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Versions: 00 01 02

Network Working Group                                         P. Wouters
Internet-Draft                                                   Red Hat
Intended status: Standards Track                       February 13, 2014
Expires: August 17, 2014


    Using DANE to Associate OpenPGP public keys with email addresses
                     draft-wouters-dane-openpgp-02

Abstract

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

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
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   This Internet-Draft will expire on August 17, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   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  . . . . . . . .   3
   3.  Location of the OpenPGPKEY record . . . . . . . . . . . . . .   4
   4.  OpenPGP Key size and DNS  . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
     5.1.  Email address information leak  . . . . . . . . . . . . .   5
     5.2.  Forward security of OpenPGP versus DNSSEC . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Generating OPENPGPKEY records  . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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, two problems need to be solved by the
   sender's email client, MUA or MTA.  Where does one find the
   recipient's public key and how does one trust that the found key
   actually belongs to the intended recipient.

   Obtaining a public key is not a straightforward process as there are
   no trusted standarized locations for publishing OpenPGP public keys
   indexed by email address.  Instead, OpenPGP clients rely on "well-
   known key servers" that are accessed using the web based HKP protocol
   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 lost their
   private key.





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   The lack of association of email address and public key lookup is
   also preventing email clients, MTAs and MUAs from encrypting a
   received message to the target receipient forcing the software to
   send the message unencryped.  Currently deployed MTA's 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 "web of trust" 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
   server.

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 public key with an email address, thus forming a
   "OpenPGP public key association".

   The type value allocated for the OPENPGPKEY RR type is [TBD].  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 is the binary OpenPGP public keyring as
   specified in [RFC4880] without any ascii armor or base64 encoding.

2.3.  The OPENPGPKEY RDATA presentation format




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   The RDATA Presentation Format, as visible in textual zone files,
   consists of the [RFC4880] formatted OpenPGP public keyring encoded in
   Base64 [RFC4648]

3.  Location of the OpenPGPKEY record

   Email addresses are mapped into DNS using the following method:

   1.  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 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.

   2.  The DNS does not allow the use of all characters that are
       supported in "local-part" of email addresses as defined in
       [RFC2822] and [RFC6530] . The SHA2-224 hashing of the user name
       ensures that none of these characters would need to be placed
       directly in the DNS.

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

   4.  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 "hugh@example.com", an OPENPGPKEY query would
   be placed for the following QNAME: "8d5730bd8d76d417bf974c03f59eedb7a
   f98cb5c3dc73ea8ebbd54b7._openpgpkey.example.com" The corresponding RR
   in the example.com zone might look like:

   8d5730bd8d76d417bf974c03f59eedb7af98cb5c3dc73ea8ebbd54b7._openpgpkey.example.com. IN OPENPGPKEY <encoded public key>


4.  OpenPGP Key size and DNS

   Although the reliability of the transport of large DNS Resoruce
   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 accomodate this
   section.





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   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.  Email address information leak

   Email addresses are not secret.  Using them causes its 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
   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 SHA2-224 hashes into common and short user names, as
   single rainbow tables can be re-used accross 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.

5.2.  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.





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   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.

   Therefor, an OpenPGP key obtained via an OPENPGPKEY record can only
   be trusted as much as the DNS domain can be trusted, and are 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.

6.  IANA Considerations

6.1.  OPENPGPKEY RRtype

   This document uses a new DNS RR type, OPENPGPKEY, whose value [TBD]
   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.

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.





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   [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

   [OPENPGPKEY-USAGE]
              Wouters, P., "Usage considerations with the DNS OPENPGPKEY
              record", draft-wouters-openpgpkey-usage (work in
              progress), January 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
              2001.

   [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 \
       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.



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   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 \
       hugh@example.com | wc -c

   gpg --export --export-options export-minimal \
       hugh@example.com | hexdump -e \
          '"\t" /1 "%.2x"' -e '/32 "\n"'



   These values can then be used to generate a generic record:


   <SHA2-224(hugh)>._openpgpkey.example.com. IN TYPE65280 \# <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
   8d5730bd8d[...]bbd54b7._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...]

   ~> openpgpkey --output generic hugh@example.com
   8d5730bd8d[...]bbd54b7._openpgpkey.example.com. IN TYPE65280 \# 2313 99008d03[...]




Author's Address

   Paul Wouters
   Red Hat

   Email: pwouters@redhat.com










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