Hardware-Backed Heist: Extracting ECDSA Keys from Qualcomm's TrustZone

Trusted Execution Environments (TEEs) such as ARM TrustZone are in widespread use in both mobile and embedded devices, and they are used to protect sensitive secrets while often sharing the same computational hardware as untrusted code. Although there has been limited research in the area, the threat of microarchitectural attacks against ARM TrustZone has not been thoroughly studied. This is not the case for other TEEs, such as Intel SGX, where the security promises of the TEE have been violated numerous times by the academic community, showing that it is possible to use side-channel attacks to gain detailed insight into the microarchitectural behavior of trusted code. In this work, we show that TrustZone is susceptible to similar attacks, and we demonstrate the ability to achieve cache attacks with high temporal precision, high spatial precision, and low noise. These tools make it easy to monitor the data flow and code flow of TrustZone code with great resolution, and we apply our techniques to investigate the security of a real-world application. We examine ECDSA signing in Qualcomm's implementation of Android's hardware-backed keystore and identify a series of vulnerabilities that leak sensitive cryptographic information through shared microarchitectural structures. By using the powerful attacks developed in this paper, we are able to successfully extract this sensitive information and fully recover a 256-bit private key from Qualcomm's version of the hardware-backed keystore.