elgamalcrypt.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.util.Random;
public class ElGamalCrypt {
    
    Parameter params;
    int pLength;
    BigInteger un=BigInteger.ONE;
    public ElGamalCrypt(Parameter params){
        this.params=params;
        pLength=params.getP().bitLength();
    }
    
    public BigInteger []chiffre(BigInteger m,PublicKey y){
        BigInteger c1,c2;
        BigInteger k=new BigInteger(pLength-1,new Random());
        c1=params.getG().modPow(k, params.getP());
        c2=y.getY().modPow(k, params.getP()).multiply(m).mod(params.getP());
        BigInteger []ret={c1,c2};
        return ret;
    }
    
    public BigInteger dechiffre(BigInteger []c,PrivateKey x){
        BigInteger c1=c[0];
        BigInteger c2=c[1];
        BigInteger tmp=c1.modPow(params.getP().subtract(x.getX()).subtract(un), params.getP());
        BigInteger m=tmp.multiply(c2).mod(params.getP());
        return m;
    }
    
}