pragma solidity ^0.4.2;

contract Login {
   event LoginAttempt(address sender, string challenge);

   function Login (string challenge) public {
       LoginAttempt(msg.sender, challenge);
   }

}

const LoginContract = new web3.eth.Contract(abiLogin);

// LoginAttempt is the name of the event that signals logins in the
// Login contract. This is specified in the login.sol file.

loginAttempt = LoginContract.LoginAttempt({});

challenges = {};
successfulLogins = {};

loginAttempt.watch({}, '', function(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;
    }
});