(Programming exercise) You will create a program decrypt.java
that will decrypt any ciphertext which was encrypted using a shift or a Vigenere
cipher. You must write the entire program yourself, and may not get any part of it
from other sources.
For each step below, answer the question and make sure your program implements
your answer. You should use the letter frequency data from Bishop, Figure 9.1.
(a) Your program should take one command-line input, which specifies a file containing 
the ciphertext. Your program should count the instances of each letter in the
input file (you may ignore non-alphabetic characters, and should not distinguish
between upper- and lower-case). Based on this data, how can your program
determine whether a substitution cipher or a Vigenere cipher was used?
(b) If the ciphertext was encrypted using a substitution cipher, how can you check
whether a shift cipher was used (recall that a shift cipher is a special type of
substitution cipher)? Assuming a shift cipher was used, how can your program
then decrypt the ciphertext?
(c) If the ciphertext was encrypted using a Vigenere cipher, how can your program
determine the length of the key that was used? Once you have determined the
key length, how can your program decrypt the entire ciph