Thank you for your answer.It was obvious but I couldn't see it... I can't see this one too: how do you get from 2^64 keystream space to 2^53 stored data points?From what I understand from the 2^64 states 2^3 states can be excluded due to the fact that after the 100 steps, only 16% of the states can exist. >From the remaining 2^61 how you end up at 2^56.5 or 2^57? Or is it that you assume that 2^57(2^56.5) gives enough success rate with the given lookup keystreams and so it suffices, and from that point on taking into account the 100 steps you end up at 2^53? Thank you.
On Fri, Jan 14, 2011 at 10:43 PM, <[email protected]> wrote: > On Fri, Jan 14, 2011 at 05:28:27PM +0200, Tari Mrkis wrote: > > Hi, > > > > I haven’t understood a very basic point. > > > > If there is the possibility of backclocking why do we have to build the > > rainbow tables to begin with. I understand that if there is a hit in the > > It is not possible to clock back keystream. You have to know a A5/1 > register assignment to be able to reach neighbouring states through > forward/backclocking. You cannot know what the A5/1 register content > is from examining keystream (because it is a cryptographic one way > function). > So with the time memory tradeoff attack you can reverse that function > and end up anywhere in the state space of internal A5/1 registers along the > path that was taken to generate said keystream; from where > it is trivial to clock to a point where the frame number has been removed > from the state. > > > table, either this is the state we are looking for (next to get rid of > the > > frame number) OR this state can be forward clocked for 100 times and then > > backwards to reach some more valid states any of which can be the desired > > one. So why don’t simply backclock the 64 bit sequence, find the valid > > states and then backclock them all for 100 without the need to build > tables. > > From what I understand there is not a huge numbers of states, 1.4 states > on > > average for the 100 forward-back clocking, so for the 64 backwards > clocking > > this number may increase a little but not too much. > > > > Also one more question; the total size of the constructed tables is about > > 1,7 GB with 40 tables of 42 GB each, is it right? > > 48 tables of 42GB each. > _______________________________________________ > A51 mailing list > [email protected] > http://lists.lists.reflextor.com/cgi-bin/mailman/listinfo/a51 >
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