Computer Science

A Vigenère cipher uses a sequence of “shift-by-n” simple substitutions, where the shifts are indexed using a keyword, with “A” representing a shift-by-0, “B” representing a shift-by-l, etc. For example, if the keyword is “DOG,” then the first letter is encrypted using a simple substitution with a shift-by-3, the second letter is encrypted using a shift-by-14, the third letter is encrypted using a shift-by-6, and the pattern is repeated—the fourth letter is encrypted using a shift-by-3, the fifth letter is encrypted using a shift-by-14, and so on. Cryptanalyze the following ciphertext, i.e., determine the plaintext and the key. This particular message was encrypted using a Vigenère cipher with a 3-letter English keyword:

CTMYR DOIBS RESRR RIJYR EBYLD IYMLC CYQXS RRMLQ FSDXF OWFKT CYJRR IQZSM X

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Suppose that you have a message consisting of 1024 bits. Design a method that will extend a key that is 64 bits long into a string of 1024 bits, so that the resulting 1024 bits can be XORed with the message, just like a one-time pad. Is the resulting cipher as secure as a one-time pad? Is it possible for any such cipher to be as secure as a one-time pad?

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