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Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: April 7, 2011                                          Kirei AB
                                                               W. Kumari
                                                              A. Langley
                                                         October 4, 2010

  Using Secure DNS to Associate Certificates with Domain Names For TLS


   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.  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

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 April 7, 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 the
   certificate, must trust the trust anchor upon which the server's
   certificate is rooted, and must perform correct validation on the

   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.

   In this document, a certificate association is based on 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.  (Note that there may be
   other methods to securely obtain certificate associations in DNS, but
   those methods are not covered by this document.)

   Certificate associations are made between a 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

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   certificates (even if they are normally accessed with different host
   names), or in the case that a server is changing from one certificate
   to another.

   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
   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 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].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This document is being discussed on the "keyassure" mailing list; see

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

   The CERT RR [RFC4398] allows expansion by defining new certificate
   types.  This document describes two new Certificate Types, TLSFP and
   TLSRQ.  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 and TLSRQ.

2.1.  The TLSFP Certificate Type

   This section describes the TLSFP certificate type of the CERT RR.
   The TLSFP certificate type is TBD1.  The key tag and algorithm fields
   are both set to zero.

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   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 TLS hash, as defined in the IANA
      registry titled "TLS HashAlgorithm Registry"
      (<http://www.iana.org/assignments/tls-parameters>).  For example,
      the value for the SHA-1 hash function is "2".

   o  A one-octet value, called "certificate type", specifying the TLS
      certificate type of the target certificate.  This value has the
      same values as those of the TLS certificate types, as defined in
      the IANA registry titled "TLS Certificate Types"
      (<http://www.iana.org/assignments/tls-extensiontype-values>).  For
      example, the value for PKIX certificates is "0".

   o  A variable-length set of bytes containing the hash of the
      associated certificate (that is, of the certificate itself, not
      the TLS ASN.1Cert object).

   An example of a fingerprint for a single certificate:

      www.example.com. IN CERT
          TLSFP 0 0 AgDne3GdTpxjwLCgMzvgpBiOSQthjg==

   An example of a fingerprint of a certificate that is found by its
   hash value:

      e77b719d4e9c63c0b0a0333be0a4188e490b618e.www.example.com. IN CERT
          TLSFP 0 0 AgDne3GdTpxjwLCgMzvgpBiOSQthjg==

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

2.2.  The TLSRQ Certificate Type

   This section describes the TLSRQ certificate type of the CERT RR.  If
   a domain has many certificates associated with it, the number of
   TLSFP CERT RRs may become impractical.  In this case, a TLSRQ
   certificate type may be used.

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   The semantics of the TLSRQ certificate type are that the requesting
   party should send another query for the CERT RR that is formed by
   prepending a label to the host name that is the hex of the SHA-1 hash
   value taken over the certificate (not the TLS ASN.1Cert object).  If
   that certificate is a valid certificate that would have been returned
   in a TLSFP certificate type, requesting it with this encoded hash
   prepended to the host name will yield a TLSFP certificate type.
   There is no security problem with using SHA-1 even if the SHA-1 hash
   function continues to weaken because the hash is simply used to
   differentiate the various certificates used by the server.

   Note that the TLSRQ certificate type can only be used if the
   requesting party knows the hash of the certificate that is being used
   by the TLS server.  This usually means that the request will be sent
   by the application acting as the TLS client after it has received the
   TLS server's Certificate message that contains the server's
   certificate.  Such a request can slow down the TLS handshake
   processing, but is required in the case where different hosts with
   different certificates respond on the same domain name.

   The typical scenario would look like the following:

   1.  The application client that is about to use TLS sends a CERT
       query for www.example.com.

   2.  The name server responds with a CERT record that has a TLSRQ
       certificate type.

   3.  The application client starts the TLS handshake, and receives a
       Certificate message from the server.

   4.  The application client takes the SHA-1 hash of the certificate,
       encodes that value as hex.  For this example, assume that encoded
       value is "e77b719d4e9c63c0b0a0333be0a4188e490b618e".

   5.  The application client sends a CERT query for
       e77b719d4e9c63c0b0a0333be0a4188e490b618e.www.example.com.  It
       receives a response with a TLSFP certificate type.

   6.  The application uses the information in the TLSFP certificate
       type and associates it with www.example.com.

   The TLSRQ certificate type has a certificate body with a single octet
   of 0.  The TLSFP certificate type is TBD2.

   For example:

      www.example.com. IN CERT TLSRQ 0 0 AA=

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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.

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

   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 process TLS in the normal fashion.

   If a match between one of the certificate association(s) and the
   server's end entity certificate in TLS is found, the TLS client
   continues the TLS handshake.  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.

3.1.  Certificate Validation by TLS Clients When Using Certificate

   TLS client policy is deliberately not prescribed by this
   specification.  A client MAY choose to trust a DNSSEC-secured
   certificate association, depending on its local policy.

3.1.1.  Use of Self-Signed Certificates

   One expected use case for this protocol is that some TLS servers will
   begin to use self-signed certificates in association with certificate
   associations.  A TLS client that is using this protocol needs to
   treat self-signed certificates as special, and thus SHOULD NOT
   attempt certificate validation on them.  (An exception to this rule
   would be clients that keep self-signed end entity certificates in its
   trust anchor store.)

3.1.2.  Ignorning Host Names in Certificates

   All data in a self-signed certificate other than the key itself can
   be ignored as untrusted unless a client validates the self-signed
   certificate to a trust anchor that is identical to the certificate.
   That means that the host name given in the self-signed certificate is
   meaningless, and that the only way to associate the public key in the
   certificate with the domain name is through the certificate
   association made in the DNS, secured with DNSSEC.

