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 article, we propose 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 have implemented the PoTR and evaluated two distinct configurations of the proof, one tailored to estimate the average latency experience by clients and the other tailored to assess its variance. 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 prototypes 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 noise introduced by edge network delays, we show that the auditor is able to effectively detect SLA violations.

In this paper we propose a novel abstraction that we have named Range-Revocable Pseudonyms (RRPs). RRPs are a new class of pseudonyms whose validity can be revoked for any time-range within its original validity period. The key feature of RRPs is that the information provided to revoke a pseudonym for a given time-range cannot be linked with the information provided when using the pseudonym outside the revoked range. We provide an algorithm to implement RRPs using efficient cryptographic primitives where the space complexity of the pseudonym is constant, regardless of the granularity of the revocation range, and the space complexity of the revocation information only grows logarithmically with the granularity; this makes the use of RRPs far more efficient than the use of many short-lived pseudonyms. We have used RRPs to design EDGAR, an access control system for VANET scenarios that offers backward unlinkability. The experimental evaluation of EDGAR shows that, when using RRPs, the revocation can be performed efficiently (even when using time slots as small as 1 second) and that users can authenticate with low latency (0.5-3.5ms ).

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.

The edge computing paradigm extends cloud computing with storage and processing capacity close to the edge of the network, which can be materialized by using many fog nodes placed in multiple geographic locations. Fog nodes are likely to be vulnerable to tampering, so it is important to protect the functions they provide from attacks. A key building block of many distributed applications is an ordering service that keeps track of cause-effect dependencies among events and that allows events to be processed in an order that respects causality. This article presents the design and implementation of a secure event ordering service for fog nodes. Our service, named Omega, leverages the availability of a Trusted Execution Environment (TEE), based on SGX technology, to offer fog clients guarantees regarding the order in which events are applied and served, even when fog nodes are compromised. We have also built OmegaKV, a key-value store that uses Omega to offer causal consistency. Experimental results show that the ordering service can be secured without violating the latency constraints of time-sensitive edge applications, despite the overhead associated with using a TEE. Omega introduces an additional latency of approximately 4ms, that contrary to cloud based solutions, allows latency values in the 5ms-30ms range, as required by time-sensitive edge applications.

Data storage has been recognized as one of the key tasks for edge/fog computing infrastructures. Keeping replicas of data near the edge has many advantages, including allowing client to be served directly from the fog layer with lower latency and avoiding short-lived data to be shipped in its entirely to the cloud servers. In both cases, edge storage can offer significant bandwidth savings in traffic to and from the cloud datacenterse. However, replica placement on the edge is challenging for multiple reasons. First, objects can be updated by many sources, unlike in classic CDN networks where most updates are centralized. Second, different objects may have different consistency requirements. Third, the number of nodes and objects is very large, which precludes the use of centralized solutions. In this paper, we propose a replica placement algorithm for the edge, named Cathode, that addresses the challenges above. Cathode is decentralized and scalable, providing fast convergence, but also achieving high quality deployments. Furthermore, when making placement decision, it takes into account the data consistency protocol, considering both the cost of update and read operations, leading to different placements for different replica-consistency algorithms. The paper offers an extensive evaluation of Cathode and show that it outperforms previous state-of-the-art replica placement algorithms.

Edge computing is a paradigm that extends cloud computing with storage and processing capacity close to the edge of the network that can be materialized by using many fog nodes placed in multiple geographic locations. Fog nodes are likely to be vulnerable to tampering, so it is important to secure the functions they provide. A key building block of many distributed applications is an ordering service that keeps track of cause-effect dependencies among events and that allows events to be processed in an order that respects causality. In this paper we present the design and implementation of a secure event ordering service for fog nodes. Our service, named Omegae, leverages the availability of a Trusted Execution Environment (TEE) based on Intel SGX technology to offer fog clients guarantees regarding the order in which events are applied and served, even when fog nodes are compromised. We have also built OmegaKV, a key-value store that uses Omega e to offer causal consistency. Experimental results show that the ordering service can be secured without violating the latency constraints of time-sensitive edge applications, despite the overhead associated with using a TEE.