What kind of tampering are you thinking about here? Aren't those messages sent from the server to the client? Thus, in-transit tampering should be prevented by

What kind of tampering are you thinking about here? Aren't those messages sent from the server to the client? Thus, in-transit tampering should be prevented by TLS. If the key isn't sent to the client, how is the client supposed to display that message? Why are these messages encoded into a cookie in the first place? I don't think I understand the concept.