On Friday, November 24, 2000, 9:58:48 PM, Thomas wrote:
JR>> I don't know enough to discuss the point mathematically or with
JR>> any great knowledge, but I have the following observations
JR>> about the one time pad if I understood what I read. In the
JR>> first place the key has to be larger than the text which makes
JR>> it a little impractical for some uses
> I didn't read that article, but in practice, people would start
> reusing the key from the beginning if the text is larger than the key.
I think you both misunderstand what one-time-pad is. :) In a true
one-time-pad system, the key is exactly of the same length of the
text. (In the article Alex quoted in his first post, the key length
is one "block" longer, and the extra block is used to signify where
in the "code book" one should start, and is not part of the
encrypting key.)
Because with the OTP system, every bit is encrypted with a random
bit, it's unbreakable ASSUMING the code book isn't intercepted,
which is the weak link as both of you have pointed out. The code
book also has to be very large because each key can be used only
once, and the key has to be as long as the text. Hence OTP, while
ideal in theory, isn't very practical in day-to-day encryption
practice. It's used only sparingly for extremely important purposes.
One other thing. OTP is a symmetric system, so you and each one
you're communicating with need a common code book to begin with.
> By the way, if anybody has ever seen the film U-571, this whole
> thing was the point of stealing the enigma machine. Messages could
> not be decrypted when intercepted, so the Americans needed the
> machine, as the one-time pads were stored in there.
I haven't seen the film, but I have seen two documentary about the
Enigma machine (one is actually about Allen Turing). The Enigma
isn't a true OTP system, since they reuse the key. And stealing the
machine itself isn't enough, they had to know the code used to
synchronize two machines in communication. Fortunately (or
unfortunately for Nazis), they were too lazy to change the code
often, and they (the Navy especially, IIRC) delivered the code in a
careless way.
> (Historically the film was nonsense, as the Americans were not
> even in the war at the time this film claims to be set. Never
> mind, that's Hollywood.)
According to the documentary I saw, the Americans, while not
officially in the war at the time, were indeed cooperating with the
Brits in solving the Enigma. (IIRC, it's a Polish underground
operation that stole the machine, and sold it to the Brits. Too bad
I've recycled the tape, and I have never been known for good
memories. :( Sorry, folks. Perhaps someone others have seen those
documentaries on PBS.)
--
Best regards,
Ming-Li
The Bat! 1.48 Beta/7 | Win2k SP1
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