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Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: August 7, 2012                                         Kirei AB
                                                        February 4, 2012


  Using Secure DNS to Associate Certificates with Domain Names For TLS
                      draft-ietf-dane-protocol-15

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.  TLSA provides 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 August 7, 2012.

Copyright Notice

   Copyright (c) 2012 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
   to this document.  Code Components extracted from this document must



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   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.
















































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Certificate Associations . . . . . . . . . . . . . . . . .  4
     1.2.  Securing Certificate Associations  . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  The TLSA Resource Record . . . . . . . . . . . . . . . . . . .  5
     2.1.  TLSA RDATA Wire Format . . . . . . . . . . . . . . . . . .  6
       2.1.1.  The Certificate Usage Field  . . . . . . . . . . . . .  6
       2.1.2.  The Selector Field . . . . . . . . . . . . . . . . . .  7
       2.1.3.  The Matching Type Field  . . . . . . . . . . . . . . .  7
       2.1.4.  The Certificate Association Data Field . . . . . . . .  7
     2.2.  TLSA RR Presentation Format  . . . . . . . . . . . . . . .  7
     2.3.  TLSA RR Examples . . . . . . . . . . . . . . . . . . . . .  8
   3.  Domain Names for TLS Certificate Associations  . . . . . . . .  8
   4.  Semantics and Features of TLSA Certificate Usages  . . . . . .  9
     4.1.  Pass PKIX Validation and Chain Through CA  . . . . . . . .  9
     4.2.  Pass PKIX Validation and Match End Entity Certificate  . .  9
     4.3.  Pass PKIX Validation and Use Trust Anchor  . . . . . . . .  9
     4.4.  Match Certificate  . . . . . . . . . . . . . . . . . . . .  9
   5.  Use of TLS Certificate Associations in TLS . . . . . . . . . . 10
   6.  TLSA and DANE Use Cases and Requirements . . . . . . . . . . . 11
   7.  Mandatory-to-Implement Algorithms  . . . . . . . . . . . . . . 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  TLSA RRtype  . . . . . . . . . . . . . . . . . . . . . . . 13
     8.2.  TLSA Usages  . . . . . . . . . . . . . . . . . . . . . . . 14
     8.3.  TLSA Selectors . . . . . . . . . . . . . . . . . . . . . . 14
     8.4.  TLSA Matching Types  . . . . . . . . . . . . . . . . . . . 14
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     11.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Operational Considerations for Deploying TLSA
                Records . . . . . . . . . . . . . . . . . . . . . . . 18
     A.1.  Creating TLSA Records  . . . . . . . . . . . . . . . . . . 18
       A.1.1.  Ambiguities and Corner Cases When TLS Clients
               Build Trust Chains . . . . . . . . . . . . . . . . . . 18
       A.1.2.  Choosing a Selector Type . . . . . . . . . . . . . . . 19
     A.2.  Provisioning TLSA Records in DNS . . . . . . . . . . . . . 20
       A.2.1.  Provisioning TLSA Records with Aliases . . . . . . . . 20
       A.2.2.  Provisioning with NS Records . . . . . . . . . . . . . 23
     A.3.  Securing the Last Hop  . . . . . . . . . . . . . . . . . . 23
     A.4.  Handling Certificate Rollover  . . . . . . . . . . . . . . 23
   Appendix B.  Pseudocode for Using TLSA . . . . . . . . . . . . . . 24
     B.1.  Helper Functions . . . . . . . . . . . . . . . . . . . . . 24
     B.2.  Main TLSA Pseudo Code  . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27



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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 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.  Section 6 covers the
   applicability of this document to the use cases in detail.

   This document applies to both TLS [RFC5246] and DTLS [RFC4347bis].
   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

   A certificate association is formed from a piece of information
   identifying a certificate with the domain name where the data is
   found.  The data used to identify the certificate consists of either
   a PKIX certificate or a hash of a PKIX certificate.  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, not certificates of other formats.

   A DNS query can return multiple certificate associations, such as in
   the case of different server software on a single host using
   different certificates, 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;
   the DNS information needs 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.

   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 that
   is 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.  This document does not specify how
   DNSSEC validation occurs because there are many different proposals
   for how a client might get validated DNSSEC results.

   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].

   This document also makes use of standard PKIX, DNSSEC, and TLS
   terminology.  See [RFC5280], [RFC4033], and [RFC5246] respectively,
   for these terms.  In addition, terms related to TLS-protected
   application services and DNS names are taken from [RFC6125].


