Building a Web3 Authentication Flow

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In the evolving landscape of decentralized applications, Web3 authentication stands as a cornerstone for secure, user-centric identity management. Unlike traditional web2 systems that rely on centralized databases to store personal information, Web3 enables users to authenticate using blockchain wallets and cryptographic proofs—putting control back into their hands.

This guide walks you through building a real-world Sign-In with Ethereum (SIWE) authentication flow in a Next.js application. You'll learn how to implement secure, non-custodial login using ethers.js, support multiple wallet providers across devices, and verify user identity server-side—all while maintaining privacy and decentralization principles.

Whether you're a developer exploring decentralized identity or building your first dApp, this tutorial offers a practical foundation in modern Web3 auth patterns.

Understanding Web3 Authentication

At its core, Web3 authentication replaces passwords with cryptographic signatures. Instead of entering credentials, users sign a message with their private key—proving ownership of a wallet address without exposing sensitive data.

This process hinges on two key operations:

This method is not only secure but also portable across platforms and applications. Users can log in seamlessly from desktop or mobile, using any compatible wallet.

👉 Discover how blockchain-powered identity verification works in practice.

Core Components of the Authentication Flow

Let’s break down the architecture:

1. Client-Side Wallet Connection

Using libraries like web3modal, users connect their wallet (e.g., MetaMask, Rainbow) through a unified interface. This abstraction supports both browser extensions and mobile wallets via WalletConnect.

2. Nonce Generation & Challenge

The server generates a one-time-use number (nonce) tied to the user’s wallet address. This prevents replay attacks and ensures each login attempt is unique.

3. Message Signing

The client requests the user to sign the nonce using their wallet. This creates a digital signature bound to their private key.

4. Server-Side Verification

The server uses ethers.utils.verifyMessage() to decode the signature. If the recovered address matches the original, authentication succeeds.

These steps form a robust, trustless login system—no passwords, no third-party trackers.

Setting Up the Project

Start by creating a new Next.js app:

npx create-next-app sign-in-with-ethereum
cd sign-in-with-ethereum

Install required dependencies:

npm install ethers web3modal @walletconnect/web3-provider @toruslabs/torus-embed

These libraries provide:

Implementing the Backend API

Create an in-memory user registry at utils/users.js:

export const users = {}
In production, replace this with a database or decentralized storage like Ceramic.

Generate Nonce (pages/api/auth.js)

import { users } from '../../utils/users'

export default function handler(req, res) {
  const { address } = req.query
  let user = users[address]

  const nonce = Math.floor(Math.random() * 10000000)

  if (!user) {
    user = { address, nonce }
  } else {
    user.nonce = nonce
  }

  users[address] = user
  res.status(200).json(user)
}

This endpoint returns a fresh nonce for each login request.

Verify Signature (pages/api/verify.js)

import { ethers } from 'ethers'
import { users } from '../../utils/users'

export default function handler(req, res) {
  const { address, signature } = req.query
  const user = users[address]

  if (!user || !signature) {
    return res.status(400).json({ authenticated: false })
  }

  const decodedAddress = ethers.utils.verifyMessage(user.nonce.toString(), signature)

  const authenticated = decodedAddress.toLowerCase() === address.toLowerCase()
  res.status(200).json({ authenticated })
}

This checks whether the signed message was created by the claimed address.

Building the Frontend Interface

Update pages/index.js with the following logic:

import React, { useState } from 'react'
import { ethers } from 'ethers'
import Web3Modal from 'web3modal'
import WalletConnectProvider from '@walletconnect/web3-provider'

const ConnectWallet = () => {
  const [account, setAccount] = useState('')
  const [connection, setConnection] = useState(false)
  const [loggedIn, setLoggedIn] = useState(false)

  async function getWeb3Modal() {
    const Torus = (await import('@toruslabs/torus-embed')).default
    return new Web3Modal({
      network: 'mainnet',
      cacheProvider: false,
      providerOptions: {
        torus: { package: Torus },
        walletconnect: {
          package: WalletConnectProvider,
          options: { infuraId: 'your-infura-id' }
        }
      }
    })
  }

  async function connect() {
    const web3Modal = await getWeb3Modal()
    const instance = await web3Modal.connect()
    const provider = new ethers.providers.Web3Provider(instance)
    const accounts = await provider.listAccounts()
    setConnection(instance)
    setAccount(accounts[0])
  }

  async function signIn() {
    const res = await fetch(`/api/auth?address=${account}`)
    const user = await res.json()
    const provider = new ethers.providers.Web3Provider(connection)
    const signer = provider.getSigner()
    const signature = await signer.signMessage(user.nonce.toString())
    const verifyRes = await fetch(`/api/verify?address=${account}&signature=${signature}`)
    const data = await verifyRes.json()
    setLoggedIn(data.authenticated)
  }

  return (
    <div style={{ width: '900px', margin: '50px auto', textAlign: 'center' }}>
      {!connection && (
        <button style={button} onClick={connect}>Connect Wallet</button>
      )}
      {connection && !loggedIn && (
        <button style={button} onClick={signIn}>Sign In</button>
      )}
      {loggedIn && <h2>Welcome, {account}</h2>}
    </div>
  )
}

const button = {
  width: '100%',
  margin: '5px',
  padding: '20px',
  border: 'none',
  backgroundColor: 'black',
  color: 'white',
  fontSize: 16,
  cursor: 'pointer'
}

export default ConnectWallet

Users now experience:

  1. Click “Connect Wallet” → choose provider.
  2. Click “Sign In” → approve signature in wallet.
  3. Upon success → see welcome message.

👉 See how leading platforms streamline crypto authentication today.

Frequently Asked Questions

Q: Is this method secure against replay attacks?
A: Yes. Each login uses a randomly generated nonce invalidated after use, preventing attackers from reusing old signatures.

Q: Can I use this in production?
A: While functional, consider upgrading the user store to a persistent database or decentralized protocol like Ceramic for scalability and reliability.

Q: Which wallets are supported?
A: MetaMask, Torus, WalletConnect-compatible apps (e.g., Rainbow, Trust Wallet), and more—thanks to web3modal.

Q: Does this work on mobile?
A: Absolutely. WalletConnect bridges web and mobile experiences seamlessly.

Q: How does this differ from OAuth?
A: Unlike OAuth, no third party holds your data. You retain full control over your identity via your wallet.

Q: What are the core keywords for SEO?
A: Web3 authentication, Sign-In with Ethereum, blockchain login, decentralized identity, SIWE, crypto authentication, Ethereum signature verification.

Final Thoughts & Next Steps

You’ve now built a fully functional Web3 authentication system. From wallet connection to cryptographic verification, this pattern empowers users with true ownership of their digital identity.

To go further:

With tools like ethers.js and web3modal, implementing secure, cross-platform authentication has never been easier.

👉 Unlock advanced identity solutions powered by blockchain technology.