Cryptography - ROT13 Algorithm

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Till now, you have learnt about reverse cipher and Caesar cipher algorithms. Now, let us discuss the ROT13 algorithm and its implementation.

So we are going to discuss the ROT13 cryptography algorithm. Rotate 13 Places is referred to as ROT13. It is a special case of the Caesar cipher encryption algorithm. It is among the simplest methods of encryption. As the algorithm is the same for both encryption and decryption, it is a symmetric key encryption technique.

Because the encryption key displays the letters A through Z as the numbers 0 through 25, the cipher text differs by 13 spaces from the plaintext letter. As an example, A turns into N, B into O, and so on. Because the encryption and decryption processes are identical, they can be done programatically.

Explanation of ROT13 Algorithm

ROT13 cipher refers to the abbreviated form Rotate by 13 places. In this particular Caesar Cipher, shift is always 13. The message is encrypted or decrypted by shifting each letter thirteen places.

Let us see the below image to know how ROT13 algorithm works −

Every letter has been moved 13 spaces to the right, as you can see. As an example, "B" will become "O," "D" will become "Q," and so on.

Features of ROT13

Here are some main features of ROT13 −

• Since ROT13 is symmetric, both encryption and decryption use the same algorithm and key.

• It works for each of the alphabetic letters, substituting each one with the letter 13 positions forward (or back, based on the decryption) in the alphabet.

• The rotation value of 13 is fixed in ROT13.

• When applying ROT13, if the rotation goes beyond 'Z', it comes back to the beginning of the alphabet 'A'.

• ROT13 is very easy to implement and use.

• It provides no security against any determined attacker, as it is easily cracked with simple frequency analysis.

Implementation using Python

So we can implement this algorithm in different ways −

Using For Loop and ord() function

We will first encrypt the given message and generate an ROT13 code using a for loop. In this program, the for loop will be used to iterate over each character in the given text. If the character is a letter, we will keep it (uppercase or lowercase) and shift it by thirteen places. Non-alphabetic characters will remain unchanged. Also we will use the ord() function of Python which is used to change a single Unicode character into its integer representation.

Example

Below is a simple Python code for ROT13 cryptography. See the code below −

# Encryption Function 
def rot13_encrypt(text):
   encrypted_text = ''
   for char in text:
      if char.isalpha():
         shifted = ord(char) + 13
         if char.islower():
            if shifted > ord('z'):
               shifted -= 26
         else:
            if shifted > ord('Z'):
               shifted -= 26
         encrypted_text += chr(shifted)
      else:
         encrypted_text += char
   return encrypted_text

# Decryption Function 
def rot13_decrypt(text):
   decrypted_text = ''
   for char in text:
      if char.isalpha():
         shifted = ord(char) - 13  # Decryption involves shifting back by 13
         if char.islower():
            if shifted < ord('a'):
               shifted += 26
         else:
            if shifted < ord('A'):
               shifted += 26
         decrypted_text += chr(shifted)
      else:
         decrypted_text += char
   return decrypted_text

# function execution
message = "Hello, Tutorialspoint!"
 encrypted_msg = rot13_encrypt(message)
print("The Encrypted message:",  encrypted_msg)  

 decrypted_msg = rot13_decrypt( encrypted_msg)
print("The Decrypted message:",  decrypted_msg)

Following is the output of the above example −

Input/Output

The Encrypted message: Uryyb, Ghgbevnyfcbvag!
The Decrypted message: Hello, Tutorialspoint!

Using list comprehension

In this example, list comprehension is used to perform the ROT13 encryption. Using the list we will iterate over each character in the input text using the for char in the text part at the end. For each character in the input text we will use a conditional expression to find the encrypted value.

Example

Below is a simple Python code for the ROT13 algorithm. See the program below −

# Encryption function
def rot13_encrypt(text):
   encrypted_text = ''.join([chr(((ord(char) - 65 + 13) % 26) + 65) if 'A' <= char <= 'Z' else 
      chr(((ord(char) - 97 + 13) % 26) + 97) if 'a' <= char <= 'z' else char for char in text])
   return encrypted_text

# Decryption function
def rot13_decrypt(text):
   decrypted_text = ''.join([chr(((ord(char) - 65 - 13) % 26) + 65) if 'A' <= char <= 'Z' else 
      chr(((ord(char) - 97 - 13) % 26) + 97) if 'a' <= char <= 'z' else char for char in text])
   return decrypted_text

# Function execution
message = "Hello, Everyone!"
 encrypted_msg = rot13_encrypt(message)
print("The Encrypted message:",  encrypted_msg)

 decrypted_msg = rot13_decrypt( encrypted_msg)
print("The Decrypted message:",  decrypted_msg)

Following is the output of the above example −

Input/Output

The Encrypted message: Uryyb, Rirelbar!
The Decrypted message: Hello, Everyone!

Using Dictionary

In this example we are going to use two dictionaries to implement the program for ROT13. So the first dictionary will map uppercase letters to their index in the alphabet. And the second dictionary will map shifted indices back to uppercase letters, from 'Z' to 'A'. The encryption function sends the resultant index back to a letter using the second dictionary after identifying each letter's index using the first dictionary and adding the shift value. And in the decryption function we will reverse the process.

