 If a fourth party, Dave, (anyone other than Alice, Charles, or Bob) requires Alice to disclose her secret key, she can defend with the argument that she used the key merely for authentication and did not intend to make the message confidential.If Dave cannot force Alice to disclose an authentication key (the knowledge of which would enable him to forge messages from Alice), then her messages will remain confidential.On the other hand, Charles does not even possess any secret keys that he could be ordered to disclose.