Ask HN: Saving the results of client-side JavaScript processing

Let's say we have server D (for Data). Data is encrypted. We jave server K (for Key). The client (your browser) requests D and K to do their decryption magic and would like to store three results (which may be large) on the hard drives. Implicit in this is that I don't really trust Server D. I can do the decryption using nifty libraries, but I don't know how to persist the content to the hard drive. Anybody have have any good ideas"?