Some feedback - this part was not clear to me:
POST the signature, the URI and the public key to the callback URL
Specifically, in what standard format are the parameters passed? Using Json? Http headers? Encoded onto the Uri? What? A tangible example of the exact HTTP POST request sent from the client, byte by byte, would have been enormously helpful here. As is now, I'm blind on the server side - I don't know what data to expect and have to guess how the client sends its data?