  Cryptocurrency wallets store the wallets private key(s), and hence, are a
lucrative target for attackers. With possession of the private key, an attacker
virtually owns all of the currency in the compromised wallet. Managing
cryptocurrency wallets offline, in isolated ('air-gapped') computers, has been
suggested in order to secure the private keys from theft. Such air-gapped
wallets are often referred to as 'cold wallets.' In this paper, we show how
private keys can be exfiltrated from air-gapped wallets. In the adversarial
attack model, the attacker infiltrates the offline wallet, infecting it with
malicious code. The malware can be preinstalled or pushed in during the initial
installation of the wallet, or it can infect the system when removable media
(e.g., USB flash drive) is inserted into the wallet's computer in order to sign
a transaction. These attack vectors have repeatedly been proven feasible in the
last decade (e.g., [1],[2],[3],[4],[5],[6],[7],[8],[9],[10]). Having obtained a
foothold in the wallet, an attacker can utilize various air-gap covert channel
techniques (bridgeware [11]) to jump the airgap and exfiltrate the wallets
private keys. We evaluate various exfiltration techniques, including physical,
electromagnetic, electric, magnetic, acoustic, optical, and thermal techniques.
This research shows that although cold wallets provide a high degree of
isolation, it is 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 of a fictional character named Satoshi within a matter of
seconds
