21st century wiretapping storing data

Ed Felten's latest post in his series about 21st century wiretapping is up.

"Suppose the government gathered information about all phone calls, including the calling and called numbers and the duration of the call, and then stored that information in a giant database, in the hope that it might prove useful later in criminal investigations or foreign intelligence. Unlike the recently disclosed NSA call database, which is apparently data-mined, we’ll assume that the data isn’t used immediately but is only stored until it might be needed. Under what circumstances should this be allowed?

We can start by observing that government should not have free rein to store any data it likes, because storing data, even if it is not supposed to be accessed, still imposes some privacy harm on citizens...

It follows that, before storing such data, government should have to make some kind of showing that the expected value of storing the data outweighs the harms, and that there should be some kind of plan for minimizing the harms, for example by storing the data securely (even against rogue insiders) and discarding the data after some predefined time interval.

The most important safeguard would be an enforceable promise by government not to use the data without getting further permission (and showing sufficient cause)...

Part of the required showing, I think, would have to be an argument that there is not some way to store much less data and still get nearly the same benefit. An alternative to storing data on everybody is to store data only about people who are suspected of being bad guys and therefore are more likely to be targets of future investigations."