/**
 *  RSA Public/Private Key Generator
 *
 *  <applet code="PubKey.class" width=600 height=270>
 *
 */

import java.math.BigInteger;
import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;
import java.util.Random;
import java.util.Date;
import java.io.*;

public class GenKey extends Applet implements ActionListener {
    static final int WIDTH = 600;
    static final int MARGIN = 5;
    static final int DY = 40;
    static final int VSKIP = 4;
    static final int HSKIP = 4;
    static final int CERTAINTY = 64;
    static final int NUM_ATTEMPTS = 32;
    static final int MAX_DIGITS = 500;

    Random rnd = new Random(new Date().getTime());

    BigInteger module;
    BigInteger euler;
    BigInteger p, q;
    BigInteger e;
    BigInteger d;
    int numDigits;

    TextField numDigitsTxt;
    TextField moduleTxt;
    TextField publicKeyTxt;
    TextField privateKeyTxt;

    Button genButton;
    Button writeButton;
    TextField fileName;

    Label message;

    public static void main(String[] args) {
        Frame f = new Frame("RSA Public/Private Key Generator");

        WindowAdapter wa = new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        };
        f.addWindowListener(wa);

        f.setFont(new Font("Helvetica", Font.PLAIN, 14));

        // Add menu bar
        final MenuBar mb = new MenuBar();
        f.setMenuBar(mb);
        final Menu fileMenu = new Menu("File");
        final MenuItem quitItem = new MenuItem("Quit");
        fileMenu.add(quitItem);
        mb.add(fileMenu);

        ActionListener al = new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                // System.out.println("Action " + e);
                if (e.getActionCommand().equals("Quit")) {
                    System.exit(0);
                }
            }
        };
        fileMenu.addActionListener(al);

        GenKey gk = new GenKey();
        f.add("Center", gk);

        gk.init();
        gk.start();

        // f.pack();
        f.setSize(
            WIDTH + 24,
            6 * (DY + VSKIP) + 2 * MARGIN +
            50                  // Menu bar
        );
        f.setVisible(true);
    }

    public GenKey() {

    }

    public void init() {
        setLayout(null);
        setBackground(Color.lightGray);

        int y = MARGIN;
        int x0 = MARGIN;
        int x1 = 90;
        int x2 = 120;
        int w = WIDTH - MARGIN - x1;

        setFont(new Font("Helvetica", Font.PLAIN, 14));

        Label l = new Label("Number of digits:");
        add(l);
        l.setBounds(x0, y, 110, DY);

        numDigitsTxt = new TextField("");
        add(numDigitsTxt);
        numDigitsTxt.setBounds(x2, y, 60, DY);

        y += DY + VSKIP;
        l = new Label("Module:");
        add(l);
        l.setBounds(x0, y, 80, DY);

        moduleTxt = new TextField("");
        add(moduleTxt);
        moduleTxt.setBounds(x1, y, w, DY);

        y += DY + VSKIP;
        l = new Label("Public key:");
        add(l);
        l.setBounds(x0, y, 80, DY);

        publicKeyTxt = new TextField("");
        add(publicKeyTxt);
        publicKeyTxt.setBounds(x1, y, w, DY);

        y += DY + VSKIP;
        l = new Label("Private key:");
        add(l);
        l.setBounds(x0, y, 80, DY);

        privateKeyTxt = new TextField("");
        add(privateKeyTxt);
        privateKeyTxt.setBounds(x1, y, w, DY);

        y += DY + VSKIP;
        genButton = new Button("Generate");
        add(genButton);
        genButton.setBounds(x0, y, 80, DY);
        genButton.addActionListener(this);

        writeButton = new Button("Write to file");
        add(writeButton);
        writeButton.setBounds(x1, y, 80, DY);
        writeButton.addActionListener(this);

        l = new Label("File name:");
        add(l);
        l.setBounds(x1 + 90, y, 70, DY);

        fileName = new TextField("KeyPair.dat");
        add(fileName);
        fileName.setBounds(
            x1 + 90 + 70, y, 
            WIDTH - (x1 + 90 + 70) - MARGIN, DY
        );

        y += DY + VSKIP;
        message = new Label("");
        add(message);
        message.setBounds(x0, y, WIDTH - 2 * MARGIN, DY);
        message.setForeground(Color.blue);
    }

    public void actionPerformed(ActionEvent e) {
        Object source = e.getSource();
        if (source == genButton) {
            onGenerate();
        } else if (source == writeButton) {
            onWrite();
        }
    }

    void onGenerate() {
        String num = numDigitsTxt.getText();
        if (num == "") return;
        message.setText("");
        try {
            numDigits = Integer.parseInt(num);
        } catch (NumberFormatException e) {
            message.setText("Illegal number of digits.");
            return;
        }

        int binaryDigits = (int)(
           numDigits * 0.5 * Math.log(10.) / Math.log(2.)
        );
        if (binaryDigits < 2 || numDigits > MAX_DIGITS) {
            message.setText("Illegal number of digits (maximum " + MAX_DIGITS + ").");
            return;
        }

        message.setText("Please, wait...");

        p = new BigInteger(
            binaryDigits,
            CERTAINTY,
            rnd
        );
        boolean ok = false;
        for (int t = 0; t < NUM_ATTEMPTS; t++) {
            q = new BigInteger(
                binaryDigits,
                CERTAINTY,
                rnd
            );
            if (!p.equals(q)) {
                ok = true;
                break;
            }
        }
        if (!ok) {
            message.setText("Alas, cannot generate...");
            System.out.println("Alas, cannot generate...");
            return;
        }

        module = p.multiply(q);
        euler = p.subtract(BigInteger.valueOf(1)).multiply(
            q.subtract(BigInteger.valueOf(1))
        );
        moduleTxt.setText("" + module);

        System.out.println("p = " + p);
        System.out.println("q = " + q);
        System.out.println("m = pq = " + module);
        System.out.println("Euler function(m) = " + euler);

        ok = false;
        for (int t = 0; t < NUM_ATTEMPTS; t++) {
            e = new BigInteger(
                binaryDigits*15/8,
                CERTAINTY,
                rnd
            );
            System.out.println("e = "+e);
            if (e.gcd(euler).equals(BigInteger.valueOf(1))) {
                ok = true;
                break;
            }
        }
        if (!ok) {
            message.setText("Alas, cannot generate...");
            System.out.println("Alas, cannot generate...");
            return;
        }

        d = e.modInverse(euler);
        System.out.println("d = " + d);

        publicKeyTxt.setText("" + e);
        privateKeyTxt.setText("" + d);

        message.setText("RSA key pair is generated.");
    }

    void onWrite() {
        String filnam = fileName.getText();
        if (filnam.equals("")) {
            message.setText("Please, write a file name.");
            return;
        }

        try {
            FileOutputStream fos = new FileOutputStream(filnam);
            DataOutputStream dos = new DataOutputStream(fos);

            dos.writeBytes("RSA Public/Private Key Pair\n");
            dos.writeBytes("prime p       = " + p + "\n");
            dos.writeBytes("prime q       = " + q + "\n");
            dos.writeBytes("module pq     = " + module + "\n");
            dos.writeBytes("public key e  = " + e + "\n");
            dos.writeBytes("private key d = " + d + "\n");

            fos.close();
            message.setText("Key pair is written to the file \"" + filnam + "\"");
        } catch (IOException e) {
            message.setText("Cannot write to the file.");
            System.out.println("Cannot write to file.\n" + e);
        } catch (SecurityException e) {
            message.setText("Cannot write to the file by security reasons.");
            System.out.println("Cannot write to file by security reasons.\n" + e);
        }
    }
}