Using DANE to Associate OpenPGP public keys with email addresses
draft-wouters-dane-openpgp-01

Network Working Group                                         P. Wouters
Internet-Draft                                                   Red Hat
Intended status: Standards Track                        October 21, 2013
Expires: April 24, 2014

    Using DANE to Associate OpenPGP public keys with email addresses
                     draft-wouters-dane-openpgp-01

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.

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   This Internet-Draft will expire on April 24, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The OPENPGPKEY record presence  . . . . . . . . . . . . . . .   3
   3.  The OPENPGPKEY Resource Record  . . . . . . . . . . . . . . .   4
     3.1.  Location of the OpenPGPKEY record . . . . . . . . . . . .   4
     3.2.  The OPENPGPKEY RDATA Format . . . . . . . . . . . . . . .   5
   4.  OpenPGP public key considerations . . . . . . . . . . . . . .   5
     4.1.  Public Key UIDs and email addresses . . . . . . . . . . .   5
     4.2.  Public Key UIDs and IDNA  . . . . . . . . . . . . . . . .   5
     4.3.  Public Key UIDs and synthesized DNS records . . . . . . .   5
     4.4.  Public Key size and DNS record size . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     5.1.  Email address information leak  . . . . . . . . . . . . .   7
     5.2.  OpenPGP security and DNSSEC . . . . . . . . . . . . . . .   7
     5.3.  MTA behaviour . . . . . . . . . . . . . . . . . . . . . .   7
     5.4.  MUA behaviour . . . . . . . . . . . . . . . . . . . . . .   8
     5.5.  Email client behaviour  . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . .   9
   7.  Generating OPENPGPKEY RRdata  . . . . . . . . . . . . . . . .   9
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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

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   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.

   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.  This is
   similar to the SSHFP [RFC4255] RRType, except that this method
   associates keys with users, not hosts.

   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 intercepted 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 record presence

   A user who publishes an OPENPGPKEY record in DNS explicitly favours
   receiving encrypted email instead of unencrypted email.

   A user who publishes an OPENPGPKEY record in DNS still expects
   senders to perform their due diligence by additional verification of
   their public key via other out-of-band methods before sending any
   confidential or sensitive information

   In other words, the OPENPGPKEY record in DNS, without any additional
   verification, should be used only as an alternative to sending
   plaintext email.  It SHOULD NOT be used to change one's opinion on
   whether it is safe or appropriate to sent the content via email in
   the first place.

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3.  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.  If an an OPENPGPKEY RR contains an expired OpenPGP
   public key, it MUST NOT be used for encryption.

3.1.  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 encoded with Base32 [RFC4648], 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 but does include any trailing equal
       signs ("=") of base64 padding.

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

   3.  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 address is "hugh@example.com", you would use
   "nb2wo2a=._openpgpkey.example.com" in the request.  The corresponding
   RR in the example.com zone might look like:

   nb2wo2a=._openpgpkey.example.com. IN OPENPGPKEY <encoded public key>

   Design note: Encoding the user name with Base32 allows local parts
   that have characters that would prevent their use in domain names.
   For example, a period (".") is a valid character in a local part, but
   would wreak havoc in a domain name.  Similarly, RFC 6530 allows non-
   ASCII characters in local parts, and encoding a local part with non-
   ASCII characters with Base32 renders the name usable in the DNS.  The
   equal sign ("=") is a valid character for a DNS label, even though it
   is not a valid character for a DNS hostname.

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3.2.  The OPENPGPKEY RDATA Format

   The RDATA (or RHS) of an OPENPGPKEY Resource Record contains a single
   value consisting of a [RFC4880] formatted OpenPGP public keyring
   encoded in base64 as specified in [RFC4648].  This is not equivalent
   to an "ascii armor export" which adds a header, a footer, and
   sometimes additional items, to the exported data.

