// Generate an Elliptic Curve key-pair, using a named curve
//
// ECgen UserID
//   will create files: UserID.private, UserID.public
//
import java.math.BigInteger;
import java.security.*;
import java.security.spec.*;
import java.security.interfaces.*;
import java.io.*;

public class ECgen
{
  public static void usage()
  {
    System.err.println( "Usage: ECgen [-c curve_name (default: secp256k1)] UserID");
    System.exit(1);
  }

  public static void main(String args[]) throws Exception
  {
    String name = "secp256k1", UserID = null;

    for( int i = 0; i < args.length; ++i)
    {
      if( args[i].equals( "-c"))
        name = args[++i];
      else if( UserID != null)
        usage();
      else
        UserID = args[i];
    }

    if( UserID == null)
      usage();

   // generate key pair
   //
    KeyPairGenerator kpg = KeyPairGenerator.getInstance( "EC");
    ECGenParameterSpec curve = new ECGenParameterSpec( name);
    kpg.initialize( curve);
    KeyPair kp = kpg.genKeyPair();
    ECPrivateKey priv = (ECPrivateKey) kp.getPrivate();
    ECPublicKey pub = (ECPublicKey) kp.getPublic();

   // save private key
   //
    FileOutputStream fos = new FileOutputStream( UserID + ".private");
    fos.write( priv.getEncoded());
    fos.close();

   // save public key
   //
    fos = new FileOutputStream( UserID + ".public");
    fos.write( pub.getEncoded());
    fos.close();

   // display private and public values
   //
    BigInteger S = priv.getS();
    ECPoint W = pub.getW();
    BigInteger WX = W.getAffineX();
    BigInteger WY = W.getAffineY();

    System.out.println( name);
    System.out.println( "  S = " + S.toString(16));
    System.out.println( "W.X = " + WX.toString(16));
    System.out.println( "W.Y = " + WY.toString(16));
  }
}