Krux
Krux is open-source firmware that enables anyone to build a Bitcoin signing device from off-the-shelf parts. It runs on cheap, general-purpose microcontrollers (e.g. M5StickV, Maix Amigo, Maix Dock) and turns them into air-gapped signing devices.
What It Is
- Open-source firmware written in MicroPython, designed for K210-based devices
- DIY build: no custom PCB, no secure element, no proprietary chips — buy commodity hardware and flash the firmware
- Air-gapped: communicates via QR codes; no USB data after setup, no wireless radios used
- Seed handling: generates and imports BIP39 seeds, displays xpubs, signs PSBTs
Key Features
- Works with coordinator wallets like Specter Desktop, Sparrow, and Nunchuk
- Supports Multisig setups as one of several signing devices
- Camera-based QR scanning (same air-gap pattern as SeedSigner)
- Fully inspectable build — firmware, schematics, and assembly are public
- Low cost: tens of dollars in parts versus $100+ for commercial hardware wallets
Security Model
Krux trades tamper-resistant hardware for transparency. There is no secure element; physical security and supply-chain verification are the user's responsibility. The trade-off is that every layer — hardware, firmware, and signing flow — can be inspected and rebuilt by the user.
Relation to Round Rock Bitcoiners
- Featured in Meetup #50 — On Self Custody & Entropy as a DIY alternative to commercial signers
- Fits the community's focus on accessible, verifiable self-custody
- Complements SeedSigner and Jade DIY in the DIY signing-device ecosystem
Resources
See also
- Custody Models — trade-offs between custody approaches
- SeedSigner — another DIY air-gapped signing device
- Specter Desktop — coordinator software for multisig