4.  OpenPGP public key considerations

   Once an OPENPGPKEY resource record has been found and the OpenPGP
   public keyring has been decoded, the right public key must be located
   inside the keyring.  For a public key in the keyring to be usable,
   the public key has to have a key uid as specified in [RFC4648] that
   matches the email address for which the OPENPGPKEY RR lookup was
   performed.

4.1.  Public Key UIDs and email addresses

   An OpenPGP public key can be associated with multiple email addresses
   by specifying multiple key uids.  The OpenPGP public key obtained
   from a OPENPGPKEY RR can be used as long as the target recipient's
   email address appears as one of the OpenPGP public key uids.  The
   name part (left of the @) should appear in the native format, not its
   base32 encoding that was used to lookup the OPENPGPKEY RR.

4.2.  Public Key UIDs and IDNA

   Internationalized domains that use non-ascii characters (U-label) are
   encoded in DNS using IDNA [RFC5891] - also referred to as punycode or
   A-label.  When matching OpenPGP public key uids, both the email
   address specified using U-label and A-label should be considered as
   valid public key uids.

4.3.  Public Key UIDs and synthesized DNS records

   CNAME's (see [RFC2181]) and DNAME's (see [RFC6672]) can be followed
   to obtain an OPENPGPKEY RR, as long as the original recipient's email
   address appears as one of the OpenPGP public key uids.  For example,
   if the OPENPGPKEY RR query for hugh@example.com
   (b2wo2a=._openpgpkey.example.com) yields a CNAME to
   b2wo2a=._openpgpkey.example.net, and an OPENPGPKEY RR for
   b2wo2a=._openpgpkey.example.net exists, then this OpenPGP public key
   can be used, provided one of the key uids contains
   "hugh@example.com".  This public key cannot be used if it would only
   contain the key uid "hugh@example.net".

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   If one of the OpenPGP key uids contains only a single wildcard as the
   LHS of the email address, such as "*@example.com", the OpenPGP public
   key may be used for any email address within that domain.  Wildcards
   at other locations (eg hugh@*.com) or regular expressions in key uids
   are not allowed, and any OPENPGPKEY RR containing these should be
   ignored.

4.4.  Public Key size and DNS record size

   Although the reliability of the transport of large DNS Resoruce
   Records has improved in the last few 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 the OpenPGP keypair should not be modified to accomodate this
   section.

   [Should a statement be made on the number of signatures left on the
   key?  Should there be _any_ signatures other than the self-signed
   one?]

   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.

5.  Security Considerations

   The main goal of the OPENPGPKEY resource record is to stop passive
   attacks against plaintext emails.  While it can also twart some
   active attacks (such as people uploading rogue keys to keyservers in
   the hopes that others will encrypt to these rogue keys), this
   resource record is not a replacement for verifying OpenPGP public
   keys via the web of trust signatures, or manually via a fingerprint
   verification.

   Various components could be responsible for encrypting an email
   message to a target recipient.  It could be done by the sender's
   email client or software plugin, the sender's Mail User Agent (MUA)
   or the sender's Mail Transfer Agent (MTA).  Each of these have their
   own characteristics.  An email client can direct the human to make a
   decision before continuing.  The MUA can either accept or refuse a
   message.  The MTA must deliver the message as-is, or encrypt the
   message before delivering.  Each of these programs should ensure that
   the security of an email message is never downgraded, and that an
   unencrypted received message will be encrypted whenever possible.

   Organisations that require to be able to read everyone's encrypted
   email should publish the escrow key as the OPENPGPKEY record.  Upon

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   receipt, such mail servers can optionally re-encrypt the message to
   the individual's OpenPGP key.

