Paper 2017/106

$\mu$chain: How to Forget without Hard Forks

Ivan Puddu, Alexandra Dmitrienko, and Srdjan Capkun

Abstract

In this paper, we explore an idea of making (proof-of-work) blockchains mutable. We propose and implement $\mu$chain, a mutable blockchain, that enables modifications of blockchain history. Blockchains are, by common definition, distributed and immutable data structures that store a history of events, such as transactions in a digital currency system. While the very idea of mutable event history may seem controversial at a first glance, we show that $\mu$chain does not undermine security guarantees provided by immutable blockchains. In particular, all mutations in our system are controlled by fiat, enforced by consensus and are verifiable in the same way as regular transactions. At the same time, $\mu$chain provides a solution to a number of challenging problems, such as the patching of vulnerable smart contracts and removal of abusive content from blockchain history. It also gives rise to new blockchain applications that were not possible with immutable blockchains. For instance, governments and companies could now maintain registers of citizens and customers, while preserving their legislated rights to be forgotten. Banks could consider consolidation of cryptocurrency with traditional payments, which is hard to achieve without the ability to revert transactions. To further illustrate the power of $\mu$chain on more concrete examples, we present two new applications, the collaborative recommendation system with the ability to censor inappropriate content, and a time-lock encryption mechanism that provides a method to decrypt messages after a certain deadline has passed.