Using DANE to Associate OpenPGP public keys with email addresses
draft-ietf-dane-openpgpkey-05

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Network Working Group                                         P. Wouters
Internet-Draft                                                   Red Hat
Intended status: Experimental                            August 28, 2015
Expires: February 29, 2016

    Using DANE to Associate OpenPGP public keys with email addresses
                     draft-ietf-dane-openpgpkey-05

Abstract

   OpenPGP is a message format for email (and file) encryption that
   lacks a standardized lookup mechanism to securely obtain OpenPGP
   public keys.  This document specifies a method for publishing and
   locating OpenPGP public keys in DNS for a specific email address
   using a new OPENPGPKEY DNS Resource Record.  Security is provided via
   DNSSEC.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 29, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The OPENPGPKEY Resource Record  . . . . . . . . . . . . . . .   3
     2.1.  The OPENPGPKEY RDATA component  . . . . . . . . . . . . .   4
       2.1.1.  The OPENPGPKEY RDATA content  . . . . . . . . . . . .   4
       2.1.2.  Reducing the Transferable Public Key size . . . . . .   5
     2.2.  The OPENPGPKEY RDATA wire format  . . . . . . . . . . . .   5
     2.3.  The OPENPGPKEY RDATA presentation format  . . . . . . . .   5
   3.  Location of the OPENPGPKEY record . . . . . . . . . . . . . .   5
   4.  Email address variants  . . . . . . . . . . . . . . . . . . .   6
   5.  Application use of OPENPGPKEY . . . . . . . . . . . . . . . .   7
     5.1.  Obtaining an OpenPGP key for a specific email address . .   7
     5.2.  Confirming the validity of an OpenPGP key . . . . . . . .   7
     5.3.  Verifying an unknown OpenPGP signature  . . . . . . . . .   7
   6.  OpenPGP Key size and DNS  . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     7.1.  Response size . . . . . . . . . . . . . . . . . . . . . .   8
     7.2.  Email address information leak  . . . . . . . . . . . . .   8
     7.3.  Storage of OPENPGPKEY data  . . . . . . . . . . . . . . .   9
     7.4.  Forward security of OpenPGP versus DNSSEC . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Generating OPENPGPKEY records  . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   OpenPGP [RFC4880] public keys are used to encrypt or sign email
   messages and files.  To encrypt an email message, or verify a
   sender's OpenPGP signature, the email client or MTA needs to locate
   the recipient's OpenPGP public key.

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   OpenPGP clients have relied on centralized "well-known" key servers
   that are accessed using either the HTTP Keyserver Protocol [HKP]
   Alternatively, users need to manually browse a variety of different
   front-end websites.  These key servers do not validate the email
   address in the User ID of the uploaded OpenPGP public key.  Attackers
   can - and have - uploaded rogue public keys with other people's email
   addresses to these key servers.

   Once uploaded, public keys cannot be deleted.  People who did not
   pre-sign a key revocation can never remove their OpenPGP public key
   from these key servers once they have lost access to their private
   key.  This results in receiving encrypted email that cannot be
   decrypted.

   Therefor, these keyservers are not well suited to support email
   clients and MTA's to automatically encrypt email - especially in the
   absence of an interactive user.

   This document describes a mechanism to associate a user's OpenPGP
   public key with their email address, using the OPENPGPKEY DNS RRtype.
   These records are published in the DNS zone of the user's email
   address.  If the user loses their private key, the OPENPGPKEY DNS
   record can simply be updated or removed from the zone.

   The proposed new DNS Resource Record type is secured using DNSSEC.
   This trust model is not meant to replace the Web Of Trust model.

1.1.  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 DNSSEC and DANE terminology.
   See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for
   these terms.

2.  The OPENPGPKEY Resource Record

   The OPENPGPKEY DNS resource record (RR) is used to associate an end
   entity OpenPGP Transferable Public Key (see Section 11.1 of [RFC4880]
   with an email address, thus forming a "OpenPGP public key
   association".  A user that wishes to specify more than one OpenPGP
   key, for example because they are transitioning to a newer stronger
   key, can do so by adding multiple OPENPGPKEY records.  A single
   OPENPGPKEY DNS record MUST only contain one OpenPGP key.

