def _PERM_OP(a, b, n, m):
 t = (((a >> n) ^ b) & m)
 b = (b ^ t)
 a = (a ^ (t << n))
 return (a, b)

def _PERM_OP ( a , b , n , m ) : t = ( ( ( a >> n ) ^ b ) & m ) b = ( b ^ t ) a = ( a ^ ( t << n ) ) return ( a , b )
cleverer bit manipulation .