Question

Write a function to compute the Affine cipher. The input parameters for the function will be a, b, m and the plaintext. The function will return the ciphertext. This function should check to make sure that a and m are relatively prime (by calling the function above). If not, it should return a blank string.

Answer 1

//CPP program to illustate Affine Cipher

#include<bits/stdc++.h>

using namespace std;

//Key values of a and b

const int a = 17;

const int b = 20;

string encryptMessage(string msg)

{

///Cipher Text initially empty

string cipher = "";

for (int i = 0; i < msg.length(); i++)

{

// Avoid space to be encrypted

if(msg[i]!=' ')

/* applying encryption formula ( a x + b ) mod m

{here x is msg[i] and m is 26} and added 'A' to

bring it in range of ascii alphabet[ 65-90 | A-Z ] */

cipher = cipher +

(char) ((((a * (msg[i]-'A') ) + b) % 26) + 'A');

else

//else simply append space character

cipher += msg[i];

}

return cipher;

}

string decryptCipher(string cipher)

{

string msg = "";

int a_inv = 0;

int flag = 0;

//Find a^-1 (the multiplicative inverse of a

//in the group of integers modulo m.)

for (int i = 0; i < 26; i++)

{

flag = (a * i) % 26;

//Check if (a*i)%26 == 1,

//then i will be the multiplicative inverse of a

if (flag == 1)

{

a_inv = i;

}

}

for (int i = 0; i < cipher.length(); i++)

{

if(cipher[i]!=' ')

/*Applying decryption formula a^-1 ( x - b ) mod m

{here x is cipher[i] and m is 26} and added 'A'

to bring it in range of ASCII alphabet[ 65-90 | A-Z ] */

msg = msg +

(char) (((a_inv * ((cipher[i]+'A' - b)) % 26)) + 'A');

else

//else simply append space characte

msg += cipher[i];

}

return msg;

}

//Driver Program

int main(void)

{

string msg = "AFFINE CIPHER";

//Calling encryption function

string cipherText = encryptMessage(msg);

cout << "Encrypted Message is : " << cipherText<<endl;

//Calling Decryption function

cout << "Decrypted Message is: " << decryptCipher(cipherText);

return 0;

}