Krux

Krux is open-source firmware, developed under the selfcustody project, that turns inexpensive off-the-shelf Kendryte K210 development boards — such as the Maix Amigo, M5StickV, and Maix Dock — into air-gapped Bitcoin signing devices. Because it runs on general-purpose hardware the user supplies, Krux is a low-cost, DIY route to a dedicated signer rather than a packaged product: you buy a hobbyist board with a screen and camera, flash verified firmware onto it, and own a signing device no vendor ever knew would become one.

How it operates

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 for file-based transfer and an optional thermal-printer output for producing paper backups of mnemonics or SeedQRs — a genuinely hacker-spirited touch. Like SeedSigner, Krux can run fully stateless, with the user loading the seed phrase each session by keypad, SeedQR scan, or even physical dice rolls for entropy at generation time; it also supports encrypted mnemonic storage for users who accept that trade. It handles single-signature and multisig wallets, BIP-39 passphrases, and works with coordinators such as Sparrow and Specter Desktop through standard PSBT exchange.

Open source, with an honest caveat

The firmware is published openly on GitHub under the selfcustody organization, with documentation covering reproducible builds and signature verification of releases — the builder can verify what they flash before trusting it. The project's own documentation has been candid that the software had not undergone a formal independent third-party audit, and that users should weigh that before trusting it with significant funds. That candor is worth respecting rather than glossing over: it is exactly the kind of honest disclosure the self-custody ecosystem should reward, and it argues for using DIY signers as one cosigner in a multisig quorum rather than a lone key over life savings.

Why commodity-hardware signers matter

Practical notes for prospective builders: the supported boards differ mainly in ergonomics — screen size, battery, enclosure — so the choice is about comfort rather than security, and the project documents the trade-offs. Flashing follows the pattern every sovereign toolchain should: download a release, verify its signature against the maintainers' published key, then write it to the board; skipping verification quietly defeats much of the DIY security argument. Krux's dice-roll seed generation deserves special mention, since it lets a user create a wallet whose entropy provably came from physical randomness they performed themselves — no RNG trust anywhere in the chain. And the optional thermal printer, charming as it is, prints plaintext seed material: treat its output like the seed itself, and prefer stamped metal for anything long-lived. Used this way — verified firmware, physical entropy, disciplined backups, one seat in a multisig quorum — a sub-fifty-dollar board holds its own in a custody architecture alongside commercial hardware many times its price.

Krux sits in the same lineage as SeedSigner and Specter DIY: projects proving that sovereign-grade signing does not require proprietary devices or sealed supply chains. Generic boards purchased from ordinary electronics channels carry little targeted supply-chain risk — no attacker knows this particular unit will hold keys — and published firmware means nothing in the stack is beyond inspection. The trade-offs are equally honest: no hardened secure element, so physical possession plus expertise is a real threat for stateful configurations, and the user carries the assembly and verification burden a vendor would otherwise shoulder. For a hands-on sovereign comfortable flashing firmware — the same person who repairs their own hashboards rather than shipping them away — that burden is the feature. See hardware wallet and self-custody for the surrounding territory.