import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Random;


public class DSASign {
  public BigInteger p,q,g;
  public BigInteger x,y;
  
  public BigInteger _randomInZq(){
   BigInteger r= null;
   do {
   System.out.print(".");
   r = new BigInteger(160, new SecureRandom());
   }while(r.compareTo(q) >=0);
   System.out.print(".");
   return r;
  }
  
  public BigInteger _hashInZq(byte m[]){
   MessageDigest md;
   try {
   md = MessageDigest.getInstance("SHA-1");
   md.update(m);
   byte b[] = new byte[17];
   System.arraycopy(md.digest(), 0, b, 1, 16);
   return new BigInteger(b);
   }catch (NoSuchAlgorithmException e){
   System.out.print("This cannot happen!");
   }
   return null;
  }
  
  public void initKeys(){
   q = new BigInteger(160, 100, new SecureRandom());
   do {
   BigInteger t = new BigInteger(512, new SecureRandom());
   p = t.multiply(q).add(BigInteger.ONE);
   System.out.println("~");
   }while(!p.isProbablePrime(100));
   BigInteger h = _randomInZq();
   g = h.modPow(p.subtract(BigInteger.ONE).divide(q), p);
   x = _randomInZq();
   y = g.modPow(x, p);
   System.out.println("p : " + p);
   System.out.println("q : " + q);
   System.out.println("g : " + g);
   System.out.println("x : " + x);
   System.out.println("y : " + y);
  }
  
  public BigInteger[] signature(byte m[]){
   BigInteger k = _randomInZq();
   BigInteger sig[] = new BigInteger[2];
   sig[0] = g.modPow(k, p).mod(q);
   sig[1] = _hashInZq(m).add(x.multiply(sig[0])).mod(q)
   .multiply(k.modInverse(q)).mod(q);
   return sig;
  }
  public boolean verify(byte m[], BigInteger sig[]){
   BigInteger w = sig[1].modInverse(q);
   BigInteger u1 = _hashInZq(m).multiply(w).mod(q);
   BigInteger u2 = sig[0].multiply(w).mod(q);
   BigInteger v = g.modPow(u1, p).multiply(y.modPow(u2, p)).mod(p).mod(q);
   System.out.println("v = " + v);
   System.out.println("r = " + sig[0]);
   return v.compareTo(sig[0]) == 0;
  }
  
  public static void main(String args[]){
   DSASign dsa = new DSASign();
   dsa.initKeys();
   String message = "My name is xxx, my student number is xxxx.";
   System.out.println(message);
   BigInteger sig[] = dsa.signature(message.getBytes());
   System.out.println("DSASignture verifies result:" + dsa.verify(message.getBytes(),sig) );
  }
  
}