Taking part in surveys, experiments, and studies is often compensated by rewards to increase the number of participants and encourage attendance. While privacy requirements are usually considered for participation, privacy aspects of the reward procedure are mostly ignored. To this end, we introduce PrePaMS, an efficient participation management system that supports prerequisite checks and participation rewards in a privacy-preserving way. Our system organizes participations with potential (dis-)qualifying dependencies and enables secure reward payoffs. By leveraging a set of proven cryptographic primitives and mechanisms such as anonymous credentials and zero-knowledge proofs, participations are protected so that service providers and organizers cannot derive the identity of participants even within the reward process. In this paper, we have designed and implemented a prototype of PrePaMS to show its effectiveness and evaluated its performance under realistic workloads. PrePaMS covers the information whether subjects have participated in surveys, experiments, or studies. When combined with other secure solutions for the actual data collection within these events, PrePaMS can represent a cornerstone for more privacy-preserving empirical research.

Privacy-oriented cryptocurrencies, like Zcash or Monero, provide fair transaction anonymity and confidentiality, but lack important features compared to fully public systems, like Ethereum. Specifically, supporting assets of multiple types and providing a mechanism to atomically exchange them, which is critical for e.g. decentralized finance (DeFi), is challenging in the private setting. By combining insights and security properties from Zcash and SwapCT (PETS 21, an atomic swap system for Monero), we present a simple zk-SNARKs based transaction scheme, called Zswap, which is carefully malleable to allow the merging of transactions, while preserving anonymity. Our protocol enables multiple assets and atomic exchanges by making use of sparse homomorphic commitments with aggregated open randomness, together with Zcash friendly simulation-extractable non-interactive zero-knowledge (NIZK) proofs. This results in a provably secure privacypreserving transaction protocol, with efficient swaps, and overall performance close to that of existing deployed private cryptocurrencies. It is similar to Zcash Sapling and benefits from existing code-bases and implementation expertise.

Decentralized token exchanges allow for secure trading of tokens without a trusted third party. However, decentralization is mostly achieved at the expense of transaction privacy. For a fair exchange, transactions must remain private to hide the participants and volumes while maintaining the possibility for noninteractive execution of trades. In this paper we present a swap confidential transaction system (SwapCT) which is related to ring confidential transactions (e.g. used in Monero) but supports multiple token types to trade among and enables secure, partial transactions for noninteractive swaps. We prove that SwapCT is secure in a strict, formal model and present its efficient performance in a prototype implementation with logarithmic signature sizes for large anonymity sets. For our construction we design an aggregatable signature scheme which might be of independent interest. Our SwapCT system thereby enables a secure and private exchange for tokens without a trusted third party.

As use of cryptography increases in all areas of computing, efficient solutions for key management in distributed systems are needed. Large deployments in the cloud can require millions of keys for thousands of clients. The current approaches for serving keys are centralized components, which do not scale as desired. This work reports on the realization of a key manager that uses an untrusted distributed key-value store (KVS) and offers consistent key distribution over the Key-Management Interoperability Protocol (KMIP). To achieve confidentiality, it uses a key hierarchy where every key except a root key itself is encrypted by the respective parent key. The hierarchy also allows for key rotation and, ultimately, for secure deletion of data. The design permits key rotation to proceed concurrently with key-serving operations. A prototype was integrated with IBM Spectrum Scale, a highly scalable cluster file system, where it serves keys for file encryption. Linear scalability was achieved even under load from concurrent key updates. The implementation shows that the approach is viable, works as intended, and suitable for high-throughput key serving in cloud platforms.

Coded caching is a communications technique that has elevated the preemptive use of memory (caching) into a powerful ingredient in general communications networks, promising to change the way networking and PHY-based communications are conducted. At the same time though - because this approach is heavily dependent on cooperation between the content provider (CP), and a centralized powerful transmitter of information (ISP), and because it is heavily dependent on users caching a variety of content that is not their own - raises privacy concerns which have the potential to compromise the applicability of coded caching. What we are showing in this early work here, is that in fact coded caching carries a distinct set of salient features that in fact boost privacy. We present a step-by-step privacy-aware content-delivery protocol that utilizes caching and which - at a small cost in performance - can safeguard against unauthorized matching of users to their requests, as well as against unauthorized knowledge of the popularity statistics of files; both crucial privacy issues in different scenarios such as video on demand. These properties include multicasting-only transmissions for continuous obfuscation of the true destination of content, an almost seamless addition of phantom users that can skew the true popularity distribution, popularity-agnostic caches, cache-agnostic ISP, and an overall minimization of data traffic between CP and ISP, and between ISP and users.