1. This question consists of three parts. Fr each part you need first to provide an answer and then an explanation as to why you believe the answer to be correct. The correct answer only, without an explanation is worth half of the respective mark. Zero mark will be awarded for an explanation only without an explicit answer. (a) Suppose that we define a block cipher with block size 5 using a look-up table. What is the number of rows in this table? (8 marks) (b) Suppose that Alice wants to send to Bob an encrypted message so that nobody but Bob could recover the original plaintext. Which of the following four encryption methods best serves this purpose? (6 marks) (i) Encrypt the message with the private key of Alice. (ii) Encrypt the message with the public key of Alice. (iii) Encrypt the message with the private key of Bob. (iv) Encrypt the message with the pubic key of Bob. (c) Suppose that Alice wants to send to Bob a message so that there will be no doubt that this message has been indeed sent by Alice and not by somebody else. Which of the following four encryption methods is the best way to achieve this goal? (6 marks) (i) Encrypt the message with the private key of Alice. (ii) Encrypt the message with the public key of Alice. (iii) Encrypt the message with the private key of Bob. (iv) Encrypt the message with the pubic key of Bob