Homework #$02$: Message Authentication Codes

The given figure below illustrates three basic uses of a Message Authentication Code $($MAC$).$

$($h$)$

a$.[5$ pts$.]$ Critically analyze the differences of security features in terms of authentication, confidentiality, and integrity provided by the three scenarios; $a,b,$ and c illustrated in the figure. What are the potential vulnerabilities in each approach?

b$.[5$ pts$.]$ In scenario $($c$)$ of the figure, encryption is combined with the MAC. Discuss the rationale behind this design choice. What would be the security implications if encryption were omitted in this context?