Acoustic Cryptanalysis: Using Electronic Noise to Crack Encryption Keys

Acoustic Cryptanalysis: Using Electronic Noise to Crack Encryption Keys
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An interesting paper titled “RSA Key Extraction via Low-Bandwidth Acoustic Cryptanalysis.” was published passed December by Daniel Genkin (Technion and Tel Aviv),  Adi Shamir (University Weizmann Institute of Science) and Eran Tromer (Tel Aviv University). The three researchers demonstrated how it’s possible to perform a cryptanalysis using the electronically noise produced by a laptop. The attack can extract full 4096-bit RSA decryption keys from laptop computers within an hour.

The useful electronic noise was characterized as the one generated “by vibration of electronic components (capacitors and coils) in the voltage regulation circuit, as it struggles to supply constant voltage to the CPU despite the large fluctuations in power consumption caused by different patterns of CPU operations. The relevant signal is not caused by mechanical components such as the fan or hard disk, nor by the laptop’s internal speaker”.

Primary tests used acoustic information below 20Khz captured using a regular phone microphone at close range (30cm/ 11.8 “) and using a parabolic microphone over 4m / 13ft.  The research paper mentions also other technics that were used to capture information fluctuations such as Power analysis which analyzes laptops DC power supply variations.

The core of the cryptanalysis technic is sending an encrypted emails engineered to produce an acoustic leakage specifically related to particular bits in the encryption key. As new crafted messages are decrypted, the key is progressively recovered. The authors mention how such emails (which are almost Trojans) can be easily categorized as spam by the user or the email program and almost not raise any suspicion.

The range of attack scenarios discussed in the paper is wide:

  • Install an attack app on your phone. Set up a meeting with the victim and place the phone on the desk next to his laptop.
  • Break into the victim’s phone, install the attack app, and wait until the victim inadvertently places his phone next to the target laptop.
  • Construct a web page use the microphone of the computer running the browser (using Flash or HTML Media Capture, under some excuse such as VoIP chat). When the user permits the microphone access, use it to steal the user’s secret key.
  • Put your stash of eavesdropping bugs and laser microphones to a new use.
  • Send your server to a colocation facility, with a good microphone inside the box, and then acoustically extract keys from all nearby servers.
  • Get near a TEMPEST/1-92 protected machine, such as the one pictured to the right, place a microphone next to its ventilation holes, and extract its supposedly-protected secrets.

The authors of the paper list possible countermeasures such as sound dampening via sound-proof boxes or redesigned circuits. Another possibility is to render the encryption algorithm (electronically) independent from the inputs it receives with nevertheless an obvious loss of performance.

As a side note, Tal Ater, a developer, recently discovered a flaw in Google’s Chrome browser which allows sites to take control of  a PC/Laptop’s microphone and record the surrounding sounds. There’s probably no need to setup a complex equipment in this case, to capture all the required acoustic signatures to retrieve the encryption key. Google dismissed the discovery as an exploit.

Source: The Blavatnik School of Computer Science, Tel Aviv.
Photos Credits: Ibid. / Main Photo: Acoustic Leakage Spectrogram: In this spectrogram, the horizontal axis (frequency) spans 40 kHz, and the vertical axis (time) spans 1.4 seconds. Each yellow arrow points to the middle of a GnuPG RSA decryption. It is easy to see where each decryption starts and ends.