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   The type value allocated for the OPENPGPKEY RR type is 61.  The
   OPENPGPKEY RR is class independent.  The OPENPGPKEY RR has no special
   TTL requirements.

2.1.  The OPENPGPKEY RDATA component

   The RDATA portion of an OPENPGPKEY Resource Record contains a single
   value consisting of a [RFC4880] formatted Transferable Public Key.

2.1.1.  The OPENPGPKEY RDATA content

   An OpenPGP Transferable Public Key can be arbitrarily large.  DNS
   records are limited in size.  When creating OPENPGPKEY DNS records,
   the OpenPGP Transferable Public Key should be filtered to only
   contain appropriate and useful data.  At a minimum, an OPENPGPKEY
   Transferable Public Key for the user hugh@example.com should contain:

        o The primary key X
          o One User ID Y, which SHOULD match 'hugh@example.com'
            o self-signature from X, binding X to Y

   If the primary key is not encryption-capable, a relevant subkey
   should be included resulting in an OPENPGPKEY Transferable Public Key
   containing:

        o The primary key X
          o One User ID Y, which SHOULD match 'hugh@example.com'
            o self-signature from X, binding X to Y
          o encryption-capable subkey Z
            o self-signature from X, binding Z to X
          o [ other subkeys if relevant ... ]

   The user can also elect to add a few third-party certifications which
   they believe would be helpful for validation in the traditional Web
   Of Trust.  The resulting OPENPGPKEY Transferable Public Key would
   then look like:

        o The primary key X
          o One User ID Y, which SHOULD match 'hugh@example.com'
            o self-signature from X, binding X to Y
            o third-party certification from V, binding Y to X
            o [ other third-party certifications if relevant ... ]
          o encryption-capable subkey Z
            o self-signature from X, binding Z to X
          o [ other subkeys if relevant ... ]

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2.1.2.  Reducing the Transferable Public Key size

   When preparing a Transferable Public Key for a specific OPENPGPKEY
   RDATA format with the goal of minimizing certificate size, a user
   would typically want to:

   o  Where one User ID from the certifications matches the looked-up
      address, strip away non-matching User IDs and any associated
      certifications (self-signatures or third-party certifications)

   o  Strip away all User Attribute packets and associated
      certifications.

   o  Strip away all expired subkeys.  The user may want to keep revoked
      subkeys if these were revoked prior to their preferred expiration
      time to ensure that correspondents know about these earlier then
      expected revocations.

   o  Strip away all but the most recent self-sig for the remaining user
      IDs and subkeys

   o  Optionally strip away any uninteresting or unimportant third-party
      User ID certifications.  This is a value judgment by the user that
      is difficult to automate.  At the very least, expired and
      superseded third-party certifcations should be stripped out.  The
      user should attempt to keep the most recent and most well
      connected certifications in the Web Of Trust in their Transferable
      Public Key.

2.2.  The OPENPGPKEY RDATA wire format

   The RDATA Wire Format consists of a single OpenPGP Transferable
   Public Key as defined in Section 11.1 of [RFC4880].  Note that this
   format is without ASCII armor or base64 encoding.

2.3.  The OPENPGPKEY RDATA presentation format

   The RDATA Presentation Format, as visible in textual zone files,
   consists of a single OpenPGP Transferable Public Key as defined in
   Section 11.1 of [RFC4880] encoded in base64 as defined in Section 4
   of [RFC4648].

