Question

What is the formula used for decoding the ciphered text using affine cipher(a,b are constants and x is the numerical equivalent of a letter to be encrypted)?

(a) a^-1(x-b)%26

(b) (ax+b)%26

(c) b^-1(x-a)%26

(d) b^-1(x-a)

I got this question by my college professor while I was bunking the class.

My question is based upon Cryptography topic in division Cryptography of Data Structures & Algorithms II

Answer 1

The correct option is (a) a^-1(x-b)%26

To explain: Affine cipher uses linear algebra for the purpose of encryption. It uses the formula  a^-1(x-b)%26 for decryption of ciphered text.