  Bitcoin relies on a peer-to-peer overlay network to broadcast transactions
and blocks. From the viewpoint of network measurement, we would like to observe
this topology so we can characterize its performance, fairness and robustness.
However, this is difficult because Bitcoin is deliberately designed to hide its
topology from onlookers. Knowledge of the topology is not in itself a
vulnerability, although it could conceivably help an attacker performing
targeted eclipse attacks or to deanonymize transaction senders.
  In this paper we present TxProbe, a novel technique for reconstructing the
Bitcoin network topology. TxProbe makes use of peculiarities in how Bitcoin
processes out of order, or "orphaned" transactions. We conducted experiments on
Bitcoin testnet that suggest our technique reconstructs topology with precision
and recall surpassing 90%. We also used TxProbe to take a snapshot of the
Bitcoin testnet in just a few hours. TxProbe may be useful for future
measurement campaigns of Bitcoin or other cryptocurrency networks.