2.  The TLSA Resource Record

   The TLSA DNS resource record (RR) is used to associate a certificate
   with the domain name where the record is found.  The semantics of how
   the TLSA RR is interpreted are given later in this document.

   The type value for the TLSA RR type is TBD.

   The TLSA RR is class independent.

   The TLSA RR has no special TTL requirements.



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2.1.  TLSA RDATA Wire Format

   The RDATA for a TLSA RR consists of a one octet usage type field, a
   one octet selector field, a one octet matching type field and the
   certificate association data field.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Usage     |   Selector    | Matching Type |               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               /
   /                                                               /
   /                 Certificate Association Data                  /
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.1.1.  The Certificate Usage Field

   A one-octet value, called "certificate usage" or just "usage",
   specifying the provided association that will be used to match the
   target certificate from the TLS handshake.  This value is defined in
   a new IANA registry (see Section 8.2) in order to make it easier to
   add additional certificate usages in the future.  The usages defined
   in this document are:

      0 -- The target certificate MUST pass PKIX validation and MUST
      chain through a CA certificate matching the TLSA record

      1 -- The target certificate MUST pass PKIX validation and MUST
      match the TLSA record

      2 -- The target certificate MUST pass PKIX validation, with any
      certificate matching the TLSA record considered to be a trust
      anchor for this validation

      3 -- The target certificate MUST match the TLSA record

   The certificate usages defined in this document explicitly only apply
   to PKIX-formatted certificates in DER encoding.  If TLS allows other
   formats later, or if extensions to this protocol are made that accept
   other formats for certificates, those certificates will need their
   own certificate usage values.

   The use of this field is described in greater detail in Section 4.







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2.1.2.  The Selector Field

   A one-octet value, called "selector", specifying what the association
   data will be matched against from the TLS certificate presented by
   the server.  This value is defined in a new IANA registry (see
   Section 8.3.  The selectors defined in this document are:

      0 -- Full certificate

      1 -- SubjectPublicKeyInfo

2.1.3.  The Matching Type Field

   A one-octet value, called "matching type", specifying how the
   certificate association is presented.  This value is defined in a new
   IANA registry (see Section 8.4).  The types defined in this document
   are:

      0 -- Exact match on selected content

      1 -- SHA-256 hash of selected content

      2 -- SHA-512 hash of selected content

   Because a client support for multiple hash algorithms might be
   limited, it is advisable to use the same hash algorithm for the
   matching type as is used for the signature in the certificate.  Doing
   so will increase the likelihood of interoperability.

2.1.4.  The Certificate Association Data Field

   The "certificate association data" to be matched.  The field contains
   the bytes to be matched or the hash of the bytes to be matched.  The
   field contains the bytes to be matched: the raw data, for matching
   type 0, or the hash of the raw data, for matching types 1 and 2.  The
   data refers to the certificate in the association, not to the TLS
   ASN.1 Certificate object.

2.2.  TLSA RR Presentation Format

   The presentation format of the RDATA portion is as follows:

   o  The certificate usage field MUST be represented as an unsigned
      decimal integer.

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




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

   o  The certificate association data field MUST be represented as a
      string of hexadecimal characters.  Whitespace is allowed within
      the string of hexadecimal characters.

2.3.  TLSA RR Examples

   An example of a hashed (SHA-256) association of a PKIX CA
   certificate:

   _443._tcp.www.example.com. IN TLSA (
      0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
            7983a1d16e8a410e4561cb106618e971 )

   An example of a hashed (SHA-512) subject public key association of a
   PKIX end entity certificate:

   _443._tcp.www.example.com. IN TLSA (
      1 1 2 92003ba34942dc74152e2f2c408d29ec
            a5a520e7f2e06bb944f4dca346baf63c
            1b177615d466f6c4b71c216a50292bd5
            8c9ebdd2f74e38fe51ffd48c43326cbc )

   An example of a full certificate association of a PKIX end entity
   certificate:

   _443._tcp.www.example.com. IN TLSA (
      3 0 0 30820307308201efa003020102020... )


3.  Domain Names for TLS Certificate Associations

   Unless there is an protocol-specific specification that is different
   than this one, TLSA resource records are stored at a prefixed DNS
   domain name.  The prefix is prepared 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.  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".



