Neat cryptography thing. When I first saw it, I was thinking it was closer to the Mastermind style of game, but it is different enough to require other skills. The menu fits well, and the central problem is tough enough to require heavy thinking.
The opponent's ability to make multiple entries every button press would make this game pretty unfair is the opponent was a human, or more understood the nature of the problem. Luckily, it appears somewhat easy to just mash some of the missing character in, finish the code, delete the missing character, and immediately enter.
Rather than have two separate lose conditions, maybe have the opponent's guesses be entered every x seconds, so that if you take too long, it'll just brute force the password, which is probably more fitting with the scenario.
High Score: 3322