Async Payjoin: The HTTPS of Bitcoin Privacy

The Payjoin Foundation has introduced Async Payjoin, a new open-source protocol designed to bring mass privacy adoption to Bitcoin. Drawing parallels to how HTTPS became the standard for secure web browsing, this technology aims to solve Bitcoin's privacy pain points through a decentralized, non-custodial standard.

Unlike previous iterations that required simultaneous online presence, Async Payjoin (Payjoin V2) operates asynchronously. It is a software library built with Bitcoin Core primitives, making it easy for wallet developers to integrate. By coordinating between sender and receiver, it disrupts the standard transaction patterns that blockchain analysts rely on, offering stronger privacy for all users as adoption grows.

The concept of Async Payjoin is being positioned as the HTTPS of Bitcoin privacy. Just as Let's Encrypt drove the mass adoption of secure web transactions in the 2010s, the Payjoin Foundation is building an open toolkit to ensure privacy at scale. This protocol is not a standalone wallet but a software library that any Bitcoin payments app can integrate.

The goal is to move away from privacy being a feature of specific, isolated wallets and toward an open standard that functions across the ecosystem. The Foundation emphasizes that this approach is modeled after the Bitcoin and Lightning dev kits, utilizing the same cryptographic primitives found in the main Bitcoin implementation to ensure seamless integration.

Key features of the standard include:

• Asynchronous operation: Users do not need to be online at the same time.
• Open integration: Available as a library for any wallet developer.
• Backwards compatibility: Users with non-supporting wallets can still send to Payjoin addresses.

Privacy on the Bitcoin blockchain is often compromised by heuristics used by chain analysts. A standard transaction typically involves one input from the sender, splitting into two outputs: one for the payment and one for the change. Analysts assume these two outputs belong to the same entity.

Async Payjoin dissolves this single-input heuristic. By facilitating coordination between the sender and the receiver, the transaction is co-created to include inputs from both parties. The result is a transaction with two inputs and two outputs, which looks different from a standard payment.

This process is entirely non-custodial and atomic. If both parties do not agree on the terms, the transaction is not valid. As more users adopt this method, the reliability of standard transaction analysis breaks down, increasing privacy for the entire network.

The development of Async Payjoin is spearheaded by Dan Gould, the Executive Director of the Payjoin Foundation. Gould has a long history in Bitcoin privacy, having worked on tools since the TumbleBit era and co-authoring BIP 77. The team also includes Yuval Kogman, advisory board member and Spiral Bitcoin Wizard, and Armin Sabouri, R&D lead.

The Foundation operates as a nonprofit organization, having applied for 501(c)(3) status. According to Gould, this structure is essential because for-profit privacy models have struggled to survive. He noted that while for-profits have an incentive to sell products that may not guarantee privacy, a nonprofit aligns incentives to solve the problem sustainably.

The project is supported by funding from:

• OpenSats
• Cake Wallet
• Spiral
• Human Rights Foundation

Gould emphasized the importance of this support, stating, "none of this work is possible without the funders."

A growing list of Bitcoin wallets already supports the Payjoin Foundation’s standards. For Async Payjoin (V2), supported wallets include Blue Wallet, Bull Bitcoin Mobile, and Cake Wallet. For the older V1 standard, support is available in BTCPay Server, Wasabi Wallet, and Sparrow Wallet among others.

Developers looking to integrate the protocol can find the technical reference at BIP 77 and access the plug-and-play dev kit on GitHub. The Rust implementation alone has seen contributions from 37 developers.

The Foundation encourages users to ask their preferred wallet providers to integrate this open-source standard, furthering the goal of widespread privacy adoption similar to the transition to HTTPS on the web.