1. The Developer's Dilemma: Authentication Overhead

When you're trying to launch a Minimum Viable Product (MVP) quickly, building the user authentication flow often brings momentum to a halt. In particular, creating the "Forgot Password" and "Password Reset" logic—sending reset emails, generating secure tokens, checking expiration times, and building UI—takes surprisingly long.

To avoid this hassle, many modern apps are adopting Passwordless SMS Login using phone numbers and OTPs (One-Time Passwords). It removes the security burden of hashing and storing passwords, and users love the frictionless experience.

However, the problem lies in the SMS API providers. If you look for a local SMS API to integrate, they almost always demand a mountain of paperwork: Business Registration Certificates, Telecom Service Usage Certificates, and identity verification. If you are a solo developer building a side project or an early-stage startup without a registered legal entity, you are blocked from even starting.

2. Zero-Paperwork SMS Authentication in 5 Minutes

In this tutorial, we will implement a passwordless login system in Next.js (App Router) using EasyAuth, an ultra-simple SMS API designed for developers. EasyAuth requires no paperwork, no caller ID pre-registration, and takes exactly 5 minutes to set up.

EasyAuth's API structure consists of just two endpoints:

• POST /send: Send the verification code
• POST /verify: Verify the code

3. Step-by-Step Implementation in Next.js

Step 1: Create the 'Send SMS' API Route (app/api/auth/send/route.ts)

This route receives the user's phone number and requests EasyAuth to send an OTP.

import { NextResponse } from 'next/server';

export async function POST(request: Request) {
  try {
    const { phone } = await request.json();

    // Send SMS via EasyAuth API
    const response = await fetch('https://api.easyauth.io/send', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'Authorization': `Bearer ${process.env.EASYAUTH_API_KEY}`,
      },
      body: JSON.stringify({ phone }),
    });

    if (!response.ok) {
      throw new Error('Failed to send SMS');
    }

    return NextResponse.json({ success: true, message: 'Verification code sent.' });
  } catch (error) {
    return NextResponse.json({ success: false, error: 'Server error occurred.' }, { status: 500 });
  }
}

Step 2: Create the 'Verify Code' API Route (app/api/auth/verify/route.ts)

This route checks the 6-digit code entered by the user.

import { NextResponse } from 'next/server';

export async function POST(request: Request) {
  try {
    const { phone, code } = await request.json();

    // Verify code via EasyAuth API
    const response = await fetch('https://api.easyauth.io/verify', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'Authorization': `Bearer ${process.env.EASYAUTH_API_KEY}`,
      },
      body: JSON.stringify({ phone, code }),
    });

    if (!response.ok) {
      return NextResponse.json({ success: false, error: 'Invalid verification code.' }, { status: 400 });
    }

    // Verification successful! Retrieve user from DB and generate JWT token here.
    // const user = await findOrCreateUser(phone);
    // const token = generateToken(user);

    return NextResponse.json({ success: true, message: 'Login successful!' }); // return token
  } catch (error) {
    return NextResponse.json({ success: false, error: 'Verification error' }, { status: 500 });
  }
}

Step 3: Frontend Login Component (app/login/page.tsx)

Now, let's create a functional client component that uses the API routes we just built.

'use client';

import { useState } from 'react';

export default function PasswordlessLogin() {
  const [phone, setPhone] = useState('');
  const [code, setCode] = useState('');
  const [step, setStep] = useState<'SEND' | 'VERIFY'>('SEND');

  const handleSendCode = async () => {
    const res = await fetch('/api/auth/send', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ phone })
    });
    
    if (res.ok) {
      alert('Code sent successfully!');
      setStep('VERIFY');
    } else {
      alert('Failed to send code.');
    }
  };

  const handleVerifyCode = async () => {
    const res = await fetch('/api/auth/verify', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ phone, code })
    });

    if (res.ok) {
      alert('Successfully logged in!');
      // TODO: Redirect to the dashboard
    } else {
      alert('Invalid code.');
    }
  };

  return (
    <div>
      <h1>SMS Login</h1>
      
      {step === 'SEND' ? (
        <div>
           setPhone(e.target.value)}
            className="p-3 border rounded"
          /&gt;
          
            Send Code
          
        </div>
      ) : (
        <div>
          <p>A code has been sent to {phone}.</p>
           setCode(e.target.value)}
            className="p-3 border rounded"
          /&gt;
          
            Verify &amp; Login
          
        </div>
      )}
    </div>
  );
}

4. Tips & Best Practices

1. Rate Limiting: Prevent malicious users from draining your SMS credits by limiting the number of API requests per IP address or phone number per day.
2. Expiration Timer: OTP codes should typically expire in 3 to 5 minutes. Implement a countdown timer in your frontend UI to visually indicate this to the user.
3. Bypass Caller ID Registration: Traditional providers require you to register a 'Sender ID' using telecom certificates. EasyAuth utilizes an 'Auto-Sender ID' feature, completely bypassing this headache.

5. Conclusion: Focus on Your Product, Not Paperwork

Implementing passwordless SMS login drastically reduces the time spent building authentication, eliminating the need for complex 'Forgot Password' flows.

If you are an indie hacker, freelancer, or building an MVP, try EasyAuth. It's the ultimate hassle-free API—you can start in 5 minutes with zero business paperwork. Plus, you get 10 free SMS credits upon signup to test your implementation immediately. Even after that, it offers highly reasonable pricing at 15~25 KRW per message (cheaper than the standard 30~50 KRW). Skip the red tape and get back to building what actually matters!