Hello,

In this article I will describe how I solved the GB – Basic GameBoy crackme challenge from Root-Me.

Before reading this article you should attempt to solve the challenge on your own. Start by reading/skimming through the GameBoy CPU manual then download an emulator such as mGba and play with the ROM. To disassemble the ROM I’ve used Ghidra and mgbdis.

After reading through the GameBoy CPU manual, I’ve opened the file in Ghidra and found these interesting strings:

If you emulate the ROM using mGba and press the Left key then the Enter key, the message “Left” will appear on the screen, for the moment I can assume that the strings are related to the message.

Ghidra doesn’t know about the GameBoy ROM structure and I don’t know either, so I’ve tried to disassemble the ROM with the help of a specialized GameBoy disassembler:

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➜  mgbdis git:(master) ✗ ./mgbdis.py crack_rom.gb 
Loading "crack_rom.gb"...
ROM MD5 hash: 51ff0d38b93107c38a753d9b0ee1576c
Generating labels...
Generating disassembly..
Disassembly generated in "/home/denis/Downloads/mgbdis/disassembly"

By opening the output of the disassembler and searching for 0459 (the address of the Nope string) I’ve found code that references it at 0415.

I went back to Ghidra and disassembled the block at 0415 by pressing the D key.

This seems to be the block that checks some values from RAM and prints the flag if the values are correct.

The first comparison is done for the Left key. It loads the value from the RAM address 0xc0b0 into the A register.

The A register is then compared to 0x32 and there’s a conditional jump to the NopeBlock. JP NZ,NN ;jumps if Z is reset

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                             LAB_ram_03c0                   
        ram:03c0 11 b0 c0        LD         DE,0xc0b0
        ram:03c3 1a              LD         A,(DE=>DAT_ram_c0b0)
        ram:03c4 4f              LD         C,A
        ram:03c5 fe 32           CP         0x32
        ram:03c7 c2 15 04        JP         NZ,NopeBlock
        ram:03ca 18 03           JR         LAB_ram_03cf
        ram:03cc c3 15 04        JP         NopeBlock

CP is a subtraction from A that doesn't update A, only the flags it would have set/reset if it really was subtracted. If A == N, then Z flag is set.

So if A = 0x32 then Z is set and the jump to NopeBlock only happens if Z is reset. Sounds easy to me. To pass the check the value at 0xc0b0 needs to be 0x32.

If we look at the memory view in the mGba emulator, the location at 0xc0b0 has the value 0x39. If we press the Left key the value is decremented.

To get the flag you must solve for the Up, Down and Right keys.

Thanks for reading! <3