SwingJwtHelper.java

import org.json.JSONObject;

import javax.swing.*;
import java.awt.*;
import java.nio.charset.StandardCharsets;
import java.util.Base64;

public class SwingJwtHelper {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(SwingAppUI::createAndShowGUI);
    }

    private static class SwingAppUI {
        private static JFrame frame;
        private static JTextArea encodedTokenTextArea;
        private static JTextArea headerTextArea;
        private static JTextArea payloadTextArea;
        private static JTextField publicKeyTextField;

        private static void createAndShowGUI() {
            // Create the main frame
            frame = new JFrame("JWT Decoder");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setSize(600, 400);
            frame.setLayout(new BorderLayout());

            // Create the encoded token text area
            encodedTokenTextArea = new JTextArea();
            encodedTokenTextArea.setLineWrap(true);
            encodedTokenTextArea.getDocument().addDocumentListener(new TokenChangeListener());

            JScrollPane encodedTokenScrollPane = new JScrollPane(encodedTokenTextArea);

            // Create the header text area
            headerTextArea = new JTextArea();
            headerTextArea.setEditable(false);

            JScrollPane headerScrollPane = new JScrollPane(headerTextArea);

            // Create the payload text area
            payloadTextArea = new JTextArea();
            payloadTextArea.setEditable(false);

            JScrollPane payloadScrollPane = new JScrollPane(payloadTextArea);

            // Create the public and private key text fields
            publicKeyTextField = new JTextField();

            // Create the Verify Signature button
            JButton verifyButton = new JButton("Verify Signature");

            // Create the left panel
            JPanel leftPanel = new JPanel(new BorderLayout());
            leftPanel.add(new JLabel("Encoded Token"), BorderLayout.NORTH);
            leftPanel.add(encodedTokenScrollPane, BorderLayout.CENTER);

            // Create the right panel
            JPanel rightPanel = new JPanel(new GridBagLayout());
            GridBagConstraints c = new GridBagConstraints();
            c.insets = new Insets(5, 5, 5, 5);

            // Add Header section
            c.gridx = 0;
            c.gridy = 0;
            c.anchor = GridBagConstraints.WEST;
            c.fill = GridBagConstraints.BOTH;
            c.weightx = 0.5;
            c.weighty = 0.0;
            rightPanel.add(new JLabel("Header"), c);

            c.gridy = 1;
            c.anchor = GridBagConstraints.EAST;
            c.fill = GridBagConstraints.BOTH;
            c.weightx = 1.5;
            c.weighty = 1.0;
            rightPanel.add(headerScrollPane, c);

            // Add Payload section
            c.gridy = 2;
            c.weighty = 0.0;
            rightPanel.add(new JLabel("Payload"), c);

            c.gridy = 3;
            c.weighty = 1.0;
            rightPanel.add(payloadScrollPane, c);

            // Add Verify Signature section
            c.gridy = 4;
            c.weighty = 0.0;
            rightPanel.add(new JLabel("Verify Signature"), c);

            c.gridy = 5;
            c.weightx = 1.0;
            c.weighty = 0.0;
            c.fill = GridBagConstraints.HORIZONTAL;
            rightPanel.add(new JLabel("Public Key"), c);

            c.gridy = 6;
            rightPanel.add(publicKeyTextField, c);

            c.gridy = 7;
            rightPanel.add(new JLabel("Private Key"), c);

            c.gridy = 8;
            rightPanel.add(verifyButton, c);

            // Add components to the main frame
            frame.add(leftPanel, BorderLayout.CENTER);
            frame.add(rightPanel, BorderLayout.EAST);

            // Display the main frame
            frame.setVisible(true);
        }


        private static class TokenChangeListener implements javax.swing.event.DocumentListener {
            @Override
            public void insertUpdate(javax.swing.event.DocumentEvent e) {
                processTokenChange();
            }

            @Override
            public void removeUpdate(javax.swing.event.DocumentEvent e) {
                processTokenChange();
            }

            @Override
            public void changedUpdate(javax.swing.event.DocumentEvent e) {
                processTokenChange();
            }

            private static void processTokenChange() {
                String encodedToken = encodedTokenTextArea.getText();

                // Reset header and payload text areas
                headerTextArea.setText("");
                payloadTextArea.setText("");

                try {
                    String[] tokenParts = encodedToken.split("\\.");
                    if (tokenParts.length == 3) {
                        String header = decodeBase64String(tokenParts[0]);
                        String payload = decodeBase64String(tokenParts[1]);

                        JSONObject headerJson = new JSONObject(header);
                        JSONObject payloadJson = new JSONObject(payload);

                        String algorithm = headerJson.optString("alg");
                        if ("RS256".equals(algorithm)) {
                            headerTextArea.setText(headerJson.toString(2));
                            payloadTextArea.setText(payloadJson.toString(2));
                        } else {
                            headerTextArea.setText("Algorithm is not RS256");
                        }
                    } else {
                        headerTextArea.setText("Invalid token format");
                    }
                } catch (Exception e) {
                    headerTextArea.setText("Error decoding token");
                }
            }

            private static String decodeBase64String(String base64String) {
                byte[] decodedBytes = Base64.getUrlDecoder().decode(base64String);
                return new String(decodedBytes, StandardCharsets.UTF_8);
            }
        }
    }
}