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Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: August 27, 2011                                        Kirei AB
                                                       February 23, 2011


  Using Secure DNS to Associate Certificates with Domain Names For TLS
                      draft-ietf-dane-protocol-05

Abstract

   TLS and DTLS use 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.  DNSSEC provides a mechanism for a zone operator to sign DNS
   information directly.  This way, bindings of keys to domains are
   asserted not by external entities, but by the entities that operate
   the DNS.  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 August 27, 2011.

Copyright Notice

   Copyright (c) 2011 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
   (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



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   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Certificate Associations . . . . . . . . . . . . . . . . .  3
     1.2.  Securing Certificate Associations  . . . . . . . . . . . .  4
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Getting TLS Certificate Associations from the DNS  . . . . . .  5
     2.1.  Requested Domain Name  . . . . . . . . . . . . . . . . . .  5
     2.2.  Format of the Resource Record  . . . . . . . . . . . . . .  5
     2.3.  Making Certificate Associations  . . . . . . . . . . . . .  6
     2.4.  Presentation Format  . . . . . . . . . . . . . . . . . . .  7
     2.5.  Wire Format  . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Use of TLS Certificate Associations in TLS . . . . . . . . . .  8
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  TLSA RRtype  . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  TLSA Certificate Types . . . . . . . . . . . . . . . . . .  9
     4.3.  TLSA Hash Types  . . . . . . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12






















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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 the
   certificate, must trust a trust anchor upon which the server's
   certificate is rooted, and must successfully validate the
   certificate.

   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.

   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".  This document only
   relates to securely associating certificates for TLS and DTLS with
   host names; 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].

1.1.  Certificate Associations

   In this document, a certificate association is based on a
   cryptographic hash of a certificate (sometimes called a
   "fingerprint") or on the certificate itself.  For a fingerprint, a
   hash is taken of the binary, DER-encoded certificate, and that hash
   is the certificate association; the type of hash function used can be
   chosen by the DNS administrator.  When using the certificate itself
   in the certificate association, the entire certificate in the normal
   format is used.  This document also only applies to PKIX [RFC5280]
   certificates.

   Certificate associations are made between a certificate or the hash
   of a certificate and a domain name.  Server software that is running
   TLS that is found at that domain name would use a certificate that
   has a certificate association given in the DNS, as described in this
   document.  A DNS query can return multiple certificate associations,
   such as in the case of different server software on a single host
   using different certificates (even if they are normally accessed with
   different host names), or in the case that a server is changing from



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   one certificate to another.

1.2.  Securing Certificate Associations

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

   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.  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 the DNS information
   for the certificate association using DNSSEC; other secure DNS
   mechanisms are out of scope.

1.3.  Terminology

   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].

   A note on terminology: Some people have said that this protocol is a
   form of "certificate exclusion".  This is true, but in a very unusual
   sense.  That is, a DNS reply that contains two of the certificate
   types defined here inherently excludes every other possible
   certificate in the universe other than those found with a pre-image
   attack against one of those two.  The certificate type defined here
   is better thought of as "enumeration" of a small number of
   certificate associations, not "exclusion" of a near-infinite number
   of other certificates.

   Some of the terminology in this draft may not match with the
   terminology used in RFC 5280.  This will be fixed in future versions
   of this draft, with help from the PKIX community.  In specific, we
   need to say (in a PKIX-appropriate way) that when we say "valid up
   to" and "chains to", full RFC 5280 path processing including
   revocation status checking is intended.







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2.  Getting TLS Certificate Associations from the DNS

   This document defines a new DNS resource record type, "TLSA".  A
   query on a prepared domain name for the TLSA RR can return one or
   more records of the type TLSA.  The TLSA RRType is TBD.

2.1.  Requested Domain Name

   Domain names are prepared for requests in the following manner.

   1.  The decimal representation of the port number on which a TLS-
       based service is assumed to exist is prepended with an underscore
       character ("_") to become the left-most label in the prepared
       domain name.

   2.  The protocol name of the transport on which a TLS-based service
       is assumed to exist is prepended with an underscore character
       ("_") to become the second left-most label in the prepared domain
       name.  The transport names defined for this protocol are "tcp",
       "udp" and "sctp".

   3.  The domain name is appended to the result of step 2 to complete
       the prepared domain name.

   For example, to request a TLSA resource record for an HTTP server
   running TLS on port 443 at "www.example.com", you would use
   "_443._tcp.www.example.com" in the request.  To request a TLSA
   resource record for an SMTP server running the STARTTLS protocol on
   port 25 at "mail.example.com", you would use
   "_25._tcp.mail.example.com".

