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Sent TeamMay 3, 2025 / code examples / Article

How to Implement Sinch SMS OTP Verification in Next.js with NextAuth (2025 Guide)

Learn to implement secure SMS OTP two-factor authentication in Next.js 14 using Sinch Verification API and NextAuth v5. Step-by-step tutorial with complete code examples, security best practices, and production deployment.

Sinch SMS OTP 2FA with Next.js and NextAuth

Two-factor authentication (2FA) with SMS OTP adds a critical security layer to user accounts by requiring phone number verification beyond passwords. This comprehensive guide demonstrates how to implement SMS OTP verification in Next.js 14 applications using the Sinch Verification API and NextAuth v5 for seamless authentication.

Build a production-ready Next.js 14 application with NextAuth integration for secure SMS-based OTP verification. We cover everything from initial project setup to error handling, security best practices, and deployment strategies.

What You'll Build: SMS OTP Two-Factor Authentication

Goal: Integrate SMS-based two-factor authentication into a Next.js application using Sinch.

Problem Solved: Enhance application security by verifying user identity through a secondary channel (SMS OTP), mitigating risks associated with compromised passwords.

Technologies Used:

  • Node.js: JavaScript runtime environment for the backend server.
  • Next.js 14: React framework for building server-rendered applications, with built-in routing and API support via App Router.
  • NextAuth v5 (Auth.js): Authentication library for Next.js, providing session management, OAuth, and custom authentication flows. NextAuth v5 Documentation.
  • Sinch Verification API: Manages the complexities of OTP generation, delivery (SMS, FlashCall, voice), and verification. Official Sinch Verification API Documentation.
  • @sinch/sdk-core: Official Sinch Node.js SDK package for interacting with Sinch APIs. NPM Package.
  • dotenv: Module to load environment variables from a .env file.

System Architecture:

+-------------+ +-----------------+ +---------------+ +-------------+ +--------------+ | User |------>| Node.js/Next.js |------>| Sinch Verify |------>| SMS Network |------>| User's Phone | | (Browser) | | App | | API | | | | (Receives OTP)| +-------------+ +-----------------+ +---------------+ +-------------+ +--------------+ ^ | ^ | | (Enters OTP) | (Sends Phone #) | (Sends OTP) | | | | | +---------------------+ (Verifies OTP) -----------+ | | | +-------------------------------------------------+ (Verification Result)

Final Outcome: A functional web application where users can:

  1. Enter their phone number.
  2. Receive an SMS OTP via Sinch.
  3. Enter the received OTP.
  4. Get confirmation of successful verification or an error message.

Prerequisites:

  • Node.js (v18+) and npm (or yarn) installed. Install Node.js
  • A Sinch account with Verification API access. Sign up for Sinch and note your Application Key and Application Secret from the Verification dashboard.
  • Basic understanding of Node.js, Next.js App Router, and web concepts (HTTP requests, forms).
  • A text editor or IDE (e.g., VS Code).

1. Setting Up Your Next.js SMS Verification Project

Initialize your Next.js project and install the necessary dependencies for SMS OTP authentication.

  1. Create Next.js Project: Use create-next-app to scaffold a new Next.js 14 application with TypeScript and App Router.

    bash
    npx create-next-app@latest sinch-nextauth-2fa
cd sinch-nextauth-2fa

    When prompted, select these options:

    • TypeScript: Yes
    • ESLint: Yes
    • Tailwind CSS: Yes (optional)
    • src/ directory: Yes (recommended)
    • App Router: Yes
    • Import alias: No (or default @/*)

  2. Install Dependencies: Install NextAuth v5 (currently in beta), the Sinch SDK, and dotenv for credential management. Sinch Node.js SDK Reference.

    bash
    npm install next-auth@beta @sinch/sdk-core dotenv
npm install --save-dev @types/node

  3. Create Project Structure: Set up directories for API routes and authentication configuration.

    sinch-nextauth-2fa/ ├── src/ │ ├── app/ │ │ ├── api/ │ │ │ ├── auth/ │ │ │ │ └── [...nextauth]/ │ │ │ │ └── route.ts │ │ │ ├── verification/ │ │ │ │ ├── request/ │ │ │ │ │ └── route.ts │ │ │ │ └── check/ │ │ │ │ └── route.ts │ │ ├── page.tsx │ │ └── layout.tsx │ ├── lib/ │ │ ├── sinch.ts │ │ └── auth.ts ├── .env.local ├── .gitignore ├── package.json └── tsconfig.json

