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Researchers Reveal How to Extract Decryption Keys With Sound

A group of cryptology experts say they have demonstrated how to extract a 4096-bit RSA key from a laptop in less than an hour using a sound-based attack technique.

A group of cryptology experts say they have demonstrated how to extract a 4096-bit RSA key from a laptop in less than an hour using a sound-based attack technique.

The research was performed by Adi Shamir, one of the inventors of the RSA algorithm, as well as researchers Daniel Genkin and Eran Tromer of Tel Aviv University. According to the group, the key extraction attack builds off research from 2004.

“Many computers emit a high-pitched noise during operation, due to vibration in some of their electronic components,” according to a paper written by the trio. “These acoustic emanations are more than a nuisance: they can convey information about the software running on the computer, and in particular leak sensitive information about security-related computations.”

“In a preliminary presentation (Eurocrypt’04 rump session), we have shown that different RSA keys induce different sound patterns, but it was not clear how to extract individual key bits,” the researchers continued. “The main problem was that the acoustic side channel has a very low bandwidth (under 20kHz using common microphones, and a few hundred kHz using ultrasound microphones), many orders of magnitude below the GHz-scale clock rates of the attacked computers.”

“The attack can extract full 4096-bit RSA decryption keys from laptop computers (of various models), within an hour, using the sound generated by the computer during the decryption of some chosen ciphertexts. 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.”

The attack relies on crafting ciphertexts that cause numerical cancellations “deep inside GnuPG’s modular exponentiation algorithm,” the researchers explained.

“This causes the special value zero to appear frequently in the innermost loop of the algorithm, where it affects control flow,” according to the paper. “A single iteration of that loop is much too fast for direct acoustic observation, but the effect is repeated and amplified over many thousands of iterations, resulting in a gross leakage effect that is discernible in the acoustic spectrum over hundreds of milliseconds.”

In order to apply the attack to GnuPG, the researchers leveraged OpenPGP encrypted email messages. Specifically, they targeted Enigmail, a plugin for Mozilla Thunderbird that allows offers OpenPGP public key email signing and encryption. Enigmail, the researchers note, automatically decrypts incoming email as long as the GnuPG passphrase is cached or empty, allowing an attacker in this case to send specially-crafted messages to the victims, wait until they reach the computer, and then observe their acoustic signature.

The attack has been disclosed to GnuPG developers and main distributors as CVE-2013-4576. 

The simplest way to mitigate the attack is to use GnuPG 2.x as opposed to the earlier version,” explained Paul Ducklin, head of technology at Sophos.

“The Version 2 branch of GnuPG has already been made resilient against forced-decryption attacks by what is known as RSA blinding,” he blogged.

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