Caesar, Shift the Cipher

Shift Cipher

The 'Shift Cipher' is one of the oldest cryptography methods. It is named after Julius Caesar. In the first century, Suetonius, a Roman historian, wrote ‘The Lives of the Twelve Caesars’. This book contains about shift cipher. It was a technique used by Julius Caesar to send secret messages.  If he had to say anything confidential, he wrote it in cipher. So in honor of Julius Ceasar, this cipher is known as the shift/caesar cipher. Later on, Suetonius explained the encryption and decryption process. He also described that encryption and description are reverse processes. 

In shift cipher, each letter of the message is shifted down by ‘K’ letters. Where ‘K’ is any number specified by the user. For example, Earlier people uses cipher disks to decrypt or encrypt the message. People rotate the disk and write the message using corresponding alphabets. The disk looks like -

Formula Used 

The formula used for encryption is - 

E(m) = (m + k) mod 26

The formula of decryption is:

D(m) = (m - k) mod 26

Where, 

E depicts encryption

D depicts decryption

m denotes the letter-value

k denotes the key-value

Let’s see an example for a better understanding. 

Example

Let’s say we want to encrypt the word ‘CODING’ and the key is 5. The table of letters used will be as follows - 

If we apply the encryption formula on the word ‘CODING’ with k = 5, then the ciphertext will be as 

So, the encrypted message is ‘HTINSL’

Using the decryption formula on this ciphertext with K = 5. The plaintext received will be as follows -

Yes, we got our original message i.e., ‘CODING’, back.

Implementation

Let’s see the implementation of shift cipher - 

Code

#include <bits/stdc++.h>
using namespace std;

// Create encryption function
string encryption(string message, int size, int key)
{
    for(int i=0;i<size;i++)  
    {  
        char c = message[i];  
        
        // Checking for lower-case letters
        if(c >= 'a' && c <= 'z')  
        {  
            c = c + key;  
            if(c > 'z')  
            c = c - 'z' + 'a' - 1;  
              
            message[i] = c;  
        }  
        
        //Checking for upper-case letters
        else if(c >= 'A' && c < 'Z')  
        {  
            c = c + key;  
            if(c > 'Z')  
            c = c = 'Z' + 'A' - 1;  
             
            message[i] = c;  
        }  
    }
    return message;
}

// Create decryption function
string decryption(string message, int size, int key)
{
    
    for(int i=0;i<size;i++)  
    {  
        char c = message[i];  
        // Checking for lower-case letters
        if(c >= 'a' && c <= 'z')  
        {  
            c = c - key;  
            if(c < 'a')  
            c = c + 'z' - 'a' + 1;  
              
            message[i] = c;  
        } 
        
        // Checking for upper-case letters
        else if(c >= 'A' && c <= 'Z')  
        {  
            c = c - key;  
            if(c < 'A')  
            c = c + 'Z' - 'A' + 1;  
              
            message[i] = c;  
        }  
    }  
    return message;
}

int main()
{
    string message;
    int key;
    
    // Taking User Input
    cout<<"Enter your message"<<endl;  
    getline(cin, message);
    
    cout<<endl;
    cout<<"Enter key"<<endl;
    cin>>key;
    
    int n=message.size();
    cout<<endl;
    
    string s = encryption(message, n, key);
    cout<<"Encrypted message: "<<s<<endl;
   
    message = decryption(s, n, key);
    cout<<"Decrypted message: "<<message<<endl;
    return 0;
}

You can also try this code with Online C++ Compiler

Run Code

Output

Advantages 

Shift cipher has the following advantages - 

• It is simple and very easy to implement.
• Only one short key is used in the entire process. The same key is used for encryption and decryption.
• If the system does not use complex coding techniques, it is the method and easy too.
• Only a few computing resources are required.

Disadvantages

Shift cipher has the following disadvantages - 

• Easy to hack. Attackers can use a brute force approach for description. 
• It is less secure.
• The key used is not different and easy to detect. 
• The message is easy to decrypt by just looking at the pattern.

Frequently Asked Questions

What is cryptography?

Cryptography is a method of encrypting and decrypting messages and information to establish a secure information exchange between the sender and the receiver.

What is shift cipher?

Shift cipher is a cryptographic algorithm. It shifts each letter of a message forward by a specified number. 

What are encryption and decryption?

Encryption is the conversion of plain text into ciphertext. In contrast, decryption is the process that converts ciphertext into plaintext.

How many different shifts are possible in a Caesar cipher?

The number of shifts possible will be one less than the number of letters present i.e., 26(0-25) in English.

What are plain text and ciphertext?

The plaintext can refer to anything that humans can understand. In contrast, the ciphertext is a series of randomized letters that don't make any sense to humans. 

Conclusion

In this article, we have discussed the shift cipher. We also discussed the formula used for encryption and decryption by this Cipher. Using this formula, we implemented shift cipher. And at last, we discussed the advantages and disadvantages of using this shift cipher. 

Do not stop learning! We recommend you read these articles -. 

1. What is Cryptography?
2. Stream ciphers
3. Keyword Cipher
4. Hill Cipher
5. Difference between Public Key and Private Key
6. Monte Carlo Simulation
    

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