Le Chiffre Indéchiffrable.

This is a simple implementation of the Vigenère cipher. The Vigenère cipher is a method of encrypting alphabetic text by using a simple form of polyalphabetic substitution. A polyalphabetic cipher uses multiple substitution alphabets to encrypt the data. The Vigenère cipher has been reinvented many times. The method is named after Blaise de Vigenère, though it was actually first described by Giovan Battista Bellaso in 1553.

The Vigenère cipher is essentially a Caesar cipher with a different shift for each letter. To encrypt, a table of alphabets can be used, termed a tabula recta, Vigenère square, or Vigenère table. It consists of the alphabet written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet, corresponding to the 26 possible Caesar ciphers. At different points in the encryption process, the cipher uses a different alphabet from one of the rows. The alphabet used at each point depends on a repeating keyword.

Examples

The Viégère cipher key is a word, such as "lemon". Each letter of the key is a shift value. In this case, the key is "lemon", which translates to [11, 4, 12, 14, 13]. The key is repeated for the length of the plaintext. The plaintext "attackatdawn" is encrypted as follows:

use vgnr::Vigenere;

let scheme = Vigenere::new("lemon");
let plaintext = "attackatdawn";

let ciphertext = scheme.encrypt(plaintext).unwrap();
assert_eq!(ciphertext, "lxfopvefrnhr");

let decrypted = scheme.decrypt(&ciphertext).unwrap();
assert_eq!(decrypted, plaintext);

The Caesar cipher is a special case of the Vigenère cipher where the key is a single letter. The Caesar cipher is a type of substitution cipher in which each letter in the plaintext is shifted a certain number of places down the alphabet.

use vgnr::Vigenere;

let scheme = Vigenere::new("d");
let plaintext = "attackatdawn";

let ciphertext = scheme.encrypt(plaintext).unwrap();
assert_eq!(ciphertext, "dwwdfndwdqgq");

let decrypted = scheme.decrypt(&ciphertext).unwrap();
assert_eq!(decrypted, plaintext);

The Vigenère cipher is simple to understand and implement. Please, do not use this for anything serious. That's a cipher invented in the 16th century. It's not secure.

Cryptanalysis of the Vigenère cipher is possible. The primary weakness of the Vigenère cipher is the repeating nature of its key. If a cryptanalyst correctly guesses the key's length, the cipher text can be broken by determining the shift value for each letter in the key.

The Kasiski examination, also called the Kasiski test, is a cryptanalytic method that breaks Vigenère ciphers. The Kasiski examination takes advantage of the fact that repeated words are, by chance, sometimes encrypted using the same key letters, leading to repeated groups in the ciphertext. For example, consider the following encryption using the keyword ABCD:

Key: ABCDABCDABCDABCDABCDABCDABCD Plaintext: cryptoisshortforcryptography Ciphertext: CSASTPKVSIQUTGQUCSASTPIUAQJB

The repeated groups in the ciphertext are "CSASTP" and "CSASTP". The distance between the two groups is 12 characters. The distance between the two groups is a multiple of the key length. In this case, the key length is 4.

See more about breaking the Vigenère cipher in the Wiki.

Use this crate as a challenge to learn more about cryptography and try to break the Vigenère cipher. It's a fun exercise.