Question
(i) (a) Give a table showing the Caesar cipher with a key 5, and then give the result of applying this cipher to the text 'I came I saw I conquered'. (You can ignore the difference between upper- and lower-case letters.)
(b) A three-letter word has been encrypted using a Caesar cipher. Someone wants to decrypt the word, and decides to simply list all three-letter combinations from which the cipher text might have been produced. (We assume that the key is unknown to this person.) How many three-letter combinations will there be in the list? Explain your answer.
(c) Which is easier to break, the cipher used by an Enigma machine or a Caesar cipher?
(ii) Digital signatures employ a message digest. Assume that you have to choose between two (admittedly rather simplistic) methods for calculating the message digest. The first method checks whether or not the message starts with an 'a', and if this is true it returns True, otherwise it returns False. The second method determines the number of 'a's in the message. So, for example, the message digest of 'Study Calendar' is False according to the first method and ture according to the second method. Which method is most secure? Explain your answer, demonstrating that you understand the notion of a message digest and its role in digital signatures.
(a) Describe in what respect asymmetric key cryptography is superior to symmetric key cryptography.
(b) What is the main drawback of asymmetric key cryptography techniques?
(c) For each of the following statements, state whether it is True, False or Indeterminate (i.e. there isn't sufficient information to tell whether it is True or False).
1- SSL uses symmetric cryptography to send a key securely over the internet.
2- SSL is very secure and efficient.
3- SSL sometimes uses the same symmetric key more than once.
4- SSL is efficient because it uses asymmetric key cryptography.
5- SSL requires the computers between which data is transferred to know each other's private keys. (iv)
(a) Which of the following passwords would you recommend, if you had to choose one? Explain your choice.
1- ^&*74446757$
2- Secret
3- A1anTur!ng
(b) Does choosing a secure password mean that you are safe once and for all?