Dead Air

The brief

At 03:17 UTC, a shortwave station that had sent the same pattern for eleven years went silent. One recording survived the shutdown. Two operators were on shift. One spoke out loud. The other never made a sound you could hear. Find out what Station KESTREL was really sending.

• dead-air.zip, 9 MB

In short

A stereo recording carried two messages. The left channel spells a key in Morse, the right channel draws an archive password into its spectrogram, and the archive holds a glyph cipher that the Morse key opens with Vigenere modulo 37.

Two operators, one stereo file

The recording is 47 seconds of 16 bit stereo. The left channel carries a 700 Hz tone and the right one sounds empty. Beside it sits a logbook archive locked with AES.

The left channel speaks Morse

The lab filtered the 700 Hz tone and timed it. Dots last 0.08 seconds and dashes about 0.27. The message reads KESTREL STATION OFF AIR KEY IS NIGHTJAR LISTEN WITH YOUR EYES.

The right channel writes in the spectrum

Listen with your eyes. The lab drew the right channel as a spectrogram between 4.5 and 14.5 kHz, and 13 bursts of sound draw the text KSTR-7742-VXQ. It opens the logbook exactly as written.

28 glyphs against a code pad

The logbook holds a page of 28 hand drawn glyphs and a code pad of 37 for A to Z, 0 to 9 and the underscore. The lab matched every glyph by cross correlation, each at 0.987 or better, and got 59R_6L3Q9BYGBQ3QZAYZMP3QIZ92.

NIGHTJAR turns it into words

The Morse key decrypts the page. Vigenere modulo 37 with NIGHTJAR, subtracted column by column, turns the 28 symbols into one sentence about silence, and that sentence is the flag.

The insight

Each channel held half the answer. The silent one carried the password as a picture, and the audible one carried the key to the cipher inside.

The flag

hiCTF{s1l3nc3_w4s_th3_m3ss4g3_6r3v}