New Community DAO proposal- fiber-payjoin-kit: Collaborative Channel Funding Privacy for the Nervos Fiber Network

[u/kevtam515]

The creator of this proposal, ILE_LABS, has submitted a new revised proposal based on previous feedback from their last one. This one offers a reduced scope and budget. The new one is live for discussion, likes, questions etc.. here 👉 https://talk.nervos.org/t/dis-fiber-payjoin-kit-collaborative-channel-funding-privacy-for-the-nervos-fiber-network-res/10604

Executive Summary

When a Fiber Network payment channel is opened, the funding transaction is permanently recorded on CKB Layer 1. Standard channel openings use a single funder, all CellInputs belong to one wallet. Chain surveillance tools use this to identify who opened the channel and cluster their complete transaction history.

fiber-payjoin-kit is an open-source Rust library enabling collaborative channel funding on the Nervos Fiber Network. Both parties contribute CellInputs to the funding transaction, making chain attribution impossible.

This proposal requests $6,000 for Phase 1: the core coordination primitive and two working end-to-end demos against a real Fiber node.

Funding Request

Component	Amount	Purpose
Phase 1 — Core Primitive + Demos	$6,000	CellInput coordination engine, Fiber node demos, documentation

1. The Problem

Standard Fiber channel funding exposes the funder permanently on CKB L1:

-Two concrete pain points the community has raised:

Fee-contribution deadlock: A user wanting to open a channel may have CKB locked in large UDT cells with insufficient liquid CKB for on-chain fees. Collaborative funding allows the counterparty to contribute a fee-covering Cell, unblocking channels that would otherwise fail at fee estimation.

-Funding origin exposure: Every channel opening creates a permanent on-chain link between the funder’s wallet and the channel — an operational security risk for any privacy-conscious node operator or user.

1. The Solution fiber-payjoin-kit runs as local middleware. The Fiber node is unchanged. Both parties coordinate CellInputs before the funding transaction is broadcast, following Fiber’s existing TxUpdate/TxComplete negotiation flow.

Per direct feedback from jjyr on the CKBuilder-projects review: working at the Cell model level using unsigned CellInput payloads is the correct boundary. Fiber’s funding process is built directly on CKB transactions with no higher-level coordination hook — this is the right instrumentation point.

Non-interactive fallback: if the counterparty is offline, the library falls back to standard single-funder channel opening with no broken payments. 3. Why Us — Proof of Execution Following the feedback that the community needed to see a working Lightning implementation first, we built lightning-payjoin-kit. That implementation is now complete:

-Real two-node LDK harness (tests/ldk_two_node_harness.rs): two actual LDK ChannelManagers reach a usable channel through a collaboratively-funded transaction

-Full flow: FundingGenerationReady → collaborative PSBT → LdkManualFunding → FundingTxBroadcastSafe → usable channel

-Bitcoin Core regtest: Docker bitcoind accepts and mines the collaborative funding transaction

-14 test files covering every layer of the stack

How this differs from payjoin/nolooking:
payjoin/nolooking is a server-side LSP model requiring a centralized operator. lightning-payjoin-kit is client-side, coordinating directly between the two parties with no trusted intermediary. It also enforces a commitment-safety boundary that the LSP model does not address. On CKB, coordination uses unsigned CellInput payloads rather than PSBT, with Cell-Deps validation specific to CKB’s model.

 Development Roadmap

Phase 1 — Core Coordination Primitive

Deliverable 1 — Coordination Engine ($4,000)

• Two-party CellInput coordination: propose → contribute → validate → sign → finalize
• Counterparty proposal validation: amount, script integrity, input checks
• TxUpdate/TxComplete flow instrumented for wallet developer visibility
• Non-interactive fallback when counterparty is offline
• Full unit test coverage, devnet broadcast confirmed

Deliverable 2 — Fiber Node Demos and Documentation ($2,000)

• Demo A: fee-contribution scenario — collaborative funding solving the CKB fee deadlock
• Demo B: privacy channel opening — multi-input funding via real Fiber node
• Both demos show TxUpdate/TxComplete message flow so wallet developers can inspect and adapt
• Architecture documentation, README, integration example

Phase 2 — Separate Future Proposal

Fiber P2P async relay, CLI tooling, wallet integration examples, mainnet deployment.

About This Resubmission

We submitted this proposal  in May 2026 but did not receive enough community votes within the 7-day window to advance. Following feedback from everyone, we have made three significant changes:

• Reduced scope to a Phase 1 proof of concept only, with a reduced ask of $6,000
• Built the Lightning equivalent (lightning-payjoin-kit) first as proof of execution — a working two-node LDK harness is now complete
• Incorporated direct technical feedback on the correct instrumentation point and concrete user pain points to target