Another good method to encrypt ASCII based text. To view this example in work, http://editor.clizware.net/?id=7 (remember to press compile)

//Copyright (C) 2012 Artem Los, All rights reserved

function mod_c(text,key,enc_dec)
{
	//only for 128 ascii values
	if (key == "")
	{
		return "Error: no key given";
	}
	if (enc_dec === true)
	{
		var result = "";

		for (var i=0; i < text.length; i++)
		{
			result += String.fromCharCode( (text.charCodeAt(i) + key.charCodeAt(i % key.length)).mod(128)    );
		}
		return result;
	}
	else if(enc_dec === false)
	{
		var result = "";

		for (var i=0; i < text.length; i++)
		{
			result += String.fromCharCode( (text.charCodeAt(i) - key.charCodeAt(i % key.length)).mod(128)  );
		}
		return result;
	}
	else
	{
		return "Error: enc_dec is not given";
	}
}

Number.prototype.mod = function(n) {
return ((this%n)+n)%n;
}