4 ms·

I think it means that the infected airgapped computer can communicate with an infected non-airgapped computer over audio. The malicious code would already need

by sehrope 13y ago

I think it means that the infected airgapped computer can communicate with an infected non-airgapped computer over audio. The malicious code would already need to be there but once it's there it could (albeit slowly given the transmission rate) send data out from the air gapped machine. I was going to add a line like "Who has a mic and speakers on their airgapped machine?!" but obviously every laptop does.

mschuster91 13y ago
And next to every PC has at least a speaker for the BIOS beep. I'm not sure how good the DAC driver chip is (especially, how they perform at ultrasonic or subsonic frequencies).
- sehrope 13y ago
  I was thinking about this too. If it's possible to send out arbitrary frequencies with it you could have a one way channel even if the air gapped machine doesn't have a mic. Only the receiver would need one. The air gapped machine would just be transmitting 24/7 hoping another machine is in range.
  - mschuster91 13y ago
    Quite like a navigation beacon...
- b409ba0801cd21 13y ago
  The PC speaker does not have much of a DAC. Before the common use of ASIC chipsets, the PC speaker was connected to one of the channels of a clock divider chip. The other channel was used to drive the timer IRQ. When a program wanted to produce a tone, the program took the input frequency of the clock divider divided by the desired frequency and programmed it into the clock divider. To control the duration of the tone, a DOS or BIOS call was available delay the program or the program could hook the timer interrupt vector. The program would then turn the speaker off or start the next tone. Programs could also turn the output bit on and off manually. Some programs could turn the bit on and off rapidly to play arbitrary sounds. There was even a Windows driver to play sound through the PC speaker, but it disabled interrupts, causing the clock, keyboard, mouse, and network to stop while the sound was playing.
  - mschuster91 13y ago
    Now that's an interesting piece of history. Thanks!
  - PeterisP 13y ago
    If the speaker is physically capable of that, the same technique can output ultrasound - with modern processor speeds, you probably could twiddle that output bit while still leaving room for normal system operation.
venomsnake 13y ago
Schneier admitted his airgapped machine was laptop. But I supposed he has the good sense to physically damage the mic, speakers and wifi and Bluetooth.

by sehrope 13y ago

mschuster91 13y ago

And next to every PC has at least a speaker for the BIOS beep. I'm not sure how good the DAC driver chip is (especially, how they perform at ultrasonic or subsonic frequencies).

sehrope 13y ago
I was thinking about this too. If it's possible to send out arbitrary frequencies with it you could have a one way channel even if the air gapped machine doesn't have a mic. Only the receiver would need one. The air gapped machine would just be transmitting 24/7 hoping another machine is in range.
- mschuster91 13y ago
  Quite like a navigation beacon...
b409ba0801cd21 13y ago
The PC speaker does not have much of a DAC. Before the common use of ASIC chipsets, the PC speaker was connected to one of the channels of a clock divider chip. The other channel was used to drive the timer IRQ. When a program wanted to produce a tone, the program took the input frequency of the clock divider divided by the desired frequency and programmed it into the clock divider. To control the duration of the tone, a DOS or BIOS call was available delay the program or the program could hook the timer interrupt vector. The program would then turn the speaker off or start the next tone. Programs could also turn the output bit on and off manually. Some programs could turn the bit on and off rapidly to play arbitrary sounds. There was even a Windows driver to play sound through the PC speaker, but it disabled interrupts, causing the clock, keyboard, mouse, and network to stop while the sound was playing.
- mschuster91 13y ago
  Now that's an interesting piece of history. Thanks!
- PeterisP 13y ago
  If the speaker is physically capable of that, the same technique can output ultrasound - with modern processor speeds, you probably could twiddle that output bit while still leaving room for normal system operation.

venomsnake 13y ago

Schneier admitted his airgapped machine was laptop. But I supposed he has the good sense to physically damage the mic, speakers and wifi and Bluetooth.