Vist Github repo for CKBunker.

"Basically the cost of a Bitcoin HSM with custom policies is now the cost of a coldcard and you don't need a thirty party to maintain it." - Francis P.

It's a python program that you run on a computer attached to a Coldcard. It will setup and operate the Coldcard in "HSM Mode" where it signs without a human pressing the OK key. To keep your funds safe, the Coldcard implements a complex set of spending rules which cannot be changed once HSM mode is started.

Using the tord (Tor deamon) you already have, the CK Bunker can make itself available as a hidden service for remote access over Tor. A pretty website for setup and operation allows access to all HSM-related Coldcard features, including:

• transaction signing, by uploading a PSBT; can broadcast signed txn using Blockstream.info (onion)
• define policy rules, spending limits, velocity controls, logging policy
• user setup (TOTP QR scan to enroll on Coldcard, or random passwords (Coldcard) or known password

The bunker encrypts its own settings and stores the private key for that inside Coldcard's storage locker (which is inside the secure element). The private key for the onion service, for example, is held on the bunker's disk encrypted like that.

Coldcard is a Cheap, Ultra-secure & Opensource Hardware Wallet for Bitcoin. Get yours at ColdcardWallet.com

Follow @COLDCARDwallet on Twitter to keep up with the latest updates and security alerts.

Do a checkout, recursively to get all the submodules:

git clone --recursive https://github.com/Coldcard/ck-bunker.git

Then:

• virtualenv -p python3 ENV (Python 3.7 or higher is required)
• source ENV/bin/activate (or source ENV/bin/activate.csh based on shell preference)
• pip install -r requirements.txt
• pip install --editable .

The executable is called ck-bunker:

$ ck-bunker --help
Usage: ck-bunker [OPTIONS] COMMAND [ARGS]...

Options:
  -s, --serial HEX  Operate on specific unit (default: first found)
  --help            Show this message and exit.

Commands:
  list     List all attached Coldcard devices
  example  Show an example config file, using the default values
  run      Start the CK-Bunker for normal operation
  setup    Configure your transaction signing policy, install it and then...

There are two modes for the Bunker: "setup" and "run mode". In setup mode, Tor connections are disabled, as is the login screen.

You would typically use the setup mode for picking the onion address, the master login password and all the details of the HSM policy.

$ ck-bunker setup

Open this URL in your local web browser (must be same machine): http://localhost:9823

Once the Coldcard is running in HSM mode, with your policy installed, it makes sense to operate in normal "run" mode:

$ ck-bunker run

You may also run with remote connections (and login) disabled. This would be useful if you have some existing web proxy already in place.

$ ck-bunker --local run

To access over Tor as a hidden service, you must have tord running on the same machine. For desktop systems, keeping TorBrowser open is enough to acheive this. On servers, start tord with default options, and ck-bunker will use the control port (localhost port 9051 or 9151).

If you use the bunker to broadcast the final (signed) transaction, the socks proxy of tord (port 9050) will also be used.

You will need:

• this code
• a Coldcard connected via USB
• tord (Tor program)
• Internet connection
• a Tor-capable brwoser, like "Tor Browser" or Tails.
• (optional) a microSD card, for logging of transactions on Coldcard

You can set a password and have your Coldcard sign anything below a custom amount provided you enter that password. Or instead require you to type a pin into the Coldcard for verification. 6102

Advanced: Your Coldcard could be in another country; you can lock Coldcard (boot-to-HSM™ feature). Remote hands can do power cycles if needed & keep Bunker running. Video-conference with them to send 6-digit code to complete a PSBT authentication (entered on Coldcard keypad). DocHex

The HSM policy on your Coldcard could enable spending to just one single cold-storage address (via a whitelist). When your warm wallet is in danger, pull this cord to collect all UTXOs and send them to safety, signed, unattended, by the COLDCARDwallet DocHex

Time-based 2FA code from the phones of 3 of these 5 executives needed to authorize spending; Each exec connects to Bunker at the same time, checks proposed transaction and adds their OTP code. Only the Coldcard and exec's phone knows the shared 2FA secret. DocHex ·

You can disable PSBT signing completely and allow automatic signatures on text messages. This turn the Coldcard wallet into an HSM for Bitcoin-based auth/attestations. Can be limited to specific BIP32 subpath derivations. Same with address generation/derived XPUBS. DocHex

There is a locker of about 400 bytes of secret storage in the Mk3 secure element. CK Bunker uses this to hold the secret that encrypts bunker's settings (when at rest), such as the private key for the address of the Tor hidden service. So a corrupt LEA can't impersonate your bunker after capture. DocHex

Heard you like co-signing! All the Bunker/HSM features work with multisig (P2SH / P2WSH) so maybe you're automatically co-signing some complex multisig from a CasaHODL quorum. DocHex

Sorry no. CK Bunker only works on Mk3 because we need the extra RAM and the newer features of the 608a secure element.

Hardware Security Module