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Bitcoin vaults

This project provides a prototype for bitcoin vaults based around the concept of sharding and pre-signed transactions. The advantage of sharding into many UTXOs with different relative timelocks is that it gives the user an opportunity to observe on-chain thefts and react to them, without losing 100% of the funds.

See a visualization of the planned transaction tree:

WARNING

This is not production-ready code. Do not use this on bitcoin mainnet or any other mainnet. In fact, the private keys are static and hard coded into the prototype. Don't use this right now. It's only been used for about ~50 vaults, and before any real-world usage there should be many thousands of tested vaults.

Background (summary)

A vault is an on-chain construct that keeps coins locked up until someone opens the vault. Upon opening the vault, the user has the option of sending coins back into a new vault configured with identical keys, sweeping the coins to a cold storage wallet or more-deep cold storage vault, or allowing the coins to exit the vault and be used by some hot wallet.

Sharding refers to a policy recommendation of splitting a bitcoin UTXO into multiple new UTXOs that can each become un-vaulted only on a specific schedule. The purpose of sharding is to minimize the total amount lost due to theft. By sampling the hot wallet with only one sharded UTXO at a time, the user can immediately push the remaining UTXOs to cold storage the moment a theft on the hot wallet is detected. Without this feature, a user might keep 100% of their funds in a single UTXO which can be stolen by that same kind of thief.

Additional background:

https://www.coindesk.com/the-vault-is-back-bitcoin-coder-to-revive-plan-to-shield-wallets-from-theft

https://bitcoinmagazine.com/articles/revamped-idea-bitcoin-vaults-may-end-exchange-hacks-good

https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-August/017229.html

https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-August/017231.html

What's in here?

This repository contains an experimental prototype of Bitcoin vaults using pre-signed transactions.

The main entry point is in initialize.py in the commands/ directory.

It works by connecting to bitcoind using RPC, mining some BTC to a specific address, and then begins planning an entire tree of pre-signed transactions that implement the vault. After tree planning, the tree is signed.

To run the vault program do something like:

mkdir -p /tmp/vaults/001/
cd /tmp/vaults/001/

vault init

This will produce a few files. The interesting one is log.txt and transaction-store.json which has the list of transactions.

In log.txt or text-rendering.txt scroll to the line that starts with "Start" and those will be the signed transactions ready for broadcast.

Modules:

Source code:

Internal details

The two main entrypoints of interest are setup_vault in the planner.py file and sign_transaction_tree in the signing.py file. In practical usage, vault init calls the entrypoint commands/initialize.py which calls the subsequent functions.

The project can only be run if bitcoind -regtest is running in the background. It currently looks for ~/bitcoin/bitcoin.conf to figure out the bitcoin RPC parameters. (TODO: Spin up regtest nodes automatically, especially for tests.)

PlannedInput, PlannedOutput, and PlannedTransaction are custom classes that represent the transaction tree, as defined in the models. The real bitcoin transactions are assembled in place hanging off of these objects. output_utxos is for the outputs on the current transaction, while child_transactions on a PlannedUTXO are a list of possible child transactions. Obviously, because double spending is forbidden, only one of those child transactions can make it into the blockchain.

ScriptTemplate and its descendents are how UTXOs can describe themselves. Each UTXO in the planned transaction tree can use one of a limited number of scripts that were used to design the transaction tree. ScriptTemplate is used to derive the scriptPubKey value and the witness necessary to spend that UTXO. The witness is obviously applied to the input that spends the UTXO.

PlannedInput.parameterize_witness_template_by_signing is responsible for parsing a witness template, signing, and producing a valid witness. This is based off of one of those ScriptTemplate classes that each UTXO picks: the input has to have a witness that satisfies that UTXO's script template and script.

After running vault init, to load the serialized data, do the following:

from vaults.experiment import *
initial_planned_transaction = load()

But it is probably simpler to do a one-liner like:

from experiment import *; initial_tx = from_dict(json.loads(open("output-auto.txt", "r").read()));

initial_tx.name
initial_tx.output_utxos
initial_tx.output_utxos[0].name
initial_tx.output_utxos[0].child_transactions
name = initial_tx.output_utxos[0].child_transactions[0].name
assert name == "Vault locking transaction"

Usage

This package installs the vault command, which is an interface for working with the vault library based on vault files stored in the current working directory. The vault subcommands are as follows:

vault init
vault info
vault broadcast

vault init turns the current working directory into a new vault with new parameters. It creates all of the pre-signed transaction trees and also creates the signatures. Note that vault init does not broadcast the transaction putting the coins into the vault-- that is the user's responsibility using the other commands, as are any other broadcast actions- again using the other available commands.

vault info gives information about the current status of the vault.

vault broadcast transmits a pre-signed bitcoin transaction to the bitcoin network.

