PoTR: Accurate and Efficient Proof of Timely-Retrievability for Storage Systems

The use of remote storage has become prevalent both by organizations and individuals. By relying on third-party storage, such as cloud or peer-to-peer storage services, availability, fault tolerance, and low access latency can be attained in a cost-efficient manner. Unfortunately, storage providers may misbehave and violate Service-Level Agreements (SLAs). In this paper, we propose, implement and evaluate a new Proof of Timely-Retrievability (PoTR) that aims at assessing whether a provider is able to retrieve data objects with a latency lower than some SLA-specific threshold δ. We leverage Trusted Execution Environments (e.g., Intel SGX) to ensure that the proof is produced by the node being audited and to reduce the communication between the auditor and the audited node. We have experimentally evaluated our design considering a challenging edge computing setting, where storage services are provided by resource-constrained fog nodes, and the distance between the auditor and the audited node can be large. Despite the variance in edge network delays, we show that the auditor is able to effectively detect SLA violations.