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Command-line, (SLIP39) sharded wallet designed for air-gapped deployments
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README.md

Hermit

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Hermit is a sharded, HD command-line wallet designed for cryptocurrency owners who demand the highest possible form of security.

Hermit implements the SLIP-0039 standard for hierarchical Shamir sharding.

Hermit is designed to operate in tandem with an online wallet which can talk to a blockchain. All communication between the user, Hermit, and the online wallet is done via QR codes, cameras, screen, and keyboard.

This means that a Hermit installation does not require WiFi, Bluetooth, or any other form of wired or wireless communication. Hermit can operate in a completely air-gapped environment.

Read on or watch these videos to learn more about Hermit:

Quickstart

$ pip install hermit                          # may need 'sudo' for this command
...
$ hermit                                      # launch hermit
...
wallet> help                                  # see wallet commands
...
wallet> shards                                # enter shard mode
...
shards> help                                  # see shard commands
...
shards> list-shards                           # see current shards
...
shards> build-family-from-random              # create new shard family from random data
...
shards> list-shards                           # see newly created shards
...
shards> write                                 # save newly created shards to disk
shards> quit                                  # back to wallet mode
wallet> export-xpub m/45'/0'/0'               # export an extended public key
...
wallet> sign-bitcoin                          # sign a bitcoin transaction
                                              # see examples/signature_requests/bitcoin_testnet.jpg

...

See more details in the "Usage" section below.

Design

Hermit follows the following design principles:

  • Unidirectional information transfer -- information should only move in one direction
  • Always-on sharding & encryption -- keys should always be sharded and each shard encrypted
  • Open-source everything -- complete control over your software and hardware gives you the best security
  • Flexibility for human security -- you can customize the sharding configuration to suit your organization

Audience

Hermit is a difficult to use but highly-secure wallet.

Hermit is not recommended for non-technical individuals with a small amount of cryptocurrency.

Hermit is designed for computer-savvy people and organizations to self-custody significant amounts of cryptocurrency.

Sharding

Hermit is different than other wallets you may have used because it always shards your key.

Sharding is done using SLIP-39 which means there are two levels of structure:

  • Groups -- some quorum of P of Q groups is required to unlock a key

  • Shards -- some quorum of n of m shards is required to unlock each group, with n and m possibly different for each group

This structure creates a lot of flexibility for different scenarios.

Hermit extends the current SLIP-39 proposal by encrypting each shard with a password.

Shards in Hermit are generally encrypted by a password. Each shard has its own password, allowing for teams to operate a key together, each team member operating a given shard (in some group). However, if the user explicitly supplies an empty string as the password when either creating a shard, or changing the password on the shard, the resulting shard will be unencrypted. While this makes the transport of the shard less safe, it does make it possible to export the shards to technologies that support only unencrypted SLIP-39 implementations.

Compatibility with other wallets

If you are using a non-sharded wallet such as a hardware wallet (Trezor, Ledger, &c.) or Electrum, you can import your key from your BIP39 "seed phrase" and Hermit will shard it for you.

You may also input a new key using whatever source of randomness you like.

Hermit will not export keys as BIP39 phrases; only as encrypted SLIP39 phrases. This means it is challenging to extract a key from a Hermit installation for use in, for example, a hardware wallet or Electrum. This constraint is present by design.

Input & Output

Hermit is designed to be deployed in air-gapped environments.

  • The only way data should be able to enter the device Hermit is running on is via keyboard and camera.

  • The only way data can leave Hermit is via the screen of the device it is running on.

Hermit has no idea what is happening on the blockchain and relies on an external, online wallet application to draft transaction requests. These transaction requests are passed to Hermit via QR codes. Signatures produced by Hermit are similarly displayed on screen as QR codes.

The usage of QR codes for transaction requests and signatures creates a limit to how complex of a transaction Hermit can sign. This limitation will be addressed by a QR code packeting algorithm one day.

