ISRO | ISRO CS 2013 | Question 21

What will be the cipher text produced by the following cipher function for the plain text ISRO with key k =7. [Consider ‘A’ = 0, ‘B’ = 1, …….’Z’ = 25]. Ck(M) = (kM + 13) mod 26
(A) RJCH
(B) QIBG
(C) GQPM
(D) XPIN

Answer: (A)
Explanation:

Ck(M) = (kM + 13) mod 26 

Here, 'A' = 0, 'B' = 1, .......'Z' = 25

I = Ck(I) = (7*8 + 13) mod 26 = 17 = R
S = Ck(S) = (7*18 + 13) mod 26 = 9 = J
R = Ck(R) = (7*17 + 13) mod 26 = 2 = C
O = Ck(O) = (7*14 + 13) mod 26 = 7 = H  

So, option (A) is correct.
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