Janiczek / elm-secret-sharing

Shamir's Secret Sharing algorithm over GF(256) in Elm

elm-secret-sharing

An implementation of Shamir's Secret Sharing: your secret is encrypted into N keys, of which only K are needed to reconstruct the original secret.

Port of simbo1905/shamir.

Example usage:

secret : String
secret =
    "Hello there!"

allKeysAndNewSeed : Result Secret.EncryptError ( List Key, Random.Seed )
allKeysAndNewSeed =
    Secret.encryptString
        -- README sin #1: Don't use a static seed in production!
        { seed = Random.initialSeed 0
        , parts = 5
        , minPartsNeeded = 3
        }
        secret

onlySomeKeys : List Key
onlySomeKeys =
    allKeys
        -- README sin #2: Don't throw away the advanced seed either!
        |> Result.map Tuple.first
        |> Result.withDefault []
        |> List.drop 2

decryptedSecret : Result Secret.DecryptError String
decryptedSecret =
    Secret.decryptString onlySomeKeys

-- decryptedSecret == Ok secret

Seed choice

Since this dives into the cryptography area, a few rules/recommendations apply whenever you're expecting safety:

• Don't use a hardcoded static seed (eg. Random.initialSeed 0)
• Don't use the current time as a seed (it's not safe enough, could be bruteforce-able)
• Use a cryptographically random value as a seed. For example, pass an integer generated by crypto.getRandomValues into your Elm app via flags:
• Don't reuse a seed. All functions that use the seed typically give you a new seed alongside the generated value (or at least they should). Replace the seed in your model with it!

const seed = crypto.getRandomValues(new Uint32Array(1))[0];
const app = Elm.Main.init({flags: {initialSeed: seed}});

type alias Flags =
    { initialSeed : Int }


type alias Model =
    { seed : Random.Seed }


init flags =
    ( { seed = Random.initialSeed flags.initialSeed }
    , Cmd.none
    )


-- later:
Secret.encryptString
    { seed = model.seed -- Tada!
    , parts = 5
    , minPartsNeeded = 3
    }
    secret

Advanced usage:

It's possible to have a tiered sharing: let's say you want to have admin keys and user keys; allowing either two admin keys or one admin key and three user keys to recover the secret.

For more info on how to do that check this link. Essentially you generate one extra higher-tiered key and instead of giving it to somebody split it into the lower-tiered keys.