CIA reveals invisible ink methods used by spies

World War I spies engraved messages on toe-nails and used lemon juice to write invisible letters, classified documents released by the CIA reveal. The six documents, amongst the oldest secret papers to be held by the agency, disclose a number of spying techniques.
The nearly century-old records include instructions “to suspect and examine every possible thing”.

Recent advancements in technology have made it possible to release the documents, the CIA said. One document suggests

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