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package math
import (
"crypto/rand"
"io"
"math/big"
)
// IsSafePrime reports whether p is (probably) a safe prime.
// The prime p=2*q+1 is safe prime if both p and q are primes.
// Note that ProbablyPrime is not suitable for judging primes
// that an adversary may have crafted to fool the test.
func IsSafePrime(p *big.Int) bool {
pdiv2 := new(big.Int).Rsh(p, 1)
return p.ProbablyPrime(20) && pdiv2.ProbablyPrime(20)
}
// SafePrime returns a number of the given bit length that is a safe prime with high probability.
// The number returned p=2*q+1 is a safe prime if both p and q are primes.
// SafePrime will return error for any error returned by rand.Read or if bits < 2.
func SafePrime(random io.Reader, bits int) (*big.Int, error) {
one := big.NewInt(1)
p := new(big.Int)
for {
q, err := rand.Prime(random, bits-1)
if err != nil {
return nil, err
}
p.Lsh(q, 1).Add(p, one)
if p.ProbablyPrime(20) {
return p, nil
}
}
}
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