Friday 22 May 2020, 09:00 BST
(24 hour open online assessment – Indicative duration 1.5 hours)
DEGREES OF MSc, MSci, MEng, BEng, BSc,MA and MA (Social Sciences)
Cryptography and Secure Development (M) COMPSCI 5079
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(Answer All 4Questions)
This examination paper is worth a total of 60 marks
(a) What is the entropy of a message space and how is it used in cryptography? Explain why the concept of entropy is useful?
(b) Calculate the entropy of the following message space in which the message “Nothing Special” occurs with probability 1/2 and the messages “Rain”, “Snow”, “Ice” and “Gales” each occur with probability 1/8.
(c) Define the term “redundancy of a language” and give a formula to calculate it in terms of various rates of a language. How can these rates be calculated?
(d) A system authenticates users by asking them to enter an 8 character password. Estimate the entropy of this password system if the users use lower case English words as their passwords. Describe how a practical attack that exploits this low entropy could be mounted.
(a) Describe with your own diagrams how a rotor machine implements an encryption algorithm, explaining how multiple rotors can be combined, how letters of the plain text are entered and letters of the resulting cipher text displayed. How is the encryption key entered? How is the cipher text decrypted to produce the original plain text?
(b) Give an example of a 4 rotor machine where each rotor implements a Caesar cipher with shifts 3, 15, 7, 4. The alphabet consists of the 26 English letters plus space, comma, fullstop. Show how your machine both encrypts and decrypts?
(c) How would you use your rotor machine to avoid the mistakes that led to the ENIGMA rotor machine being broken.
(a) An simple version of the RSA public key system is based on the two prime numbers 5 and 11. The encryption key is 3, calculate the decryption key. State the public and secret keys.
(b) Explain why 4 cannot be an encryption or decryption key. List all the possible encryption and decryption keys.
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(c) Calculate the cipher text if the plaintext is 4. Explain why this system will not encrypt the plain text 45 properly and state 2 more plain text values that cannot be encrypted properly.
(a) What is a moral hazard, also known as a perverse incentive? Give 3 examples of moral
hazards with a security context.
(b) What is a security threat model and how can it constructed? How is it related to an attack surface and a security target? Give an example (not one from your course notes).
(c) Give an example of a Misuse Cases and show how can it be used in an Agile development setting.
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