Category: Hardware
Team Name : Windows 9/11
Author: Nayanesh

CTF Writeup: Open The Flood Gates Hardware Challenge

Challenge Description

Franky used to work at a Dockyard, but never understood how the complex network of gates and channels worked there. So, the Dock Master drew a diagram for him which he could understand. This helped him instantly understand the workings, are you as smart as Franky?

Provided Information/Files

Image of a circuit containing BJT connections with 6 inputs and 1 output (either 1 or 0).
A xlsx file containing input values.

Initial Clues & Analysis

The provided circuit image suggested a logical computation based on BJT connections. Given that the circuit had 6 inputs and 1 binary output, my first step was to analyze the logic governing the output.

Investigation Path

Step 1: Circuit Analysis

Since analyzing the circuit manually would be time-consuming, I used Claude AI to examine the circuit. I provided the circuit image along with the prompt:

"Explain this circuit and all outputs for all possible inputs. Trace the signal path and provide the 64 outputs."

Claude AI provided the following Boolean expression:

(NOT A AND NOT B) OR (NOT C AND NOT D) OR (NOT E AND NOT F)

which can be rewritten as:

(!A • !B) + (!C • !D) + (!E • !F)

Step 2: Constructing a Truth Table

Using this Boolean expression, I generated the truth table for all 64 possible input combinations. The function's logic stated that the output is 1 if at least one of the three conditions holds:

Both A and B are 0
Both C and D are 0
Both E and F are 0

Otherwise, the output is 0.

Step 3: Processing Given Input File

Using the given input file, I applied the derived Boolean function to determine the output for each set of inputs. The results were stored as a binary sequence.

Step 4: Converting Binary to ASCII

The binary sequence had 249 outputs of 0s and 1s. Upon reviewing the pattern, I suspected it was an encoded message. To decode it, I used dcode.fr to convert the binary string into ASCII text. The decoded text revealed the flag.

Finding the Flag

After converting the binary output to ASCII, I successfully extracted the flag:

REDFOX{Unl0ck_7he_L0gic_Puzz1e}

Tools & Resources Used

Claude AI (for circuit analysis)
Python (to apply Boolean logic on the input dataset)
dcode.fr (for binary-to-ASCII conversion)

Lessons Learned / Takeaways

Circuit Analysis: Using AI tools can significantly speed up the process of understanding complex circuit behavior.
Boolean Logic: Breaking down the circuit into a logical expression helped determine the output efficiently.
Binary Encoding: Recognizing binary output as potential ASCII characters was key to extracting the flag.

Rabbit Holes / Dead Ends

Initially, I considered manually tracing the circuit behavior, but it proved time-consuming and complex.
I attempted decoding the binary using different encoding schemes before realizing it was ASCII-encoded.

This challenge reinforced the importance of logical deduction and efficient problem-solving techniques in hardware-based CTF challenges.

Cyberstorm 2025 Writeup: Open The Flood Gates [by Windows 9/11]