Token payment systems were the first application of blockchain technology and are still the most widely used one. Early implementations of such systems, like Bitcoin or Ethereum, provide virtually no privacy beyond basic pseudonymity: all transactions are written in plain to the blockchain, which makes them linkable and traceable.

Fabric is a modular and extensible open-source system for deploying and operating permissioned blockchains and one of the Hyperledger projects hosted by the Linux Foundation (www.hyperledger.org).

Cloud computing is a dominant trend in computing for the foreseeable future; e.g., major cloud operators are now estimated to house over a million machines each and to host substantial (and growing) fractions of our IT and web infrastructure. CCSW is a forum for bringing together researchers and practitioners to discuss the implications of this trend to the security of cloud operators, tenants, and the larger Internet community. CCSW welcomes submissions on new threats, countermeasures, and opportunities brought about by the move to cloud computing, with a preference for unconventional approaches, as well as measurement studies and case studies that shed light on the security implications of clouds.

Bitcoin is a decentralized payment system that relies on Proof-of-Work (PoW) to resist double-spending through a distributed timestamping service. To ensure the operation and security of Bitcoin, it is essential that all transactions and their order of execution are available to all Bitcoin users. Unavoidably, in such a setting, the security of transactions comes at odds with transaction privacy. Motivated by the fact that transaction confirmation in Bitcoin requires tens of minutes, we analyze the conditions for performing successful double-spending attacks against fast payments in Bitcoin, where the time between the exchange of currency and goods is short (in the order of a minute). We show that unless new detection techniques are integrated in the Bitcoin implementation, double-spending attacks on fast payments succeed with considerable probability and can be mounted at low cost. We propose a new and lightweight countermeasure that enables the detection of double-spending attacks in fast transactions. In light of such misbehavior, accountability becomes crucial. We show that in the specific case of Bitcoin, accountability complements privacy. To illustrate this tension, we provide accountability and privacy definition for Bitcoin, and we investigate analytically and empirically the privacy and accountability provisions in Bitcoin.

Recent incidents of data-breaches from the cloud suggest that users should not trust the cloud provider to enforce access control on their data. We focus on mitigating trust to the cloud in scenarios where granting access to data not only considers user identities (as in conventional access policies), but also contextual information such as the user's location and time of access. Previous work in this context assumes a fully trusted cloud that is further capable of locating users. We introduce LoTAC, a novel framework that seamlessly integrates the operation of a cloud provider and a localization infrastructure to enforce location- and time-based access control to cloud-stored data. In LoTAC, the two entities operate independently and are only trusted to offer their basic services: the cloud provider is used and trusted only to reliably store data, the localization infrastructure is used and trusted only to accurately locate users. Furthermore, neither the cloud provider nor the localization infrastructure can access the data, even if they collude. LoTAC protocols require no changes to the cloud provider and minimal changes to the localization infrastructure. We evaluate our protocols using a cellular network as the localization infrastructure and show that they incur in low communication and computation costs and scale well with a large number of users and policies.

Bitcoin is a decentralized payment system that relies on Proof-of-Work (PoW) to verify payments. Nowadays, Bitcoin is increasingly used in a number of fast payment scenarios, where the time between the exchange of currency and goods is short (in the order of few seconds). While the Bitcoin payment verification scheme is designed to prevent double-spending, our results show that the system requires tens of minutes to verify a transaction and is therefore inappropriate for fast payments. An example of this use of Bitcoin was recently reported in the media: Bitcoins were used as a form of \emph{fast} payment in a local fast-food restaurant. Until now, the security of fast Bitcoin payments has not been studied. In this paper, we analyze the security of using Bitcoin for fast payments. We show that, unless appropriate detection techniques are integrated in the current Bitcoin implementation, double-spending attacks on fast payments succeed with overwhelming probability and can be mounted at low cost. We further show that the measures recommended by Bitcoin developers for the use of Bitcoin in fast payments are not always effective in detecting double-spending; we show that if those recommendations are integrated in future Bitcoin implementations, double-spending attacks on Bitcoin will still be possible. Finally, we propose and implement a modification to the existing Bitcoin implementation that ensures the detection of double-spending attacks against fast payments.