マジで恥ずかしいミス

ARM-based Android smartphones rely on the TrustZone hardware support for a Trusted Execution Environment (TEE) to implement security-sensitive functions. The TEE runs a separate, isolated, TrustZone Operating System (TZOS), in parallel to Android. The implementation of the cryptographic functions within the TZOS is left to the device vendors, who create proprietary undocumented designs.
In this work, we expose the cryptographic design and implementation of Android’s Hardware-Backed Keystore in Samsung’s Galaxy S8, S9, S10, S20, and S21 flagship devices. We reversed-engineered and provide a detailed description of the cryptographic design and code structure, and we unveil severe design flaws. We present an IV reuse attack on AES-GCM that allows an attacker to extract hardware-protected key material, and a downgrade attack that makes even the latest Samsung devices vulnerable to the IV reuse attack. We demonstrate working key extraction attacks on the latest devices. We also show the implications of our attacks on two higher-level cryptographic protocols between the TrustZone and a remote server: we demonstrate a working FIDO2 WebAuthn login bypass and a compromise of Google’s Secure Key Import.
We discuss multiple flaws in the design flow of TrustZone based protocols. Although our specific attacks only apply to the ≈100 million devices made by Samsung, it raises the much more general requirement for open and proven standards for critical cryptographic and security designs.

As we discussed in Section 3, the wrapping key used to encrypt the key blobs (HDK) is derived using a salt value computed by the Keymaster TA. In v15 and v20-s9 blobs, the salt is a deterministic function that depends only on the application ID and application data (and constant strings), which the Normal World client fully controls. This means that for a given application, all key blobs will be encrypted using the same key. As the blobs are encrypted in AES-GCM mode-of-operation, the security of the resulting encryption scheme depends on its IV values never being reused.