  4. Configure Environment Variables: Create a .env.local file in the project root. Add your Sinch Application Key and Secret. Never commit this file to version control. Get credentials from Sinch Dashboard.

    dotenv
    # .env.local
SINCH_APPLICATION_KEY=YOUR_APPLICATION_KEY_HERE
SINCH_APPLICATION_SECRET=YOUR_APPLICATION_SECRET_HERE
NEXTAUTH_SECRET=GENERATE_A_RANDOM_SECRET_HERE
NEXTAUTH_URL=http://localhost:3000

    Replace YOUR_APPLICATION_KEY_HERE and YOUR_APPLICATION_SECRET_HERE with actual credentials from your Sinch Verification dashboard. Generate a secure random string for NEXTAUTH_SECRET using:

    bash
    openssl rand -base64 32

  5. Update .gitignore: Ensure .env.local is listed in your .gitignore file (Next.js includes this by default).

    text
    # .gitignore
.env.local
.env*.local
node_modules/
.next/

  6. Create Sinch Client Library (src/lib/sinch.ts): Initialize the Sinch SDK client for reuse across API routes.

    typescript
    // src/lib/sinch.ts
import { SinchClient, Verification } from '@sinch/sdk-core';

// Validate environment variables at module load
if (!process.env.SINCH_APPLICATION_KEY || !process.env.SINCH_APPLICATION_SECRET) {
  throw new Error('Missing SINCH_APPLICATION_KEY or SINCH_APPLICATION_SECRET in environment variables');
}

// Initialize Sinch client with Application credentials
// Reference: https://developers.sinch.com/docs/verification/sdk/node/syntax-reference
export const sinchClient = new SinchClient({
  applicationKey: process.env.SINCH_APPLICATION_KEY,
  applicationSecret: process.env.SINCH_APPLICATION_SECRET,
});

// Export verification service for direct access
export const verificationService = sinchClient.verification;

// Helper to build SMS verification request
export function buildSmsVerificationRequest(phoneNumber: string, reference?: string) {
  return Verification.startVerificationHelper.buildSmsRequest(phoneNumber, reference);
}

// Helper to build SMS report request
export function buildSmsReportRequest(id: string, code: string) {
  return Verification.reportVerificationByIdHelper.buildSmsRequest(id, code);
}

    Why this approach?

    • Centralized Configuration: Single point for SDK initialization and credential validation.
    • Type Safety: TypeScript ensures correct usage of the Sinch SDK.
    • Helper Functions: Simplify common operations like building request payloads per Sinch SDK Helper Methods.
    • Early Validation: Fails fast if credentials are missing, preventing runtime errors.

2. Building the SMS OTP Verification API Routes

Build the API routes for the SMS OTP 2FA flow using Next.js 14 App Router and the Sinch Verification API.

2.1 Create SMS Verification Request Route

This endpoint initiates the SMS verification process and sends the OTP to the user's phone.

File: src/app/api/verification/request/route.ts

typescript
// src/app/api/verification/request/route.ts
import { NextRequest, NextResponse } from 'next/server';
import { verificationService, buildSmsVerificationRequest } from '@/lib/sinch';
import { z } from 'zod';

// Input validation schema using Zod
const RequestSchema = z.object({
  phoneNumber: z.string()
    .regex(/^\+[1-9]\d{1,14}$/, 'Phone number must be in E.164 format (e.g., +14155551234)')
});

export async function POST(request: NextRequest) {
  try {
    // Parse and validate request body
    const body = await request.json();
    const validation = RequestSchema.safeParse(body);

    if (!validation.success) {
      return NextResponse.json(
        { error: 'Invalid phone number format. Use E.164 format (e.g., +14155551234).' },
        { status: 400 }
      );
    }

    const { phoneNumber } = validation.data;

    // Log only last 4 digits for privacy
    console.log(`[Verification Request] Phone ending: ...${phoneNumber.slice(-4)}`);

    // Build request using Sinch SDK helper
    // Reference: https://developers.sinch.com/docs/verification/getting-started/node-sdk/sms-verification
    const requestData = buildSmsVerificationRequest(phoneNumber);

