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ISRO | ISRO CS 2014 | Question 24

In a system an RSA algorithm with p=5 and q=11, is implemented for data security. What is the value of the decryption key if the value of the encryption key is 27?
(A) 3
(B) 7
(C) 27
(D) 40

Answer: (A)
Explanation: p = 5 and q = 11, encryption key, e = 27

n = p * q = 5 * 11 = 55
z = (p-1) * (q-1) = 4 * 10 = 40

Let the value of decryption key be ‘d’ such that:

e * d mod z = 1
27 * d mod 40 = 1
d = 3

So, the correct answer is (A)
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