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   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 Usages

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

4.1.  Pass PKIX Validation and Chain Through CA

   Certificate usage 0 is used to specify a CA certificate, or the
   public key of such a certificate, that must be found in any of the
   PKIX validation chains for the end entity certificate given by the
   server in TLS.  This usage is sometimes referred to as "CA
   constraint" because it limits which CA can be used to issue
   certificates for a given host name.

4.2.  Pass PKIX Validation and Match End Entity Certificate

   Certificate usage 1 is used to specify an end entity certificate, or
   the public key of such a certificate, that must be matched with the
   end entity certificate given by the server in TLS.  This usage is
   sometimes referred to as "service certificate constraints" because it
   limits which end entity certificate may be used by a given host name.

4.3.  Pass PKIX Validation and Use Trust Anchor

   Certificate usage 2 is used to specify a certificate, or the public
   key of such a certificate, that must be used as a trust anchor when
   validating the end entity certificate given by the server in TLS.
   This usage allows a domain name administrator to specify a new trust
   anchor, such as if the domain issues its own certificates under its
   own CA that is not expected to be in the end users collection of
   trust anchors.

4.4.  Match Certificate

   Certificate usage 3 is used to specify a certificate, or the public
   key of such a certificate, that must match the end entity certificate
   given by the server in TLS.  This usage is sometimes referred to as



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   "domain-issued certificate" because it allows for a domain name
   administrator to issue certificates for a domain without involving a
   third-party CA.


5.  Use of TLS Certificate Associations in TLS

   Section 2.1 of this document defines the mandatory matching rules for
   the data from the TLS certificate associations and the certificates
   received from the TLS server.

   The TLS session that is to be set up MUST be for the specific port
   number and transport name that was given in the TLSA query.  The
   matching or chaining MUST be done within the life of the TTL on the
   TLSA record; using a TLSA record past the lifetime specified in the
   TTL can expose the the TLS client to many types of attacks.

   Some specifications for applications that run under TLS, such as
   [RFC2818] for HTTP, require the server's certificate to have a domain
   name that matches the host name expected by the client.  Some
   specifications such as [RFC6125] detail how to match the identity
   given in a PKIX certificate with those expected by the user.

   An application that complies with this document first requests TLSA
   records in order to make certificate associations.

   Determining whether a TLSA RRset can be used depends on the DNSSEC
   validation state (as defined in [RFC4033]).

   o  A TLSA RRset whose DNSSEC validation state is secure MUST be used
      as a certificate association for TLS unless a local policy would
      prohibit the use of the specific certificate association in the
      secure TLSA RRset.

   o  If the DNSSEC validation state on the response to the request for
      the TLSA RRset is bogus, this MUST cause TLS not to be started or,
      if the TLS negotiation is already in progress, MUST cause the
      connection to be aborted.

   o  A TLSA RRset whose DNSSEC validation state is indeterminate or
      insecure cannot be used for TLS and MUST be marked as unusable.

   If an application receives zero usable certificate associations, it
   processes TLS in the normal fashion without any input from the TLSA
   records; otherwise, that application attempts to match each
   certificate association with the TLS server's end entity certificate.

   Clients that validate the DNSSEC signatures themselves MUST use



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   standard DNSSEC validation procedures.  Clients that rely on another
   entity to perform the DNSSEC signature validation MUST use a secure
   transport between themselves and the validator.  Examples of secure
   transports include TSIG [RFC2845], SIG(0) [RFC2931], and IPsec
   [RFC6071].  Note that it is not sufficient to use secure transport to
   a DNS resolver that does not do DNSSEC signature validation.

   If a certificate association contains a certificate usage, selector,
   or matching type that is not understood by the TLS client, that
   certificate association MUST be marked as unusable.  If the
   comparison data for a certificate is malformed, the certificate
   association MUST be marked as unusable.  If a certificate association
   contains a matching type or certificate association data that uses a
   cryptographic algorithm that is considered too weak for the TLS
   client's policy, the certificate association MUST be marked as
   unusable.


6.  TLSA and DANE Use Cases and Requirements

   The different types of certificates associations defined in TLSA are
   matched with various sections of [DANEUSECASES].  Three use cases
   from Section 3 of [DANEUSECASES] are covered in this protocol as
   follows:

   3.1 CA Constraints  -- Implemented using certificate usage 0.

   3.2 Certificate Constraints  -- Implemented using certificate usage
      1.

   3.3 Domain-Issued Certificates  -- Implemented using certificate
      usage 3.

   The requirements from Section 4 of [DANEUSECASES] are covered in this
   protocol as follows:

   Multiple Ports  -- The TLSA records for different application
      services running on a single host can be distinguished through the
      service name and port number prefixed to the host name (see
      Section 3).

