[Q] 128-bit symmetric encryption.

[Denis McCauley]

On Sat, 31 May 2003 15:24:22 -0400
Daniel Carrera <dcarrera@math.umd.edu> wrote:

> Yes, I'm aware of that.  But what I said is that a 128-bit key takes
2**64 
> times longer to crack by brute force than a a 64-bit key.  The 50%
factor 
> gets accounted for in the time that it takes to brute force a 64-bit
key.
> 
The point I was trying to make is that there's no "impossible" with
brute force attacks. An attacker COULD break the encryption at the very
first attempt, though the probability is extremely remote.

It's the improbability, not the impossibility, which makes a brute force
attack on 128-bit encryption not worth trying. Taking your base of the
cracking of a 64-bit key, trying 2**64 combinations on a 128-bit key
would almost certainly take longer and would have a probability of less
than one in a million million million of succeeding.

Which, I guess, answers your question anyway: 

> What I'm trying to find out is if there is any point at all in having
> a key length longer than 128 bits.

Cheers
--
=====================================
Denis McCauley
GPG/PGP keys at http://www.djmccauley.tk
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