Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: September 13, December 5, 2011                                       Kirei AB
                                                          March 12,
                                                            June 3, 2011

  Using Secure DNS to Associate Certificates with Domain Names For TLS
                      draft-ietf-dane-protocol-06
                      draft-ietf-dane-protocol-07

Abstract

   TLS and DTLS use PKIX 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  TLSA provides a mechanism for a zone operator to sign DNS
   information directly.  This way, bindings of keys to domains that 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 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 September 13, December 5, 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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   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  The TLSA Resource Record . . . . . .  4 . . . . . . . . . . . . .  5
     2.1.  Requested Domain Name  TLSA RDATA Wire Format . . . . . . . . . . . . . . . . . .  5
     2.2.  Format of the Resource Record
       2.1.1.  The Certificate Type Field . . . . . . . . . . . . . .  5
     2.3.  Making Certificate Associations
       2.1.2.  The Reference Type Field . . . . . . . . . . . . . . .  6
       2.1.3.  The Certificate for Association Field  . . . . . . . .  6
       2.3.1.
     2.2.  TLSA RR Presentation Format of Certificates Used to Identify End
               Entities  . . . . . . . . . . . . . . .  6
     2.3.  TLSA RR Examples . . . . . . . . . . . . . . . . . . . . .  7
   3.  Domain Names for TLS Certificate Associations  . . . . . . . .  7
     2.4.  Presentation Format
   4.  Semantics and Features of TLSA Certificate Types . . . . . . .  7
     4.1.  End Entity Certificate . . . . . . . . . . . . . . . . . .  8
     2.5.  Wire Format
     4.2.  Certification Authority Certificate  . . . . . . . . . . .  8
     4.3.  Certificate Public Key . . . . . . . . . . . . . . . . . .  8
   3.
     4.4.  Use of TLS Certificate Associations in TLS . . . . . . . .  9
   5.  TLSA and Use Cases and Requirements  . . . . . . . .  9
   4. . . . . . 10
   6.  Mandatory-to-Implement Algorithms  . . . . . . . . . . . . . .  9
   5. 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
     5.1. 11
     7.1.  TLSA RRtype  . . . . . . . . . . . . . . . . . . . . . . . 10
     5.2. 11
     7.2.  TLSA Certificate Types . . . . . . . . . . . . . . . . . . 10
     5.3. 11
     7.3.  TLSA Hash Types  . . . . . . . . . . . . . . . . . . . . . 10
   6. 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7. 12
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   8. 13
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1. 13
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2. 13
     10.2. Informative References . . . . . . . . . . . . . . . . . . 13 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 14

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 a CA. an external certificate authority (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.

   There are many use cases for such functionality.  [DANEUSECASES]
   lists the ones that the protocol in this document is meant to apply
   to.  [DANEUSECASES] also lists many requirements, most of which the
   protocol in this document is believed to meet.

   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 and other forms of
   identification of TLS servers (such as IP addresses) 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")
   "fingerprint"), a public key, or on the certificate itself.  For a
   fingerprint, a hash is taken of the binary, DER-encoded certificate, certificate
   or public key, 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 only
   applies to PKIX [RFC5280] certificates.

   Certificate associations are made between a certificate or the hash
   of a certificate public key
   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),
   certificates, or in the case that a server is changing from 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 DNS;
   the DNS information may need to be be 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.

   [[ IMPORTANT NOTE FOR THIS DRAFT: There is still confusing and likely
   wrong wording about DNSSEC.  The editors acknowledge that we have not
   completely specified where DNSSEC is and is not needed.  We solicit
   wording that will make this clearer. ]]

   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 that use DNSSEC in order to assure the
   proof of origin of the data.  More detail is given in this document
   when DNSSEC is and is not required for securing certificate
   associations.

   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 only in the sense
   that 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 on 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.

2.  Getting TLS Certificate Associations from the DNS

   This document defines a new  The TLSA Resource Record

   The TLSA DNS resource record type, "TLSA".  A
   query on (RR) is used to associate a prepared certificate
   with the domain name for where the TLSA RR can return one or
   more records record is found.  The semantics of how
   the type TLSA.  The TLSA RRType RR is TBD.

