Affine_substitution_Cipher.cpp

#include <bits/stdc++.h>
using namespace std;

const int a = 19;
const int b = 20;
const int m = 26;

string encryption(string message) {
  string cipher;
  /*
   Encrypted Message(x) = ( a * x + b ) mod m
  modulus m: size of the alphabet, here 26
  a and b: key of the cipher.
  a and m should be coprime.
  */
  for (int i = 0; i < message.size(); ++i) {
    if (message[i] != ' ') { // Spaces should not be encrypted
      cipher += (char)((a * (message[i] - 'A') + b) % 26 + 'A');
      //to make it in range of ASCII alphabet addition of "A" is done
    }
    else {
      cipher += message[i];
    }
  }
  return cipher;
}

string decryption(string cipher) {
  string message;
  int a_inv = 0;
  for (int i = 0; i < 26; ++i) {
    if ((a * i) % 26 == 1) {
      a_inv = i;
      break;
    }
  }
  /*
  Decrypted Message(x) = a_inv ( x - b ) mod m
  a_inv : modular multiplicative inverse of a modulo m
  1 = (a * a_inv) mod m .
  */
  for (int i = 0; i < cipher.size(); ++i) {
    if (cipher[i] != ' ') { // Spaces were not encrypted
      message += (char)(a_inv * (cipher[i] + 'A' - b) % 26 + 'A');
      //to make it in range of ASCII alphabet addition of "A" is done
    }
    else {
      message += cipher[i];
    }
  }
  return message;
}

int main() {
  string message = "TESSERACTCODINGNEOALGO";
  //use only uppercase characters (you can modify the code for lowercase characters)
  string new_message = encryption(message);
  cout << "Encrypted Message: " << new_message << "\n";
  string original_message = decryption(new_message);
  cout << "Decrypted Message: " << original_message;
  return 0;
}

/*
OUTPUT:
Encrypted Message: RSYYSFUGRGAZQHEHSAUVEA
Decrypted Message: TESSERACTCODINGNEOALGO
*/