Have You Ever Hit a Wall Trying to Add User Authentication to Your Side Project?

"I just wanted to add identity verification to block fake users and bots, but the API provider asked for a business registration certificate and telecom subscription proof. We haven't even incorporated our startup yet!"

For developers building a Minimum Viable Product (MVP) or working on side projects, user authentication is often one of the biggest bottlenecks just before launch. Here's the bottom line: for early-stage services, lightweight SMS Authentication (OTP) is overwhelmingly more advantageous than heavy and complex identity verification systems like Korea's PASS.

In this article, we'll compare PASS identity verification with SMS authentication for MVPs, and show you how to implement a fully working SMS authentication system in Next.js in under 5 minutes—without submitting a single document.

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1. The Breakdown: PASS Verification vs SMS OTP

PASS Identity Verification (KCB, NICE, etc.)

• Data Verified: Real name, gender, exact date of birth, citizenship status.
• Setup Process: Requires business registration and lengthy contracts with PG (Payment Gateway) or verification agencies (takes 1-2 weeks).
• Costs: High. Often includes an initial setup fee ($40-$100), and costs $0.03-$0.05 per request with a monthly minimum fee.
• Best For: Financial technology services, age-restricted e-commerce (19+).

SMS Authentication (OTP)

• Data Verified: Possession of the specific phone number (great for unique user identification and bot prevention).
• Setup Process: Traditionally requires telecom proofs due to strict caller ID registration laws in Korea.
• Costs: Low. No setup fee, usually around $0.015-$0.025 per message.
• Best For: Startup MVPs, indie hackers, side projects, and fast-launching platforms.

The bottleneck with traditional SMS APIs in Korea is the paperwork. Anti-spam laws require developers to pre-register a sender ID (caller ID) using telecommunications certificates before sending a single message.

Meet EasyAuth: The Developer-First SMS API with Zero Paperwork

EasyAuth was built specifically for developers without business registrations or telecommunication certificates.

• Zero Paperwork: Skip the business registration and caller ID proof submissions.
• Instant Setup: Get an API key upon signup and start sending within 5 minutes. We provide automatic shared sender numbers.
• Cost-Effective: At just 15~25 KRW ($0.01) per message, it's nearly half the price of traditional APIs.
• Free Trial: Get 10 free SMS credits immediately upon registration for testing.

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2. Next.js SMS Authentication Tutorial (5-Minute Integration)

EasyAuth's API architecture is incredibly straightforward, consisting of just two endpoints: POST /send and POST /verify. Let's implement this in a Next.js (App Router) environment.

Step 1. The Send API Route (/send)

// app/api/auth/send/route.ts
import { NextResponse } from 'next/server';

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

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

    if (response.ok) {
      return NextResponse.json({ success: true, message: 'A 6-digit code has been sent.' });
    }
    return NextResponse.json({ success: false, message: 'Failed to send' }, { status: 400 });
  } catch (error) {
    return NextResponse.json({ success: false, message: 'Server error' }, { status: 500 });
  }
}

Step 2. The Verify API Route (/verify)

// app/api/auth/verify/route.ts
import { NextResponse } from 'next/server';

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

  try {
    // Call EasyAuth Verify API
    const response = await fetch('https://api.easyauth.kr/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: true, message: 'Verification successful.' });
    }
    return NextResponse.json({ success: false, message: 'Invalid code.' }, { status: 400 });
  } catch (error) {
    return NextResponse.json({ success: false, message: 'Server error' }, { status: 500 });
  }
}

Step 3. Client-Side Implementation

'use client';
import { useState } from 'react';

export default function AuthPage() {
  const [phone, setPhone] = useState('');
  const [code, setCode] = useState('');
  const [step, setStep] = useState(1);

  const handleSend = async () => {
    const res = await fetch('/api/auth/send', {
      method: 'POST',
      body: JSON.stringify({ phone })
    });
    if (res.ok) setStep(2);
  };

  const handleVerify = async () => {
    const res = await fetch('/api/auth/verify', {
      method: 'POST',
      body: JSON.stringify({ phone, code })
    });
    if (res.ok) alert('Verified! Proceeding to signup.');
  };

  return (
    <div>
      <h2>SMS Authentication</h2>
      {step === 1 ? (
        &lt;&gt;
           setPhone(e.target.value)} placeholder="01012345678" /&gt;
          Send Code
        &lt;/&gt;
      ) : (
        &lt;&gt;
           setCode(e.target.value)} placeholder="123456" /&gt;
          Verify
        &lt;/&gt;
      )}
    </div>
  );
}

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3. Best Practices for SMS Auth Security

1. Rate Limiting: Protect your budget from SMS-pumping bots by limiting the number of SMS requests per IP address or session per day.
2. Expiration Time: Set your OTPs to expire within 3 to 5 minutes. EasyAuth automatically handles this expiration logic on the backend for enhanced security.
3. Resend Cooldown: Implement a UI cooldown (e.g., 60 seconds) on the "Resend Code" button to prevent users from spamming the API.

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4. Conclusion: Speed is Everything for an MVP

When building a startup MVP or a toy project, speed to market is your biggest asset. Unless your core business heavily relies on extracting strict demographic data like exact age or legal name, SMS OTP is far superior to heavy PASS identity verification.

If you were hesitating because traditional SMS APIs still demanded paperwork and caller ID setups, EasyAuth is your answer. You can build a secure, reliable SMS verification system in just 5 minutes without a single piece of paper, all at an industry-low 15 KRW per message.

Sign up today, grab your 10 free test credits, and focus on building what truly matters—your core product!