2.2.  Format of the Resource Record

   The format of the data in the resource record is a binary record with
   three values, which MUST be in the order defined here:

   o  A one-octet value, called "certificate type", specifying the
      provided association that will be used to match the target
      certificate.  This will be an IANA registry in order to make it
      easier to add additional certificate types in the future.  The
      types defined in this document are:

         1 -- An end-entity certificate in DER encoding

         2 -- A certification authority's certificate in DER encoding






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   o  A one-octet value, called "reference type", specifying how the
      certificate association is presented.  This value is defined in a
      new IANA registry.  The types defined in this document are:

         0 -- Full certificate

         1 -- SHA-1 hash of the certificate

         2 -- SHA-256 hash of the certificate

         3 -- SHA-384 hash of the certificate

      Using the same hash algorithm as is used in the signature in the
      certificate will make it more likely that the TLS client will
      understand this TLSA data.

   o  The "certificate for association".  This is the bytes containing
      the full certificate or the hash of the associated certificate
      (that is, the certificate or the hash of the certificate itself,
      not of the TLS ASN.1Cert object).

   Certificate types 1 and 2 explicitly only apply to PKIX-formatted
   certificates.  If TLS allows other formats later, or if extensions to
   this protocol are made that accept other formats for certificates,
   those certificates will need certificate types.

2.3.  Making Certificate Associations

   A TLS client conforming to this protocol MUST treat the certificate
   for association in a TLSA resource record for a domain name as a
   trust anchor for that domain name at the specific port number and
   transport name that was queried.  This trust anchor MUST only be used
   for the domain name of the resource record.  The trust anchor MUST
   NOT be loaded for longer than the TTL on the TSLA record.

   The TLS client determines whether or not the certificate offered by
   the TLS server matches the trust anchor received in the TLSA resource
   record.  If the certificate from the TLS server matches, the TLS
   client accepts the certificate association.  Each certificate type
   has a different method for determining matching.

   Certificate type 1 (end-entity certificate) is matched against the
   first certificate offered by the TLS server.  The certificate for
   association is used only for exact matching, not for chained
   validation.  With reference type 0, the certificate association is
   valid if the certificate in the TLSA data matches to the first
   certificate offered by TLS.  With reference types other than 0, the
   certificate association is valid if the hash of the first certificate



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   offered by the TLS server matches the value from the TLSA data.

   Certificate type 2 (certification authority's certificate) can be
   used in one of two ways.  With reference type 0, the certificate in
   the TLSA resource record is used in chaining from the end-entity
   given in TLS.  The certificate association is valid if the first
   certificate in the certificate bundle can be validly chained to the
   trust anchor from the TLSA data.  With reference types other than 0,
   if the hash of any certificate past the first in the certificate
   bundle from TLS matches the trust anchor from the TLSA data, and the
   chain in the certificate bundle is valid up to that TLSA trust
   anchor, then the certificate association is valid.  Alternately, if
   the first certificate offered chains to an existing trust anchor in
   the TLS client's trust anchor repository, and the hash of that trust
   anchor matches the value from the TLSA data, then the certificate
   association is valid.

   [[ Need discussion of self-signed certificates being CA certificates.
   Need to be sure that this discussion uses correct PKIX terminology
   and is carefully explained. ]]

2.4.  Presentation Format

   The RDATA of the presentation format of the TLSA resource record
   consists of two numbers (certificate and hash type) followed by the
   bytes containing the certificate or the hash of the associated
   certificate itself, presented in hex.  An example of a SHA-256 hash
   (type 2) of an end-entity certificate (type 1) would be:

   _443._tcp.www.example.com. IN TLSA (
      1 2 5c1502a6549c423be0a0aa9d9a16904de5ef0f5c98
          c735fcca79f09230aa7141 )

   An example of an unhashed CA certificate (type 2) would be:

   _443._tcp.www.example.com. IN TLSA (
      2 0 308202c5308201ada00302010202090... )

   Because the length of hashes and certificates can be quite long,
   presentation format explicitly allows line breaks and white space in
   the hex values; those characters are removed when converting to the
   wire format.

2.5.  Wire Format

   The wire format is:





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                        1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Cert type   |   Hash type   |                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
   /                                                               /
   /                    Certificate for association                /
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The wire format for the RDATA in the first example given above would
   be:

   _443._tcp.www.example.com. IN TYPE65534 \# 34 ( 01025c1502a6549c42
                 3be0a0aa9d9a16904de5ef0f5c98c735fcca79f09230aa7141 )

   The wire format for the RDATA in the second example given above would
   be:

   _443._tcp.www.example.com. IN TYPE65534 \# 715 0200308202c5308201a...

