Krux is an open-source DIY hardware signer for Bitcoin that can sign for multisignature and single-key wallets. It is a low-cost airgapped device built from off-the-shelf parts that communicates with wallet software via QR codes and wipes its memory after every session.

Disclaimer

WARNING: This software has not yet been audited by a third party. Use at your own risk!

Getting Started

Instructions for building and running Krux can now be found on our GitHub Pages site:

https://selfcustody.github.io/krux/

The instructions below are intended for developers who wish to contribute to the project.

Development

Fetch the code

Run the following:

git clone --recurse-submodules https://github.com/selfcustody/krux

This will pull down the Krux source code as well as the code for all its dependencies and put them inside a new krux folder.

Note: When you wish to pull down updates to this repo, you should run the following:

git pull origin main && git submodule update --init --recursive

This will make sure that all submodules (and their submodules, etc.) are pulled down and updated.

Install krux and dev tools

The krux code is a Python package that should be installed with Poetry.

poetry install

This will also install all development tools so that you can run tests, run pylint, format code with black, etc.

Note that you can run poetry install after making a change to the krux code if you wish to test a change in the interpreter.

Format code

poetry run black ./src && poetry run black ./tests

Run pylint

poetry run pylint ./src

Run tests

poetry run pytest --cache-clear --cov src/krux --cov-report html ./tests

This will run all tests and generate a coverage report you can browse to locally in your browser at file:///path/to/krux/htmlcov/index.html.

For more verbose test output (e.g., to see the output of print statements), run:

poetry run pytest --cache-clear --cov src/krux --cov-report html --show-capture all --capture tee-sys -r A ./tests

Use the Python interpreter (REPL)

This can be useful for testing a change to the krux code without having to run a full build and flash:

poetry run python

Python 3.9.1
Type "help", "copyright", "credits" or "license" for more information.
>>> from krux.key import Key
>>> Key("olympic term tissue route sense program under choose bean emerge velvet absurd", False).xpub()
'tpubDCDuqu5HtBX2aD7wxvnHcj1DgFN1UVgzLkA1Ms4Va4P7TpJ3jDknkPLwWT2SqrKXNNAtJBCPcbJ8Tcpm6nLxgFapCZyhKgqwcEGv1BVpD7s'
>>>

Live debug a device

If you've made a fresh build and flashed it to your device, you can connect to the device over serial connection with:

screen /dev/tty.usbserial-device-name 115200

If you see a Resource is busy message, make sure to shut down the Vagrant box and try again:

vagrant halt

If successful, the device should restart and you should see:

K210 bootloader by LoBo v.1.4.1

* Find applications in MAIN parameters
0: '       firmware', @ 0x00080000, size=XXX, app_size=XXX, App ok, ACTIVE
* Loading app from flash at 0x00080000 (XXX B)
* Starting at 0x80000000 ...


[MAIXPY] Pll0:freq:XXX
[MAIXPY] Pll1:freq:XXX
[MAIXPY] Pll2:freq:XXX
[MAIXPY] cpu:freq:XXX
[MAIXPY] kpu:freq:XXX
[MAIXPY] Flash:0xef:0x17
[MaixPy] gc heap=0x8029f430-0x8036f430(851968)
init i2c:2 freq:XXX
[MAIXPY]: find ov7740
[MAIXPY]: find ov sensor

From here, you can use the device as normal and, if you added any print statements to the code, they should appear whenever your code is reached.

You can also drop into a live Python REPL at any point by issuing an interrupt with Ctrl-C:

Traceback (most recent call last):
  File "boot.py", line 38, in <module>
  File "krux/pages/__init__.py", line 192, in run
  File "krux/pages/__init__.py", line 207, in run_loop
  File "krux/input.py", line 27, in wait_for_button
KeyboardInterrupt:
MicroPython; Sipeed_M1 with kendryte-k210
Type "help()" for more information.
>>>
>>>

Inspired by these similar projects:

• https://github.com/SeedSigner/seedsigner for Raspberry Pi (Zero)
• https://github.com/diybitcoinhardware/f469-disco for the F469-Discovery board

Contributing

Issues and pull requests welcome! Let's make this as good as it can be.

Support + Community

For support installing or using Krux, please join #krux:matrix.org and ask questions there. We do not use GitHub issues for support requests, only for bug reports and feature requests.

You can also post a question in our Discussions forum here on GitHub.

If you're on Telegram, check out the DIYbitcoin group, a broader community of tinkerers, builders, hackers, etc.