1 package math
2 3 import (
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.
13 func 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.
21 func 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 }
35