You know that Secretary of Defense Gates had his e-mail hacked, a couple of years ago? Things have improved somewhat, I gather, but online security will never be perfect, so another way to protect data confidentiality is to keep it on disks never linked to the web.
In addition you may look up somewhat more advanced search terms, in case you are interested: "program obfuscation" and "key-exposure resilient cryptography." Of course these techniques would have to be applied before the data is actually saved on the disk. Btw, I loved the term "program 'obfuscation'", and I conscientiously had it spelled for me, thinking maybe it's some obscure technical term meaning something different from the everyday meaning of the word, but nah! it means exactly the same.