Cryptography Analysis The operator of a Vigenere encryption machine is bored and encrypts a plaintext consisting of the same letter of the alphabet repeated several hundred times. The key is a six-letter English word. Eve knows that the key is a word but does not yet know its length. • (a) What property of the ciphertext will make Eve suspect that the plaintext is one repeated letter and will allow her to guess that the key length is six? • (b) Once Eve recognizes that the plaintext is one repeated letter, how can she determine the key? (Hint: You need the fact that no English word of length six is a shift of another English word.) • (c) Suppose Eve doesn’t notice the property needed in part (a), and therefore uses the method of displacing then counting matches for finding the length of the key. What will the number of matches be for
Cryptography Analysis
The operator of a Vigenere encryption machine is bored and encrypts a plaintext consisting of the same letter of the alphabet repeated several hundred times. The key is a six-letter English word. Eve knows that the key is a word but does not yet know its length.
• (a) What property of the ciphertext will make Eve suspect that the plaintext is one repeated letter and will allow her to guess that the key length is six?
• (b) Once Eve recognizes that the plaintext is one repeated letter, how can she determine the key? (Hint: You need the fact that no English word of length six is a shift of another English word.)
• (c) Suppose Eve doesn’t notice the property needed in part (a), and therefore uses the method of displacing then counting matches for finding the length of the key. What will the number of matches be for the various displacements? In other words, why will the length of the key become obvious by this method?
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