## 3 Related Work
### Decentralized Privacy-Aware Data Storage  
In this section, we overview the most relevant work that focuses on privacy-aware and decentralized data storage. Most existing solutions rely on a combination of decentralized ledger technology to enable decentralized trust, combined with a Distributed Hash Table (DHT) for data storage.  
The authors in [38] propose an Ethereum-based blockchain framework. Although the solution is decentralized, the data storage part relies on a MySQL database to store the data while a hash sum of the data is sent to the blockchain. The main limitation of this work is the MySQL database that becomes a single point of failure. In addition, the hash stored on the Ethereum blockchain leaves information about the data storage operations publicly available.  
In [39], the authors propose a blockchain-based framework that allows users to control and keep the track of the data they share with their friends, family, and others. The proposed framework uses a software client to manage the record of the private key that is shared with the circle members. It also encrypts the data with the circle's public key before sharing it. However, the paper mentions private key sharing, which is a major security risk that could lead to key leakage and data security issues.  
The authors in [40] develop a food traceability framework based on the blockchain and smart contracts. The proposed framework stores original data on the IPFS, whereas the corresponding hash of the data is stored on the blockchain. A smart contract is used to handle traceability information. However, the proposed framework modules are managed by a manufacturer node server that is subject to a single point of failure. In addition, data stored on the smart contract are immutable and cannot be updated later. One more limitation of the IPFS network is the immutability that stores file permanently [9]. On the other hand, we use DHT that allows modification in the data after storing it in the database.  
In [41], the authors combine a blockchain with a Distributed Hash Table (DHT) to manage personal data. A DHT is used to store encrypted data and a symmetric key (a single shared key for encryption/decryption) while the data hash is stored to the blockchain. The proposed framework allows both the service and the user to query data using a data hash. However, the authors did not discuss any solution to protect symmetric keys from unauthorized access. Additionally, a fine-grained access control model is used to access the blockchain. However, the proposed solution records permissions on the blockchain that is subject to immutability concerns.  
The authors in [42] propose a blockchain-based data management and access control framework. A blockchain is used to store access control permissions. However, the permissions are stored on the blockchain are immutable and publicly accessible, which raises privacy issues.  
A decentralized data storage for PingER (Ping End-to-End Reporting) is presented in [43]. The metadata of PingER files is stored on a permissioned blockchain, while actual data references are stored in a DHT. The work is quite similar to ours but does not consider security aspects.  
In summary, the most relevant data storage frameworks rely somehow on a single point of failure at one stage or another of the proposed solution. Another concern is scalability for the solution that stores data on the blockchain. In addition, a good part of them stores data publicly without any security mechanisms, which highlights the need for a privacy-aware, decentralized solution.