GnuPG and OpenPGP
See RFC-4880 for a description of OpenPGP. These notes are older
than RFC-4880 and refer to the predecessor of the specs (RFC-2440).
GnuPG (>=1.0.3) is in compliance with RFC2440 despite these exceptions:
* (9.2) states that IDEA SHOULD be implemented. This is not done
due to patent problems.
UPDATE: Since version 1.4.13 (or GnuPG 2.x with Libgcrypt 1.6)
IDEA support has been added to allow decryption of old
PGP-2 encrypted material.
All MAY features are implemented with this exception:
* multi-part armored messages are not supported.
MIME (rfc2015) should be used instead.
Most of the OPTIONAL stuff is implemented.
There are a couple of options which can be used to override some
RFC requirements. This is always mentioned with the description
of that options.
A special format of partial packet length exists for v3 packets
which can be considered to be in compliance with RFC1991; this
format is only created if a special option is active.
UPDATE: This support has been removed with version 1.3.6.
GnuPG uses a S2K mode of 101 for GNU extensions to the secret key
protection algorithms. This number is not defined in OpenPGP, but
given that this number is in a range which is used at many other
places in OpenPGP for private/experimental algorithm identifiers,
this should be not a too bad choice. The 3 bytes "GNU" are used to
identify this as a GNU extension - see the file DETAILS for a
definition of the used data formats.
Some Notes on OpenPGP / PGP Compatibility:
* PGP 5.x does not accept V4 signatures for anything other than
key material. The GnuPG option --force-v3-sigs mimics this
* PGP 5.x does not recognize the "five-octet" lengths in
new-format headers or in signature subpacket lengths.
* PGP 5.0 rejects an encrypted session key if the keylength
differs from the S2K symmetric algorithm. This is a bug in its
* PGP 5.0 does not handle multiple one-pass signature headers and
trailers. Signing one will compress the one-pass signed literal
and prefix a V3 signature instead of doing a nested one-pass
* When exporting a private key, PGP 2.x generates the header
"BEGIN PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY
BLOCK". All previous versions ignore the implied data type, and
look directly at the packet data type.
* In a clear-signed signature, PGP 5.0 will figure out the correct
hash algorithm if there is no "Hash:" header, but it will reject
a mismatch between the header and the actual algorithm used. The
"standard" (i.e. Zimmermann/Finney/et al.) version of PGP 2.x
rejects the "Hash:" header and assumes MD5. There are a number
of enhanced variants of PGP 2.6.x that have been modified for
* PGP 5.0 can read an RSA key in V4 format, but can only recognize
it with a V3 keyid, and can properly use only a V3 format RSA
* Neither PGP 5.x nor PGP 6.0 recognize ElGamal Encrypt and Sign
keys. They only handle ElGamal Encrypt-only keys.
Parts of this document are taken from:
OpenPGP Message Format
Copyright 1998 by The Internet Society. All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.