package org.bouncycastle.crypto.generators;
import java.math.BigInteger;
import java.security.SecureRandom;
import org.bouncycastle.crypto.params.DHParameters;
public class DHParametersGenerator
{
private int size;
private int certainty;
private SecureRandom random;
private static BigInteger ONE = BigInteger.valueOf(1);
private static BigInteger TWO = BigInteger.valueOf(2);
public void init(
int size,
int certainty,
SecureRandom random)
{
this.size = size;
this.certainty = certainty;
this.random = random;
}
/**
* which generates the p and g values from the given parameters,
* returning the DHParameters object.
* <p>
* Note: can take a while...
*/
public DHParameters generateParameters()
{
BigInteger g, p, q;
int qLength = size - 1;
//
// find a safe prime p where p = 2*q + 1, where p and q are prime.
//
for (;;)
{
q = new BigInteger(qLength, certainty, random);
p = q.multiply(TWO).add(ONE);
if (p.isProbablePrime(certainty))
{
break;
}
}
//
// calculate the generator g - the advantage of using the 2q+1
// approach is that we know the prime factorisation of (p - 1)...
//
for (;;)
{
g = new BigInteger(qLength, random);
if (g.modPow(TWO, p).equals(ONE))
{
continue;
}
if (g.modPow(q, p).equals(ONE))
{
continue;
}
break;
}
return new DHParameters(p, g, q, 2);
}
}