Consider the following protocol:

A $->$ KDC :IDA$||$IDB$||$N$1$

KDC $->$ A :E$($ka$,[$ks$||$IDB$||$N$1||$E$($kb$,[$ks$||$IDA$]))$

A $->$ B :E$($kb$,[$ks$||$IDA$])(1)$ B $->$ A :E$($ks$,$N$2)$

A $->$ B :E$($ks$,$f$($N$2))$

$($a$)$ Explain the protocol. $(5$ points$)$

$($b$)$ Can you think of a possible attack on this protocol? Explain how it can be done. $(5$ points$)$

$($c$)$ How can we prevent this attack? just write the basics of the idea. $(5$ points$)$