In the network of seven nodes shown below, a one$-$on$-$one secure communication has to be established between every pair of nodes. That is$,$ A and B have to securely communicate with each other, $B$ and $C$ have to securely communicate with each other, and so on$.$

a$.$ What is the total number of keys required if symmetric key cryptography is used?

b$.$ What is the total number of keys required if asymmetric key cryptography is used?