martinivanov / sskr-tool

A tool for splitting and recovering BIP-39 mnemonics according to the SSKR standard

sskr-tool

This is a simple command-line wrapper for bc-sskr-rust, which implements the SSKR standard for splitting and recombining secrets using Shamir's secret sharing algorithm. Shares produced by this tool are interoperable with other SSKR implementations such as seedtool-cli.

A common motivation for using this technique to back up a seed phrase is to avoid storing it in a single place (thus introducing a single point of failure), while ensuring recoverability by sufficient members of a trusted group. This is useful for inheritance, so that family or friends can recover your funds only if working together. This eliminates the possibility of a rogue individual stealing the secured funds, since the group needs to work together to recover the seed phrase.

One major downside of this approach over a multisig setup is that it requires the mnemonic to be assembled in one place before being used. This risk is minimized by only using this tool on an offline device and then moving the funds after recovery, but if this is unacceptable other alternatives should be explored.

Installation

$ cargo install sskr-tool

Usage

The tool is intended to be used on a secure, offline computer. The generation of a random mnemonic is there for convenience and testing, but should not be relied upon for funds storage. The ideal usage of this tool is:

1. Install this tool on an offline-only computer that won't be used for anything else, possibly by compiling it on another computer and then copying over the binary.

2. Determine the group and threshold parameters that are appropriate for your use-case.

3. Test this tool on the offline computer, specifying the groups and group threshold but leaving off the mnemonic (a randomly-generated one will be used). For example, this invocation creates shares where either 2 from the first group or 3 from the second group may recover the mnemonic:

    $ sskr-tool 2of3,3of5 1
   

4. Attempt recovery of this random mnemonic using the tool to make sure the process is understood and the tool works on the intended device. For the example above, place a sufficient number of the generated shares in a file such as shares.txt (one per line), and then run:

    $ sskr-tool recover shares.txt
   

5. Verify that the mnemonic is recovered and that it matches the original.

6. Once you are comfortable with the process, generate a mnemonic using a dedicated hardware wallet device such as a ColdCard.

7. Split the mnemonic using this tool using the same parameters as before, but adding the mnemonic from the hardware wallet (in quotes):

    $ sskr-tool 2of3,3of5 1 "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
   

8. Engrave the shares in metal and distribute them according to your use-case, Making sure to indicate what they are for and provide instructions for recovery.

Tests are included that round-trip share splitting and recovery with a variety of parameters. Before relying on the shares produced by this tool, test recovery (ideally with multiple SSKR implementations).