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cc-implementation/README.md

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+# COLDCARD Website Implementation
+
+This is the code for the NFC Push TX implementation at <https://coldcard.com/pushtx>.
+
+## Overview
+
+We use the APIs of mempool.space and blockstream.info to broadcast the transaction
+and for retrieving additional transaction details afterwards.
+
+In an edge case where the transaction is already confirmed (e.g. same Push TX URL
+used twice), the APIs respond with a 400 error. If that happens, we use
+[bitcoinjs-lib](https://github.com/bitcoinjs/bitcoinjs-lib) to compute the TXID
+and try to fetch the details anyway.
+
+## How to Build
+
+To build, you need Node.js and NPM installed. There's a `Makefile` target for
+installing dependencies and running the build:
+
+```
+make build
+```
+
+This builds two versions:
+
+- Separate JS and CSS files that power <https://coldcard.com/pushtx> - saved to `build/`
+- A self-contained HTML where all JS and CSS is inlined, which is useful for easy
+  self-hosting - saved to `build-single-file/`.
+
+## Don't Trust, Verify
+
+The `SHA256SUMS` file includes hashes of all important files in this repo and is
+signed with the PGP key `3F4D 2119 6E14 FD39 3835  492D F960 23C5 8E14 72C9` in
+`SHA256SUMS.asc` (detached signature).
+
+To check the signature:
+
+```
+gpg --verify SHA256SUMS.asc
+```
+
+To check the hashes match:
+
+```
+shasum -a 256 -c SHA256SUMS
+```