ROT13 cipher
ROT13 cipher(read as – “rotate by 13 places”) is a special case of the Ceaser cipher in which the shift is always 13.
So every letter is shifted 13 places to encrypt or to decrypt the message.
You must think that it is just another caesar cipher so what’s different this time? Well the difference is in its implementation. The approach is to use two separate python dictionaries.
- First one to lookup the various letters according to their place in the English alphabets to get the shifted number
- Second one to get the letters which correspond to those shifted numbers.
Implementation:
C++
// CPP program to implement // ROT13 Caesar Cipher #include<bits/stdc++.h> using namespace std; // Map to lookup the index of alphabets map < char , int > dict1; // Map to lookup alphabets corresponding // to the index after shift map < int , char > dict2; // Function to create map to lookup void create_dict() { for ( int i = 1; i < 27; i++) dict1[ char (64 + i)] = i; dict2[0] = 'Z' ; for ( int i = 1; i < 26; i++) dict2[i] = char (64 + i); return ; } // Function to encrypt the string // according to the shift provided string encrypt(string message, int shift) { string cipher = "" ; for ( int i = 0; i < message.size(); i++) { // Checking for namespace if (message[i] != ' ' ) { // looks up the map and // adds the shift to the index int num = (dict1[message[i]] + shift) % 26; // looks up the second map for the // shifted alphabets and adds them cipher += dict2[num]; } else { // adds space cipher += " " ; } } return cipher; } // Function to decrypt the string // according to the shift provided string decrypt(string message, int shift) { string decipher = "" ; for ( int i = 0; i < message.size(); i++) { // checks for space if (message[i] != ' ' ) { // looks up the map and // subtracts the shift to the index int num = (dict1[message[i]] - shift + 26) % 26; // looks up the second map for the // shifted alphabets and adds them decipher += dict2[num]; } else { // adds space decipher += " " ; } } return decipher; } // Driver code int main() { create_dict(); string message = "Beginner FOR Beginner" ; int shift = 13; cout << encrypt(message, shift) << "\n" ; message = "TRRXF SBE TRRXF" ; shift = 13; cout << decrypt(message, shift) << "\n" ; return 0; } // This code is contributed by Sachin Bisht |
Java
// java program for the above approach import java.util.*; public class Main { // Map to lookup the index of alphabets static Map<Character, Integer> dict1 = new HashMap<>(); // Map to lookup alphabets corresponding // to the index after shift static Map<Integer, Character> dict2 = new HashMap<>(); // Function to create map to lookup static void create_dict() { for ( int i = 1 ; i < 27 ; i++) dict1.put(( char )( 64 + i), i); dict2.put( 0 , 'Z' ); for ( int i = 1 ; i < 26 ; i++) dict2.put(i, ( char )( 64 + i)); } // Function to encrypt the string // according to the shift provided static String encrypt(String message, int shift) { String cipher = "" ; for ( int i = 0 ; i < message.length(); i++) { // Checking for namespace if (message.charAt(i) != ' ' ) { // looks up the map and // adds the shift to the index int num = (dict1.get(message.charAt(i)) + shift) % 26 ; // looks up the second map for the // shifted alphabets and adds them cipher += dict2.get(num); } else { // adds space cipher += " " ; } } return cipher; } // Function to decrypt the string // according to the shift provided static String decrypt(String message, int shift) { String decipher = "" ; for ( int i = 0 ; i < message.length(); i++) { // checks for space if (message.charAt(i) != ' ' ) { // looks up the map and // subtracts the shift to the index int num = (dict1.get(message.charAt(i)) - shift + 26 ) % 26 ; // looks up the second map for the // shifted alphabets and adds them decipher += dict2.get(num); } else { // adds space decipher += " " ; } } return decipher; } // Driver code public static void main(String[] args) { create_dict(); String message = "Beginner FOR Beginner" ; int shift = 13 ; System.out.println(encrypt(message, shift)); message = "TRRXF SBE TRRXF" ; shift = 13 ; System.out.println(decrypt(message, shift)); } } // This code is contributed by prince |
Python3
