(JavaScript) 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

Note

This example is intended for running within a Chilkat.Js embedded JavaScript engine. All Chilkat JavaScript examples require Chilkat v11.4.0 or greater.

var success = false;

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

var 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.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

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

success = fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg");
if (success == false) {
    console.log(fac.LastErrorText);
    return;
}

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

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

    var hashHex = crypt.HashBdENC(bd);
    console.log(i + ": " + hashHex);
    bdHashes.AppendEncoded(hashHex,"hex_lower");

    i = i+1;
}

fac.FileClose();

// Hash the concatenated SHA-256 hashes.
var content_hash = crypt.HashBdENC(bdHashes);
console.log("content_hash = " + content_hash);