Very informative article. Has particular relevance to the cloud and to privacy. Essentially, you can put a file onto a cloud or service (e. g. Facebook), but you can't necessarily truly delete it securely.

He discusses a project, http://vanish.cs.washington.edu/index.html (Vanish) that does true deletion.

If I'm understanding this right, someone can download and cache the decryption key (while it's available) and then decrypt the cipher text at any time they want in the future, right?

I don't think it's beyond the NSA's ability to just pre-emptively cache every decryption key, just in case they need it later.

Is it just me, or does Vanish sound completely impractical?

Approximately one hour after Vanish catches on, the hottest new firefox plugin is going to be "Vanish Key Preserver" that archives the keys needed to decrypt every Facebook photo you look at.

To stop a deletion, you would have to get everyone who has a part of the file on their computer to preserve it. Since there are a large number of unrelated parties running these computers, and the uploader can choose which to use when uploading, an attacker trying to prevent deletion will have quite a lot of fun convincing all these people to cooperate.

I was skeptical at first too. But it might just work in practice.