PushTX.org Introduces NFC-Based Bitcoin Transaction Broadcasting for Coldcard Users

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.

#COLDCARD Firmware Update

# Shared Mk4 & Q

New Features:

📳 PushTX: 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) June 26, 2024

This feature is designed for use with Coldcard's Mk4 and Q models and aims to streamline the transaction process while maintaining a focus on security and privacy.

Overview of the NFC Push TX Feature

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

By tapping the Coldcard against a compatible smartphone, the phone's browser detects 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.

When the URL is loaded in the phone's browser, it connects to a Bitcoin node and broadcasts the transaction to the network. This process is intended to simplify 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 for broadcasting transactions, including mempool.space/pushtx, coldcard.com/pushtx, and its own platform (pushtx.org/pushtx).

Additionally, users have the option to host their own backend, which may appeal to those seeking enhanced privacy.

Technical Considerations

There are some technical considerations associated with the NFC Push TX feature.

For instance, 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.

The introduction of NFC-based transaction broadcasting by PushTX.org offers Coldcard users a new method for sending Bitcoin transactions.