Hactober-2023-1/affine.cpp at main · Parvm1102/Hactober-2023-1 · GitHub

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//CPP program to illustrate Affine Cipher

#include<bits/stdc++.h>

using namespace std;

//Key values of a and b

const int a = 11;

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 character

            msg += cipher[i];

    }


    return msg;
}

//Driver Program

int main(void)
{

    string msg = "Computer";


    //Calling encryption function

    string cipherText = encryptMessage(msg);

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


    //Calling Decryption function

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


    return 0;
}