# A Cross-Chain-Based Access Control Framework for Cloud Environment
## 1 Introduction  
The growth of diverse, well-known businesses today leans on cloud computing, permitting clients to access their tailored requested services with ease. Cloud computing can offer on-demand access to resources and can handle vast amounts of shared data over distributed servers in the network. The security of cloud computing is based on several aspects [1], such as data security, network security, threats related to the cloud infrastructure, and access control (AC) mechanisms [2]. The AC, a fundamental pillar in cloud security, directs the process in which a subject (a cloud client) has authorization to execute privileges on an object (cloud data or resources). It takes into consideration the data holder or service provider access policies that the request has to satisfy, intending to prevent unauthorized access, disclosure, or modification of data and resources. Conventional AC mechanisms for cloud computing--mandatory access control (MAC), discretionary access control (DAC), role-based access control (RBAC), attribute-based access control (ABAC), attribute-based access encryption (ABE), and federated identity management (FIM)--grant access control functionalities to cloud environments, but they embody a multitude of shortcomings [3]. MAChas flexibility issues and a fixed count of users. DAC has scalability and maintenance problems. RBAC is prone to errors. ABAC is complex to maintain, and FIM has trust management issues. Blockchain (BC) gained popularity with the development of cryptocurrencies, such as Bitcoin, and the evolution of web technology with version 3.0 [4]. In this new era, BC is considered a basic infrastructure for the development of decentralized applications and smart contracts. Thus, numerous industrial solutions (Ethereum, Cosmos, Avalanche, etc.) have emerged [5], using different approaches for managing and maintaining their ledgers. These solutions endorse their own coins and provide tamper evidence by linking blocks of signed transactions, using consensus mechanisms for validation and cryptographic techniques for robust security. However, we can extend this promising technology to a multitude of sectors [6]. It grants traceability, transparency, and auditability, ensuring that no entity has total control over the blockchain ledger. Thus, researchers have adopted BC technology, leveraging its strengths to propose access control mechanisms that tackle the shortcoming of the traditional ACs mentioned above. The stated blockchain-based solutions operate autonomously. Thus, cross-chain technology, with its mechanisms and protocols, comes to allow communication within BC ecosystems. In a basic scenario, it involves two blockchains: one serving as the transaction's source and the other as the target, with the source BC initiating the transaction for execution on the target BC without the need for intermediaries. In recent years, researchers have utilized BC technology to provide more advanced control mechanisms for the cloud. In certain cases, researchers have utilized smart contracts in conjunction with third-party key management authorities and other data transport solutions, such as the interplanetary file system (IPFS). However, leaning on certification authorities represents a single point of failure for the proposed solutions, as the other peer-to-peer (P2P) propositions have BC communication interoperability issues. Therefore, in our contribution, we aim to address this research gap. * We are introducing an access control framework for the cloud, which incorporates a BC-based, fine-grained ABAC mechanism. Cross-chain technology is used to connect the front- and back-end cloud as BC zones, thereby minimizing the attack surface on cloud assets. * We also enhanced the ABAC mechanism with semantic reasoning based on ontologies. This combination gives our framework a higher level of adaptivity to any cloud service consumer (CSC) business needs.