O Quiet Time

To mitigate the risk of human-generated noise, like that of typing, you have a few options. Using a silent, rubber keyboard or an on-screen or touch-screen keyboard are both noiseless options. Pen-based computing is also quiet. None of these options is anywhere near as comfortable as typing on a real keyboard, though. Perhaps the better option is to try to ensure that you use sufficient sound proofing in the area to prevent the attacker from obtaining the source material to analyze.

To reduce the risk of attacks on machine-generated acoustics, either use sound dampening equipment that will make the sound inaudible or mask it with a broad spectrum noise creation device. Another option, like with all of the other side-channel attacks, is to reduce the available data by introducing obfuscating factors in the code or by running additional operations simultaneously. Furthermore, hardware noises can be changed or disabled via Linux kernel modules and software.

Most sounds on a standard Linux system are made by the speakers, the fan, and the hard drive. You can change or disable these sounds as described next.

The System Fan You can usually control the system fan via the libsensorsO library, which is used to display temperature, voltage, and fan sensors. The kernel drivers, lm-sensors, will do the same as well. You can find more information at http://www.lm-sensors.nu/. Additionally, you can also control the fans through ACPI power management controls.

The Hard Drive The hard drive noise comes from the spinning of the disks. You can purchase special "quiet" disks. You can also obtain large flash memory cards, which have no moving parts, make no noise, and produce very little heat, to reduce or eliminate noise.

For spinning disks, the hdparm command allows you to set the disk spin, which can also stop the disk from spinning when not in use. Use the -S, -y, and -Y flags to change the IDE drive access modes for a standby timeout, forcing the hard drive to immediately enter low-power use and spin down and immediately enter sleep mode.

PC Speaker Unplugging the speaker is an option, however, not everyone may want to or even be able to get access to the motherboard. From the console, use the command setterm -blength 0 and for the X Window System, xset b off to turn the bell off. You can find more details on disabling speaker noise the Visible-Bell-mini-Howto by Alessandro Rubini at http://tldp.org/HOWTO/Visual-Bell.html.

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