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README.md

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-# push-tx
-NFC Push TX
+# NFC Push TX
+## _NEW_ Coldcard Feature
+
+This feature allows single-tap broadcast of the freshly-signed transaction.
+
+`PSBT ==[SD|QR|NFC]==> COLDCARD signed TXN ==[NFC tap]==> Phone Browser ==> Server TXN Broadcast`
+
+Once enabled with a URL, the COLDCARD will show the NFC animation
+after signing the transaction. When the user taps their phone, the
+phone will see an NFC tag with URL inside. That URL contains the
+signed transaction ready to go, and once opening in the mobile
+browser, that URL will load. The landing page will connect to a
+Bitcoin node (or similar) and send the transaction on the public
+Bitcoin network.
+
+This feature is available on Q and Mk4 and requires NFC to be enabled.
+See `Advanced/Tools > NFC Push Tx`
+
+## Protocol Spec
+
+The COLDCARD needs a URL prefix. To that it appends some values:
+
+- `t=...` 
+  - this is the transaction, in binary encoded with
+    [base64url](https://datatracker.ietf.org/doc/html/rfc4648#section-5)
+
+- `&c=...`
+    - the rightmost 8 bytes of SHA256 over the transaction. Also `base64url` encoded.
+
+- `&n=XTN`
+    - if, and only if, the COLDCARD is set for Testnet, this value is appended to
+      indicate that the transaction is for Testnet3 and not MainNet.
+    - when RegTest is enabled, the value will be `XRT`
+
+We provide a few default URL values to our customers, including one backend we
+will operate on `colcard.com`. The URL can also be directly entered by the
+customer. On the Q, it can be scanned from a QR code.
+
+For COLDCARD backend, the url used is:
+
+    https://coldcard.com/pushtx#
+
+The complete URL with a typical transaction might look like this (but longer):
+
+    https://coldcard.com/pushtx#t=AgAAAAMNCxXtp2GVYVhkRXHLMmdZFs4p3kbFK ⋯ ABf&c=uiSVRda-1tw
+
+We are using hash symbol here so that our server logs do not get
+contaminated with the arguments. The landing page uses javascript
+to read the hash part of the URL and decodes from there. If you
+prefer, your URL can end with `?` and then the arguments will be
+sent by the phone's browser to your server. Your processing can be
+entirely done in the backend in this case.
+
+## Expectations for the Backend
+
+Your code should decode the transaction and check the SHA-256 hash
+matches. If it does not match, or if `c` value is missing, assume
+the URL has been truncated and report that to the user.
+
+Once decoded, your code should immediately broadcast the transaction.
+A confirmation step is not required in our opinion. Once it is
+submitted to Bitcoin Core (or other API), any status response should
+be decoded and shown to the user so they know it is on it's way.
+If it was not accepted, please report the error to the user as
+clearly as possible.
+
+Next, it would make sense to either link to the TXID on a block
+explorer to provide further proof that it has been sent and that
+it is now waiting in the mempool.
+
+## Backend Implementations
+
+- Mempool.space's [implementation of this feature](https://github.com/mempool/mempool/pull/5132).
+
+- A single-file (html and javascript) file is available
+  at [coldcard.com/static/coldcard-pushtx.html](https://coldcard.com/static/coldcard-pushtx.html).
+  You can host this file anywhere your phone can reach, and then use that URL in your
+  COLDCARD settings. It uses your phone's browser to submit directly
+  to `mempool.space` and `blockstream.info` sites (both at same time). It is equivalent
+  to the page hosted at `https://coldcard.com/pushtx#`
+
+### Notes
+
+- Complete URL might be as large as 8,000 bytes. Some web servers will not support beyond
+  4k bytes and the NFC implementation of the phone may also have limits.
+- The service URL provided must end in `?` or `#` or `&`.
+- `base64url` values from COLDCARD will not have padding (`=` bytes) at end.
+- POST cannot be used directly because the expect the phone to do a GET on the URL provided.
+- Honest backends will not log the IP address of incoming transactions, but there is
+  no way to enforce that, and CloudFlare sees all.
+
+## Example URL
+
+```
+https://mempool.space/pushtx#t=AgAAAAOHqK3w3hC6PSC0buthnJA5R9Y88WAlEvm9cifNVUPhIwAAAABqRzBEAiB-M9YprNYoohqHdQHg4wY_qcEMwDmyIQH8prykk8-0KwIgARxcojKrtixicouiUxhk4jQq_MAl11ptIgHDlRjgk5ABIQM4bgMAVDbDSr_9CvLjbg5nxrWnDGI-kVmkfL81GXZtCf____8OaH0RxW7DjZKdIF6rvbHvvyFGCBQ0PTgpx20nA_wbLgAAAABqRzBEAiBwUFigORJDPK8ptnYPAntjV-RUn1jAuzphicQstwVv-QIgEbMC8FWXQ5Jve5DaAqKJsqoj3peK83iub_oOkmbiYg4BIQO5Ehn2t0oUG3hnK4cBnwCwMc33DcdJ8aSMWzRQ_wjZL_____-UG6M-eBeAun-EZp6EbVypvVJ3mXCQrN_fUDn-kwoEnQAAAABqRzBEAiAgFAtVTpQYTKplc9NuV7Ws7ZFYeNO8BCS4ozgWrgd2ogIgGTTcw98xQdcGWeWQhVfVm_vZorBIOYovQPQeK0Lg9t8BIQLPWPioVWvj1z4NMHBCkeirYOUalCa83wbSH0CREnGZvv____8CjM_wCAAAAAAZdqkUIJA8_yqzaj0NzhvYVEIBno5gETGIrIzP8AgAAAAAGXapFEaV7xTyleuEX9OejdlUlsz7RTr0iKwAAAAA&c=hre47vyMC78&n=XTN
+```
+
+- this transaction doesn't have valid inputs, and will cause an error
+- mempool.space will redirect this to a testnet endpoint (because ends with `n=XTN`)
+
+