hahaha, this comment is hilarious.  Your parent comment shared an x-ray level of detail about the tech including naming the credit card reader model (MagTek), and a complete inventory of their technical stack (&quot;Node.js on the Pis, NaCl for all crypto, TypeScript on the server (and Node.js), Stripe for handling payments, GraphQL for query handling for the iOS clients&quot;) and technical implementaiton details (QR code display and reading to distribute public keys during setup, Bluetooth LE data transfer code between Node.js on the Pis and iOS) in addition to the point of the whole solution (&quot;sending the data to the server over Wi-Fi and in the streetcar, over Bluetooth LE to a co-located iPad Pro, and from there we used its cellular connection to talk to the server.&quot;)<p>What more details could you possibly want?  He&#x27;s gone above and beyond in sharing.<p>Thanks, erichocean