3.  Location of the OPENPGPKEY record

   The DNS does not allow the use of all characters that are supported
   in the "local-part" of email addresses as defined in [RFC5322] and
   [RFC6530].  Therefore, email addresses are mapped into DNS using the
   following method:

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   o  The user name (the "left-hand side" of the email address, called
      the "local-part" in the mail message format definition [RFC5322]
      and the local-part in the specification for internationalized
      email [RFC6530]) should already be encoded in UTF-8 (or its subset
      ASCII).  If it is written in another encoding it should be
      converted to UTF-8 and then hashed using the SHA2-256 [RFC5754]
      algorithm, with the hash truncated to 28 octets and represented in
      its hexadecimal representation, to become the left-most label in
      the prepared domain name.  Truncation comes from the right-most
      octets.  This does not include the at symbol ("@") that separates
      the left and right sides of the email address.

   o  The string "_openpgpkey" becomes the second left-most label in the
      prepared domain name.

   o  The domain name (the "right-hand side" of the email address,
      called the "domain" in RFC 5322) is appended to the result of step
      2 to complete the prepared domain name.

   For example, to request an OPENPGPKEY resource record for a user
   whose email address is "hugh@example.com", an OPENPGPKEY query would
   be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35
   eec8f72e57f9eec01c1afd6._openpgpkey.example.com".  The corresponding
   RR in the example.com zone might look like (key shortened for
   formatting):

   c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY <base64 public key>

4.  Email address variants

   Mail systems usually handle variant forms of local-parts.  The most
   common variants are upper and lower case, often automatically
   corrected when a name is recognized as such.  Other variants include
   systems that ignore "noise" characters such as dots, so that local
   parts johnsmith and John.Smith would be equivalent.  Many systems
   allow "extensions" such as john-ext or mary+ext where john or mary is
   treated as the effective local-part, and the ext is passed to the
   recipient for further handling.  This can complicate finding the
   OPENPGPKEY record associated with the dynamically created email
   address.

   [RFC5321] and its predecessors have always made it clear that only
   the recipient MTA is allowed to interpret the local-part of an
   address.  A client supporting OPENPGPKEY therefor MUST NOT perform
   any kind of mapping rules based on the email address.

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5.  Application use of OPENPGPKEY

   The OPENPGPKEY record allows an application or service to obtain or
   verify an OpenPGP public key.  The lookup result MUST pass DNSSEC
   validation; if validation reaches any state other than "Secure", the
   verification MUST be treated as a failure.

5.1.  Obtaining an OpenPGP key for a specific email address

   If no OpenPGP public keys are known for an email address, an
   OPENPGPKEY lookup MAY be performed to discover the OpenPGP public key
   that belongs to a specific email address.  This public key can then
   be used to verify a received signed message or can be used to send
   out an encrypted email message.  An application that confirms the
   lack of an OPENPGPKEY record SHOULD remember this for some time to
   avoid sending out a DNS request for each email message that is sent
   out as this constitutes a privacy leak.

5.2.  Confirming the validity of an OpenPGP key

   Locally stored OpenPGP public keys are not automatically refreshed.
   If the owner of that key creates a new OpenPGP public key, that owner
   is unable to securely notify all users and applications that have its
   old OpenPGP public key.  Applications and users can perform an
   OPENPGPKEY lookup to confirm the locally stored OpenPGP public key is
   still the correct key to use.  If verifying a locally stored OpenPGP
   public key and the OpenPGP public key found through DNS is different
   from the locally stored OpenPGP public key, the verification MUST be
   treated as a failure.  An application that can interact with the user
   MAY ask the user for guidance.  For privacy reasons, an application
   MUST NOT attempt to validate a locally stored OpenPGP key using an
   OPENPGPKEY lookup at every use of that key.

5.3.  Verifying an unknown OpenPGP signature

   Storage media can be signed using an OpenPGP public key.  Even if the
   OpenPGP public key is included on the storage media, it needs to be
   independently validated.  OpenPGP public keys contain one or more IDs
   than can have the syntax of an email address.  An application can
   perform a lookup for an OPENPGPKEY at the expected location for the
   specific email address to confirm the validity of the OpenPGP public
   key.  Once the key has been validated, all files on the storage media
   that have been signed by this key can now be verified.

