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:
➜ 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
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
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! ❤