Program contains the main function that uses BigInteger class. It gets the
user input for two numbers and operator, creates two BigInteger objects, and
then applies the operator by calling a function in BigInteger.It finally prints
the expression and result to the screen.
Implements a four-square cipher algorithm. Program asks the user for both keys,
and whether they want to encrypt or decrypt.
If they want to encrypt, program asks for the original message,
and then print the encrypted result.
If user wants to decrypt, program asks for the encrypted message and then print
the decrypted result. All spaces are ignored in all messages and results.