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Krux is open-source firmware that enables anyone to build their own Bitcoin signing device via off-the-shelf parts. It runs on Kendryte K210 devices such as the M5StickV and Maix Amigo, converting them into airgapped devices that can sign transactions for multisignature and single-signature wallets.

You can even attach a thermal printer and print out the QR codes that it generates.

To learn more about how Krux works, check out Getting Started.