2.1.  Requested Domain Name

   Domain names interpreted are prepared for requests given later in the following manner.

   1. this document.

   The decimal representation of type value for the port number on which a TLS-
       based service TLSA RR type is assumed to exist TBD.

   The TLSA RR is prepended with an underscore
       character ("_") to become the left-most label in the prepared
       domain name.

   2. class independent.

   The protocol name TLSA RR has no special TTL requirements.

2.1.  TLSA RDATA Wire Format

   The RDATA for a TLSA RR consists 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 one octet certificate type
   field, a one octet reference type field and the prepared domain
       name.  The transport names defined certificate for this protocol are "tcp",
       "udp" and "sctp".

   3.  The domain name is appended to the result of step
   association field.

                        1 1 1 1 1 1 1 1 1 1 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 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   |   Ref type    |                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
   /                                                               /
   /                    Certificate 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 association                /
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.1.1.  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 Certificate Type Field

   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 -- A PKIX certificate that identifies an end entity

      2 -- A PKIX certification authority's certificate

      Both

      3 -- A public key expressed as a PKIX SubjectPublicKeyInfo
      structure

   All three types are structured using the RFC 5280 formatting rules
   and use the DER encoding.  As described later

   The three certificate types defined in this document, type 1 document 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 do not will need to correctly use all PKIX semantics.

   o their own
   certificate types.

2.1.2.  The Reference Type Field

   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-256 hash of the certificate

      2 -- SHA-512 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

2.1.3.  The Certificate for Association Field

   The "certificate for association".  This is the bytes containing the
   full certificate certificate, SubjectPublicKeyInfo or the hash of the associated
   certificate
      (that is, the certificate or the hash of SubjectPublicKeyInfo.  For certificate types 1 and 2,
   this 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 object.

2.2.  TLSA RR Presentation Format

   The presentation format of the RDATA portion is as follows:

   o  The two certificate types for TLS have very different semantics.  A
   TLS client conforming to this protocol receiving a type field MUST be represented as an unsigned
      decimal integer.

   o  The reference type field MUST be represented as an unsigned
      decimal integer.

   o  The certificate for association of type 1 field MUST compare it, using be represented as a
      string of hexadecimal characters.  Whitespace is allowed within
      the specified string of hexadecimal characters.

2.3.  TLSA RR Examples

   An example of a SHA-256 hash type,
   with the (type 1) of an end entity certificate received in TLS.  A TLS client
   conforming to this protocol receiving a certificate for association
   (type 1) would be:

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

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

   _443._tcp.www.example.com. IN TLSA (
      2 MUST treat it as a trust anchor 0 308202c5308201ada00302010202090... )

3.  Domain Names for that TLS Certificate Associations

   TLSA resource records are stored at a prefixed DNS domain name.

   Certificate type 1 (a certificate that identifies an end entity) 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
   prefix is valid if the certificate prepared in the TLSA data matches to following manner:

   1.  The decimal representation of the first certificate 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.  This number has no leading zeros.

   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".

4.  Semantics and Features of TLSA Certificate Types

   The three certificate types have very different semantics, but also
   have features common to all three types.

4.1.  End Entity Certificate

   Certificate type 1 (a certificate that identifies an end entity) 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 offered by the TLS server matches the value from the TLSA
   data.

4.2.  Certification Authority Certificate

   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.

   The end entity certificate from TLS, regardless of whether it was
   matched with a TLSA

4.3.  Certificate Public Key

   Certificate type 1 certificate or chained to 3 (public key expressed as a TLSA type 2 CA
   certificate, must have at least one identifier in PKIX
   SubjectPublicKeyInfo structure) is used to assert that the subject or
   subjectAltName field public key
   will appear in one of the matched certificates matches received from the expected
   identifier server.  A
   server might choose this type for many reasons, including (but not
   limited to):

   o  the trust anchor to which TLS server.  Further, server's certificate chains might
      change without the trust anchor's public key changing

   o  the TLS session server is using a self-signed certificate that is not
      marked as a CA certificate

   A TLS client conforming to
   be set up MUST be for the specific port number and transport name this protocol that was given receives a public key
   in the TLSA query.  The matching or chaining MUST a type 3 certificate for association must be
   done within able to extract the life
   SubjectPublicKeyInfo from each of the TTL on certificates presented to it by
   the TSLA record.

