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Network Working Group                                         A. Smasher
Internet-Draft                                              S. Josefsson
Intended status: Informational                           August 28, 2014
Expires: March 1, 2015

                The "OpenPGP" mail and news header field


   This document describes the "OpenPGP" mail and news header field.
   The field provide information about the sender's OpenPGP key.

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 March 1, 2015.

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
   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.

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Table of Contents

   1.  Preface  . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Background and Motivation  . . . . . . . . . . . . . . . . . .  3
   3.  OpenPGP Header Field . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Primary Key ID field: id . . . . . . . . . . . . . . . . .  5
     3.2.  Key URL field: url . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Protection Preference Field: preference  . . . . . . . . .  6
   4.  Comments . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Copying conditions  . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Preface

   This document define the "OpenPGP" message header field.  This field
   is suitable for both mail [RFC5322] and netnews [RFC1036] messages,
   and is used to provide information about the sender's OpenPGP
   [RFC4880] key.

   This document should be interpreted within the context of [RFC5322].
   In the event of a discrepancy, refer to that document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Background and Motivation

   There are quite a few PGP and GnuPG users who add header fields with
   information about the sender's OpenPGP key.  Fields in current use
   include "X-PGP:", "X-PGP-Key:", "X-Request-PGP:", "X-PGP-KeyID:", and
   "X-PGP-Fingerprint:".  The fields are not standardized, so they
   cannot be reliably parsed automatically by applications, only parsed
   by humans.

   Since both PGP and GnuPG rely on the OpenPGP protocol, it appears
   preferable to use the term "OpenPGP" rather than "PGP", or "GPG", in
   the field name.  The latter forms appear as underhanded attempts to
   advocate specific applications, rather than the open standard they
   both share.  The field specified here is named "OpenPGP".

   The OpenPGP field is not a required part of successful use of OpenPGP
   in e-mail or other messages.  It is intended as a convenience, in
   those situations where the user experience may be enhanced by using
   the information in the field.  Consequently, the information in the
   field should not disrupt the normal OpenPGP key retrieval and web of
   trust mechanism.  Neither the integrity nor the authenticity of the
   information in the field should be assumed to be correct or trust-

   This document neither suggests a specific scenario of when the field
   should be used, nor how it should be used.  It is acknowledged that
   the dominant use of the information in the field may be by humans and
   not applications.

   To promote good use of the field, care should be taken so that
   applications do not trigger error messages that may annoy the user,
   when an error condition arises during handling of the OpenPGP field.
   It is generally recommended that an implementation ignore the

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   presence of an OpenPGP field if it would cause an error condition.
   Since the field is optional, this approach should not be difficult to
   implement.  The philosophy here is to enable an enhanced user
   experience.  Error messages rarely contribute to that goal.

3.  OpenPGP Header Field

   The OpenPGP header field is intended to present characteristics of
   the sender's OpenPGP key.  The field typically contains the Key ID
   and the URL where the key can be retrieved.

   Because the mail header is typically not integrity protected, the
   information conveyed in the OpenPGP header field MUST NOT be trusted
   without additional verification.  Some of the information given in
   this field may also be given in the OpenPGP key itself.  When these
   two sources conflict, users SHOULD favor the information from the
   OpenPGP key, as that information can be cryptographically protected.

   The field is of a "structured" type (see section 2.2.2 of RFC 5322).
   In general, the structure consist of one or more parameters, each
   consisting of one attribute and one value.  The terminology and
   format of the field was inspired by MIME [RFC2045].  The various
   provisions of RFC 2045 apply.  In particular, the value part of
   parameters may be quoted; whitespace, folding and comments may occur
   in the middle of parameters; except as noted below.

   The OpenPGP header field is defined below in the Augmented BNF
   [RFC5234] notation.  By itself, however, this grammar is incomplete.
   It refers by name to syntax rules that are defined in [RFC5322] and
   [RFC3986].  Rather than reproduce those definitions here, and risk
   unintentional differences between the two, this document refers the
   reader to the other documents for the definition of non-terminals.

   Implementations MUST understand the "id", "url", and "preference"
   attributes.  Parameter with unrecognized attributes MUST be ignored.
   The grammar permits unknown parameters to allow for future
   extensions.  Each parameter attribute (e.g., "url") MUST NOT occur
   more than once in any single instance of the OpenPGP field.  The
   OpenPGP field itself MAY occur more than once in a single email (for
   example if the sender has multiple keys).

