ISRO-2013-21

Question

What will be the cipher text produced by the following cipher function for the plain text ISRO with key $k=7$. [ Consider $'A'=0, 'B'=1,....'Z'=25$]

$C_{k} (M) = (kM + 13) \mod 26$

• A. RJCH
• B. QIBG
• C. GQPM
• D. XPIN

Answer 1

Answer is A

See A=0 B=1 C=2 .......... you will get I = 8 ................

Now you just apply the cipher function character wise ,

BY THE WAY IN THE ABOVE FUNCTION M STANDS FOR  1 character one by one of input

So our first character is I

C( I ) = ( KM+13) MOD 26 =(7*8+13)MOD 26 = 17  and if you see 17 is R value

So just from the first character you can get answer RJCH

Comments

the position of `i` is 9. so w.r.t. function C(i) = (7*9+13)MOD 26 = 24 which is X. so option D should be correct.

---

The main thing to notice here is that alphabets are starting from 0 not from 1.

So 'i' will get 8 not 9. Similarly numbers will be assigned to other characters.