Virtual Machine 0 picoCTF 2023 Solution

Description

1. Step 1Open the model in Blender and count the gear teeth
   Import the .dae file into Blender. The model contains two gears: a large red gear and a small blue gear. Delete non-essential parts to expose the gears, then count the teeth on each. The large gear has 40 teeth and the small gear has 8 teeth, giving a gear ratio of 5:1.
   bash

   # In Blender:

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   bash

   # File -> Import -> Collada (.dae) -> select the .dae file

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   bash

   # Use X to delete non-gear objects

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   bash

   # Count the cogs/teeth on each gear manually

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   COLLADA (.dae)is an XML-based 3D model interchange format. Blender can import it directly via File -> Import -> Collada. The model in this challenge is a Lego-style gear assembly representing a simple machine.

   Gear ratios work as follows: if the large gear has 40 teeth and the small gear has 8 teeth, one full rotation of the large gear causes 5 full rotations of the small gear (40 / 8 = 5). So when the red axle (attached to the large gear) rotates by some amount, the blue axle (attached to the small gear) rotates 5 times as much.

   In this challenge the relationship is inverted: the input is the red axle (large gear) rotation and we must find the blue axle rotation. Since the small gear spins 5x faster, multiply the red rotation by 5 to get the blue rotation, or equivalently divide the very large input number to reverse-engineer what the output axle value corresponds to.

2. Step 2Apply the gear ratio and convert to the flag
   Take the large input number from the challenge description. Divide it by the gear ratio (5) to get the blue axle output value. Convert the result from decimal to hexadecimal, then interpret the hex as ASCII to reveal the flag.
   python

   python3 - <<'PY'
   # Replace INPUT with the large number given in the challenge
   INPUT = 0  # paste the number here
   
   ratio = 5  # 40 teeth / 8 teeth = 5
   output = INPUT // ratio
   
   # Convert to hex and then to ASCII
   hex_str = hex(output)[2:]  # strip '0x'
   flag = bytes.fromhex(hex_str).decode('ascii', errors='replace')
   print(f"Output: {output}")
   print(f"Hex: {hex_str}")
   print(f"Flag: {flag}")
   PY

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   The gear ratio translates the very large input rotation number into a smaller output number. That output number, when viewed in hexadecimal and decoded as ASCII, reveals a picoCTF flag string like picoCTF{g34rs_0f_m0r3}.

   This challenge teaches that CTF problems can come in non-traditional formats. Instead of a binary or web app, the "virtual machine" is a physical gear system modeled in 3D. The mathematical relationship (gear ratio) is the "instruction set" of this machine, and computing the output given the input is the "execution."

Flag

The gear ratio is 5 (40-tooth gear drives an 8-tooth gear). Divide the input by 5, convert to hex, decode as ASCII.