Everyone hates a tattle-tale. Many of us have shared sensitive information with a friend in confidence, only to feel betrayed when that two-faced friend tells someone else behind your back. In personal relationships, this can be frustrating, but when it comes to state secrets, the consequences of information leaks can be absolutely devastating. Rather than dealing with embarrassment, you'll be dealing with lost lives.
As a result of the potential for these dangerous information leaks, espionage agencies have developed a technique known as the canary trap. This technique is designed to expose traitors and double-agents by selectively sharing several distinct versions of documents or other info with various members of a group. Depending on the version of your bait that gets leaked, you'll know who the traitor was.
Here's a simplified example of how it works. Let's say you have 3 friends: Tom, Dick, and Harry. You're planning a party, and don't want any uninvited guests, but you know Tom, Dick, or Harry will leak the details. So, you decide to tell each of your friends a false version of the evening's plan:
The party is actually planned for 5pm. So, if everyone shows up at their respective times without additional guests, you'll know all three friends were honest and didn't leak the details to their friends. If a big crowd of guests show up at 6, you'll know Tom was the leak, and so on.
The canary trap technique has been used extensively to find spies, in both civilian and military applications. Saddam Hussein famously used a canary trap to avoid assassination in 2003, by telling each of his staff members about a different house he'd supposedly be hiding in. When only one of those houses was bombed, he knew who had leaked his plans.
So, if you're in a survival situation, and you don't know who you can trust, remember the canary trap. It can help you distinguish friends from enemies—and, once you've found a leak, you can intentionally mislead that individual to throw off your enemies.