History1917-1918 (declassified 2011)United Stateshigh confidence

One of the declassified documents outlines the chemicals and techniques necessary for developing certain types of secret writing ink and a method for opening sealed letters without detection.

Notes on verification

Multiple independent outlets (Reuters, The Guardian, NBC) corroborate the CIA's 2011 declassification announcement describing secret ink formulas and covert letter-opening techniques from WWI-era documents.

Sources