Free Republic
Browse · Search
News/Activism
Topics · Post Article

To: BigBobber
Could the Quoran be an obvious but readily available one-time-pad for the terrorists?

No, since other people have copies of the exact same material. A one-time pad is absolutely secure if, and only if, the key is generated completely randomly and each part of it is used [at most] once and then never against used for any purpose whatsoever. If two messages are sent using the same pad, it will in many cases be possible for an interceptor to decrypt both of them.

It should be noted that the typical cryptosystem used with a one-time pad makes it trivially easy to derive the key given a ciphertext and the corresponding plaintext. If someone has a copy of the ciphertext and can make a good guess as to the original message, one will in so doing be able to make a good guess as to the part of the key used in processing that text. The basis for the security of the one-time pad is that being able to guess the key in such circumstances buys a would-be codebreaker absolutely nothing.

58 posted on 10/04/2001 11:43:32 PM PDT by supercat
[ Post Reply | Private Reply | To 57 | View Replies ]


To: supercat
Could the Quoran be an obvious but readily available one-time-pad for the terrorists?

To clarify another reason why this would be bad, suppose someone fond of 'book cyphers' encrypts a message using "alphabetic addition" and we intercept it. We intercept the message "CYEXEQTNLQHKHUIRYHROIMAVCPHKNFVRYO" and know that this person often begins his message with "GREETINGSX". So we subtract "GREETINGSX" from the letters at the start of the encrypted message and we get "WHATLIGHTT". If we guess the next word of his message might be "STRIKE", we end up with a key that starts "WHATLIGHTTPRQMYN". Not very promising. But if we happen to guess the next word is "ATTACK", we end up with a key that starts "WHATLIGHTTHROUGH".

At that point, it becomes very easy to guess that the key might continue "YONDERWINDOWBREAKS". If we guess that to be the case, decoding the rest of the message yields "ATELEVENPMTOMORROW".

Although I used Shakespeare for this example, the Quran is no less succeptible to that sort of analysis.

59 posted on 10/05/2001 12:01:23 AM PDT by supercat
[ Post Reply | Private Reply | To 58 | View Replies ]

Free Republic
Browse · Search
News/Activism
Topics · Post Article


FreeRepublic, LLC, PO BOX 9771, FRESNO, CA 93794
FreeRepublic.com is powered by software copyright 2000-2008 John Robinson