As blockchain technology advances, an increasing number of applications require interactions between smart contracts across multiple blockchains. However, existing cross-chain solutions exhibit limited scalability due to heterogeneous blockchain environments and diverse application requirements. A fundamental challenge lies in the absence of a unified resource definition for cross-chain processes, impeding moderate resource allocation and effective conflict resolution. Specifically, when extended to general cross-chain transactions involving invocations among multiple contracts, these methods lack the capability to correctly handle state transitions for all related contracts. This paper proposes AtomXross, a novel cross-chain scheme that supports arbitrary combinations of smart contracts during the cross-chain process. We build a scalable cross-chain architecture based on a relay chain and a cluster of cross-chain nodes to provide better scalability. We propose a unified definition for cross-chain resources within the system and implement an adaptive resource management mechanism on the relay chain, enabling it to record the invocation relationships of contract functions. When a cross-chain transaction involves multiple contract calls, AtomXross can index the calls and generate the corresponding call tree. To address the challenges posed by potential mutual invocations between smart contracts, we design an atomic transaction protocol based on buckle-lock, an ordered two-tier pessimistic locking mechanism. AtomXross ensures that cross-chain transactions do not conflict with each other while remaining compatible with non-cross-chain calls that may occur at any time. Furthermore, we propose a universal programming template for on-chain smart contracts, which enables ordinary smart contracts to acquire cross-chain capabilities. We implement AtomXross based on Hyperledger Fabric and FiscoBCOS. In comparison to WeCross, AtomXross lowers the gas cost on system initialization and incurs only a 14% increase in transaction latency while supporting inter-contract calls.

Difficulty adjustment algorithm (DAA) is critical for PoW blockchain systems in currency issuance, transaction throughput, and blockchain security. Most DAAs concentrate on the unique goal of block-making stability from seconds to minutes. In contrast, we propose a new DAA with the goal of stabilizing orphan rates among the blocks made recently, observing that the orphan rate is more indicative since too high orphan rate indicates poor blockchain security and waste of honest hash power whereas too low orphan rate indicates poor blockchain throughout and wast of time awaiting other blocks. We notice a security defect of the recent orphan-rate DAA (NDSS 2022) in which an attacker may continually create orphan blocks and mislead miners to wrongly increase the difficulty. To fix this defect, our enhanced DAA takes into the orphan rate variation during the historical slots with smooth exponential decay affection on current difficulty adjustment. Theoretical and experimental analyses show that our DAA is effective and secure.