4 See RFC-4880 for a description of OpenPGP. These notes are older
5 than RFC-4880 and refer to the predecessor of the specs (RFC-2440).
10 GnuPG (>=1.0.3) is in compliance with RFC2440 despite these exceptions:
12 * With GnuPG >= 2.1.0 all support for version 3 keys has been
13 removed. Thus there is no more compatibility with PGP-2. Users
14 who need to be able to decrypt old PGP 2 messages should use
15 GnuPG 1.4.x along with the option --allow-weak-digest-algos.
17 * With GnuPG >= 2.1.0 all signatures (on messages and keys) are
18 created using version 4 signatures. Support for verifying
19 version 3 signature is still available.
21 * (9.2) states that IDEA SHOULD be implemented. This is not done
22 due to patent problems.
23 UPDATE: Since version 1.4.13 (or GnuPG 2.x with Libgcrypt 1.6)
24 IDEA support has been added to allow decryption of old
25 PGP-2 encrypted material.
27 All MAY features are implemented with this exception:
29 * multi-part armored messages are not supported.
30 MIME (rfc2015) should be used instead.
32 Most of the OPTIONAL stuff is implemented.
34 There are a couple of options which can be used to override some
35 RFC requirements. This is always mentioned with the description
38 A special format of partial packet length exists for v3 packets
39 which can be considered to be in compliance with RFC1991; this
40 format is only created if a special option is active.
41 UPDATE: This support has been removed with version 1.3.6.
43 GnuPG uses a S2K mode of 101 for GNU extensions to the secret key
44 protection algorithms. This number is not defined in OpenPGP, but
45 given that this number is in a range which is used at many other
46 places in OpenPGP for private/experimental algorithm identifiers,
47 this should be not a too bad choice. The 3 bytes "GNU" are used to
48 identify this as a GNU extension - see the file DETAILS for a
49 definition of the used data formats.
52 Some Notes on OpenPGP / PGP Compatibility:
53 ==========================================
55 * PGP 5.x does not accept V4 signatures for anything other than
56 key material. The GnuPG option --force-v3-sigs mimics this
59 * PGP 5.x does not recognize the "five-octet" lengths in
60 new-format headers or in signature subpacket lengths.
62 * PGP 5.0 rejects an encrypted session key if the keylength
63 differs from the S2K symmetric algorithm. This is a bug in its
66 * PGP 5.0 does not handle multiple one-pass signature headers and
67 trailers. Signing one will compress the one-pass signed literal
68 and prefix a V3 signature instead of doing a nested one-pass
71 * When exporting a private key, PGP 2.x generates the header
72 "BEGIN PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY
73 BLOCK". All previous versions ignore the implied data type, and
74 look directly at the packet data type.
76 * In a clear-signed signature, PGP 5.0 will figure out the correct
77 hash algorithm if there is no "Hash:" header, but it will reject
78 a mismatch between the header and the actual algorithm used. The
79 "standard" (i.e. Zimmermann/Finney/et al.) version of PGP 2.x
80 rejects the "Hash:" header and assumes MD5. There are a number
81 of enhanced variants of PGP 2.6.x that have been modified for
84 * PGP 5.0 can read an RSA key in V4 format, but can only recognize
85 it with a V3 keyid, and can properly use only a V3 format RSA
88 * Neither PGP 5.x nor PGP 6.0 recognize ElGamal Encrypt and Sign
89 keys. They only handle ElGamal Encrypt-only keys.
92 Parts of this document are taken from:
93 ======================================
95 OpenPGP Message Format
96 draft-ietf-openpgp-formats-07.txt
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