In order to propagate transactions and blocks, today’s blockchain systems rely on unstructured peer-to-peer overlay networks. In such networks, broadcast is known to be an inefficient operation in terms of message complexity and overhead. In addition to the impact on the system performance, inefficient or delayed block propagation may have severe consequences regarding security and fairness of the consensus layer. In contrast, the Kadcast protocol is a structured peer-to-peer protocol for block and transaction propagation in blockchain networks. Kadcast utilizes the well-known overlay topology of Kademlia to realize an efficient broadcast operation with tunable overhead. We study the security and privacy of the Kadcast protocol based on probabilistic models and analyze its resilience to packet losses and node failures. Moreover, we evaluate Kadcast’s block delivery performance, broadcast reliability, efficiency, and security based on advanced network simulations. Lastly, we introduce a QUIC-based prototype implementation of the Kadcast protocol and show its merits through deployment in a global-scale cloud-based testbed.

In recent years, research has shown the networking layer’s significant influence on the scalability, security, and privacy of blockchain systems. Such large-scale networks however exhibit a degree of complexity that demands model-based simulations as real-world experiments are often not possible. In this work, we methodically characterize blockchain networks by reference to the paradigmatic Bitcoin peer-to-peer network, explore the state-of-the-art protocols, and emphasize this key design space. To this end, we conducted a longitudinal measurement study on the Bitcoin network, from which we extract a comprehensive network model and implement it as part of the bns network simulation framework. We validate the model in comparison to real-world measurements as well as to results from related work. Moreover, we experimentally show how network utilization and miners’ geographical location impact the block propagation characteristics.

The Lightning Network is a scaling solution for Bitcoin that promises to enable rapid and private payment processing. In Lightning, multi-hop payments are secured by utilizing Hashed Time-Locked Contracts (HTLCs) and encrypted on the network layer by an onion routing scheme to avoid information leakage to intermediate nodes. In this work, we however show that the privacy guarantees of the Lightning Network may be subverted by an on-path adversary conducting timing attacks on the HTLC state negotiation messages. To this end, we provide estimators that enable an adversary to reduce the anonymity set and infer the likeliest payment endpoints. We developed a proof-of-concept measurement node that shows the feasibility of attaining time differences and evaluate the adversarial success in model-based network simulations. We find that controlling a small number of malicious nodes is sufficient to observe a large share of all payments, emphasizing the relevance of the on-path adversary model. Moreover, we show that adversaries of different magnitudes could employ timing-based attacks to deanonymize payment endpoints with high precision and recall.

As online sources are becoming more prevalent in journalism and scientific literature, the ephemeral nature of the World Wide Web is becoming an increasingly serious issue for their verifiability, replicability, and reproducibility. The architecture of Webchain, a new system enabling source and reference verifiability on the Web, is combining distributed ledger technologies with secure timestamping to ensure the history of creation, ownership, and referential integrity of online resources. We present the architecture and system extensions, conduct a security analysis, and evaluate the Webchain system based on a comprehensive prototype implementation. The results confirm the feasibility and robustness of our approach.

In order to propagate transactions and blocks, today's blockchain systems rely on unstructured peer-to-peer overlay networks. In such networks, broadcast is known to be an inefficient operation in terms of message complexity and overhead. In addition to the impact on the system performance, inefficient or delayed block propagation may have severe consequences regarding security and fairness of the consensus layer. Therefore, we introduce Kadcast, a novel peer-to-peer protocol for block propagation in blockchain networks. Kadcast utilizes the well-known structured overlay topology of Kademlia to realize an efficient broadcast operation with tunable overhead. As our protocol is based on UDP, we incorporate forward error correction (FEC) to increase reliability while still maintaining its lightweight protocol architecture. To this end, we build a probabilistic model to analyze Kadcast's resilience to packet losses and node failures. Moreover, we evaluate Kadcast's block delivery performance, broadcast reliability, efficiency, and security based on advanced network simulations, which confirm the merits of the Kadcast protocol.

Zcash is a privacy-preserving cryptocurrency that provides anonymous monetary transactions. While Zcash's anonymity is part of a rigorous scientific discussion, information on the underlying peer-to-peer network are missing. In this paper, we provide the first long-term measurement study of the Zcash network to capture key metrics such as the network size and node distribution as well as deeper insights on the centralization of the network. Furthermore, we present an inference method based on a timing analysis of block arrivals that we use to determine interconnections of nodes. We evaluate and verify our method through simulations and real-world experiments, yielding a precision of 50% with a recall of 82% in the real-world scenario. By adjusting the parameters, the topology inference model is adaptable to the conditions found in other cryptocurrencies and therefore also contributes to the broader discussion of topology hiding in general.