Indomit's solution to my KeyGenMe #5

CrackMe Location :: KeyGenMe #5 @ Crackmes.de.
Solution Location :: Solution by indomit.

So, the main goal of crackme is win the game. As we knew in the Tic-tac-toe game (3x3 grid) the best strategy of both parties leads to a draw. So, to beat the computer there must be a weakness in algorithm. And there it is! If we put "X" in center, the computer always bring to a draw; If we put "X" in any side but not corner, the computer always win; But if we put "X" in corner, there is one weakness that helps us win:

After we put "X" in corner, computer always put "O" in center:
X - -
- O -
- - -

Then, if we want win, the best way is put "X" in an opposite corner:
X - -
- O -
- - X

After this, the best computer move is put "O" at any side (not corner!), but in crackme it moves into corner:
X - O
- O -
- - X

At this point we have already won ;). Just put "X" in empty corner and there is fork :)
X - O
- O -
X - X

Now, computer can't do anything to win or draw :)
This was the main logic. Read the complete solution (in my box or from crackmes.de) for further details.