#!/usr/bin/env python
#
# bitdiff -- Determine differing bits in hexadecimal hash values
#
# From: Rick van Rein <rick@openfortress.nl>


import sys

print "This program demonstrates the sensitivity of hash values in hex representation"
print "The expectation value of changing bits between different inputs is always 50.0%%"

def countbits (nr):
	if nr < 0:
		raise 'Counting bits of negative numbers is undefined'
	bits = 0
	while nr > 0:
		if nr & 0x01:
			bits = bits + 1
		nr = nr >> 1
	return bits


def hex2long (strval):
	numval = 0L
	for ch in strval:
		numval = numval << 4
		if ch in '0123456789':
			numval = numval + ord (ch) - ord ('0')
		elif ch in 'abcdef':
			numval = numval + ord (ch) - ord ('a') + 10
		elif ch in 'ABCDEF':
			numval = numval + ord (ch) - ord ('A') + 10
		else:
			raise ("Naughty character '%c' found in hexstring" % ch)
	return numval


sys.stdout.write ('Hash value #1: ');
h1 = sys.stdin.readline ()
if h1 [-1] == '\n':
	h1 = h1 [:-1]

sys.stdout.write ('Hash value #2: ');
h2 = sys.stdin.readline ()
if h2 [-1] == '\n':
	h2 = h2 [:-1]

if len (h1) != len (h2):
	raise "Hash lengths differ -- cannot continue"

if len (h1) == 0:
	raise "Hashes are empty -- not very convincing, ej?"

numbits = 4*len (h1)
hash1 = hex2long (h1)
hash2 = hex2long (h2)
diffbits = countbits (hash1 ^ hash2)
perc = (100.0 * diffbits) / (1.0 * numbits)

print "%d out of %d bits different, or %3.1f%%\n" % (diffbits, numbits, perc)