Network Working Group A. Smasher
Internet-Draft S. Josefsson
Expires: November 26, 2005 May 25, 2005
The OpenPGP mail and news header
draft-josefsson-openpgp-mailnews-header-01
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes the OpenPGP mail and news header field. The
field provide information about the sender's OpenPGP key.
See <http://josefsson.org/openpgp-header/> for more information.
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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 . . . . . . . . . . . . . . . . . . . . 5
4. Comments . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7
10. Copying conditions . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1 Normative References . . . . . . . . . . . . . . . . . . 8
11.2 Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . 10
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1. Preface
This document is intended to define the "OpenPGP" message header.
This header should be considered "informational" (and "optional"),
and be suitable for both mail [8] and netnews [1] messages. This
header should be used to provide information about the sender's
OpenPGP [7] key. This header MAY be used in any message.
This document should be interpreted within the context of RFC 2822.
In the event of a discrepancy, refer to that document.
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 [4].
2. Background and Motivation
There are quite a few PGP and GnuPG users who add headers with
information about the sender's OpenPGP key. Headers in current use
include "X-PGP:", "X-PGP-Key:", "X-Request-PGP:", "X-PGP-KeyID:", and
"X-PGP-Fingerprint:". The headers lack standardization, which
prevent them from being reliably parsed automatically by
applications, rather than manually parsed by humans.
Since both PGP and GnuPG rely on the OpenPGP protocol, it appear
preferable to use the term "OpenPGP" rather than "PGP", or "GPG", in
the header name. The latter forms appear as underhanded attempts to
advocate specific applications, rather than the open standard they
both share. The header specified here is named "OpenPGP".
The OpenPGP header is not a required part of successful use of
OpenPGP in e-mail. It is intended as a convenience, in those
situations where the user experience may be enhanced by using the
information in this header. Consequently, the information in this
header should not disrupt the normal OpenPGP key retrieval and web of
trust mechanism. Neither the integrity nor the authenticity of the
information in this header should be assumed to be correct or be
trust-worthy.
No specific scenario when the header should be used, nor how it
should be used in that scenario, are suggested by this document. It
is acknowledged that the dominant use of the information in this
header may be by humans and not applications.
To promote good use of this header, care should be taken so that
applications do not trigger error messages that may annoy the user,
when an error condition arise during handling of the OpenPGP header.
It is generally recommended that an implementation ignore the
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presence of the OpenPGP header if an error condition occur. Since
the header 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
This header is intended to present characteristics of the sender's
OpenPGP key. It may contain the Key ID and the URL where the key can
be retrieved.
This header is of a "structured" type (see section 2.2.2 of RFC
2822). In general, the structure consist of one or more parameters,
each consisting of one attribute and one value. The terminology and
format of the header was inspired by MIME [2]. The various
provisions of RFC 2045 apply. In particular, the value part of all
parameters may be quoted; whitespace, foldoing and comments may occur
in the middle of parameters. The provisions of MIME [3] also apply;
in particular it deals with handling parameters of excessive length.
In the Augmented BNF [5] notation, an OpenPGP header field is defined
as below. By itself, however, this grammar is incomplete. It refers
by name to several syntax rules that are defined by RFC 2822 and the
URI syntax document [6]. Rather than reproduce those definitions
here, and risk unintentional differences between the two, this
document refer the reader to RFC 2822 and RFC 2396 for the definition
of non-terminals.
Unrecognized parameters MUST be ignored. The grammar permit them to
allow for future extensions. This header SHOULD NOT appear more than
once within a message. A given parameter type (i.e., "id" or "url")
may appear no more than once.
openpgp := "OpenPGP:" id-or-url /
(openpgp-parameter *(";" openpgp-parameter))
CRLF
id-or-url := id / url
id := *HEXDIG
url := absoluteURI ; Defined in RFC 2396.
openpgp-parameter
:= ("id" "=" id) /
("url" "=" url) /
parameter ; See RFC 2045 for definition of parameter.
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3.1 Primary Key ID field: id
The "id" attribute=value pair, if present, MUST define the primary
key ID. The value MUST identify the key ID (in either short or long
form) or the fingerprint, all using the hexadecimal [14] notation.