   Note that in the preceding examples, "TYPE65534" is given as an
   example.  That RR Type is in the IANA "private use" range; the real
   RR Type for TLSA will be issued by IANA, as described in the IANA
   Considerations section below.


3.  Use of TLS Certificate Associations in TLS

   In order to use one or more TLS certificate associations described in
   this document obtained from the DNS, an application MUST assure that
   the certificates were obtained using DNS protected by DNSSEC.  TLSA
   records must only be trusted if they were obtained from a trusted
   source.  This could be a localhost DNS resolver answer with the AD
   bit set, an inline validating resolver library primed with the proper
   trust anchors, or obtained from a remote nameserver to which one has
   a secured channel of communication.

   If a certificate association contains a hash type that is not
   understood by the TLS client, that certificate association MUST be
   marked as unusable.

   An application that requests TLS certificate associations using the
   method described in this document obtains zero or more usable
   certificate associations.  If the application receives zero usable
   certificate associations, it processes TLS in the normal fashion.

   If a match between one of the certificate association(s) and the



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   server's end entity certificate in TLS is found, the TLS client
   continues the TLS handshake.  If no match between the usable
   certificate association(s) and the server's end entity certificate in
   TLS is found, the TLS client MUST abort the handshake with an
   "access_denied" error.


4.  IANA Considerations

4.1.  TLSA RRtype

   This document uses a new DNS RRType, TLSA, whose value is TBD.  A
   separate request for the RRType will be submitted to the expert
   reviewer, and future versions of this document will have that value
   instead of TBD.

4.2.  TLSA Certificate Types

   This document creates a new registry, "Certificate Types for TLSA
   Resource Records".  The registry policy is "RFC Required".  The
   initial entries in the registry are:

   Value        Short description                         Ref.
   -------------------------------------------------------------
   0            Reserved                                  [This]
   1            End-entity certificate                    [This]
   2            CA's certificate                          [This]
   3-254        Unassigned
   255          Private use

   Applications to the registry can request specific values that have
   yet to be assigned.

4.3.  TLSA Hash Types

   This document creates a new registry, "Hash Types for TLSA Resource
   Records".  The registry policy is "Specification Required".  The
   initial entries in the registry are:

   Value        Short description       Ref.
   -----------------------------------------------------
   0            No hash used            [This]
   1            SHA-1                   NIST FIPS 180-2
   2            SHA-256                 NIST FIPS 180-2
   3            SHA-384                 NIST FIPS 180-2
   4-254        Unassigned
   255          Private use




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   Applications to the registry can request specific values that have
   yet to be assigned.


5.  Security Considerations

   The security of the protocols described in this document relies on
   the security of DNSSEC as used by the client requesting A/AAAA and
   TLSA records.

   A DNS administrator who goes rogue and changes both the A/AAAA and
   TLSA records for a domain name can cause the user to go to an
   unauthorized server that will appear authorized, unless the client
   performs certificate validation and rejects the certificate.  That
   administrator could probably get a certificate issued anyway, so this
   is not an additional threat.

   The values in the TLSA data will be normally entered in the DNS
   through the same system used to enter A/AAAA records, and other DNS
   information for the host name.  If the authentication for changes to
   the host information is weak, an attacker can easily change any of
   this information.  Given that the TLSA data is not easily human-
   readable, an attacker might change those records and A/AAAA records
   and not have the change be noticed if changes to a zone are only
   monitored visually.

   If the authentication mechanism for adding or changing TLSA data in a
   zone is weaker than the authentication mechanism for changing the
   A/AAAA records, a man-in-the-middle who can redirect traffic to their
   site may be able to impersonate the attacked host in TLS if they can
   use the weaker authentication mechanism.  A better design for
   authenticating DNS would be to have the same level of authentication
   used for all DNS additions and changes for a particular host.

   SSL proxies can sometimes act as a man-in-the-middle for TLS clients.
   In these scenarios, the clients add a new trust anchor whose private
   key is kept on the SSL proxy; the proxy intercepts TLS requests,
   creates a new TLS session with the intended host, and sets up a TLS
   session with the client using a certificate that chains to the trust
   anchor installed in the client by the proxy.  In such environments,
   the TLSA protocol will prevent the SSL proxy from functioning as
   expected because the TLS client will get a certificate association
   from the DNS that will not match the certificate that the SSL proxy
   uses with the client.  The client, seeing the proxy's new certificate
   for the supposed destination will not set up a TLS session.






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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,
   Phill Hallam-Baker, Simon Josefsson, Warren Kumari, Adam Langley,
   Ilari Liusvaara, Scott Schmit, and Ondrej Sury.


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.

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

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

   [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,



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              January 2006.


Authors' Addresses

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org


   Jakob Schlyter
   Kirei AB

   Email: jakob@kirei.se




































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