Level: Medium
Tags: picoCTF 2023, Reverse Engineering, Analog
Author: LT 'SYREAL' JONES
Description:
Can you crack this black box?
We grabbed this design doc from enemy servers: Download.
We know that the rotation of the red axle is input and the rotation of the blue axle is output.
The following input gives the flag as output: Download.
Hints:
1. Rotating the axle that number of times is obviously not feasible.
Can you model the mathematical relationship between red and blue?
Challenge link: https://play.picoctf.org/practice/challenge/385
Unzipping the given Zip-file gives you a .dae file. A format I had previously never heard about. So I googled for a program to open it with and found Blender which is free and open source.
Start Blender.
Then in the File menu, choose Import and Collada (.dae). Select the Virtual-Machine-0.dae file.
Zoom in and you should see the "black box" machine
Now you need to dismantle the black box by selecting components and moving them away. The navigation is a bit wierd and this takes time.
You need to isolated the red and blue gearwheels good enough to be able to count their number of cogs.
The blue gear has 8 cogs and the red gear has 40 cogs.
The difference in the number of cogs is 5 (40 / 8 = 5).
Then calculate an assumed hex-encoded flag in Python
>>> input = 39722847074734820757600524178581224432297292490103996089444214757432940313
>>> difference = 5
>>> hex_flag = hex(input * difference)[2:]
>>> bytes.fromhex(hex_flag).decode()
'picoCTF{<REDACTED>}'
>>> For additional information, please see the references below.