#!/usr/bin/env python
# crypt.py

import string
characters=list(string.printable)[:-3]
#       Instead of using ASCII values for this program, I have designed it to encrypt/decrypt
#       from a given character set(list(string.printable)[:-3]), allowing the replacement
#       of the above with any other desired.

def displace(list, value, distance):
    "Retrieves a specified value from a list and returns the value offset from it by distance"
    if list.index(value)+distance<len(list):
        return list[list.index(value)+distance]
    else:
        return list[(list.index(value)+distance)-len(list)]

def pair(text, key):
    "Pair one letter of key with each of text, returns dictionary of pairs"
    text=list(text)
    key=list(key)
    if len(key)<=len(text):
        key=key*(len(text)//len(key))
        for i in range(len(text)-len(key)):
            key.append(key[i])
        return zip(text, key)
    else:
        return zip(text, key)

def encrypt(text, key):
    "Encrypts text using key"
    ctext=[]
    for a,b in pair(text, key):
        ctext.append(displace(characters,a,characters.index(b)-len(key)))
    return "".join(ctext)

def decrypt(text, key):
    "Decrypts text using key"
    dtext=[]
    for a,b in pair(text, key):
        dtext.append(displace(characters,a,-characters.index(b)+len(key)))
    return "".join(dtext)

def getinput():
    "Continuously retrieves data from the user until stopped"
    choice=raw_input("Would you like to encrypt [2], decrypt [1], or exit [0]? ")
    if choice=='2':
        #Encrypt the text that is input
        print encrypt(raw_input("Enter the text to encrypt: "), raw_input("Enter the encryption key: "))
        print '\n'
        getinput()
    elif choice=='1' :
        #Decrypt the text that is input
        print decrypt(raw_input("Enter the text to decrypt: "), raw_input("Enter the decryption key: "))
        print '\n'
        getinput()
    else:
        print "Exiting..."

if __name__=="__main__": getinput()