Question

1. Implementing a cryptosystem:

Choose one of the following cryptography techniques and implement it using any programming language you prefer. Your program should provide the user with two options Encryption and Decryption, with a simple UI to get the input from the user, and view the result. You can use any restriction you need for the user input but you need to clarify that and validate the user input base on your restriction.

● Feistel

● Keyword columnar

● Any cryptosystem of your choice (needs to be approved by the instructor)

Notes:

• Submission materials:
  • Your source code
  • Report containing Description of your cryptosystem, the encryption and decryption processes, screenshots of all the execution results.
• Make sure your code is defensive code against any programming flaws.
• Test your program with different input.
• You must reference your work if you used any external sources for the code or report.

Answer 1

In below code I have used Caesar cipher as a cryptography techniques for encryption and decryption of plain text and C++ language to implement the code.

Caesar_cipher.cpp

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

using namespace std;

int main() {
    // User input to Encrypt text
   cout<<"Enter your Plain text:";
   char text[50];
   cin.getline(text,50);
   int i, j,n,choice,key;
   // Key or shift value which used to encrypt text
   cout << "Enter key: ";
   cin >> key;
   n = strlen(text);
   while(1){
        //user menu
   cout<<"\n1.Encryption\n2.Decryption\n3.Exit\n";
   cout<<"Enter your choice:";
   cin>>choice;

   if (choice==1){
      char word;
      for(int i = 0; text[i] != '\0'; ++i) {
         word = text[i];
        // encryption for lowercase letters
         if (word >= 'a' && word <= 'z'){
            word = word + key;
            if (word > 'z') {
               word = word - 'z' + 'a' - 1;
            }
            text[i] = word;
         }
         // encryption for uppercase letters
         else if (word >= 'A' && word <= 'Z'){
            word = word + key;
            if (word > 'Z'){
               word = word - 'Z' + 'A' - 1;
            }
            text[i] = word;
         }
      }
      // Encrypted text or result
     cout<<"\nEncrypted text:"<<text<<endl;
   }
   else if (choice == 2) {
      char word;
      for(int i = 0; text[i] != '\0'; ++i) {
         word = text[i];
        //decryption for lowercase letters
         if(word >= 'a' && word <= 'z') {
            word = word - key;
            if(word < 'a'){
               word = word + 'z' - 'a' + 1;
            }
            text[i] = word;
         }
         //decryption for uppercase letters
         else if(word >= 'A' && word <= 'Z') {
            word = word - key;
            if(word < 'A') {
               word = word + 'Z' - 'A' + 1;
            }
            text[i] = word;
         }
      }
        //  Decrypted text or result
      cout <<"\nDecrypted text:"<<text<<endl;

   }
   // For exiting the menu on operation complete
   else if (choice == 3) {
            exit(0);}
   }

}

Output :- You can see the below screenshot for output of above code.

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