An Metet wrote:
By GINA KOLATA
http://www.nytimes.com/2001/02/20/science/20CODE.html?pagewanted=all
essentially, a one-time-pad with a central source of randomness, the key being the point in the random-number-stream that you start with.
"It's a cute idea, but it's simply unmanageable," Dr. Deavours said.
I agree completely on that, but for very different reasons. a) ok, it *is* unbreakable, since it's a one-time-pad. however, that only refers to mathematics/cryptoanalysis, not to the wide variety of other attacks. b)
He explained: "You can still get the message, but maybe not by cryptanalysis. If you're in this business, you go after a reasonably cheap, reliable method. It may be one of the three B's: burglary, bribery or blackmail. Those are right up there along with cryptanalysis in their importance."
of course, that's the first thing coming to mind. however, I have other reservations: c) the system's security rests entirely on the fact that the data volume is so "enormous" (10 mio. mio bits per second). or is it? high-energy research (CERN, etc.) today generates terrabytes of data everytime they hit the "on" switch. the computer centres there are equipped to handle two-digit terrabyte volumes in very short times. the problem is that the security of your message increases linear with the time you wait before sending it, because said time determines the likeliness that eve has run out of storage space and has started to overwrite old data. d) that, however, isn't even the worst problem. since bob also has to record the stream in order to decrypt the message, alice has to say "start" at a time X. all eve has to do is also hit the "record" button.(*) what this system does protect against is eve finding the message and her desire to decrypt it AFTER the fact. but it doesn't offer any advantage over existing systems in this area. the main danger of them is that either alice or bob store the one-time-pad or plaintext somewhere. that's a danger entirely outside the cryptosystem and not dealt with in this one, either. (*) one might think that no explicit "start" is required, since bob can just start grabbing the stream "live" while receiving the message. however, that requires perfect syncrocity(sp?) between alice and bob, something that is quite impractical at the requested flow rate. a solution here would be to have the random numbers broadcasted over the very medium which is used to transport the messages, much like the cycling in your PC. practical problems of a wide-area useage of this aside, it would only make it even easier for eve to decrypt the message, since she receives the key right alongside it. in other words: the whole encryption business would be a trivial waste of computing power. if that's not enough: e) the central source of randomness is sure to be a major target of any attacker. just by replacing the randomness with a seemingly-random function that you can easily recreate, eve would save tremendous amounts of storage space while lulling everyone in the impression of having an "unbreakable" cipher.