Encryption and Decryption using Polyalphabetic Ciphers by using C++ programming

Question

Encryption and Decryption using Polyalphabetic Ciphers by using C++ programming language:

• Input
– A given text file for plaintext or ciphertext (assume only 26 letters, no special characters, numbers and punctuations)
– 3 substitution ciphers, M1,M2,M3
• M1 – left shift 3 letters
• M2 – Plain: a b c d e f g h i j k l m n o p q r s t u v w x y z
Cipher: D K V Q F I B J W P E S C X H T M Y A U O L R G Z N
• M3 – right shift 5 letters
– cycling pattern
• n=3: M1,M3,M2,M3; M1,M3,M2,M3;
• Output
– Encrypted ciphertext or decrypted plaintext

Explanation / Answer

#include<iostream>
#include<ctype.h>
#include<string.h>
#include<stdlib.h>
#include<fstream>

using namespace std;
int main(){
    int k;
    string msg;
    string line;
    string str1 = "abcdefghijklmnopqrstuvwxuz";
    string str2 = "DKVQFIBJWPESCXHTMYAUOLRGZN";
    ifstream fin("input.txt");
    if (!fin){
        cout << "Error opening file ";
        return 0;
    }
    while (getline(fin,line)){
        //cout << "Enter a message:" << endl;
      
        //getline(fin,line);
        msg = line;      
        cout << "1-Encrypt 2-Decrypt:" << endl;
        int en;
        getline(cin,line);
        en = atoi(line.c_str());

        cout << "1.M1 (left shift 3 letters)" << endl;
        cout << "2.M2 (abcdefghijklmnopqrstuvwxyz)" << endl;
        cout << "     (DKVQFIBJWPESCXHTMYAUOLRGZN)" << endl;
        cout << "3.M3 (left shift 3 letters)" << endl;
        cout << "4.Exit" << endl;
        cout << "Enter choice:";
        int n;
       
        //cin.ignore();
        getline(cin,line);
        n = atoi(line.c_str());
        if (n == 4)
            break;
        if (n == 1){
           //cout << "------------------------- ";
           string ans = "";
           //cout << msg << endl;
           for (int i = 0; i<msg.length(); i++){
               for (int j = 0; j<str1.length(); j++){
                   if (!isalpha(msg[i])){
                      ans = ans + msg[i];
                      break;
                   }
                    
                   if (str1[j] == msg[i]){
                        if (en == 1){
                            k = (j - 3) % 26;
                            if (k < 0)
                               k = k + 26;
                            ans = ans + str1[k];
                        }
                        else if (en == 2){
                            k = (j + 3) % 26;
                            ans = ans + str1[k];
                        }
                   }  
               }
           }
         
           cout << ans << endl;
        }
        if (n == 2){
           string ans = "";
           for (int i =0; i<msg.length(); i++){
               for (int j = 0; j<str1.length(); j++){
                   if (!isalpha(msg[i])){
                      ans = ans + msg[i];
                      break;
                   }
                   if (en == 1){
                      if (tolower(str1[j]) == msg[i]){
                         ans = ans + str2[j];
                      }
                   }
                   if (en == 2){
                      if (tolower(str2[j]) == msg[i]){
                         ans = ans + str2[j];
                      }
                   }
               }
           }
           cout << ans << endl;
        }
        if (n == 3){
           string ans = "";
           for (int i =0; i<msg.length(); i++){
               for (int j = 0; j<str1.length(); j++){
                   if (!isalpha(msg[i]))
                      ans = ans + msg[i];
                   if (en == 1){
                      if (str1[j] == msg[i]){
                           k = (j + 5) % 26;
                           ans = ans + str1[j];
                      }
                   }
                   if (en == 2){
                      if (str1[j] == msg[i]){
                           k = (j - 5) % 26;
                           if (k < 0)
                              k = 26 + j;
                           ans = ans + str1[k];
                      }
                   }
               }
           }
           cout << ans << endl;
        }
    }
    fin.close()
}

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