NFC Push TX -- Coldcard Feature (new)

This feature allows single-tap broadcast of freshly-signed transactions.

PSBT ==[SD|QR|NFC]==> COLDCARD signed TXN ==[NFC tap]==> Phone Browser ==> TXN Broadcast by node

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 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 to enable.

URL Protocol Spec

The URL contains the signed transaction in base64url and an truncated SHA-256 checksum.

Read the full specification here.

Public Services

We know of the following services that your Coldcard can use:

`coldcard.com/pushtx'
`mempool.space/pushtx'

You can also host your own backend and there may be other services out there we don't know about.

A single-file (html and javascript) file is available at 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#

Roll Your Own

This github repo contains the source for the code hosted at Coldcard.com/pushtx for your review or to replace and personalize.

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. Truncation errors will be detected by the checksum.
The service URL provided must end in ? or # or &.
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)

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