Trace Labs — Challenge 45 — Writeup

Target file: voicemail_007_wav.wav Path: ~/OSINT/TraceLabs/challenge_45/ Date: 2026-08-14 Methodology: Spectrogram Analysis → Voice Transcript → Morse Decoding

1. Spectrogram — Locating Voice vs. Tone

Reading the spectrogram:

0–11.5s: complex harmonic energy (formants) → spoken voice
11.5–12.5s: silence
12.5–20s: narrow, steady band at ~1 kHz, regular on/off pattern → Morse tone

2. Voice Transcript

Given how short the segment is (11.5s), I transcribed it using transcribe.org.

Transcript:

(00:00) "Hey, it's me, don't call this number again, okay?" (00:04) "A shipment for the facility gets moved Thursday night."

3. Morse Decoding — Python Script

I wrote a custom script using numpy/scipy (Hilbert transform for the envelope, thresholding to binarize on/off, run-length classification into dots/dashes/spaces).

Dependencies (via apt, no PEP 668 issues):

sudo apt install -y python3-numpy python3-scipy

sudo apt install -y python3-numpy python3-scipy

Script: decode_morse.py (attached)

morse_tail.wav is an isolated segment extracted from the original voicemail_007.mp3, trimmed down to just the ~12.5–20s tone portion and exported as WAV for the numpy/scipy pipeline.

Logic:

hilbert() → signal envelope
smoothing with a moving average (5ms)
threshold at 30% of peak amplitude → binary on/off sequence
run-length encoding of on/off sequences (in ms)
base unit = shortest ON duration (a dot)
classification: ON ≥3 units → dash (-), otherwise dot (.); gap ≥7 units → word space, gap ≥3 units → letter space
translate Morse table → ASCII

Run it:

python3 decode_morse.py morse_tail.wav

python3 decode_morse.py morse_tail.wav

Output:

Raw Morse: -.. --- -.-. -.- --...
Decoded: DOCK7

Raw Morse: -.. --- -.-. -.- --...
Decoded: DOCK7

4. Robustness Check

To rule out the result being an artifact of the arbitrarily chosen threshold (30%), I re-ran the script with a lower threshold (20%):

sed 's/threshold = envelope.max() \* 0.3/threshold = envelope.max() * 0.2/' decode_morse.py > decode_morse_v2.py
python3 decode_morse_v2.py morse_tail.wav

sed 's/threshold = envelope.max() \* 0.3/threshold = envelope.max() * 0.2/' decode_morse.py > decode_morse_v2.py
python3 decode_morse_v2.py morse_tail.wav

Identical output: DOCK7 — stable result across two different thresholds, high confidence.

5. Final Result

Flag/Answer: DOCK7

Reconstructed OSINT context:

Voicemail from an unidentified contact ("don't call this number again")
References a "shipment" being moved to a "facility," Thursday night
The Morse tail specifies the exact location: Dock 7