Consensus encalpsulates the inherent problems of building fault tolerant distributed systems. In this context, the classic model of Byzantine faulty processes can be restated such that messages from a subset of processes can be arbitrarily corrupted (including addition and omission of messages).We consider the case of dynamic and transient faults, that may affect all processes and that are not permanent, and we model them via corrupted communication. For corrupted communication it is natural to distinguish between the safety of communication, which is concerned with the number of altered messages, and the liveness of communication, which restricts message loss.We present two consensus algorithms, together with sufficient conditions on the system to ensure correctness. Our first algorithm needs strong conditions on safety but requires weak conditions on liveness in order to terminate. Our second algorithm tolerates a lower degree of communication safety at the price of stronger liveness conditions.Our algorithms allow us to circumvent the resilience lower bounds from Santoro/Widmayer and Martin/Alvisi.

Consensus is one of the key problems in fault tolerant distributed computing. A very popular model for solving consensus is the failure detector model defined by Chandra and Toueg. However, the failure detector model has limitations. The paper points out these limitations, and suggests instead a model based on communication predicates, called HO model. The advantage of the HO model over failure detectors is shown, and the implementation of the HO model is discussed in the context of a system that alternates between good periods and bad periods. Two definitions of a good period are considered. For both definitions, the HO model allows us to compute the duration of a good period for solving consensus. Specifically, the model allows us to quantify the difference between the required length of an initial good period and the length of a non initial good period.

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In the area of the Internet of Things, cloud-based camera surveillance systems are ubiquitously available for industrial and private environments. However, the sensitive nature of the surveillance use case imposes high requirements on privacy/confidentiality, authenticity, and availability of such systems. In this work, we investigate how currently available mass-market camera systems comply with these requirements. Considering two attacker models, we test the cameras for weaknesses and analyze for their implications. We reverse-engineered the security implementation and discovered several vulnerabilities in every tested system. These weaknesses impair the users' privacy and, as a consequence, may also damage the camera system manufacturer's reputation. We demonstrate how an attacker can exploit these vulnerabilities to blackmail users and companies by denialof-service attacks, injecting forged video streams, and by eavesdropping private video data -even without physical access to the device. Our analysis shows that current systems lack in practice the necessary care when implementing security for IoT devices.

Abstract. The paper considers the consensus problem in a partially synchronous system with Byzantine processes. In this context, the literature distinguishes authenticated Byzantine faults, where messages can be signed by the sending process (with the assumption that the signature cannot be forged by any other process), and Byzantine faults, where there is no mechanism for signatures (but the receiver of a message knows the identity of the sender). The paper proposes an abstraction called weak interactive consistency (WIC ) that unifies consensus algorithms with and without signed messages. WIC can be implemented with and without signatures.The power of WIC is illustrated on two seminal Byzantine consensus algorithms: the Castro-Liskov PBFT algorithm (no signatures) and the Martin-Alvisi FaB Paxos algorithms (signatures). WIC allows a very concise expression of these two algorithms.

Aguilera et al. and Malkhi et al. presented two system models, which are weaker than all previously proposed models where the eventual leader election oracle can be implemented, and thus, consensus can also be solved. The former model assumes unicast steps and at least one correct process with f outgoing eventually timely links, whereas the latter assumes broadcast steps and at least one correct process with f bidirectional but moving eventually timely links. Consequently, those models are incomparable. In this paper, we show that can also be implemented in a system with at least one process with f outgoing moving eventually timely links, assuming either unicast or broadcast steps. It seems to be the weakest system model that allows to solve consensus via -based algorithms known so far. We also provide matching lower bounds for the communication complexity of in this model, which are based on an interesting "stabilization property" of infinite runs. Those results reveal a fairly high price to be paid for this further relaxation of synchrony properties.

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