Storage

Hermit uses 3 storage locations:

 _________               _____________                  ____________
|         |             |             |                |            |
| memory  | -- write -> | filesystem  | -- persist --> | data store |
|_________|             |_____________|                |____________|

When Hermit first boots, shards from the data store or filesystem (in that order) are loaded into memory. Changes made to shards are always made in memory and will not survive Hermit restarts.

To save data across Hermit restarts, ensure you write it to the filesystem.

If your Hermit lives on a read-only filesystem, to save data across Hermit machine restarts, ensure you persist it to the data store.

Modes

Hermit is a modal application with two modes:

  • wallet mode is where you will spend most of your time, signing transactions and exporting public keys
  • shards mode is accessed when you need to import keys or shards, export shards, or otherwise change something about how your key is unlocked

Bitcoin Only

As a sharded, HD wallet, Hermit is a tool that can be used with any cryptocurrency that operates with the BIP32 standard.

But Hermit also ships with a sign-bitcoin command that will sign Bitcoin (BTC) transactions.

You can extend Hermit for other cryptocurrencies if you need; see the "Plugins" section below.

Usage

Installation

Installing Hermit can be done via pip:

$ pip install hermit

If you want to develop against Hermit, see the "Developers" section below for a different way of installing Hermit.

Starting Hermit

To start Hermit, just run the hermit command.

$ hermit

You can also pass a configuration file

$ HERMIT_CONFIG=/path/to/hermit.yml hermit

See the "Configuration" section below for more information on how to configure Hermit.

Creating a Key

Before you can do much with Hermit you'll need to create or import at leaest one key. To do so, first enter shards mode:

$ hermit
...
wallet> shards

Two shard mode commands will let you import a key:

  • build-family-from-phrase -- enter a BIP39 phrase. This is useful if you are importing a key from a hardware wallet such as Trezor or Ledger or from another software wallet such as Electrum.
  • build-family-from-random -- enter random characters. This is useful if you want to generate your own entropy (from, say, rolling dice)

Whichever you choose, you will be prompted to enter a shard configuration.

Creating a secure shard set from a key requires additional randomness beyond the seed of the key. So even if you choose to build-family-from-phrase, you will still be asked to input random characters. Ensure you are prepared to do so using a good source of randomness (such as rolling dice).

Exporting Public Keys

Hermit can export public keys (or extended public keys) from the key it protects. These are useful for other applications which want to refer to Hermit's key but, obviously, can't be allowed to see its private contents.

Two wallet mode commands are useful for this:

  • export-xpub -- exports an extended public key
  • export-pub -- exports a public key

Each of these commands expects a BIP32 path as an argument and each will display its data as a QR code.

Signing Transactions

The whole point of Hermit is to ultimately sign transactions. Transaction signature requests must be created by an external application. You can also use a test signature request available at examples/signature_requests/bitcoin.jpg.

Once you have a signature request, and you're in wallet mode, you can run sign-bitcoin to start signing a Bitcoin transaction.

Configuration

Hermit looks for a configuration file at /etc/hermit.yml by default. You can change this path by passing theHERMIT_CONFIG environment variable when you start the hermit program:

$ HERMIT_CONFIG=/path/to/hermit.yml hermit

The configuration file is a YAML file. See the documentation for the HermitConfig class for details on allowed configuration settings.

Developers

Developers will want to clone a copy of the Hermit source code:

$ git clone --recursive https://github.com/unchained-capital/hermit
$ cd hermit
$ make

NOTE: To develop using the Hermit code in this directory, run source environment.sh. This applies to all the commands below in this section.

Testing

Hermit ships with a full pytest suite. Run it as follows:

$ source environment.sh
$ make test
$ make lint

Hermit has been tested on the following platforms:

  • OS X High Sierra 10.13.6
  • Linux Ubuntu 18.04
  • Linux Slax 9.6.4

There is also a full-flow example script provided at (tests/test_script.md)[tests/test_script.md] that you can follow to see and test all functionality.

