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Advancements in Ecash
Ecash is a digital cash system based on David Chaum's work from the 1980s, using blind signatures to create and transfer bearer tokens without revealing spenders. Cashu adapts this model by issuing bitcoin-backed tokens redeemable through the Lightning Network, while Fedimint applies a federated Chaumian design that allows communities to issue and redeem private bearer tokens. Both of these approaches aim to deliver private, fast, and offline-capable transactions.
As David Chaum once observed, the stakes of digital money go far beyond convenient payments.
The choice between keeping information in the hands of individuals or of organizations is being made each time any government or business decides to automate another set of transactions. In one direction lies unprecedented scrutiny and control of people's lives, in the other, secure parity between individuals and organizations. The shape of society in the next century may depend on which approach predominates.
—David Chaum
Over the past year, OpenSats has given long-term and targeted support to ecash developers and projects. This led to contributors proposing new protocol standards, improving reference implementations, enhancing wallet usability, and adding entry points for merchants and services. Several new standards now span multiple projects, signaling adoption, interoperability, and the growth of bitcoin-based ecash infrastructure.
The nine initiatives highlighted in this impact report are:
The cumulative advancement of these projects marks a shift from experimentation towards reliable infrastructure, showing that ecash is becoming a more dependable part of Bitcoin's payment landscape.
Let's take a closer look at how each of them is making an impact.
Cashu
Over the past year, Cashu's developments focused on strengthening reliability, expanding interoperability, and supporting new use cases. With long-term support from OpenSats, Calle advanced new privacy-preserving standards, improved Lightning settlement, and introduced features that make multi-mint and shared custody practical. These efforts moved Cashu toward infrastructure that operators can run and wallets can trust. Many of the improvements were formalized as new NUT standards—which define the Notation, Usage, and Terminology of the Cashu protocol.
Among the changes, cached responses (NUT-19) allow wallets to recover after a failed transaction rather than risking loss of funds. Calle also authored clear authentication (NUT-21) and blind authentication flows, which allow mints to restrict access while preserving transaction privacy. An extension to NUT-11 introduced the SIG_ALL option, enabling shared custody and escrow by requiring multiple parties to authorize a spend. Work on payment requests (NUT-18) also provided Cashu with an equivalent of a Lightning invoice, giving wallets a standardized way to request tokens over different communication channels, such as HTTP or nostr.
These specifications transitioned into production. The reference implementation, Nutshell, added support for multi-mint payments and a Lightning path-finding loop, allowing a single invoice to be split across several custodians while improving payment success rates across multiple routes. The 0.17.x release series consolidated these advances into tagged builds (0.17.0), bringing Redis-backed caching, Keycloak-based authentication, and expanded quote responses that ease wallet integration in stateless environments.
Alongside protocol changes, Calle developed tools to improve reliability and adoption. The Cashu Auditor automates test payments between mints and exposes uptime through an API, giving wallet UIs visibility into mint health. NFChat demonstrated NFC-based transfers between Android wallets, and the Cashu Decoder made it easier for developers to inspect and debug tokens.
Cashu has grown into a true pillar of the Bitcoin open-source movement, driving innovations once thought impossible. The free and open-source project has brought countless new developers into Bitcoin. Today, Cashu powers Bitcoin communities worldwide, and is already at the heart of multiple companies’ products. This momentum is only possible thanks to our incredible grassroots community and the freedom-driven contributors who make Cashu what it is today.
—Calle
Work is underway on a proposal for a nostr-based backup format for mint lists, providing multi-mint users a portable, censorship-resistant way to restore their setup. Together with the Auditor, NFChat, and Decoder, these contributions strengthened Cashu's foundations, improved operator confidence, and moved the protocol closer to production-ready ecash infrastructure.
Cashu Dev Kit
The Cashu Development Kit (CDK) makes it easier for developers to build apps on top of the Cashu protocol. In 2025, the team delivered one of the most requested features: mobile bindings, which allow Cashu's core functions to run directly in iOS and Android apps through CDK Swift and CDK Kotlin.
These bindings make CDK the most accessible path for developers who want to create Cashu-powered experiences on mobile.
Mobile bindings have been the number one requested CDK feature for some time. With the release of our Swift and Kotlin bindings, CDK is the easiest way to get started building Cashu apps. As Bitcoin monetizes, Cashu will be the protocol which day-to-day and online-native payments are settled.
