Substitution Cipher

Hiding some data is known as encryption. When plain text is encrypted it becomes unreadable and is known as ciphertext. In a Substitution cipher, any character of plain text from the given fixed set of characters is substituted by some other character from the same set depending on a key. For example with a shift of 1, A would be replaced by B, B would become C, and so on. 

Note: Special case of Substitution cipher is known as Caesar cipher where the key is taken as 3.

Mathematical representation

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:

Plain Text: I am studying Data Encryption
Key: 4
Output: M eq wxyhCmrk Hexe IrgvCtxmsr

Plain Text: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Key: 4
Output: EFGHIJKLMNOPQRSTUVWXYZabcd

Algorithm for Substitution Cipher:

Input: 

• A String of both lower and upper case letters, called PlainText. 
• An Integer denoting the required key. 

Procedure:

• Create a list of all the characters. 
• Create a dictionary to store the substitution for all characters. 
• For each character, transform the given character as per the rule, depending on whether we’re encrypting or decrypting the text. 
• Print the new string generated. 

Below is the implementation.

C++

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

int main()
{
    string all_letrs = "abcdefghijklmnopqrstuvwxyzABCDEFGHI"
                       "JKLMNOPQRSTUVWXYZ";

    // create a dictionary to store the substitution for the
    // given alphabet in the plain text based on the key
    map<char, char> dict1;
    int key = 4;
    for (int i = 0; i < all_letrs.length(); i++) {
        dict1[all_letrs[i]]
            = all_letrs[(i + key) % all_letrs.length()];
    }

    string plain_txt = "I am studying Data Encryption";
    string cipher_txt;

    // loop to generate ciphertext
    for (char& c : plain_txt) {
        if (all_letrs.find(c) != string::npos) {
            cipher_txt += dict1;
        }
        else {
            cipher_txt += c;
        }
    }

    cout << "Cipher Text is: " << cipher_txt << endl;

    // create a map to store the substitution for the given
    // alphabet in the cipher text based on the key
    map<char, char> dict2;
    for (int i = 0; i < all_letrs.length(); i++) {
        dict2[all_letrs[i]]
            = all_letrs[(i - key) % all_letrs.length()];
    }

    string decrypt_txt;

    // loop to recover plain text
    for (char& c : cipher_txt) {
        if (all_letrs.find(c) != string::npos) {
            decrypt_txt += dict2;
        }
        else {
            decrypt_txt += c;
        }
    }

    cout << "Recovered plain text : " << decrypt_txt
         << endl;

    return 0;
}

Java

// Java implementation of Substitution Cipher
import java.io.*;
import java.util.*;
import java.util.HashMap;
import java.util.Map;

public class CaesarCipher {
    public static void main(String[] args) {
        String allLetters = "abcdefghijklmnopqrstuvwxyzABCDEFGHI" + "JKLMNOPQRSTUVWXYZ";

        // create a dictionary to store the substitution for the given alphabet in the plain text based on the key
        Map<Character, Character> dict1 = new HashMap<>();
        int key = 4;
        for (int i = 0; i < allLetters.length(); i++) {
            dict1.put(allLetters.charAt(i),
                    allLetters.charAt((i + key) % allLetters.length()));
        }

        String plainText = "I am studying Data Encryption";
        StringBuilder cipherText = new StringBuilder();

        // loop to generate ciphertext
        for (char c : plainText.toCharArray()) {
            if (allLetters.indexOf(c) != -1) {
                cipherText.append(dict1.get(c));
            } else {
                cipherText.append(c);
            }
        }

        System.out.println("Cipher Text is: " + cipherText);

        // create a map to store the substitution for the given alphabet in the cipher text based on the key
        Map<Character, Character> dict2 = new HashMap<>();
        for (int i = 0; i < allLetters.length(); i++) {
            dict2.put(allLetters.charAt(i),
                    allLetters.charAt((i - key + allLetters.length()) % allLetters.length()));
        }

        StringBuilder decryptedText = new StringBuilder();

        // loop to recover plain text
        for (char c : cipherText.toString().toCharArray()) {
            if (allLetters.indexOf(c) != -1) {
                decryptedText.append(dict2.get(c));
            } else {
                decryptedText.append(c);
            }
        }

        System.out.println("Recovered plain text: " + decryptedText);
    }
}

Python3

# Python program to demonstrate
# Substitution Cipher


import string


# A list containing all characters
all_letters= string.ascii_letters


"""
create a dictionary to store the substitution
for the given alphabet in the plain text
based on the key
"""


dict1 = {}
key = 4

for i in range(len(all_letters)):
    dict1[all_letters[i]] = all_letters[(i+key)%len(all_letters)]


plain_txt= "I am studying Data Encryption"
cipher_txt=[]

