JNTU B.Tech IV Year Supplementary Examinations, February 2008
Network Security And Cryptography
(Computer Science & Engineering)
1. This problem provides a numerical example of encryption using a one-round version
of DES. We start with the same bit pattern for the key and plain text, namely,
In hexadecimal notation 0 1 2 3 4 5 6 7 8 9 A B C D E F
Inbinary notation: 0000 0001 0010 0011 0100 0101 0110 0111
1000 1001 1010 1011 1100 1101 1110 1111
(a) Derive K1, the first – round subkey.
(b) Derive L0,R0
(c) Expand R0 to get E[R0]
(d) Calculate A=E[R0] _ K1.
(e) Group the 48-bit result of (d) into sets of 6 bits and evaluate the corresponding
(f) Cancatenate the results of (e) to get a 32-bit result,B.
(g) Apply the permutation to get P(B).
(h) Calculate R1=P(B) _ L0
(i) write down the cipher text.
2. (a) List the characteristics of advanced block ciphers.
(b) What are the differences between RC5 and RC2?
3. (a) Illustrate Diffie-hellman Key Exchange scheme for GF(P).
(b) Consider a diffie-helman scheme with a common prime q=11 and a primitive
i. If user A has public key YA = 9, what is A’s private key XA ?
ii. If user B has public key YB = 3, what is the shared key K?
4. (a) State properties of Modular Arithmetic.
(b) What is difference between modular arithmetic and ordinary arithmetic?
(c) Prove the following :
i. a _b mod n implies b _ a mod n
ii. a _ b mod n and b _ c mod n imply a _ c mod n.
5. (a) Is it necessary to recover the secret key in order to attack a MAC algorithm?
(b) What characteristics are needed in a secure hash function?
6. (a) What is R64 conversion?
(b) Why is R64 conversion useful for an e-mail application?
(c) Why is the segmentation and reassembly function in PGP needed?
7. Write note on Encapsulating security payload.
8. (a) List and briefly define three classes of intruders.
(b) What are two common techniques used to protect a password file?