The Central Intelligence Agency has “declassified the United States Government’s six oldest classified documents, dating from 1917 and 1918,” it says.
According to the CIA, “one document outlines the chemicals and techniques necessary for developing certain types of secret writing ink and a method for opening sealed letters without detection. Another memorandum dated June 14, 1918 — written in French — reveals the formula used for German secret ink.”
The secrets can now be revealed, CIA Director Leon Panetta says in a statement, because “recent advancements in technology” apparently make the methods obsolete. (via NPR)