Your goal is to break a four-digit colour code in as few moves as possible. As soon as you find out the correct strategy, you should be able to usually win each game after about eight or nine guesses.
Download the game and the source from my Sourceforge site:
A separate ZX81 Chroma version is also included (colourized, a native version within a single .p file should follow one day).
I hope the ZX80 screen rolling after each keyboard input is not too annoying, at least in the EightyOne emulator the game is still quite playable.