    // Start SMS verification
    const response = await verificationService.verifications.startSms(requestData);

    if (!response.id) {
      throw new Error('Sinch API did not return a verification ID');
    }

    console.log(`[Verification Request] ID: ${response.id}, Status: ${response.status}`);

    return NextResponse.json({
      success: true,
      verificationId: response.id,
      message: 'Verification code sent successfully'
    });

  } catch (error: any) {
    console.error('[Verification Request Error]:', error);

    // Handle Sinch API errors
    if (error.statusCode) {
      return NextResponse.json(
        { error: error.message || 'Verification request failed' },
        { status: error.statusCode }
      );
    }

    // Generic error response
    return NextResponse.json(
      { error: 'An unexpected error occurred. Try again.' },
      { status: 500 }
    );
  }
}

2.2 Create OTP Verification Check Route

This endpoint verifies the SMS OTP code submitted by the user.

File: src/app/api/verification/check/route.ts

typescript
// src/app/api/verification/check/route.ts
import { NextRequest, NextResponse } from 'next/server';
import { verificationService, buildSmsReportRequest } from '@/lib/sinch';
import { z } from 'zod';

// Input validation schema
const CheckSchema = z.object({
  verificationId: z.string().min(1, 'Verification ID is required'),
  code: z.string()
    .regex(/^\d{4,6}$/, 'Code must be 4–6 digits')
});

export async function POST(request: NextRequest) {
  try {
    // Parse and validate request body
    const body = await request.json();
    const validation = CheckSchema.safeParse(body);

    if (!validation.success) {
      return NextResponse.json(
        { error: validation.error.errors[0].message },
        { status: 400 }
      );
    }

    const { verificationId, code } = validation.data;

    console.log(`[Verification Check] ID: ${verificationId}`);

    // Build report request using Sinch SDK helper
    // Reference: https://developers.sinch.com/docs/verification/sdk/node/syntax-reference
    const requestData = buildSmsReportRequest(verificationId, code);

    // Report the SMS code for verification
    const response = await verificationService.verifications.reportSmsById(requestData);

    // Check response status
    // Possible statuses: SUCCESSFUL, PENDING, FAIL, ERROR, DENIED, ABORTED
    // Reference: https://developers.sinch.com/docs/verification/api-reference
    if (response.status === 'SUCCESSFUL') {
      console.log(`[Verification Check] Success – ID: ${verificationId}`);
      return NextResponse.json({
        success: true,
        status: response.status,
        message: 'Phone number verified successfully'
      });
    } else {
      console.warn(`[Verification Check] Failed – ID: ${verificationId}, Status: ${response.status}`);
      return NextResponse.json(
        {
          success: false,
          status: response.status,
          error: 'Invalid or expired verification code'
        },
        { status: 400 }
      );
    }

  } catch (error: any) {
    console.error('[Verification Check Error]:', error);

    // Handle Sinch API errors
    if (error.statusCode) {
      return NextResponse.json(
        { error: error.message || 'Verification check failed' },
        { status: error.statusCode }
      );
    }

    // Generic error response
    return NextResponse.json(
      { error: 'An unexpected error occurred. Try again.' },
      { status: 500 }
    );
  }
}

Why these choices?

  • Next.js App Router: Uses the new app/api directory structure with route.ts files for API endpoints (Next.js 14 standard).
  • Type Safety: TypeScript with Zod for runtime validation ensures type-safe API contracts.
  • E.164 Format: Phone numbers must be in E.164 international format (e.g., +14155551234) as required by Sinch.
  • Helper Methods: Uses Sinch SDK helper functions (buildSmsRequest, buildSmsReportRequest) for correct payload structure per SDK documentation.
  • Status Handling: Checks response status (SUCCESSFUL, PENDING, FAIL) as documented in Sinch Verification API Reference.
  • Privacy: Logs only last 4 digits of phone numbers and never logs verification codes.

2.3 Install Additional Dependencies

Add Zod for validation:

bash
npm install zod

3. Creating the SMS OTP Verification UI

Create React components for the phone number input and OTP verification forms.

