Extracting RSA Key From GnuPG by capturing Computer Sounds
'RSA Key Extraction via Low-Bandwidth Acoustic Cryptanalysis', is an interesting paper recently published by Three Israeli Security Researchers at Tel Aviv University.

They claimed that, they have successfully broken one of the most secure encryption algorithms, 4096-bit RSA, just by capturing Computer's CPU Sound while it runs decryption routines.
Daniel Genkin, Adi Shamir (who co-invented RSA), and Eran Tromer, uses a side channel attack and through a process called "acoustic cryptanalysis", they successfully extracted 4096-bit RSA key From GnuPG.
"We experimentally demonstrate that such attacks can be carried out, using either a plain mobile phone placed next to the computer, or a more sensitive microphone placed 4 meters away,"
Extracting RSA Key From GnuPG by capturing Computer Sounds

The paper specifies some possible implementations of this attack. Some email-client softwares i.e. Enigmail can automatically decrypt incoming e-mail (for notification purposes) using GnuPG. An attacker can e-mail suitably-crafted messages to the victims, wait until they reach the target computer, and observe the acoustic signature of their decryption, thereby closing the adaptive attack loop.
"The acoustic signal of interest is generated by vibration of electronic components (capacitors and coils) in the voltage regulation circuit, as it struggles to maintain a 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."
The security researchers listen to the high-pitched (10 to 150 KHz) sounds produced by your computer as it decrypts data and warned that a variety of other applications are also susceptible to the same acoustic cryptanalysis attack.
"We observe that GnuPG's RSA signing (or decryption) operations are readily identified by their acoustic frequency spectrum. Moreover, the spectrum is often key-dependent, so that secret keys can be distinguished by the sound made when they are used. The same applies to ElGamal decryption."
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Here in the above picture, a mobile phone (Samsung Note II) is placed 30 cm (nearly 12 inches) from a target laptop. The phone's internal microphone points toward the laptop's fan vents. Full key extraction is possible in this configuration and distance.

They have notified GnuPG about the vulnerability and If you want to keep your data secure, please follow recommended countermeasures:
"One obvious countermeasure is to use sound dampening equipment, such as "sound-proof" boxes, designed to sufficiently attenuate all relevant frequencies. Conversely, a sufficiently strong wide-band noise source can mask the informative signals, though ergonomic concerns may render this unattractive. Careful circuit design and high-quality electronic components can probably reduce the emanations."
GnuPG team has developed a patch for the vulnerability to defend against key extraction attacks and released GnuPG 1.4.16.

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