Vigenere with a known key

vigenere_key.py

#!/usr/bin/env python

def crack(x, y):
	"""For the length of string i, get the value of the character A=1, B=2 etc
	Subtract this from the value of the code at the relevant position
	Modulo 26 to remove an remainders/negatives."""
	result = ""
	for i in range(len(x)):
		result += chr((((ord(x[i].lower())) - (ord(y[i % 21].lower()))) % 26) + 97).upper()
	return result