He who holds the private keys owns all of the bitcoins. For those who manage their cryptocurrency in offline, or “cold,” wallets under the premise that they cannot be compromised, recent news from researcher Dr. Mordechai Guri from Ben-Gurion University of the Negev, Israel, raises some alarms. Guri demonstrated that cold wallets can be infected with malicious code, allowing an attacker to access the wallet’s private keys.
Because cold wallets are presumably safer than storing their keys in “hot,” or online, wallets, many cryptocurreny owners keep their bitcoin wallets isolated in air-gapped PCs so that they are away from the internet and not connected to any network, Wi-Fi or Bluetooth.
In addition to publishing a white paper, Guri also demonstrated the attack method’s effectiveness using malware called bridgeware, which successfully leaks the bitcoin private key over air gap via ultrasonic signals in only 3 seconds.
The discovery isn’t new, nor is it the first time a hacking technique was used to compromise an isolated machine. Rather, Guri’s experiment showed that private cryptocurrency keys can be stolen using out-of-band communication methods.
Malware can be preinstalled, delivered during the initial installation of the wallet, or pushed through a removable media. Once the malware is installed, there are a variety of exfiltration methods an attacker can use, and Guri evaluated several, including physical, electromagnetic, electric, magnetic, acoustic, optical and thermal.
“This research shows that although cold wallets provide a high degree of isolation, it’s not beyond the capability of motivated attackers to compromise such wallets and steal private keys from them. We demonstrate how a 256-bit private key (e.g., bitcoin’s private keys) can be exfiltrated from an offline, air-gapped wallet … within a matter of seconds,” Guri noted.
The PC and keyboard are removed in the second video to demonstrate an additional exfiltration method – a technique known as a RadIoT attack. In about 15 seconds, Guri successfully transmits private keys from a Raspberry Pi to a nearby smartphone over air gap by way of electromagnetic signals.
“I think that the interesting issue is that the airgap attacks that were thought to be exotic issues for high-end attacks may become more widespread,” Guri wrote in an email to Ars Technica. “While airgap covert channels might be considered somewhat slow for other types of information, they are very relevant for such brief amounts of information. I want to show the security of ‘cold wallet’ is not hermetic given the existing air-gap covert channels.”