Shamir Threshold Secret Sharing using Galois Field 2^8

Uses Shamir's secret sharing method to allows space-efficient secret sharing (splitting) requiring a minimum threshold of shares in order to unlock the original secret.

• Wikipedia on Shamir's Secret Sharing
• Octect-based Shamir Secret Sharing patterned after IETF informational draft Threshold Secret Sharing
• The Laws of Cryptography

Example

import * as shamir_tss from 'shamir-tss-gf256'
// or const shamir_tss = require('shamir-tss-gf256')
// or <script src='https://unpkg.com/shamir-tss-gf256@0.4.0/umd/shamir-tss-gf256.min.js' ></script>

const secret = shamir_tss.randomBytes(16) // or any Uint8Array/Buffer

// split into 10 shares, allowing any 3 to unlock the secret
const shares = shamir_tss.generateShares_b64(secret, 3, 10)
// shares = [
//   'AwH9nHA-35qs4JvF8qkfngTK',
//   'AwI3E71t-b3c2odZ_2pNcT-3',
//   'AwO1ZJY6wtOKODOfme1m1wP0',
//   'AwSoWHr_aQSotSWu6Pl60tEj',
//   'AwUqL1GoUmr-V5Fojn5RdO1g',
//   'AwbgoJz7dE2ObY30g70Dm9Yd',
//   'Awdi17esTyPYjzky5TooPepe',
//   'AwjN7ewQD5wmByQm8vxuDECz',
//   'AwlPmsdHNPJw5ZDglHtFqnzw',
//   'AwqFFQoUEtUA34x8mbgXRUeN' ]

const res_under_a = shamir_tss.unlockShares([shares[2], shares[7]], false)
// res_under_a = false

try {
  const res_under = shamir_tss.unlockShares([shares[2], shares[7]])
} catch (err) {
  // Number of shares did not meet threshold to unlock shared secret. (2 of 3, 0 duplicates)
}

const res_min = shamir_tss.unlockShares([shares[2], shares[7], shares[4]])
// res_min = Uint8Array [ 127, 235, 91, 105, 228, 244, 250, 2, 47, 3, 148, 46, 52, 56, 56, 137 ]

const res_over = shamir_tss.unlockShares([shares[2], shares[7], shares[4], shares[9], shares[1]])
// res_over = Uint8Array [ 127, 235, 91, 105, 228, 244, 250, 2, 47, 3, 148, 46, 52, 56, 56, 137 ]

API

Primary

generateShares(secret, thresholdShares, totalShares)

Transforms secret into a Galois Field polynomial according to Shamir's secret sharing method, returning the specified number of totalShares where any unique subset of thresholdShare shares will unlock the original secret.

Returns the shares as array of Uint8Array.

generatorShares(secret, thresholdShares, [totalShares])

generateShares() returning an iterator of Uint8Array shares.

unlockShares(shares, [valueIfUnderThreshold])

Computes and returns the secret using provided shares array, given sufficient number of shares. Otherwise, valueIfUnderThreshold is returned.

Cautionary Note: There are no self-consistency checks built into Shamir Threshold Secret Shares -- combining shares from different source will result in unlocking some undefined "secret".

generateShares_b64(secret, thresholdShares, totalShares)

generateShares() returning an array of Base64 encoded shares.

generatorShares_b64(secret, thresholdShares, [totalShares])

generateShares() returning an iterator of Base64 encoded shares.

unlockShares_b64(shares, [valueIfUnderThreshold])

unlockShares() with result Base64 encoded.

Utilities

isBinarySecret(arg)

True if arg is a Buffer or Uint8Array, false otherwise.

randomBytes(n_bytes)

In Browser environments, generates a Uint8Array of specified byte length using crypto.getRandomValues().

In NodeJS, generates a Uint8Array of specified byte length using crypto.randomBytes().

u8_to_base64(u8)

Converts a Uint8Array or Buffer to a Base64 (URL safe) encoded string.

base64_to_u8(string_b64)

Converts a Base64 encoded string to a Uint8Array or Buffer.

Internal API

The internal class ShamirSecretShare (alias ShamirTSS) is exposed. Please read the source for understanding and details.

Galois Field 256 (2^8) math primitives

function compute_poly(x, poly_coeff) {}
function lagrange_basis_poly_at_zero(u) {}
function lagrange_interpolate(u, v_iterable) {}

function add(x, y) {}
function mul(x, y) {}
function div(x, y) {}

To use:

const gf256 = require('shamir-tss-gf256/cjs/gf256')
// or import * as gf256 from 'shamir-tss-gf256/esm/gf256'
// or <script src='https://unpkg.com/shamir-tss-gf256@0.4.0/umd/gf256.js' ></script>

License

ISC