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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.4.

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package org.bouncycastle.crypto.generators;

import java.math.BigInteger;
import java.security.SecureRandom;

import org.bouncycastle.math.ec.WNafUtil;
import org.bouncycastle.util.BigIntegers;

class DHParametersHelper
{
    private static final BigInteger ONE = BigInteger.valueOf(1);
    private static final BigInteger TWO = BigInteger.valueOf(2);

    /*
     * Finds a pair of prime BigInteger's {p, q: p = 2q + 1}
     * 
     * (see: Handbook of Applied Cryptography 4.86)
     */
    static BigInteger[] generateSafePrimes(int size, int certainty, SecureRandom random)
    {
        BigInteger p, q;
        int qLength = size - 1;
        int minWeight = size >>> 2;

        for (;;)
        {
            q = new BigInteger(qLength, 2, random);

            // p <- 2q + 1
            p = q.shiftLeft(1).add(ONE);

            if (!p.isProbablePrime(certainty))
            {
                continue;
            }

            if (certainty > 2 && !q.isProbablePrime(certainty - 2))
            {
                continue;
            }

            /*
             * Require a minimum weight of the NAF representation, since low-weight primes may be
             * weak against a version of the number-field-sieve for the discrete-logarithm-problem.
             * 
             * See "The number field sieve for integers of low weight", Oliver Schirokauer.
             */
            if (WNafUtil.getNafWeight(p) < minWeight)
            {
                continue;
            }

            break;
        }

        return new BigInteger[] { p, q };
    }

    /*
     * Select a high order element of the multiplicative group Zp*
     * 
     * p and q must be s.t. p = 2*q + 1, where p and q are prime (see generateSafePrimes)
     */
    static BigInteger selectGenerator(BigInteger p, BigInteger q, SecureRandom random)
    {
        BigInteger pMinusTwo = p.subtract(TWO);
        BigInteger g;

        /*
         * (see: Handbook of Applied Cryptography 4.80)
         */
//        do
//        {
//            g = BigIntegers.createRandomInRange(TWO, pMinusTwo, random);
//        }
//        while (g.modPow(TWO, p).equals(ONE) || g.modPow(q, p).equals(ONE));


        /*
         * RFC 2631 2.2.1.2 (and see: Handbook of Applied Cryptography 4.81)
         */
        do
        {
            BigInteger h = BigIntegers.createRandomInRange(TWO, pMinusTwo, random);

            g = h.modPow(TWO, p);
        }
        while (g.equals(ONE));


        return g;
    }
}