   No Downgrade  -- Section 5 specifies the conditions under which a
      client can process and act upon TLSA records.  Specifically, if
      the DNSSEC status for the TLSA resource record set is determined
      to be bogus, TLS is not attempted at all.






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   Encapsulation  -- Covered in the TLSA response semantics.

   Predictability  -- The appendixes of this specification provide
      operational considerations and implementation guidance in order to
      enable application developers to form a consistent interpretation
      of the recommended DANE client behavior.

   Opportunistic Security  -- If a client conformant to this
      specification can reliably determine the presence of a TLSA
      record, it will attempt to use this information.  Conversely, if a
      client can reliably determine the absence of any TLSA record, it
      will fall back to processing TLS in the normal fashion.  This is
      discussed in Section 5.

   Combination  -- Multiple TLSA records can be published for a given
      host name, thus enabling the client to construct multiple TLSA
      certificate associations that reflect different DANE assertions.
      No support is provided to combine two TLSA certificate
      associations in a single operation.

   Roll-over  -- Section 5 states that applications must not cache TLSA
      records beyond their TTL expiration.  This ensures that clients
      will not latch onto assertions made by expired TLSA records, and
      will be able to transition between using one DANE mechanism to
      another.

   Simple Key Management  -- The SubjectPublicKeyInfo selector in the
      TLSA record provides a mode that enables a domain holder to only
      have to maintain a single long-lived public/private key pair
      without the need to manage certificates.  Appendix A outlines the
      usefulness and the potential downsides to using this mode.

   Minimal Dependencies  -- This specification relies on DNSSEC to
      protect the origin authenticity and integrity of the TLSA resource
      record set.  Additionally, if DNSSEC validation is not performed
      on the system that wishes to use TLSA certificate bindings, this
      specification requires that the "last mile" be over a secure
      transport.  There are no other deployment dependencies for this
      approach.

   Minimal Options  -- The operating modes map precisely to the DANE use
      cases and requirements.  DNSSEC use is mandatory in that this
      specification encourages applications to use TLSA records that are
      only shown to be validated.







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   Wild Cards  -- Covered in a limited manner in the TLSA request
      syntax; see Appendix A.

   Redirection  -- Covered in the TLSA request syntax; see Appendix A.


7.  Mandatory-to-Implement Algorithms

   A system creating TLSA records that conforms to this specification
   MUST be able to create TLSA records containing certificate usages 0,
   1, 2, and 3.  A system creating TLSA records that conforms to this
   specification MUST be able to create TLSA records with selectors 0
   (full certificate) and 1 (SubjectPublicKeyInfo).  A system creating
   TLSA records that conforms to this specification MUST be able to
   create TLSA records using matching type 0 (no hash used) and matching
   type 1 (SHA-256), and SHOULD be able to create TLSA records using
   matching type 2 (SHA-512).

   TLS clients conforming to this specification MUST be able to
   correctly interpret TLSA records with certificate usages 0, 1, 2, and
   3.  TLS clients conforming to this specification MUST be able to
   compare a certificate association with a certificate from the TLS
   handshake using selectors type 0 and 1, and matching type 0 (no hash
   used) and matching type 1 (SHA-256), and SHOULD be able to make such
   comparisons with matching 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.


8.  IANA Considerations

8.1.  TLSA RRtype

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

   In the following sections, "RFC Required" was chosen TLSA usages, and
   "Specification Required" for selectors and hashes, because of the
   amount of detail that is likely to be needed for implementers to
   correctly implement new usages as compared to new matching types and
   hash types.




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8.2.  TLSA Usages

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

   Value    Short description                       Reference
   ----------------------------------------------------------
   0        Pass PKIX and chain through CA          [This]
   1        Pass PKIX and match EE                  [This]
   2        Pass PKIX and trusted via certificate   [This]
   3        Match certificate                       [This]
   4-254    Unassigned
   255      Private use

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

8.3.  TLSA Selectors

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

   Value    Short description                       Reference
   ----------------------------------------------------------
   0        Full Certificate                        [This]
   1        SubjectPublicKeyInfo                    [This]
   2-254    Unassigned
   255      Private use

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

8.4.  TLSA Matching Types

   This document creates a new registry, "Matching 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




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


9.  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.