# loop to generate ciphertext

for char in plain_txt:
    if char in all_letters:
        temp = dict1[char]
        cipher_txt.append(temp)
    else:
        temp =char
        cipher_txt.append(temp)

cipher_txt= "".join(cipher_txt)
print("Cipher Text is: ",cipher_txt)


"""
create a dictionary to store the substitution
for the given alphabet in the cipher
text based on the key
"""


dict2 = {}
for i in range(len(all_letters)):
    dict2[all_letters[i]] = all_letters[(i-key)%(len(all_letters))]

# loop to recover plain text
decrypt_txt = []

for char in cipher_txt:
    if char in all_letters:
        temp = dict2[char]
        decrypt_txt.append(temp)
    else:
        temp = char
        decrypt_txt.append(temp)

decrypt_txt = "".join(decrypt_txt)
print("Recovered plain text :", decrypt_txt)

C#

using System;
using System.Collections.Generic;
using System.Text;

namespace SubstitutionCipher
{
    class Program
    {
        static void Main(string[] args)
        {
            string allLetters = "abcdefghijklmnopqrstuvwxyzABCDEFGHI" + "JKLMNOPQRSTUVWXYZ";

            // create a dictionary to store the substitution for the given alphabet in the plain text based on the key
            Dictionary<char, char> dict1 = new Dictionary<char, char>();
            int key = 4;
            for (int i = 0; i < allLetters.Length; i++)
            {
                dict1.Add(allLetters[i],
                    allLetters[(i + key) % allLetters.Length]);
            }

            string plainText = "I am studying Data Encryption";
            StringBuilder cipherText = new StringBuilder();

            // loop to generate ciphertext
            foreach (char c in plainText)
            {
                if (allLetters.IndexOf(c) != -1)
                {
                    cipherText.Append(dict1);
                }
                else
                {
                    cipherText.Append(c);
                }
            }

            Console.WriteLine("Cipher Text is: " + cipherText);

            // create a dictionary to store the substitution for the given alphabet in the cipher text based on the key
            Dictionary<char, char> dict2 = new Dictionary<char, char>();
            for (int i = 0; i < allLetters.Length; i++)
            {
                dict2.Add(allLetters[i],
                    allLetters[(i - key + allLetters.Length) % allLetters.Length]);
            }

            StringBuilder decryptedText = new StringBuilder();

            // loop to recover plain text
            foreach (char c in cipherText.ToString())
            {
                if (allLetters.IndexOf(c) != -1)
                {
                    decryptedText.Append(dict2);
                }
                else
                {
                    decryptedText.Append(c);
                }
            }

            Console.WriteLine("Recovered plain text: " + decryptedText);
        }
    }
}

Javascript

// JavaScript program to demonstrate
// Substitution Cipher

// A string containing all characters
const all_letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

/*
Create a map to store the substitution for the given alphabet in the plain text based on the key
*/
const dict1 = {};
const key = 4;

for (let i = 0; i < all_letters.length; i++) {
dict1[all_letters[i]] = all_letters[(i + key) % all_letters.length];
}

const plain_txt = "I am studying Data Encryption";
let cipher_txt = "";

// loop to generate ciphertext
for (let i = 0; i < plain_txt.length; i++) {
const char = plain_txt[i];
if (all_letters.includes(char)) {
const temp = dict1[char];
cipher_txt += temp;
} else {
cipher_txt += char;
}
}

console.log("Cipher Text is: ", cipher_txt);

/*
Create a map to store the substitution for the given alphabet in the cipher text based on the key
*/
const dict2 = {};

for (let i = 0; i < all_letters.length; i++) {
dict2[all_letters[i]] = all_letters[(i - key + all_letters.length) % all_letters.length];
}

// loop to recover plain text
let decrypt_txt = "";

for (let i = 0; i < cipher_txt.length; i++) {
const char = cipher_txt[i];
if (all_letters.includes(char)) {
const temp = dict2[char];
decrypt_txt += temp;
} else {
decrypt_txt += char;
}
}

console.log("Recovered plain text :", decrypt_txt);

Output

Cipher Text is:  M eq wxyhCmrk Hexe IrgvCtxmsr
Recovered plain text : I am studying Data Encryption

 Time Complexity: O(n)
Auxiliary Space: O(n)