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