When a web application is connected to Phantom, it can also request that the user signs a given message. Applications are free to write their own messages which will be displayed to users from within Phantom's signature prompt. Message signatures do not involve network fees and are a convenient way for apps to verify ownership of an address.
In order to send a message for the user to sign, a web application must:
Provide a hex or UTF-8 encoded string as a Uint8Array.
Request that the encoded message is signed via the user's Phantom wallet.