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10. Suppose there is a language that has only the letters a and b. The frequency of the letter a is .1 and the frequency of b is .9. A message is encrypted using a Vigenre cipher (working mod 2 instead of mod 26). The ciphertext is BABABAAABA. (a) Show that the key length is probably 2. (b) Using the information on the frequencies of the letters, determine the key and decrypt the message. 11. Suppose you have a language with only the 3 letters a, b, c, and they occur with frequencies .7, .2, .1, respectively. The following ciphertext was encrypted by the Vigenre method (shifts are mod 3 instead of mod 26, of course): ABCBABBBAC