Let's assume a dishonest Alice who sends, encrypts & digital signs a message to Bob.
Bob stores the decrypted message and the digital signature in a database.
However Alice is a bad girl and erases all data from her side, including her private/public keys. She then claims she never sent the message.
Is there a solution to this case?
In my case, Alice uses a communication tool that Bob has provided, where the private/public key are generated within the context of the tool (with no 'public key' advertised somewhere else, hence making it even more difficult to prove anything)