Information Assurance Homework 3: Cryptography Questions - Prof. David M. Nicol, Assignments of Electrical and Electronics Engineering

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Information Assurance: Homework 3

Due February 19, 2009 on compass.

1. In a monoalphabetic cryptosystem, LKZLP can be the cipher text for which of the following plain texts? a) array b) atlas c) ample d) angry 2. An English word has been encrypted to QAOPERTYEIAS. What type of cipher is used? a) Transposition Cipher b) Substitution Cipher 3. In the Vigenere cryptosystem, what is the cipher text corresponding to the plaintext “HELLO WORLD” and the key “ENG”? a) PLOOK WKQOI b) LWRRT ILSRT c) LRRPB CSERH d) DRFHB QKEFZ 4. In the DES cryptosystem, S-boxes are: a) Non-linear and one-to-one functions b) Linear and one-to-one functions c) Non-linear and many-to-one functions d) Linear and many-to-one functions

5. Which property of the f function in a Feistel network is crucial to its working and security: a) Invertible b) Non-linear c) Both a and b d) None of the above 6. The text “Brain: an apparatus with which we think we think” is encrypted using a block cipher to “FGRJIJUPO LPUZSUYTJ HGTLKYQTR KOUJFCZAQ GZRALKJUK PKIOPUXSA LOSREKHTA PKIOPUXSA LOSREKHTA”. Which block cipher mode was used? (Assume that the block size is a word of 9 letters. If a word is shorter, it is padded to 9 letters) a) ECB b) CBC 7. In a 128-bit LFSR which of the following is true about its period? a) Period is 2 128 - b) Period is 2 64 - c) Period can be larger than 2 128 - d) Period cannot be determined with the information provided 8. In a Diffie-Hellman key exchange session, if p=73, public key of Alice=18, private key of Alice=34, and private key of Bob=23, what is the shared key? a) 36 b) 43 c) 57 d) The information is not enough; we have to know the value of g 9. Which of the following is true for an integer w>0: a) Φ(w) = w-1 (Φ denotes the Totient function) b) w can be used as the modulus in the RSA algorithm c) If s is prime number (w<s), then w s^ ≡ w (mod s) d) All of the above

10. In an RSA cryptosystem, p=23, q=5, Alice’s public key= (5, 115), and Alice’s private key=53. Bob wants to send a number to Alice which only Alice can read. He encrypts it and sends the encrypted number 96 to Alice. What is the plaintext number? a) 33 b) 34 c) 35 d) 36