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Comment by paxys | Hacker News Reader

Journalists/whistleblowers have had to deal with the same set of issues for digital images and other documents & media for a while now. Visible & invisible watermarks, custom metadata and even non-standard binary manipulation means that shared files are pretty much fully trackable, and complete anonymization is out of reach for everyone but the most technical users.

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I'll take this opportunity to shill my little tool I wrote for this purpose https://github.com/wyager/metastrip

It removes metadata (in the dumbest way possible - convert to raster and back) and also inserts random noise to defeat straightforward stego.

Dumb question. Could one take additional measures with media to disguise watermarking (e.g. rather than uploading an image, take a crap photo of the image on a screen using a physical camera)

Going to analog and back by eg. photographing the screen or printing then re-scanning a document is a good way to ensure you've removed all document metadata, but brings its own challenges (see the article, where it mentions source camera identification), not to mention that the camera/scanner itself may add back its own metadata.

Watermarking may or may not survive that kind of process - depending on the kind of watermark it might be designed to still be detectable even in low-quality copies.

The printer adds metadata as well: https://en.wikipedia.org/wiki/Machine_Identification_Code

The more coarsely you filter the data (reducing resolution is essentially a low-pass filter on the image data), the more you reduce the bandwidth for a watermarking signal, but using spread spectrum and forward error correction techniques, the ratio of watermark to data can be brought arbitrarily low. There's no amount of obfuscation that will defeat watermarking if they algorithm/key is unknown and a huge amount of data needs to be released.

That is, maybe you can use video and audio filtering/manipulation to push the watermark bandwidth down to one bit of watermark per 1 GB of data, but with 100 participants and 7 GB of data, 7 bits is enough to identify the leaker.

Good thing a journalist doesn't need to release a huge amount of the data then, only enough to be plausible.

Much of the works that makes up good journalism is the summarisation in a comprehensible manner of the full data they gathered.

You can take additional measures, but you can never be sure that you've avoided all forms of watermarking. As an extreme example, suppose a company distributed 4 versions of a document to employees, each printed in red, blue, green, or black ink (a variant of steganography). Even though you've taken a crap photo, you still have the color in evidence, and they know which subset of the employee pool the leak came from.

Any number of techniques like this could be used, such that you can't be sure the "watermark" is gone without making the document functionally useless - especially if you don't know what the watermark is. e.g. maybe you take a black and white photo of the document above, but they also changed some of the wording on the page in each version.

+1 to my sibling comment's point about how each digital device you use to cover your trail will do its best to leave a new trail.

Another technique is variation of language (synonyms), orthotypography, variations in the white space of the document... so you can take a blurry B&W picture but they still got you because your version was the one saying "amazing" and not "fantastic" in the third paragraph and using the Oxford comma in the fourth paragraph, etc.

Exactly.

Thinking about this starts to border on paranoia, but if the document is written by a sufficiently determined organization, it's all possible.

One I vaguely remember reading about years ago (not as an example, as an actual instance) included where the word wrap occurred, so on some copies a word ended a line and on others the word started the next line.

Depends on the sophistication of the watermarking. Watermarks embedded in the media itself (audio or images) can often be more robust than the information in the media itself (i.e. even if you blur it so as to be unreadable the watermark can be extracted). It basically boils down to encoding a very small amount of data in the media as robustly as possible, and modern signal processing is very good at that.

Not exactly. Whonix makes it easy — or at least as easy as possible. If you’re in need of such services, I encourage you to visit their wiki.

I believe parent is referring to journalists having trouble keeping their sources anonymous (b/c watermarks), not themselves...

I’m aware. They make it easy to scrub metadata.

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