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. 

1. First one to lookup the various letters according to their place in the English alphabets to get the shifted number
2. 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 = "GEEKS FOR GEEKS";
    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

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 = "GEEKS FOR GEEKS"
    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()

Output

TRRXF SBE TRRXF
GEEKS FOR GEEKS

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. 

This article is contributed by Palash Nigam . If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.