2. (a) What is a block cipher. (5 marks) (b) Why are blocks ciphers with small block lengths not regarded suitable for the modern cryptography? (5 marks) (c) Consider a block cipher with block length 100 designed as follows. Each plaintext of length 100 is mapped into an encrypted word of length 100 using a table with two columns where the first column contains all possible plaintexts of length 100 and the second column contains their corresponding encryptions. What is the drawback of such a design that makes it impossible to implement on any real computer? (10 marks)