An air-gapped machine has no network — the strongest isolation there is. Malware can still get in (a USB stick crossed the gap into Stuxnet's centrifuges). The hard part is getting data back out with no network and no speaker. The trick: don't add a transmitter — weaponize the noise the machine already makes. Spin the fan, seek the disk, and the secret rides out on a sound the computer was always going to produce.
① the lineage
note. Fansmitter, DiskFiltration, and MOSQUITO are all from Mordechai Guri's group at Ben-Gurion University — three of dozens of air-gap "Bridgeware" channels they built (others leak over electromagnetic, optical, magnetic, thermal, and power lines).
set 8 bits — the fan whine carries them, no speaker involved
③ the family — noise made to talk
FansmitterModulate the fan's RPM, and its blade-pass tone shifts — FSK on a whirr. Audio-less PC to a phone in the room, 1–8 m, up to ~60 bit/min per fan.emitter · CPU/GPU/chassis fan
DiskFiltrationDrive the hard-drive head through chosen seeks; the actuator's click-and-grind becomes the carrier. Works on a speakerless machine — fails against an SSD.emitter · HDD actuator
MOSQUITORetask a speaker as a microphone in software, and two air-gapped PCs talk to each other directly over ultrasound — no mic, no network, no human hears it.emitter · speaker-to-speaker
…and beyond soundThe same lab leaks over LEDs, magnetic fields, heat, and power lines — "Bridgeware." Acoustics is just the loudest member of a silent family.emitter · everything that radiates
④ the air gap isn't airtight — and your key can leave
This is Paper III run in reverse: there the attacker read a machine's involuntary leak; here malware manufactures one. It defeats the strongest isolation we have because the threat model never included the room's acoustics — only its wires. The rates are punishing (bits per minute, not per second), but a key is only a few hundred bits, and the attacker has all night. For attribution this is the exit wound: the same secured machine that holds your signing key can hum it out through its own cooling fan. Defenses match the channel — an "audio gap" (no phones or mics near the secure host), fan-speed and disk-pattern monitoring, acoustic enclosures, ambient jamming. The lesson is blunt: a machine that can make noise is never fully offline.
what's authentic. the BPF formula (RPM × blades ÷ 60), the Fansmitter / DiskFiltration / MOSQUITO mechanisms and the ≈60 bit/min figure are real and sourced from Guri's group. the demo genuinely encodes 8 bits as two fan-tone frequencies inside synthesized fan noise and recovers them by Goertzel analysis — verified, not faked.
honest frame. a real fan changes speed slowly (mechanical inertia) and hides in HVAC and room noise, so true exfiltration runs at bits-per-minute and needs error coding — i sped it up and cleaned the carrier so the mechanism is audible. the physics (data in the blade-pass frequency) is exactly right; the ease is not.
PAPER IV · AIR-GAP EXFILTRATION — make a silent machine talknext ▸ V · ADVERSARIAL AUDIO — a message only a machine hears