2.3.1.  Format TLS server.  It then does a bit-for-bit comparison between the
   certificate for association and the SubjectPublicKeyInfos in the
   certificates; if it does not find a match, the client aborts the TLS
   handshake.

4.4.  Use of Certificates Used TLS Certificate Associations in TLS

   A TLS client conforming to Identify End Entities

   When presented with this protocol receiving a certificate for
   association of type 1 certificate, MUST compare it for equality, using the
   specified reference type, with the end entity certificate received in
   TLS.  A TLS client conforming to this protocol receiving a
   certificate for association of type 2 MUST NOT
   verify the correct PKIX semantics treat it as a trust anchor
   for the keyCertSign bit that domain name.  A TLS client conforming to this protocol
   receiving a certificate for association of the
   keyUsage extension, nor type 3 MUST find a
   matching SubjectPublicKeyInfo structure in one of the certificates
   offered by the basicConstraints extension.  This
   is because PKIX (RFC 5280) makes TLS server.

   The end entity certificate from TLS, regardless of whether it clear that all self-signed
   certificates are CA certificates and cannot be end entity
   certificates.  The last paragraph of section 3.2 of RFC 5280 says:

   "This specification covers two classes of certificates: CA
   certificates and end entity certificates.  CA certificates may be
   further divided into three classes: cross-certificates, self-issued
   certificates, and self-signed certificates. ...  Self-issued
   certificates are was
   matched with a TLSA type 1 certificate or chained to a TLSA type 2 CA certificates
   certificate, might have at least one identifier in which the issuer and subject are or
   subjectAltName field of the same entity. ...  Self-signed certificates are self-issued matched certificates where the digital signature may be verified by that matches the
   public key bound into
   expected identifier for the certificate.  Self-signed certificates are
   used to convey a public key TLS server.  Some specifications for use to begin certification paths.
   End entity certificates are issued to subjects that are not
   authorized to issue certificates."

   This means
   applications that a self-signed certificate (one where the subject and
   issuer are the same, and the public key in the certificate can be
   used to directly evaluate the signature on the certificate) must
   follow all the PKIX semantics rules run under TLS, such as [RFC2818] for CAs, and probably need to
   follow all HTTP, requires
   the policy rules as well.  This is clearly not what people
   who want server's certificate have a simple way to associate their public signing key with
   their domain name in an end entity certificate that can be used in
   TLS.

   Because of these PKIX requirements on end entity certificates, the
   processing rules for TLSA are very different for certificates that
   identify end entities directly and CA certificates that can be used
   to validate PKIX end entity certificates.  The rules here allow self-
   signed certificates offered as type 1 certificates to not follow all
   the PKIX semantics rules.

2.4.  Presentation Format

   The RDATA of the presentation format of matches the TLSA resource record
   consists of two numbers (certificate and hash type) followed host
   name expected 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 1) of an end entity certificate (type 1) would be:

   _443._tcp.www.example.com. IN TLSA (
      1 1 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 client.  Further, the hex values; those characters are removed when converting TLS session that is to the
   wire format.

2.5.  Wire Format

   The wire format is:

                        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 ( 01015c1502a6549c42
                 3be0a0aa9d9a16904de5ef0f5c98c735fcca79f09230aa7141 )

   The wire format be
   set up MUST be 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 specific port number and transport name that
   was given in the IANA "private use" range; the real
   RR Type for TLSA will query.  The matching or chaining MUST be issued by IANA, as described in done
   within the IANA
   Considerations section below.

3.  Use life of TLS Certificate Associations in TLS the TTL on the TLSA record.

   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 reference 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 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 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.

