In permissionless blockchains, transaction issuers include a fee to incentivize miners to include their transactions. 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：在无需许可的区块链中，交易发起方通过支付手续费激励矿工将其交易纳入区块。为准确估算交易的优先级费用，交易发起方（或更广义的区块链参与者）依赖两个基本透明度概念，即竞争透明度和优先级透明度。竞争透明度要求参与者知悉所有可能与给定交易竞争上链的待处理交易；优先级透明度则要求参与者知悉每笔竞争交易所支付的交易费用或优先级费用。然而这两种透明度目前均未能实现。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——这两种方式均非公开可见。本研究系统揭示了比特币和以太坊因上述实践导致的竞争透明度和优先级透明度缺失现象。我们证明私有中继网络被广泛使用，且隐私交易极为普遍。研究表明，透明度的缺失为矿工合谋创造了条件：用户即便使用这些几乎无法保证交易优先级的私有中继网络，仍可能面临超额收费。区块链中这些透明度的缺失对交易发起方以及区块链稳定性均具有关键影响。最后，我们将数据集和脚本开源共享。