3.1 Main Page with SMS Verification Flow

File: src/app/page.tsx

typescript
'use client';

import { useState } from 'react';

export default function Home() {
  const [step, setStep] = useState<'phone' | 'code' | 'success'>('phone');
  const [phoneNumber, setPhoneNumber] = useState('');
  const [code, setCode] = useState('');
  const [verificationId, setVerificationId] = useState('');
  const [error, setError] = useState('');
  const [loading, setLoading] = useState(false);

  const handleRequestCode = async (e: React.FormEvent) => {
    e.preventDefault();
    setError('');
    setLoading(true);

    try {
      const response = await fetch('/api/verification/request', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ phoneNumber }),
      });

      const data = await response.json();

      if (response.ok) {
        setVerificationId(data.verificationId);
        setStep('code');
      } else {
        setError(data.error || 'Failed to send verification code');
      }
    } catch (err) {
      setError('Network error. Try again.');
    } finally {
      setLoading(false);
    }
  };

  const handleVerifyCode = async (e: React.FormEvent) => {
    e.preventDefault();
    setError('');
    setLoading(true);

    try {
      const response = await fetch('/api/verification/check', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ verificationId, code }),
      });

      const data = await response.json();

      if (response.ok && data.success) {
        setStep('success');
      } else {
        setError(data.error || 'Invalid verification code');
      }
    } catch (err) {
      setError('Network error. Try again.');
    } finally {
      setLoading(false);
    }
  };

  const reset = () => {
    setStep('phone');
    setPhoneNumber('');
    setCode('');
    setVerificationId('');
    setError('');
  };

  return (
    <main className="flex min-h-screen flex-col items-center justify-center p-24">
      <div className="w-full max-w-md space-y-8">
        <div className="text-center">
          <h1 className="text-3xl font-bold">Phone Verification</h1>
          <p className="mt-2 text-gray-600">Secure 2FA with Sinch SMS OTP</p>
        </div>

        {error && (
          <div className="bg-red-50 border border-red-200 text-red-700 px-4 py-3 rounded">
            {error}
          </div>
        )}

        {step === 'phone' && (
          <form onSubmit={handleRequestCode} className="space-y-4">
            <div>
              <label htmlFor="phone" className="block text-sm font-medium mb-2">
                Phone Number (E.164 format)
              </label>
              <input
                id="phone"
                type="tel"
                value={phoneNumber}
                onChange={(e) => setPhoneNumber(e.target.value)}
                placeholder="+14155551234"
                className="w-full px-4 py-2 border rounded-lg focus:ring-2 focus:ring-blue-500"
                required
                disabled={loading}
              />
              <p className="mt-1 text-sm text-gray-500">
                Include country code (e.g., +1 for US)
              </p>
            </div>
            <button
              type="submit"
              disabled={loading}
              className="w-full bg-blue-600 text-white py-2 px-4 rounded-lg hover:bg-blue-700 disabled:opacity-50"
            >
              {loading ? 'Sending...' : 'Send Verification Code'}
            </button>
          </form>
        )}

        {step === 'code' && (
          <form onSubmit={handleVerifyCode} className="space-y-4">
            <div>
              <label htmlFor="code" className="block text-sm font-medium mb-2">
                Verification Code
              </label>
              <input
                id="code"
                type="text"
                value={code}
                onChange={(e) => setCode(e.target.value)}
                placeholder="Enter 4–6 digit code"
                className="w-full px-4 py-2 border rounded-lg focus:ring-2 focus:ring-blue-500"
                maxLength={6}
                pattern="\d{4,6}"
                required
                disabled={loading}
              />
              <p className="mt-1 text-sm text-gray-500">
                Code sent to {phoneNumber}
              </p>
            </div>
            <button
              type="submit"
              disabled={loading}
              className="w-full bg-blue-600 text-white py-2 px-4 rounded-lg hover:bg-blue-700 disabled:opacity-50"
            >
              {loading ? 'Verifying...' : 'Verify Code'}
            </button>
            <button
              type="button"
              onClick={reset}
              className="w-full bg-gray-200 text-gray-700 py-2 px-4 rounded-lg hover:bg-gray-300"
            >
              Use Different Number
            </button>
          </form>
        )}

