Ohdave JavaScript crypto library:
Example:
<script type="text/javascript" src="js/rsa.js"></script>
<script type="text/javascript" src="js/Barret.js"></script>
<script type="text/javascript" src="js/BigInt.js"></script>
<script type="text/javascript">
// key
var key = createKey(1024);
/*** encrypt */
var ciphertext = encryptedString(key, "plaintext");
/*** decrypt */
var plaintext = decryptedString(key, ciphertext);
</script>