示例#1
0
文件: rsa.go 项目: ivanwyc/google-go
// randomNumber returns a uniform random value in [0, max).
func randomNumber(rand io.Reader, max *big.Int) (n *big.Int, err os.Error) {
	k := (max.Len() + 7) / 8

	// r is the number of bits in the used in the most significant byte of
	// max.
	r := uint(max.Len() % 8)
	if r == 0 {
		r = 8
	}

	bytes := make([]byte, k)
	n = new(big.Int)

	for {
		_, err = io.ReadFull(rand, bytes)
		if err != nil {
			return
		}

		// Clear bits in the first byte to increase the probability
		// that the candidate is < max.
		bytes[0] &= uint8(int(1<<r) - 1)

		n.SetBytes(bytes)
		if n.Cmp(max) < 0 {
			return
		}
	}

	return
}