Question:

(a) What is the major problem with public key encryption when compared to symmetric key encryption?

(b) Consider the following protocol for communication between two parties, for example, user A wishing to send message M  to user B (messages exchanged are in the format (sender's name, text, receiver's name)):

A sends to B the following block: (A, Ekub[M, A]), B).

B acknowledges receipt by sending to A the following block: (B, Ekua([M, B]), A).

The above protocol is criticized to contain a certain redundancy and is simplified to the following:

A sends B the block: (A, Ekub (M), B).

B acknowledges receipt by sending to A the block: (B, Ekua (M), A).

Describe how the simplified version  of the protocol allows another user (attacker) of the network (has his own public encryption key pair and is able to send his own messages to A or B and to receive theirs) to obtain message M, which user A has previously sent to B. Explain why this attack is not successful in the original protocol. 

(c) Briefly list four functions of a public key infrastructure (PKI).

(d) Which entity can assist the certificate authority in the PKI certificate process?

(e) Which protocol allows to verify whether a certificate has been revoked or not?