vigenere_cipher.cpp File Reference

Implementation of Vigenère cipher algorithm. More...

#include <iostream>
#include <string>
#include <cassert>

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Namespaces
namespace	ciphers
	Algorithms for encryption and decryption.
namespace	vigenere
	Functions for vigenère cipher algorithm.

Functions
std::string	ciphers::vigenere::encrypt (const std::string &text, const std::string &key)
std::string	ciphers::vigenere::decrypt (const std::string &text, const std::string &key)
void	test ()
int	main ()

Detailed Description

Implementation of Vigenère cipher algorithm.

The Vigenère cipher is a method of encrypting alphabetic text by using a series of interwoven vigenere ciphers, based on the letters of a keyword. It employs a form of polyalphabetic substitution.

Algorithm

The encryption can also be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A → 0, B → 1, ..., Z → 25. Encryption of \(i^{th}\) character in Message M by key K can be described mathematically as,

\[ E_{K}(M_{i}) = (M_{i} + K_{i})\;\mbox{mod}\; 26\]

while decryption of \(i^{th}\) character in Cipher C by key K can be described mathematically as,

\[ D_{k}(C_{i}) = (C_{i} - K_{i} + 26)\;\mbox{mod}\; 26\]

Where \(K_{i}\) denotes corresponding character in key. If \(|key| < |text|\) than same key is repeated untill their lengths are equal.

For Example, If M = "ATTACKATDAWN" and K = "LEMON" than K becomes "LEMONLEMONLE".

Note

Rather than creating new key of equal length this program does this by using modular index for key (i.e. \((j + 1) \;\mbox{mod}\; |\mbox{key}|\))

This program implements Vigenère cipher for only uppercase English alphabet characters (i.e. A-Z).

Author

Deep Raval

Function Documentation

decrypt()

std::string ciphers::vigenere::decrypt	(	const std::string &	text,
		const std::string &	key )

Decrypt given text using vigenere cipher.

Parameters

text	text to be decrypted
key	key to be used for decryption

Returns

new decrypted text

                                                                        {
            // Going through each character of text and key
            // Note that key is visited in circular way hence  j = (j + 1) % |key|
            std::string decrypted_text = ""; // Empty string to store decrypted text
            for(size_t i = 0, j = 0; i < text.length(); i++, j = (j + 1) % key.length()) {
                int place_value_text = get_value(text[i]); // Getting value of character in text
                int place_value_key = get_value(key[j]); // Getting value of character in key
                place_value_text = (place_value_text - place_value_key + 26) % 26; // Applying decryption
                char decrypted_char = get_char(place_value_text); // Getting new character from decrypted value
                decrypted_text += decrypted_char; // Appending decrypted character
            }        
            return decrypted_text; // Returning decrypted text
        }

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encrypt()

std::string ciphers::vigenere::encrypt	(	const std::string &	text,
		const std::string &	key )

Encrypt given text using vigenere cipher.

Parameters

text	text to be encrypted
key	to be used for encryption

Returns

new encrypted text

                                                                        {
            std::string encrypted_text = ""; // Empty string to store encrypted text
            // Going through each character of text and key
            // Note that key is visited in circular way hence  j = (j + 1) % |key|
            for(size_t i = 0, j = 0; i < text.length(); i++, j = (j + 1) % key.length()) {
                int place_value_text = get_value(text[i]); // Getting value of character in text
                int place_value_key = get_value(key[j]); // Getting value of character in key
                place_value_text = (place_value_text + place_value_key) % 26; // Applying encryption
                char encrypted_char = get_char(place_value_text); // Getting new character from encrypted value
                encrypted_text += encrypted_char; // Appending encrypted character
            }
            return encrypted_text; // Returning encrypted text
        }

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main()

int main

(

void

)

Driver Code

           {
    // Testing
    test();
    return 0;
}

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test()

void test

(

)

Function to test above algorithm

            {
    // Test 1
    std::string text1 = "NIKOLATESLA";
    std::string encrypted1 = ciphers::vigenere::encrypt(text1, "TESLA");
    std::string decrypted1 = ciphers::vigenere::decrypt(encrypted1, "TESLA");
    assert(text1 == decrypted1);
    std::cout << "Original text : " << text1;
    std::cout << " , Encrypted text (with key = TESLA) : " << encrypted1;
    std::cout << " , Decrypted text : "<< decrypted1 << std::endl;
    // Test 2
    std::string text2 = "GOOGLEIT";
    std::string encrypted2 = ciphers::vigenere::encrypt(text2, "REALLY");
    std::string decrypted2 = ciphers::vigenere::decrypt(encrypted2, "REALLY");
    assert(text2 == decrypted2);
    std::cout << "Original text : " << text2;
    std::cout << " , Encrypted text (with key = REALLY) : " << encrypted2;
    std::cout << " , Decrypted text : "<< decrypted2 << std::endl;
}

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