   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.  Thus,
   such proxies might choose to aggressively block TLSA requests and/or
   responses, even though this is not a recommended practice.

   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.

   If a server's certificate is revoked, or if an intermediate CA in a
   chain between the end entity and a trust anchor has its certificate
   revoked, a TLSA record with a certificate type of 2 that matches the
   revoked certificate would in essence override the revocation because



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   the client would treat that revoked certificate as a trust anchor and
   thus not check its revocation status.  Because of this, domain
   administrators need to be responsible for being sure that the key or
   certificate used in TLSA records with a certificate type of 2 are in
   fact able to be used as reliable trust anchors.

   Certificates that are delivered in TLSA with usage type 2
   fundamentally change the way the TLS server's end entity certificate
   is evaluated.  For example, the server's certificate might chain to
   an existing CA through an intermediate CA that has certain policy
   restrictions, and the certificate would not pass those restrictions
   and thus normally be rejected.  That intermediate CA could issue
   itself a new certificate without the policy restrictions and tell its
   customers to use that certificate with usage type 2.  This in essence
   allows an intermediate CA to be come a trust anchor for certificates
   that the end user might have expected to chain to an existing trust
   anchor.


10.  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, Ondrej Mikle, Scott Schmit, Ondrej Sury,
   Richard Barnes, Jim Schaad, Stephen Farrell and Suresh Krishnaswamy.

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


11.  References

11.1.  Normative References

   [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.



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   [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.

   [RFC4347bis]
              Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security version 1.2", draft-ietf-tls-rfc4347-bis (work in
              progress), July 2010.

   [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.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

11.2.  Informative References

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

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

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake, D., and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, May 2000.

   [RFC2931]  Eastlake, D., "DNS Request and Transaction Signatures (
              SIG(0)s)", RFC 2931, September 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.



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   [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
              RFC 4641, September 2006.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6071]  Frankel, S. and S. Krishnan, "IP Security (IPsec) and
              Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
              February 2011.


Appendix A.  Operational Considerations for Deploying TLSA Records

A.1.  Creating TLSA Records

   When creating TLSA record with certificate usage type 0 (CA
   Certificate) or type 2 (Trust Anchor), one needs to understand the
   implications when choosing between selector type 0 (full certificate)
   and 1 (SubjectPublicKeyInfo).  A careful choice is required because
   the different methods for building trust chains are used by different
   TLS clients.  The following outlines the cases that one should be
   aware of and discusses the implications of the choice of selector
   type.

   Certificate usage 2 is not affected by the different types of chain
   building when the end entity certificate is the same as the trust
   anchor certificate.

A.1.1.  Ambiguities and Corner Cases When TLS Clients Build Trust Chains

   TLS clients may implement their own chain-building code rather than
   rely on the chain presented by the TLS server.  This means that,
   except for the end entity certificate, any certificate presented in
   the suggested chain may or may not be present in the final chain
   built by the client.

   Certificates that the client can use to replace certificates from
   original chain include:

   o  Client's trust anchors

   o  Certificates cached locally

   o  Certificates retrieved from a URI listed in an Authority
      Information Access X.509v3 extension

   CAs frequently reissue certificates with different validity period,
   signature algorithm (such as an different hash algorithm in the



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   signature algorithm), CA key pair (such as for a cross-certificate),
   or PKIX extensions where the public key and subject remain the same.
   These reissued certificates are the certificates TLS client can use
   in place of an original certificate.

   Clients are known to exchange or remove certificates that could cause
   TLSA association that rely on the full certificate to fail.  For
   example:

   o  The client considers the signature algorithm of a certificate to
      no longer be sufficiently secure

   o  The client may not have an associated root certificate in trust
      store and instead uses a cross-certificate with an identical
      subject and public key.

A.1.2.  Choosing a Selector Type

   In this section, "false-negative failure" means that a client will
   not accept the TLSA association for certificate designated by DNS
   administrator.  Also, "false-positive acceptance" means that the
   client accepts a TLSA association for a certificate that is not
   designated by the DNS administrator.

A.1.2.1.  Selector Type 0 (Full Certificate)

   The "Full certificate" selector provides the most precise
   specification of a TLS certificate association, capturing all fields
   of the PKIX certificate.  For a DNS administrator, the best course to
   avoid false-negative failures in the client when using this selector
   are:

   o  If a CA issued a replacement certificate, don't associate to CA
      certificates that have a signature algorithm with a hash that is
      considered weak (such as MD2 and MD5).

   o  Determine how common client applications process the TLSA
      association using a fresh client installation, that is, with the
      local certificate cache empty.

