero or more key associations.  If the
   application receives zero key associations, it process TLS in the
   normal fashion.  If one or more key associations are received from
   the DNS:

   o  If the PKIX certificate given by the TLS server is signed by a CA
      trusted by the client, the application compares each key
      association with the the hash of the key from the certificate,
      using the same hash function that is given in the key association
      type.  If a match is found, the TLS handshake continues as normal,
      including the TLS client doing all PKIX validation checks.

   o  If the PKIX certificate given by the TLS server is not signed by a
      CA trusted by the client, the application compares each key
      association with the the hash of the key from the certificate,
      using the same hash function that is given in the key association
      type.  If a match is found, the TLS handshake continues using the
      key from the certificate, but with no PKIX validations checks
      being performed.

   In either of the above cases, if a match between the key
   association(s) is not found, the TLS client MUST abort the handshake
   with an "access_denied" error.


4.  IANA Considerations

   [[ TBD.  Will include the registration for the TLSFP RR if that is
   the style chosen, as well as a new registry for hash algorithm types,
   depending on what style is decided on. ]]





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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 TLSFP
   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, and Jeff Hodges,
   among others.


7.  References

7.1.  Normative References

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

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [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.

   [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.

   [RFC4634]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, July 2006.

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



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   [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, May 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 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 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 fingerprints.  Briefly: TLS requires that
      the server gives a PKIX 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  After a format for the information is chosen, the other two listed
      earlier will go into this appendix.








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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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