So essentially we're relying on a one-way hash function, which is what I am thinking too. (In Quantum Information Theory, the question whether or not there exis

So essentially we're relying on a one-way hash function, which is what I am thinking too. (In Quantum Information Theory, the question whether or not there exists true one-way hash functions is still unanswered, but for the time being, current cryptographic functions are obviously working well enough). I feel like this wouldn't work with humans though. For instance, if I was torturing you and knew that your brother was the other agent, I might think to ask you for any inside jokes between you and your brother. There's a chance you wouldn't think of the right one (i.e. segfault analogy), but also a chance that you might, so you haven't really "forgotten" the answer. On top of that, I feel like something straight out of Harry Potter probably isn't the answer :) I like the idea (in the comments on OP's blog) of using a very unique smell. You can explain what's happening in a picture or hum a melody, but maybe a smell is more difficult to describe.