A.1.2.2.  Selector Type 1 (SubjectPublicKeyInfo)

   A SubjectPublicKeyInfo selector gives greater flexibility in avoiding
   some false-negative failures caused by trust-chain-building
   algorithms used in clients.

   One specific use-case should be noted: creating a TLSA association to
   certificate I1 that directly signed end entity certificate S1 of



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   server.  Since the key used to sign S1 is fixed, association to I1
   must succeed: if a client swaps I1 for I2 (a different certificate),
   its SPKI must match SPKI of I1.  Such association would not suffer
   from a false-negative failure on client's side if the client uses a
   reissued CA certificate I2 in place of I1.

   The attack surface is a bit broader compared to "full certificate"
   selector: the DNS administrator might unintentionally specify an
   association that would lead to false-positive acceptance.

   o  The administrator must know or trust the CA not to engage in bad
      practices, such as not sharing key of I1 for unrelated CA
      certificates leading to trust-chain redirect.  If possible, the
      administrator should review all CA certificates that have the same
      SPKI.

   o  The administrator should understand whether some PKIX extension
      may adversely affect security of the association.  If possible,
      administrators should review all CA certificates that share the
      SubjectPublicKeyInfo.

   Using the SubjectPublicKeyInfo selector for association with a
   certificate in a chain above I1 needs to be decided on a case-by-case
   basis: there are too many possibilities based on the issuing CA's
   practices.  Unless the full implications of such an association are
   understood by the administrator, using selector type 0 is a better
   option from a security perspective.

A.2.  Provisioning TLSA Records in DNS

A.2.1.  Provisioning TLSA Records with Aliases

   The TLSA resource record is not special in the DNS; it acts exactly
   like any other RRtype where the queried name has one or more labels
   prefixed to the base name, such as the SRV RRtype [RFC2782].  This
   affects the way that the TLSA resource record is used when aliasing
   in the DNS.

   Note that the IETF sometimes adds new types of aliasing in the DNS.
   If that happens in the future, those aliases might affect TLSA
   records, hopefully in a good way.

A.2.1.1.  Provisioning TLSA Records with CNAME Records

   Using CNAME to alias in DNS only aliases from the exact name given,
   not any zones below the given name.  For example, assume that a zone
   file has only the following:




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   sub1.example.com.          IN CNAME sub2.example.com.

   In this case, a request for the A record at "bottom.sub1.example.com"
   would not return any records because the CNAME given only aliases the
   name given.  Assume, instead, the zone file has the following:

   sub3.example.com.          IN CNAME sub4.example.com.
   bottom.sub3.example.com.   IN CNAME bottom.sub4.example.com.

   In this case, a request for the A record at bottom.sub3.example.com
   would in fact return whatever value for the A record exists at
   bottom.sub4.example.com.

   Application implementations and full-service resolvers request DNS
   records using libraries that automatically follow CNAME (and DNAME)
   aliasing.  This allows hosts to put TLSA records in their own zones
   or to use CNAME to do redirection.

   If the owner of the original domain wants a TLSA record for the same,
   they simply enter it under the defined prefix:

   ; No TLSA record in target domain
   ;
   sub5.example.com.            IN CNAME sub6.example.com.
   _443._tcp.sub5.example.com.  IN TLSA 1 1 1 308202c5308201ab...
   sub6.example.com.            IN A 192.0.2.1
   sub6.example.com.            IN AAAA 2001:db8::1/32

   If the owner of the original domain wants to have the target domain
   host the TLSA record, the original domain uses a CNAME record:

   ; TLSA record for original domain has CNAME to target domain
   ;
   sub5.example.com.            IN CNAME sub6.example.com.
   _443._tcp.sub5.example.com.  IN CNAME _443._tcp.sub6.example.com.
   sub6.example.com.            IN A 192.0.2.1
   sub6.example.com.            IN AAAA 2001:db8::1/32
   _443._tcp.sub6.example.com.  IN TLSA 1 1 1 536a570ac49d9ba4...

