Tarek said:
"...and even then, a key piece of information will be missing from the file."
If having the document is important, and the missing information is important to understanding and using the document correctly, then the document is useless to you as well.
If having the document is important and the missing information is not important to understanding and using the document correctly, what good is it to leave that information out?
If the document isn't important... why have it in the first place?
Ultimately, there's no such thing as perfect security, though. I guess my evil overlord data retention rule would be something like the following:
All data which needs not be retained shall not be retained. All data which must be retained will be kept on contemporary media with the most advanced crytography currently practical. Periodically (lets say every year-- I'm an evil overlord with a lavish volcano lair, I've got cash to spare), all of the archives will be translated to whatever the current media of choice is, and as part of this process will be unencrypted and re-encrypted with then-current encryption. Old copies will be disposed of in an appropriate way which physically destroys all data previously on them.
Of course, this opens issues about who has access to the keys, and how we safeguard the data during translation, and what protocols are enacted to ensure that the old copies are adequately disposed of, but that's beyond the scope of this rule, I s'pose.