The Needham$-$Schroeder protocol is reproduced below.

$AS$:$,A,B,N_A$

$SA$:$,\{N,B,K_{AB},\{K_{AB},A\}{K}_{BS}\}{K}_{AS}$

$AB$:$,\{K_{AB},A\}{K}_{BS}$

$BA$:$,\{N_B\}{K}_{AB}$

$AB$:$,\{N_B-1\}{K}_{AB}$

For each of the boxed fields, describe $($briefly$)$ an attack that may be possible if the field

were omitted.

a$.$ B

b$.NA$

c$.N{B}_B-1$