   Note that it is acceptable for both the original domain and the
   target domain to have TLSA records, but the two records are
   unrelated.  Consider the following:









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   ; TLSA record in both the original and target domain
   ;
   sub5.example.com.            IN CNAME sub6.example.com.
   _443._tcp.sub5.example.com.  IN TLSA 1 1 1 308202c5308201ab...
   sub6.example.com.            IN A 192.0.2.1
   sub6.example.com.            IN AAAA 2001:db8::1/32
   _443._tcp.sub6.example.com.  IN TLSA 1 1 1 ac49d9ba4570ac49...

   In this example, someone looking for the TLSA record for
   sub5.example.com would always get the record whose value starts
   "308202c5308201ab"; the TLSA record whose value starts
   "ac49d9ba4570ac49" would only be sought by someone who is looking for
   the TLSA record for sub6.example.com, and never for sub5.example.com.
   One should note that deploying different certificates for multiple
   services located at a shared TLS listener often requires the use of
   TLS SNI (Server Name Indication) [RFC6066].

   Note that these methods use the normal method for DNS aliasing using
   CNAME: the DNS client requests the record type that they actually
   want.

A.2.1.2.  Provisioning TLSA Records with DNAME Records

   Using DNAME records allows a zone owner to alias an entire subtree of
   names below the name that has the DNAME.  This allows the wholesale
   aliasing of prefixed records such as those used by TLSA, SRV, and so
   on without aliasing the name itself.  However, because DNAME can only
   be used for subtrees of a base name, it is rarely used to alias
   individual hosts that might also be running TLS.

   ; TLSA record in target domain, visible in original domain via DNAME
   ;
   sub5.example.com.            IN CNAME sub6.example.com.
   _tcp.sub5.example.com.       IN DNAME _tcp.sub6.example.com.
   sub6.example.com.            IN A 192.0.2.1
   sub6.example.com.            IN AAAA 2001:db8::1/32
   _443._tcp.sub6.example.com.  IN TLSA 1 1 1 536a570ac49d9ba4...

A.2.1.3.  Provisioning TLSA Records with Wildcards

   Wildcards are generally not terribly useful for RRtypes that require
   prefixing because you can only wildcard at a layer below the host
   name.  For example, if you want to have the same TLSA record for
   every TCP port for www.example.com, you might have

   *._tcp.www.example.com.    IN TLSA 1 1 1 5c1502a6549c423b...

   This is possibly useful in some scenarios where the same service is



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   offered on many ports.

A.2.2.  Provisioning with NS Records

   [[ THIS SECTION NEEDS TO BE WRITTEN OR REMOVED.  This was proposed,
   and questioned, and not yet followed through on.  This section will
   be removed in the next draft if no one volunteers to write it. ]]

A.3.  Securing the Last Hop

   As described in Section 5, an application processing TLSA records
   must know the DNSSEC validity of those records.  There are many ways
   for the application to securely find this out, and this specification
   does not mandate any single method.

   Some common methods for an application to know the DNSSEC validity of
   TLSA records include:

   o  The application can have its own DNS resolver and DNSSEC
      validation stack.

   o  The application can communicate through a trusted channel (such as
      requests to the operating system under which the application is
      running) to a local DNS resolver that does DNSSEC validation.

   o  The application can communicate through a secured channel (such as
      requests running over TLS, IPsec, TSIG or SIG(0)) to a non-local
      DNS resolver that does DNSSEC validation.

   o  The application can communicate through a secured channel (such as
      requests running over TLS, IPsec, TSIG or SIG(0)) to a non-local
      DNS resolver that does not do DNSSEC validation, but gets
      responses through a secured channel from a different DNS resolver
      that does DNSSEC validation.

A.4.  Handling Certificate Rollover

   Certificate rollover is handled in much the same was as for rolling
   DNSSEC zone signing keys using the pre-publish key rollover method
   [RFC4641].  Suppose example.com has a single TLSA record for a TLS
   service on TCP port 990:

   _990._tcp.example.com IN TLSA 1 1 1 1CFC98A706BCF3683015...

   To start the rollover process, obtain or generate the new certificate
   or SubjectPublicKeyInfo to be used after the rollover and generate
   the new TLSA record.  Add that record alongside the old one:




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   _990._tcp.example.com IN TLSA 1 1 1 1CFC98A706BCF3683015...
   _990._tcp.example.com IN TLSA 1 1 1 62D5414CD1CC657E3D30...

