$($RSA digital signature$)$ Bob chooses the secret primes $p=3$ and $q=7.$

Bob chooses the verification exponent $e=5.$

Bob publishes $N=21$ and $e=5.$

$($a$)$ Compute $d$ such that

$de-=1,(mod(p-1)(q-1))$

$($b$)$ Sign the document $D=3$ by computing $S-=D^d(modN).$

$($c$)$ Verify the correctness of the signature by computing $S^e(modN)$