Run it: apt install g++ geany. Open the .cpp in Geany. Hit F9 once. F5 to run.

airgapFTP.av1.mp4

🔪 Gut a keyboard & sand off black coating

Solder photoresistors to leads who type if shorted

Tape photoresistors to eyes of the sending machine

Or 345B/s (press enter & play wav at the same time)

This method proves that modern generic photoresistors are sensitive to at least 5,520 adjustments in brightness per second (an opposite bit is appended to each data bit to keep the LED brightness normalized.) The bottleneck is the LED; it cannot become darker fast enough after each flash of light.

LED.av1.mp4

See LED.cpp. (May need to tilt photoresistor away as in video above. Use it in a dark enclosure.)
_{*Receiving machine must have audio recording hardware at least like that of the Dell Latitude E5500 (made in 2008.)
If you own a modern thousand-dollar laptop, this won't work; your mic line is always noisy, even when disabled.}

Or 1kB/s (press enter & play wav at the same time)

See aux.cpp
_{*Receiving machine must have audio recording hardware at least like that of the Dell Latitude E5500 (made in 2008.)
If you own a modern thousand-dollar laptop, this won't work; your mic line is always noisy, even when disabled.}

Send files to sending machine via local network.

• Sending machine should have Gnome. Go to Settings, Sharing. Enable sharing, then enable "File Sharing" without password. This creates a "Public" folder in /home/user/ if missing.
• Sending machine should run these tools in a new folder in /home/user/Public.
• Now any machine on that network has read / write access to that "Public" folder. Find it in your files browser in "Browse Network" or "Other Locations" and keep dropping files in folder "Send".

Appendix

airgapFTP has been written about on HACKADAY. Thank you for the recognition.