When choosing a user authentication method for your application, you usually have several options: develop your own system for identification, authentication, and authorization, or use a ready-made solution. A ready-made solution means that the user already has an account on an external system such as Google, Facebook, or GitHub, and you use the appropriate mechanism, most likely OAuth, to provide limited access to the user’s protected resources without transferring the username and password to it. The second option with OAuth is easier to implement, but there is a risk for your user if the user's account is blocked and the user will lose access to your site. Also, if I, as a user, want to enter a site that I do not trust, I have to provide my personal information, such as my email and full name, sacrificing my anonymity.
In this article, we’ll build an alternative login method for Spring using the MetaMask browser extension. MetaMask is a cryptocurrency wallet used to manage Ethereum assets and interact with the Ethereum blockchain. Unlike the OAuth provider, only the necessary set of data can be stored on the Ethereum network. We must take care not to store secret information in the public data, but since any wallet on the Ethereum network is in fact a cryptographic strong key pair, in which the public key determines the wallet address and the private key is never transmitted over the network and is known only by the owner, we can use asymmetric encryption to authenticate users.