5.  TLSA and Use Cases and Requirements

   The different types of certificates for association defined in TLSA
   are matched with various sections of [DANEUSECASES]. [[ IMPORTANT
   NOTICE, DANGER OF MOVING PARTS: this draft of the protocol is based
   on the -02 version of [DANEUSECASES].  As that document changes in
   the WG and IETF Last Call, this protocol might change as well. ]]

   Certificate type 1 (end entity certificate) is used for "certificate
   constraints".  Certificate type 2 (CA certificate) is used for "CA
   constraints".  Certificate type 3 (public key structure) is used for
   "CA constraints" and "certificate constraints", depending on which
   certificate the public key is extracted from.  All three types are
   also used for "domain-issued certificates if the domain owner creates
   its own CA certificate and then issues and end entity certificate
   from that CA.  Note that [DANEUSECASES] discusses "CA constraints"
   and "certificate constraints" in terms of a "well-known CA"; TLSA
   extends this in some cases to allow domain-issued (not-well-known)
   CAs.

   As described in [DANEUSECASES], when TLSA is deployed for CA
   constraints, DNSSEC is not required.  Both type 2 and type 3 can be
   used for CA constraints, but because type 3 is only used for CA
   constraints in some cases.  This can easily be confusing in
   deployments, so this particular lack of need for DNSSEC is not
   emphasized in the rest of the certificate association(s) this document.

   TLSA allows delegated services.  It also supports opportunistic
   security and web services if the
   server's end entity domain uses a certificate in TLS that
   chains to a well-known CA that 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 trusted in
   TLS is found, the "legacy" TLS client MUST abort
   application.  It also meets all the handshake requirements listed except for
   being compatible with an
   "access_denied" error.

4. DNS wildcards.

6.  Mandatory-to-Implement Algorithms

   DNS systems conforming to this specification MUST be able to create
   TLSA records containing certificate types 1 and 2.  DNS systems
   conforming to this specification MUST be able to create TLSA records
   using hash reference type 0 (no hash used) and hash reference type 1 (SHA-256),
   and SHOULD be able to create TLSA records using hash reference type 2
   (SHA-512).

   TLS clients conforming to this specification MUST be able to
   correctly interpret TLSA records containing certificate types 1 and
   2.  TLS clients conforming to this specification MUST be able to
   compare a certificate for association with a certificate from TLS
   using hash reference type 0 (no hash used) and hash reference type 1 (SHA-256),
   and SHOULD be able to make such comparisons with hash reference type 2
   (SHA-512).

   At the time this is written, it is expected that there will be a new
   family of hash algorithms called SHA-3 within the next few years.  It
   is expected that some of the SHA-3 algorithms will be mandatory
   and/or recommended for TLSA records after the algorithms are fully
   defined.  At that time, this specification will be updated.

5.

7.  IANA Considerations

5.1.

7.1.  TLSA RRtype

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

5.2.

7.2.  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                       Reference
   ----------------------------------------------------------
   0        Reserved                                [This]
   1        Certificate to identify an end entity   [This]
   2        CA's certificate                        [This]
   3        Public key as SubjectPublicKeyInfo      [This]
   3-254    Unassigned
   255      Private use

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

5.3.

7.3.  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    Reference
   ---------------------------------------------
   0        No hash used         [This]
   1        SHA-256              NIST FIPS 180-3
   2        SHA-512              NIST FIPS 180-3
   3-254    Unassigned
   255      Private use

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

6.

8.  Security Considerations

   [[ NOTE: Some of the text here is wrong in that DNSSEC does not need
   to be used in all cases.  This will be much better delineated and
   described in a future version of the spec. ]]

   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.

7.  Thus,
   such proxies might choose to aggressively block TLSA requests and/or
   responses.

   Client treatment of any information included in the trust anchor is a
   matter of local policy.  This specification does not mandate that
   such information be inspected or validated by the domain name
   administrator.

9.  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, Ben
   Laurie, Ilari Liusvaara, Scott Schmit, and Ondrej Sury.

8.

   This document has also been greatly helped by many active
   participants of the DANE Working Group.

10.  References

8.1.

10.1.  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.

   [DANEUSECASES]
              Barnes, R., "Use Cases and Requirements for DNS-based
              Authentication of Named Entities (DANE)",
              draft-ietf-dane-use-cases (work in progress), 2011.

   [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.

8.2.

10.2.  Informative References

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [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.

Authors' Addresses

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org

   Jakob Schlyter
   Kirei AB

   Email: jakob@kirei.se