# File: caesardec.py
# Program to encrypt using Caesar cipher
#
# Matt Bishop, MHI 289I, Fall 2022
#

#
# 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 encipherment function
#
def encipher(p, k):
        return (p + k) % 26
#
# now the main routine
#
def main():
        # ask user for input message (plaintext)
        try:
                plain = input("Enter your message here: ")
        except:
                return

        # initialize output (ciphertext)
        cipher = ""

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

        # print it, surrounded by quotes

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