Dr. Artzt described the roles and the responsibilities of blockchain participants under the GDPR and similar international data protection regimes.Miners validate transactions by solving a mathematical puzzle defined by the applicable consensus mechanism used by the blockchain. Miners do not determine the specific purpose of any data processing activity nor do they carry out any specific services based on instructions of controllers. Therefore miners are not controllers or processors under the GDPR.Nodes refer to the decentralized computers that each store a copy of the blockchain. As an IT operation, storing does not implicate decision-making processes. Therefore nodes are not controllers or processors. Indeed, it is difficult to interpret merely participating in the blockchain network as determining the means and purpose of a specific data processing activity.Wallets store and manage asymmetric keys and addresses used for transactions at the application level. Wallets allow blockchain users to control their own private keys and interact with the network. As a software package, wallets merely pass data from users to miners and therefore do not by themselves determine the purpose or means of processing and are not controllers.Users – Users participate in transactions on a blockchain network. Users could be controllers if (i) personal information is submitted to the ledger and (ii) the processing activity has a professional or commercial background only. However, if the processing of information is in the course of a “purely personal or household activity,” it may fall under the GDPR’s household exception. Art. 2(2)(c). In that case, the GDPR doesn´t apply.Smart Contract Developers create algorithms to use in “smart contract” programs stored on the blockchain which provide certain functionality to blockchain users. Notably, they do not operate the software they write and therefore do not determine the purpose and means of the processing activity.Oracles refer to agents that allow the transfer of external data feeds to the blockchain to leverage smart contracts. Oracles are necessary to process external real-world events for the network and have a strong influence on the data processing operation. Therefore, oracles can qualify as controllers if they have a commercial interest in the outcome of the related data processing.Governance Bodies monitor blockchain transactions and define the roles of the participants. Existing only in private blockchains, a governance body qualifies as a controller if it has control over the processing of personal data. Governance bodies can also determine one participant to act as a controller and become processors or joint controllers as a result.The masterclass on blockchain technology completes the VUB-FPF Digital Data Flows Masterclass series.