Coldcard Firmware Update: Shared Mk4 & QNew Features: ? PushTX

PushTX.org has introduced a new feature that allows Coldcard hardware wallet users to broadcast freshly-signed Bitcoin transactions using near-field communication (NFC) technology.

hardware wallet Coldcard users now have a new way to broadcast their freshly signed Bitcoin transactions, thanks to a new feature introduced by PushTX.org.

This feature, which is available for Coldcard Mk4 and Q models, aims to streamline the transaction process with a focus on security and privacy.

Here's a closer look at how the NFC Push TX feature works and its technical considerations.

PushTX.org service now supports Coldcard Mk4 & Q models with a new feature: NFC Push TX. Once enabled with a service provider's URL, you can tap the COLDCARD and your phone will open a webpage that transmits your freshly-signed transaction onto the blockchain.

See Settings > NFC Push Tx to enable… pic.twitter.com/6sajvEzT7G

— COLDCARD (@Coldcardwallet) August 11, 2023

NFC Push TX for Coldcard Hardware Wallets

The NFC Push TX feature can be enabled in the Coldcard device settings. Once activated, the Coldcard will display an NFC animation after a transaction is signed, indicating that it is ready to be broadcast.

When the user taps the Coldcard against a compatible smartphone, the phone's browser will detect an NFC tag that contains a URL. This URL includes the signed transaction encoded in base64url format, along with a truncated SHA-256 checksum for verification.

As the URL is loaded in the phone's browser, it connects to a Bitcoin node and broadcasts the transaction to the network. This process simplifies the steps required to send Bitcoin transactions, potentially enhancing the user experience.

Public Services and Customization Options

PushTX.org offers support for various public services to broadcast transactions, including mempool.space/pushtx, coldcard.com/pushtx, and its own platform (pushtx.org/pushtx).

Moreover, users can host their own backend, which may be鏽 for those who prioritize privacy.

Technical Considerations for NFC Push TX

There are some technical considerations to be aware of when using the NFC Push TX feature.

The complete URL for the NFC Push TX feature can be as large as 8,000 bytes, which may exceed the capacity of some web servers that do not support URLs beyond 4,000 bytes.

Additionally, the NFC implementation on some smartphones may also have limits. Any truncation errors in the URL will be detected by the checksum included with the transaction.

The service URL provided must end with a ?, #, or &. While honest backends are expected not to log the IP address of incoming transactions, there is no way to enforce this, and it is important to note that services like CloudFlare will still see all traffic.

With the introduction of NFC-based transaction broadcasting by PushTX.org, Coldcard users now have a new method for sending Bitcoin transactions.