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.

//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;  

}  

1. a

2. a

3. d

explanation: