Problem $2\mathrm{.}36($page $91)$

a$.$

The protocol is secure because AES$-128$ is secure and r is randomly generated.

b$.$

The protocol is not secure because it leaks a plaintext$-$ciphertext pair $($r$,$rA$)$ if Alice and Bob do share the same AES$-128$ key, which may be used to launch known$-$plaintext attacks.

c$.$

The protocol is not secure because r is short.

d$.$

The protocol is secure because the secret key KA and KB cannot be learned from the communications by the attacker