6.  OpenPGP Key size and DNS

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   Due to the expected size of the OPENPGPKEY record, applications
   SHOULD use TCP - not UDP - to perform queries for the OPENPGPKEY
   Resource Record.

   Although the reliability of the transport of large DNS Resource
   Records has improved in the last years, it is still recommended to
   keep the DNS records as small as possible without sacrificing the
   security properties of the public key.  The algorithm type and key
   size of OpenPGP keys should not be modified to accommodate this
   section.

   OpenPGP supports various attributes that do not contribute to the
   security of a key, such as an embedded image file.  It is recommended
   that these properties are not exported to OpenPGP public keyrings
   that are used to create OPENPGPKEY Resource Records.  Some OpenPGP
   software, for example GnuPG, have support for a "minimal key export"
   that is well suited to use as OPENPGPKEY RDATA.  See Appendix A.

7.  Security Considerations

   OPENPGPKEY usage considerations are published in [OPENPGPKEY-USAGE].

7.1.  Response size

   To prevent amplification attacks, an Authoritative DNS server MAY
   wish to prevent returning OPENPGPKEY records over UDP unless the
   source IP address has been verified with [DNS-COOKIES].  Such servers
   MUST NOT return REFUSED, but answer the query with an empty Answer
   Section and the truncation flag set ("TC=1").

7.2.  Email address information leak

   The hashing of the user name in this document is not a security
   feature.  Publishing OPENPGPKEY records however, will create a list
   of hashes of valid email addresses, which could simplify obtaining a
   list of valid email addresses for a particular domain.  It is
   desirable to not ease the harvesting of email addresses where
   possible.

   The domain name part of the email address is not used as part of the
   hash so that hashes can be used in multiple zones deployed using
   DNAME [RFC6672].  This does makes it slightly easier and cheaper to
   brute-force the SHA2-256 hashes into common and short user names, as
   single rainbow tables can be re-used across domains.  This can be
   somewhat countered by using NSEC3.

   DNS zones that are signed with DNSSEC using NSEC for denial of
   existence are susceptible to zone-walking, a mechanism that allows

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   someone to enumerate all the OPENPGPKEY hashes in a zone.  This can
   be used in combination with previously hashed common or short user
   names (in rainbow tables) to deduce valid email addresses.  DNSSEC-
   signed zones using NSEC3 for denial of existence instead of NSEC are
   significantly harder to brute-force after performing a zone-walk.

7.3.  Storage of OPENPGPKEY data

   Users may have a local key store with OpenPGP public keys.  An
   application supporting the use of OPENPGPKEY DNS records MUST NOT
   modify the local key store without explicit confirmation of the user,
   as the application is unaware of the user's personal policy for
   adding, removing or updating their local key store.  An application
   MAY warn the user if an OPENPGPKEY record does not match the OpenPGP
   public key in the local key store.

   Applications that do not have users associated with, such as daemon
   processes, SHOULD store OpenPGP public keys obtained via OPENPGPKEY
   up to their DNS TTL value.  This avoids repeated DNS lookups that
   third parties could monitor to determine when an email is being sent
   to a particular user.  If TLS is in use between MTA's, only the DNS
   lookup could happen unencrypted.

7.4.  Forward security of OpenPGP versus DNSSEC

   DNSSEC key sizes are chosen based on the fact that these keys can be
   rolled with next to no requirement for security in the future.  If
   one doubts the strength or security of the DNSSEC key for whatever
   reason, one simply rolls to a new DNSSEC key with a stronger
   algorithm or larger key size.  On the other hand, OpenPGP key sizes
   are chosen based on how many years (or decades) their encryption
   should remain unbreakable by adversaries that own large scale
   computational resources.

   This effectively means that anyone who can obtain a DNSSEC private
   key of a domain name via coercion, theft or brute force calculations,
   can replace any OPENPGPKEY record in that zone and all of the
   delegated child zones, irrespective of the key size of the OpenPGP
   keypair.  Any future messages encrypted with the malicious OpenPGP
   key could then be read.

