Blockchain Access Privacy: Challenges and Directions

Privacy, facilitated by a confluence of cryptography and decentralization, is one of the primary motivations for the adoption of cryptocurrencies like Bitcoin. Alas, Bitcoins privacy promise has proven illusory, and despite growing interest in privacy-centric blockchains, most blockchain users remain susceptible to privacy attacks that exploit network-layer information and access patterns that leak as users interact with blockchains. Understanding if and how blockchain-based applications can provide strong privacy guarantees is a matter of increasing urgency. Many researchers advocate using anonymous communications networks, such as Tor, to ensure access privacy. We challenge this approach, showing the need for mechanisms through which non-anonymous users can (i) publish transactions that cannot be linked to their network addresses or to their other transactions, and (ii) fetch details of specific transactions without revealing which transactions they seek. We hope this article inspires blockchain researchers to think beyond Tor and tackle these important access privacy problems head-on.

Cryptography to the Rescue?

Most blockchains are, at their core, massively distributed and publicly accessible databases; therefore, beyond ensuring that the data they store does not, in and of itself, betray user privacy, any research program that seeks to fully address blockchain privacy must additionally consider (at the very least) privacy for two fundamental types of transactions: reading data from and writing data to a blockchain.