Filesystem

The vault init should be run after creating a new directory via the mkdir command. Inside of the directory, run vault init and it will check for the presence of a vaultfile file. The purpose of the vaultfile is to place a marker on the filesystem indicating that the folder already contains a vault and that the program should not overwrite it. Inside the file is some versioning information for future upgrade and file format version management reasons.

Installation

# clone the repository
git clone ...

# setup a new virtualenv to avoid system-wide installation
python3 -m venv venv/

# enter into the virtualenv
source ./venv/bin/activate

# install all python requirements
pip3 install -r requirements.txt

# Use install to copy the code into the virtualenv's python modules directory,
# or "develop" to use symlinks such that updates to the code propagate without
# requiring re-installation.
python3 setup.py install

bitcoin.conf

bitcoind configuration must include:

regtest=1

# Data directory: where should this regtest blockchain live?
#datadir=/tmp/bitcoin/regtest/

txindex=1

debug=1
logtimestamps=1
printtoconsole=1

server=1
listen=1
maxconnections=500
noconnec=1

# Override fallback fee estimation.
# (perhaps it can be disabled with =0?)
fallbackfee=0.0001

# To allow for creating zero-value CPFP hook outputs.
acceptnonstdtxn=1

# To allow for zero-fee transactions.
minrelaytxfee=0

# To mine zero-fee transactions (CPFP not implemented yet)
blockmintxfee=0

Run with bitcoind -regtest if this is your ~/.bitcoin/bitcoin.conf file, otherwise you will have to run with -conf=/path/to/bitcoin.conf each time.

bitcoin-cli -regtest getblockchaininfo

Testing

Run the tests like this:

python3 -m unittest

For regtest testing, some setup is required (and this changes based on the secret key, and also based on the amount that is being inserted into the vault as defined in the initialization function):

bitcoin-cli -regtest generatetoaddress 100 bcrt1q08alc0e5ua69scxhvyma568nvguqccrvah6ml0
bitcoin-cli -regtest sendtoaddress bcrt1q08alc0e5ua69scxhvyma568nvguqccrvah6ml0 5

Private key

Pre-signed vaults are reliant on secure key deletion of a private key.

In python-vaults, the prototype has two ways to provide a private key-- one way is the default private key is constructed using the "correct horse battery staple" passphrase, and the other way is to pass --private-key=$regtestprivatekey to the vault init command.

In the prototype, all of the keys- including for cold storage- are equal to the given private key. In the future, this needs to be updated and configured to include real values for the user's cold storage system and the other required keys.

The default private key is not secure. Instead of passing an override private key, another option (in the future) would be to implement something that uses system entropy to generate keys. However, the keys for cold storage and the hot wallet should be from other machines.

License

BSD

Is this ready for production?

No, this software is not ready for production. There are a number of things that need to be done before this could be used in production. This list is a good starting point, but it is not exhaustive.

Private keys: The way that private keys are handled should be improved. Secure key deletion is currently not happening. Private keys could be generated from some entropy- or from dice rolls- but this is also currently not occurring.

Multi-client protocol: Right now the prototype assumes control of two private keys for the pre-signed transaction tree. However, in practice, there should be multiple clients that pass data off to each other.

Extensive testing: There should be substantial more unit testing of all the various functions, and a test framework to simulate many thousands of different scenarios with different initial configurations or parameters. Also, these tests should all be performed on not just the regtest network but also signet and testnet, and only then moving on to small-scale mainnet tests.

Improved initialization: The current workflow for working with Bitcoin Core wallets is a little weird-- based on using RPC against a bitcoin wallet with bitcoin stored on the wallet. Instead, the initial transaction should be constructed another way, perhaps with some steps that the user has to manually run to feed in a txid, vout, etc.

User guides: User documentation should be written for all procedures and operations, including information about how to operate airgapped devices and how to store the key material.

Watchtower: A watchtower implementation is required. This also needs to be tested and setup for production use.

Rust implementation: Using python in production may not be a good idea. Evaluate which segments of the source code should be written in rust, and then build a rust implementation. Also convert all appropriate tests.

Code review: This work should be extensively reviewed for best practices with a deep knowledge of bitcoin software development.

The burn transactions are pretty dangerous. Users should store those in either their cold storage system or in a similar secure offline system.

See also

Other pre-signed vault implementations

There are also two related upcoming manuscripts presently nearing final draft status. (As of 2020-03-28)

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Prototype bitcoin vault: cold storage and theft minimization

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