Example

Following is a simple Python program of ROT13 algorithm in which will use two dictionaries. Check the code below −

# Dictionary to lookup the index 
dictionary1 = {'A': 1, 'B': 2, 'C': 3, 'D': 4, 'E': 5,
   'F': 6, 'G': 7, 'H': 8, 'I': 9, 'J': 10,
   'K': 11, 'L': 12, 'M': 13, 'N': 14, 'O': 15,
   'P': 16, 'Q': 17, 'R': 18, 'S': 19, 'T': 20,
   'U': 21, 'V': 22, 'W': 23, 'X': 24, 'Y': 25, 'Z': 26}

# Dictionary to lookup alphabets 
dictionary2 = {0: 'Z', 1: 'A', 2: 'B', 3: 'C', 4: 'D', 5: 'E',
   6: 'F', 7: 'G', 8: 'H', 9: 'I', 10: 'J',
   11: 'K', 12: 'L', 13: 'M', 14: 'N', 15: 'O',
   16: 'P', 17: 'Q', 18: 'R', 19: 'S', 20: 'T',
   21: 'U', 22: 'V', 23: 'W', 24: 'X', 25: 'Y'}

# Encryption Function 
def encrypt(msg, shift):
   cipher = ''
   for letter in msg:
      # check for space
      if letter != ' ':
         num = (dictionary1[letter] + shift) % 26
         cipher += dictionary2[num]
      else:
         cipher += ' '

   return cipher

# Decryption Function 
def decrypt(msg, shift):
   decipher = ''
   for letter in msg:
      # checks for space
      if letter != ' ':
         num = (dictionary1[letter] - shift + 26) % 26
         decipher += dictionary2[num]
      else:
         decipher += ' '

   return decipher

msg = "Hey Tutorialspoint"
shift = 13
result = encrypt(msg.upper(), shift)
print("The Encrypted message: ", result)

msg = "URL GHGBEVNYFCBVAG"
shift = 13
result = decrypt(msg.upper(), shift)
print("The Decrypted message: ", result)

Following is the output of the above example −

Input/Output

The Encrypted message:  URL GHGBEVNYFCBVAG
The Decrypted message:  HEY TUTORIALSPOINT

Implementation using C++

To implement the ROT13 algorithm we will use C++ programming langugage. Using the rot13Func function we will encrypt the given string message. Each letter in the given input text is iterated over. To get the matching ROT13 character, it alters the character by 13 positions if it is an alphabet letter. The main function calls the rot13Func function and outputs the encrypted text.

Example

#include <iostream>
#include <string>

using namespace std;

// Function to perform rot13Func encryption
string rot13Func(string text) {
   for (char& c : text) {
      if (isalpha(c)) {
         char base = islower(c) ? 'a' : 'A';
         c = (c - base + 13) % 26 + base;
      }
   }
   return text;
}

int main() {
   string plaintext = "Hello this world is so beautiful";
   cout << "The Plaintext Message is: " << plaintext << endl;

   string encrypted_text = rot13Func(plaintext);
   cout << "Encrypted text: " << encrypted_text << endl;

   return 0;
}

Following is the output of the above example −

Input/Output

The Plaintext Message is: Hello this world is so beautiful
Encrypted text: Uryyb guvf jbeyq vf fb ornhgvshy

Implementation using Java

In this implementation we are going to use Java programming langugage to create ROT13 algorithm. Using a string text as input, the rot13Func method encrypts it using ROT13. Every character in the input text is iterated over. To get the matching ROT13 character, it alters the character by 13 positions if it is an alphabet letter. After receiving input message, the main method calls the rot13 function and outputs the encrypted text.

Example

public class ROT13Class {
   // Function to perform ROT13 encryption
   public static String rot13Func(String text) {
      StringBuilder result = new StringBuilder();
      for (char c : text.toCharArray()) {
         if (Character.isLetter(c)) {
            char base = Character.isLowerCase(c) ? 'a' : 'A';
            c = (char) (((c - base + 13) % 26) + base);
         }
         result.append(c);
      }
      return result.toString();
   }

   public static void main(String[] args) {
      String plaintext = "The world is so beautiful!";
      System.out.println("The Plain Text Message: " + plaintext);
      String encryptedText = rot13Func(plaintext);
      System.out.println("The Encrypted text: " + encryptedText);
   }
}

Following is the output of the above example −

Input/Output

The Plain Text Message: The world is so beautiful!
The Encrypted text: Gur jbeyq vf fb ornhgvshy!

Drawback

As the ROT13 cipher is really a special case application of the Caesar cipher, it is not very secure. Although the ROT13 cipher can be cracked by simply moving the letters 13 positions, the Caesar cipher can only be cracked via frequency analysis or by trying all 25 keys. It is therefore useless in real life.

Analysis of ROT13 Algorithm

ROT13 cipher algorithm is considered as special case of Caesar Cipher. It is not a very secure algorithm and can be broken easily with frequency analysis or by just trying possible 25 keys whereas ROT13 can be broken by shifting 13 places. Therefore, it does not include any practical use.

Summary

We have examined the Python cryptography ROT13 algorithm in this article. ROT13 is a quick and efficient method of encrypting a given message by moving each letter in the alphabet by 13 positions. It is mostly used for basic encryption tasks. We have implemented different ways in this chapter so you can now encrypt and decrypt messages using the ROT13 algorithm in Python.