Deniable Cryptography [was winnowing, chaffing etc]

[Julian Assange <proff@iq.org>]

mgraffam@mhv.net writes:

> I figure the best we can do is to hide the contents of S with crypto and
> hide its existence through other means. Traditional stego works well 
> for this latter goal, but it does not give us a way to cough up something
> meaningful in place of S, which could be very handy.
> 
> In short, certainly the existence of S needs to be hidden, and it would be
> best to do hide it in plain sight as it were, in a big junk pile with
> everything else on the drive. 
> 
> Indexing this huge mess of data to allow for a practical system to work
> with is certainly a challenge, and in all likelyhood impossible given the
> parameters of the system.
> 

Marutukku (my rubber-hose proof filing system) addresses most of these
technical issues, but I'd like to just comment on the best strategy
game-theory wise, of the person wielding the rubber-hose.

In Marutukku the number of encrypted extents (deniable "virtual"
partitions) defaults to 16 (although is theoretically unlimited). As
soon as you get over about 4 pass-phrases, the excuse "I can't recall"
or "there's nothing else there" starts to sounding highly plauseable.

Ordinarily best strategy for the rubber-hose wielder is to keep on
beating keys out of (let us say, Alice) indefinitely till there are no
keys left. However, and importantly, in Marutukku, *Alice* can never
prove that she has handed over the last key. As Alice hands over more
and more keys, her attackers can make observations like "the keys
Alice has divulged correspond to 85% of the bits". However at no point
can her attackers prove that the remaining 15% isn't simply
unallocated space, and at no point can Alice, even if she wants to,
divulge keys to 100% of the bits, in order to get the un-divulged
portion down to 0%. An obvious point to make here is that
fraction-of-total-data divulged is essentially meaningless, and both
parties know it - the launch code extent may only take up .01% of the
total bit-space.

What I find interesting, is how this constraint on Alice's behaviour
actually protects her from revealing her own keys, because each party,
at the outset can make the following observations:

Rubber-hose-squad: We will never be able to show that Alice has
		   revealed the last of her keys. Further, even if
		   Alice has co-operated fully and has revealed all of
		   her keys, she will not be able to prove it.
		   Therefor, we must assume that at every stage that
		   Alice has kept secret information from us, and
		   continue to beat her, even though she may have
		   revealed the last of her keys. But the whole time
		   we will feel uneasy about this because Alice may
		   have co-operated fully. Alice will have realised this
		   though, and so presumably it's going to be very hard
		   to get keys out of her at all.


Alice:		   (Having realised the above) I can never prove that I
		   have revealed the last of my keys. In the end I'm
		   bound for continued beating, even if I can buy
		   brief respites by coughing up keys from time to
		   time. Therefor, it would be foolish to divulge my
		   most sensitive keys, because (a) I'll be that much
		   closer to the stage where I have nothing left to
		   divulge at all (it's interesting to note that this
		   seemingly illogical, yet entirely valid argument of
		   Alice's can protect the most sensitive of Alice's
		   keys the "whole way though", like a form
		   mathematical induction), and (b) the taste of truly
		   secret information will only serve to make my
		   aggressors come to the view that there is even
		   higher quality information yet to come, re-doubling
		   their beating efforts to get at it, even if I have
		   revealed all. Therefor, my best strategy would be
		   to (a) reveal no keys at all or (b) depending on
		   the nature of the aggressors, and the psychology of
		   the situation, very slowly reveal my "duress" and
		   other low-sensitivity keys.

Alice certainly isn't in for a very nice time of it (although she
she's far more likely to protect her data).

On the individual level, you would have to question whether you might
want to be able to prove that, yes, infact you really have surrendered
the last remaining key, at the cost of a far greater likelihood that
you will. It really depends on the nature of your opponents.  Are they
intelligent enough understand the deniable spect of the cryptosystem
and come up with the above strategy? Determined to the extent they
are will to invest the time and effort in wresting the last key out of
you? Ruthless - do they say "Please", hand you a Court Order, or is it
more of a Room 101 affair?

But there's more to the story.

Organisations and groups may have quite different goals in terms of
key retention vs torture relief to the individuals that comprise them,
even if their views are otherwise co-aligned. I'm not talking about
some mega-complex multinational 8 level hierarchy. A simple democratic
union of two or more people will exhibit this behaviour.

When a member of a group, who uses conventional cryptography to
protect group secrets is rubber-hosed, they have two choices (1)
defecting (by divulging keys) in order to save themselves, at the cost
of selling the other individuals in the group down the river or (2)
staying loyal, protecting the group and in the process subjugating
themselves continued torture.

With Marutukku-style deniable cryptography, the benefits to the
individual derived from choosing tactic (1) are largely
eliminated. Individuals that are "otherwise loyal" to the group, will
realise this and choose tactic (2).

Presumably most people in the group do not want to be forced to give
up their ability to choose defection. On the other hand, no one in the
group wants anyone (other than themselves) in the group to be given
the option of defecting against the group (and thus
themselves). Provided no individual is certain* they are to be
rubber-hosed, every individual will support the adoption of a
group-wide Marutukku-style cryptographically deniable crypto-system.

* Actually a complicated threshold.

Cheers,
Julian