        {step === 'success' && (
          <div className="text-center space-y-4">
            <div className="text-6xl">✅</div>
            <h2 className="text-2xl font-bold text-green-600">
              Verification Successful!
            </h2>
            <p className="text-gray-600">
              Your phone number has been verified.
            </p>
            <button
              onClick={reset}
              className="bg-blue-600 text-white py-2 px-6 rounded-lg hover:bg-blue-700"
            >
              Start Over
            </button>
          </div>
        )}
      </div>
    </main>
  );
}

3.2 Run the Application

Start the development server:

bash
npm run dev

Navigate to http://localhost:3000 and test the verification flow:

  1. Enter a phone number in E.164 format (e.g., +14155551234)
  2. Click "Send Verification Code"
  3. Check your phone for the SMS
  4. Enter the code received
  5. See the success message upon verification

4. Integrating SMS OTP with NextAuth v5

To integrate this SMS OTP verification flow with NextAuth for actual authentication, create a custom credentials provider.

File: src/lib/auth.ts

typescript
// src/lib/auth.ts
import NextAuth from 'next-auth';
import Credentials from 'next-auth/providers/credentials';

export const { handlers, signIn, signOut, auth } = NextAuth({
  providers: [
    Credentials({
      name: 'Phone Verification',
      credentials: {
        phoneNumber: { label: 'Phone Number', type: 'tel' },
        verificationId: { label: 'Verification ID', type: 'text' },
        code: { label: 'Code', type: 'text' },
      },
      async authorize(credentials) {
        // In production, verify the code via your API and look up/create the user in your database

        const response = await fetch(`${process.env.NEXTAUTH_URL}/api/verification/check`, {
          method: 'POST',
          headers: { 'Content-Type': 'application/json' },
          body: JSON.stringify({
            verificationId: credentials.verificationId,
            code: credentials.code,
          }),
        });

        const data = await response.json();

        if (response.ok && data.success) {
          // Return user object – in production, fetch from database
          return {
            id: credentials.phoneNumber as string,
            phone: credentials.phoneNumber as string,
          };
        }

        return null; // Authentication failed
      },
    }),
  ],
  pages: {
    signIn: '/auth/signin',
  },
  callbacks: {
    async jwt({ token, user }) {
      if (user) {
        token.phone = user.phone;
      }
      return token;
    },
    async session({ session, token }) {
      if (token.phone) {
        session.user.phone = token.phone as string;
      }
      return session;
    },
  },
});

File: src/app/api/auth/[...nextauth]/route.ts

typescript
// src/app/api/auth/[...nextauth]/route.ts
import { handlers } from '@/lib/auth';

export const { GET, POST } = handlers;

5. SMS OTP Security Best Practices

5.1 Rate Limiting for SMS Verification

Implement rate limiting to prevent abuse. Install and configure @upstash/ratelimit:

bash
npm install @upstash/ratelimit @upstash/redis

Update verification request route:

typescript
import { Ratelimit } from '@upstash/ratelimit';
import { Redis } from '@upstash/redis';

const ratelimit = new Ratelimit({
  redis: Redis.fromEnv(),
  limiter: Ratelimit.slidingWindow(5, '1 h'), // 5 requests per hour
  analytics: true,
});

export async function POST(request: NextRequest) {
  // Rate limit by IP
  const ip = request.ip ?? '127.0.0.1';
  const { success } = await ratelimit.limit(ip);

  if (!success) {
    return NextResponse.json(
      { error: 'Too many requests. Try again later.' },
      { status: 429 }
    );
  }

  // ... rest of the handler
}

5.2 Environment Security

  • Never expose credentials: Keep .env.local out of version control.
  • Use secrets management: In production, use services like AWS Secrets Manager, Azure Key Vault, or Vercel Environment Variables.
  • HTTPS only: Always use HTTPS in production to encrypt data in transit.
  • NEXTAUTH_SECRET: Generate a strong random secret for NextAuth session encryption.

5.3 Input Validation

  • E.164 Format: Always validate phone numbers are in proper E.164 international format.
  • Code Format: Limit OTP codes to 4–6 digits as per Sinch API specifications.
  • Sanitize Inputs: Use Zod or similar validation libraries to prevent injection attacks.

5.4 Verification Best Practices

  • Time Limits: Sinch verification codes typically expire after 2–5 minutes (configurable in dashboard).
  • Attempt Limits: Limit failed verification attempts to 3–5 tries before requiring a new code.
  • Session Management: Store verification IDs securely in session storage or encrypted cookies.
  • Cost Management: Monitor SMS usage to prevent unexpected charges. Sinch SMS pricing varies by country.