Developers

Integrating

Hermit needs an external, online wallet application in order to work. This application has a few ways it may need to integrate with Hermit:

In all cases, Hermit uses the same scheme to encode/decode data into QR codes. The pipelines look like this:

  • To create a QR code from a string: utf-8 encode -> gzip-compress -> Base32 encode
  • To parse a QR code string: Base32 decode -> gzip-decompress -> utf-8 decode

The string data may sometimes itself be JSON.

Plugins

Hermit allows you to write plugins to extend its functionality. This is chiefly so that you can write Signer classes for cryptocurrencies beyond Bitcoin (BTC).

The default directory for plugin code is /var/lib/hermit. Any *.py files in this directory will be loaded by Hermit when it boots (though you can customize this directory; see the "Configuration" section above).

An example signer class is below

#
# Example signer class for a putative "MyCoin" currency.
#
# Put in /var/lib/hermit/mycoin_signer.py
#

from hermit.errors import InvalidSignatureRequest
from hermit.signer.base import Signer
from hermit.ui.wallet import wallet_command
import hermit.ui.state as state

# Some library for MyCoin
from mycoin_lib import sign_mycoin_transaction

class MyCoinSigner(Signer):
    """Signs MyCoin transactions"""

    def validate_request(self) -> None:
        """Validates a MyCoin signature request"""
	# This is built into the Signer class
	self.validate_bip32_path(request.get('bip32_path'))

	# this isn't great validation code, but you get the point...
	if 'input' not in self.request:
	    raise InvalidSignatureRequest("The param 'input' is required.")
	if 'output' not in self.request:
	    raise InvalidSignatureRequest("The param 'output' is required.")
	if 'amount' not in self.request:
	    raise InvalidSignatureRequest("The param 'amount' is required.")

	self.bip32_path = self.request['bip32_path']
	self.input = self.request[input]
	self.output = self.request['output']
	self.amount = self.request['amount']
		
    def display_request(self) -> None:
        """Displays the transaction to be signed"""
        print("""
        INPUT:  {}
	OUTPUT: {}
	AMOUNT: {}
        SIGNING AS: {}
	""".format(self.input,
                 self.output,
                 self.amount,
	           self.bip32_path))
				
    def create_signature(self) -> None:
        """Signs a transaction"""
	keys = self.generate_child_keys(self.bip32_path)
	# Here is the magic of MyCoin...
        self.signature = sign_mycoin_transaction(self.input, self.output, self.amount, keys)

@wallet_command('sign-mycoin')
def sign_mycoin():
    """usage:  sign-mycoin

  Create a signature for a MyCoin transaction.

  Hermit will open a QR code reader window and wait for you to scan an
  Ethereum transaction signature request.

  Once scanned, the details of the signature request will be displayed
  on screen and you will be prompted whether or not you want to sign
  the transaction.

  If you agree, Hermit will open a window displaying the signature as
  a QR code.

  Creating a signature requires unlocking the wallet.

    """
    MyCoinSigner(state.Wallet, state.Session).sign()

Contributing to Hermit

Unchained Capital welcomes bug reports, new features, and better documentation for Hermit. To contribute, create a pull request (PR) on GitHub against the Unchained Capital fork of Hermit.

Before you submit your PR, make sure to lint your code and run the test suite!

$ source environment.sh
$ make test
$ make lint

(Linting is done with flake8 and mypy.)

Key rotation

Each individual share should be managed by a team. Each team has multiple copies of the passphrase to decrypt the share. The share only exists on the Hermit device, and it's encrypted. Rotating out a member is achieved by using one of the other team members to decrypt the share and then re-encrypting the share with a new passphrase, thus excluding the previous user.

TODO

  • Validate wallet public keys/signatures against the provided redeem script in the bitcoin signer.
  • Re-do QR-code protocol details once a standard emerges
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