Locally calculate dropbox hash of files

Question

Dropbox rest api, in function metatada has a parameter named "hash" https://www.dropbox.com/developers/reference/api#metadata

Can I calculate this hash locally without call any remote api rest function?

I need know this value to reduce upload bandwidth.

Answer 1

https://www.dropbox.com/developers/reference/content-hash explains how Dropbox computes their file hashes. A Python implementation of this is below:

import hashlib
import math
import os

DROPBOX_HASH_CHUNK_SIZE = 4*1024*1024

def compute_dropbox_hash(filename):
    file_size = os.stat(filename).st_size
    with open(filename, 'rb') as f:
        block_hashes = b''
        while True:
            chunk = f.read(DROPBOX_HASH_CHUNK_SIZE)
            if not chunk:
                break
            block_hashes += hashlib.sha256(chunk).digest()
        return hashlib.sha256(block_hashes).hexdigest()

Answer 2

The "hash" parameter on the metadata call isn't actually the hash of the file, but a hash of the metadata. It's purpose is to save you having to re-download the metadata in your request if it hasn't changed by supplying it during the metadata request. It is not intended to be used as a file hash.

Unfortunately I don't see any way via the Dropbox API to get a hash of the file itself. I think your best bet for reducing your upload bandwidth would be to keep track of the hash's of your files locally and detect if they have changed when determining whether to upload them. Depending on your system you also likely want to keep track of the "rev" (revision) value returned on the metadata request so you can tell whether the version on Dropbox itself has changed.