GnuPG and OpenPGP
See RFC2440 for a description of OpenPGP. We have an annotated version
of this RFC online: http://www.gnupg.org/rfc2440.html
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.
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.
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 the fact that this number is in a range which used at many
other places in OpenPGP for private/experimenat algorithm identifiers,
this should be not a so 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
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