Question $1(20$ marks$)$

a$.$ Under what circumstances would you choose a symmetric cipher over an asymmetric cipher, and vice versa? Explain your reasoning. $(3$ marks$)$

b$.$ Contrast block ciphers and stream ciphers. Provide two $(2)$ examples of each. $(4$ marks$)$

c$.$ Explain the concept of differential cryptanalysis and how it can be used to exploit weaknesses in block ciphers. Provide an example of a vulnerable cipher. $(3$ marks$)$

d$.$ Describe how a frequency analysis attack can be carried out on a substitution cipher. $(2$ marks$)$

e$.$ How does brute force differ from other cryptanalysis techniques? Provide an example of a cipher vulnerable to brute force. $(2$ marks$)$

f$.$ Modes of operation are algorithms that specify how a block cipher is used to encrypt a message that is longer than the block size. Briefly discuss the three $(3)$ modes of operation. $(6$ marks$)$