Caesar Cipher in Cryptography

The Caesar Cipher technique is one of the earliest and simplest methods of encryption technique. It’s simply a type of substitution cipher, i.e., each letter of a given text is replaced by a letter with a fixed number of positions down the alphabet. For example with a shift of 1, A would be replaced by B, B would become C, and so on. The method is apparently named after Julius Caesar, who apparently used it to communicate with his officials. 

Thus to cipher a given text we need an integer value, known as a shift which indicates the number of positions each letter of the text has been moved down. 
The encryption can be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B = 1,…, Z = 25. Encryption of a letter by a shift n can be described mathematically as. 

(Encryption Phase with shift n)

(Decryption Phase with shift n)

Examples :

Text : ABCDEFGHIJKLMNOPQRSTUVWXYZ
Shift: 23
Cipher: XYZABCDEFGHIJKLMNOPQRSTUVW

Text : ATTACKATONCE
Shift: 4
Cipher: EXXEGOEXSRGI

Advantages:

• Easy to implement and use thus, making suitable for beginners to learn about encryption.
• Can be physically implemented, such as with a set of rotating disks or a set of cards, known as a scytale, which can be useful in certain situations.
• Requires only a small set of pre-shared information.
• Can be modified easily to create a more secure variant, such as by using a multiple shift values or keywords.

Disadvantages:

• It is not secure against modern decryption methods.
• Vulnerable to known-plaintext attacks, where an attacker has access to both the encrypted and unencrypted versions of the same messages.
• The small number of possible keys means that an attacker can easily try all possible keys until the correct one is found, making it vulnerable to a brute force attack.
• It is not suitable for long text encryption as it would be easy to crack.
• It is not suitable for secure communication as it is easily broken.
• Does not provide confidentiality, integrity, and authenticity in a message.

Algorithm for Caesar Cipher:
Input:

1. A String of lower case letters, called Text.
2. An Integer between 0-25 denoting the required shift.

Procedure:

• Traverse the given text one character at a time .
• For each character, transform the given character as per the rule, depending on whether we’re encrypting or decrypting the text.
• Return the new string generated.

A program that receives a Text (string) and Shift value( integer) and returns the encrypted text. 

C++

// A C++ program to illustrate Caesar Cipher Technique
#include <iostream>
using namespace std;

// This function receives text and shift and
// returns the encrypted text
string encrypt(string text, int s)
{
    string result = "";

    // traverse text
    for (int i = 0; i < text.length(); i++) {
        // apply transformation to each character
        // Encrypt Uppercase letters
        if (isupper(text[i]))
            result += char(int(text[i] + s - 65) % 26 + 65);

        // Encrypt Lowercase letters
        else
            result += char(int(text[i] + s - 97) % 26 + 97);
    }

    // Return the resulting string
    return result;
}

// Driver program to test the above function
int main()
{
    string text = "ATTACKATONCE";
    int s = 4;
    cout << "Text : " << text;
    cout << "\nShift: " << s;
    cout << "\nCipher: " << encrypt(text, s);
    return 0;
}

Java

//A Java Program to illustrate Caesar Cipher Technique
class CaesarCipher
{
    // Encrypts text using a shift od s
    public static StringBuffer encrypt(String text, int s)
    {
        StringBuffer result= new StringBuffer();

        for (int i=0; i<text.length(); i++)
        {
            if (Character.isUpperCase(text.charAt(i)))
            {
                char ch = (char)(((int)text.charAt(i) +
                                  s - 65) % 26 + 65);
                result.append(ch);
            }
            else
            {
                char ch = (char)(((int)text.charAt(i) +
                                  s - 97) % 26 + 97);
                result.append(ch);
            }
        }
        return result;
    }

    // Driver code
    public static void main(String[] args)
    {
        String text = "ATTACKATONCE";
        int s = 4;
        System.out.println("Text  : " + text);
        System.out.println("Shift : " + s);
        System.out.println("Cipher: " + encrypt(text, s));
    }
}

