In permissionless blockchains, transaction issuers include a fee to incentivize miners to include their transaction. To accurately estimate this prioritization fee for a transaction, transaction issuers (or blockchain participants, more generally) rely on two fundamental notions of transparency, namely contention and prioritization transparency. Contention transparency implies that participants are aware of every pending transaction that will contend with a given transaction for inclusion. Prioritization transparency states that the participants are aware of the transaction or prioritization fees paid by every such contending transaction. Neither of these notions of transparency holds well today. Private relay networks, for instance, allow users to send transactions privately to miners. Besides, users can offer fees to miners via either direct transfers to miners' wallets or off-chain payments -- neither of which are public. In this work, we characterize the lack of contention and prioritization transparency in Bitcoin and Ethereum resulting from such practices. We show that private relay networks are widely used and private transactions are quite prevalent. We show that the lack of transparency facilitates miners to collude and overcharge users who may use these private relay networks despite them offering little to no guarantees on transaction prioritization. The lack of these transparencies in blockchains has crucial implications for transaction issuers as well as the stability of blockchains. Finally, we make our data sets and scripts publicly available.

翻译：在无许可区块链中，交易发起者会包含一笔手续费以激励矿工打包其交易。为准确估算某项交易的优先级费用，交易发起者（或更广义的区块链参与者）依赖于两个基本透明度概念，即竞争透明度和优先级透明度。竞争透明度意味着参与者知晓所有将与指定交易竞争打包资格的未处理交易。优先级透明度则表明参与者知晓每一笔具有竞争关系的交易所支付的交易费或优先级费用。目前，这两种透明度概念均未得到良好实现。例如，私有中继网络允许用户私下向矿工发送交易。此外，用户可通过直接转账至矿工钱包或链下支付的方式向矿工提供费用——这两种方式均不公开。本研究揭示了此类行为导致比特币和以太坊中竞争透明度与优先级透明度的缺失。我们证明私有中继网络被广泛使用，且私有交易十分普遍。研究表明，透明度的缺失促使矿工合谋，对使用这些几乎不提供交易优先级保障的私有中继网络的用户进行超额收费。区块链中此类透明度的缺失对交易发起者及区块链的稳定性均具有重大影响。最后，我们公开了数据集和脚本。