def _xor_block(a, b):
    """ XOR two blocks of equal length. """
    return ''.join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a, b)])
def _xor_block ( a , b ) : return '' . join ( [ chr ( ord ( x ) ^ ord ( y ) ) for ( x , y ) in zip ( a , b ) ] )
XOR two blocks of equal length.