Marcus rubbed his eyes, the blue glow of the spectrogram software burning into his retinas. He was an acoustic cryptanalyst for the Bureau, one of a dying breed. The world had moved on to quantum encryption and photon keys, but the old-school spies knew better. They knew that sometimes, the best way to hide a message wasn't to lock it in a safe, but to bury it in the noise.

He extracted the square wave data, converted it back into binary, and unpacked the archive. A single PDF opened on his screen.

He watched the spectrogram, the visual representation of the frequency spectrum. Usually, this looked like a chaotic blur. But as the audio stretched, the blur resolved into distinct, geometric shapes.

He dialed the number.

He translated furiously.

He switched tactics. He ran the file through a high-pass filter, stripping away the low rumbles. Then, he applied a phase inversion. Still nothing but snow on the screen.

Several methods are employed in audio steganography to embed secret information: