From: Adam Back <aba@atlas.ex.ac.uk>
Date: Fri, 19 Jul 1996 08:52:00 +0800
To: jim@ACM.ORG
Subject: Re: Opiated file systems
In-Reply-To: <199607162030.NAA10344@mycroft.rand.org>
Message-ID: <199607181001.LAA00078@server.test.net>
MIME-Version: 1.0
Content-Type: text/plain



Jim Gillogly <jim@acm.org> writes:
> "Deranged Mutant" <WlkngOwl@unix.asb.com> writes:
> >A problem with a c'punk-style encrypted fs with source code and wide 
> >distribution is, of course, that attackers will KNOW that there is a 
> >duress key.
>
> Good point.  This suggests a design desideratum for any such system should
> be that the user may choose not to have a duress key, maintaining
> semi-plausible deniability for those who choose to have one.

For plausibility it would probably be best if very few people used the
duress key feature.

If PGP had an infrequently used duress key feature, it would provide
quite a bit of plausible deniability: lots of people have PGP.

This was the basis for comments earlier in this thread about it being
desirable to have a very popular file system with these features
included.  The more users (mostly for it's normal features) the less
suspicious having the software on your system becomes.

One problem is that some of the additional requirements to do a good
job of obscuring whether or not there is data in the unused part of an
encrypted file system add overheads.

For example re-encrypting the unused data with random IVs so that it
doesn't appear stale even if the duress key feature was not requested.
If that overhead is too great it will be annoying for people who do
not wish to use the duress key feature.  It might possibly be a good
idea to do re-encrypting of the blocks anyway as it would obscure
usage patterns.  (eg I am thinking when the disk starts up it will be
cold, as it warms up the heads will be positioned fractionally
differently, and from this kind of analysis it might be possible to
make inferences about the amount of data used in the file system,
etc.)

Adam
--
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