Compression compatability [was Re: 10x compression factor]

[David Shaw]

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On Mon, Feb 09, 2004 at 08:46:00PM -1000, Maxine Brandt wrote:
> On Mon, 9 Feb 2004  David Shaw wrote
> Subject: Re: 10x compression factor
> 
> > If they are compressing within PGP, then it's ZIP compression, as
> > that's the only compression algorithm that PGP supports.  
> 
> This raises a point about compatability of GPG keys with PGP
> 8.x. The default compression preferences in GPG are Z2 Z1 (ZLIB ZIP)
> and decompression failures with GPG-encrypted messages were reported
> by PGP 8.0.3 users in a recent thread on the PGP-Basics list.
> 
> PGP keys have no compression (or hash) preferences as far as I can
> see by importing a PGP key and doing gpg --pref, so it seems GPG
> uses ZLIB even if PGP 8 can't handle it, and apparently has no way
> of finding from the PGP key that it shouldn't.

This isn't what happens.  GnuPG treats having no compression
preferences as a preference for ZIP.  This is part of the OpenPGP
standard.  Thus, it will always use ZIP when encrypting to a PGP key,
and PGP has ZIP, of course.

There are occasional problems when someone generates a key in GnuPG
(thus having a "ZLIB ZIP" preference), then moves that key over to PGP
for use without updating their preferences.  Programs encrypting to
that key see the ZLIB preference and use it.

The problem here is actually in PGP, which should warn the user when
they import the key that they are advertising preferences that cannot
be honored.  The new OpenPGP draft standard recommends this.

Nevertheless, since PGP does not currently have the ability to rewrite
preferences, this has to be done on the GnuPG side.  It's a FAQ, in
fact:

   http://www.gnupg.org/(en)/documentation/faqs.html#q5.9

David
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