Answer:
//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;
}
