Post-Mix Best Practices
After you have completed a CoinJoin, your bitcoin is in a "post-mix" state. How you handle these post-mix UTXOs is critical - careless spending can completely undo the privacy you gained from mixing.
What Is a Post-Mix UTXO?
A post-mix UTXO is an output from a CoinJoin transaction. It has been mixed with other participants' bitcoin and has a certain level of privacy.
The Golden Rule
Never spend 2+ post-mix UTXOs from different CoinJoin rounds in the same transaction.
This is the single most damaging mistake a CoinJoin user can make. It completely destroys the anonymity set gained from mixing.
Why Post-Mix Spending Is Dangerous
When you spend 2+ post-mix UTXOs together, the Common Input Ownership Heuristic links them. An adversary can then trace backward through each CoinJoin to the pre-mix inputs, collapsing the anonymity set to 1.
Post-mix consolidation is one of the most damaging mistakes you can make. It completely undoes the privacy you worked hard to achieve through CoinJoin.
Once post-mix UTXOs are linked, every subsequent transaction from those UTXOs is also compromised. The damage spreads forward through your entire transaction history.
Post-Mix Best Practices
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Spend Each UTXO Independently
Each post-mix UTXO should be spent in its own transaction. Never combine them.
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Use PayJoin When Possible
PayJoin (Stowaway) adds an extra layer of privacy to post-mix spending.
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Use Tor
Always route post-mix spending through Tor to hide your IP address.
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Label Your Post-Mix UTXOs
Keep track of which UTXOs are post-mix and which round they came from.
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Wait Between Spending
Do not spend all your post-mix UTXOs in quick succession. Wait between spends.
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Use Ricochet for Extra Distance
Ricochet adds 4 hops between your post-mix and the final destination.
Post-Mix Wallet Management
Samourai Wallet has a dedicated post-mix wallet that keeps your mixed UTXOs separate from your premix and regular balances.
Features: - Separate post-mix wallet - Stowaway (PayJoin) support - Ricochet support - UTXO freeze for dust
Sparrow Wallet supports Whirlpool and has coin control features for managing post-mix UTXOs.
Features: - Whirlpool integration - Coin control - UTXO labeling - Transaction preview
Common Post-Mix Mistakes
Spending 2+ post-mix UTXOs in a single transaction. This is the worst mistake you can make.
Sending post-mix bitcoin to a KYC exchange or other known entity links your mixed bitcoin to your identity.
Spending all your post-mix UTXOs in quick succession creates a temporal link that can be analyzed.
Never spend post-mix UTXOs alongside premix UTXOs. This links your mixed and unmixed bitcoin.