NumoPay: Tap-to-Pay Bitcoin with Calle | SLP728

**Calle** (0:00)
We wanted to see Tap2Pay in Bitcoin. We're all kind of jealous about the fiat payment experience with, you know, you tap your phone and the payment goes through instantly and takes maybe a second or two.

**Stephan Livera** (0:10)
Hi, everyone, and welcome back to Stephan Livera Podcast. Today, we're rejoined by our friend Calle. Calle is known for a bunch of things, probably mostly known for his work on Cashu, but also working on BitChat. And I know Calle recently put out this new thing, it's called Numopay. And I wanted to chat about that because I thought it's fascinating. I had a quick chance to play around with it. First off, welcome back to the show, Calle.

**Calle** (0:32)
Hey, thanks for having me, Stephan.

**Stephan Livera** (0:35)
So let's just start with the obvious, like why Numopay? What's the story behind it?

**Calle** (0:40)
So Numopay is a payment terminal that we've been working on for quite a while in the background in stealth mode in the last couple months or so. And it's a payment terminal, open-source payment terminal that anyone can download and install in their Android phone. And you can use it to receive Bitcoin in your brick and mortar store, basically. So you can run it on any Android phone out there or on a typical POS device, these open-source, open hardware POS devices that are basically everywhere. And the differentiating factor and the reason why we've been working on Numopay was because we wanted to see tap to pay in Bitcoin. We're all kind of jealous about the fiat payment experience with you tap your phone and the payment goes through instantly and takes maybe a second or two. And with Bitcoin, we kind of achieved good UX over the last couple of years, but we stagnated a little bit in terms of we all converge to QR codes and scanning, showing QR codes. And that is pretty common in many parts of the world. But all across the world, people are using more and more tap to pay using their credit cards. And with Numopay, that's our approach to bring that kind of experience into the Bitcoin world. And how we were able to achieve this is by leveraging the features or the properties of Cashu itself. So Cashu is a Taumin eCash system that is built on top of Bitcoin. It kind of gives you a private, instant, cheap way to make Bitcoin payments using payment rails that are built on top of Bitcoin.
It is a custodial system. That means you have to trust a mint operator with the Bitcoin when you deposit Bitcoin and receive eCash for it. The eCash itself is highly privacy preserving and it's a bare asset that you store on the device that you hold it on. So, for example, that could be your phone. And because the eCash is stored on the payer's phone, we can now build these experiences where if you tap that phone onto a NUMO terminal, the eCash can travel from one phone to another using the NFC chip. And that is exactly kind of the ingredients that you need in order to build this tap to pay UX flow for Bitcoin. But now with the twist that we have an open protocol that supports this, it can be built on top of basically anything. Obviously, we choose to build on Bitcoin and it is highly privacy preserving. That means kind of we have this rare occurrence where UX and privacy are perfectly aligned. Where the best privacy option basically also can give you the best UX option. And I think that that's not typical. Usually, you kind of have to make a trade-off between the two, but not in the case of E-cash.

**Stephan Livera** (3:23)
Excellent. And so, yeah, there's a lot in there. We're going to dive into some of those things, but I guess we'll talk through what it looks like. Now, I had just played around with it on my phone. I found it very simple. Basically, I mean, I could probably pull it up just on the screen. But basically, the gist of it is you download, you install this app, and it allows you to very quickly show a QR. That QR can show a Cashu, basically like a terminal, that people, listeners can see if you're watching on the video. But basically, it looks like just think you're at the farmer's market, you're the guy selling meat or whatever. You can literally type in, I want to receive whatever, one thousand one hundred cents or whatever it is, and just it shows a QR that I guess to your point, Calle, the difference is here, you can also not just scan to pay like we're used to in Bitcoin with Lightning, but you can also tap to pay. So just elaborate a little bit on what that looks like for listeners.