1. What is the dierence between passive and active security threats? 2. List and briey dene categories of passive and active security attacks. Active attacks: masquerade, replay, modication of messages, and denial of service. 3. List and briey dene categories of security services. 4. Consider an automated teller machine (ATM) in which users provide a personal identication number (PIN) and a card for account access. Give examples of condentiality, integrity, and availability requirements associated with the system and, in each case, indicate the degree of importance of the requirement. 5. Consider a desktop publishing system used to produce documents for various organizations. (a) Give an example of a type of publication for which condentiality of the stored data is the most important requirement. (b) Give an example of a type of publication in which data integrity is the most important requirement. (c) Give an example in which system availability is the most important requirement. 6. What are the essential ingredients of a symmetric cipher? 7. What are the two basic functions used in encryption algorithms? 8. One way to solve the key distribution problem is to use a line from a book that both the sender and the receiver possess. Typically, at least in spy novels, the rst sentence of a book serves as the key. The particular scheme discussed in this problem is from one of the best suspense novels involving secret codes, Talking to Strange Men, by Ruth Rendell. Work this problem without consulting that book! Consider the following message: SIDKHKDM AF HCRKIABIE SHIMC KD LFEAILA This ciphertext was produced using the rst sentence of The Other Side of Silence (a book about the spy Kim Philby): The snow lay thick on the steps and the snowakes driven by the wind looked black in the headlights of the cars. A simple substitution cipher was used. (a) What is the encryption algorithm? (b) How secure is it? (c) To make the key distribution problem simple, both parties can agree to use the rst or last sentence of a book as the key. To change the key, they simply need to agree on a new book.The use of the rst sentence would be preferable to the use of the last. Why? 9. (a) Construct a Playfair matrix with the key largest. (b) Construct a Playfair matrix with the key occurrence. Make a reasonable assumption about how to treat redundant letters in the key. 10. (a) Encrypt the message meet me at the usual place at ten rather than eight oclock q using the Hill cipher with the key : 9 4 5 7 Show your calculations and the result. (b) The inverse matrix of the key is 5 12 15 25 mod 26. Show the calculations for the corresponding decryption of the ciphertext to recover the original plaintext.