A canary trap is a method for exposing an information leak, which involves giving different versions of a sensitive document to each of several suspects and seeing which version gets leaked.
The term was coined by Tom Clancy in his novel Patriot Games, though Clancy did not invent the technique. The actual method (usually referred to as a Barium meal test in espionage circles) has been used by intelligence agencies for many years. The fictional character Jack Ryan describes the technique he devised for identifying the sources of leaked classified documents:
Each summary paragraph has six different versions, and the mixture of those paragraphs is unique to each numbered copy of the paper. There are over a thousand possible permutations, but only ninety-six numbered copies of the actual document. The reason the summary paragraphs are so lurid is to entice a reporter to quote them verbatim in the public media. If he quotes something from two or three of those paragraphs, we know which copy he saw and, therefore, who leaked it.
A refinement of this technique uses a thesaurus program to shuffle through synonyms, thus making every copy of the document unique.
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According to the book Spycatcher by Peter Wright (published in 1987) the technique is standard practice which has been used by MI5 (and other intelligence agencies) for many years, under the name "Barium meal test". A Barium meal test is more sophisticated than a canary trap because it is flexible and may take many different forms. However, the basic premise is to reveal a secret to a suspected enemy (but nobody else) then monitor whether there is evidence of the fake information being utilised by the other side. For example, the double agent could be offered some tempting "bait" e.g. be told that important information was stored at a dead drop site. The fake dead drop site could then be periodically checked for signs of disturbance. If the site showed signs of being disturbed (in order to copy the microfilm stored there) then this would confirm that the suspected enemy really was an enemy e.g. a double agent.
The technique of embedding significant information in a hidden form in a medium has been used in many ways, which are usually classified according to intent:
The canary trap was also used in Clancy's (chronologically) earlier novel, Without Remorse, when a CIA official alters a report given to a senator, revealing an internal leak who was giving information to the KGB.
Barium meals are also administered in Robert Littel's book The Company, and later in the TV short-series with same name.
The technique (not named) was used in the 1970s BBC television serial 1990. The same unnamed technique also appeared in Irving Wallace's book The Word (1972).
A variation of the canary trap was used in Miami Vice, with various rendezvous dates leaked to different groups.
In the third-season finale of The Mentalist, the characters use a canary trap (giving different hotel room numbers to different suspects) to uncover a mole within their agency. A similar ruse was used in the TV series Ashes to Ashes.
In A Clash of Kings; the second book in A Song of Ice and Fire, Tyrion Lannister uses the trap to find out which member of the King's Privy Council is reporting to his sister, the Queen Regent Cersei Lannister. To Grand Maester Pycelle he tells of a plot to marry his niece Princess Myrcella to a prince from House Martel. To Lord Peter Baelysh, he claims he will instead send Myrcella to be raised by Lysa Arryn and married to her son Robert. To Varys, he says his plan is to send his nephew Tommen to the Martels. When Cersei confronts him, and knows only of the plan to send Myrcella to the Martels, Tyrion knows Pycelle to be the leak.
When distributing Broken to friends, Trent Reznor claims that he watermarked the tapes with dropouts at certain points so that he could identify if a leak would surface.
Screener versions of DVDs are often marked in some way so as to allow the tracking of unauthorised releases to their source.