Abstract
Time is used in secure systems to validate security properties. Consequently, it is vital to protect the integrity of time information. Intel SGX enables building secure applications inside a Trusted Execution Environment (TEE), called an enclave, isolated from the untrusted OS. However, accessing time information from the enclave remains challenging as the OS controls the system time. Previous versions of the SGX SDK provided the sgx_get_trusted_time function as an alternative to OS time. However, Intel removed the API in 2020, without providing an alternative. This paper examines trusted time challenges in SGX and presents TPM-based Trusted Time Extensions (T3E), a novel solution that builds on readily available hardware. T3E leverages TPM functionality to provide trusted time services in enclaves while protecting against common attacks. It offers better time granularity and lower latency than Intel’s sgx_get_trusted_time implementation. Unlike related work, it does not rely on deprecated features or hardware/firmware modifications.
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Notes
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Removed as of Intel SGX SDK for Linux version 2.8 (https://github.com/intel/linux-sgx/releases/tag/sgx_2.8).
- 2.
A post on the Intel Developer Forum by a moderator suggests that the reason of the removal might be linked to the fact that the CSME is not available on server platforms, and due to licensing issues. https://community.intel.com/t5/Intel-Software-Guard-Extensions/The-delay-attack-towards-the-trusted-time/m-p/1343497.
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Acknowledgements
This research is partially funded by the Research Fund KU Leuven, and by the Flemish Research Program Cybersecurity. The authors thank all reviewers, who provided constructive feedback to improve our paper. In addition, the first author would like to also thank Dody Suhendra from Sandhiguna for triggering the discussion leading to this research question.
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A Appendix: Comparison with Related Solutions
A Appendix: Comparison with Related Solutions
The features of T3E are comparable to the related works presented in Sect. 3. However, T3E can be implemented without requiring intrusive changes to hardware or software. It does not rely on deprecated infrastructure or APIs and does not require modifications to commodity hardware such as custom-made firmware. T3E is an alternative for the deprecated sgx_get_trusted_time function, offering better security guarantees and a higher clock granularity.
T3E builds on the security properties of the TPM and SGX to ensure the clock’s monotonicity and authenticity. A trusted channel is set up between the TPM and the enclave. This approach is similar to the TimeSeal solution, where the time information originates from the PSE.
To mitigate delay attacks, T3E maintains a maximum use count for each set of time information received from the TPM. While TimeSeal considered a scheduling delay, it did not consider the possibility of the entire application thread being suspended, pausing the time progression altogether. S-FaaS can detect the pause attack, but it cannot determine the duration of the delay, making it impossible to use it as a reliable tick source for a clock. TrustedClock does not have this issue because the tick generator is completely isolated from the untrusted OS. However, a powerful adversary can still suspend the enclave execution right before the trusted time is used, allowing TOCTTOU (Time-of-Check to Time-of-Use) attacks. We summarize our comparison in Table 3 and Table 4.
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Hamidy, G.M., Philippaerts, P., Joosen, W. (2023). T3E: A Practical Solution to Trusted Time in Secure Enclaves. In: Li, S., Manulis, M., Miyaji, A. (eds) Network and System Security. NSS 2023. Lecture Notes in Computer Science, vol 13983. Springer, Cham. https://doi.org/10.1007/978-3-031-39828-5_17
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