6. Error Handling and Logging

6.1 Comprehensive Error Types

Common Sinch Verification API errors and how to handle them:

Error StatusDescriptionHandling Strategy
400 Bad RequestInvalid phone number or parametersValidate input format before API call
401 UnauthorizedInvalid Application Key/SecretCheck environment variables
429 Too Many RequestsRate limit exceededImplement client-side rate limiting and retry logic
500 Internal Server ErrorSinch service issueRetry with exponential backoff

Reference: Sinch API Error Codes

6.2 Structured Logging

Use a logging library like Winston or Pino for production:

bash
npm install winston
typescript
import winston from 'winston';

export const logger = winston.createLogger({
  level: process.env.LOG_LEVEL || 'info',
  format: winston.format.combine(
    winston.format.timestamp(),
    winston.format.json()
  ),
  transports: [
    new winston.transports.Console(),
    new winston.transports.File({ filename: 'error.log', level: 'error' }),
    new winston.transports.File({ filename: 'combined.log' }),
  ],
});

7. Testing SMS OTP Verification

7.1 Unit Tests

Test the API routes using Jest and Supertest:

bash
npm install --save-dev jest @types/jest ts-jest supertest @types/supertest

7.2 Integration Tests

Test the full verification flow:

typescript
describe('Verification Flow', () => {
  it('should send verification code', async () => {
    const response = await fetch('/api/verification/request', {
      method: 'POST',
      body: JSON.stringify({ phoneNumber: '+14155551234' }),
    });

    expect(response.status).toBe(200);
    const data = await response.json();
    expect(data.verificationId).toBeDefined();
  });
});

8. Production Deployment

8.1 Environment Configuration

Set environment variables in your hosting platform (Vercel, AWS, Azure):

bash
SINCH_APPLICATION_KEY=<your_production_key>
SINCH_APPLICATION_SECRET=<your_production_secret>
NEXTAUTH_SECRET=<strong_random_secret>
NEXTAUTH_URL=https://yourdomain.com

8.2 Performance Optimization

  • Caching: Cache Sinch client initialization.
  • Connection Pooling: Reuse HTTP connections for API calls.
  • CDN: Serve static assets via CDN.
  • Monitoring: Set up monitoring for API response times and error rates.

8.3 Compliance Considerations

  • GDPR/Privacy: Obtain user consent before sending SMS messages.
  • TCPA (US): Comply with Telephone Consumer Protection Act requirements.
  • Data Retention: Implement policies for storing phone numbers and verification logs.
  • Audit Logs: Maintain logs of all verification attempts for security audits.

9. SMS Verification Cost Considerations

Sinch charges per verification attempt. Costs vary by country:

  • US SMS: Approximately $0.02–$0.04 per verification.
  • International: Varies significantly by destination country.
  • FlashCall: Often cheaper alternative to SMS in supported regions.

Monitor usage in the Sinch Dashboard and set up alerts for unexpected spikes. Reference: Sinch SMS Pricing.

10. Alternative Verification Methods

Sinch supports multiple verification methods beyond SMS:

Frequently Asked Questions (FAQ)

How do I implement SMS OTP in Next.js?

To implement SMS OTP in Next.js, use a verification service like Sinch Verification API with NextAuth. Create API routes for requesting and verifying OTP codes, integrate with NextAuth credentials provider, and implement rate limiting for security. Follow the complete implementation guide above for step-by-step instructions.

What is the difference between SMS OTP and 2FA?

SMS OTP (One-Time Password) is a specific type of two-factor authentication (2FA). While 2FA refers to any authentication method requiring two verification factors, SMS OTP specifically uses text messages to deliver temporary verification codes to users' mobile devices.

How secure is SMS-based two-factor authentication?

SMS-based 2FA adds significant security compared to passwords alone but is vulnerable to SIM swapping attacks. For critical applications, consider combining SMS OTP with additional security measures like rate limiting, device fingerprinting, or using alternative methods like TOTP (Time-based One-Time Password) authenticator apps.

How much does Sinch SMS verification cost?

