parameter.java

ElGamal method is an aymetric method to crypt message. You can use ElGamal to crypt the key session

package elgamal;
/*
import java.math.BigInteger;
import java.security.SecureRandom;

public class Parameter
{
	private BigInteger g;
	private BigInteger p;
	
	private int size;
	private int certainty;
	private SecureRandom random;
	
	private static final BigInteger ONE = BigInteger.valueOf(1);
	private static final BigInteger TWO = BigInteger.valueOf(2);
	
	public Parameter(int size, int certainty, SecureRandom random)
	{
		this.size = size;
		this.certainty = certainty;
		this.random = random;
		generateParameters();
	}
	
	private void generateParameters()
	{
		BigInteger g, p, q;
		int qLength = size - 1;
		
		while(true)
		{
			q = new BigInteger(qLength, 1, random);			
			if (q.bitLength() != qLength)
			{
				continue;
			}
			if (!q.isProbablePrime(certainty))
			{
				continue;
			}
			p = q.multiply(TWO).add(ONE);
			if (p.isProbablePrime(certainty))
			{
				break;
			}
		}
		this.p = p;
		
		while (true)
		{
			g = new BigInteger(qLength, random);
			if (g.modPow(TWO, p).equals(ONE))
			{
				continue;
			}
			if (g.modPow(q, p).equals(ONE))
			{
				continue;
			}
			break;
		}
		this.g = g;
	}
	
	
	public BigInteger getP()
	{
		return p;
	}
	
	public BigInteger getG()
	{
		return g;
	}
}


*/
import java.math.BigInteger;
import java.util.Random;

public class Parameter
{

    private BigInteger g;
    private BigInteger p;
    private BigInteger q;
    private int pLength;
    private int certainty;
    private static final BigInteger UN = BigInteger.ONE;
    private static final BigInteger DEUX = new BigInteger("2");

    public Parameter(int pLength, int certainty)
    {
        this.pLength = pLength;
        this.certainty = certainty;
        generateParameters();
    }

    private void generateParameters()
    {
         BigInteger k;
         while(true){
            k = new BigInteger(8,new Random());
            q = new BigInteger(pLength-1-k.bitLength(),certainty, new Random());
            p = q.multiply(DEUX).multiply(k).add(UN);

            while(!p.isProbablePrime(certainty))
            {         
                k = k.add(UN);
                p = DEUX.multiply(q).multiply(k).add(UN);
                if(p.bitLength()>pLength)break;
            }
            if(p.bitLength()==pLength)break;
         }
        BigInteger expo[] = new BigInteger[5];
        expo[0] = k.multiply(q);
        expo[1] = DEUX.multiply(k);
        while(k.mod(DEUX).intValue() == 0) k = k.divide(DEUX);
        int i = 2;
        BigInteger diviseur = DEUX.add(UN);
        while(diviseur.compareTo(k) <= 0) 
        {
            if(k.mod(diviseur).intValue() == 0)
            {
                expo[i++] = DEUX.multiply(q).multiply(k.divide(diviseur));
                while(k.mod(diviseur).intValue() == 0) k = k.divide(diviseur);
            }
            diviseur = diviseur.add(DEUX);
        }

        boolean ok = true;
        while(ok)
        {
            g = new BigInteger(pLength-1, new Random());
            int j = 0;
            while(true)
            {
                if(j >= i)
                {
                    break;
                }
                BigInteger h = g.modPow(expo[j], p);
                if(h.equals(UN))
                {
                    ok = false;
                    break;
                }
                j++;
            } 
            if(ok)
            {
                break;
            }
            ok = true;
        } 
        
       
    }

    public BigInteger getP()
    {
        return p;
    }

    public BigInteger getG()
    {
        return g;
    }

}