As new hardware launches, many readers ask whether they produce any noticeable form of coil whine. For instance, this is an issue for graphics cards that are outputting a very high frame rate. The electronics create sound from the current oscillating as it flows through them. It can also be an issue for motherboards or power supplies as well. You can check out this fairly old video from LinusTechTips for a demonstration.
Image Credit: ACM
It turns out that, because this whine is related to the signal flowing through the oscillating circuit, security researchers are looking into the types of information that can be inferred from the whine. In particular, the Association for Computing Machinery (ACM) published a paper called Physical Key Extraction Attacks on PCs. It discusses several methods of attacking a device, such as reading minor fluctuations in its grounding plug or monitoring induced radiation with an antenna. Its headlining method is “Acoustic” though, which listens to coil whine sound produced by the computer, as it decrypts RSA messages that are sent to it, to gather the RSA secret key from it.
While they have successfully demonstrated the attack using a parabolic microphone at 33ft away, and a second demonstration using a mobile phone at 1ft away, the news should be taken with a grain of salt. Mostly, it's just interesting to realize that there's nothing really special about a computer. All it does is stores and processes data on whatever physical state we have available in the world. Currently, that's almost always radio-frequency radiation flowing through semiconductors. Whatever we use will have consequences. For instance, as transistors get smaller, to push more complex signals through a given surface area and power, we'll eventually run out of atoms.
This is just another, often forgotten side-effect: electric signals induce the transfer of energy. It could be electromagnetic, acoustic, or even thermal. In the realm of security, this could, itself, carry some of the data that we attached to our world's state, and allow others to access it (or sometimes modify it) without our knowledge or consent.