  Recently, Decentralized Finance (DeFi) platforms on Ethereum are booming, and
numerous traders are trying to capitalize on the opportunity for maximizing
their benefits by launching front-running attacks and extracting Miner
Extractable Values (MEVs) based on information in the public mempool. To
protect end users from being harmed and hide transactions from the mempool,
private transactions, a special type of transactions that are sent directly to
miners, were invented. Private transactions have a high probability of being
packed to the front positions of a block and being added to the blockchain by
the target miner, without going through the public mempool, thus reducing the
risk of being attacked by malicious entities.
  Despite the good intention of inventing private transactions, due to their
stealthy nature, private transactions have also been used by attackers to
launch attacks, which has a negative impact on the Ethereum ecosystem. However,
existing works only touch upon private transactions as by-products when
studying MEV, while a systematic study on private transactions is still
missing. To fill this gap and paint a complete picture of private transactions,
we take the first step towards investigating the private transactions on
Ethereum. In particular, we collect large-scale private transaction datasets
and perform analysis on their characteristics, transaction costs and miner
profits, as well as security impacts. This work provides deep insights on
different aspects of private transactions.
