I posted a similar question but I see that I did it incorrectly. For this reason I post it again hoping not to cause any problem.

I have the following situation:
Alice often sends funny jokes to Bob. She does not care about the confidentiality of these messages but wants to get credit for the jokes to prevent Bob from claiming authorship of or modifying them. How can public-key cryptography achieve this?

I have provided the following answer but I need feedback from an expert:
It is clear that we are talking about an ideal situation because Bob can create a new message and type each joke entirely. However, we can do the following taking in consideration some restrictions for Bob.

If confidentiality is not important then it is not necessary to encrypt the messages. What Alice can do is to include a text at the end of each joke stating she is the author, for example, “Alice is the author of this joke”. After that, Alice digitally signs each message with her private key producing a hash of the message and sends it to Bob along with her public key. When Bob sends a joke to Carol, he sends Alice’s original message including Alice's signature and Alice's public key. This way, if Bob modifies the message, the digital signature will not be successfully validated by Carol because there will be a different result of the hash meaning that the original message suffered a change in some way.

I will very much appreciate your comments.


2 Answers 2


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:

  1. Ask both Alice and Bob to send the joke as was originally signed by the trusted escrow.

  2. If one of them fails to provide a signed joke then we assume that she/he is an imposer.

  3. 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.


Probably the best (although not totally reliable) solution is using steganography to secretly watermark your jokes. For instance, write your jokes such that the first letter of every third word spells out your name. Then hope Bob doesn't notice, and doesn't rewrite your jokes so that it destroys your watermark.


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