BIP 78 Payjoin Explained: Interactive Bitcoin Payments and Privacy

BIP 78 Payjoin explained: see how receivers add inputs, break ownership heuristics, enforce sender checks and secure the interactive endpoint.

What the standard solves

Payjoin, also called P2EP, is an interactive payment in which the receiver contributes one or more inputs to a transaction originally proposed by the sender. The final transaction still pays the requested amount. But the common-input-ownership heuristic no longer reliably says that every input belongs to the sender.

Start from the primary BIP because a product label does not prove that every required field or rule is implemented.

Start with the validating node, because the ASIC only hashes the candidate header it receives. Record the node release and the pool component that assembled the block. If those facts are unknown, the operator cannot show which rules were actually applied before electricity was committed to the work.

Core data and construction

The sender creates an original PSBT and posts it to the receiver endpoint from a payment URI. The receiver adds its input, adjusts outputs and returns a proposal. The sender validates strict invariants, signs only its own inputs, finalises and broadcasts. The original transaction remains a fallback under defined failure handling.

Preserve the exact serialised data for audit. A friendly wallet summary can omit the field that explains why a signer accepted or rejected the operation.

Treat status dashboards as observations, not as the source of truth. Compare them with an independently operated node and retain the raw deployment or template response. Period boundaries, chain reorganisations and cached pool pages can otherwise make a correct-looking percentage describe the wrong state.

Security boundary

The receiver is untrusted from the sender’s perspective. The sender must confirm its input set, payment output, allowed fee contribution, sequence and locktime policy, no unexpected keypath leakage and that receiver-added inputs are valid. HTTPS authenticates the endpoint but cannot replace transaction-level checks. Separate watching, policy, key custody and final signing so no single convenience interface silently expands authority.

Build the failure response before the boundary arrives. Define which rejection messages trigger an alert, who can pause a template source and how failover is prevented from returning miners to the same faulty validation stack. A second hostname is not independent when both endpoints share one node.

Privacy and operational trade-offs

Payjoin breaks a common chain-analysis assumption and can improve both parties’ privacy set, yet the receiver and network endpoint observe interaction metadata. Amounts and later spending can still reveal relationships. Direct clearnet requests can expose IP information. So transport and server logging policy are part of the design.

The improvement is conditional, not magic: network observers, counterparties and compromised endpoints may still infer information outside the mechanism.

Separate readiness, signalling and enforcement in the operating log. Readiness is a claim about software and process, signalling is data carried by blocks, and enforcement is a validation result. Combining them into a single supported or unsupported label hides the point at which revenue is actually at risk.

Failure modes to avoid

A malicious proposal may increase fees, substitute outputs, consume sender change, request unauthorised signatures or create a transaction that will not relay. Endpoint probing can also reveal receiver activity. Implement exact BIP limits, timeouts and fallback, and never broadcast both original and modified transactions casually. Test negative cases and cancellation paths before moving value, and never treat successful parsing as proof of safe intent.

Map responsibility across the full path: validating node, template server, pool protocol, proxy, firmware and ASIC. For each layer, state what it can alter and what it merely relays. This prevents a version-bit setting in firmware from being mistaken for complete consensus-rule support. Relate that responsibility map to the pool and job-control boundary in our Stratum V2 guide.

Compatibility and deployment

BIP 78 describes Payjoin version one over HTTP with PSBT. Adoption is optional and sender and receiver wallets must both support it. Later Payjoin designs can use different transport or asynchronous mechanisms. So a wallet claiming Payjoin should state which protocol and endpoint security model it implements.

Record software and hardware versions because optional fields, draft changes and vendor support differ across otherwise compatible-looking tools.

Test the primary and failover paths with the same checks. Compare chain tip, chainwork, deployment state, required rules and template age, then save the result with a timestamp. The process should be repeatable by another operator without relying on an undocumented pool conversation.

Verification checklist

Run an end-to-end test with separate sender, receiver and validating nodes. Inspect the original and proposal diffs, enforce maximum extra fee and reject every prohibited mutation. Simulate endpoint failure and confirm safe fallback. Verify the final input ownership, outputs, fee and confirmation without logging sensitive production PSBTs.

Rehearse the complete workflow with test funds, preserve checksums and raw artefacts, then confirm the final transaction independently on a validating node.

Turn the conclusion into a business decision. State which chain and settlement venues the operation intends to serve, the maximum acceptable stale-block exposure and the point at which mining pauses. This connects protocol evidence to electricity cost, pool revenue and payout finality.

Operator decision record

A concise decision record for BIP 78 Payjoin should name the source documents, their dates, the node release tested, the responsible pool or template provider and the exact trigger for action. Include screenshots or machine-readable output for the deployment state. But keep the raw node response as the stronger evidence.

State whether a change affects policy, block construction or consensus validity, because those layers have different failure costs.

Run the check on every production and failover path. Confirm that monitoring alerts on stale templates, unexpected chain tips, rejected proposals and a rise in stale shares. Keep rollback instructions for node and pool configuration. But do not roll back across an active consensus boundary without understanding the rules the older release enforces.

If the evidence conflicts, pause the affected path and investigate before committing more electricity to uncertain work.

For related background, read our plain-English BIP-110 guide and technical BIP-110 review. Those articles use a modern proposal to show why signalling, activation, template construction and accepted chain history must be examined separately.

Conclusion

Bip 78 payjoin is best understood as a defined interaction between validating software, mining infrastructure and economic acceptance. The safest operator does not infer consensus from a dashboard percentage or a pool slogan. They verify the rule source, the activation boundary, the template fields and the chain their payouts ultimately settle on. That discipline reduces the chance of hashing an invalid or commercially unwanted block.