The recent outbreak of COVID-19 has taken the world by surprise, forcing lockdowns and straining public health care systems. COVID-19 is known to be a highly infectious virus, and infected individuals do not initially exhibit symptoms, while some remain asymptomatic. Thus, a non-negligible fraction of the population can, at any given time, be a hidden source of transmissions. In response, many governments have shown great interest in smartphone contact tracing apps that help automate the difficult task of tracing all recent contacts of newly identified infected individuals. However, tracing apps have generated much discussion around their key attributes, including system architecture, data management, privacy, security, proximity estimation, and attack vulnerability. In this article, we provide the first comprehensive review of these much-discussed tracing app attributes. We also present an overview of many proposed tracing app examples, some of which have been deployed countrywide, and discuss the concerns users have reported regarding their usage. We close by outlining potential research directions for next-generation app design, which would facilitate improved tracing and security performance, as well as wide adoption by the population at large.

Bitcoin is a popular cryptocurrency that records all transactions in a distributed append-only public ledger called blockchain. The security of Bitcoin heavily relies on the incentivecompatible proof-of-work (PoW) based distributed consensus protocol, which is run by network nodes called miners. In exchange for the incentive, the miners are expected to honestly maintain the blockchain. Since its launch in 2009, Bitcoin economy has grown at an enormous rate, and it is now worth about 170 billions of dollars. This exponential growth in the market value of Bitcoin motivates adversaries to exploit weaknesses for profit, and researchers to discover new vulnerabilities in the system, propose countermeasures, and predict upcoming trends.In this paper, we present a systematic survey that covers the security and privacy aspects of Bitcoin. We start by presenting an overview of the Bitcoin protocol and its major components along with their functionality and interactions within the system. We review the existing vulnerabilities in Bitcoin and its underlying major technologies such as blockchain and PoW based consensus protocol. These vulnerabilities lead to the execution of various security threats to the normal functionality of Bitcoin. We then discuss the feasibility and robustness of the state-of-theart security solutions. Additionally, we present current privacy and anonymity considerations in Bitcoin and discuss the privacyrelated threats to Bitcoin users along with the analysis of the existing privacy-preserving solutions. Finally, we summarize the critical open challenges and suggest directions for future research towards provisioning stringent security and privacy techniques for Bitcoin.

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