The core element of the architecture is the blockchain. After a technological study, a solution based on the implementation of the Ethereum blockchain is chosen. In addition to having a complete language for the implementation of smart contracts, Solidity , this blockchain implementation has recently changed its proof of work to a consensus protocol based on Proof of Stake , which makes its consumption is greatly reduced and thus its environmental impact, compared to other PoW-based implementations such as Bitcoin .

The Ethereum-based blockchain has three main features that solve an important part of the non-functional requirements for us:

• Decentralization: these networks can be made up of a large number of nodes, distributed geographically. 
• Availability: Both the exchanged symmetric keys and the messages sent are stored in the blockchain as contract events, thus freeing users from the obligation to always be connected to avoid missing an event, as happens with technologies such as P2P. Thus, it would suffice to retrieve the events of the smart contract and check if there is any new conversation or any new message. 
• Reliability: this technology guarantees that no transaction can be falsified using decentralized validation methods.

Although the characteristics mentioned are typical of any blockchain network, not all networks allow the creation of complex systems that take advantage of them. The Ethereum network or similar have an element called smart contract that allows you to integrate your own code fragments to carry out more complex transactions.

This component was uploaded to the Cities2030 website, S2CP  section. In addition, it was presented at the Third WP6 workshop “Analyze the challenge” that took place on June 10, 2022, on the occasion of the R113 milestone according to the WP6 Gantt diagram, which was incorporated into the deliverable D6.1, Section 2.3.

Labs attended a tutorial on the use of this component, in which its developers presented some installation instructions, and a demo of its use to enable private messaging in Cities2030. Video recordings and presentation materials were available to all Cities2030 participants through the official Cities2030 data management platform: Correlate.