Re: Exchange random numbers (was: Re: netscape's response)

[Christian Wettergren <cwe@it.kth.se>]

| > Giving out contribution: 
| >      MD5(select_bits(my_seed, start_bit, stop_bit)) -> remote
| > Taking in contribution : 
| >      my_seed = my_seed XOR 
| >      ((select_low_bits(remote_contrib, contrib_width) << contrib_area)
| > 
| 
| People seem to think this kind of thing is obviously safe.  I'm not yet
| convinced.

Well, I'm not either, actually. But I think this might be better
than the current state of affairs, where every bit of your seed
is almost guessable. And it might also be an intermediate solution
until there is a good random seed hardware generator in every computer.

| By xoring in a quantity *chosen by your adversary*, you're essentially
| allowing related-key attacks on your stream cipher.  (Your PRNG is just
| a stream cipher, keyed with my_seed.)

I think you mustn't allow the any external partner to "contribute" at a
known and/or chosen offset into the buffer. You mustn't either accept 
"too much" contribution.

| Noone knows how secure most ciphers are against related-key attacks:
| related-key attacks are known to be very powerful (often more powerful
| than any other type); but very little research on this topic is available.
| You're treading on unknown ground.

Yes. But I wonder whether this isn't really about the battle between
"the pragmatists" vs "the purists" point of view wrt security? I see so
many very unsophisticated attacks out there that a related-key attack,
although possible and powerful, still is rather unlikely.

Could you quantify how powerful a related-key attack is, compared to
some other kind of attack? I don't know anything about this kind of 
attack, do you have any references?

| There's the also a small error in your specific algorithm.  Let
| 	 n = stop_bit - start_bit;
| presumably n is much less than the length of your seed.  Then a brute-force
| search over n bits will recover n bits of the seed -- this is a much faster
| cryptanalysis than a brute force over all bits of the seed.  This can
| probably be fixed by something like
| 	MD5(select_bits(MD5(my_seed))) -> remote,
| but the related-key uncertainties still remain.

Ok, noted. Maybe I should try to write down this "idea" for a proper review?
Hmmm.

/Christian