5.1.  Email address information leak

   DNS zones that are signed with DNSSEC using NSEC for denial of
   existence are susceptible to zone-walking, a mechanism that allow
   someone to enumerate all the names in the zone.  Someone who wanted
   to collect email addresses from a zone that uses OPENPGPKEY might use
   such a mechanism.  DNSSEC-signed zones using NSEC3 for denial of
   existence are significantly less susceptible to zone-walking.
   Someone could still attempt a dictionary attack on the zone to find
   OPENPGPKEY records, just as they can use dictionary attacks on an
   SMTP server or grab the entire contents of existing PGP key servers
   to see which addresses are valid.

5.2.  OpenPGP security and 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.

   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 length strength of the
   OpenPGP keypair.

   Therefor, DNSSEC is not an alternative for the "web of trust" or for
   manual fingerprint verification by humans.  It is a solution aimed to
   ease obtaining someone's public key, and without manual verification
   should be treated as "better then plaintext" only.  While this twarts
   all passive attacks that simply capture and log all plaintext email
   content, it is not a security measure against active attacks.

5.3.  MTA behaviour

   An MTA could be operating in a stand-alone mode, without access to
   the sender's OpenPGP public keyring, or in a way where it can access
   the user's OpenPGP public keyring.  Regardless, the MTA MUST NOT
   modify the user's OpenPGP keyring.

   An MTA sending an email MUST NOT add the public key obtained from an
   OPENPGPKEY resource record to a permanent public keyring for future
   use beyond the TTL.

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   If the obtained public key is revoked, the MTA MUST NOT use the key
   for encryption, even if that would result in sending the message in
   plaintext.

   If a message is already encrypted, the MTA SHOULD NOT re-encrypt the
   message, even if different encryption schemes or different encryption
   keys were used.

   If an OPENPGPKEY resource record is received without DNSSEC
   protection, it MAY still be used for encryption.

   If the DNS request for an OPENPGPKEY returned an "indeterminate" or
   "bogus" answer, the MTA MUST NOT sent the message and queue the
   plaintext message for delivery at a later time.  If the problem
   persists, the email should be returned via the regular bounce
   methods.

   If multiple non-revoked OPENPGPKEY resource records are found, the
   MTA SHOULD pick the most secure RR based on its local policy.  [or
   should it encrypt to both?]

5.4.  MUA behaviour

   If the public key for a recipient obtained from the locally stored
   sender's public keyring differs from the recipient's OPENPGPKEY RR,
   the MUA MUST NOT accept the message for delivery.

   If the public key for a recipient obtained from the locally stored
   sender's public keyring contains contradicting properties for the
   same key obtained from an OPENPGPKEY RR, the MUA SHOULD NOT accept
   the message for delivery.

   If multiple non-revoked OPENPGPKEY resource records are found, the
   MUA SHOULD pick the most secure OpenPGP public key based on its local
   policy.

5.5.  Email client behaviour

   Email clients should adhere to the above listed MUA behaviour.
   Additionally, an email client MAY interact with the user to resolve
   any conflicts between locally stored keyrings and OPENPGPKEY RRdata.

   An email client that is encrypting a message SHOULD clearly indicate
   to the user the difference between encrypting to a locally stored and
   humanly verified public key and encrypting to an unverified (by the
   human sender) public key obtained via an OPENPGPKEY resource record.

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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.  Generating OPENPGPKEY RRdata

   The commonly available GnuPG software can be used to generate the
   RRdata portion of an OPENPGPKEY record:

   gpg --export --export-options export-minimal \
       your@email.com | base64

   The --armor or -a option should NOT be used.  While it also provides
   a base64 encoded copy of the binary openpgk key data, it adds a
   header and footer to the output.

8.  Acknowledgements

   This document is based on RFC-4255 and draft-ietf-dane-smime whose
   authors are Paul Hoffman, Jacob Schlyter and W. Griffin.

9.  References

9.1.  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.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

9.2.  Informative References

   [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.

   [RFC4255]  Schlyter, J. and W. Griffin, "Using DNS to Securely
              Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
              January 2006.

   [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.

Author's Address

   Paul Wouters
   Red Hat

   Email: pwouters@redhat.com

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