Compression compatability [was Re: 10x compression factor]

David Shaw dshaw at
Tue Feb 10 13:39:01 CET 2004

On Tue, Feb 10, 2004 at 08:17:31AM -1000, Maxine Brandt wrote:
> On Tue, 10 Feb 2004 David Shaw wrote:
> > 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.
> Thanks for the precision, David. That was my understanding of the
> situation, until recently I had this problem occur in corresponding
> with a PGP 8 user whose key was definitely PGP-created. All my
> messages failed to decrypt because of a decompression failure. When
> I changed the preferences on my key, putting ZIP first, the problem
> disappeared.
> The thread I cited was started by a user of both GPG and PGP who had
> imported his GPG key into PGP without changing the preferences, but
> the same problem was reported in the thread by an organisation which
> used only PGP. Strange.

There are unfortunately many ways people can shoot themselves in the
foot with algorithm choices.  For example, the --compress-algo option
overrides the preferences and allows you do make a message that cannot
be understood by the recipient.

If GnuPG ever creates a ZLIB message when sending to a ZIP-only key,
that would be a bug.  I'd love to see a message like that, and what
settings were used to create it.


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