BA-10 · LESSON 02

CoinJoin

Collaborative transactions break simple ownership heuristics but introduce coordination, amount, and operational considerations.

45 min Difficulty 3/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · privacy boundaryObservable evidence creates inferences, not automatic proofSelect any state to inspect its exact role.

RELATIONSHIP MAPCompeting paths and resolution

01
OBSERVABLEAmount, script, timing
Chain and network observers see different subsets of transaction and propagation data.
02
HEURISTICProbabilistic link
Wallet patterns can support a guess but do not prove a real-world identity.
03
RISKCoinJoin
The CoinJoin lesson identifies which evidence or failure is in scope.
04
CONTROLReduce or separate evidence
Address discipline, coin selection, local verification, and routing choices change exposure.
05
RESIDUALExplicit remaining observer
Every mitigation leaves assumptions and observers that must be named honestly.

Rule shown: Privacy improves by reducing linkable evidence and separating observers, while preserving verifiable spending rules.

STAGE 1 OF 5OBSERVABLE: Amount, script, timing

Chain and network observers see different subsets of transaction and propagation data.

Governing relationship: Privacy improves by reducing linkable evidence and separating observers, while preserving verifiable spending rules. Complete path: 1. OBSERVABLE, Amount, script, timing. Chain and network observers see different subsets of transaction and propagation data. 2. HEURISTIC, Probabilistic link. Wallet patterns can support a guess but do not prove a real-world identity. 3. RISK, CoinJoin. The CoinJoin lesson identifies which evidence or failure is in scope. 4. CONTROL, Reduce or separate evidence. Address discipline, coin selection, local verification, and routing choices change exposure. 5. RESIDUAL, Explicit remaining observer. Every mitigation leaves assumptions and observers that must be named honestly.

1. State transitionWhat exists, what changes, and what results

BEFORE

Five participants each receive one 100,000-sat CoinJoin output, while unequal change outputs are separately identifiable.

MECHANISM

Participants prove control of proposed inputs so an attacker cannot register arbitrary victim coins. Registration communication can itself leak network metadata.

AFTER

CoinJoin changes the transaction graph; it does not erase external identity or guarantee future unlinkability.

Text equivalent: Starting from Five participants each receive one 100,000-sat CoinJoin output, while unequal change outputs are separately identifiable. The mechanism begins when Participants prove control of proposed inputs so an attacker cannot register arbitrary victim coins. Registration communication can itself leak network metadata. The resulting lesson is CoinJoin changes the transaction graph; it does not erase external identity or guarantee future unlinkability.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Register inputs with ownership proof

Participants prove control of proposed inputs so an attacker cannot register arbitrary victim coins. Registration communication can itself leak network metadata.

Complete path: 1. Register inputs with ownership proof: Participants prove control of proposed inputs so an attacker cannot register arbitrary victim coins. Registration communication can itself leak network metadata. 2. Obtain output credentials: Privacy-preserving coordination separates input registration from output registration so the coordinator cannot trivially map them. 3. Construct one transaction: The coordinator assembles registered inputs, equal-value mix outputs, change where applicable, and fees. 4. Verify before signing: Each participant checks its output, fee, input, and complete transaction. If anything is wrong, it withholds its signature. 5. Broadcast and preserve privacy: After all signatures arrive, the transaction broadcasts. Later merging equal outputs, spending change with them, or exposing wallet metadata can undo privacy gains.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

A CoinJoin candidate set is counted only after positive safe-domain inputs and outputs conserve value, fees reconcile, and equal denominations are separated from change.

CHANGED CONDITION

An output total above inputs fails closed with no anonymity credit, while later combining denomination outputs degrades the candidate set.

Important distinction: CoinJoin makes bitcoin untraceable after one round. It creates ambiguity under stated assumptions. Amount fingerprints, coordinator metadata, input history, change, later consolidation, and external records can reduce it.

Text equivalent: The accepted path is enforced by A CoinJoin candidate set is counted only after positive safe-domain inputs and outputs conserve value, fees reconcile, and equal denominations are separated from change. If the required condition changes, An output total above inputs fails closed with no anonymity credit, while later combining denomination outputs degrades the candidate set.

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Learn the idea

CoinJoin is collaborative transaction construction in which multiple participants contribute inputs and receive outputs in one Bitcoin transaction. Equal-output designs create an anonymity set: several outputs are deliberately indistinguishable by amount. The coordinator can organize messages but need not custody funds if each participant verifies and signs only after seeing the complete transaction.

GUIDED EXPLANATION1/5 ideas inspected

Register inputs with ownership proof

Participants prove control of proposed inputs so an attacker cannot register arbitrary victim coins. Registration communication can itself leak network metadata.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“CoinJoin makes bitcoin untraceable after one round.”

It creates ambiguity under stated assumptions. Amount fingerprints, coordinator metadata, input history, change, later consolidation, and external records can reduce it.

FINISH LEARNING FIRST

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