Re: Double DES calculations

[Hal <hfinney@shell.portal.com>]

I missed the start of this double-des thread due to system problems and
being gone, and I've never been able to pick up the main point since.  It
sounds like some kind of meet-in-the-middle attack is being discussed.
It is true that with current technology MITM generally seems more costly
in terms of space than time.  However, I have seen references to techniques
which shift this tradeoff some, costing more time and less space.  Un-
fortunately, I can't remember where I saw them!

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 moral is, be cautious about feeling safe against MITM attacks purely
because of memory limitations.  If you don't have protection on the time
costs as well there may be a tradeoff which can kill you.

Hal