—David Caseria, CDK contributor
Beyond bindings, the project also worked on simplifying multi-mint wallet support. A new implementation will allow developers to handle balances across different mints more seamlessly, supporting common wallet actions without requiring manual mint management. This functionality will be built into the bindings once complete and released in a future version of CDK.
With OpenSats' support, the team is also building Sats App, a native wallet that applies CDK's new bindings on iOS and Android. An iOS TestFlight is targeted for late 2025.
CDK is emerging as the developer toolkit that connects protocol advancements with real-world applications, ensuring that Cashu can move from research and wallets into broader usage across mobile platforms.
Nutshell
As maintainer of the Nutshell reference implementation, lollerfirst focused on making Cashu reliable in practice. His work turned the protocol's advancements into production code, added safeguards around issuance, and started research into privacy and programmability features.
Using support from OpenSats, lollerfirst implemented cached requests and responses (NUT-19) and signatures on quotes (NUT-20) in production, ensuring wallets could safely retry failed operations and that minted tokens reached the intended recipient. Nutshell also added recipient-key locking for safer direct payments tied to the intended recipient's key, and simple shared-fund setups.
In addition, support for keyset management was added, with new identifiers, expirations, rotation tools, and per-keyset balance views. Operators now have clearer control, while users know their tokens remain valid as keys rotate.
Lightning settlement reliability has also been improved. Nutshell now retries different routes and splits payments into smaller parts, allowing them to be processed through multiple channels. This improves how Cashu wallets interact with Lightning, increasing the chances of settlement. The browser-based Cashu.me wallet adopted these upgrades, alongside NFC tap-to-pay, multi-mint balances, and simpler onboarding.
Efficiency improved with binary token serialization, reducing token size by approximately one-third and enabling smaller, faster QR codes that are easier to scan under real-world conditions.
On the research side, there was an exploration of anonymous-credential ecash (KVAC), which aims to unlink the act of receiving tokens from spending them, so mints cannot connect issuance to use. Prototypes were also tested for conditional payments, where Discrete Log Contracts (DLCs) make tokens spendable only if a specified condition is met, and synthetic-dollar backends with LNMarkets, which allow tokens to represent a stable-value balance rather than bitcoin's fluctuating price.
By mid-2025, Nutshell shipped stable versions that added wallet authentication, recoverable payments after failures, safer mint quotes, and support for multi-party custody. It also delivered stronger operator controls and research prototypes. Nutshell remains a core implementation in the ecosystem and a testbed for new ideas.
Minibits Wallet
Minibits is a Cashu wallet developed by independent contributor Misovan. It runs on mobile devices and keeps payments private while preserving the bearer nature of ecash tokens.
With support from OpenSats, Minibits expanded beyond Android this year. An iOS version launched via TestFlight, and EU users can also download it from the Freedom Store. The project updated its React Native codebase and libraries for compatibility with modern devices.
The wallet added migration and transfer features. Users can export and import tokens, mint lists, and contacts between devices, while keeping the same public wallet address when switching phones. It also adopted the compact v4 token format for all supporting mints and animated QR codes (NUT-16) for scanning and display, making large token sets easier to scan and share.
Reliability improved with WebSocket (NUT-17) updates, which instantly push balances, and the adoption of cached responses (NUT-19), which allows wallets to retry incomplete payments after a crash or disconnect—a safeguard now used across the ecosystem. On Android, the Nostr Wallet Connect commands have been adjusted to minimize the likelihood of system-level interruptions disrupting a payment in progress.
Minibits can now create and pay for payment requests (NUT-18), aligning with other wallets that adopted the same format. Tokens can also be sent over nostr direct messages, making it simple to pass ecash between people already using nostr clients. Another addition allows senders to lock tokens to a recipient's public key, with an optional expiration if the token remains unclaimed.
One of the most notable improvements was the ability to make offline transfers. A sender can prepare a payment while online, then go offline. The recipient can later connect to validate and redeem the tokens, or keep them circulating as ecash.
Thanks to the OpenSats grant, I was able to focus on the niche area of offline ecash payments. For those of us used to fast internet, this may seem like a curiosity, but in many underdeveloped regions it's a real necessity. Minibits has become a pioneer in showcasing the unique properties of ecash for moving bitcoin. The wallet enables the payer to be completely offline, leveraging ecash's nature as a true bearer token stored directly in the wallet.
—Misovan
By broadening platform support, improving reliability, enabling sender-locked and offline payments, and making transfers easier across devices, Minibits moved closer to being a daily-use mobile wallet for ecash.
