In the modern web development landscape, JSON Web Tokens (JWT) have become a cornerstone of secure authentication and authorization systems. While there are existing tools and libraries for decoding JWTs, building your own custom JWT decoder can provide a tailored solution for specific use cases. In this article, we’ll guide you through creating a robust, online JWT decoding tool using Firebase Functions for the backend and React for the frontend.

Understanding JWT

Before diving into the implementation, it’s essential to understand the basics of JWT. A JWT consists of three parts: the header, the payload, and the signature, separated by dots. The header typically contains the type of token and the algorithm used for signing. The payload holds the actual claims or data, and the signature ensures the integrity and authenticity of the token.

Structure of a JWT

<base64UrlEncoded(header)>.<base64UrlEncoded(payload)>.<base64UrlEncoded(signature)>

The decoding process involves base64 URL decoding each part and then validating the signature if the token is signed.

Setting Up the Project

Prerequisites

  • A Firebase account
  • Node.js and npm installed
  • Basic understanding of React and JavaScript

Initializing the Project

  1. Create a Firebase Project:

    • Go to the Firebase Console.
    • Create a new project and enable Firebase Authentication.

  2. Set Up the Backend:

    • Install Firebase CLI and initialize your project: 
      npm install -g firebase-tools
firebase login
firebase init
    • Create a new directory for your backend functions.

  3. Set Up the Frontend:

    • Create a new React project: 
      npx create-react-app jwt-decoder
cd jwt-decoder

Building the Backend with Firebase Functions

Firebase Functions provide a serverless environment to run your backend logic. We’ll create a function to handle the JWT decoding request.

Writing the Firebase Function

  1. Create a new function in your Firebase project:

    // functions/src/index.ts
import * as functions from 'firebase-functions';
import * as admin from 'firebase-admin';

admin.initializeApp();

exports.decodeJWT = functions.https.onRequest(async (req, res) => {
  if (req.method !== 'POST') {
    res.status(405).send('Method Not Allowed');
    return;
  }

  const { token } = req.body;
  if (!token) {
    res.status(400).send('Token is required');
    return;
  }

  try {
    // Split the token into header, payload, and signature
    const parts = token.split('.');
    if (parts.length !== 3) {
      throw new Error('Invalid JWT format');
    }

    // Base64 decode the header and payload
    const decodedHeader = Buffer.from(parts[0], 'base64url').toString('utf8');
    const decodedPayload = Buffer.from(parts[1], 'base64url').toString('utf8');

    // Return the decoded parts
    res.json({
      header: JSON.parse(decodedHeader),
      payload: JSON.parse(decodedPayload),
    });
  } catch (error) {
    res.status(500).send(error.message);
  }
});

  2. Deploy the function:

    firebase deploy --only functions

Building the Frontend with React

Now, let’s create the user interface for our JWT decoder.

Creating the React Component

  1. Install Required Dependencies:

    npm install axios

  2. Create the Decoder Component:

    // src/components/JWTDecoder.tsx
import React, { useState } from 'react';
import axios from 'axios';

export default function JWTDecoder() {
  const [token, setToken] = useState('');
  const [decodedToken, setDecodedToken] = useState(null);
  const [error, setError] = useState('');

  const decodeToken = async () => {
    try {
      const response = await axios.post('your-firebase-function-url', { token });
      setDecodedToken(response.data);
      setError('');
    } catch (error) {
      setError(error.response ? error.response.data : 'Failed to decode token');
      setDecodedToken(null);
    }
  };

  return (
    <div className="container mt-5">
      <h1>JWT Decoder</h1>
      <div className="mb-3">
        <textarea
          className="form-control"
          value={token}
          onChange={(e) => setToken(e.target.value)}
          placeholder="Paste your JWT token here..."
          rows={3}
        ></textarea>
      </div>
      <button
        className="btn btn-primary mb-3"
        onClick={decodeToken}
        disabled={token === ''}
      >
        Decode
      </button>
      {error && <div className="alert alert-danger">{error}</div>}
      {decodedToken && (
        <div className="card">
          <div className="card-header">
            Decoded Token
          </div>
          <div className="card-body">
            <h5 className="card-title">Header</h5>
            <pre>{JSON.stringify(decodedToken.header, null, 2)}</pre>
            <h5 className="card-title">Payload</h5>
            <pre>{JSON.stringify(decodedToken.payload, null, 2)}</pre>
          </div>
        </div>
      )}
    </div>
  );
}

  3. Update the App Component:

    // src/App.tsx
import React from 'react';
import JWTDecoder from './