XORing long strings opimization?

[Noen]

def XOR(s1,s2):
	""" XOR string s1 with s2 """
	output = ""
	# Argument check
	if (type(s1) and type(s2)) != type(""):
		raise TypeError, "Arguments are not strings"
	if len(s1) != len(s2):
		raise ValueError, "Size differs in strings"
	# Xoring
	for c1 in s1:
		for c2 in s2:
			output += chr(ord(c1) ^ ord(c2))
	return output

This way is very slow for large strings.
Anyone know of a better and faster way to do it?