Sinch SMS verification costs vary by country, typically ranging from $0.02–$0.04 per verification in the US. International rates vary significantly. Consider using FlashCall verification in supported regions for lower costs. Monitor usage in the Sinch Dashboard to manage expenses.

Conclusion

You now have a complete, production-ready SMS OTP 2FA implementation using Sinch Verification API, Next.js 14, and NextAuth v5. This solution provides:

✅ Secure phone number verification with SMS OTP ✅ Type-safe TypeScript implementation ✅ Modern Next.js App Router architecture ✅ Proper error handling and logging ✅ Security best practices (rate limiting, input validation) ✅ Production deployment guidance

Additional Resources

For questions or issues, consult the Sinch Developer Forum or contact Sinch support.

Frequently Asked Questions

How to add two-factor authentication to Node.js

Two-factor authentication (2FA) can be added to your Node.js Express app using the Vonage Verify API. This involves sending a one-time password (OTP) to the user's phone via SMS, adding an extra layer of security beyond just a password. This guide provides a step-by-step tutorial for implementing this security measure.

What is Vonage Verify API used for in Node.js

The Vonage Verify API is used to manage the complexities of OTP generation, delivery (via SMS or voice call), and verification within your Node.js application. It simplifies the process by handling the OTP lifecycle, so you don't have to build and maintain this logic yourself.

Why use two-factor authentication with Node.js Express?

2FA enhances security by verifying user identity through a secondary channel (SMS OTP). This mitigates the risks associated with compromised passwords, protecting user accounts more effectively.

When should I implement 2FA in my Node.js app?

Implement 2FA as early as possible in your development process to prioritize user account security from the start. This proactive approach minimizes vulnerabilities and reinforces trust with your users.

Can I customize the branding in Vonage Verify API SMS?

Yes, you can customize the "brand" name that appears in the SMS message sent to the user during the 2FA process. Set the `brand` parameter in the `vonage.verify.start()` method to your app's name, enhancing the user experience.

How to install necessary dependencies for Vonage 2FA

Use `npm install express ejs body-parser @vonage/server-sdk dotenv` in your terminal to install the required packages for a Node.js 2FA project using Vonage. This command sets up Express for the server, EJS for templating, body-parser for handling forms, the Vonage SDK, and dotenv for environment variables.

What is the project structure for Vonage 2FA tutorial?

The project utilizes a structured approach with directories for views (EJS templates), a .env file for credentials, .gitignore for excluding files from version control, server.js for the main application logic, and the standard package.json and node_modules folders.

How to set up environment variables for Vonage API

Create a `.env` file in your project's root directory and add your `VONAGE_API_KEY`, `VONAGE_API_SECRET`, and desired `PORT`. Load these variables into your `server.js` file using `require('dotenv').config();`.

How to handle Vonage API errors in Node.js

Check the `status` property in the Vonage API response. A non-zero status indicates an error. Log the `status` and `error_text` and display a user-friendly error message based on these values.

How to start the Vonage verification process in my app

Initiate 2FA by calling `vonage.verify.start()` with the user's phone number and your app's brand name. This sends the OTP to the user's device. The function returns a `request_id` which you need to verify the OTP later.

How to verify the OTP sent by Vonage Verify API

Call `vonage.verify.check()` with the `request_id` (obtained from `vonage.verify.start()`) and the OTP entered by the user. This confirms if the entered code matches the one sent.

What is the purpose of vonage.verify.cancel() function?

The `vonage.verify.cancel()` function is used to explicitly cancel an ongoing verification request. This is useful if the user navigates away from the verification process or requests a new code. This can be implemented as a GET route in the Express application.

What does a '0' status code mean in Vonage Verify API response

A status code of '0' in the Vonage Verify API response signifies success. Any other status code indicates an error, which can be debugged using the accompanying error text (`error_text`) from the API response.

How to handle network errors when using Vonage Verify API

Wrap Vonage Verify API calls within a `try...catch` block to handle potential network errors. Use a retry mechanism with exponential backoff for transient network issues.

What are the prerequisites for Vonage 2FA tutorial

You'll need Node.js and npm (or yarn) installed, a Vonage API account (sign up for free at [https://dashboard.nexmo.com/sign-up](https://dashboard.nexmo.com/sign-up)), basic understanding of Node.js, Express, and web concepts, and a text editor or IDE.

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