At 08:12 PM 4/16/96 -0400, Perry E. Metzger wrote:
Are you sure you want to claim that the text of Hamlet would make a good key for a one-time pad?
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It is far, far more probable for the cryptanalyst, thinking the key was "Hamlet", to get out a plausible but totally bogus text, than it is for the key to actually be "Hamlet".
I can agree with this.
Of course, it is also far, far more probable for you to be stupid than for a random number generator to put out "Hamlet".
I agree here too. I've been stupid many times, but I never expect to see a fair random number generator produce Hamlet. (I should live so long!)
but if you go around getting rid of RNGs that produce "Hamlet" or anything close, you have in theory given information to the attacker that gives them a slightly better chance of attacking you since your pads are no longer purely random.
And I could agree with this too, except that cryptanalysts do not consider every string to be equally likely. If they did, they would never even bother to look at XORing a bitstream with ciphertext to produce plaintext.
The reason all this isn't stupid to discuss and actually has some importance is just this fact. If you build a system that discards things that "don't look like they have enough entropy" (which certain people around here have proposed), you are giving the cryptanalyst a very strong piece of information about the key, so your key is no longer totally unpredictable.
This is true. But it is also unavoidable. Actually, I'm pleased to give up one-percent of my keyspace, if that's the one-percent that an analyst will check first. Another example: What if I selected a nonsense passphrase, "Dagmar shaved Howard's cocker spaniel" Not great, but adequate for my needs. If, by some wild coindence, a book by that title became a best seller, I would change my passphrase. A cryptanalyst who knew that was my feeling could simplify his cracking by not bothering to search for best selling book titles. On the other hand, a cryptanalyst who was not so convinced of my paranoia, and who DID check book titles, would not find my passphrase. I assume that BOTH philosophies would be used in a serious attack. When I do the math, it says that, assuming BOTH types of attack are done, it is better to have a passphrase that is not the title of a book.
An irony, but something important to keep in mind. Every once in a while (once in every four billion bits, or so) your random number generator will put out 32 1's in a row if it is functioning properly.
Agreed. And if that produces a "weak key" for your cipher, you'll get broken.
Any given small segment of the output of a good RNG might not look "random", but "random" isn't a property of a given number -- it is the property of the infinite sequence itself.
I agree here too. But the analyst doesn't see the infinite sequence, only the number itself. I am enjoying this discussion, but I feel like I'm running out of useful new ways to try to express this idea. If I don't reply, it doesn't mean you have convinced me. :)