## 2 Related Work  
During the last few years, several research attempts have tried to provide fine-grained access control in the dynamic multi-authority and multi-domain setting, while maintaining interoperability [10]. Some of these works have focused on decentralization and privacy-preserving encryption [11; 12; 13]. Such targeted solutions enable adequate compatibility and fine-grained access control; however, they fail to combine credentials issued by independent authorities. Other solutions rely on privacy-preserving encryption, such as Attribute-Based Access Control (ABAC). For example, Ref. [14] proposes a decentralized MA-ABAC (DMA-ABAC) scheme for multi-domain healthcare ecosystems. Despite the fact that authorities can independently control their security settings and enforce cross-domain policies, the lack of a mechanism obliging users to use the system for accessing the data, in combination with the absence of global verifiability, leads to strong trust assumptions. In [15], an ABAC solution is designed for the shared multi-owner setting, assuming a distributed setting with multiple authorities. The authorities own pieces of data and may issue attribute keys that users may combine to access data belonging to different authorities. Even though the proposed solution strengthens the restrictions for accessing data, it falls short on addressing the challenge of inter- and cross-domain policy enforcement, i.e., dynamically defining access policies both to control access for all authorities within a domain or for all authorities belonging in different domains. Many works try to solve this problem by using Multi-Authority Ciphertext Policy Attribute-Based Encryption (MA-CP-ABE) schemes. For example, the authors of [9] present a hierarchical multichain access model suitable for multi-owner setting specifically for healthcare environments. With the use of MA-CP-ABE, in encrypting the data, the proposed model achieves both inter- and cross-domain policy management. However, since multi-domain and multi-authority environments are inherently large scale, the lack of an end-to-end implementation makes it hard to actually assess the efficiency and practicality of [9] in actual applications. In [16], a MA-CP-ABE scheme is proposed that supports range policy, which, however, maintains the need for a trusted central authority. In [17], authors adopt the idea of hybrid encryption to reduce the computational overhead of data encryption, using a symmetric key to encrypt data and a MA-CP-ABE mechanism to encrypt the symmetric key. The authors in [18], also propose a MA-CP-ABE scheme combined with an outsourced decryption and zero knowledge proof for the Internet of mobile things. In [19], another MA-CP-ABE scheme is proposed suitable for IoT applications and devices with low computational capabilities. Decryption operations are outsourced to fog for efficiency reasons, but no policy management is supported. The authors of [20] introduce a token-based access control scheme which uses smart contract and blockchain to generate decryption keys according to verified user attributes. In [21], Elliptic Curve Cryptography (ECC) and MA-CP-ABE are combined to create an access control system that supports the setting of multiple authorities, but the addition or removal of authorities remains inefficient. In [22], the authors propose a privacy-preserving MA-CP-ABE scheme for blockchain-based applications in the supply chain. This model achieves fine-grained access control and versatile authorization and also protects user's private key from leakage even when some attributes authorities fail. However, in most of the above solutions, trust expectations about global transaction verifiability persist. Strong trust assumptions are required to ensure that all access transactions to encrypted data are immutably logged and may be globally verified by all entities.