From: hal@finney.org
Date: Thu Sep 27 2001 - 15:35:34 MDT
Harvey writes:
> Nothing is uncrackable these days, even with brute-force. Encryption is a
> temporary solution at best. We must assume that at some date in the future
> supercomputers or networks of computers will scan all Dejanews/Google
> archives and decrypt all stored messages in the history of the Internet. No
> one should send anything encrypted over the Internet that you don't want
> read publicly in the future.
I think there is a lot of truth to this. (I would note though that not
many people send encrypted messages via Usenet or any other method which
gets into public archives.)
The largest key sizes in common use today are 256 bits. If quantum
computers become possible they effectively halve the key length, so it
would require 2^128 operations to brute force such a key. According to
Nanosystems, "a 1000 MIPS computer can occupy less than one cubic micron
and consume less than 0.1 microwatt of power" (page 19). One cubic
kilometer of such devices executes 2^120 instructions per second.
If you could create a quantum computer of the size and speed of Drexler's
nanocomputer, and put together a cubic kilometer of them, you could
brute force a 256 bit key in a matter of hours, consuming 1e8 terawatts.
So given the computational abilities which will be available in principle
over the next century, virtually no messages being sent today can be
expected to be secure from brute-forcing, assuming that quantum computers
are possible.
Now, you could construct a cipher with an even larger key, say 512 bits,
and some such do exist. It is not too difficult to calculate a key size
for which, even assuming quantum computers of unrealistically large power
(single-atom computers for example), there would not be enough atoms
and time in the entire universe to brute-force such a cipher.
Hal
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