How long would it take?

[Perry E. Metzger]

Derek Atkins says:
> Breaking IDEA would take a brute force attack (2^128 keys) unless
> something better comes up.

Its generally unwise to make the assumption that the only possible
attack on your conventional scheme is a brute force attack. Certainly
the attacks used on many previous generations of cryptosystems were
never brute force -- and certainly every generation of naive
cryptographer has said "well, using brute force it would take N years
to break my cypher". A simple vingenere cypher with a 12 letter key
would seem to be very strong indeed (stronger than DES), and yet we
know you can break one in a few moments because there are better
attacks than brute force.

We have suprisingly little in the way of general theory on what would
or would not make a conventional cryptosystem strong.  Certainly
differential cryptanalysis will not be the last thing people come up
with. Until we know everything the NSA knows, I will be hesitant to
say "unless something better comes up" and more comfortable saying
"until something better comes up."

Perry