Recall Needham-Schroeder protocol we studied in Week 3. Let us consider Eve, an eavesdropper who has been logging all messages sent and received by Alice. Since Eve does not know Ka,kdc, she cannot learn any useful information. Later time, she is successful to somehow learn (or steal) Ka,kdc. Fortunately, shortly after that, Alice noticed that her key has been compromised and then contacted KDC to update her key to Ka,kdc.

a. The key update does not completely prevent Eve from impersonating Alice against Bob. Please explain how Eve can take advantage of knowledge of Ka,kdc and recorded messages (to/from Alice) to achieve it?

b. Please discuss a possible countermeasure.