I'm looking to setup a monero address for each of my users on my website so they can sign a message to prove that they own the monero address. Since you cannot sign a message with your payment_id, this is the only way it seems. My main concern is if a user can prove that they own that specific address, by signing the a message.
A good example is by signing the message of a bitcoin address, it proves that the monero user owns that particular bitcoin address. This is just one of the use cases I have.