Dart

Dropbox Content Hash

See more Dropbox Examples

Demonstrates how to compute the Dropbox content_hash of a file. (This is the "content_hash" found in the Dropbox FileMetadata object.

Note: This example requires Chilkat v9.5.0.79 or greater because it uses the fac.ReadBlockBd method.

Dart

import 'package:chilkat/chilkat.dart';

void main() {
  // This example requires the Chilkat API to have been previously unlocked.
  // See Global Unlock Sample for sample code.

  final crypt = CkCrypt2();

  // To calculate the content_hash of a file:
  // Split the file into blocks of 4 MB (4,194,304 or 4 * 1024 * 1024 bytes). The last block (if any) may be smaller than 4 MB.
  // Compute the hash of each block using SHA-256.
  // Concatenate the hash of all blocks in the binary format to form a single binary string.
  // Compute the hash of the concatenated string using SHA-256. Output the resulting hash in hexadecimal format.

  crypt.hashAlgorithm = 'sha256';
  crypt.encodingMode = 'hex_lower';

  // We're going to calculate the content_hash for the Milky Way JPG image linked here:  https://www.dropbox.com/developers/reference/content-hash

  final fac = CkFileAccess();
  final bd = CkBinData();
  final bdHashes = CkBinData();

  try {
    fac.openForRead('qa_data/jpg/milky-way-nasa.jpg');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  final blockSize = 4194304;
  final numBlocks = fac.getNumBlocks(blockSize);

  var i = 0;
  while (i < numBlocks) {
    // Read the next 4MB block into bd.
    bd.clear();
    fac.readBlockBd(i, blockSize, bd);

    final hashHex = crypt.hashBdENC(bd);
    print('$i: $hashHex');
    bdHashes.appendEncoded(hashHex, 'hex_lower');

    i++;
  }

  fac.fileClose();

  // Hash the concatenated SHA-256 hashes.
  final contentHash = crypt.hashBdENC(bdHashes);
  print('content_hash = $contentHash');
}