Q3. Suppose EK is a block cipher with a block size of 128 bits and a key size of 80 bits.

(i) Explain how to use Ek to encrypt a message of length 400 bits using CTR mode.

You may use a diagram to illustrate your answer. (6 marks)

(ii) In contrast to CTR mode, using CBC mode requires padding. State FOUR proper-

ties that a padding scheme must satisfy. (4 marks)

(iii) Explain how to use Ek to encrypt a message of length 200 bits using CBC mode.

You may assume a suitable padding scheme. You may use a diagram to illustrate

your answer. (6 marks)

(iv) Give THREE examples of application contexts where CTR mode may be preferred

to CBC mode, justifying your answer in each case. (9 marks)