Replace UUID with public key as unique user ID

[LGro]

UUIDs are more prone to impersonation because they are not backed up by ownership of a private key like a public key would be.

Attack scenario: I learn about a user's UUID and create a new account with that UUID, attempting to connect with others.

Whereas with a public key, as long as it also comes with a signature that can be verified, this attack is not possible.