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system in Ethereum that allows special data to be logged. Events are generally watched by clients monitoring the evolution of the blockchain. This allows actions to be taken by clients when events are created. In our case, whenever a user attempts to login, an event created with the challenge is broadcast. We only care about receiving a call from the rightful owner of the Ethereum address that was passed to the third party website. And, thanks to the way Ethereum works, we can be sure the sender was the one who performed the call. In addition to the sender’s address, the challenge is also broadcast. This means anyone watching the blockchain now knows the challenge. However, this cannot be used on its own to impersonate a user: a user can only interact with the backend through the session JWT. This means an attacker must know three pieces of information to impersonate a user: the Ethereum address, the challenge AND the JWT issued with the challenge. Since JWTs are signed, an attacker cannot create a valid JWT to impersonate an user, even with access to the challenge. What follows is our backend code. First, let’s see how to watch for Ethereum events: const LoginContract = require('./login_contract.js'); const loginContract = LoginContract.at(process.env.LOGIN_CONTRACT_ADDRESS || '0xf7b06365e9012592c8c136b71c7a2475c7a94d71'); // LoginAttempt is the name of the event that signals logins in the // Login contract. This is specified in the login.sol file. const loginAttempt = loginContract.LoginAttempt(); const challenges = {}; const successfulLogins = {}; loginAttempt.watch((error, event) => { if(error) { console.log(error); return; } console.log(event); const sender = event.args.sender.toLowerCase(); // If the challenge sent through Ethereum matches the one we generated, // mark the login attempt as valid, otherwise ignore it. if(challenges[sender] === event.args.challenge) { successfulLogins[sender] = true; } }); The login_contract.js file contains what is needed to inter-operate with our contract. Let’s take a look: // web3 is an Ethereum client library 41PDF Image | Intro to Ethereum and Smart Contracts
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