CoinJoin — the heavy one, and why most readers should leave it alone

A number of people put coins into a single transaction at the same time, and equal-sized amounts come back out to each of them. Because every output is the same size, there is nothing to match them up by. Anyone watching can see the transaction happened and can see who put coins in — what they cannot do is say which output came back to which person, beyond a one-in-however-many guess.

Nobody can take your coins, and this is the property that makes it a real tool rather than a trust exercise. Each participant signs only their own input, and only after checking that one of those equal outputs pays an address of theirs. Whoever is organising the round can refuse to run it, can stall, and can lie to you about how many of the other participants are also them — but they cannot walk off with anything, because they never hold anything.

What this buys is a break in the chain going forward. Everything before the transaction is still visible and still attached to whatever it was attached to. What stops is the ability to follow those particular coins onward through it.

And it is not subtle. A transaction with fifteen identical outputs does not look like anything else on the chain. Anyone can tell one happened and can tell you took part; what they lose is the thread afterwards. That visibility is not a side effect to be engineered away — it is a permanent property of the coins that come out, and the reason for most of what follows.

Do not acquire coins tied to your name in the first place. If buying without an identity check is available where you are, it achieves in advance what mixing tries to achieve afterwards, for less money and with none of the aftermath. This is the option most people consider last, and it is the one we would consider first.

The everyday habits, plus keeping coins from different sources apart. A fresh address every time, not being publicly known as a holder, and not carelessly combining coins that came from different places. Most of the available benefit, none of the cost. The privacy lesson covers all three.

Then the four mistakes, in the order people make them:

Mixing and then depositing to an exchange. Several exchanges freeze deposits that have been through one, and you may be asked to account for coins you have deliberately made hard to account for. If you are going to do this at all, work out how the coins come back off the chain before they go on it.

Spending mixed coins alongside unmixed ones. The moment a later payment combines one of each, the assumption that put them together is the same assumption you just paid to defeat. It re-links them, and it undoes the whole exercise quietly. Mixed coins live in their own wallet, forever, or the money was wasted.

Believing the advertised numbers. Crowd sizes have been measured by researchers and they came out materially smaller than the figures being marketed. Assume you are hidden among considerably fewer people than the tool claims, and decide whether it is still worth it on that basis.

Expecting it to erase anything. It does not reach backwards, it does not touch what an exchange recorded when you bought, and it does not remove your name from anywhere it already appears.

For almost everyone reading this: not this. Not because it does not work — it does — but because it is a practice rather than a setting, and a practice half-kept is worse than none: you will have paid the fees, taken the permanent mark, and undone the benefit the first time you spent without thinking. Nothing else on this site asks for ongoing discipline in order not to backfire.

If your situation genuinely calls for it — your holdings are already publicly attached to your name, you run a business whose counterparties you would rather not have reading your accounts, or being known to hold Bitcoin where you live is a physical risk rather than an inconvenience — then learn it properly before you touch it, plan the way out first, keep what comes out in its own wallet permanently, and expect rougher tools than existed a few years ago.

On the legal position, plainly: we are not lawyers, and nothing here is legal advice. What we can give you is the record, and the record is below. Nothing in it says that using one of these is an offence. Nothing in it settles the question either — the case that would have tested it ended in guilty pleas before it was argued. If that distinction could matter to you, it is a question for a lawyer where you live, not for a guide on the internet.

What we will not do is tell you it is fine, or tell you it is shady. It is a tool with a real purpose, whose usefulness has gone down and whose surrounding risk has gone up, and a reader deciding about it deserves both halves of that in one place rather than whichever half the page they landed on preferred.