A formula for invisible ink used by spies during the First World War and espionage techniques involving messages etched on toenails are among classified documents released by the CIA.
Six of the oldest files held by the agency, which predate it by several decades, were made public this week.
One paper from 1918 lists the chemicals and techniques used to create "secret writing". Another from 1914 and written in French gives the formula the Germans were believed to have used for their invisible ink.
"When historical information is no longer sensitive, we take seriously our responsibility to share it with the American people," said Leon Panetta, the CIA director.