*Contributions*

The contributions of this work include the description of a high-level solution for a decentralized personal data marketplace involving the use of DLTs, DFS and smart contracts for the creation of a PIMS: (i) data owners store their personal data in PDSs implemented using a DFS, such as the InterPlanetary File System (IPFS) [20]; (ii) this storage is complemented by the use of a DLT, which enables data integrity validation and indexing of the data, i.e., the IOTA DLT [21]; (iii) a distributed authorization mechanism enables access control of this data, thanks to the decentralized execution of immutable instructions of smart contracts implemented using the Ethereum protocol [22]; our authorization blockchain network executes access control to enact data owners' preferences and to verify the authenticity of the claims; the nodes of such a network employ a cryptographic schema that enables the protection of data owner's personal data (i.e., based on the use of a Threshold Proxy Re-Encryption [23]); (iv) finally, we provide a system for the search for data according to their content or meaning, relying on the use of a Distributed Hash Table (DHT) as a layer placed over the DLTs; indeed, data inserted into the DLTs and DFS is usually unstructured and no efficient decentralized mechanisms are present to query a certain kind of data; in our solution, data stored in DLT can be searched in a decentralized way through keywords stored in the DHT; the distinctive feature of our DHT network is that it is essentially a hypercube overlay structure [24], in which each node indexes objects representing specific indexes and addresses of a DLT using keywords.

Moreover, in this paper, we address a use case that assists us in describing the implementation of the decentralized personal data marketplace. Since our main driving force is facilitating the use of private data for the social good, we present a use case where citizens can use and give access to their personal data to produce new data, creating value for governments, businesses and other citizens as well. Through practices that promote a collaborative and co-creative approach to working together on the management, control and governance of the use of data, people and society can influence and shape data

governance processes and can support greater social and economic equity [2,25]. By implementing this use case, we demonstrate the ability of our system to support these processes. In particular, our system allows citizens to take part in an organization for the creation of new datasets and to steer the developmental decisions through a Decentralized Autonomous Organization (DAO) [26]. Our system provides the elements of a framework for participatory data stewardship [2], being transparent, i.e., informing individuals about their data, and collaboration, i.e., enabling individuals to take action [25], at its core.

The original contributions and novelties of our work are summarized as follows:

	- • Our PDS is implemented using a DFS for storing personal data;
	- • We use a DLT for providing data integrity, validation and indexing;
	- • Our smart contract-based authorization system executes distributed data access control;
	- • Our hypercube DHT enables a decentralized way of searching for data in DLTs. In particular, we provide a detailed description of the protocols behind the authorization blockchain and the hypercube DHT.

The remainder of the paper is organized as follows: Section 2 provides a background on the main concepts and technologies used, while Section 3 focuses on related work. Section 4 presents a description of the decentralized personal data marketplace architecture. Section 5 provides the description of a citizen-generated data creation use case with the intent to present our marketplace implementation. In Section 6, the evaluation of our proposal is shown and the results are discussed. Finally, Section 7 provides the concluding remarks.
