Rating:
> Molly was able to take pictures of a strange digital circuit sketch, along with an also strange message. All of these things were inside an envelope in a safe, which was labeled "Top Secret".
>
> We believe it might contain Butcher Corp's plans for the future, can you help us read the message?
>
> [Strange Circuit](https://0x99.net/wp-content/uploads/2017/10/StrangeCircuit.jpg)
>
> [Message](https://0x99.net/wp-content/uploads/2017/10/Message.txt)
To start off we are given a circuit diagram and an input file. Taking a look at the circuit, I recognized the JK Flip-Flop zero to seven counter in the upper right, with the current value being fed into a MUX. Following this, we see that the MUX is sending a positive value to one of eight transistors which in turn provide a ground to one row of diodes. Looking over the circuit, we can note that there are 9 (0-8) inputs, this matches the *Message.txt* file we got. Input 0 is used to clock the JK Flip-Flop circuit, while inputs 1-8 are fed to the grid of diodes.
At this point, I was a little stuck. I didn't understand how to get anything useful from this. So I showed the diagram to an Electrical Engineering friend of mine. He suggested that the diode grid was actually a *Light Emitting* Diode grid. Immediately I realized that had to be it, it's an LED matrix! So some quick python code to read in the file and...
Well that doesn't look right... I think I see "CTF" in there, but... Well no, something went wrong. Back to the circuit! Then I realized, inputs 2,3,7, and 8 are inverted! They go through two transistors! Let's invert those inputs in the code!
There it is! We got a flag!
```python
from PIL import Image
#Black and white pixels
OFF = (0,0,0)
ON = (255,255,255)
#Read in the file and split it into "letters"
with open("Message.txt", 'r') as f:
blocks = f.read().split("\n\n")
#Create an empty image with the right dimensions
img = Image.new("RGB", (len(blocks)*8,8))
for n,block in enumerate(blocks):
#Set the x offset depending on the character
dx = n*8
#Split the block into lines only using positive clocked lines
#We don't care about the zero lines
chunked = []
for line in block.split("\n"):
if line[0] == "1":
chunked.append(line.split(" ")[1:])
print(chunked)
#Draw the pixels
for y,chunk in enumerate(chunked):
for x,val in enumerate(chunk):
#Should this one be inverted?
if x in [1,2,6,7]:
if val == "0":
pix = OFF
else:
pix = ON
else:
if val == "1":
pix = OFF
else:
pix = ON
img.putpixel((x+dx,y), pix)
img.save("solved.png")
```
