Algorithms_in_C++  1.0.0
Set of algorithms implemented in C++.
vigenere_cipher.cpp File Reference

Implementation of Vigenère cipher algorithm. More...

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

 ciphers
 Algorithms for encryption and decryption.
 
 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
texttext to be decrypted
keykey to be used for decryption
Returns
new decrypted text
92  {
93  // Going through each character of text and key
94  // Note that key is visited in circular way hence j = (j + 1) % |key|
95  std::string decrypted_text = ""; // Empty string to store decrypted text
96  for(size_t i = 0, j = 0; i < text.length(); i++, j = (j + 1) % key.length()) {
97  int place_value_text = get_value(text[i]); // Getting value of character in text
98  int place_value_key = get_value(key[j]); // Getting value of character in key
99  place_value_text = (place_value_text - place_value_key + 26) % 26; // Applying decryption
100  char decrypted_char = get_char(place_value_text); // Getting new character from decrypted value
101  decrypted_text += decrypted_char; // Appending decrypted character
102  }
103  return decrypted_text; // Returning decrypted text
104  }
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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
texttext to be encrypted
keyto be used for encryption
Returns
new encrypted text
73  {
74  std::string encrypted_text = ""; // Empty string to store encrypted text
75  // Going through each character of text and key
76  // Note that key is visited in circular way hence j = (j + 1) % |key|
77  for(size_t i = 0, j = 0; i < text.length(); i++, j = (j + 1) % key.length()) {
78  int place_value_text = get_value(text[i]); // Getting value of character in text
79  int place_value_key = get_value(key[j]); // Getting value of character in key
80  place_value_text = (place_value_text + place_value_key) % 26; // Applying encryption
81  char encrypted_char = get_char(place_value_text); // Getting new character from encrypted value
82  encrypted_text += encrypted_char; // Appending encrypted character
83  }
84  return encrypted_text; // Returning encrypted text
85  }
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◆ main()

int main ( void  )

Driver Code

131  {
132  // Testing
133  test();
134  return 0;
135 }
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◆ test()

void test ( )

Function to test above algorithm

111  {
112  // Test 1
113  std::string text1 = "NIKOLATESLA";
114  std::string encrypted1 = ciphers::vigenere::encrypt(text1, "TESLA");
115  std::string decrypted1 = ciphers::vigenere::decrypt(encrypted1, "TESLA");
116  assert(text1 == decrypted1);
117  std::cout << "Original text : " << text1;
118  std::cout << " , Encrypted text (with key = TESLA) : " << encrypted1;
119  std::cout << " , Decrypted text : "<< decrypted1 << std::endl;
120  // Test 2
121  std::string text2 = "GOOGLEIT";
122  std::string encrypted2 = ciphers::vigenere::encrypt(text2, "REALLY");
123  std::string decrypted2 = ciphers::vigenere::decrypt(encrypted2, "REALLY");
124  assert(text2 == decrypted2);
125  std::cout << "Original text : " << text2;
126  std::cout << " , Encrypted text (with key = REALLY) : " << encrypted2;
127  std::cout << " , Decrypted text : "<< decrypted2 << std::endl;
128 }
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std::string
STL class.
std::string::length
T length(T... args)
ciphers::vigenere::encrypt
std::string encrypt(const std::string &text, const std::string &key)
Definition: vigenere_cipher.cpp:73
ciphers::vigenere::decrypt
std::string decrypt(const std::string &text, const std::string &key)
Definition: vigenere_cipher.cpp:92
std::cout
test
void test()
Definition: vigenere_cipher.cpp:111
std::endl
T endl(T... args)