Alice $($A$)$ and Bob $($B$)$ exchange messages.

a$.$What is happening in the following exchange? Describe each step. M is a message and n is a serial number.

$(4$ marks$)$

i$.$A $->$ B : R

ii$.$B $->$ A: $\{\{$M$1\}$K$,$ n$1\}$K$($B pr$)$

b$.$The serial number provides a guarantee of freshness. Name two other ways of guaranteeing freshness that Bob and Alice could have used