I guess I would have failed to create complexity theory with my solution --- but everyone involved would have understood it, no long calculations required: give the guard a lock and the spy a key. It's still a sort of one-way function (easier to make a lock from a key then a key for a lock), but it doesn't require squaring 100 digit numbers in the dead of night before deciding whether to shoot a man.
Practical, literal-minded solutions are not the kind of thing that typically inspires theoretical insight into mathematics, though. That's like saying Euler could have just swum across some of the rivers in Koenigsberg.
For what its worth, my solution was to give the guard a set of sealed envelopes. Each envelope having a password written on the outside and a password written on a piece of paper inside. The spy would give the guard the word that is written on one of the envelopes, the guard would then open the envelope and ask the spy for the password contained inside.
