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Aperi'Solve v.3.4.1

Spectrogram

When you upload an audio file (WAV, MP3, FLAC, OGG, M4A...), Aperi'Solve renders two visualizations of the sound: a spectrogram (frequency content over time) and a waveform (amplitude over time). Audio steganography in CTFs is overwhelmingly visual — the flag is drawn into the spectrogram — so this is the first thing to check on any audio challenge.

What Aperi'Solve runs

The analyzer is pure Python (NumPy + Pillow), no external DSP library:

  • PCM WAV is decoded directly with the standard-library wave module (8/16/24/32-bit, mono or stereo, averaged to mono).
  • Every other container, and non-PCM WAV, is transcoded to a temporary PCM WAV with ffmpeg first.
  • The spectrogram is a Hann-windowed Short-Time Fourier Transform (N_FFT = 1024, hop 512), converted to relative decibels with an -80 dB floor and colored with a viridis palette. Low frequencies are at the bottom, time runs left to right.
  • The waveform is a min/max envelope of the signal.

Reading the output

  • Spectrogram — bright shapes against a dark background are energy at that frequency and time. Text, QR codes and simple drawings placed here are read directly by eye.
  • Waveform — the overall loudness shape. Long silences, abrupt bursts or a signal that clips the top and bottom hint at something deliberate.
  • The metadata table reports sample rate, channel count, bit depth, duration and frame count.

Common CTF patterns

  • The flag is written in the spectrogram — the classic. A tone is swept to spell out flag{...}; it is invisible in the waveform and often inaudible. Aperi'Solve draws it for you.
  • A hidden image or QR code in the high frequencies of an otherwise ordinary song.
  • DTMF / Morse — dial tones or short/long bursts visible as discrete frequency bands; decode them from the pattern.
  • Stereo tricks — data hidden in only one channel, or in the difference between channels (Aperi'Solve averages to mono, so also inspect the raw channels in Audacity if nothing shows).

Doing it yourself

Audacity (Analyze → Spectrogram view) and Sonic Visualiser give an interactive, zoomable spectrogram with adjustable window size and color scale — useful when the payload sits in a narrow band the fixed rendering flattens.

Limitations

  • The rendering uses a fixed FFT size and color scale; a faint payload may need a different window length or contrast to pop — reach for Audacity.
  • Channels are averaged to mono, so purely per-channel or phase-based hiding will not appear here.
  • Only audio is visualized; data appended after the audio stream is the job of binwalk and strings, which also run on your upload.