Decentralization in open quorum systems: Limitative results for ripple and stellar

Decentralisation is one of the promises introduced by blockchain technologies: fair and secure interaction amongst peers with no dominant positions, single points of failure or censorship. Decentralisation, however, appears difficult to be formally defined, possibly a continuum property of systems that can be more or less decentralised, or can tend to decentralisation in their lifetime. In this paper we focus on decentralisation in quorum-based approaches to open (permissionless) consensus as illustrated in influential protocols such as the Ripple and Stellar protocols. Drawing from game theory and computational complexity, we establish limiting results concerning the decentralisation vs. safety trade-off in Ripple and Stellar, and we propose a novel methodology to formalise and quantitatively analyse decentralisation in this type of blockchains

Evolução do grau de eficiência do mercado de moedas criptográficas de 2014 a 2020: uma análise baseada em seus componentes fractais

Objective: This study aims to analyze the evolution of the cryptocurrency market efficiency         based on fractal aspects of the historical price series of 15 cryptocurrencies and a benchmark developed for this market (CRIX). Methodology: The proposed analyses start from the efficiency index proposed by Kristoufek and Vosvrda (2013), which captures long-and short-term memory biases as well as first-order autocorrelation. The database covers the period from 08/02/2014 to 12/31/2020. Using structural breakout analysis for time series, it was possible to divide the sample into five periods of analysis, and the efficiency index was calculated for each one. Findings: It was identified the existence of oscillations between the efficiency indexes over the analyzed periods, verifying a greater inefficiency at times of market upswing. In addition, it can be seen that in general this market has been gaining efficiency over the years, although it has not yet reached the absence of inefficiency. This conclusion corroborates studies on the adaptation of market efficiency based on its investors and agents. Finally, one can characterize the current scenario as a speculative bubble, which, due to the presence of the herd effect, enables the existence of arbitrage. Originality: The research in this area is still recent, as it is a new financial segment, so there are several doubts and gaps in the literature. In this sense, the adoption of a longitudinal approach to identify the evolution of efficiency of this market is not only interesting but it is also an approach little explored by the literature.Objetivo: Este estudo visa analisar a evolução da eficiência do mercado criptoativos com base em aspectos fractais da série histórica de preços de 15 criptomoedas e um índice de referência desenvolvido para este mercado (CRIX). Metodologia: As análises propostas partem do índice de eficiência proposto por Kristoufek e Vosvrda (2013), que captura os vieses de memória de longo e curto prazo, bem como a autocorrelação de primeira ordem. O banco de dados cobre o período de 02/08/2014 a 31/12/2020. Usando a análise de quebra estrutural para séries temporais, foi possível dividir a amostra em cinco períodos de análise, e o índice de eficiência foi calculado para cada um deles. Resultados: Foi identificada a existência de oscilações entre os índices de eficiência ao longo dos períodos analisados, verificando uma maior ineficiência em momentos de ascensão do mercado. Além disso, pode-se observar que, em geral, este mercado vem ganhando eficiência ao longo dos anos, embora ainda não tenha alcançado a ausência de ineficiência. Esta conclusão corrobora os estudos sobre a adaptação da eficiência do mercado com base em seus investidores e agentes. Finalmente, pode-se caracterizar o cenário atual como uma bolha especulativa, o que, devido à presença do efeito de manada, permite a existência de arbitragem. Originalidade: Pesquisas nesta área ainda são recentes, pois se trata de um novo segmento financeiro, portanto existem várias dúvidas e lacunas na literatura. Neste sentido, a adoção de uma abordagem longitudinal para identificar a evolução da eficiência deste mercado não só é interessante como também é uma abordagem pouco explorada pela literatura

Open Heterogeneous Quorum Systems

In contrast to proof-of-work replication, Byzantine replicated systems maintain consistency with higher throughput, modest energy consumption, and deterministic liveness guarantees. If complemented with open membership and heterogeneous trust, they have the potential to serve as a global financial infrastructure. This paper presents a general model of heterogeneous quorum systems, where each participant can declare its own quorums, and captures the consistency, availability, and inclusion properties of these systems. In order to support open membership, it then presents reconfiguration protocols for heterogeneous quorum systems: joining and leaving of a process, and adding and removing of a quorum. It presents trade-offs for the properties that reconfigurations can preserve, and accordingly, presents reconfiguration protocols and proves their correctness. It further presents a graph characterization of heterogeneous quorum systems, and its application for reconfiguration optimization

Decentralization in Open Quorum Systems: Limitative Results for Ripple and Stellar

Decentralisation is one of the promises introduced by blockchain technologies: fair and secure interaction amongst peers with no dominant positions, single points of failure or censorship. Decentralisation, however, appears difficult to be formally defined, possibly a continuum property of systems that can be more or less decentralised, or can tend to decentralisation in their lifetime. In this paper we focus on decentralisation in quorum-based approaches to open (permissionless) consensus as illustrated in influential protocols such as the Ripple and Stellar protocols. Drawing from game theory and computational complexity, we establish limiting results concerning the decentralisation vs. safety trade-off in Ripple and Stellar, and we propose a novel methodology to formalise and quantitatively analyse decentralisation in this type of blockchains