# Privacy-Preserving Electricity Trading for Connected Microgrids
## 2 Related Work  
The scientific literature has plenty of research papers addressing security and/or privacy issues in smart metering [12] and decentralized energy trading [13]. The authors in [10] present a proposal in which smart-grid users purchase electricity tokens (Chaum's e-coins) and use them to pay for the electricity they consume when submitting their measurement reports. Privacy requires trust in a third party, an aggregator,who acts as an intermediary between smart meters and the utility provider. The proposal also enables the utility provider to purchase energy from users, but it does not consider trading among users. Chaum's anonymous e-coin payments are also used in [14] in a system designed for customer-to-electricity-provider payments. Privacy preservation requires the presence of a trusted aggregator who prevents the operation centre from learning user consumption details. After analysing the proposal in depth, one can see that the proposal includes an additional actor, the bank, who can learn the consumption profile of every household from the e-coin withdrawal queries it receives. This review continues by addressing proposals allowing energy trading among users. The authors of [15] present a blockchain-based electricity trading platform. It includes the functionality to withdraw financial assets from an account managed by the DSO into anonymous addresses on the blockchain, where they can be transferred among addresses as a result of trade operations. Financial assets from the blockchain can later be deposited into an account. This solution prevents households from learning the real identity of the participants involved in each transaction. Unfortunately, all the identities are available to the DSO who is able to trace all the transactions carried out by each participant. In [16], a decentralized privacy-preserving TTP-free energy trading system based on a broadcast channel for message interchange is proposed. Payments are made using the Bitcoin cryptocurrency. The system is designed to provide privacy by means of the use of pseudonyms that hide the real identity of the parties involved in each transaction. Unfortunately, Bitcoin payments make the system vulnerable to Bitcoin value fluctuations caused by events unrelated to the electricity market. In fact, those households that usually consume more electricity than they produce will regularly run out of Bitcoins and will need to acquire them through third parties at a difficult-to-predict price. Last but not least, blockchain-based cryptocurrencies allow some degree of traceability [17] since all the transactions are anonymous yet publicly accessible, so the real degree of privacy is difficult to assess. A similar proposal was made in [18] where energy offers and demands are matched by a transaction server. Payments are made through a blockchain-based e-coin system that includes a bank actor able to trace all the payments. The security analysis made by the authors focuses on data integrity and authentication, while the privacy of consumption patterns is not addressed. The authors in [19] propose a system which makes use of a permissioned blockchain for managing energy allocation transactions. Privacy and security are enforced by means of an identity validation mechanism based on the use of group signatures together with a pseudonym-based node identification system. In spite of considering privacy, the proposal does not address energy payments. Consequently, it is not clear whether privacy could be preserved after the inclusion a billing procedure. Another blockchain-based proposal is presented in [20]. The participants in energy trading protect their privacy by using pseudonyms. After an offer-demand matching, the supply user sends their account information to the acquiring user. The paper does not address the fact that account information is a quasi-identifier which can be used for reidentification. A blockchain-oriented energy trading system focused on privacy is detailed in [21].