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Comment by limmeau | Hacker News Reader

As far as I understand the original posting, you can download any file from Dropbox's servers if you know its Dropbox hash, which apparently is a sequence of SHA256 hashes of 4MB blocks.

If you have a sub-4MB sensitive file, and you publish its SHA256, and the Dropbox protocol applies the hash function in the same way as file hashing tools (e.g. doesn't include a tag meaning "this hash is computed particularly for Dropbox deduplication" into the SHA computation), yes, then apparently people can download your file.

However, I rarely see SHA256 checksums along with download links; more SHA1 and MD5.

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That's still a bit worrying though; do people stop to consider that publishing a SHA256 hash bears the risk of being equivalent of publishing the file itself (assuming someone uploads it to dropbox)?

Another related attack could be to start with a known file (say, your employment contract), swap out the name with a colleague and generate a bunch of files with different salary amounts, essentially bruteforcing sha256 sums. If dropbox suddenly coughs up a file, you've revealed his salary!

The employment contract scenario doesn't require download-by-hash, only deduplication. You could just measure the amount of network traffic the client needs to "upload" your file.

Assuming you know the exact structure of the file this would be a perfectly valid attack. There could be a lot of variance in rich formats like PDF files from things like compression, etc, so this might be expensive to perform on non-plaintext files.

Dropbox effectively acts as an "existence oracle". You can't ask it to cough up a file you don't have, but you can ask it if a given file exists anywhere in the system.

This would be an effective way for law enforcement or copyright civil enforcement to check for content that is clearly illegal or a certainly copyright violation to possess. They would need to query for a set of hashes of the given illegal content. If any matches returned positive data, they would be able to issue a subpoena for all users who stored the given content in their dropbox folder and pursue them further.

> for content that is clearly illegal or a certainly copyright violation to possess

How can something be "clearly" a violation? If I have an album, but copy someone else's rip instead of making my own - is that "clearly" a violation? Alternatively if I used the same application, I'd probably obtain the exact same file - is that clearly a violation too?

(grooveshark kind of operates on the assumption that it's ok)

I'm thinking of something like a pre-release album, a theatre rip of a movie, etc. Not a rip of something legitimately licensed to you, but of something not officially released to the public.

The Perkeo database used by some German polices contains hashes of known child-porn image files. Probably not SHA256, though, given that it was started in 1998.

Just read the reappeared sourcecode (assuming it works as advertised): The hash is an SHA256 of pure 4MB blocks in the input file. They add no message type information which could prevent mixups between Dropbox-deduplication hashes and hashes computed for other purposes.

The following dropship file was assembled using only shasum, ls and vi:

         {"blocks": ["f3f754a5dcd93f271ad013a5ee84f495a36da84f152e0a1fec4646345b0c10d6"], "name": "ostseestrand.jpg", "size": 514779}

Could someone who has never shared files with me verify that it indeed produces a picture of a beach?

Yes, I've got the beach image in my Dropbox folder now :)

Yeah, Canon PowerShot A60 ;)

When I run dropship with a file containing the JSON you quoted, it prints, "('Oops, blocks are not known: %s', [u'8_dUpdzZPyca0BOl7oT0laNtqE8VLgof7EZGNFsMENY'])".

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