C++ Program to Implement Caesar Cypher

It is a mono-alphabetic cipher wherein each letter of the plaintext is substituted by another letter to form the ciphertext. It is a simplest form of substitution cipher scheme.

This cryptosystem is generally referred to as the Shift Cipher. The concept is to replace each alphabet by another alphabet which is ‘shifted’ by some fixed number between 0 and 25.

For this type of scheme, both sender and receiver agree on a ‘secret shift number’ for shifting the alphabet. This number which is between 0 and 25 becomes the key of encryption.

The name ‘Caesar Cipher’ is occasionally used to describe the Shift Cipher when the ‘shift of three’ is used.

Process

• In order to encrypt a plaintext letter, the sender positions the sliding ruler underneath the first set of plaintext letters and slides it to LEFT by the number of positions of the secret shift.

• The plaintext letter is then encrypted to the ciphertext letter on the sliding ruler underneath. The result of this process is depicted in the following illustration for an agreed shift of three positions. In this case, the plaintext ‘tutorial’ is encrypted to the ciphertext ‘wxwruldo’. Here is the ciphertext alphabet for a Shift of 3 −

• On receiving the ciphertext, the receiver who also knows the secret shift, positions his sliding ruler underneath the ciphertext alphabet and slides it to RIGHT by the agreed shift number, 3 in this case.

• He then replaces the ciphertext letter by the plaintext letter on the sliding ruler underneath. Hence the ciphertext ‘wxwruldo’ is decrypted to ‘tutorial’. To decrypt a message encoded with a Shift of 3, generate the plaintext alphabet using a shift of ‘-3’ as shown below −

Here is the implementation of above process in C++.

Steps and pseudocodes

Take the message and key as input −

For encryption

• Input: tutorial.
• Output: wxwruldo

For decryption

• Input: wxwruldo
• Output: tutorial

For encryption

Begin
   For i = 0 to msg[i] != '\0'
      ch = msg[i]
   //encrypt for lowercase letter
      If (ch >= 'a' and ch <= 'z')
         ch = ch + key
         if (ch > 'z')
            ch = ch - 'z' + 'a' - 1
         done
         msg[i] = ch
   //encrypt for uppercase letter
      else if (ch >= 'A' and ch <= 'Z')
         ch = ch + key
         if (ch > 'Z')
            ch = ch - 'Z' + 'A' - 1
         done
         msg[i] = ch
      done
   done
   Print Encrypted message
End

For decryption

Begin
   For i = 0 to msg[i] != '\0'
      ch = msg[i]
   //decrypt for lowercase letter
      if(ch >= 'a' and ch <= 'z')
         ch = ch - key
         if (ch < 'a')
            ch = ch +'z' - 'a' + 1
      done
      msg[i] = ch
   //decrypt for uppercase letter
      else if (ch >= 'A' and ch <= 'Z')
         ch = ch + key
         if (ch < 'A')
            ch = ch + 'Z' - 'A' + 1
         done
         msg[i] = ch
      done
   done
   Print decrypted message
End

Example

#include<iostream>
#include<string.h>
using namespace std;
int main() {
   cout<<"Enter the message:\n";
   char msg[100];
   cin.getline(msg,100); //take the message as input
   int i, j, length,choice,key;
   cout << "Enter key: ";
   cin >> key; //take the key as input
   length = strlen(msg);
   cout<<"Enter your choice \n1. Encryption \n2. Decryption \n";
   cin>>choice;
   if (choice==1) //for encryption{
      char ch;
      for(int i = 0; msg[i] != '\0'; ++i) {
         ch = msg[i];
         //encrypt for lowercase letter
         If (ch >= 'a' && ch <= 'z'){
            ch = ch + key;
            if (ch > 'z') {
               ch = ch - 'z' + 'a' - 1;
            }  
            msg[i] = ch;
         }
         //encrypt for uppercase letter
         else if (ch >= 'A' && ch <= 'Z'){
            ch = ch + key;
            if (ch > 'Z'){
               ch = ch - 'Z' + 'A' - 1;
            }
            msg[i] = ch;
         }
      }
      printf("Encrypted message: %s", msg);
   }
   else if (choice == 2) { //for decryption
      char ch;
      for(int i = 0; msg[i] != '\0'; ++i) {
         ch = msg[i];
         //decrypt for lowercase letter
         if(ch >= 'a' && ch <= 'z') {
            ch = ch - key;
            if(ch < 'a'){
               ch = ch + 'z' - 'a' + 1;
            }
            msg[i] = ch;
         }
         //decrypt for uppercase letter
         else if(ch >= 'A' && ch <= 'Z') {
            ch = ch - key;
            if(ch < 'A') {
               ch = ch + 'Z' - 'A' + 1;
            }
            msg[i] = ch;
         }
      }
      cout << "Decrypted message: " << msg;
   }
}

Output

For encryption:
Enter the message:
tutorial
Enter key: 3
Enter your choice
1. Encryption
2. Decryption
1
Encrypted message: wxwruldo

For decryption:
Enter the message:
wxwruldo
Enter key: 3
Enter your choice
1. Encryption
2. Decryption
2
Decrypted message: tutorial