# File: caesardec.py
# Program to decrypt using Caesar cipher
#
# Matt Bishop, MHI 289I, Fall 2021
#

#
# this corresponds to a key of 'D' ('A' = 0, ... 'Z' = 25)
# it's near the top so we can find (and change) it easily
#
key = 3
#
# the decipherment function
#
def decipher(c, k):
        return (c + 26 - k) % 26
#
# now the main routine
#
def main():
        # ask user for input message (plaintext)
        try:
                cipher = input("Enter your message here: ")
        except:
                return

        # initialize output (ciphertext)
        plain = ""

        #
        # encipher each character and append it to current ciphertext
        #
        for i in cipher:
                # leave non-letters alone
                if i in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
                        # map the letter into 0 . . 25
                        n = ord(i) - ord('A')
                        # shift it
                        p = decipher(n, key)
                        # map it back into a letter
                        lett = chr(ord('A') + p)
                elif i in "abcdefghijklmnopqrstuvwxyz":
                        # map the letter into 0 . . 25
                        n = ord(i) - ord('a')
                        # shift it
                        p = decipher(n, key)
                        # map it back into a letter
                        lett = chr(ord('a') + p)
                else:
                        lett = i
                # now append it
                plain = plain + lett

        # print it, surrounded by quotes

        print("'%s' --> '%s'" % (cipher, plain))
#
# run the program
#
main()