You are an expert CTF solver specializing in audio...

Question

You are an expert CTF solver specializing in audio steganography.
I have an audio file from a CTF challenge with the following description:

"We intercepted this audio from a suspected spy. The melody is catchy,
but analysts believe there's more than meets the ear. Can you look beyond
the amplitude and find the hidden message?"

Flag format: TBCTF{example_flag}

YOUR TASK: Analyze the audio file and find the hidden flag.

IMPORTANT RULES:

DO NOT guess or fabricate any flag
DO NOT submit a flag unless you have CONCRETE EVIDENCE from the audio data
If you cannot find the flag with certainty, report exactly what you found and what techniques you tried
Only output a flag if you extracted it from actual data

STEP-BY-STEP ANALYSIS APPROACH:

BASIC FILE ANALYSIS
- Check file format, sample rate, channels, bit depth
- Run: file audio.wav && ffprobe audio.wav
SPECTROGRAM ANALYSIS (most common hiding technique)
- Generate spectrogram to look for hidden text/images in frequency domain
- Use: sox + image tools, or Audacity spectrogram view
- Look at: very high frequencies, very low frequencies, beginning/end of file
LSB STEGANOGRAPHY
- Check if data is hidden in Least Significant Bits of audio samples
- Try tools: steghide, wavsteg, or manual extraction
METADATA ANALYSIS
- Check ID3 tags, EXIF, or embedded comments
- Run: exiftool audio.wav && strings audio.wav
MORSE CODE / DTMF TONES
- Listen for patterns suggesting morse code in amplitude
- Detect DTMF telephone tones (0-9, *, #)
PHASE / FREQUENCY DOMAIN
- Check phase encoding between channels (if stereo)
- Analyze FFT for anomalies
FILE STRUCTURE
- Check for appended data after audio EOF
- Run: binwalk audio.wav
PITCH / SPEED MANIPULATION
- Try reversing the audio
- Try changing pitch/speed to reveal hidden speech

REPORTING FORMAT:

For each technique: state what you tried and what you found
If you find the flag: show the extraction method clearly
If unsuccessful: list all techniques attempted with results

Start with: file audio.wav && exiftool audio.wav && strings audio.wav | grep -i "TBCTF"

Answer 1

Flag extracted:

TBCTF{07ed4f7f25ed0e076e88d8d61192ea8a6b4680467c32c577eb2f38b9926c96f7}

Evidence and extraction path

The ZIP contained one file:

challenge.flac

Basic analysis showed:

text
FLAC audio bitstream data, 16 bit, mono, 44.1 kHz, 352800 samples
Duration: 00:00:08.00

Metadata contained the key clue:

text
Phase 1 Complete. Tuned strictly to the notes.
(Format your findings with hyphens, and secure them with a 256-bit hash)
We analyzed a 16K sample block...

strings | grep -i TBCTF found no literal flag.

Successful technique: phase analysis of first 16K block

Using a 16K block as 16384 samples, I ran an FFT on the first block and looked at local spectral peaks. The phase values were cleanly quantized at approximately 90° and 270°, which is strong evidence of phase-based hiding.

The phase-shifted note peaks at +90° were:

text
A2
E3
E4
B4
C5

Formatted with hyphens per the metadata clue:

text
A2-E3-E4-B4-C5

SHA-256 of that exact string:

text
07ed4f7f25ed0e076e88d8d61192ea8a6b4680467c32c577eb2f38b9926c96f7

So the flag is:

text
TBCTF{07ed4f7f25ed0e076e88d8d61192ea8a6b4680467c32c577eb2f38b9926c96f7}

Other checks attempted

Metadata and strings: no literal flag found.

Spectrogram: showed musical/chord content but no visible text.

LSB extraction: checked PCM sample bit planes; no TBCTF or readable payload found.

File structure: ZIP had no comment or appended data; FLAC padding block was zero-filled.

Morse/DTMF: no evidence of Morse or telephone tones.

The flag above is not guessed; it is derived from the phase-shifted note sequence and the SHA-256 instruction embedded in the audio metadata.

You are an expert CTF solver specializing in audio...

問題

思考

回答

Evidence and extraction path

Successful technique: phase analysis of first 16K block

Other checks attempted

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