DeGoogle Yourself

You're not going to find something outside of official large companies. The reason they work at all is because the money never goes through your account during that transaction, but between the bank and Google/Apple/whoever. It's all handwaving and trust relationships. A typical tap goes through (basically) these steps.

• Tap
• The phone requests a unique one-time token from Google wallet, which is an intent to pay, signalling that the payment will be authorized
• The POS system takes the token through the tap, sends it to their banking sofware
• The banking system then asks Google/Apple/whoever "Hey is this token legit". They then validate it
• The money transfers between Google -> Bank. (Google will probably front the actual deployment of money from your bank -> Google -> Their bank)

This is why it's the most secure, because nowhere in that process can anyone intercept anything about you, all they would get is a one-time use token, which is only good from that specific POS system. Even the POS doesn't really get much about you except for what Google/Apple responds with.

It's because of this behind-the-scenes transaction that makes it almost impossible to have an open source version. You would need to convince all of the existing banking infrastructure (who remember still use ~~csv files~~ "wires") to move money around in batches at night to not only implement Google, Apple, and Samsung, but also the open source app that is totally secure and legit, you can trust us.

Not only that, but you immediately open yourself up to issues with money too. Accepting money is easy, sending money out is just 3 1/2 headaches. Not only do you have the risk, you have security and compliance, you have agencies, you have auditing, FINRA, the SEC can get involved, and that's only in America. If you want to go international? Hoo boy, just take all of that neasea and personal liability and turn it up to 1000 because now you have to think about every country and their laws.

So, probably more than you wanted to know, but that's why you're not going to find one.

Source - worked FinTech for about 7 years, learned way too much about finance back then