Exam Slot: E
06EC6152 Reg Number. . . . . . . . .
Name. . . . . . . . . . . . . . .
A P J ABDUL KALAM TECHNOLOGICAL UNIVERSITY
M.TECH DEGREE EXAMINATION, MAY/JUNE 2019
SECOND SEMESTER
Communication Engineering
Principles of Secure Communication
Time: 3 Hrs Maximum Marks:60
PART A
Answer ALL Questions
1. a)Define Euler’s totient function or phi function and determine Euler’s Totient
function of (231).
(3 marks)
b) Determine gcd(4655,12075)
(2 marks)
2. a) What are polyalphabetic ciphers? Give examples.
(2 marks)
b)Employ RAILFENCE cipher method to encrypt the message:“A P J ABDUL
KALAM TECHNOLOGICAL UNIVERSITY”;ignore spaces
(3 marks)
3. What are the requirements of hash function ?Differentiate between MAC and
Hash function.
(2+3 marks)
4. What are viruses? What are the different types of viruses? What are the pre-
cautions to be taken against virus?
(1+2+2 marks)
4 x 5 marks = 20 marks
PART B
5. a) What is a ring? List its properties. When does a ring become a field?
(1+2+2 marks)
201
b)State Fermat’s theorem. Using Fermat’s theorem, find 3 mod 11
(2+3 marks)
OR
1
6. a) State and explain the importance of Chinese Remainder theorem in Cryptog-
raphy
(5 marks)
b)(ii) Using CRT,find x from the equations, x ≡ 88 mod 6 and x ≡ 100 mod 15
(5 marks)
7. a)Use Playfair cipher to encrypt the message “We live in an insecure world ”with
secret key “GUIDANCE”. Decrypt the Playfair code“SYICHOLER ”using the
keyword alphabet.
(3+2 marks)
b)Write about Mix-Columns Transformation and Add-Round Key Transforma-
tion of AES cipher.
(2+3 marks)
OR
8. What is the purpose of the S-boxes in DES? Draw the general structure of DES
and explain the encryption and decryption process.
(5+5 marks)
9. Discuss RSA with computations for public key cryptography. Also perform the
Encryption and decryption for p =7 , q = 11, e = 17 and m = 8.
(5+5 marks)
OR
10. What are the basic mechanisms involved in digital signatures?List the properties
of digital signature scheme. Explain the two approaches to digital signature.
(2+2+6 marks)
11. a)Using elliptic curve cryptography, explain how secret keys are exchanged and
messages are encrypted.
(5 marks)
b)Users A and B use Diffie-Hellman key exchange scheme using prime q=71 and
primitive root α =2.
i)User A has private key Xa=5, What is the public key of A: Ya?
ii)User B has private key Xb=12, what is the public key of B: Yb?
iii)What is the shared secret key?
(2+2+1 marks)
OR
12. What is a firewall? What is the need for firewalls? Describe different techniques
used by firewalls to control access and enforce a security policy.
(2+2+6 marks)
4 x 10 marks = 40 marks
