This document contains a Python program that implements Caesar's cipher to encrypt and decrypt messages. It uses lists of uppercase and lowercase letters to shift letters by a key value provided by the user. The program gets a message, key, and encryption/decryption mode from the user and prints the translated text. It then asks if the user wants to encrypt another message or quit.

1. #Marie Dimond
#11-12-2015
#dimond_caesar_cipher.py
#Caesar's Cipher using lists and while loops, encodes a message
#Constants
#Uppercase alpabet list for uppercase letters
UPPER_ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
#Uppercase alpabet list for uppercase letters
LOWER_ALPHABET = "abcdefghijklmnopqrstuvwxyz"
#Repeats game until user says no
while True:
#Get user message
def getMessage():
print("Enter your message:")
return input()
#Get user key
def getKey():
key = 0
while True:
print ("Enter the key number (1-26):")
try:
key = int(input())
except ValueError:#Checks if value is interger
print("Please enter a valid number.")
continue
if key >= 1 and key <= 26:#Between 1 and 26
print("Key accepted")#Breadcrumb
return key
break
else:
print ("Please enter a number between 1 and 26")
def getMode():
while True:# <=== Need this for repition
print("Do you wish to encrypt (e) or decrypt (d) a message?")
mode = input().lower()
if mode == 'd':
print("Decrypt") #Breadcrumb
return mode #mode 'd'
print()
break #Exit

2. elif mode == 'e':
print("Encrypt")#Breadcrumb
return mode #mode 'e'
print()
break
else:
print("Please enter (e) or (d).")#error
#Making the funtion returns equal the variables
message = getMessage()
print()
key = getKey()
print()
mode = getMode()
print()
#Easy to read lise for reader's convience
print("Message: " + str(message))
print("Key: " + str(key))
print("Mode: " + str(mode))
print()
#Translate messages using two lists and indexing
def Translation(message, key, mode):
cipher = " "
if mode [0] == "d":
key = -key
for c in message:
if c in UPPER_ALPHABET:
cipher += UPPER_ALPHABET[(UPPER_ALPHABET.index(c)+key)
%(len(UPPER_ALPHABET))]
elif c in LOWER_ALPHABET:
cipher += LOWER_ALPHABET[(LOWER_ALPHABET.index(c)+key)
%(len(LOWER_ALPHABET))]
else:
cipher = cipher + c #Does nothing to empty spaces
return cipher
#Prints translated message
print ("Your translated text is:")
print()
print(Translation(message, key, mode))
print()
print("Do you wish to encrypt or decrypt another message?")
print("(y) Yes, (n) No")

3. mode = input().lower()
if mode == 'y':
print("Great!")
print()
continue
elif mode == 'n':
print("Thank you for playing.")
break
else:
print("Please enter (y) or (n).")
print()
#Notes
#Sources:
#https://www.youtube.com/watch?v=WXIHuQU6Vrs
#http://inventwithpython.com/hacking/chapter17.html(Free online book)
#https://inventwithpython.com/chapter14.html
#https://courses.cs.washington.edu/courses/cse142/11au/python/week4/encrypt.py
#Above used for cipher = cipher + c
#Glossary
#Breadcrumb = Developement tool to check functions,
#can be removed post-beta test