python-shamir-mnemonic

Reference implementation of SLIP-0039: Shamir's Secret-Sharing for Mnemonic Codes

Abstract

This SLIP describes a standard and interoperable implementation of Shamir's secret sharing (SSS). SSS splits a secret into unique parts which can be distributed among participants, and requires a specified minimum number of parts to be supplied in order to reconstruct the original secret. Knowledge of fewer than the required number of parts does not leak information about the secret.

Specification

See https://github.com/satoshilabs/slips/blob/master/slip-0039.md for full specification.

Security

This implementation is not using any hardening techniques. Secrets are passed in the open, and calculations are most likely trivially vulnerable to side-channel attacks.

The purpose of this code is to verify correctness of other implementations. It should not be used for handling sensitive secrets.

Installation

With pip from PyPI:

$ pip3 install shamir-mnemonic[cli]  # for CLI tool

From local checkout for development:

Install the [Poetry](https://python-poetry.org/) tool, checkout python-shamir-mnemonic from git, and enter the poetry shell:

$ pip3 install poetry
$ git clone https://github.com/trezor/python-shamir-mnemonic
$ cd python-shamir-mnemonic
$ poetry install
$ poetry shell

CLI usage

CLI tool is included as a reference and UX testbed.

Warning: this tool makes no attempt to protect sensitive data! Use at your own risk. If you need this to recover your wallet seeds, make sure to do it on an air-gapped computer, preferably running a live system such as Tails.

When the shamir_mnemonic package is installed, you can use the shamir command:

$ shamir create 3of5   # create a 3-of-5 set of shares
$ shamir recover       # interactively recombine shares to get the master secret

You can supply your own master secret as a hexadecimal string:

$ shamir create 3of5 --master-secret=cb21904441dfd01a392701ecdc25d61c

You can specify a custom scheme. For example, to create three groups, with 2-of-3, 2-of-5, and 4-of-5, and require completion of all three groups, use:

$ shamir create custom --group-threshold 3 --group 2 3 --group 2 5 --group 4 5

Use shamir --help or shamir create --help to see all available options.

If you want to run the CLI from a local checkout without installing, use the following command:

$ python3 -m shamir_mnemonic.cli

Test vectors

The test vectors in vectors.json are given as a list of quadruples: * The first member is a description of the test vector. * The second member is a list of mnemonics. * The third member is the master secret which results from combining the mnemonics. * The fourth member is the BIP32 master extended private key derived from the master secret.

The master secret is encoded as a string containing two hexadecimal digits for each byte. If the string is empty, then attempting to combine the given set of mnemonics should result in error. The passphrase "TREZOR" is used for all valid sets of mnemonics.