func gf2MatrixTimes(mat []uint64, vec uint64) uint64 {
	var sum uint64

	for vec != 0 {
		if vec&1 != 0 {
			sum ^= mat[0]
		}
		vec >>= 1
		mat = mat[1:]
	}
	return sum
}

func gf2MatrixSquare(square, mat []uint64) {
	if len(square) != len(mat) {
		panic("square matrix size mismatch")
	}
	for n := range mat {
		square[n] = gf2MatrixTimes(mat, mat[n])
	}
}