brea_xor.py

#!/usr/bin/env python
import sys

def score(text):
        charset = "_{}abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789,.'\n"
        p = 0
        for s in text:
                if s in charset or s == ' ' or s == '\'':
                        p+=1
        return p

# function that performs XOR operation on two strings
def xor(s1, s2):
        res = ""
        for i in range(0, len(s1)):
                res += chr(ord(s1[i]) ^ ord(s2[i%len(s2)]))

        return res

def main():
        best = ""
        b = 0

    # bruteforcing all possible values
        for i in range(1, 256):
                c = xor(sys.argv[1].decode('hex'), chr(i))
                if score(c) > b:
                        b = score(c)
                        best = c

        print "Plaintext: {}".format(best)

if __name__ == "__main__":
        main()