Security researchers have successfully broken one of the most secure encryption algorithms, 4096-bit RSA, by listening – yes, with a microphone — to a computer as it decrypts some encrypted data. The attack is fairly simple and can be carried out with rudimentary hardware. The repercussions for the average computer user are minimal, but if you’re a secret agent, power user, or some other kind of encryption-using miscreant, you may want to reach for the Rammstein when decrypting your data.
This acoustic cryptanalysis, carried out by Daniel Genkin, Adi Shamir (who co-invented RSA), and Eran Tromer, uses what’s known as a side channel attack. A side channel is an attack vector that is non-direct and unconventional, and thus hasn’t been properly secured. For example, your pass code prevents me from directly attacking your phone — but if I could work out your pass code by looking at the greasy smudges on your screen, that would be a side channel attack. In this case, the security researchers listen to the high-pitched (10 to 150 KHz) sounds produced by your computer as it decrypts data.
This might sound crazy, but with the right hardware it’s actually not that hard. For a start, if you know exactly what frequency to listen out for, you can use low- and high-pass filters to ensure that you only have the sounds that emanate from your PC while the CPU decrypts data. (In case you were wondering, the acoustic signal is actually generated by the CPU’s voltage regulator, as it tries to maintain a constant voltage during wildly varied and bursty loads). Then, once you have the signal, it’s time for the hard bit: Actually making sense of it.