Computer Science Assessment - Network Security

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201390 ITC593 Assessment Item 2 Value: 20% Due date: 24-Dec-2013 Return date: 10-Jan-2014 Submission method options only EASTS (online) Tasks Task 1 (25 marks) a) Consider the table in Figure 2.5 on page 66 of the textbook showing the relative frequencies of letters in English. Arrange the frequencies from largest to smallest, and make a cumulative plot, using Excel for example. Which smallest group of letters accounts for at least 50% of all letters used in English? b) An affine cipher scheme can be mathematically expressed in the following form Here and are numbers selected in the range , and represents a letter converted to a number using the rule . What are the and values for the Caesar cipher? Encrypt the plaintext “MOUNTPANORAMA” using . Why is not a good choice of affine cipher parameters when there are 26 letters in the alphabet? c) Do exercise 2.3 on page 85 of the textbook. Task 2: 20 marks The keyspace for a cipher is the set all of possible keys that can be used for the cipher. A DES key consists of 56 (independent) bits. Express the size of the DES keyspace to the nearest power of 10, and find three examples of physical quantities, or other known values, that are approximately the same size. Consider the collection of passwords defined by combinations of digits and letters (both upper and lower case). What password length would be required so that the number of possible passwords is at least the size of the DES keyspace? Encryption is only useful if there is a matching reversible operation called decryption. In a Feistel cipher, the F-function or round function need not be reversible. Show that F-function of DES is not reversible Explain how a Feistel cipher whose main component is not reversible can actually support decryption. Task 3: 20 marks a) Let denote the DES encryption of the plaintext P by the key K. If we set K to be the key consisting of all zeroes, explain why encrypting twice with this key for DES has...