As Distributed Ledger Technologies (DLTs) mature, the inherent performance and scalability shortcomings of linearly structured blockchain designs become better understood. Protocols based on Directed Acyclic Graphs (DAGs) have been proposed to address such shortcomings. DAG-based protocols differ from traditional DLTs in the way they build and represent knowledge about transactions and relations between them. While traditional DLTs have straightforward homogeneous semantic attached to blocks and links between blocks, the semantic of vertices and edges in DAG-based protocols is nuanced and varied. In this work, we identify almost a dozen of knowledge-building dimensions in DAG-based DLTs, none of which have been studied before. Knowledge-building is important in DAG-based DLTs because of its significant impact on the size of the DAG, the pace at which new transactions are added, the finality of transactions, and so on. We analyze 40 DAG-based DLTs from this perspective, summarize our results in a taxonomy, and identify a number of research gaps.

Systems managing Verifiable Credentials are becoming increasingly popular. Unfortunately, their support for revoking previously issued credentials allows verifiers to effectively monitor the validity of the credentials, which is sensitive information. While the issue started to gain recognition, no adequate solution has been proposed so far. In this work, we propose a novel framework for time-limited continuous verification. The holder is able to individually configure the verification period when sharing information with the verifier, and the system guarantees proven untraceability of the revocation status after the verification period expires. Differently from existing systems, the implementation adopts a more scalable blacklist approach where tokens corresponding to revoked credentials are stored in the registry. The approach employs ZK proofs that allow holders to prove non-membership in the blacklist. In addition to theoretically proving security, we evaluate the approach analytically and experimentally and show that it significantly improves bandwidth consumption on the holder while being on par with state-of-the-art solutions with respect to the other performance metrics.

This paper is a Systematization of Knowledge (SoK) that focuses on Directed Acyclic Graph (DAG)-based consensus protocols in Distributed Ledger Technologies (DLTs). Our study evaluates their impact on performance and their tradeoffs concerning consistency, availability, and partition tolerance, as postulated by the CAP theorem. We delineate the key functionalities and tradeoffs of DAGbased consensus protocols, highlighting iterative improvements and deviations from foundational models. Additionally, we identify research gaps and suggest directions for future work to refine DAG-based consensus mechanisms.

Efficient data storage and query processing systems play a vital role in many different research areas. Blockchain technology and distributed ledgers attract massive attention and trigger multiple projects in various industries. Nevertheless, blockchain still lacks the features of a Database Management System (DBMS or simply databases), such as high throughput, low latency, and high capacity. For that purpose, there have been many proposed approaches for handling data storage and query processing solutions in the blockchain. This paper presents a complete overview of many different DBMS types and how these systems can be used to implement, enhance, and further improve blockchain technology. More concretely, we give an overview of 10 transactional, an extensive overview of 14 analytical, 9 hybrids, i.e., translytical, and 13 blockchain DBMSs. We explain how database technology has influenced the development of blockchain technology by unlocking different features, such as Atomicity, Consistency, Isolation, and Durability (ACID), transaction consistency, rich queries, real-time analysis, and low latency. Using a relaxation approach analogous to the one used to prove the Consistency, Availability, Partition tolerance (CAP)-theorem, we postulate a “Decentralization, Consistency, and Scalability (DCS)-satisfiability conjecture” and give concrete strategies for achieving the relaxed DCS conditions. We also provide an overview of the different DBMSs, emphasizing their architecture, storage manager, query processing, and implementation.