Python3

#A python program to illustrate Caesar Cipher Technique
def encrypt(text,s):
    result = ""

    # traverse text
    for i in range(len(text)):
        char = text[i]

        # Encrypt uppercase characters
        if (char.isupper()):
            result += chr((ord(char) + s-65) % 26 + 65)

        # Encrypt lowercase characters
        else:
            result += chr((ord(char) + s - 97) % 26 + 97)

    return result

#check the above function
text = "ATTACKATONCE"
s = 4
print ("Text  : " + text)
print ("Shift : " + str(s))
print ("Cipher: " + encrypt(text,s))

C#

// A C# Program to illustrate Caesar Cipher Technique
using System;
using System.Text;

public class CaesarCipher
{
    // Encrypts text using a shift od s
    public static StringBuilder encrypt(String text, int s)
    {
        StringBuilder result= new StringBuilder();

        for (int i=0; i<text.Length; i++)
        {
            if (char.IsUpper(text[i]))
            {
                char ch = (char)(((int)text[i] +
                                s - 65) % 26 + 65);
                result.Append(ch);
            }
            else
            {
                char ch = (char)(((int)text[i] +
                                s - 97) % 26 + 97);
                result.Append(ch);
            }
        }
        return result;
    }

    // Driver code
    public static void Main(String[] args)
    {
        String text = "ATTACKATONCE";
        int s = 4;
        Console.WriteLine("Text : " + text);
        Console.WriteLine("Shift : " + s);
        Console.WriteLine("Cipher: " + encrypt(text, s));
    }
}

/* This code contributed by PrinciRaj1992 */

PHP

<?php
// A PHP program to illustrate Caesar
// Cipher Technique

// This function receives text and shift
// and returns the encrypted text
function encrypt($text, $s)
{
    $result = "";

    // traverse text
    for ($i = 0; $i < strlen($text); $i++)
    {
        // apply transformation to each
        // character Encrypt Uppercase letters
        if (ctype_upper($text[$i]))
            $result = $result.chr((ord($text[$i]) +
                               $s - 65) % 26 + 65);

    // Encrypt Lowercase letters
    else
        $result = $result.chr((ord($text[$i]) +
                           $s - 97) % 26 + 97);
    }

    // Return the resulting string
    return $result;
}

// Driver Code
$text = "ATTACKATONCE";
$s = 4;
echo "Text : " . $text;
echo "\nShift: " . $s;
echo "\nCipher: " . encrypt($text, $s);

// This code is contributed by ita_c
?>

Javascript

<script>
//A Javascript Program to illustrate Caesar Cipher Technique

    // Encrypts text using a shift od s
    function encrypt(text, s)
    {
        let result=""
        for (let i = 0; i < text.length; i++)
        {
            let char = text[i];
            if (char.toUpperCase(text[i]))
            {
                let ch =  String.fromCharCode((char.charCodeAt(0) + s-65) % 26 + 65);
                result += ch;
            }
            else
            {
                let ch = String.fromCharCode((char.charCodeAt(0) + s-97) % 26 + 97);
                result += ch;
            }
        }
        return result;
    }

    // Driver code
    let text = "ATTACKATONCE";
    let s = 4;
    document.write("Text  : " + text + "<br>");
    document.write("Shift : " + s + "<br>");
    document.write("Cipher: " + encrypt(text, s) + "<br>");

    //  This code is contributed by avanitrachhadiya2155
</script>

Output

Text : ATTACKATONCE
Shift: 4
Cipher: EXXEGOEXSRGI

Time complexity: O(N) where N is length of the given text
Auxiliary space: O(N)

How to decrypt?
We can either write another function decrypt similar to encrypt, that’ll apply the given shift in the opposite direction to decrypt the original text. However we can use the cyclic property of the cipher under modulo, hence we can simply observe 

Cipher(n) = De-cipher(26-n)

Hence, we can use the same function to decrypt, instead, we’ll modify the shift value such that shift = 26-shift (Refer to this for a sample run in C++).

This article is contributed by Ashutosh Kumar. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.