removed dependency to old code

utils.go

@@ -11,6 +11,31 @@ var ONE = big.NewInt(1)
 var TWO = big.NewInt(2)
 var FOUR = big.NewInt(4)
 
+//  returns n! = n*(n-1)*(n-2)...3*2*1
+func Factorial(n int) *big.Int {
+	ret := big.NewInt(1)
+	for i := 1; i <= n; i++ {
+		ret = new(big.Int).Mul(ret, big.NewInt(int64(i)))
+	}
+	return ret
+}
+
+//  Returns 2 primes such that p = 2 * q + 1 and that the length of
+//  p is nbits.  `p` is called a safe prime
+func GenerateSafePrimes(nbits int, random io.Reader) (p, q *big.Int, err error) {
+	for {
+		q, err = rand.Prime(random, nbits-1)
+		if err != nil {
+			return
+		}
+		p = (new(big.Int)).Mul(q, big.NewInt(2))
+		p = p.Add(p, big.NewInt(1))
+		if p.ProbablyPrime(50) { //a probability of 2**-100 of not being prime
+			return
+		}
+	}
+}
+
 // Generate a random element in the group of all the elements in Z/nZ that
 // has a multiplicative inverse.
 func GetRandomNumberInMultiplicativeGroup(n *big.Int, random io.Reader) (*big.Int, error) {
@@ -26,3 +51,14 @@ func GetRandomNumberInMultiplicativeGroup(n *big.Int, random io.Reader) (*big.In
 	return r, nil
 
 }
+
+//  Return a random generator of RQn with high probability.  THIS METHOD
+//  ONLY WORKS IF N IS THE PRODUCT OF TWO SAFE PRIMES! This heuristic is used
+//  threshold signature paper in the Victor Shoup
+func GetRandomGeneratorOfTheQuadraticResidue(n *big.Int, rand io.Reader) (*big.Int, error) {
+	r, err := GetRandomNumberInMultiplicativeGroup(n, rand)
+	if err != nil {
+		return nil, err
+	}
+	return new(big.Int).Mod(new(big.Int).Mul(r, r), n), nil
+}