Java

Dropbox Content Hash

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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.

Chilkat2 Java

import com.chilkatsoft.chilkat2.BinData;
import com.chilkatsoft.chilkat2.ChilkatException;
import com.chilkatsoft.chilkat2.Crypt2;
import com.chilkatsoft.chilkat2.FileAccess;

public class ChilkatExample {

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

        Crypt2 crypt = new Crypt2();

        // 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.setHashAlgorithm("sha256");
        crypt.setEncodingMode("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

        FileAccess fac = new FileAccess();
        BinData bd = new BinData();
        BinData bdHashes = new BinData();

        try {
            fac.openForRead("qa_data/jpg/milky-way-nasa.jpg");
        } catch (ChilkatException e) {
            System.out.println(e.getLastErrorText());
            return;
        }

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

        int i = 0;
        while (i < numBlocks) {
            // Read the next 4MB block into bd.
            bd.clear();
            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);
    }
}