Nutstash
Nutstash is a web-based Cashu wallet maintained by Gandlaf. Over the past year, with support from OpenSats, the project gained new features for multi-mint payments, offline use, protocol compatibility, and security.
The 2.2.0 release introduced multi-mint payments, allowing a single Lightning invoice to be paid using balances from multiple mints. Nutstash also added Lightning Address support, real-time balance updates over WebSockets, and compatibility with mints that use clear authentication and blind authentication.
Offline features advanced as well. Users can now receive tokens without an internet connection, with the wallet confirming validity before adding them to the balance. A coin selector has been added, allowing users to choose which tokens to spend when sending offline. QR code and NFC transfers were updated to support these flows, making ecash more practical in weak-connectivity environments. Support for payment requests was also added, joining other wallets in offering a standard invoice format.
Beyond the wallet itself, Gandlaf worked on supporting tools. The Cashu Brrr project introduced a new physical note template for printed tokens and launched OpenPleb, a self-hosted service that lets users pay fiat QR bills with Bitcoin via Lightning or ecash.
Community involvement remained a focus. Gandlaf gave the talk "Ecash on Bitcoin — Cashu New Frontiers" at Bitcoin Seoul 2025 and organized monthly Bitcoin Pusan meetups, offering developers and users practical exposure to ecash.
By mid-2025, Nutstash had achieved support for multi-mint payments, offline transfers, standardized payment requests, shared custody arrangements, and improved security. These features demonstrated the practical, reliable, and private use of ecash in real-world applications.
Npub.cash
Npub.cash makes it possible to receive Lightning payments as Cashu ecash. When someone sends a Lightning payment to a Lightning Address, npub.cash receives it, and converts it into tokens, holding them until the user returns online to collect them.
Within the past year, Egge released npub.cash v2, a major update that made the system easier to run. The new version dropped dependencies on third-party services and instead connects directly to Cashu mints. It also introduced mint quotes instead of proofs, added more secure login options, and reorganized the codebase for simpler deployment.
Users gained more control with this release. Npub.cash v2 allows users to select the mint they want to use and verifies the connection. It also added locked quotes, which require the wallet to sign before the tokens can be redeemed. This ensures tokens are only issued to the rightful user, extending safeguards from wallets to the npub.cash server. Egge also introduced Cashu-402 (NUT-24), a way for services to charge for access directly through the Cashu protocol.
A WebSocket endpoint has been added, allowing wallets to receive instant notifications when a payment arrives, rather than waiting for periodic refreshes. To handle many concurrent payments reliably, Egge also created almnd, a scheduling tool that spaces out requests to mints, allowing the system to track thousands of payments without overloading servers.
Other wallets began integrating these improvements. Cashu.me added npub.cash v2 as an experimental feature, and the Sovran wallet listed support for using npub.cash addresses with different mints. Egge also built nut-bridge, a prototype that forwards nostr zaps as ecash "NutZaps" and issues receipts.
Egge's work on npub.cash is tied to his role as a maintainer of cashu-ts, the main TypeScript library for the protocol. He contributed hundreds of changes, helped lead multiple releases, and launched a new documentation site.
With npub.cash v2, Lightning Addresses are no longer tied to a specific wallet. Users can get an address right away, wallets can support whichever address their users prefer, and providers can launch their own services in minutes.
—Egge
Npub.cash v2 made Lightning Addresses portable and easy to host. It allows people to receive Lightning payments as ecash without staying online, while giving them the choice to use a public service or run their own.
Hashpool
Hashpool is an accountless mining pool that uses ecash tokens to represent mining shares. Instead of tracking balances in a custodial database, the pool issues miners these private, bearer-style tokens for their work. When blocks are found, the tokens can be redeemed for bitcoin.
With support from OpenSats, developments have focused on integrating ecash issuance directly into the Stratum V2 protocol.
The Cashu mining share payment method protocol has evolved significantly and is close to being PR-ready. Major improvements include rewriting the cdk branch to conform to best practices within the cdk repo and eliminating the need for a quote lookup API, which significantly increased the privacy and security attack surface of the protocol. Work on Hashpool has resulted in a number of merged commits to the cdk project.
The mint was refactored into a proper Stratum V2 role using all new extension message types, and the hashpool repository was upgraded with Nix Flakes, Bitcoin regtest setups, and command-line utilities for testing wallet balances and redemption. These changes make Hashpool easier to test and operate under real-world mining conditions.
