Basics of TEE

Trusted Execution Environments (TEE) are technologies that leverage hardware resources to isolate the code execution from malicious software. The main TEE technology used in cloud computing is the Software Guard Extensions (SGX), from Intel. SGX is an extension of the x86 and x86-64 instruction sets for creating isolated regions in the computer’s main memory, called enclaves.

Enclaves Implementation

Enclaves are implemented as shared objects (or dynamic libraries, in Windows OS) containing code executed in a processor’s reserved memory. The access to this reserved memory space from a code ruining outside of the enclave is denied at microarchitectural level. The main advantage of using enclaves is the protection against high privileged software, such as super-users or operating systems. The enclave code can only be called through special instructions (ECALL), which involves security checks.

Attestation

The enclave initialization instructions measure the enclave state at microarchitectural level, such that no software can tamper with the measurement. This measurement is signed using a special system enclave (the Quoting Enclave) using a key provided by Intel during a setup phase (or provisioning phase). The signed measurement is forwarded to the client, which can verify if the enclave was correctly initialized in a genuine plattform. The enclave and the client can perform a symetric key exchange for creating an encrypted channel. However, the client send its data only if the enclave authenticity was successfuly attested.

Sealing

Enclave data is encrypted with a sealing key before beeing stored in the disk. This sealing key is derived from a hardware-specific key, and only the enclave can use the sealing key. The key is never accessible to other software.