# Cryptography: Overview and Classical Encryption Techniques

All answers must be in complete sentences or, if it is a problem, work must be shown.

1.What is the OSI security architecture?

2.What is the difference between passive and active security threats?

3.Consider an automated teller machine (ATM) in which users provide a personal identification number (PIN) and a card for account access. Give examples of confidentiality, integrity, and availability requirements associated with the system and, in each case, indicate the degree of importance of that requirement.

4.Consider a desktop publishing system used to produce documents for various organizations.
1.Give an example of a type of publication for which confidentiality of the stored data is the most important requirement.
2.Give an example of a type of publication in which data integrity is the more important requirement.
3.Give an example in which system availability is the most important requirement.

5.Draw a matrix similar to Table 1.4 that shows the relationship between security services and attacks.

6.What are the essential ingredients of a symmetric cipher?

7.What are the two general approaches to attacking a cipher?

8.One way to solve the key distribution problem is to use a line from a book that both the send and the receiver possess. Typically, at least in spy novels, the first of a book serves as the key. The particular scheme discussed in this problem is one of the best suspense novels involving secret codes, Talkingking to Strange Men, by Ruth Rendell. Work this problem without consulting that book!

Consider the following message:

SIDKHKDM AF HCRKIABIE SHIMC KD LFEAILA

The ciphertext was produced using the first sentence of The Other Side of Silence (a book about the spy Kim Philby):

The snow lay thick on the steps and the snowflakes 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 first 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 first sentence would be preferable to the use of the last. Why?

9. When PT-109 American patrol boat, under the command of Lieutenant John F. Kennedy, was sun by a Japanese destroyer, a message was received at an Australian wireless station in Playfair code:

KXJEY UREBE ZWEHE WRYTU HEYFS
KREHE GOYFI WTTTU OLKSY CAJPO
BOTEI ZONTX BYBNT GONEY CUZWR
GDSON SXBOU YWRHE BAAHY USEDQ

The key used was royal new Zealand navy. Decrypt the message. Translate TT into tt.

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