| Category | Difficulty | Solves | Author |
|---|---|---|---|
forensics |
medium |
10 | Artemis |
Description
We found some microcontrollers the hackers were using to send each other secret messages.
I wonder what this one says?The lights seem to alternate in colour.
Note: there are two versions of the exact same recording.
One is in grayscale (-gray), the other in full RGB. You only need one to solve the challenge.
Players are given two files to download: blinkenlights-0.mp4 and blinkenlights-0-gray.mp4.
Analysis
The provided video shows a circuit board with a couple of active LEDs: one green and one red.
The LEDs blink at a consistent frequency, with one of the LEDs blinking during each clock interval.
Together, the LEDs encode a simple bitstream containing the flag, where:
- red translates to a “0”
- green translates to a “1”
Solution
My full solve script can be viewed here.
It requires Python 3.10+, Pillow, and PyAV: pip install pillow av.
I’ve tuned the thresholds to ensure it works properly on both videos.
Here’s the high-level outline of my script:
- Iterate through each video frame and extract the values of a pixel at each LED.
- Translate the varying “analog” color intensity values to boolean state transitions (with deadzones for debouncing).
- Translate these state transitions to a bitstream.
- Red going high emits a 0, green going high emits a 1.
- Display the recovered bitstream as a string.
Conclusion
This challenge is completely solvable by hand.
However, the next challenges in this series are significantly longer and more complex, so writing a script for this challenge is a good idea.
I’m quite happy with the ergonomics of the “chained generators” pattern I used. It made debugging pretty simple, helps organize the code, and makes it easier to reuse for the next two challenges.
Also, PyAV runs circles around OpenCV, from what I’ve seen :)