3.playfaircipher.cpp

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

class Playfair{
    string key;
    int find_in_key(char a){
        for(int i=0; i<25; i++){
            if(a==key[i])
            return i;
        }
        return -1;
    }

    pair<char, char> encipher_pair(char a, char b){
        int a_id = find_in_key(a), b_id = find_in_key(b);
        int x_a=a_id/5, y_a=a_id%5, x_b=b_id/5, y_b=b_id%5;
        
        if(x_a == x_b)
        return make_pair(key[x_a*5 + (y_a+1)%5], key[x_b*5 + (y_b+1)%5]);

        if(y_a == y_b)
        return make_pair(key[((x_a+1)%5)*5 + y_a], key[((x_b+1)%5)*5 + y_b]);

        else
        return make_pair(key[x_a*5 + y_b], key[x_b*5 + y_a]);
    }

    pair<char, char> decipher_pair(char a, char b){
        int a_id = find_in_key(a), b_id = find_in_key(b);
        int x_a=a_id/5, y_a=a_id%5, x_b=b_id/5, y_b=b_id%5;
        if(x_a==x_b)
        return make_pair(key[x_a*5 + (y_a-1)%5], key[x_b*5 + (y_b-1)%5]);
        if(y_a == y_b)
        return make_pair(key[((x_a-1)%5)*5 + y_a], key[((x_b-1)%5)*5 + y_b]);

        else
        return make_pair(key[x_a*5 + y_b], key[x_b*5 + y_a]);
    }

    public:
    Playfair(string KEY){key = KEY;}

    string encipher(string plainText){
        if(plainText.length()%2 == 1)
        plainText += "x";
        string cipherText = plainText;
        for(int i=0; i<plainText.length(); i+=2){
            pair<char, char> t = encipher_pair(plainText[i], plainText[i+1]);
            cipherText[i] = t.first;
            cipherText[i+1] = t.second;
        }
        return cipherText;
    }

    string decipher(string cipherText){
        if(cipherText.length()%2 == 1)
        cipherText += "x";
        string plainText = cipherText;
        for(int i=0; i<cipherText.length(); i+=2){
            pair<char,char> t = decipher_pair(cipherText[i], cipherText[i+1]);
            plainText[i] = t.first;
            plainText[i+1] = t.second;
        }
        return plainText;
    }
};

int main(){
    string key = "alonpzmihxvyrswukdfteqqcb";
    Playfair p(key);
    string plainText;
    cout<<"Enter the plain text : ";
    cin>>plainText;
    cout<<"*******************************"<<endl;
    cout<<"Encoded cipher text : "<< p.encipher(plainText) << endl;
    cout<<"Deciphered plain text : "<< p.decipher(p.encipher(plainText)) << endl;
    return 0;
}