The design flaws primarily affect devices that use ARM's TrustZone technology: the hardware support provided by ARM-based Android smartphones (which are the majority) for a Trusted Execution Environment (TEE) to implement security-sensitive functions. TrustZone splits a phone into two portions, known as the Normal world (for running regular tasks, such as the Android OS) and the Secure world, which handles the security subsystem and where all sensitive resources reside. The Secure world is only accessible to trusted applications used for security-sensitive functions, including encryption.
Matthew Green, associate professor of computer science at the Johns Hopkins Information Security Institute, explained on Twitter that Samsung incorporated "serious flaws" in the way its phones encrypt key material in TrustZone, calling it "embarrassingly bad." "They used a single key and allowed IV re-use," Green said. "So they could have derived a different key-wrapping key for each key they protect," he continued. "But instead Samsung basically doesn't. Then they allow the app-layer code to pick encryption IVs." The design decision allows for "trivial decryption," he said.
Samsung responded to the academics' disclosure by issuing a patch for affected devices that addressed CVE-2021-25444: an IV reuse vulnerability in the Keymaster Trusted Application (TA) that runs in the TrustZone. Keymaster TA carries out cryptographic operations in the Secure world via hardware, including a cryptographic engine. The Keymaster TA uses blobs, which are keys "wrapped" (encrypted) via AES-GCM. The vulnerability allowed for decryption of custom key blobs. Then, in July 2021, the researchers revealed a downgrade attack -- one that lets attacker trigger IV reuse vulnerability with privileged process. Samsung issued another patch -- to address CVE-2021-25490 -- that remoged the legacy blob implementation from devices including Samsung's Galaxy S10, S20 and S21 phones.
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