Hashpool developer vnprc also published a rust crate for transmitting bitcoin transactions via nostr, and continues to experiment with using nostr as a payment forwarding layer. He is also working on a new user interface, which will be crucial to improving Hashpool's user experience and tooling.
A related project called Axepool applies the same design to small-scale miners, particularly those running Bitaxe hardware. By issuing eHash (ecash tokens backed by proof of work) directly for contributed hashpower, Axepool removes the need for miners to run a hosted service and enables direct ecash payouts, making it easier for everyday miners to receive frequent payouts. The Axepool design is still in the conceptual phase and undergoing rapid iteration. Together, Hashpool and Axepool demonstrate how ecash can replace traditional pool accounting with direct, accountless mining rewards that are private and transferable.
Fedimint
Fedimint is an ecash protocol based on a federated Chaumian ecash framework, designed to let communities pool custody and issue private bearer tokens. With OpenSats' support, contributors made progress delivering software and tooling.
The Lightning Gateway project, spearheaded by m1sterc001guy, introduced LNv2, a new payment system that fixes malleability and simplifies invoices. Operators also gained a self-contained LDK mode that eliminates the need for an external Lightning node, along with improved fee handling, authenticated endpoints, and simple 12-word recovery for both ecash and on-chain funds.
Networking and deployment were improved by Harbor and core contributors, led by oleonardolima. Tor support was added for private connectivity, while Iroh peer-to-peer networking removed the need for domain names or port forwarding. Packaging advances delivered Docker Compose files and a Start9 service, making federations easier to launch and manage.
Wallet safety and upgrades were the focus of core releases coordinated by Bradley Stachurski, while database migrations and recovery tools were introduced. Real-world federations tested and hardened these changes.
Experiments such as ROASTr using nostr-signing, conducted by m1sterc001guy, and Ethan Tuttle, demonstrated how federations could collectively sign and broadcast authenticated messages. Testing utilities like Devimint and packaging for StartOS simplified onboarding providing operators with plug-and-play options for deployment.
Together, with support from OpenSats, these efforts delivered real progress on gateways, wallets, networking, and new modules to the broader ecash landscape.
OpenCash
The OpenCash Association is a nonprofit dedicated to advancing research and development of open-source ecash systems built on Bitcoin. Using support from OpenSats, its mission is to fund contributors, coordinate community efforts, and create the conditions for Cashu to move from prototypes into tools with real-world applications.
In early 2025, OpenCash launched a bounty for a BTCPay Server plugin that would allow merchants already using BTCPay to accept Cashu payments. Community developers delivered an early beta plugin, BTCNutServer, aligned with that scope. The plugin allows merchants to either accept and hold ecash directly or automatically convert tokens into Lightning invoices. Mint whitelists give operators the ability to specify which issuers they trust.
OpenCash also supported adoption by sponsoring design work. It backed a Cashu track at the 2025 Bitcoin Designathon, which brought UX specialists into the ecosystem. The work addressed wallet setup, backup and recovery, handling multiple mints, and NFC-based payments, producing design patterns that wallet teams could adopt directly.
Coordination across the developer community remained a priority. OpenCash organized recurring public developer calls, publishing minutes and agendas. These calls facilitated the adoption of standards such as WebSocket (NUT-17) updates and Payment Requests (NUT-18), ensuring implementations remained aligned.
To create a clear entry point for newcomers, OpenCash maintained opencash.dev as the hub for grants, bounties, and updates. It also contributed to resources such as cashu.space and docs.cashu.space, shortening the path for both developers and users who want to get involved.
With long-term support from OpenSats, OpenCash advanced Cashu at several levels: it funded the first merchant bridge through BTCPay, sponsored design work, coordinated protocol development through public calls, and maintained a stable entry point for newcomers. These steps strengthened the foundation for Cashu as a privacy-preserving part of the Bitcoin ecosystem.
Ecash offers a path toward practical, privacy-preserving payments layered on Bitcoin. It combines the speed and liquidity of Lightning with strong privacy guarantees, bearer-style tokens, and the ability to transfer value offline or asynchronously.
The advancements outlined above show a clear trend: as protocol standards mature, wallets are becoming more interoperable, mints more resilient, and tooling more user-friendly. These infrastructure developments allow merchants to begin relying on ecash, while user-facing improvements are making wallets faster, safer, and easier to use.
Much work remains, but the direction is clear: ecash is evolving into a durable infrastructure that expands the payment capabilities of bitcoin as an asset. OpenSats will continue to support the contributors and maintainers who are driving this progress.
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