function vigenere(input, key, forward) {
	if (key == null)
		key = "";
	var alphabet =   "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
	               + "abcdefghijklmnopqrstuvwxyz";

	// Validate key:
	key = key.toUpperCase();
	var key_len = key.length;
	var i;
	var adjusted_key = "";
	for (i = 0; i < key_len; i ++) {
		var key_char = alphabet.indexOf(key.charAt(i));
		if (key_char < 0)
			continue;
		adjusted_key += alphabet.charAt(key_char);
	}
	key = adjusted_key;
	key_len = key.length;
	if (key_len == 0) {
		alert ('You forgot to supply a key!');
		key = "a";
		key_len = 1;
	}

	// Transform input:
	var input_len = input.length;
	var output = "";
	var key_index = 0;
	var in_tag = false;
	for (i = 0; i < input_len; i ++) {
		var input_char = input.charAt(i);
		if (input_char == "<")
			in_tag = true;
		else if (input_char == ">")
			in_tag = false;
		if (in_tag) {
			output += input_char;
			continue;
		}
		var input_char_value = alphabet.indexOf(input_char);
		if (input_char_value < 0)
		{
			output += input_char;
			continue;
		}
		var lowercase = input_char_value >= 26 ? true : false;
		if (forward)
			input_char_value += alphabet.indexOf(key.charAt(key_index));
		else
			input_char_value -= alphabet.indexOf(key.charAt(key_index));
		input_char_value += 26;
		if (lowercase)
			input_char_value = input_char_value % 26 + 26;
		else
			input_char_value %= 26;
		output += alphabet.charAt(input_char_value);
		key_index = (key_index + 1) % key_len;

	}
	
	return output;
}