Protecting authorship of a message
Public key cryptography per-ce cannot protect the authorship. In your example, e.g., Bob could re-type Alice' joke, sign it with his own valid key, and claim that he thought about it before Alice sent him the joke.

A possible solution could be to use a trusted escrow: Any party who invents a joke and wants to hold title to it has to send the joke to the trusted escrow before telling the joke to anyone else. The escrow timestamps the joke and signs the timestamped joke with their private key.

Now if Carrol receives the same joke from both Alice and Bob, who both claim title to it, and Carrol wants to know who is the real author, then she goes as follows:


Ask both Alice and Bob to send the joke as was originally signed by the trusted escrow.
If one of them fails to provide a signed joke then we assume that she/he is an imposer.
If both send a signed joke then Carrol will:

a. Verify with the trusted escrow's public keys that both signatures are valid.

b. Compare the time stamps.


We formally define the author of the joke as the one whose timestamp is the earliest.