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I understand that the common input ownership heuristic states that in a transaction all inputs are deemed to be controlled by the same person. But given that a signature must be provided for each individual input it is possible that multiple parties (each owning an input) collaborate and produce a transaction. This is the basis of coinjoin.

Given this, why is the common input ownership heuristic so heavily relied upon when conducting blockchain analysis and identifying clusters? Aside from address reuse (which is much more clear cut), it is one of the more commonly used heuristics for linking addresses. So why is it that if a coinjoin service is identified any common input heuristic is discarded, but for some random transaction it is deemed to apply?

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There isn't much more to say about this than that it's a heuristic, it doesn't need to be perfect.

The reality is that a large majority of transactions are not CoinJoins, and thus, if someone wants to make a guess about the nature of a transactions, it's not unreasonable to assume that all inputs had a common owner. It won't constitute a proof, but in many cases it can help discern what is going on.

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  • I fully understand that it is a heuristic and does not constitute proof. Ultimately blockchain analysis is a series of educated guesses. Out of curiosity, other than the dedicated coinjoin wallets like Wasabi, do many "normal" wallets support "coinjoin by agreement" type transactions? Ie Alice and Bob want to buy a house together and they each have Bitcoin in say an Exodus and Trezor wallet respectively. Being that Alice and Bob are regular Bitcoin owners but don't hang out on Bitcoin Stackexchange, can they easily construct a coinjoin transaction or is it a more specialised skill? Commented Apr 4 at 1:08
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    @user2334659 there are some wallets that can communicate under the hood to craft txs that include inputs from multiple users, but generally that is done in the background in order to achieve some bigger-picture goal (eg privacy. for example, see: payjoin/P2EP txs). Average users can partake simply by using the right wallets, but otoh, manually creating a payment in the way you've described would be a more specialized skill.
    – chytrik
    Commented Apr 4 at 6:03

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