1package math
 2
 3import (
 4	"crypto/rand"
 5	"io"
 6	"math/big"
 7)
 8
 9// IsSafePrime reports whether p is (probably) a safe prime.
10// The prime p=2*q+1 is safe prime if both p and q are primes.
11// Note that ProbablyPrime is not suitable for judging primes
12// that an adversary may have crafted to fool the test.
13func IsSafePrime(p *big.Int) bool {
14	pdiv2 := new(big.Int).Rsh(p, 1)
15	return p.ProbablyPrime(20) && pdiv2.ProbablyPrime(20)
16}
17
18// SafePrime returns a number of the given bit length that is a safe prime with high probability.
19// The number returned p=2*q+1 is a safe prime if both p and q are primes.
20// SafePrime will return error for any error returned by rand.Read or if bits < 2.
21func SafePrime(random io.Reader, bits int) (*big.Int, error) {
22	one := big.NewInt(1)
23	p := new(big.Int)
24	for {
25		q, err := rand.Prime(random, bits-1)
26		if err != nil {
27			return nil, err
28		}
29		p.Lsh(q, 1).Add(p, one)
30		if p.ProbablyPrime(20) {
31			return p, nil
32		}
33	}
34}