CIA declassifies oldest documents

The CIA's Freedom of Information Act Electronic Reading Room provides public access to CIA information that has been released through the FOIA and other disclosure statutes. Recently they added the US government's 6 oldest classified documents, which have finally been declassified. The documents date from 1917 and 1918, and were the only documents remaining classified from the World War I era.  These documents all describe methods of "secret writing" that spies might use to sneak information past the enemy.   Document 4, for example, has several methods - tedious and involving chemicals and hot irons - for exposing hidden writing. Document 4 also describes "how to open sealed letters without detection" which involves copper acetyl arsenite, acetone and amyl alcohol (though there is a notation in the margin saying "Tried - not successful").  Document 6 warns to "Suspect fresly-painted metal walls on ships, etc. - Often... they cover communications scratched on the underlying metal surface". It also gives instructions on "how to make microscopic writing on a 2-cent red U.S. postage stamp", and warns that "spies and smugglers" sometimes engrave messages on toe-nails.