A simple, but effective, approach to a stress-inducing timed condition to input codes rapidly before the time runs out.
However, I've come across some bugs that didn't play fair to the experience, like numbers being diled twice with one mouse click, or the game freezing while inputing the code - and besides these problems being what induced the "stress" the most in my experience, I think maybe some other mechanic or the time being shorter could've been more interesting.
But for its size, it's simple and effective.