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   openpgp     = "OpenPGP:" o-params CRLF
                   ; CFWS is defined in RFC 5322.
                   ; CRLF is defined in RFC 5234.

   o-params    = (o-parameter *(";" o-parameter))

   o-parameter = *CFWS "id" "=" id *CFWS
               / *CFWS "url" "=" url *CFWS
               / *CFWS "preference" "=" preference *CFWS
               / *CFWS parameter *CFWS ; normally unused, for extensions
                   ; parameter is defined in RFC 2045.

   id          = 1*(8HEXDIG)
                   ; HEXDIG is defined in RFC 5234.
                   ; Matching of value is case-insensitive.

   url         = folded-uri / quoted-url
                   ; If the URL contains the character ";",
                   ; the quoted-url form MUST be used.

   quoted-url  = DQUOTE folded-uri DQUOTE
                   ; DQUOTE is defined in RFC 5234.

   folded-uri  = <absolute-URI, but free insertion of FWS permitted>
                   ; absoluteURI is defined in RFC 3986.
                   ; FWS is defined in RFC 5234.

   preference  = "sign" / "encrypt" / "signencrypt" / "unprotected"
                   ; Matching of values is case-insensitive.

   The folded-URI MAY contain folding whitespace (FWS, [RFC5322]), which
   is ignored.  To convert a folded-URI to a absolute-URI, first apply
   standard [RFC5322] unfolding rules (replacing FWS with a single SP),
   and then delete any remaining un-encoded SP characters.  Folding may
   be used to shorten long lines.

3.1.  Primary Key ID field: id

   The "id" parameter, if present, MUST hold the Key ID or key
   fingerprint for the primary key.  The value uses the hex [RFC4648]
   notation.  The parameter value is case-insensitive.

   The length of the field determines whether it denotes a Key ID (8 hex
   symbols), a long Key ID (16 hex symbols), a v3 key fingerprint (32
   hex symbols), or a v4 key fingerprint (40 hex symbols).

   Note that each of the following examples includes a comment, which is

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       id=12345678 (short key ID)
       id=1234567890ABCDEF (long key ID)
       id=1234567890abcdef0123456789ABCDEF01234567 (v4 fingerprint)
       id=1234567890ABCDEF0123456789ABCDEF (v3 fingerprint, deprecated)

3.2.  Key URL field: url

   The "url" parameter, if present, MUST specify a URL where the public
   key can be found.  It is RECOMMENDED to use a common URL family, such
   as HTTP [RFC2616] or FTP [RFC0959].  The URL MUST be fully qualified,
   MUST explicitly specify a protocol and SHOULD be accessible on the
   public Internet.

   The content of where the URL points SHOULD be either an ASCII armored
   or binary OpenPGP packet containing the key.  A valid reason for
   storing something else may be if the key has been revoked.

   For example:


   If the URL contains the character ';' the entire URL MUST be quoted,
   as illustrated in the example.

3.3.  Protection Preference Field: preference

   The "preference" parameter, if present, specify the quality of
   protection preferred by the sender.  The parameter value is case-

   The available values are as follows.  The "unprotected" token means
   that the sender prefers not to receive OpenPGP protected e-mails.
   The "sign" token means that the sender prefers to receive digitally
   signed e-mails.  The "encrypt" token means that the sender prefers to
   receive encrypted e-mails.  A "signencrypt" token means that the
   sender prefers to receive encrypted and signed e-mails.

   Note that there is no normative requirement on the receiver to follow
   the stated preference.

   For example:


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   As discussed in section 3.2.2 of RFC 5322, comments may appear in
   header field bodies.  Comments are not intended to be interpreted by
   any application, but to provide additional information for humans.

   The following are examples of OpenPGP fields with comments:

     id=B565716F (key stored on non-networked laptop)
     id=12345678 (1024 bit RSA Key for Encrypt or Sign)
     id=ABCD0123 (created Sun Jan  2 02:25:15 CET 2005)

5.  Examples

   These are valid examples of how the field may be used.  This list is
   not meant to be exhaustive, but to reflect expected typical usages.

