$ cat writeup.md…
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broncoctf2026
Task: a stereo WAV containing a garbled AI-remix of a well-known song, with a hidden message drawn into the ultrasonic 14-24 kHz spectrogram band. Solution: render a clean high-contrast SoX spectrogram of that band, read the block-letter text 'THE FLAG IS THE RELEASE YEAR OF THE SONG THE LYRICS ARE FROM' (Never Gonna Give You Up -> 1987).
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So there's this guy SG who just sent me a copy of their new remix. Knowing what they are like with their remixes, I don't trust it, in more ways than one. Whatever remix, I always feel like they modified the audio specifically for me somehow. Maybe you should have a listen. Wrap the secret you find in bronco{}.
Given file: sg_remix.wav — RIFF WAVE, Microsoft PCM, 16-bit, stereo, 48000 Hz,
~82 s, ~15.7 MB. The audible track is a garbled / AI-remixed version of Rick
Astley's "Never Gonna Give You Up". Goal: find the hidden secret and wrap it in
bronco{}.
The prompt drops two strong hints: "have a listen" (audio-domain inspection) and "in more ways than one" / "modified the audio specifically for me" (there are two layers — the audible rickroll AND a second hidden layer).
A high-contrast spectrogram of the L+R mix reveals a dense, structured band of energy in the ultrasonic 14–24 kHz range, present only from about t = 13.75 s to t = 38.7 s. This is clearly artificial — normal 48 kHz music does not carry deliberate, block-shaped structure that high.
The key realization: those shapes are not a data-over-sound modulation — they are block letters literally drawn into the frequency/time plane. Rendering a clean, high dynamic-range spectrogram of the exact band and simply reading it left-to-right spells out a plain English sentence:
THE FLAG IS THE RELEASE YEAR OF THE SONG THE LYRICS ARE FROM
The garbled lyrics in the remix are Rick Astley — "Never Gonna Give You Up",
released in 1987. Therefore the flag is bronco{REDACTED}.
...
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