One Click Login With MetaMask

The basic idea is that it’s cryptographically easy to prove the ownership of an account by signing a piece of data using a private key.
If you manage to sign a precise piece of data generated by our back end, then the back end will consider you the owner of that public address. Therefore, we can build a message-signing-based authentication mechanism with a user’s public address as their identifier.
Please note that while we will be using tools connected to the Ethereum blockchain (MetaMask, Ethereum public addresses), this login process does not actually need the blockchain:
It only needs its cryptography functions. That being said, with MetaMask becoming such a popular extension, now seems a good time to introduce this login flow.

MetaMask is a browser plugin, available as the MetaMask Chrome extension or Firefox Add-on.
At its core, it serves as an Ethereum wallet: By installing it, you will get access to a unique Ethereum public address, with which you can start sending and receiving ether or tokens. But MetaMask does something more than an Ethereum wallet.
As a browser extension, it can interact with the current webpage you’re browsing. It does so by injecting a JavaScript library called web3.js in every webpage you visit. Once injected, a web3 object will be available via window.web3 in the JavaScript code of this website.
To have a look at what this object looks like, just type window.web3 in the Chrome or Firefox DevTools console, if you have MetaMask installed.