Encryption key: <input id="key" size=80 value=test /><br/>
Decrypted: <textarea id="input" cols=80 rows=5 ></textarea><br/>
Encrypted: <textarea id="output" cols=80 rows=5 ></textarea><br/>
<input type=button value=Go />
<script>
	function $(id) {
		return document.getElementById(id);
	}
	
	$("input").onchange = function() {
		$("output").value = encrypt($("input").value, $("key").value);
	};
	
	$("output").onchange = function() {
		$("input").value = decrypt($("output").value, $("key").value);
	};
	
	function encrypt(str, key) {
		var encrypted = "";
		
		for (var i = 0; i < str.length; i++) {
			var tmp = (str[i].charCodeAt(0) ^ key.charCodeAt(i % key.length)).toString(16);
			if (tmp.length == 1)
				encrypted += "0" + tmp;
			else
				encrypted += tmp;
		}
		
		return encrypted;
	}
	
	function decrypt(str, key) {
		var decrypted = "";
		
		for (var i = 0; i < str.length; i += 2) {
			var b8 = parseInt(str[i] + str[i + 1], 16);
			decrypted += String.fromCharCode(key.charCodeAt((i / 2) % key.length) ^ b8);
		}
		
		return decrypted;
	}
</script>