From: Adam Back <aba@dcs.ex.ac.uk>
Date: Sat, 20 Jul 1996 15:30:12 +0800
To: WlkngOwl@unix.asb.com
Subject: Re: Opiated file systems
In-Reply-To: <199607191718.NAA04087@unix.asb.com>
Message-ID: <199607191806.TAA00542@server.test.net>
MIME-Version: 1.0
Content-Type: text/plain



Rob <WlkngOwl@unix.asb.com> writes:
> On 18 Jul 96 at 11:01, Adam Back wrote:
> 
> > For plausibility it would probably be best if very few people used the
> > duress key feature.
> 
> And how can you guarantee that?

User apathy, people not reading documentation, documenting it as an
advanced feature... etc.

How many people actually generate a PGP key revocation cert in advance
in case of losing the key for instance?

> Also: an attacker doesn't care about what percentage of (other)
> users use duress feature of not.  His concern is whether you use it.

Good point.  But what other data does the attacker have aside from how
many others do?  Even knowing how many others do would be
tricky.. are they telling the truth when they say they aren't?

> Note that you'd have to be careful of what you say and do over email 
> in the clear (or encrypted to someone cooperating with an attacker): 
> if you post an excerpt of source code or maybe somehting like 
> Edupage, or if you save mail, there might be reason enough for the 
> attacker to expect to see some of that on your encrypted fs after 
> he's rubber-hosed your key from you.  If he doesn't, and he knows you 
> have a possibility of using the duress-key feature...

Smart analysis, yes you'd have to be very careful to partition the way
you used the two file systems.  You'd have to pretend that the 2nd
partition did not exist when comunicating with any one who you didn't
trust.  Perhaps you could have some assistance even... making the
duress file system read only when you have the hidden fs mounted as an
option to remove the chance of accidentally copying something from the
hidden fs that you couldn't (otherwise) explain being your possesion?

Someone cooperating with the attacker could be tricky though,
ultimately there's not much you can do about infiltration aside from
always using a nym for correspondence to do with your hidden persona
which goes with your hidden fs.

> Oh yeah. Psychology is a good way of determining the likelihood of 
> using a duress system.

Hmm, the psychological aspect of your plausible deniability.  Don't
think cryptographic protocols can do much about that.

> With the extra work and overhead of a duress system, you're better 
> off using stego on some gifs or graphics files.

But I don't think stego solves your whole problem: you still have to
have software to access the stegoed data.  Where do you store this?
Nearly back to square one.  (If the answer is on a floppy this applies
equally to a duress file system).

The one advantage of stegoed data is that you expect the least
sig. bits in image files to be random, where-as you don't expect the
LSBs in unused space (even in encrytped file systems once you're
inside the encryption layer) to be random.

However the disadvantage is 8 - 24 times reduction in space
efficiency.  (Your earlier point).

Adam