def add512bit(a, b):
    """ Add two 512 integers
    """
    a = bytearray(a)
    b = bytearray(b)
    cb = 0
    res = bytearray(64)
    for i in range(64):
        cb = a[i] + b[i] + (cb >> 8)
        res[i] = cb & 255
    return res
