$(16$pts$)$ Answer the following questions on certificates.

$($a$)$ Why are the certificates needed?

It binds a public key to an individual.

$($b$)$ Give two models for implementing public$-$key infrastructure.

Monarchy, anarchy.

$($c$)$ Explain the trust model of the Web.

Similar to Oligarchy. Many root CAs exists.

$($d$)$ What is certificate revocation?

Revoking a certificate because of various reasons: expiration, key compro$-$

mise, etc.