Re: Double DES calculations

[DAVESPARKS@delphi.com]

Hal Finney wrote:

> I'll give you one similar example, though.  I think this is the technique
> used in Pollard "rho" factoring.  You have an iterated series, x=f(x), and
> you want to know if it has any cycles, any values which are eventually
> repeated.  At first glance you might think that to look for a cycle of
> length N you would have to store N values of the series and check each
> value for a match, taking order of N in time and space.  The Pollard tech-
> nique instead runs two copies of the iteration at once, one twice as fast
> as the other: x=f(x) and y=f(f(y)).  Each time you just compare x and y
> for a match.  This takes about twice as long but uses no memory.

The thread was concerning the vulnerability of Double-DES with an
intermediate layer of IDEA in the middle.  It was proposed that if IDEA
could ultimately be TRIVIALLy cracked, then DES-IDEA-DES was no stronger
than Double-DES.  At that point I did some "back of the envelope"
calculations on the cost of breaking Double-DES using a MITM attack.

I'm not sure how "cycles" fit into DES.  The brute-force technique I was
hypothesizing involved trying all possible keys on the encrypt and decrypt
sides, storing them the resultant 64 bit blocks (all 2^60 bytes of them),
then comparing them.  How would Pollard rho speed that up?

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