Here's an interesting (and controversial) proposal and discussion: "Decentralised crime fighting using private set intersection protocols" - Mike Hearn https:

Here's an interesting (and controversial) proposal and discussion: "Decentralised crime fighting using private set intersection protocols" - Mike Hearn https://bitcointalk.org/index.php?topic=157130.0 https://bitcointalk.org/index.php?topic=157130.0