Collection of functions I wrote while taking the cryptography classes from Khan Academy.

ciphers.js

var alphabet = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'];

function encryption ( word, shift ) {
	var encryptedWord = '';

	for (var i = 0; i < word.length; i++) {
		var letter = word[i].toUpperCase();
		var letterIndex = alphabet.indexOf(letter);
		var shiftedIndex = letterIndex+shift;
		var newLetterIndex = shiftedIndex % alphabet.length;

		// if ( shiftedIndex > alphabet.length ) newLetterIndex--;

		var newLetter = alphabet[newLetterIndex];

		encryptedWord += newLetter;
	}

	return encryptedWord;
}

function decryption ( word, shift ) {
	var decryptedWord = '';

	for (var i = 0; i < word.length; i++) {
		var letter = word[i].toUpperCase();
		var letterIndex = alphabet.indexOf(letter);
		var shiftedIndex = letterIndex-shift;
		var newLetterIndex = shiftedIndex % alphabet.length;

		if ( shiftedIndex < 0 ) newLetterIndex = alphabet.length + newLetterIndex;

		var newLetter = alphabet[newLetterIndex];
		decryptedWord += newLetter;		
	}

	return decryptedWord;
}

function convertKeywordToIndexes( keyword ){
	var indexes = [];

	for (var i = 0; i < keyword.length; i++) {
		var letter = keyword[i].toUpperCase();
		var letterIndex = alphabet.indexOf(letter);
		indexes.push(letterIndex);
	};

	return indexes;
}

function vigenereCipher( keyword, phrase ){
	var encryptedPhrase = '';
	var indexesKeyword = convertKeywordToIndexes( keyword );
	var clonedIndexesKeyword = indexesKeyword.slice();

	for (var i = 0; i < phrase.length; i++) {
		var letter = phrase[i];

		if ( letter === ' ' ) {
			encryptedPhrase += ' ';
			continue;
		}

		var shift = clonedIndexesKeyword.shift();
		var newLetter = encryption( letter, shift );

		encryptedPhrase += newLetter;

		if ( clonedIndexesKeyword.length === 0 ) clonedIndexesKeyword = indexesKeyword.slice();
	}

	return encryptedPhrase;
}