Payjoin 0.1.0

Payjoin C# Bindings

Welcome to the C# language bindings for the Payjoin Dev Kit!

Payjoin lets the receiver of a Bitcoin transfer contribute inputs to the sender's transaction. The result looks like any other transaction, which preserves privacy by poisoning the common-input-ownership heuristic that chain surveillance depends on, and it lets the receiver batch its own operations into the same transaction. These bindings implement both BIP 78 (synchronous payjoin) and BIP 77 (asynchronous payjoin, where sender and receiver exchange the transaction through an untrusted directory and never need to be online at the same time), and ship with native libraries for every supported platform, so no Rust toolchain is required.

Install

dotnet add package Payjoin --prerelease

Requires .NET 10.0 or later, on one of:

End to end example

The usage reference is the commented walkthrough in IntegrationTests.cs: TestIntegrationV2ToV2 drives a complete payjoin from both sides, executed by CI on every change so it cannot go stale, and narrates each protocol step, from opening and persisting the session through the receiver checklist to signing the proposal.

Receive a payjoin

A receiver session produces a BIP 21 URI to show the sender. It works with every wallet: a payjoin-aware sender upgrades to a payjoin, any other wallet simply sends to the address as usual.

A session starts with the directory's OHTTP keys, fetched through an OHTTP relay so the directory never learns your IP address, then builds a receiver for your address and persists every step to an event log so your app can crash or restart and resume where it left off. From there the session advances through a typestate flow: each state hands you a request to relay with your own HTTP client, and the response moves you to the next state, through checking the sender's original transaction, contributing inputs, and posting the proposal.

Send a payjoin

A sender session starts from a BIP 21 URI scanned from the receiver (Payjoin.Uri.Parse(...).CheckPjSupported()), which is used to create and sign the original PSBT, posted to the receiver's mailbox on the directory. The sender then polls for the receiver's payjoin proposal through the same request/response flow, and once it arrives, signs it and broadcasts it to the network. The sender half of the same walkthrough shows every step.

Resume after a restart

Sessions persist each step to an event log through a persister you implement over your own storage; replaying the log with PayjoinMethods.ReplayReceiverEventLog recovers the current state after a crash or restart. Every Save has a SaveAsync counterpart, with async persister interfaces for database-backed storage.

Stability

The package is pre-1.0 while the C# API stabilizes; expect breaking changes between 0.x releases. The version's build metadata names the wrapped payjoin core release, so 0.1.0+payjoin-1.0.0 packages payjoin 1.0.0, the first stable payjoin release.

Documentation and help

• Payjoin Dev Kit for protocol background and guides
• rust-payjoin is the Rust core these bindings are generated from, with the issue tracker for bugs and feature requests

To build the bindings from source, run the tests, or produce the NuGet package locally, see CONTRIBUTING.md.