1. Which of the following statements are true, and which are false? Justify your answers.

The encryption speed in OFB mode of a block cipher can be increased by precomputing the key stream.

Non-repudiation is not required if authentication of the origin by the receiver is possible.

The AKS test for primality is more reliable than the Miller-Rabin test.

The pseudo-random sequence generated by a single LFSR is strongly secure.

There is at least one finite field with fifteen elements.

Generate the first 10 terms of the LFSR sequence with characteristic polynomial x^5 x^3 with starting values x1, x2, x3, X4) 0).

Compute 5^21 (mod 41) using the repeated sqaring algorithm.

Asha and Bano generate a common key by using the Diffie-Hellman protocol for secure communication. Suppose Asha chooses p 41, a 6 (mod x and Bano chooses y 5. Find the value of the common key, showing all the steps.

If 00 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff is the message, and
round key 00 05 0a 0f 04 09 0e 03
08 0d 02 07 0c 01 06 0f, then

apply a shift row transformation of AES on the message;

apply round key addition on the result of above;

apply a subbyte transformation on the output of(ii) above,

where i 1 (mod 256).

Let n =2911 and =2800. Factorise n into two primes.

In RSA, what is the advantage of choosing an encryption exponent with a low number of ones in its binary representation Is it advisable to have a small decryption exponent? JuStify your answer.

The cipher text EDYW was encrypted using the affine cipher. The plain text starts with HA. Find the complete message.

Find b 10 such that 91 is a pseudoprime to base b.

What is Merkle-Damgard strengthening? Illustrate this method with the string "SCRAMBLEDEGGS", assuming a block size of 64 bits.

Check that € F3 [xl is irreducible. Is it primitive? Justify your answer.

Decrypt the Vigenere cipher, OQLTHEQTFBYZOL, where the keyword is "SMART". Is the Vigenere cipher a polyalphabetic or monoalphabetic substitution cipher? Justify your answer.

Compute the multiplicative inverse of in

Factorise 221 using the Fermat factorisation method.