Blockchain-based end-to-end encryption for Matrix instant messaging

Privacy and security in online communication is an important topic today, especially in the context of instant messaging. A lot of progress has been made in recent years to ensure that conversations are secure against attacks by third parties, but privacy from the service provider itself remains difficult. There are a number of solutions offering end-to-end encryption, but most of them rely on a centralized server, proprietary clients, or both. In order to have fully secure instant messaging conversations, a decentralized and end-to-end encrypted communication protocol is needed. This means there is no single point of control, and each message is encryped directly on the user's device such that only the recipient can decrypt it. This work proposes an end-to-end encryption system for the Matrix protocol based on blockchain technology. Matrix is a decentralized protocol and network for real-time communication that is currently mostly used for instant messaging. This protocol was selected because of its versatility and extensibility. Using the Secret Store feature in OpenEthereum, the proposed system encrypts data using keys stored on the Ethereum blockchain. Access control to the keys is also handled by the Secret Store via a smart contract. The proposed encryption system has multiple advantages over alternative schemes: The underlying blockchain technology reduces the risk of data loss because of its decentralized and distributed nature. Thanks to the use of smart contracts this system also allows for the creation of an advanced access control system to decryption keys. In order to test and analyze the proposed design, a reference implementation was created in the form of a library. This library can be used for future research, but also as a building block for different applications to easily implement end-to-end encryption based on blockchain technology.