import java.math.BigInteger;
import java.security.SecureRandom;
import java.security.KeyPairGenerator;
import java.security.PublicKey;
import java.security.PrivateKey;
import java.security.Signature;
import java.security.KeyPair;
import java.security.interfaces.DSAPrivateKey;

class DSASigGen {
   public static void print_hex(byte buffer[])
      {
      for(int i = 0; i != buffer.length; ++i)
         {
         // There must be an easier way to do this (right?)
         int intval = buffer[i];

         if(intval < 0)
            intval += 256;

         String s = Integer.toHexString(intval);

         if(s.length() == 1)
            s = "0" + s;

         System.out.print(s);
         }
      System.out.println("");
      }

   /*
    Generate a DSA key and print 2 lines to stdout in hex
       ASN.1 DSA public key
       DSA signature of empty string with that key (exact string doesn't matter)
   */
   public static void main(String[] args)
      {
      try
         {
         KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DSA");

         /* JCE docs have prng setting a 'user seed', though of course
            without any guidance about how to actually create one.
            Doesn't seem to make any difference, but doing it here
            anyway. The guessing code does not make any reliance on
            what is set here since it only affects the key gen,
            feel free to modify this.
         */
         SecureRandom prng = SecureRandom.getInstance("SHA1PRNG");
         byte[] user_seed = prng.generateSeed(20);
         prng.setSeed(user_seed);
         keyGen.initialize(1024, prng);

         KeyPair pair = keyGen.generateKeyPair();

         Signature dsa = Signature.getInstance("SHA1withDSA");


         PrivateKey priv = pair.getPrivate();
         dsa.initSign(priv);

         BigInteger x = ((DSAPrivateKey)priv).getX();

         System.err.println("Private key is " + x);

         /* The Java standard lib does not seem to include either
            a) Support for writing PEM format keys
            b) A base64 encoder/decoder

          ?!? WTF Sun, seriously.

          Just write the raw ASN.1 as hex and decode it on the other
          side for minimal hassles. Could use
          org.apache.commons.codec.binary.Base64 or whatever and form
          PEM headers by hand, but doesn't seem worth the bother.
         */
         PublicKey pubKey = pair.getPublic();
         byte[] encKey = pubKey.getEncoded();
         print_hex(encKey);

         byte[] data = new byte[0];

         dsa.update(data);
         byte[] sig = dsa.sign();

         print_hex(sig);
         }
      catch(Exception e)
         {
         System.out.println(e);
         }
      }

   }