Routing Around Dropbox’s “Restricted Content” Fail for Ebooks

I am an avid reader and have about 17,000 books on my phone (because why not?) Recently I bought a Sandisk 400gb MicroSD card for my Note 8 and started syncing my Dropbox /books/ folder with the card when I started seeing an error message I’d never seen before:

File not downloadable from Dropbox, “restricted_content”

What’s going on here is that Dropbox is creating hashes of all the files in user accounts. It then compares the hashes of the files you have uploaded to Dropbox with the hashes that have been submitted to it by copyright holders of files that are allegedly being made available illegally on torrent and other sites.

If a file in your Dropbox folders matches the hash of one of these copyrighted files, then Dropbox will restrict what you can do with the file (specifically, making it difficult to sync or share in some contexts).

Fortunately, for a lot of types of content this is fairly easy to fix. Two compare the hash functions of two files, the two files have to be bit-for-bit copies. Even slight changes in a file will lead to an entirely different hash, which will then no longer flag the file.

So, for example, 90 percent of the books in my collection are EPUB files. An EPUB file is actually just a ZIP container that includes the book text, images, and other data for that format. So to avoid the “restricted_content” nonsense I:

  1. Renamed bookxyz.epub to
  2. Created a text file called “lol.txt” that just contained the text “lol” (you could probably just leave the file blank, and this would still work).
  3. Copied the lol.txt file into the zip archive.
  4. Renamed back to bookxyz.epub

Now the hashes for the old and new versions of the file are wildly different, so Dropbox no longer flags the files as “restricted_content”


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One thought on “Routing Around Dropbox’s “Restricted Content” Fail for Ebooks”

  1. With how easy it is to bypass, it seems like a sort of silly way to attempt to restrict copyrighted content, doesn’t it? But I suppose it’s better than nothing.


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