   After the new records have propagated to the authoritative
   nameservers and the TTL of the old record has expired, switch to the
   new certificate on the TLS server.  Once this has occurred, the old
   TLSA record can be removed:

   _990._tcp.example.com IN TLSA 1 1 1 62D5414CD1CC657E3D30...

   This completes the certificate rollover.


Appendix B.  Pseudocode for Using TLSA

   This appendix describes the interactions given earlier in this
   specification in pseudocode format.  This appendix is non-normative.
   If the steps below disagree with the text earlier in the document,
   the steps earlier in the document should be considered correct and
   this text incorrect.

   Note that this pseudocode is more strict than the normative text.
   For instance, it forces an order on the evaluation of criteria which
   is not mandatory from the normative text.

B.1.  Helper Functions


   // implement the function for exiting
   function Finish (F) = {
     if (F == ABORT_TLS) {
       abort the TLS handshake or prevent TLS from starting
       exit
     }

     if (F == NO_TLSA) {
       fall back to non-TLSA certificate validation
       exit
     }

     if (F == ACCEPT) {
       accept the TLS connection
       exit
     }

     // unreachable
   }




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   // implement the selector function
   function Select (S, X) = {
     // Full certificate
     if (S == 0) {
       return X
     }

     // SubjectPublicKeyInfo
     if (S == 1) {
       return X.SubjectPublicKeyInfo
     }

     // unreachable
   }

   // implement the matching function
   function Match (M, X, Y) {
     // Exact match on selected content
     if (M == 0) {
       return (X == Y)
     }

     // SHA-256 hash of selected content
     if (M == 1) {
       return (SHA-256(X) == Y)
     }

     // SHA-512 hash of selected content
     if (M == 2) {
       return (SHA-512(X) == Y)
     }

     // unreachable
   }


B.2.  Main TLSA Pseudo Code

   TLS connect using [transport] to [name] on [port] and receiving end
   entity cert C for the TLS server:



   (TLSArecords, ValState) = DNSSECValidatedLookup(
     domainname=_[port]._[transport].[name], RRtype=TLSA, class=IN)

   // check for states that would change processing
   if (ValState == BOGUS) {



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     Finish(ABORT_TLS)
   }
   if ((ValState == INDETERMINATE) or (ValState == INSECURE)) {
     Finish(NO_TLSA)
   }
   // if here, ValState must be SECURE

   for each R in TLSArecords {
     // unusable records include unknown certUsage, unknown
     // selectorType, unknown matchingType, erroneous RDATA, and
     // prohibited by local policy
     if (R is unusable) {
       remove R from TLSArecords
     }
   }
   if (length(TLSArecords) == 0) {
     Finish(NO_TLSA)
   }

   // A TLS client might have multiple trust anchors that it might use
   //    when validating the TLS server's end entity certificate. Also,
   //    there can be multiple PKIX validation chains for the
   //    certificates given by the server in TLS. Thus, there are
   //    possibly many chains that might need to be tested during
   //    PKIX validation.

   for each R in TLSArecords {

     // pass PKIX validation and chain through CA cert from TLSA
     if (R.certUsage == 0) {
       for each PKIX validation chain H {
         if (C passes PKIX validation in H) {
           for each D in H {
             if ((D is a CA certificate) and
                 Match(R.matchingType, Select(R.selectorType, D),
                       R.cert)) {
               Finish(ACCEPT)
             }
           }
         }
       }
     }

     // pass PKIX validation and match EE cert from TLSA
     if (R.certUsage == 1) {
       for each PKIX validation chain H {
         if ((C passes PKIX validation in H) and
                 Match(R.matchingType, Select(R.selectorType, C),



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                 R.cert)) {
             Finish(ACCEPT)
         }
       }
     }

     // pass PKIX validation using TLSA record as trust anchor
     if (R.certUsage == 2) {
       for each PKIX validation chain H that has R as the trust anchor {
         if (C passes PKIX validation in H) and
                Match(R.matchingType, Select(R.selectorType, C),
                R.cert)) {
           Finish(ACCEPT)
         }
       }
     }

     // match the TLSA record and the TLS certificate
     if (R.certUsage == 3) {
       if Match(R.matchingType, Select(R.selectorType, C), R.cert)
         Finish(ACCEPT)
       }
     }

   }

   // if here, then none of the TLSA records ended in "Finish(ACCEPT)"
   //   so abort TLS
   Finish(ABORT_TLS)


Authors' Addresses

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org


   Jakob Schlyter
   Kirei AB

   Email: jakob@kirei.se








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