Dead Air Morse, spectrogram glyphs and Vigenere

forensics
flag hiCTF{s1l3nc3_w4s_th3_m3ss4g3_6r3v}

Station KESTREL. Flag format hiCTF{}.

Artifacts

dead-air.zip ->

  • kestrel_last_recording.wav - stereo 16-bit PCM, 44100 Hz, 47.24 s
  • kestrel_logbook.7z - password protected

Chain

  1. WAV is two operators on two stereo channels.

    • Ch0 (~700 Hz carrier, on/off keyed) = the operator who "spoke out loud". OOK envelope -> Morse: KESTREL STATION OFF AIR KEY IS NIGHTJAR LISTEN WITH YOUR EYES
      • key = NIGHTJAR
      • "listen with your eyes" = look at the spectrogram
    • Ch1 = the operator who "never made a sound you could hear". Full-band spectrogram draws large glyphs at 5-8 kHz: KSTR-7742-VXQ

  2. 7z password = KSTR-7742-VXQ. Extracts:

    • logbook_page.png - 28 coded glyphs (01..28)
    • operator_codepad.png - code pad mapping glyph -> char for the alphabet ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_ (37 symbols, index 0..36)

  3. Glyph decoding. Each cell encodes its symbol in TWO redundant ways: the black stroke + start-dot shape, and a faint 3x3 dot constellation. Both agree. Template-matching each logbook glyph (rotation/flip invariant) to the pad glyphs yields ciphertext: 59R_6L3Q9BYGBQ3QZAYZMP3QIZ92

  4. Vigenère over the same 37-symbol alphabet, key NIGHTJAR: P = (C - K) mod 37 -> S1L3NC3_W4S_TH3_M3SS4G3_6R3V = "SILENCE WAS THE MESSAGE" (+ a trailing fragment 6R3V).

The two images from the logbook archive:

The logbook page: 28 numbered cells, each holding a coded glyph

The operator code pad mapping each glyph to a letter, digit or underscore

Flag

hiCTF{s1l3nc3_w4s_th3_m3ss4g3_6r3v} (ACCEPTED - the decode lowercased; the uppercase form was rejected)

Note: the trailing 6R3V decodes deterministically (glyphs 25-28 = I,Z,9,2, each verified against the pad at high confidence and via their unique dot constellations reached only by the Z/3,I/G disambiguation that keeps the first 23 chars perfectly clean). Every alternative disambiguation corrupts earlier words (S1L3NCZ / THZ / W4SBTH3), so this reading is the only consistent one.