Acoustic cryptanalysis of hard drive
Acoustic cryptanalysis of hard drive

A team of researchers - including Adi Shamir, one of the trio of researchers that came up with the RSA encryption algorithm - have confirmed a theory they developed a decade ago, namely that it is possible to identify the encryption key of an 4,096 RSA encryption key from the sound that the hard drive makes when encrypting the data on the disk surface.

The RSA algorithm was first published in 1977 by Ron Rivest, Adi Shamir, and Leonard Adleman - all researchers with MIT at the time - and the letters RSA are the initials of their surnames, listed in the same order as on their original paper.

A decade ago, Shamir - now with the Weizmann Institute of Science - along with Daniel Genkin of Technion and Tel Aviv University, plus Eran Tromer, also of Tel Aviv University, theorised that the high-pitched noise made during a computer's operation could be used to create a unique digital fingerprint for the PC in question - and that this data could be used to extract data on what the computer is doing.

Now the researchers have successfully extracted a 4,096 bit RSA key from several laptops, using a mobile phone placed next to the computer, as well as using a more sensitive microphone placed some four metres away.

Given the fact that smartphones can now be infected to become an infinity bug - as described in some depth by Trend Micro's security evangelist Rik Ferguson in the summer of last year at the B-Able security conference - this means that an infected Android handset could be used to extract an RSA crypto key remotely, SCMagazineUK.com notes.

The researchers call this attack vector an `acoustic cryptanalysis key extraction attack' and, despite sounding like a very early April Fool joke, the attack vector is explained in some depth in their paper on the topic.

The paper states that the acoustic vibrations from a PC can convey information about the software running on the computer, and in particular, leak sensitive information about security-related computations.

"In a preliminary presentation, we have shown that different RSA keys induce different sound patterns, but it was not clear how to extract individual key bits. The main problem was that the acoustic side channel has a very low bandwidth (under 20 kHz 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," say the researchers in their paper.

"In this paper we describe a new acoustic cryptanalysis key extraction attack, applicable to GnuPG's current implementation of RSA. 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," the paper goes on to say.

The good news is that the researchers have come up with several methods of mitigating the security issue, including the use of acoustic shielding; using the PC in a noisy environment; and randomising the cipher text of the crypto algorithm.

Commenting on the breakthrough - which has profound implications for the physical security of cryptography in computers - Professor John Walker of the Nottingham-Trent University Facility of Engineering, told SCMagazineUK.com that the research goes to show there will always be security attack vectors that people do not think about when developing their security technologies.

"We are in an era of ever-evolving security attacks and this attack vector highlights the ingenuity of the cybercriminals - who are able to think laterally and beat systems that were previously seen as uncrackable," he said.

Professor Walker, who is also CTO of Integral Security Xssurance, went on to say that today's security arena is now driven by commercial considerations, rather than the more practical methods of defence that are available.

"We have all been too satisfied with the security we have had available. This attack breakthrough clearly shows us that," he said.