Krux: install and first use

Krux is not a product you buy. It is open-source firmware that turns an off-the-shelf Kendryte K210 board — a class of cheap Chinese microcontroller device sold for machine-vision experiments — into an air-gapped bitcoin signer.

That distinction shapes everything. There is no Krux hardware, no Krux company, and no Krux packaging to inspect. You choose a supported device, put verified firmware on it, and what you end up with is as trustworthy as those two decisions were.

In exchange you get a signer for well under a hundred dollars, often with a colour touchscreen, and a feature set that is frankly broader than devices costing five times more. Budget an evening for the first run.

A Krux-running K210 touchscreen device on a dark desk showing a QR code, beside a D20 die and a handwritten column of numbers.

Image to come

1Choosing what to run it on

The supported devices differ more than the firmware does. Krux behaves the same on all of them; what changes is how pleasant the experience is and how much assembly you are signing up for.

• Ready-to-use touchscreen devices arrive assembled in a case with a battery. You unbox, flash, and use. For most people this is the correct choice and the reason to prefer Krux over a build-it-yourself signer.
• Development board kits are cheaper and more open, at the cost of assembling and enclosing them yourself.
• Screen size is not cosmetic here. You will be reading transaction details and scanning QR codes on this screen, repeatedly. A larger display is a security feature as much as a comfort one.

This is the main practical difference from SeedSigner, which is otherwise a close cousin: SeedSigner is three parts you assemble and possibly solder, while Krux is firmware for a device that may already be finished when it arrives.

Krux documentation   Krux source and releases

2Install the firmware, and verify it first

There is no tamper-evident bag in this story either, so the signature on the firmware is the only integrity check available. It is not optional.

Krux offers four installation routes: the Krux Installer GUI application, a pre-built official release, a pre-built test release, and building from source.

• The Krux Installer GUI is the sensible default. It downloads the release, verifies it, and flashes your device, which removes most of the ways this goes wrong by hand.
• Take the official release, not the test one. Beta builds exist for people testing Krux, not for people securing savings.
• Verify the signature before flashing if you are installing manually. This is the step that distinguishes running Krux from running something that says it is Krux.
• After the first install you can update by microSD, which means every subsequent firmware update can happen without the device ever touching a computer again.

3Generating a seed

Krux will not silently produce a seed from a chip you cannot inspect. It asks where the randomness should come from, and it shows its working.

The dice options are unusually generous, and Krux is one of the few signers that takes a twenty-sided die:

A D20 cuts a 24-word seed from ninety-nine rolls to sixty, which is a meaningful saving when you are doing it by hand. The rules from rolling your own entropy apply unchanged: record every roll, never re-roll a result you dislike, and remember the rolls are working paper rather than a backup.

The alternative is image entropy: photograph something chaotic and Krux derives the seed from a hash of the image's raw sensor bytes. Krux displays live quality indicators while you frame the shot — and its own documentation is careful to say these are approximations meant to guide your choice of image, not absolute measurements of cryptographic entropy. Treat them as a nudge away from photographing a blank wall, not as a score to optimise.

Whichever source you pick, Krux hashes it with SHA256, shows you the hash, and converts it deterministically into BIP39 words. That displayed hash is the device showing its work, and you can check the conversion independently later if you want to.

4The stored-mnemonic feature, and what it is not

Here is where Krux departs from its stateless cousins, and it is the part of the device most worth understanding properly.

Krux can store your mnemonic, encrypted, either in the device's own flash memory or on a microSD card. You choose a key, and the mnemonic comes back when you supply that key again. It is a real convenience: no re-typing twenty-four words at the start of every session.

The encryption is not decorative. Krux uses AES, and your key is not used directly — it is stretched through many rounds of PBKDF2 first, specifically so that guessing at it is slow.

Krux's own documentation says it plainly: stored mnemonics are for convenience only and should not be considered a form of backup.

That warning deserves repeating in the project's own terms, because the feature is genuinely easy to mistake for a backup. Encrypted storage lives on a device that can be lost, dropped, wiped by a firmware update, or simply fail. It protects the mnemonic while the device works. It does nothing at all once the device does not.

• Make a physical backup regardless — words on paper or metal, independent of any electronics.
• Test recovering from that physical backup before you send funds, exactly as you would with any other wallet.
• A forgotten encryption key is a lost mnemonic. The PBKDF2 stretching that protects you from an attacker protects the device from you too.
• Storing on the device concentrates risk. An encrypted mnemonic on the same device you carry through an airport is a different proposition from one on a card in a drawer.

5Signing a transaction

The signing loop is the familiar air-gapped round trip, with Krux offering two ways across the gap rather than one.

• Your coordinator wallet builds an unsigned transaction — a PSBT — and displays it as a QR code, or writes it to a microSD card.
• You load your mnemonic on the Krux device and either scan the code with its camera or read the file from the card.
• Check the amount, destination, and fee on the Krux screen. This is the whole reason the device has a display.
• Approve, and Krux returns the signed transaction the same way it came in — as a QR code, or back onto the card.
• Your coordinator reads the signature and broadcasts it.

Krux works with the mainstream coordinator wallets and supports multisig, so it slots into an existing setup rather than demanding its own.

6The unusual extras

Krux carries several features you will not find on most signers. None of them are required, and a first-time user should ignore all of them, but they are worth knowing exist:

• Printing and CNC engraving. Krux can drive a thermal printer, or output files for a CNC machine, to produce physical backups of a mnemonic or a QR code.
• Tamper detection. An experimental check intended to reveal whether the device has been interfered with between sessions.
• Mnemonic XOR. Splitting a mnemonic into parts that are individually useless and only reconstruct the original when combined.
• QR transcription tools. Guided help for copying a QR code onto a physical medium by hand.

Each of these adds a way to lose access if you misunderstand it. Get a plain wallet working and verified first; the extras will still be there next month.

7Verify before you fund it

On firmware you flashed yourself, running on hardware nobody certified, this stage is the one that converts hope into knowledge.

• Export the public keys to your coordinator wallet as a watch-only wallet, and confirm the fingerprint on the device matches the one in the software.
• Power the device off, then reload the mnemonic from your written words — not from encrypted storage — and confirm you reach the same wallet. This tests the backup you will actually need.
• Send a small test amount, confirm it arrives, then sign a transaction sending it back out.
• Note the firmware version and the device model, since both belong to the wallet you just made.

If the fingerprints disagree, stop and find out why before going any further. The software would be watching a different wallet from the one the device signs for, and every address it showed you would be wrong.

The short version

Krux is firmware, so the device is your choice and the verification is your job. Flash a verified official release, generate a seed from dice or a photograph, write the words down on something physical, and treat the encrypted-storage feature as the convenience its authors say it is.