CIA opens spy chest to reveal WWI-era secret writing techniques

The CIA has declassified the six oldest documents in its secret vault revealing a host of invisible ink recipes and one way to open sealed envelopes without being caught.

Once reserved for covert operatives and other secret-keepers the documents in some cases are a chemists dream and the stuff of spy novels. One document outlines the chemicals and techniques necessary for developing certain types of secret writing ink and one memorandum dated June 14, 1918 -- written in French -- reveals the formula used for German secret ink and how to use it.

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In another document, the user would need to assemble hydrogen sulfide, potassium ferrocyanide and iron sulfate to disclose a number of different chemicals that might be used in various secret writings. For example, hydrogen sulfide combined with a little ammonia would reveal any writing that used an iron, tin or copper base.

There are simpler techniques described as well: One involves dusting the letter with powdered charcoal to reveal paraffin-based writings.

In an interview about the papers on National Public Radio, the CIA has been managing the declassification of secret documents since the agency came to be in 1947. As to why it took so long for these documents from the First World War to be released, the CIA says that only recently has this particular aspect of spycraft become obsolete. A CIA spokesman said, "In recent years, the chemistry of making secret ink and the lighting used to detect it have greatly improved." Sheryl Shenberger of the National Archives says the tendency when dealing with national security secrets is to err on the side of caution.

An Associated Press story on the release said the CIA was not always so eager to share these particular secrets, according to Steve Aftergood of the Federation for American Scientists. He says the CIA resisted a Freedom of Information Act request in 2002 to release the records. "Invisible ink was rendered obsolete by digital encryption long ago, not in the last few years," Aftergood said.

For a look at all six documents click here and scroll down.

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