PRA-TLS: Attestation of a Client Application for TEE

Trusted Execution Environments (TEEs) are secure foundations for protecting sensitive information and executing computations over confidential data. Processing in an isolated execution environment (enclave) is invoked by an untrusted application in the Rich Execution Environment (REE). Then the enclave returns the execution results to the untrusted application. Even if the enclave has been attested, the boundary between the enclave and the untrusted application poses risks, such as tampering with input arguments or return values and manipulating the order in which the application invokes functions. Therefore, it is necessary to establish the authenticity and integrity of not only the enclave itself but also the application and its execution environment. In this study, we propose an attestation protocol, Portable Remote Attestation TLS (PRA-TLS), for a client application that invokes an enclave. We introduce a daemon that acts as an attester. PRA-TLS uses a trusted Attester Daemon to measure the state of the host environment and application code at runtime, providing these measurements as attestation evidence for verification by a remote Verifier. This mechanism allows the enclave to proceed only when the software environment is in the expected state and the application code is verified as legitimate. We define attack models and security requirements for the proposed protocol and formally evaluate its security using the Tamarin Prover. Furthermore, we implement a prototype of PRA-TLS using Intel SGX and evaluate its performance.