Replacing Firebase Phone Auth: Integrate Custom SMS Authentication in 5 Minutes (Zero Paperwork)

Does SMS Authentication Really Need to Be This Complicated?

When building a side project or a startup MVP, Firebase Phone Auth is often the go-to choice for user authentication. However, developers quickly run into friction in production: annoying reCAPTCHA challenges that increase user drop-off rates, rigid UI constraints, and pricing that scales poorly as your app grows.

So, you decide to switch to a local or traditional SMS gateway. But then you hit a massive wall: Paperwork.

• Submitting original Business Registration Certificates
• Acquiring Telecommunication Service Certificates
• Pre-registering sender phone numbers and waiting days for approval

If you are an indie hacker, a freelancer, or an early-stage startup without an incorporated entity, integrating traditional SMS APIs is practically an impossible hurdle.

Today, we are going to replace Firebase Phone Auth completely by building a custom SMS authentication server using EasyAuth—a developer-first SMS API that requires zero paperwork and can be integrated in under 5 minutes.

---

What You Will Learn

1. How to get SMS API keys instantly without tedious paperwork.
2. Implementing the send and verify OTP logic in Node.js (Express).
3. Frontend integration tips and ready-to-use boilerplate code for your production app.

---

1. Getting Started with EasyAuth (Zero Paperwork!)

The biggest advantage of EasyAuth is its developer-friendly onboarding. There is absolutely no need to submit business licenses or go through manual sender number verifications. You get an auto-assigned sender number instantly.

1. Sign up on the EasyAuth website. (You get 10 free credits immediately upon registration).
2. Grab your API KEY from the developer dashboard.
3. That's it! You are ready to start coding.

> 💡 Cost Advantage: EasyAuth costs only about 1525 KRW per message, which is nearly half the price of traditional competitors that charge up to 3050 KRW. This makes it perfect for growing e-commerce or platform services.

---

2. Understanding the API Structure

EasyAuth doesn't force you to handle complex session states, Redis caches, or database OTP storage on your own. The entire authentication flow is completed with just two simple endpoints:

• POST /send : Dispatches a 6-digit OTP to the user's mobile number.
• POST /verify : Validates the OTP code inputted by the user.

---

3. Step-by-Step Implementation (Express.js)

Step 3.1: Project Setup

First, initialize your project and install the necessary dependencies.

npm init -y
npm install express axios dotenv

Step 3.2: Writing the Backend Send & Verify Endpoints

Below is the complete, working Express.js code. You can literally copy and paste this into your application.

// server.js
require('dotenv').config();
const express = require('express');
const axios = require('axios');

const app = express();
app.use(express.json());

// EasyAuth API Configuration
const EASYAUTH_API_KEY = process.env.EASYAUTH_API_KEY;
const EASYAUTH_URL = 'https://api.easyauth.kr'; // Base API URL

/**
 * 1. Send OTP Endpoint
 * Receives a phone number from the client and requests EasyAuth to send an OTP.
 */
app.post('/api/auth/send', async (req, res) => {
  const { phoneNumber } = req.body;

  try {
    await axios.post(`${EASYAUTH_URL}/send`, {
      phone: phoneNumber
    }, {
      headers: { Authorization: `Bearer ${EASYAUTH_API_KEY}` }
    });
    
    res.json({ 
      success: true, 
      message: 'OTP successfully sent to your phone.' 
    });
  } catch (error) {
    console.error('Send Error:', error.response?.data || error.message);
    res.status(500).json({ success: false, message: 'Failed to send OTP.' });
  }
});

/**
 * 2. Verify OTP Endpoint
 * Validates the OTP code entered by the user via the EasyAuth API.
 */
app.post('/api/auth/verify', async (req, res) => {
  const { phoneNumber, code } = req.body;

  try {
    const response = await axios.post(`${EASYAUTH_URL}/verify`, {
      phone: phoneNumber,
      code: code
    }, {
      headers: { Authorization: `Bearer ${EASYAUTH_API_KEY}` }
    });
    
    // Handle validation response
    if (response.data.isValid) {
      // 🎯 You can issue a JWT or handle user login/registration logic here!
      res.json({ success: true, message: 'Phone number verified successfully.' });
    } else {
      res.status(400).json({ success: false, message: 'Invalid OTP code.' });
    }
  } catch (error) {
    res.status(500).json({ success: false, message: 'Server error during verification.' });
  }
});

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server is running on port ${PORT}`);
});

Step 3.3: Frontend Client Call Example (React / Next.js)

Once the backend is ready, connecting the frontend is incredibly straightforward.

// Inside your Next.js component
const handleSendOTP = async (phoneNumber) => {
  const res = await fetch('/api/auth/send', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ phoneNumber })
  });
  
  if (res.ok) {
    alert('A 6-digit OTP has been sent!');
    // Trigger your countdown timer UI here
  }
};

---

4. Tips & Best Practices for Production

1. Security & Rate Limiting
   To prevent malicious users from abusing your SMS endpoint (which could cause a massive API bill), always implement rate limiting. Use middleware like express-rate-limit to restrict the number of /send requests per IP address (e.g., max 5 requests per hour).

2. Better UX
   Instead of relying on Firebase's rigid default pop-ups, build a native custom UI that matches your branding. Implement a 3-minute countdown timer after the user clicks "Send OTP", and seamlessly transition the button to a "Resend" state when the timer expires.

---

Conclusion

Migrating away from Firebase Phone Auth and building your own custom SMS authentication is much easier than it sounds. If you use EasyAuth, you can completely bypass the notorious paperwork and pre-registration processes that plague traditional SMS gateways.

With just two simple backend endpoints, you can have a fully functional, highly customizable SMS verification system running in under 5 minutes.

If you are a solo developer, freelancer, or building a startup MVP that needs to validate user phone numbers quickly, try out EasyAuth today and get 10 free SMS credits immediately upon signup!