A JS & jQuery implementation for the Belaso-Vigenère Cipher.
Belaso-Vigenère is a simple shift cipher and can be considered a natural evolution of the Caesar cipher. Instead of using a single-valued key, this cipher uses a whole sequence of characters. As such, the key can be defined as a word in the alphabet. More information can be found here.
The JS implementation has ~160 lines. However the core algorithm is only a few lines long:
Note. This is a functional example but the exact implementation is subject to change.
const encrypt = (plaintext, key, alphabetSize, letterToCode, codeToLetter) => {
const cipher = [];
let blockLength = key.length;
for (let i = 0, k = 0; i < plaintext.length; i++, k = (k + 1) % blockLength) {
const shiftedCode = (letterToCode[plaintext[i]] + letterToCode[key[k]]) % alphabetSize;
cipher.push(codeToLetter[shiftedCode]);
}
return cipher.join('');
};
We use two dictionaries: one that maps from letter to numeric (code) values
and a reverse dictionary: letterToCode, and codeToLetter respectively.
The mapping between letters and codes is pre-computed at the start, given the alphabet.
Each block of plaintext is then encrypted using the key. We denote with blockLength the key length.
For each individual letter within the block, we take the code and shift it.
The shift is given by the current position in the key (key[k]).
The key is simply rotated using k = (k + 1) % blockLength.
The resulted code is piped through a modulo (over alphabetSize) to keep results in the same space.
After the final code is obtained - we re-map the code to a letter.
Internally, all the operations above occur on lists. Calling .join(''); concatenates the elements and the
final result is a string - either the plaintext (in case of decrypt) or the ciphertext.
The raw input is fetched via jQuery from the HTML form. The raw values are split into lists. The following invariants need to be respected:
- the alphabet is a sequence of unique characters (letters)
- the key, plaintext and ciphertext only contain characters from the alphabet
Depending on the version of ECMAScript running in the browser, one can get wrong results when computing the modulo of a
negative integer. In order to correct the negative modulo problem, I implemented a small modulo function.