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Date: Wed, 6 Jul 2022 15:01:19 -0400
From: Demi Marie Obenour <>
 Denial of service in  GnuPG

On Wed, Jul 06, 2022 at 04:50:11PM +0200, Solar Designer wrote:
> On Wed, Jul 06, 2022 at 09:47:28AM -0400, Demi Marie Obenour wrote:
> > On Wed, Jul 06, 2022 at 03:38:10PM +0200, Solar Designer wrote:
> > > On Wed, Jul 06, 2022 at 07:02:59AM -0400, Demi Marie Obenour wrote:
> > > > Was adding compression to PGP even a good idea in the first place?
> > > 
> > > I think actually yes, it was, especially back then.  It has probably
> > > helped more than it hurt in PGP's lifetime so far.
> > 
> > Interesting.  Why do you say that?
> Oh, I didn't feel this even needed explanation, and I feel silly writing
> the below and don't really have time for it (lesson re-learned: should
> have stayed silent), but well.
> PGP is commonly used on compressible data (such as text), and PGP
> messages are then transferred over a network and finally stay in
> people's mailboxes or such.  Bandwidth was commonly low back then, and
> storage much more limited than today's.
> Some compression existed for unencrypted messages - some network links
> somewhat compressed (e.g., V.42bis), some mail clients supported mailbox
> compression, and of course a mailbox could also be compressed manually.
> Obviously, already encrypted content is not compressible.
> Without built-in compression in PGP, its messages would be slower to
> transfer and larger to store.  Compression would need to be performed
> before PGP, which would be an inconvenience and would lead to similar
> risks, especially if automated, and would often not be done.  In PGP,
> it's just one standard way to do it, not more than one.
> So compression was of some benefit to a lot of people.  We could argue
> that it's little benefit, but multiplied by the number of people it's
> significant.  Was compression also a problem for a lot of people?
> Theoretically, yes, but in practice those attacks were not common.
> We could also argue that PGP never became popular, MUA integrations are
> poor, etc., and as a consequence that its individual features were not
> of a lot of benefit to computer users at large.  While true, that
> argument also means the risks associated with those features did not
> apply to most computer users.  So it's irrelevant.
> What I say is that for the geeks using PGP, compression was overall of
> more benefit than risk.
> Oh, and I'm also grateful for compression in SSH, despite of my own
> criticism of its effect on security.
> Alexander

Thanks!  I had not considered this at all, and it makes a ton of sense.
Being able to compress data prior to encryption is indeed necessary if
one wants to avoid using more space than a message compressed using
normal methods.
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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