elgamalsign.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 ElGamalSign {
    private BigInteger r;
    private BigInteger s;
    private static final BigInteger UN = BigInteger.ONE;
    public ElGamalSign(Parameter params,PrivateKey x,byte[]M){
        
        sha256 sh=new sha256(M);
        int[]sha=sh.getHash();
        byte[]hash=new byte[32];
        int i=0;
        for(int j=0;j<8;j++){
            hash[i++]=(byte)(sha[j] & 0xFF);
            hash[i++]=(byte)((sha[j]>>>8) & 0xFF);
            hash[i++]=(byte)((sha[j]>>>16) & 0xFF);
            hash[i++]=(byte)((sha[j]>>>24) & 0xFF);
        }
        BigInteger m=new BigInteger(hash);
        BigInteger k=new BigInteger(params.getP().bitLength()-1,new Random());
        BigInteger p1=params.getP().subtract(UN);
        while(!k.gcd(p1).equals(UN)){
            k=new BigInteger(params.getP().bitLength()-1,new Random());
        }
        r=params.getG().modPow(k,params.getP());
        k=k.modInverse(p1);
        s=k.multiply(m.subtract(r.multiply(x.getX()))).mod(p1);
    }
    
    public BigInteger getR(){
        return r;
    }
    public BigInteger getS(){
        return s;
    }
}