In the public-key certificate system, suppose that the certificate authority (CA) employs DSS signature. Assume that CA's private key and public key pair is denoted by (skCA, pkcA). Bob requests a public-key certificate for his key pair (skB, pkB), which is a RSA key pair. (a) Explain what Bob should submit to the CA to get the certificate for his public key pkB. (b) How does CA generate the certificate of Bob's public key? (You only need to specify the format of the certificate.) (c) When Alice wishes to send some sensitive information to Bob using Bob's public key, what does she need to do before she performs the RSA encryption using Bob's public key? (d) Why is a certificate authority necessary for a public-key system?