[Daniel, Adi, and Eran], students researchers at Tel Aviv University and the Weizmann Institute of Science have successfully extracted 4096-bit RSA encryption keys using only the sound produced by the target computer. It may sound a bit like magic, but this is a real attack – although it’s practicality may be questionable. The group first described this attack vector at Eurocrypt 2004. The sound used to decode the encryption keys is produced not by the processor itself, but by the processor’s power supply, mainly the capacitors and coils. The target machine in this case runs a copy of GNU Privacy Guard (GnuPG).

During most of their testing, the team used some very high-end audio equipment, including Brüel & Kjær laboratory grade microphones and a parabolic reflector. By directing the microphone at the processor air vents, they were able to extract enough sound to proceed with their attack. [Daniel, Adi, and Eran] started from the source of GnuPG. They worked from there all the way down to the individual opcodes running on the x86 processor in the target PC. As each opcode is run, a sound signature is produced. The signature changes slightly depending on the data the processor is operating on. By using this information, and some very detailed spectral analysis, the team was able to extract encryption keys. The complete technical details of the attack vector are available in their final paper (pdf link).

Once  they had the basic methods down, [Daniel, Adi, and Eran] explored other attack vectors. They were able to extract data using ground fluctuations on the computers chassis. They even were able to use a cell phone to perform the audio attack. Due to the cell phone’s lower quality microphone, a much longer (on the order of several hours) time is needed to extract the necessary data.

Thankfully [Daniel, Adi, and Eran] are white hat hackers, and sent their data to the GnuPG team. Several countermeasures to this attack are already included in the current version of GnuPG.