slides6 - CS 531 Fall 2007 One-time Pad The one-time pad...

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T 1 One-time Pad CS 531, Fall 2007 Copyright © William C. Cheng i.e., if a cryptanalyst has c 1 c 2 ...c t , the cryptanalyst can do no better than guess at the plaintext i.e., any t - bit binary strings are equally likely as plaintext The one-time pad can be shown to be theoretically unbreakable the plaintext can be any binary string of length t It has been proven that to realize an unbreakable system requires a random key of the same length as the message this reduces the practicality of one-time pad Myth or reality? (until very recently) communication line between Moscow and Washington was secured by a one-time pad transport of key done by trusted courier
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T 2 The Key Space CS 531, Fall 2007 Copyright © William C. Cheng a key is typically a compact way to specify the encryption transformation Size of key space is the number of encryption/decryption key pairs that are available in the cipher system e.g., a transposition cipher of block length t has t! encryption functions to choose from, each can be simply described by a permutation which is called the key a necessary, but usually not sufficient , condition for an encryption scheme to be secure is that the key space be large enough to preclude exhaustive search
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