     OpenPGP: id=12345678
     OpenPGP: url=http://example.com/key.txt
     OpenPGP: preference=unprotected
     OpenPGP: url=http://example.com/key.txt; id=12345678
     OpenPGP: id=12345678; url=http://example.com/key.txt;
     OpenPGP: url=http://example.com/key.txt (down 2-3pm UTC);
              id=12345678 (this key is only used at the office);
              preference=sign (unsigned emails are filtered away)
     OpenPGP: id=12345678; url="http://example.com/openpgp;key.txt"

6.  Acknowledgements

   The content of this document builds on discussions with (in
   alphabetical order) Christian Biere, Patrick Brunschwig, Jon Callas,
   Dave Evans, Alfred Hoenes, Peter J. Holzer, Ingo Klocker, Werner
   Koch, Jochen Kupper, William Leibzon, Charles Lindsey, Aleksandar
   Milivojevic, Xavier Maillard, Greg Sabino Mullane, Tim Polk, Thomas
   Roessler, Moritz Schulte, Olav Seyfarth, David Shaw, Thomas Sjogren,
   Paul Walker, and Steve Youngs.  No doubt the list is incomplete.  We
   apologize to anyone we left out.

7.  Security Considerations

   The OpenPGP header field is intended to be a convenience in locating
   public keys; it is neither secure nor intended to be.  Since the
   message header is easy to spoof, information contained in the header
   should not be trusted.  The information must be verified.

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   Applications that interpret the field MUST NOT assume that the
   content is correct, and MUST NOT present the data to the user in any
   way that would cause the user to assume that it is correct.
   Applications that interpret the data within the field SHOULD alert
   the user that this information is not a substitute for personally
   verifying keys and being a part of the web of trust.

   If an application receives a signed message and uses the information
   in the field to automatically retrieve a key, the application MAY
   ignore the retrieved key if it is not the same key used to sign the
   message.  This SHOULD be done before the newly retrieved key is
   imported into the user's keyring.

   The use of HTTPS [RFC2818], DNSSEC [RFC4033], SMTP STARTTLS
   [RFC3207], IMAP/POP3 STARTTLS [RFC2595] and other secure protocols,
   may enhance the security of information conveyed through this field,
   but does not guarantee any level of security or authenticity.
   Developers and users must remain aware of this.

   Version 3 OpenPGP keys can be created with a chosen key id (aka "the
   0xDEADBEEF attack").  Verifying the Key ID of a retrieved key against
   the one provided in the field is thus not sufficient to protect
   against a man-in-the-middle attack.  Instead, the web-of-trust
   mechanism should be used.

   If an attacker wants to check the validity of email addresses, he
   might email arbitrary addresses with a unique OpenPGP header field
   URL (presumably an URL under the attacker's control).  The attacker
   can verify the liveness of each email address by checking if the URL
   for each particular recipient has been retrieved.  To protect against
   this, implementations MUST inform the user of that potential privacy
   issue when retrieving keys from an URL provided by the field of an
   inbound email message: either when the feature is enabled or to be
   used for the first time or every time the MUA detects an unknown key.

   Given the flexibility of the syntax of the field, slightly varying
   the content between messages can be used as a covert channel.  This
   is already possible using other header fields in email, and thus the
   OpenPGP field does not introduce a new vulnerability here.

8.  IANA Considerations

   The IANA is asked to register the OpenPGP header field, using the
   template as follows, in accordance with RFC 3864 [RFC3864].

   Header field name: OpenPGP

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   Applicable protocol: mail, netnews

   Status: informational

   Author/Change controller: IETF

   Specification document(s): This document.

   Related information: None

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9.  References

9.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

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

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

9.2.  Informative References

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.

   [RFC1036]  Horton, M. and R. Adams, "Standard for interchange of
              USENET messages", RFC 1036, December 1987.

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

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

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

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   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

Appendix A.  Copying conditions

   Regarding this entire document or any portion of it, the authors
   makes no guarantees and is not responsible for any damage resulting
   from its use.  The authors grants irrevocable permission to anyone to
   use, modify, and distribute it in any way that does not diminish the
   rights of anyone else to use, modify, and distribute it, provided
   that redistributed derivative works do not contain misleading author
   or version information.  Derivative works need not be licensed under
   similar terms.

Authors' Addresses

   Atom Smasher

   Email: atom@smasher.org (762A3B98A3C396C9C6B7582AB88D52E4D9F57808)

   Simon Josefsson

   Email: simon@josefsson.org (9AA9BDB11BB1B99A21285A330664A76954265E8C)

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