(a) What is the main difference between a block cipher and a stream cipher? (b) Give a formal definition of a block cipher. Your answer should make reference to the block size n and the key size k. (c) In the context of a block cipher, explain what is meant by the following terms: (i) known plain text attack; (ii) chosen plain text attack; known cipher text attack. (d) To what extent are the three different attack models above realistic? Illustrate your answer with an example for each model.[6 marks] (e) You are given the task of selecting a block cipher to be used in an app that will run on smart phones. The block cipher will be used to encrypt users' passwords as they are sent from the phone to a remote server. Which block cipher would you choose for this purpose, and why?[4 marks] (f) Explain why modes of operation are usually needed when using block ciphers. (g) Define Counter (CTR) mode encryption. (h) Explain what the principal security requirement for using CTR mode is. Describe two different methods by which this requirement can be met, commenting briefly on any issues that may arise with each method.[5 marks] (i) CTR mode is vulnerable to "bit flipping" attacks. Explain, in general terms, what is meant by this statement, and which security property the attack violates (beyond confidentiality).[2 marks]