Think of DH key agreement, for example (although this is not precise). Once the agreement has been reached you might "verify" the sender by asking him the key (

Think of DH key agreement, for example (although this is not precise). Once the agreement has been reached you might "verify" the sender by asking him the key (if he knows it, he is the sender) but you cannot create any binding "signed" certification that he is the sender (knowing a secret you already know is not a signature, you might have faked the communication).