Function Sign

cipher/dsa/dsa.go:125–149

Sign is signature generation for DSA 1. Choose a random integer k from the range [1, q-1] 2. Compute r = (g^k mod p) mod q 3. Compute s = (k^-1 * (H(m) + x*r)) mod q

(m []byte, p, q, g, x *big.Int)

// 2. Compute r = (g^k mod p) mod q
// 3. Compute s = (k^-1 * (H(m) + x*r)) mod q
func Sign(m []byte, p, q, g, x *big.Int) (r, s *big.Int) {
	// 1. Choose a random integer k from the range [1, q-1]
	k, err := rand.Int(rand.Reader, new(big.Int).Sub(q, big.NewInt(1)))
	if err != nil {
		panic(err)
	}

	// 2. Compute r = (g^k mod p) mod q
	r = new(big.Int).Exp(g, k, p)
	r.Mod(r, q)

	// 3. Compute s = (k^-1 * (H(m) + x*r)) mod q
	h := new(big.Int).SetBytes(m)     // This should be the hash of the message
	s = new(big.Int).ModInverse(k, q) // k^-1 mod q
	s.Mul(
		s,
		new(big.Int).Add( // (H(m) + x*r)
			h,
			new(big.Int).Mul(x, r),
		),
	)
	s.Mod(s, q) // mod q

	return r, s
}

// Verify is signature verification for DSA
// 1. Compute w = s^-1 mod q