picoGym | m00nwalk Write-up

Challenge description

Decode this message from the moon.

Category: Forensics


This challenge provided us with a .wav file, commonly used to store audio. Playing the file with any media player use prefer and you’ll be greeted with some high-pitched beeping noises, and they were loud. (highly not recommended)

So, the flag is surely hidden within the file somehow. Let’s check the hints first. The first hint asked:

“How did pictures from the moon landing get sent back to Earth?”

Interesting, let’s take a look online. Within the top search results for the above question, I found a stack exchange post that discussed the matter in impressive detail.

This section, in particular, was intriguing

Looking up this “oddball format” yielded results for the slow-scan television (SSTV) format. Armed with this knowledge, I next searched for “.wav to SSTV converters”. After some digging around, another article seems to hold the key I’ve been looking for: QSSTV, a linux application.

For one reason or another, this app did NOT like my EndeavorOS system, refusing to properly install in my first few attempts. After finally getting it to play nice, I followed the instructions in the above article without much trouble and began to try and decode the file.

Coming back to the challenge’s hints, the second mentions Carnegie Mellon University (the people behind picoCTF)’s mascot. A quick search revealed the mascot’s name to be Scotty the Scottish Terrier. Looking around QSSTV’s controls, it looks like “Scottie” is an option inside the mode category.

With everything now (seemingly) set up, I began to decode the file. And here is where I blundered this challenge. Changing nothing from the default settings and using the “Scottie” mode, I only managed to partially decode the image, which didn’t give me a lot to work with.

After several long and frustrating attempts at messing with the settings, I decided to call it quits and looked up others’ write-ups for this challenge. And it was a huge face-palm moment where I realized I was doing just about everything right, except for ticking the “Auto Slant” option (like I did in the image above). Trying to decode the .wav file using this option easily netted me the flag image.

Just need to flip it upside-down


It was rather embarassing to miss the goal after getting so, so close. Perhaps this can serve as a lesson for me in future challenges, not to give up until I have exhausted every options available. Or at the very least, all of the ones staring me in the face 😢

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: