5. Triple DES is a widely used block cipher whose design is motivated by practical considerations. (a) Explain what is meant by backwards compatibility. (b) Describe three different ways in which keys could be chosen in order to make Triple DES backwards compatible with (single) DES

9. AES is a very popular symmetric encryption algorithm. (a) What applications of cryptography that you have used support the use of AES, and which key lengths do they recommend? (b) AES is under great scrutiny from the cryptographic research community. Provide a laymans guide to the latest cryptanalytic attacks that have been announced against AES and comment on their practical relevance. (c) Suppose that one day an announcement is made that AES is too insecure for widespread use. What do you think the likely reaction will be from the various communities that this will affect?

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At least in theory, the design of a block cipher is generally made more secure by: increasing the number of rounds used in a blockcipher; using a block cipher with alarger blocksize. (a) For each of these techniques, explain why it might be more secure. (b) For each of these techniques, what are the associated practical disadvantages? (c) For each of these techniques, explain which of the three significant problems with ECB mode that we identified in Section 4.6.1 might be reduced. (d) Explain why it is inadvisable to apply either of these techniques to a published (standardised) block cipher such as AES.