$($a$)$ There are three typical ways to use nonces as challenges. Suppose $N_a$ is a nonce generated by $A,A$

and $B$ share key $K,$ and $f(*)$ is a function $($such as an increment$).$ The three usages are

Describe situations for which each usage is appropriate.

$($b$)$ Encode the message $"101100110110000110101010"$ using "Radix$-64/$Base$-64$ encoding scheme.

$($c$)$ In Kerberos, when Bob receives a Ticket from Alice, how does he know it is genuine?