Revmeplx

REV | 100 pts - 201 solves

Description: What could possibly be hidden inside a diving logbook? author: Skipper|RedRocket

Running the elf, it asks us for the name of a diver:

$ ./rev 
| >>> REEF RANGERS Dive Panel <<< |
| ------------------------------- |
|    Please provide Diver Name:   |

Not knowing them we can launch strings on the executable and see what we can find:

$ strings ./rev 
[...]
CSR{
_submarines_
_solved_n1c3!}
Jeremy
Simon
Adminiman
Your dive count is: 81
Welcome instructor!
Your dive count is: 410
Your dive count is: 0
To show today's drydock report, please enter passcode:
No diving recore of diver 
 found!
| >>> REEF RANGERS Dive Panel <<< |
| ------------------------------- |
|    Please provide Diver Name:   |
[...]

The flag appears to be there, but it won't work if we try to submit it. however, we also find names of the divers. Opening the executable in Ghidra we immediately notice that it's a cpp executable. The main() function, looks a bit confusing but we notice that there's a call to an interesting door_lock() function following an if statement. After a couple of attempts we discover that the diver triggering that function is Jeremy. Looking more closely at the door_lock() function, we know that it takes param_1 as input and does the following check:

if (param_1 * 2 >> 8 == 0x539) {

Right-shifting a number of n bits also means that it's being divided by 2 to the power of n.

In [1]: (0x539 * (2**8)) / 2
Out[1]: 171136.0

Or

In [2]: (0x539 << 8) / 2
Out[2]: 171136.0

Will get us the magic number.

$ ./rev   
| >>> REEF RANGERS Dive Panel <<< |
| ------------------------------- |
|    Please provide Diver Name:   |
Jeremy
Your dive count is: 0
To show today's drydock report, please enter passcode:
171136.0
CSR{11_submarines_45864441_solved_n1c3!

CSR{11_submarines_45864441_solved_n1c3!