Today we will be making a caesar cipher to encrypt and decrypt messages. a caesar cipher is a simple cipher that translates one letter to another via shifting the letter a few spaces in the alphabet. for example: a caesar cipher with a shift of 3 would turn a into d, b into e, c into f, and so on. your task is to use string methods and two user defined functions (an encrypt function and a decrypt function) to produce an encrypted message from a "plaintext" message and a "plaintext" message from an encrypted message. the program should use a menu such as: 1. encrypt2. decrypt3. exitthe menu should be displayed in a loop until the user chooses to exit. when the user chooses the encrypt option, your program should prompt the user to input a "plaintext" string. then the program should display what the encrypted text would be. when the user chooses the decrypt option, your program should prompt the user for an encrypted message and then display the "plaintext" version of that message to the user. for this program each message should be a single word and your cipher should use a shift of 3.any message entered by the user should be capitalized using a user defined function that will take a string in by reference and use the toupper() function from to capitalize each letter of the string.

#include <iostream>

#include <stdio.h>

#include <ctype.h>

using namespace std;

string

encrypt (string text, int s)

{  string resultingString = "";

 for (int i = 0; i < text.length (); i++)

   {      if (!isspace (text[i]))

{

  if (isupper (text[i]))

    resultingString += char (int (text[i] + s - 65) % 26 + 65);

  else

  resultingString += char (int (text[i] + s - 97) % 26 + 97);

}

     else

{

  resultingString += " ";

}

  }

 return resultingString;

}

string

decrypt (string text, int s)

{

 string resultingString = "";

 for (int i = 0; i < text.length (); i++)

   {

     if (!isspace (text[i]))

{

  if (isupper (text[i]))

    resultingString += char (int (text[i] + s - 65) % 26 + 65);

  else

  resultingString += char (int (text[i] + s - 97) % 26 + 97);

}

     else

{

  resultingString += " ";

}

   }

 return resultingString;

}

string upper(string str){

   for (int i=0;i<str.length();i++){

       str[i]=toupper(str[i]);

   }

   return str;

}

int

main ()

{

 string text = "This is test text string";

 int s = 3;

 string cipherText = "";

 string decipherText = "";

int menu=-1;

while (menu!=3){

   cout<<"1. Encrypt"<<endl;

   cout<<"2. Decrypt"<<endl;

   cout<<"3. Exit"<<endl;

   cin >>menu;

   cin.ignore();

   if(menu==1){

       cout<<"Enter Plain string "<<endl;

       getline(cin,text);

       text=upper(text);

         cipherText = encrypt (text, s);

           cout << "cipher text: " << cipherText << endl;

   }

   else if(menu==2){

       cout<<"Enter Encrypted string "<<endl;

       getline(cin,cipherText);

               cipherText=upper(cipherText);

         decipherText = decrypt (cipherText, 26 - s);

 cout << "decipher text: " << decipherText << endl;

   }

   else {

       cout<<"Not valid"<<endl;

   }

}

 return 0;

}

Explanation:

Display menu with options 1 encrypt, 2 decrypt, 3 exit. Write a function to translate string to upper case. Iterate through string and for each index use toupper function to translate alphabet to upper case and store it back to current index of string.On exiting from loop return string. Pass upper case string to encrypt function.

In encrypt function pass string by reference and shift value. Create an empty string resultingString. Iterate through string for each character check if its space, if its a space add it to resulting string otherwise using ascii values convert char to its ascii add shift value and subtract 65 (staring ascii value for capital alphabet) take modules with 26 for new character and than add 65 to get alphabet.Now add this alphabet to resulting string. Upon loop completion return resultingString.

For decryption use same function as encryption. We are using cyclic property to decrypt using same function therefor subtract shift from 26 and pass resulting shift value to decrypt function.