The Caesar Cipher, also known as a shift cypher, is one of the oldest and simplest forms of encrypting a message. It is a type of substitution cypher where each letter in the original message (which in cryptography is called the plaintext) is replaced with a letter corresponding to a certain number of letters shifted up or down in the alphabet.

For each letter of the alphabet, you would take its position in the alphabet, say 3 for the letter ‘C’, and shift it by the key number. If we had a key of +3, that ‘C’ would be shifted down to an ‘F’ – and that same process would be applied to every letter in the plaintext.

Mathematically,

Encryption = E n = (x + k) mod 26

Decryption = D n = ( x + k −1 ) mod 26

Where x is the input alphabet ASCII representation and k the key, k -1 is the additive inverse of k.

Code

#include "stdc++.h"
using namespace std;
string encrypt(string message, int key)
{
   for (int i = 0; i < message.size(); i++)
   {
       message[i] = char(int(message[i] + key - 97) % 26 + 97);
   }
   return message;
}
string decrypt(string cipher, int key)
{
   string message;
   for (int i = 0; i < cipher.size(); i++)
   {
       int id = (cipher[i] - 'a' - key) % 26;
       if (id >= 0)
           message.push_back('a' + id);
       else
           message.push_back('z' + id);
   }
   return message;
}
int main()
{
   cout << "Enter the text message to be encrypted: "
        << "\n";
   string message;
   cin >> message;
   int key;
   cout << "Enter the shift key: ";
   cin >> key;
   string cipher = encrypt(message, key);
   cout << "The cipher is " << encrypt(message, key);
   cout << "\nThe plaintext is " << decrypt(cipher, key);
   return 0;
}

Output