Network Working Group A. Smasher
Internet-Draft S. Josefsson
Expires: July 8, 2005 January 7, 2005
The OpenPGP mail and news header
draft-josefsson-openpgp-mailnews-header-00
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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 Primary Key Algorithm field: algo . . . . . . . . . . . . 5
3.3 Primary Key Size field: size . . . . . . . . . . . . . . . 5
3.4 Key URL field: url . . . . . . . . . . . . . . . . . . . . 6
3.5 Key Creation Date field: created . . . . . . . . . . . . . 6
4. Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Copying conditions . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1 Normative References . . . . . . . . . . . . . . . . . . . . 8
10.2 Informative References . . . . . . . . . . . . . . . . . . . 8
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 [6] and netnews [1] messages. This
header should be used to provide information about the sender's
OpenPGP [5] 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 [2].
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, if used, is intended to present characteristics of the
sender's OpenPGP key, such as the primary key ID (or fingerprint),
algorithm, size and URL where the key can be found. This header is
of a "structured" type (see section 2.2.2 of RFC 2822). In general,
the structure consist of one or more attribute and value pairs. The
terminology and format of the header was inspired by MIME [8].
This header SHOULD NOT appear more than once within a message.
In the Augmented BNF [3] 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 [4]. 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.
openpgp := "OpenPGP:" id-or-url / (parameter *(";" parameter))
CRLF
id-or-url := id / url
id := ["0x"] (4*HEXDIG / 8*HEXDIG / 32*HEXDIG / 40*HEXDIG)
url := absoluteURI ; Defined in RFC 2396.
algo := *DIGIT ; Value in RFC 2440 section 9.1.
size := *DIGIT ; Primary key size in bits.
created := *DIGIT ; Correspond to four-octet number in
; RFC 2440 V3/V4 key packet that indicate
; the time the key was created, expressed as
; an unsigned decimal integer.
parameter := ("id" "=" id) /
("url" "=" url) /
("algo" "=" algo) /
("size" "=" size) /
("created" "=" created)
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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 MAY be prefixed with "0x". The value MUST
identify the key ID (in either short or long form) or the
fingerprint, all using the hexadecimal [13] notation.
A short key ID is a 32 bit key ID, represented as 8 characters.
A long key ID is a 64 bit key ID, represented as 16 characters.
A v4 fingerprint is a 160 bit key ID, represented as 40 characters.
A v3 fingerprint is a 128 bit key ID, represented as 32 characters.
Note that each of the following examples includes a comment, which is
optional.
id=12345678 (short key ID, no 0x prefix)
id=0x12345678 (short key ID)
id=0x1234567890ABCDEF (long key ID)
id=0x1234567890ABCDEF01234567890ABCDEF0123456 (v4 fingerprint)
id=0x1234567890ABCDEF01234567890ABCDE (v3 fingerprint)
3.2 Primary Key Algorithm field: algo
The "algo" attribute=value pair, if present, MUST specify the
algorithm of the primary key. The algorithm of the primary key MUST
be presented as the value defined in section 9.1 of RFC 2440 (Public
Key Algorithms). The value MUST be presented as an integer in
decimal notation.
Note that each of the following examples includes a comment, which is
optional.
algo=1 (RSA)
algo=17 (DSA)
3.3 Primary Key Size field: size
The "size" attribute=value pair, if present, MUST specify the size of
the primary key. The size of the primary key MUST be presented as
the number of bits in the key's modulus. The value MUST be presented
as an integer in decimal notation.
Note that one of the following examples includes a comment, which is
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optional.
size=1024
size=2048 (bits)
3.4 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 [11] or FTP [7]. 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
3.5 Key Creation Date field: created
This "created" attribute=value pair, if present, MUST define the
creation date of the primary key. The creation date of the primary
key MUST be presented as specified in section 5.5.2 of RFC2440
(Public Key Packet Formats). The value MUST be presented as a
integer in decimal notation.
Note that the following example includes a comment, which is
optional.
created=1104629466 (Sun Jan 2 01:31:06 UTC 2005)
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=0xB565716F (key stored on non-networked laptop)
id=0x12345678; algo=1 (RSA Encrypt or Sign)
id=0xABCD0123; created=1104629115 (Sun Jan 2 02:25:15 CET 2005)
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5. Examples
These are valid examples of ways in which this header may be used.
This list of examples is not meant to be exhaustive.
OpenPGP: id=0x12345678
OpenPGP: id=0x12345678; algo=1 (RSA); size=2048
OpenPGP: url=http://example.com/key.txt
OpenPGP: url=http://example.com/key.txt;
id=0x12345678 (this key is only used at the office)
6. Open Issues
Should the algo/size/created fields be included? They are supposedly
only there for v3 keys.
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?
7. Acknowledgements
The content of this document builds on discussions with (in
alphabetical order) Christian Biere, Patrick Brunschwig, Peter J.
Holzer, Ingo Klocker, Werner Koch, Jochen Kupper, Aleksandar
Milivojevic, Xavier Maillard, Greg Sabino Mullane, Thomas Roessler,
Moritz Schulte, 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 [12], DNSSEC [9], SMTP STARTTLS [10], 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.
Given the flexibility of the syntax of the header, slightly varying
the content between messages can be used as a covert channel.
9. 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
Copying and distribution of the work, with or without
modification, are permitted in any medium without royalty
provided the copyright notice and this notice are preserved.
10. References
10.1 Normative References
[1] Horton, M. and R. Adams, "Standard for interchange of USENET
messages", RFC 1036, December 1987.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[4] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[5] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
Message Format", RFC 2440, November 1998.
[6] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
10.2 Informative References
[7] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,
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RFC 959, October 1985.
[8] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[9] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[10] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595,
June 1999.
[11] 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.
[12] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[13] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
Authors' Addresses
Atom Smasher
EMail: atom@smasher.org (0x762A3B98A3C396C9C6B7582AB88D52E4D9F57808)
Simon Josefsson
EMail: simon@josefsson.org (0x0424D4EE81A0E3D119C6F835EDA21E94B565716F)
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