A canary trap is a method for exposing an information leak, that involves giving different versions of sensitive information to each of a group of suspects and seeing which version gets leaked.

The term was coined by Tom Clancy in his novel Patriot Games. The hero, Jack Ryan, describes the technique he devised for identifying the sources of leaked classified documents. By making sure that each copy of the document differs slightly in its wording, if any copy is leaked then it is possible to determine who 'sang':

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—well, lurid, I guess—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. They've got an even more refined version of the trap working now. You can do it by computer. You use a thesaurus program to shuffle through synonyms, and you can make every copy of the document totally unique.

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:

• Watermarks are used to show that items are authentic and not forged.
• Steganography is used to hide a secret message in an apparently innocuous message, in order to escape detection
• A canary trap hides information in a document that uniquely identifies it, so that copies of it can be traced.

External link

• Fingerprinting gives a good overview of different kinds of Canary trap techniques.