MetaMask is compatible with Sign In With Ethereum (SIWE), a web3-native method of managing your digital identity.
What does Sign In With Ethereum mean?
Conventionally, online identity is proven through intermediaries—identity providers—or by providing your email. Identity providers such as Apple, Facebook, or Google hold sufficient personal information to be able to prove, theoretically, that it's you who is interacting with a product or service online. This is why you often have the option to create accounts and log into websites using their identity services (single sign-on/SSO services).
This model relies on intermediaries holding your personal data and information — mainly your email and name. With SIWE, you can use your Ethereum address to sign into a compatible dapp instead, allowing greater privacy, with no requirement to hand over personal information simply to interact with the web.
Rather than placing data in the custody of intermediaries, the sign-in process is self-custodial.
If you're interested in learning more about the technical details of SIWE, check out EIP-4361, the Ethereum Improvement Proposal that describes it. This explanation of its significance from Spruce, a team leading its development, is also very useful.
How does it look in MetaMask?
SIWE is a feature that developers can choose to make available to users of their dapps. Accordingly, you won't find it on every dapp you visit, but only on those where it's implemented.
Signing in with SIWE looks similar in MetaMask to many other signature requests:
What does this mean? Well, you're essentially just signing a transaction to demonstrate your identity. Nothing more, nothing less. Using your private key to sign a transaction proves that you're the account holder, and have the authority to use that account on the dapp.