secret-shamiracle

Simple builder and utility functions for creating Shamir secret shares and secret reconstruction.

Credits

• Shamir secret sharing wiki
• Credit to Stackoverflow user JerzySkalski for providing a working example which limits the finite field with prime modulus division.

Components

• Shamir.java: Minimal static methods to facilitate the creation of shares and the reconstruction of a secret.
• ShamirSharesBuilder.java: A mutable builder to help coordinate the state needed to create and validate shares.

Shares Builder Usage

• Required Shares: The minimum number of shares needed to reconstruct the free coefficient secret.
• Total Shares: The total number of shares to generate.
• Prime:
  • Must be supplied directly, or indirectly as a Mersenne Prime exponent.
  • Must be larger than all secrets used.
• Random: Defaults to new SecureRandom()
• Secret:
  • May be provided as a byte[] or a BigInteger to initSecrets(secret).
  • Defaults to a random value in the range (0, prime) with a call to initSecrets().

var sharesBuilder = Shamir.buildShares()
  .numRequiredShares(3)
  .numShares(5)
  .mersennePrimeExponent(521)
  .initSecrets("Shamir's Secret".getBytes(UTF_8));

BigInteger[] shares = sharesBuilder.createShares();

// Validate secret reconstruction for all share combinations of size 'numRequiredShares'.
// Throws an IllegalStateException if any reconstructed secret does not equal the original.
sharesBuilder.validateShareCombinations(shares);

// Reconstruct secret.
var coordinates = Map.of(BigInteger.valueOf(1), shares[0],
                         BigInteger.valueOf(3), shares[2],
                         BigInteger.valueOf(5), shares[4]);
var secret = Shamir.reconstructSecret(coordinates, sharesBuilder.getPrime());
var secretString = new String(secret.toByteArray(), UTF_8);