Description
Find the flag in the audio file. This is an SSTV transmission.
Setup
Download the audio file.
wget <url>/message.wavSolution
Want to try it yourself first?
The guided walkthrough reveals hints one step at a time.
Step 1Identify the SSTV signal
ObservationThe description calls the file an SSTV transmission, and the name 'm00nwalk' nods to Apollo. The first move is to play the WAV and confirm the characteristic SSTV chirp.Play the audio file. You will hear the characteristic chirp and tone sequence of an SSTV (Slow Scan Television) transmission. The audio encodes an image by mapping pixel brightness values to audio frequencies over time.Learn more
SSTV (Slow Scan Television) is a method used by amateur radio operators to transmit images over audio channels. Different SSTV modes (Martin 1, Scottie 1, Robot 36, etc.) use different image sizes and encoding timings. The mode is announced at the start of the transmission with a VIS (Vertical Interval Signaling) code.
Step 2Decode with QSSTV or RX-SSTV
ObservationThe audio carries the recognizable SSTV tone sequence. A dedicated decoder like QSSTV or RX-SSTV turns those frequencies back into a picture.Install QSSTV (Linux) or RX-SSTV (Windows). On Linux, configure QSSTV to receive from a virtual audio loopback device, then play the WAV file to the loopback. On Windows, set RX-SSTV to listen to the sound card and play the WAV file.bashsudo apt install qsstvbash# Configure: set audio input to virtual loopbackbash# Then play: aplay message.wavWhat didn't work first
Tried: Run multimon-ng directly on the WAV file without converting it to raw PCM first.
multimon-ng's -t raw expects headerless signed 16-bit PCM on stdin, not a WAV container. Pipe the WAV in directly and you get garbage or an immediate error. Run it through sox first to strip the header and emit raw samples at the expected rate.
Tried: Open QSSTV and start receiving with the default audio input set to the physical microphone, then play the WAV file through the speakers.
A microphone picks up room noise and speaker distortion instead of the clean signal, so QSSTV shows a blank or corrupted image. Route the playback through a virtual audio loopback (PulseAudio's null-sink, or VB-Cable on Windows) so QSSTV gets a lossless digital copy.
Learn more
A simpler approach: use the command-line tool
qsstvor the Python librarypysstv. Another popular approach is to open the WAV in Audacity and visually inspect the spectrogram - SSTV appears as distinctive horizontal lines in the frequency domain.You can also pipe the audio directly:
sox message.wav -t raw -r 44100 -e signed -L -b 16 - | multimon-ng -t raw -a SSTV -using the multimon-ng tool.Step 3Read the flag from the decoded image
ObservationQSSTV renders the signal as a complete image, so the flag should simply be readable in that picture. No further processing needed.QSSTV will render the audio as an image. The flag text is visible in the image that appears.Learn more
SSTV was originally developed for transmitting images from the Moon during the Apollo missions - hence the challenge name. The Apollo 7 mission used SSTV to broadcast the first live TV from an American spacecraft in 1968.
Interactive tools
- StegallDrop any file and Stegall runs every applicable steg technique in parallel: LSB sweeps, bit planes, spectrograms, polyglot carving, metadata, whitespace decode, and a 6-layer base/ROT/XOR/zlib cascade. Recursively unpacks results and surfaces flag matches.
- Strings ExtractorPull printable text from any binary, library, or image. ASCII and UTF-16 detection, configurable minimum length, flag-like highlight, no command line needed.
Flag
Reveal flag
picoCTF{beep_boop_im_in_space}
Decode the SSTV audio transmission using QSSTV or multimon-ng to reveal the image containing the flag.