For those who want to keep their digital correspondence a secret from the feds, or anyone else who may be watching, a document prepared by the U.S. Department of Justice in August and pointed out by Wired's Threat Level today is crucial reading. The document, dated August 2010, has been floating around the web for some time (you can find PDF's of it here and here) and details how long cellular service providers store information on users's correspondence, web behavior, and bill payment. Depending on which of those areas are most important to keep private, the results are a mixed bag, as Wired's David Kravets explains:

Verizon, for example, keeps a list of everyone you’ve exchanged text messages with for the past year, according to the document.  But  T-Mobile stores the same data up to five years. It’s 18 months for Sprint, and seven years for AT&T.

That makes Verizon appear to have the most privacy-friendly policy. Except that Verizon is alone in retaining the actual contents of text messages. It allegedly stores the messages for five days, while T-Mobile, AT&T, and Sprint don’t store them at all.

For those concerned with companies tracking their movements via cell site, however, Verizon does seem to be the best option. "Verizon keeps that data on a one-year rolling basis; T-Mobile for 'a year or more;' Sprint up to two years, and AT&T indefinitely, from July 2008." Which means that time you called in sick to go to Atlantic City two years ago could still be hanging around in AT&T's records, should your boss have the subpoena power to ask for it.

It's much easier to read the document as a PDF, but here's a smaller version: