Selfish & Opaque Transaction Ordering in the Bitcoin Blockchain: The Case for Chain Neutrality

TL;DR

This paper investigates transaction ordering in Bitcoin, revealing miners often deviate from fee-based norms for selfish or opaque reasons, highlighting the need for chain neutrality standards.

Contribution

It provides the first statistical evidence of miners manipulating transaction order for personal or hidden interests, challenging assumptions of fee-based prioritization.

Findings

Miners deviate from fee-based norms for selfish interests.

Evidence of dark-fee payments influencing transaction order.

Call for establishing neutrality norms in blockchain transaction ordering.

Abstract

Most public blockchain protocols, including the popular Bitcoin and Ethereum blockchains, do not formally specify the order in which miners should select transactions from the pool of pending (or uncommitted) transactions for inclusion in the blockchain. Over the years, informal conventions or "norms" for transaction ordering have, however, emerged via the use of shared software by miners, e.g., the GetBlockTemplate (GBT) mining protocol in Bitcoin Core. Today, a widely held view is that Bitcoin miners prioritize transactions based on their offered "transaction fee-per-byte." Bitcoin users are, consequently, encouraged to increase the fees to accelerate the commitment of their transactions, particularly during periods of congestion. In this paper, we audit the Bitcoin blockchain and present statistically significant evidence of mining pools deviating from the norms to accelerate the commitment of transactions for which they have (i) a selfish or vested interest, or (ii) received dark-fee payments via opaque (non-public) side-channels. As blockchains are increasingly being used as a record-keeping substrate for a variety of decentralized (financial technology) systems, our findings call for an urgent discussion on defining neutrality norms that miners must adhere to when ordering transactions in the chains. Finally, we make our data sets and scripts publicly available.