   Therefore, an OpenPGP key obtained via an OPENPGPKEY record can only
   be trusted as much as the DNS domain can be trusted, and is no
   substitute for in-person key verification of the "Web of Trust".  See
   [OPENPGPKEY-USAGE] for more in-depth information on safe usage of
   OPENPGPKEY based OpenPGP keys.

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8.  IANA Considerations

8.1.  OPENPGPKEY RRtype

   This document uses a new DNS RR type, OPENPGPKEY, whose value 61 has
   been allocated by IANA from the Resource Record (RR) TYPEs
   subregistry of the Domain Name System (DNS) Parameters registry.

9.  Acknowledgments

   This document is based on RFC-4255 and draft-ietf-dane-smime whose
   authors are Paul Hoffman, Jacob Schlyter and W. Griffin.  Olafur
   Gudmundsson provided feedback and suggested various improvements.
   Willem Toorop contributed the gpg and hexdump command options.
   Daniel Kahn Gillmor provided the text describing the OpenPGP packet
   formats and filtering options.  Edwin Taylor contributed language
   improvements for various iterations of this document.  Text regarding
   email mappings was taken from draft-levine-dns-mailbox whose author
   is John Levine.

10.  References

10.1.  Normative References

   [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.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/
              RFC4880, November 2007,
              <http://www.rfc-editor.org/info/rfc4880>.

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   [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
              Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
              2010, <http://www.rfc-editor.org/info/rfc5754>.

10.2.  Informative References

   [DNS-COOKIES]
              Eastlake, Donald., "Domain Name System (DNS) Cookies",
              draft-ietf-dnsop-cookies (work in progress), August 2015.

   [HKP]      Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)",
              draft-shaw-openpgp-hkp (work in progress), March 2013.

   [OPENPGPKEY-USAGE]
              Wouters, P., "Usage considerations with the DNS OPENPGPKEY
              record", draft-ietf-dane-openpgpkey-usage (work in
              progress), October 2014.

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <http://www.rfc-editor.org/info/rfc3597>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <http://www.rfc-editor.org/info/rfc5321>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI
              10.17487/RFC5322, October 2008,
              <http://www.rfc-editor.org/info/rfc5322>.

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
              February 2012, <http://www.rfc-editor.org/info/rfc6530>.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
              <http://www.rfc-editor.org/info/rfc6672>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

Appendix A.  Generating OPENPGPKEY records

   The commonly available GnuPG software can be used to generate a
   minimum Transferable Public Key for the RRdata portion of an
   OPENPGPKEY record:

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   gpg --export --export-options export-minimal,no-export-attributes \
       hugh@example.com | base64

   The --armor or -a option of the gpg command should NOT be used, as it
   adds additional markers around the armored key.

   When DNS software reading or signing the zone file does not yet
   support the OPENPGPKEY RRtype, the Generic Record Syntax of [RFC3597]
   can be used to generate the RDATA.  One needs to calculate the number
   of octets and the actual data in hexadecimal:

   gpg --export --export-options export-minimal,no-export-attributes \
       hugh@example.com | wc -c

   gpg --export --export-options export-minimal,no-export-attributes \
       hugh@example.com | hexdump -e \
          '"\t" /1 "%.2x"' -e '/32 "\n"'

   These values can then be used to generate a generic record (line
   break has been added for formatting):

   <SHA2-256-trunc(hugh)>._openpgpkey.example.com. IN TYPE61 \# \
       <numOctets> <keydata in hex>

   The openpgpkey command in the hash-slinger software can be used to
   generate complete OPENPGPKEY records

   ~> openpgpkey --output rfc hugh@example.com
   c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...]

   ~> openpgpkey --output generic hugh@example.com
   c9[..]d6._openpgpkey.example.com. IN TYPE61 \# 2313 99008d03[...]

Author's Address

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Internet-Draft        DANE for OpenPGP public keys           August 2015

   Paul Wouters
   Red Hat

   Email: pwouters@redhat.com

Wouters                 Expires February 29, 2016              [Page 13]