  Privacy-seeking cryptocurrency users rely on anonymization techniques like
CoinJoin and ring transactions. By using such technologies benign users
potentially provide anonymity to bad actors. We propose overlay protocols to
resolve the tension between anonymity and accountability in a peer-to-peer
manner. Cryptocurrencies can adopt this approach to enable prosecution of
publicly recognized crimes. We illustrate how the protocols could apply to
Monero rings and CoinJoin transactions in Bitcoin.
