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Shamir JS

This is an implementation of Shamir's Secret Sharing algorithm in javascript designed to run on NodeJS.

Usage

generateShares(secret, totalShares, requiredShares, padLength)

Splits the given hexadecimal secret into a given totalShares wherein at least requiredShares shares must later be combined to recover the secret.

Example:

let secret = 'shamirSecretTest';
let hexSecret = Buffer.from(secret).toString('hex');
let shares = Shamir.generateShares(hexSecret, 5, 3);

console.log(shares);
// [ '01cce9a3904d0b9b334f75be3b4d9083b4a2fededacafca5ce6a202f43dd449003',
//   '025c7fbf732b041ebec19c1c64f1a417f797b41d7d622855a32ccfe9e6c23cb417',
//   '0390961ce3660f858d8ee9a25ecf5cf52e5c3890c2cba69519128ab5d170533c4f',
//   '04fb04d4c3a840e826fee0d18fb5690b9041c8f6a444b78b05b4acd29a5f06e314',
//   '0537ed7753e54b7315b1956fb58b91e9498a447b1bed394bbf8ae98eaded696b4c' ]

deriveSecret(shares)

Combines the given shares array to derive the original secret. You must provide at least requiredShares for this operation to succeed.

Example:

let secretDataHex = Shamir.deriveSecret([
  '01cce9a3904d0b9b334f75be3b4d9083b4a2fededacafca5ce6a202f43dd449003',
  '025c7fbf732b041ebec19c1c64f1a417f797b41d7d622855a32ccfe9e6c23cb417',
  '0537ed7753e54b7315b1956fb58b91e9498a447b1bed394bbf8ae98eaded696b4c'
]);

let secret = Buffer.from(secretDataHex, 'hex').toString('utf-8');
console.log(secret);

// shamirSecretTest

How it works

Effectively since the input secret is a string and Shamir requires an integer we cannot directly apply it. Instead, we apply Shamir to each individual byte of the secret. This also adds a bit of security, as if any individual polynomial is weak in some way then at most one byte of the secret is derived.To do this, we first require the secret be converted to hexadecimal, which makes it semi numeric. Next we cut the input secret into individual bytes which is pretty easy with the hex. We then create a randomized polynomial function for each byte, where f(0) = the byte. Each output share then gets a coordinate corresponding to that polynomial function for each byte in the secret. This means that ultimately each share will be almost the same length as the original secret, within margin of padding which is up to 16 bytes.

Each polynomial function is calculated within a finite field of size 256, so the maximum participants is 255.

Security

This code has not been audited in any way. Use this at your own risk.

Attributions

Lots of inspiration from the following: