Blockchain interoperability has become an essential functionality, which enables asset/data transfers across different blockchains. Sidechains have been deemed as a key technique to provide interoperability. However, sidechains are rarely used in practice, this is because sidechain technologies are impractical and non cost-efficient. To make sidechains practical, in this paper, we design a universal sidechain construction named USSC, which applies to a variety of blockchains without forking them. USSC also enables interoperability across heterogeneous blockchains regardless of underlying consensus. This is facilitated by three components: i) a committee selection method, ii) a cross-chain certificate, and iii) a cross-chain bridge based on smart contracts. The proposed committee-selection method guarantees an honest majority within a committee. Through a concrete implementation of USSC, we outline how the proof-of-stake (PoS) and the proof-of-work (PoW) blockchains enable asset transfers. Furthermore, our USSC is more storage-efficient because it produces a smaller size of certificate and only needs partial nodes instead of all sidechain nodes following a blockchain. Thus, USSC can reduce the overhead of storage and communication of nodes. In addition, we prove that USSC achieves a secure sidechain construction with desirable security properties. Finally, we develop a proof-of-concept implementation of USSC using Cardano and Ethereum. Experimental results demonstrate that USSC outperforms PoW and PoS sidechains, in terms of the certificate size.

Sidechains have been widely used to improve the interoperability and scalability of blockchain systems. Despite several interesting sidechain constructions have been proposed in the literature, they suffer from the following downsides: (1) their designs do not easily support pluggable consensus mechanisms, and (2) their communication and storage costs for cross-chain operations are not yet optimized. In this work, we first propose Ge-Co, a generic sidechain construction to realize secure asset transfers between blockchains, supporting different consensus algorithms, such as Proof-of-Stake (PoS) and Proof-of-Work (PoW). Our design is built on top of the proposed voting committee selection approach and threshold signature schemes (TSS) and meanwhile, it achieves optimally succinct and constant proof size, only yielding lightweight communication and storage costs. Ge-Co works in the semi-adaptive corruption model. To provide stronger security, we further propose PoS-Co, a PoS-based sidechain construction in the fully-adaptive corruption model. PoS-Co is based on the proposed anonymous committee selection approach, and preserves optimally succinct proof. We also formally prove that Ge-Co can achieve the security properties of atomicity and timeliness. Finally, we develop a proof-of-concept (PoC) implementation for Ge-Co, and the results demonstrate that the design is efficient and practical.

Abstract With the rapid development of blockchains, blockchain systems are moving on from a stand-alone manner to cross-chain interactions, and achieving interoperability is emerging as one of the essential features of blockchains. Unfortunately, existing mechanisms such as XCLAIM mostly focus on exchanging assets between two blockchains and it is slow and expensive to process each cross-chain trade among more than two blockchains as multiple transactions are required. In this paper, we present Interopera, a decentralized and efficient cross-chain trading protocol among two or more blockchains. Interopera atomically processes each cross-chain trade faster and more cheaply with fewer transactions by a two-phase lock/unlock process. Interopera also achieves efficient cross-chain communication by our presented Partitioned-FlyClient and Tx-FlyClient. Partitioned-FlyClient is based on FlyClient but more efficient with smaller proof size, reducing the storage and bandwidth overheads. Tx-FlyClient maintains efficiency even when cross-chain trades become frequent, instead of other mechanisms only being effective under low cross-chain trades volumes. We also develop a proof-of-concept implementation and the results demonstrate high efficiency of our protocol.

Blockchain technologies have drawn a lot of attentions, and its immutability is paramount to applications requiring persistent records. However, tremendous real-world incidents have exposed the harm of strict immutability, such as the illicit data stored on Bitcoin and the loss of millions of dollars in vulnerable smart contracts. Moreover, “Right to be Forgotten” has been imposed in new General Data Protection Regulation (GDPR) of European Union, which is incompatible with blockchain's immutability. Therefore, it is imperative to design efficient redactable blockchain in a controlled way. In this paper, we present a generic design of redactable blockchain protocols in the permissionless setting, applied to both proof-of-stake and proof-of-work blockchains. Our protocol can (1) maintain the same adversary bound requirement as the underlying blockchain, (2) support various network environments, (3) offer public verifiability for any redaction, and (4) achieve instant redaction, even only within one slot in the best case, which is desirable for redacting harmful data. Furthermore, we define the first ideal protocol of redactable blockchain and conduct security analysis following the language of universal composition. Finally, we develop a proof-of-concept implementation showing that the overhead remains minimal for both online and re-spawning nodes, which demonstrates the high efficiency of our design.

With the rapid evolution of the blockchain technologies, the interoperability of different blockchain systems is emerging as one of the essential features of blockchains. Sidechains, a mechanism providing communications between different blockchains, have been heralded as the crucial factor of blockchain interoperability. However, there are still issues that need to be addressed in terms of security and feasibility. In this article, for proof-of-stake (PoS) and proof-of-work (PoW) blockchains, we propose efficient sidechain constructions with fast cross-chain transfers and small proof size by novel cross-chain certificate generation process and committee selection methods. Moreover, we also provide an extra functionality of supporting instant cross-chain transfers, such that emergent cross-chain transactions can be processed immediately. Compared to prior sidechains, our PoS sidechain construction can achieve faster cross-chain transfers, which improves the promptness of cross-chain transfers. While our PoW sidechain construction is more efficient with smaller proof size, reducing the storage and bandwidth overhead. Furthermore, we formally prove our sidechain constructions satisfying the properties of atomicity and timeliness. Finally, we develop a proof-of-concept implementation of our sidechains, and the experimental results show our constructions is not only faster, but also efficient with low storage and bandwidth overhead.