Cashu at bitcoin++ Berlin: Highlights from the Payments Edition
From October 1 to 3, bitcoin++ payments edition took place in Berlin: a three-day event all about Bitcoin payments. Talks and workshops covered topics including Ark, Ecash, Lightning, privacy and what it takes for merchants to accept Bitcoin. Here are our Cashu highlights from the week:
In keeping with tradition, as in the past two years, we kicked off the week on September 30 with an Ecash Hackday, a satellite event bringing together developers from different projects to hack electronic cash.
We were happy to see so many participants, who presented 17 projects by the end of the day. A few examples:
- Find My Nuts uses Apple's Find My network to send Cashu tokens without an internet connection.
- A project that turns an M5Stack PaperS3 e-ink touch device into a Cashu wallet.
- A Fedimint running across three implementations.
- Peppa Pay, a browser extension that removes advertisements in exchange for Cashu payments.
- Cashu-XX, a game where you collect Cashu tokens and learn how to merge new NUTs into the spec.
The conference opened with two Cashu talks on day one. In Federated Cashu, Calle showed how a single Cashu mint can be run by multiple members:
the federation requires agreement from its members to issue blind signatures, uses consensus to order operations, and relies on a threshold key to protect the reserves. It takes advantage of Cashu's ongoing work, shifting to BLS and away from BDHKE in upcoming v3 keysets.
Following Calle's technical breakdown, Erik walked us through a presentation on Numo, an open-source Bitcoin point-of-sale app built on Cashu that lets merchants accept Bitcoin with tap to pay. A key part of development was putting Numo in merchants' hands and testing it on the ground. Their feedback shaped how Numo was built and led to new features.
One early user, for example, needed to tag individual transactions and export those tags for accounting before they could use Numo in their day-to-day work. Now each payment can be labeled directly in the POS.
Two workshops focused on Cashu libraries: TheMHV covered building a Cashu wallet with the Cashu Dev Kit (CDK), while Egge showed how easily Cashu payments can be added to a web application using Coco, a TypeScript library built on cashu-ts.
During another workshop, Hynek Jina introduced Linky, a mobile-first PWA for contacts, private Nostr messaging and Cashu payments.
In a presentation on Nucula, zeugmaster demonstrated how to build a Cashu payment terminal on an ESP32 microcontroller, from the cryptography behind Cashu to the hardware itself.
A great wrap-up was the hackathon awards ceremony, with two Cashu projects among the winners:
The team behind the Nodesignal podcast took first place by adding micropayment streaming to their podcast web player using Cashu Spilman channels. This approach allows individual micropayments to be sent without contacting the mint; the mint is involved only when a channel is opened or closed.
Another winning project, Nonfungible.cash, turns any picture into a private bearer collectible built on Cashu ecash. Because the ownership token travels in the image's EXIF header, sending the JPG transfers the NFT.
Thank you to everyone who hacked with us, gave talks, ran workshops, listened and joined in. This was fun!
