Question

The plaintext message consist of single letters with 5-bit numerical equivalents from (00000)2 to (11001)2. The secret deciphering key is the superincreasing 5-tuple (2, 3, 7, 15, 31), m = 61 and a = 17. Find the ciphertext for the message “WHY”.

(a) C= (148, 143, 50)

(b) C= (148, 143, 56)

(c) C= (143, 148, 92)

(d) C= (148, 132,92)

I have been asked this question during an online interview.

Question is from Overview in chapter Cryptography Overview, TCP/IP and Communication Networks of Cryptograph & Network Security

Answer 1

Right choice is (a) C= (148, 143, 50)

Easy explanation: {wi }= {a vi mod m}

{wi} = { 17×2 mod 61, 17×3 mod 61, 17×7 mod 61, 17×15 mod 61, 17×31 mod 61}

{wi} = {34, 51, 58, 11, and 39}

PlainText    In binary     Ci

W- 22  10110  148

H – 7  00111  143

Y – 24  11000  50

So that the ciphertext sent will be C= (148, 143, 50).