decodeShareBase64 method

List<List<List<BigInt>>> decodeShareBase64(List<String> shares)

Takes a string array of shares encoded in Base64 created via Shamir's Algorithm; each string must be of equal length of a multiple of 88 characters as a single 88 character share is a pair of 256-bit numbers (x, y).

Implementation

List<List<List<BigInt>>> decodeShareBase64(List<String> shares) {
  String first = shares[0];
  int parts = first.length ~/ 88;

  // Recreate the original object of x, y points, based upon number of shares
  // and size of each share (number of parts in the secret).
  //
  // points[shares][parts][2]
  var points = List<List<List<BigInt>>>.generate(
      shares.length, (i) => List<List<BigInt>>.generate(parts, (j) => List<BigInt>.generate(2, (k) => BigInt.zero)));

  // For each share...
  for (int i = 0; i < shares.length; i++) {
    // ensure that it is valid
    if (isValidShareBase64(shares[i]) == false) {
      throw new Exception("one of the shares is invalid");
    }

    // find the number of parts it represents.
    String share = shares[i];
    int count = share.length ~/ 88;

    // and for each part, find the x,y pair...
    for (int j = 0; j < count; j++) {
      String cshare = share.substring(j * 88, (j + 1) * 88);
      // decoding from Hex.
      points[i][j][0] = fromBase64Url(cshare.substring(0, 44));
      points[i][j][1] = fromBase64Url(cshare.substring(44, 88));
    }
  }
  return points;
}