Something Only They Would Say: A number of intelligence agencies and other organizations use this kind of call-and-response code as a non-technology-dependent shibboleth, but for the sake of extra security, they’ve more or less perfected the technique of embedding them into subconscious levels of the mind-state, so that the person using them doesn’t actually know what they are to give them away, even under duress.
This is a companion discussion topic for the original entry at https://eldraeverse.com/2015/10/22/trope-a-day-something-only-they-would-say