Can coinjoin and payjoin transactions be spotted on the blockchain?

Question

Can coinjoin and payjoin transactions be spotted on the blockchain?

Have centralized exchanges declined to accept these types of transactions?

Answer 1

The primary characteristic of a coinjoin is that multiple users are collaborating in a transaction that pays each at least one output of an amount shared with another output. In other words, a coinjoin transaction must have at least 2 inputs, at least 2 outputs, and at least 2 of those outputs need to have the same amount.

Transactions with outputs of equal amounts stand out. Of the 460,264 transactions made in the last day, 14,463 conformed to these three conditions. However, many of these have equal amounts because they're creating multiple tiny outputs (example). When we only consider outputs larger than 10,000 sats, the transaction count goes down to 2415. This includes small transactions (example) to transactions with hundreds of inputs and outputs (example). Note that even if a transaction looks like a coinjoin, anyone can create such a transaction by themselves, so false positives can never be eliminated.

Payjoins are similar, except since they include a transfer between parties, the outputs no longer need to have equal amounts. There were 63,599 transactions made in the last day that had at least 2 inputs and 2 outputs, and clearly the vast majority of them aren't actually payjoins.

6102bitcoin maintains a list of instances where exchanges have flagged users who had used coinjoin. Consequences have ranged from being questioned over the source of funds to having their account closed.