(Java) Dropbox Content Hash

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.

For more information, see https://www.dropbox.com/developers/reference/content-hash

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import com.chilkatsoft.*;

public class ChilkatExample {

  static {
    try {
        System.loadLibrary("chilkat");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load.\n" + e);
      System.exit(1);
    }
  }

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

    CkCrypt2 crypt = new 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.put_HashAlgorithm("sha256");
    crypt.put_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

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

    boolean success = fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg");
    if (success == false) {
        System.out.println(fac.lastErrorText());
        return;
        }

    int blockSize = 4194304;
    int numBlocks = fac.GetNumBlocks(blockSize);

    int i = 0;
    while ((i < numBlocks)) {
        // Read the next 4MB block into bd.
        bd.Clear();
        success = fac.ReadBlockBd(i,blockSize,bd);

        String hashHex = crypt.hashBdENC(bd);
        System.out.println(i + ": " + hashHex);
        bdHashes.AppendEncoded(hashHex,"hex_lower");

        i = i+1;
        }

    fac.FileClose();

    // Hash the concatenated SHA-256 hashes.
    String content_hash = crypt.hashBdENC(bdHashes);
    System.out.println("content_hash = " + content_hash);
  }
}