#autokey cipher

#The idea is to shift the plaintext using the plaintext itself as a key,
#but to prepend a keyword to the plaintext before encryption.  So in a way
#it resembles a one-time pad.

#The true onetime pad, with len(plain)<=len(key)

import vigenere

def onetime(plain,key):
    cipher=''
    length=len(plain)
    for j in range(length):
        textletter=ord(plain[j])-ord('a')
        keyletter=ord(key[j])-ord('a')
        shiftvalue=(textletter+keyletter)%26
        cipherletter= chr(ord('a')+shiftvalue)
        cipher +=cipherletter
    return cipher

def autokey_encrypt(plain,key):
    return onetime(plain,key+plain)

def autokey_decrypt(cipher,key):
    plain=''
    for j in range(len(key)):
        plain+=chr(ord('a')+(ord(cipher[j])-ord(key[j]))%26)
    for j in range(len(key),len(cipher)):
        plain+=chr(ord('a')+(ord(cipher[j])-ord(plain[j-len(key)]))%26)
    return plain