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76 lines
3.6 KiB
Markdown
76 lines
3.6 KiB
Markdown
# NFC Push TX -- Coldcard Feature _(new)_
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This feature allows single-tap broadcast of freshly-signed transactions from a [COLDCARD](https://coldcard.com).
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`PSBT ==[SD|QR|NFC]==> COLDCARD signed TXN ==[NFC tap]==> Phone Browser ==> TXN Broadcast by node`
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Once enabled with a URL, the COLDCARD will show the NFC animation
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after signing the transaction. When the user taps their phone, the
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phone will see an NFC tag with URL inside. That URL contains the
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signed transaction ready to go, and once opening in the mobile
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browser of the phone, that URL will load. The page will connect to
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a Bitcoin node and send the transaction on the public Bitcoin network.
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This feature is available on Q and Mk4 and requires NFC to be enabled.
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See `Advanced/Tools > NFC Push Tx` to enable.
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## URL Protocol Spec
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The URL contains the signed transaction in
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[base64url](https://datatracker.ietf.org/doc/html/rfc4648#section-5)
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and an truncated SHA-256 checksum.
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Read the [full specification here](https://github.com/Coldcard/firmware/blob/master/docs/nfc-pushtx.md).
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## Public Services
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We know of the following services that your Coldcard can use:
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- `mempool.space/pushtx`
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- `coldcard.com/pushtx`
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- `pushtx.org/pushtx`
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You can also host your own backend and there may be other services out there
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we don't know about.
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## Roll Your Own
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This [github repo](https://github.com/Coldcard/push-tx) contains the source for
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the code hosted at `Coldcard.com/pushtx` for your review or to replace
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and personalize. It also is the source for this website (*pushtx.org*).
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A single-file (html and javascript) file is available
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at [from pushtx.org](cc-implementation/build-single-file/index.html)
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or slightly more directly
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[from the github repo.](https://github.com/Coldcard/push-tx/blob/master/cc-implementation/build-single-file/index.html)
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You can host this file anywhere your phone can reach, and then use that URL in your
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COLDCARD settings. It uses your phone's browser to submit
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to `mempool.space` and `blockstream.info` sites (both at same time). It is equivalent
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to the page hosted at `https://coldcard.com/pushtx#`
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It's not hard to write your own code to submit a signed transaction to your
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node directly using the RPC-JSON protocol. For maximum privacy, that would
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be best, since there is no need to involve outside services.
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### Notes
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- Complete URL might be as large as 8,000 bytes. Some web servers will not support beyond
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4k bytes and the NFC implementation of the phone may also have limits. Truncation
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errors will be detected by the checksum.
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- The service URL provided must end in `?` or `#` or `&`.
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- Honest backends will not log the IP address of incoming transactions, but there is
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no way to enforce that, and CloudFlare sees all.
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## Example URL
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```
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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
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```
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- this transaction doesn't have valid inputs, and will cause an error
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- mempool.space will redirect this to a testnet endpoint (because ends with `n=XTN`)
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