BIP 126 Mixed-Input Transaction Privacy Explained

BIP 126 Mixed-Input Transaction Privacy: This guide explains BIP 126 Best Practices for Heterogeneous Input Script Transactions in plain English. It covers the problem behind the BIP, why it matters and whether the proposal is part of Bitcoin today.

What heterogeneous inputs reveal

Spending outputs locked to different scripts in one transaction supports the common-input-ownership heuristic. Fragmented balances sometimes make this unavoidable. A wallet can reduce unnecessary leakage, but neither standard form nor equal outputs proves that observers cannot combine timing, amounts, change behaviour and later spends.

Standard form rules

The number of unique output scripts equals the number of unique input scripts, every output script is unique, at least two outputs share a value and the largest output belongs to an equal-value set. Including all UTXOs with an already-used input script aimed to limit address-reuse consequences and avoid net UTXO growth.

Alternate construction

When standard form is impossible or consolidation is desired, the procedure builds repeated spend-and-change pairs with equal standard outputs. Unique input scripts exceed outputs, output scripts remain unique and at least one partition must reconcile inputs, standard output, change and fees. Some zero-value change exists only analytically.

Intentional and unavoidable ambiguity

Intentional privacy transactions were expected to sometimes use the alternate form so unavoidable transactions were not uniquely fingerprinted. A rigid wallet that always chooses one form can create the very classifier the BIP sought to avoid. Random selection still needs fee, dust and user-intent constraints.

Interaction with BIP 69

The proposal says to construct its form before applying BIP 69 ordering. Ordering inputs too early can change the grouping logic; deterministic ordering afterward can also contribute a wallet fingerprint when other implementations differ. Test the whole transaction, not each convention independently.

Limits and UTXO effects

A payment larger than half the spendable balance or an unsuitable distribution can make a compliant form impossible. The wallet must report that boundary rather than split funds deceptively or claim privacy it cannot provide. Fees, dusting inputs and future spending can undo apparent ambiguity.

Wallet experiment

Use labelled testnet UTXOs across reused and unique scripts to create standard, alternate and impossible cases. Decode values, scripts, fees and ordering, then apply common clustering heuristics. Confirm the wallet requests informed consent before extra consolidation and records that BIP 126 remains Draft.

Conclusion

BIP 126 describes thoughtful ways to make mixed-input spending less distinctive, but a Draft transaction shape cannot guarantee anonymity. Wallets should minimise joins, explain unavoidable leakage and use maintained privacy analysis.