gnupg/doc/OpenPGP

		    GnuPG and OpenPGP
		    =================

   See RFC2440 for a description of OpenPGP.  I have an annotated version
   of this RFC online:	http://www.d.shuttle.de/isil/gnupg/rfc2440.html



  Compatibility Notes
  ===================
   GnuPG (>=0.4.1) is in compliance with RFC2440 despite these exeptions:

    ===> Please can someone check this <=========

    * (5.1) The critical bit in signature subpackets is currently
      ignored.	This will be fixed soon.


    * (5.2) GnuPG generates V4 signatures for all V4 keys.  The option
      --force-v3-sigs allows to override.

    * (5.3) GnuPG has an option to use simple S2K for "Symmetric-Key
      Encrypted Session-Key Packets"; however a warning message is
      issued if this option is active.

    * (5.5.2) states that an implementaion MUST NOT create a v3 key
      with an algorithm other than RSA.  GnuPG has an option to
      create an ElGamal key in a v3 packet; the properties of such
      a key are as good as a v4 key.  RFC1991 does not specifiy how
      to create fingerprints for algorithms other than RSA and so it
      is okay to choose a special format for ElGamal.

    * (9.1) states that RSA SHOULD be implemented.  This is not done
      (except with an extension, usable outside the U.S.) due to
      patent problems.

    * (9.2) states that IDEA SHOULD be implemented.  This is not done
      due to patent problems.

    * (12.1) states that an implementaion MUST NOT use a symmetric
      algorithm which is not in the preference list.  GnuPG has an
      option to override this.

    * 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.

   All MAY features are implemented with this exception:

    * multi-part armored messages are not supported.
      MIME should be used instead.

   Most of the OPTIONAL stuff is implemented.




  Some Notes on OpenPGP / PGP Compatibility:
  ==========================================

     * PGP 5.x does not accept V4 signatures for anything other than
       key material.

     * 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
       validation function.

     * 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
       signature.

     * 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
       SHA-1 signatures.

     * 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
       key.

     * 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
		   draft-ietf-openpgp-formats-07.txt


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