###### Abstract  
Although there are several access control systems in the literature for flexible policy management in multi-authority and multi-domain environments, achieving interoperability and scalability, without relying on strong trust assumptions, is still an open challenge. We present HMBAC, a distributed fine-grained access control model for shared and dynamic multi-authority and multi-domain environments, along with Janus, a practical system for HMBAC policy enforcement. The proposed HMBAC model supports: (a) dynamic trust management between different authorities; (b) flexible access control policy enforcement, defined at the domain and cross-domain level; (c) a global source of truth for all entities, supported by an immutable, audit-friendly mechanism. Janus implements the HMBAC model and relies on the effective fusion of two core components. First, a _Hierarchical Multi-Blockchain_ architecture that acts as a single access point that cannot be bypassed by users or authorities. Second, a _Multi-Authority Attribute-Based Encryption_ protocol that supports flexible shared multi-owner encryption, where attribute keys from different authorities are combined to decrypt data distributedly stored in different authorities. Our approach was implemented using Hyperledger Fabric as the underlying blockchain, with the system components placed in Kubernetes Docker container pools. We experimentally validated the effectiveness and efficiency of Janus, while fully reproducible artifacts of both our implementation and our measurements are provided. access control; blockchain; multi-blockchain; multi-authority; multi-domain; Attribute-Based Encryption 2022 2  
administators for connected medical devices. In these ecosystems, _granular access_ to data and _interoperability_ of services are two basic prerequisites that greatly impact the privacy and security aspects of these systems. Traditionally, access control systems have been implemented within enterprises. The initial methods for domain-specific access control were based on centralized cloud solutions [2]. Such solutions require centralized data centers to store and handle various types of data, including user identities, cryptographic keys, and access rights, as well as a trusted cloud system administration to control the user's access rights and authorization. Although these systems offer interoperability, fundamental challenges of data security and management persisted. Specifically, there are three major concerns with this design approach. First, an attack on the centralized data center might result in a single point of failure, leading to massive data compromise [3]. Second, strong trust assumptions persist for the cloud infrastructure [4], e.g., cloud administrators are trusted not to abuse their position to gain unauthorized access to resources or interfere with the access privileges of legitimate users. Finally, it is not possible to maintain a globally trusted and immutable access log to allow auditing for possible data access privacy violations. **Motivation. ** As a result of the preceding discussion, more emphasis is required on innovative, decentralized, interoperable and adaptable trust management systems. The following are two motivating examples. _Example 1: Fine-grained access to healthcare data._ The medical sector includes a variety of domains, such as regulators, hospitals, manufacturers and insurance providers. At the same time, privacy-sensitive, health-related data may be created by various medical IoT devices, such as health monitoring devices (e.g., glucose level, blood pressure, or sleep monitoring systems), or treatment devices (e.g., medical infusion pumps). Users of one authority may require granular access to data maintained by multiple authorities (stakeholders) and domains.