Blockchain aggregators play an instrumental role in the evolution of blockchain technology, serving as pivotal enablers of interoperability, efficiency, and user accessibility in an increasingly decentralized digital world. However, the literature on this emerging technology is scarce and is not systematized, making it harder for practitioners and researchers to understand the field. In this paper, we systematize bridge aggregators, a type of blockchain aggregators. We present an exhaustive analysis of a diverse array of token and message aggregators, each distinguished by its unique architecture. Our research delves into critical aspects of these aggregators, encompassing their functionality, security measures, pricing models, and latency. This research aims to provide readers, users, and developers with insightful and actionable information, facilitating informed navigation through the complex landscape of blockchain aggregators. We explore our findings and compare them with our intuitive expectations. We show that there is a value in centralizing token aggregators. Message aggregators are found to be more powerful but less efficient in transaction cost and latency. Finally, we propose a set of future research directions for practitioners.

Recent years have witnessed significant advancements in cross-chain technology. However, the field faces two pressing challenges when it comes to security and privacy. Hacks on cross-chain bridges have led to monetary losses of around 3 billion USD, highlighting flaws in security models governing interoperability mechanisms (IMs) and the ineffectiveness of incident response frameworks. Additionally, the exploration of privacy within cross-chain scenarios remains relatively unexplored. Hackers benefit from extensive privacy, whereas users and bridge operators experience restricted privacy, thereby broadening the potential attack surface for adversaries. In this paper, we present the most comprehensive study to date on the security and privacy of blockchain interoperability. Our study employs a rigorous systematic literature review, yielding a corpus of 178 relevant documents, including 58 academic papers and 120 gray literature documents, out of a pool of 531. We systematically categorize 56 interoperability solutions based on a newly created taxonomy focusing on security and privacy considerations. Our dataset, comprising academic research, disclosures from bug bounty programs, and audit reports, exposes 45 cross-chain vulnerabilities, 25 theoretical attacks, and 88 mitigation strategies. Leveraging this data, we analyze 14 notable bridge hacks accounting for over USD 2.9 billion in losses, mapping them to the identified vulnerabilities. Our findings reveal that a substantial portion (65.8\%) of stolen funds originates from projects secured by permissioned intermediary networks with unsecured cryptographic key operations. Regarding privacy, we demonstrate that achieving unlinkability in cross-chain transactions is contingent on the underlying ledgers providing some form of confidentiality. In conclusion, our study offers critical insights into the challenges and vulnerabilities within the realm of cross-chain interoperability. We pinpoint promising directions for future research that can guide both industry practitioners and academics toward substantial advancements in this field. Our work underscores the urgency of enhancing security and privacy measures in cross-chain technology to mitigate the substantial financial risks associated with bridge hacks and to foster user trust in the blockchain ecosystem.

With the growing interest in blockchain technology, researchers and developers in different industries are shifting their attention to creating interoperability mechanisms. Existing mechanisms usually encompass asset exchanges, asset transfers, and general data transfers. However, most of the solutions based on these mechanisms only work for two permissionless blockchains falling short in use cases requiring more complex business relationships. Also, contrary to existing legacy systems, there is little standardization for cross-chain communication. Here we present MP-SATP, a resilient multi-party asset transfer protocol built on top of the Secure Asset Transfer Protocol (SATP). Furthermore, we enhance SATP’s crash recovery mechanism that directly influences the reliability and performance of our solution. Using MP-SATP, we show how to perform N-to-N resilient asset transfers in permissioned environments by decoupling them into multiple 1-to-1 asset transfers. Our results demonstrate that the latency of the protocol is driven by the latency of the slowest 1-to-1 session; and how the usage of backup gateways avoid the overhead caused by rollbacks. Enterprise-grade environments such as supply-chain management systems can immediately leverage our solution to perform atomic multi-party asset transfers as shown by our use case.

The last few years have seen a steep increase in blockchain interoperability research. Most solutions connect public blockchains; hence, the main cross-chain use case is token transfer. By-design platform transparency, tamper-resistance, and auditability make blockchains an infrastructure candidate for Central Bank Digital Currencies (CBDCs), but bridging CBDCs is an important missing piece in general. In this paper, we leverage an asset transfer protocol, ODAP/SATP, to define an extendable and dependable blockchain interoperability middleware that can bridge CBDC from Hyperledger Fabric to EVM-based permissioned blockchains. The key interoperation enabler in the solution is a shared asset definition enforced by both sides of the bridge, accompanied by a mapping between Fabric Identities and Ethereum addresses for Identity management. We implement our design for the CBDC use case utilizing Hyperledger Cactus. Through a preliminary performance evaluation, we show that the underlying ledgers heavily influence the latency of the solution, not the bridging components.