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## Anna University Questions - CS9402 Cryptography and Security April May 2014, Computer Science and Engineering (CSE), Seventh Semester, Regulation 2008

Exam
B.E/B.Tech. (Full Time) DEGREE END SEMESTER EXAMINATIONS
April May 2014
Subject Code

Subject Name

## Cryptography and Security

Branch
Computer Science and Engineering
Semester
Seventh Semester
Regulation
2008

B.E / B.Tech. (Full Time) DEGREE END SEMESTER EXAMINATIONS, APRIL / MAY 2014
Computer Science and Engineering
Seventh Semester
CS9402 CRYPTOGRAPHY AND SECURITY
(Regulation 2008)
Time : 3 Hours                      Answer A L L Questions                Max. Marks 100
PART-A (10 x 2 = 20 Marks)

1. Suppose we work mod 27 instead of mod 26 for affine ciphers. How many keys are possible?
2. Determine (231). (Eulers Totient function)
3. Give the differences between RC4 and RC5.
4. Discuss the timing attack on RSA and how it can be overcome.
5. What is meant by birthday attack in hash functions?
6. How MAC is different from hash techniques?
7. What is the purpose of X.509 certificate?
8. What problem was Kerberos designed to address?
9. List the key features of a trusted operating system.
10. What are the factors to be considered while determining the sensitive data in data base security?

Part-B (5* 16 = 80 Marks)

11. (i) Suppose we build an LFSR machine that works mod 3 instead of mod 2. It uses a recurrence of length 2 of the form                                                 (8)
XN+2 =C0Xn+ C1Xn+1 (mod 3)
To generate the sequence 1,1,0,2,2,0,1,1. Set up and solve the matrix equation to find the coefficients C0 and C1.
(ii) A group of people are arranging themselves for a parade. If they line up three to a row, one person is left over. If they line up four to a row, two people are left over and if they line up five to a row three people are left over. What is the smallest possible number of people? What is the next smallest number? (interpret this problem in terms of the CRT)                               (8)

12. a) (i) Show how 12 is transformed to C9 by subbyte routine using GF(28) field with the irreducible polynomial (x8+x4+x3+x+1) in AES algorithm. (8)
(ii) Discuss the key expansion in AES - 128. (8)
OR
b) (i) Discuss any one primality testing algorithm that is used to test whether a given number is a prime or a composite number. (8)
(ii) Perform encryption and decryption using the RSA algorithm p = 7,q = 11,e = 17,M = 8                      (8)

13. a) (i) Consider a Diffie-Hellman scheme with a common prime q = 11 and a primitive root a = 2.
• Show that 2 is a primitive root of 11.                                                     (2)
• If user A has public key YA = 9, what is A's private key XA?            (3)
• If user B has public key YB = 3, what is the shared secret key K?    (3)
(ii) Discuss in detail about the secure hash algorithm. (8)
OR
b) (i) Discuss in detail about the digital signature standard algorithm. (8)
(ii) Discuss how the key exchange is done using elliptic curve cryptography. (8)

14. a) Discuss briefly about the PGP used for E-mail security. (16)
OR
b) Discuss briefly about IP security architecture. (16)

15. a) Discuss briefly about the security models involved in trusted operating systems. (16)
OR
b) Discuss in detail about the multilevel secure database. (16)

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