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   If a TLS client fully trusts the association between a domain name
   and the certificate that was provided by the DNS, then that client
   MUST ignore the domain name that is given in the certificate.  That
   is, the certificate might contain a domain name that is different
   than the one that was used to get the TLSFP record, but if the client
   is trusting the TLSFP record, it doesn't matter what domain name is
   used in the certificate.  An expected use case for this protocol is
   to allow someone who controls the private key on a certificate to use
   that certificate for multiple TLS servers.  These servers might be on
   a single computer that has many domain names (such as a computer that
   is both a web host and a mail host, and is known by both
   "www.example.com" and "smtp.example.com"), or they might be on
   different computers (such as multiple computers that all respond IP
   addresses reachable as "www.example.com").

3.1.3.  Use of Local Trust Anchors

   Another expected use case for this protocol is that some TLS servers
   will use certificates that chain to a trust anchor that might not be
   one that is trusted by the TLS client, such as a local certificate
   authority (CA) that is administered by the organization that runs the
   TLS server.  Because of this, a TLS client that is using this
   protocol that performs certificate validation on server certificates
   MAY have a method to communicate with the user that differentiates
   between validation failures that occur on certificates that have had
   secure certificate associations and those that have not.  If it does
   not have such a method of communication, the failure to validate
   SHOULD cause the same error as for any other certificate validation.

3.1.4.  Use of Additional Certificate Data

   Some TLS clients extract data from the certificate other than the key
   to show to the user; for example, most modern web browsers have the
   ability to show an extended validation (EV) name that is embedded in
   a certificate.  Because this data comes from a trusted third party
   and not the TLS server itself, TLS clients that extract additional
   information from TLS server certificates MUST validate those
   certificates in the normal fashion.

4.  IANA Considerations

   This document requests that IANA allocates two certificate types from
   the CERT RR certificate type registry
   (<http://www.iana.org/assignments/cert-rr-types>).  The types are to
   be allocated from the 'IETF Consensus' range.

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   Decimal type: TBD1
   Type:         TLSFP
   Meaning:      TLS certificate associations

   Decimal type: TBD2
   Type:         TLSRQ
   Meaning:      Client should request TLS certificate associations
                 retrieved using hashes

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 and CERT

   A DNS administrator who goes rogue and changes both the A and CERT
   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.

   The SHA-1 hash used in the queries after the TLSRQ certificate type
   is only used to differentiate certificates.  If there is a collision
   between the SHA-1 hashes of two certificates used by the servers that
   are at the host name, there is no problem because both of those
   certificates will have the same association to the domain name.

   The values of the TLSFP and TLSRQ records 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 TLSFP and TLSRQ
   records are 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 TLSFP and
   TLSRQ records in a zone is weaker than the authentication mechanism
   for changing the A/AAAA records, an 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.

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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,
   Simon Josefsson, Phill Hallam-Baker, Ilari Liusvaara, 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.

   [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.

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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 certificate
      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
      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

   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

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      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

   o  Having an "always uses TLS" flag in the TLSFP record.  Briefly:
      the policy of always using TLS should be carried elsewhere because
      it does not line up exactly with TLSFP.  Having this flag in the
      TLSFP record can lead to silly states if a site has multiple TLSFP
      records that have the flag set differently.  It is completely
      unclear what such a flag will mean for SMTP, which uses a STARTTLS
      mechanism built into the unprotected protocol.  If the TLSFP
      record does not apply to a specific service, then all services on
      that hose that could use TLS must do so or not do so together.
      Note, however, that we believe that an "always uses TLS" statement
      should be available in the DNS.

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

   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.

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   Added Simon Josefsson to the acknowledgements.

Appendix C.  Changes between -01 and -02

   Added the TLSRQ certificate type and its semantics.

   Pointed to the IANA registry for DS hash types.

   Added Phill Hallam-Baker to the acknowledgements.

Appendix D.  Changes between -02 and -03

   In 1, added "In this document" to clarify that there may be other
   types of certificate associations described elsewhere.  Also moved
   the sentence from 3 to 1 to help make that point earlier.

   Added a paragraph to 1 to emphasize that multiple TLSFP records can
   be returned for cases where different server software on one host
   uses different certificates.

   In 2.1, got rid of "validation preference".

   In 2.1, changed the values for the hash type from being from the IANA
   registry for DNS to the IANA registry for TLS.  This caused a change
   in the examples from "0" to "2".

   In 2.1, added a one-octet value for "certificate type".

   At the end of 2.1, added a paragraph about the term "exclusion".

   Clarified section 3 to make it clear that the certificate
   associations being described are only the ones from this document.
   Also clarified the semantics of finding a match.

   Removed the last paragraph of 3 (about validation preference) and
   created 3.1 to talk about validation.

   In 5, added "unless the client performs certificate validation and
   rejects the certificate" to the second paragraph.

   In 5, added a new paragraph about monitoring zone changes visually,
   and added a new paragraph about separate authentication mechanisms
   for the TLS-specific records.

   Added Ilari Liusvaara to acknowledgments.

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   In Appendix A, added a paragraph about why we do not have a "always
   uses TLS" flag in this protocol.

Authors' Addresses

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org

   Jakob Schlyter
   Kirei AB

   Email: jakob@kirei.se

   Warren Kumari

   Email: warren@kumari.net

   Adam Langley

   Email: agl@google.com

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