Nerd of the Month: Payjoin

Bitcoin gets better when we pay together.

Written by Conor Okus

In this new recurring newsletter, we’ll be highlighting one grantee or project that our grant program supports per month. This month, it’s Payjoin. You know it, you love it, but you might not know anything about it.

Payjoin is a giant leap forward for bitcoin transactions, letting senders and receivers construct transactions together. The opportunity to batch together to optimize fees reduces transaction costs and improves network efficiency. Beyond cost savings, when both parties contribute inputs, Payjoin preserves privacy by breaking common blockchain surveillance assumptions, making it harder for a third party to assume payment amounts and cluster private spending behavior.

To date, we have given four grants to developers working on Payjoin: Dan Gould, jbesraa, and Spacebear. These grants have enabled the building of PDK (Payjoin Dev Kit), which allows wallet developers to implement Payjoin more easily in bitcoin products, ultimately providing the project’s myriad and essential benefits to more bitcoin users. For help visualizing how a world where Payjoin is thoroughly integrated might look, check out the Bitcoin Design Guide’s Payjoin Section.

The Payjoin development nerds closed out 2024 with a significant milestone, successfully launching Bull Bitcoin Mobile with Async Payjoin integrations. This achievement resulted from an intensive collaboration between Bull Bitcoin and LTBL, which saw them leverage the Payjoin-flutter library to seamlessly integrate Payjoin into their wallet. The team plans to continue focusing on these and several other refactor and stabilization efforts well into the future.

Speaking of, looking ahead to Q1 2025, the team is prioritizing wider Payjoin adoption while working to stabilize their V2 API for a 1.0 release and upcoming audit. Several promising integrations are in progress, including work with Cake Wallet and hopefully with LDK Node for lightning-payjoin integration. The team is also focusing on establishing best practices for session persistence and improving their development toolkit through initiatives like a new Nix Flake, placing special emphasis on ensuring that the V2 API maintains compatibility with future V3 goals, preventing the need for complete rewrites by implementors.

So, Payjoin. Good, right? As always, there is little to no downside to making bitcoin cheaper to use, more censorship-resistant, and private. That’s why the Payjoin development team is our Nerd, or Nerds, of the Month.