# Python program to implement # ROT13 Caesar cipher '''This script uses dictionaries instead of 'chr()' & 'ord()' function''' # Dictionary to lookup the index of alphabets dict1 = { '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 # corresponding to the index after shift dict2 = { 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' } # Function to encrypt the string # according to the shift provided def encrypt(message, shift): cipher = '' for letter in message: # checking for space if (letter ! = ' ' ): # looks up the dictionary and # adds the shift to the index num = ( dict1[letter] + shift ) % 26 # looks up the second dictionary for # the shifted alphabets and adds them cipher + = dict2[num] else : # adds space cipher + = ' ' return cipher # Function to decrypt the string # according to the shift provided def decrypt(message, shift): decipher = '' for letter in message: # checks for space if (letter ! = ' ' ): # looks up the dictionary and # subtracts the shift to the index num = ( dict1[letter] - shift + 26 ) % 26 # looks up the second dictionary for the # shifted alphabets and adds them decipher + = dict2[num] else : # adds space decipher + = ' ' return decipher # driver function to run the program def main(): # use 'upper()' function to convert any lowercase characters to uppercase message = "Beginner FOR Beginner" shift = 13 result = encrypt(message.upper(), shift) print (result) message = "TRRXF SBE TRRXF" shift = 13 result = decrypt(message.upper(), shift) print (result) # Executes the main function if __name__ = = '__main__' : main() |
C#
using System; using System.Collections; using System.Collections.Generic; using System.Linq; // C# program for the above approach class HelloWorld { // Map to lookup the index of alphabets public static Dictionary< char , int > dict1 = new Dictionary< char , int >(); // Map to lookup alphabets corresponding // to the index after shift public static Dictionary< int , char > dict2 = new Dictionary< int , char >(); // Function to create map to lookup public static void create_dict() { for ( int i = 1; i < 27; i++) dict1.Add(( char )(64 + i), i); dict2.Add(0, 'Z' ); for ( int i = 1; i < 26; i++) dict2.Add(i, ( char )(64 + i)); } // Function to encrypt the string // according to the shift provided public static string encrypt( string message, int shift) { string cipher = "" ; for ( int i = 0; i < message.Length; i++) { // Checking for namespace if (message[i] != ' ' ) { // looks up the map and // adds the shift to the index int num = (dict1[message[i]] + shift) % 26; // looks up the second map for the // shifted alphabets and adds them cipher += dict2[num]; } else { // adds space cipher += " " ; } } return cipher; } // Function to decrypt the string // according to the shift provided public static string decrypt( string message, int shift) { string decipher = "" ; for ( int i = 0; i < message.Length; i++) { // checks for space if (message[i] != ' ' ) { // looks up the map and // subtracts the shift to the index int num = (dict1[message[i]]- shift + 26) % 26; // looks up the second map for the // shifted alphabets and adds them decipher += dict2[num]; } else { // adds space decipher += " " ; } } return decipher; } static void Main() { create_dict(); string message = "Beginner FOR Beginner" ; int shift = 13; Console.WriteLine(encrypt(message, shift)); message = "TRRXF SBE TRRXF" ; shift = 13; Console.WriteLine(decrypt(message, shift)); } } // The code is contributed by Nidhi goel. |
Javascript
// Dictionary to lookup the index of alphabets const dict1 = { '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 // corresponding to the index after shift const dict2 = {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' }; // Function to encrypt the string // according to the shift provided function encrypt(message, shift) { let cipher = '' ; for (let i = 0; i < message.length; i++) { const letter = message[i]; // checking for space if (letter !== ' ' ) { // looks up the dictionary and // adds the shift to the index const num = (dict1[letter] + shift) % 26; // looks up the second dictionary for // the shifted alphabets and adds them cipher += dict2[num]; } else { // adds space cipher += ' ' ; } } return cipher; } // Function to decrypt the string // according to the shift provided function decrypt(message, shift) { let decipher = '' ; for (let i = 0; i < message.length; i++) { const letter = message[i]; // checks for space if (letter !== ' ' ) { // looks up the dictionary and // subtracts the shift to the index const num = (dict1[letter] - shift + 26) % 26; // looks up the second dictionary for the // shifted alphabets and adds them decipher += dict2[num]; } else { // adds space decipher += ' ' ; } } return decipher; } // driver function to run the program function main() { // use 'toUpperCase()' function to convert any lowercase characters to uppercase let message = "Beginner FOR Beginner" ; let shift = 13; let result = encrypt(message.toUpperCase(), shift); console.log(result); message = "TRRXF SBE TRRXF" ; shift = 13; result = decrypt(message.toUpperCase(), shift); console.log(result); } main(); // This code is contributed by adityashatmfh |
TRRXF SBE TRRXF Beginner FOR Beginner
Analysis: The ROT13 cipher is not very secure as it is just a special case of the Caesar cipher. The Caesar cipher can be broken by either frequency analysis or by just trying out all 25 keys whereas the ROT13 cipher can be broken by just shifting the letters 13 places. Therefore it has no practical use.
Application: ROT13 was in use in the net.jokes newsgroup by the early 1980s.
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