In eight hours, these words will disappear.

Well, not quite. But that's the premise behind a new system that makes electronic communications like e-mail messages and Facebook posts "self destruct" after a set period of time.

The University of Washington computer scientists who developed the prototype saw it as a way to maintain privacy in a world where more and more private information is stored on the Internet, beyond our control.

But before you start getting all Mission Impossible on any work e-mails, you might want to check your university's policy on record retention. Lawyers with whom the Washington team has consulted said the prototype, called Vanish, is "ahead of the law in many ways," said Roxana Geambasu, a doctoral student who worked on the project.

"The state university, for example, has these requirements to retain all e-mails," she said. "It's not very clear if using Vanish would be legal or not."

The researchers framed their innovation in the context of a digital landscape transitioning to "a future based on cloud computing," with massive data centers running most applications and storing most of our information. In the past, you might simply have deleted a document or photo on your personal computer, Ms. Geambasu points out. Now we entrust gobs of information to distant servers-think Google Docs or Facebook photos.

"It's really become impossible for you to control your personal data," Ms. Geambasu said. "The effect of that is that personal data really sticks around forever."

The Washington team's system for stamping an expiration date on that data is a free, open-source tool that works with text uploaded to Web services through a browser. Senders encrypt the information by highlighting the text and pressing the "vanish" button. The result is electronic communications that become ephemeral, like phone calls, negating the risk of having your messages exposed by lost laptops or hackers or subpoenas.

For those interested in the under-the-hood details, Ms. Geambasu likens the system to writing a message in the sand that is then washed away by the tide. It erodes data by taking advantage of the churn on global peer-to-peer file-sharing systems. Vanish encrypts messages with secret keys for each communication. It then chops up the key and scatters the parts on random computers in the file-sharing networks. As computers join and leave the network, parts of the key become irretrievable, and the original message cannot be read once enough parts are lost.