Hi Tenbao,
You can certainly encrypt and decrypt without prompting the user for a password, i.e. by using something that is automatically determined. This could be the userName() or some code or web service that calls active directory.
We don't have an example or a document on this, but it should be possible. If you have DBXL, that comes with the AD web service, making this even easier.
I know this is vague information, but hope it helps get you going. Good luck!