I one time saw a story on 60 Minutes many years ago that was either about the CIA or the NSA. It doesn't really matter. During the interview, the reporter asked what the oldest thing still classified was. It's from World War I, and refers to sources and methods. When pressed that everyone involved is dead, and the former enemies are now allies, the answer came back that it was sources and methods.

Apparently, it's recipe for invisible ink.[0]

[0] http://www.fas.org/sgp/news/jmpoldest.html