Definition
Krux is open-source firmware, developed under the selfcustody project, that turns inexpensive off-the-shelf Kendryte K210 boards, such as the Maix Amigo, M5StickV, and Maix Dock, into air-gapped Bitcoin signing devices. Because it runs on general-purpose hardware that the user supplies, Krux is positioned as a low-cost, DIY route to a dedicated signer rather than a single packaged product.
Operation
Krux performs all signing offline. It reads unsigned transaction data and wallet descriptors as QR codes through the board's camera and outputs signed data as QR codes on its screen, with an optional microSD path and an optional thermal printer for backing up mnemonics or SeedQRs. It supports single-signature and multisig wallets and works with coordinators such as Sparrow and Specter through standard PSBT exchange.
Open-source status
The firmware is published openly on GitHub at the selfcustody organization, with documentation covering reproducible builds and signature verification of releases. The project's own documentation notes that the software had not been formally audited by an independent third party, so users are advised to understand that caveat before trusting it with significant funds.
For background, see air-gapped signing and seed phrase.
In Simple Terms
Krux is open-source firmware, developed under the selfcustody project, that turns inexpensive off-the-shelf Kendryte K210 boards, such as the Maix Amigo, M5StickV, and Maix Dock,…