Network Working Group                                         P. Wouters
Internet-Draft                                                   Red Hat
Intended status: Standards Track                          March 09, 2015
Expires: September 10, 2015

    Using DANE to Associate OpenPGP public keys with email addresses


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

Status of This Memo

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   This Internet-Draft will expire on September 10, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The OPENPGPKEY Resource Record  . . . . . . . . . . . . . . .   3
     2.1.  The OPENPGPKEY RDATA component  . . . . . . . . . . . . .   3
     2.2.  The OPENPGPKEY RDATA wire format  . . . . . . . . . . . .   3
     2.3.  The OPENPGPKEY RDATA presentation format  . . . . . . . .   4
   3.  Location of the OPENPGPKEY record . . . . . . . . . . . . . .   4
     3.1.  Email address variants  . . . . . . . . . . . . . . . . .   4
   4.  OpenPGP Key size and DNS  . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
     5.1.  Response size . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Email address information leak  . . . . . . . . . . . . .   5
     5.3.  Forward security of OpenPGP versus DNSSEC . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Generating OPENPGPKEY records  . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   To encrypt a message to a target recipient using OpenPGP [RFC4880],
   possession of the recipient's OpenPGP public key is required.  To
   obtain that public key, the sender's email client or MTA needs to
   know where to find the recipient's public key.  Once obtained, it
   needs to find some proof that the public key found actually belongs
   to the intended recipient.

   Obtaining a public key is not a straightforward process as there are
   no trusted standardized locations for publishing OpenPGP public keys
   indexed by email address.  Instead, OpenPGP clients rely on "well-
   known key servers" that are accessed using the HTTP Keyserver
   Protocol ("HKP") or manually by users using a variety of differently
   formatted front-end web pages.

   Currently deployed key servers have no method of validating any
   uploaded OpenPGP public key.  The key servers simply store and
   publish.  Anyone can add public keys with any identities and anyone
   can add signatures to any other public key using forged malicious
   identities.  Furthermore, once uploaded, public keys cannot be
   deleted.  People who did not pre-sign a key revocation can never
   remove their public key from these key servers once they lose their
   private key.

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   The lack of a secure means to look up a public key for an email
   address also prevents email clients and MUAs from encrypting a
   received email to the target recipient, forcing the software to send
   the message unencrypted.  Currently deployed MTAs only support
   encrypting the transport of the email, not the email contents itself.

   This document describes a mechanism to associate a user's OpenPGP
   public key with their email address, using a new DNS RRtype.

   The proposed new DNS Resource Record type is secured using DNSSEC.
   This trust model is not meant to replace the Trust Signature model.
   However, it can be used to encrypt a message that would otherwise
   have to be sent out unencrypted, where it could be monitored by a
   third party in transit or located in plaintext on a storage or email

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 public key with an email address, thus forming a
   "OpenPGP public key association".

   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 (or RHS) of an OPENPGPKEY Resource Record contains a single
   value consisting of a [RFC4880] formatted OpenPGP public keyring.

2.2.  The OPENPGPKEY RDATA wire format

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

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2.3.  The OPENPGPKEY RDATA presentation format

   The RDATA Presentation Format, as visible in textual zone files,
   consists of a single OpenPGP public key as defined in
   Section of [RFC4880] encoded in Base64 [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 [RFC2822] and
   [RFC6530].  Therefore, email addresses are mapped into DNS using the
   following method:

   o  The user name (the "left-hand side" of the email address, called
      the "local-part" in the mail message format definition [RFC2822]
      and the "local part" in the specification for internationalized
      email [RFC6530]), is hashed using the SHA2-224 [RFC5754]
      algorithm, with the hash being represented in its hexadecimal
      representation, to become the left-most label in the prepared
      domain name.  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 2822) 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 "", an OPENPGPKEY query would
   be placed for the following QNAME: "8d5730bd8d76d417bf974c03f59eedb7a".  The corresponding
   RR in the zone might look like (key shortened for

   8d[..]  IN OPENPGPKEY <base64 public key>

3.1.  Email address variants

   Some email service providers and email software perform automatic
   mappings of email addresses based on special characters.  This can
   complicate finding the OPENPGPKEY record associated with the
   dynamically created email address.  Some well known examples are
   listed below

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   o  The LHS is case insensitive, and
      map to  Some email clients also automatically
      uppercase the first letter of an email address when typing it in.

   o  Everything after a "+" symbol is dynamc.
      maps to

   o  Dots are optional. maps to

   Software implementing DNS lookup for the OPENPGPKEY RRtype MAY
   perform similar translations rules while trying to find the
   OPENPGPKEY record.

4.  OpenPGP Key size and DNS

   Due to the expected size of the OPENPGPKEY record, it is recommended
   to perform DNS queries for the OPENPGPKEY record using TCP, not UDP.

   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

   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.

5.  Security Considerations

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

5.1.  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).

5.2.  Email address information leak

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   Email addresses are not secret.  Using them causes their publication.
   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

   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 SHA2-224 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
   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.

5.3.  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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6.  IANA Considerations


   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.

7.  Acknowledgements

   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.  Edwin
   Taylor contributed language improvements for various iterations of
   this document.

8.  References

8.1.  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, October 2006.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.

   [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
              Message Syntax", RFC 5754, January 2010.

8.2.  Informative References


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              Eastlake, Donald., "Domain Name System (DNS) Cookies",
              draft-ietf-dnsop-cookies (work in progress), February

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

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, September 2003.

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, February 2012.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, June 2012.

   [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 the
   RRdata portion of an OPENPGPKEY record:

   gpg --export --export-options export-minimal \ | 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:

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   gpg --export --export-options export-minimal \ | wc -c

   gpg --export --export-options export-minimal \ | 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):

   <SHA2-224(hugh)> 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
   8d[...] IN OPENPGPKEY mQCNAzIG[...]

   ~> openpgpkey --output generic
   8d[...] IN TYPE61 \# 2313 99008d03[...]

Author's Address

   Paul Wouters
   Red Hat


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