please answer it with full details to understand

4) Hash chains: [20p] a) Alice and Bob are securely communicating via a secret key encryption scheme. Currently, Alice has to generate a secret key ski and send it to Bob, before sending each ciphertext ci This is costly as each time the key ski is sent is has to be encrypted at Alice's side and decrypted on Bob's side. Explain how we can use hash chains to solve this problem. [12p] b) There are a number of trade-offs in using hash chains (in general, NOT only concerning the above case), explain TWO advantages and TWO disadvantages of using hash chains. [8p]