The length of the field imply the kind of key id, i.e., short or long
form, or a v3 or v4 key.
Note that each of the following examples includes a comment, which is
optional.
id=12345678 (short key ID)
id=1234567890ABCDEF (long key ID)
id=1234567890ABCDEF01234567890ABCDEF0123456 (v4 fingerprint)
id=1234567890ABCDEF01234567890ABCDE (v3 fingerprint, deprecated)
3.2 Key URL field: url
The "url" attribute=value pair, 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 [12] or FTP [9]. The URL MUST be fully
qualified, MUST explicitly specify a protocol and SHOULD be
accessible on the public Internet.
For example:
url=http://example.org/pgp.txt
4. Comments
As discussed in section 3.2.3 of RFC 2822, 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 header field bodies 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 ways in which this header may be used.
This list is not meant to be exhaustive, but do reflect expected
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typical usages.
OpenPGP: 12345678
OpenPGP: id=12345678
OpenPGP: http://example.com/key.txt
OpenPGP: url=http://example.com/key.txt
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)
6. Open Issues
Should there be a "supports" field, that signal whether the sender
support inline PGP or PGP/MIME? As in supports="inline, mime" or
similar. Should it be in preferred priority order?
The ABNF definition is known to be under-specified.
7. Acknowledgements
The content of this document builds on discussions with (in
alphabetical order) Christian Biere, Patrick Brunschwig, Jon Callas,
Peter J. Holzer, Ingo Klocker, Werner Koch, Jochen Kupper, Charles
Lindsey, Aleksandar Milivojevic, Xavier Maillard, Greg Sabino
Mullane, Thomas Roessler, Moritz Schulte, Olav Seyfarth, Thomas
Sjogren, Paul Walker, and Steve Youngs. No doubt the list is
incomplete. We apologize to anyone we left out.
8. Security Considerations
These headers are intended to be a convenience in locating public
keys: They are neither secure nor intended to be. Since header
information is easy to spoof, information contained in headers should
not be trusted: The information must be verified. How the
information is verified is left as an exercise for the reader.
Applications that interpret the data within this header MUST NOT
assume that the data 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 this header
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 this header to retrieve a key, the application MAY ignore the
retrieved key if it is not the same key used to sign the message.
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This SHOULD be done before the newly retrieved key is imported into
the user's keyring.
The use of HTTPS [13], DNSSEC [10], SMTP STARTTLS [11], and other
secure protocols, may enhance the security of information conveyed
through this header, 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 retrived key against
the one provided in this header 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 send out junk email to arbitrary addresses and collect those
that report back to the crafted OpenPGP URL. To protect against
this, implementations MUST inform the user of that potential privacy
issue when retrieving keys from an URL provided by the mail headers
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 header, slightly varying
the content between messages can be used as a covert channel.
9. IANA Considerations
The IANA is asked to register the OpenPGP header, using the template
as follows, in accordance with RFC 3864 [15].
Header field name: OpenPGP
Applicable protocol: mail, netnews
Status: informational
Author/Change controller: IETF
Specification document(s): This document.
Related information: None
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10. Copying conditions
In addition to the IETF/ISOC copying conditions, the following
statement grant third parties further rights to this document.
Copyright (C) 2004 Atom Smasher
Copyright (C) 2004, 2005 Simon Josefsson
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.
11. References
11.1 Normative References
[1] Horton, M. and R. Adams, "Standard for interchange of USENET
messages", RFC 1036, December 1987.
[2] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[3] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word
Extensions: Character Sets, Languages, and Continuations",
RFC 2231, November 1997.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[6] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[7] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP
Message Format", RFC 2440, November 1998.
[8] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
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11.2 Informative References
[9] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,
RFC 959, October 1985.
[10] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[11] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595,
June 1999.
[12] 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.
[13] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[14] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[15] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
Authors' Addresses
Atom Smasher
Email: atom@smasher.org (0x762A3B98A3C396C9C6B7582AB88D52E4D9F57808)
Simon Josefsson
Email: simon@josefsson.org (0x0424D4EE81A0E3D119C6F835EDA21E94B565716F)
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