A reused address links payments a CoinJoin cannot unlink

Inherited archive entry · retained from the earlier edition. Its factual, legal and policy claims have not all been reverified for 19 September 2026. Research scope.

What a public ledger makes the default

Every transaction and every address balance on Bitcoin's ledger is visible to anyone. The bitcoin.org privacy guidance, describing the position as retrieved on 16 September 2026, states that anyone can see the balance and all transactions of any address, and warns that once an address has been used, it becomes tainted by the history of all transactions it is involved with; publishing an address and later moving funds through it links that later activity back publicly. Reusing one address for multiple payments, rather than generating a new one each time as wallets are designed to do, is what turns an otherwise pseudonymous ledger into a running, linkable account statement for anyone watching.

What CoinJoin actually changes

The construction now called CoinJoin was set out in an August 2013 forum post by Gregory Maxwell, posted 22 August at 02:32 UTC. It rests on a detail of the transaction format: the signatures, one per input, inside a transaction are completely independent of each other, so separate participants can agree on a shared set of inputs and outputs, sign only their own inputs, and merge the results into one transaction. In the design described, N participants agree a uniform output size, each contributes input value at least that large, and the transaction pays out N same-size outputs plus change; an observer is left asking, as the post frames it, which of two participants owns which of two equally sized outputs, with no answer available from the transaction alone.

What it leaves visible

The same post is explicit about the limit: hiding which input funded which output does nothing about network-level identity. As the discussion in the post acknowledges, any transaction privacy system that hopes to hide a user's addresses should start with some kind of anonymity network, since a participant broadcasting or being asked for a transaction still exposes an IP address to whoever is listening, separately from anything the transaction's structure obscures. The bitcoin.org guidance carries a parallel caveat about mixing generally: it can break traceability for small amounts, but becomes increasingly difficult to do the same for larger transactions, and requires trusting whoever operates the mixing itself, in a manner whose legality varies by jurisdiction.

Questions to carry forward

• Does a wallet claiming CoinJoin privacy generate a fresh address for every output, or does address reuse elsewhere undo the benefit?
• Is the equal-output-size assumption underlying a given CoinJoin round actually met, or could an outlier output value re-identify a participant?
• Has anything been done about network-level exposure, such as connecting only over an anonymity network, separately from the transaction structure itself?

A CoinJoin transaction changes what the ledger's structure alone can prove about ownership; it was never presented, in the post that first described it, as changing what an observer of the network itself can see. The same transactions guide that defines an unspent output and a change output explains why a reused address stays exposed: every output sent to it remains attached to that one key for as long as the ledger exists.