Problem 1: Given a language with only three letters a, b, and c that occur with
frequencies .7, .2, .1, resp. The following cipher text was generated by a
Vigenere cipher (shifts mod 3 instead of 26):

CAAABBCACBCABACAABCCCACA.

Show that the likely key length is 2 and determine the most likely key,
all with explanation of course.