How we found rogerkver’s obfuscated wallet private key | Hacker News Reader

How we found rogerkver’s obfuscated wallet private key

medium.freecodecamp.org

301 pointsmonort54 comments

This is a great reminder to follow the advice of a previous article here about the blurring of sensitive data.

1) If you care about the data, don't blur. Black it out, entirely. 2) If you don't want to black it out for aesthetic reasons, then you don't care enough about the data being private. 3) Black out every pixel, every time.

Depending on your image editor, be sure to collapse PNG layers between the black bars and underlying image as well.

Take a picture of the screen or a screenshot if you are truly paranoid.

Pixelation is better than blurring but also not sufficient unless you use huge blocks.

If it's an aesthetic thing, someone could make a tool that looks like it's blurring, but actually puts in fake data and then blurs that.

But anyone who knows or cares enough to go download a purpose-made tool for censoring images probably just opened it in Paint and drew a black rectangle.

> If it's an aesthetic thing, someone could make a tool that looks like it's blurring

The classy thing to do would be to replace it with a real QR code that spells out "remember to drink your ovaltine" but is damaged enough you need about a million brute force attempts to decode it.

Yes please... I need to go make that now.

Crop it and replace the hole with a black-filled box. I find that a better option and much easier to accomplish.

The key is just to overwrite the pixels. Black box is the easiest. Could be any other color box if you want, or a picture of your dog, etc

until you run into a government agency that thinks drawing a black box over something in a PDF is acceptable redaction, where the PDF is exported/published to the public in its full vector/metadata markup format, not a raster image... hilarious when you can just highlight and copy/paste to get whatever text is hidden. Have seen it several times.

be careful asking non technical employees, sans training, to "just draw a black box over it", is what I'm saying.

Apocryphally some places have a policy that all redacted documents must be printed then rescanned before release.

because fuck you, trees! right?

It's the simplest method to wipe out any metadata from the original document

Reality Winner missed that day at NSA training!

She did print it and probably was caught because of that

http://mashable.com/2017/06/06/printer-dots-nsa-leak.amp

Doesn't that reveal the serial numbers of the printers they use?

They just print it again and re-scan. It's printers all the way down.

Maybe they don't care? Learning this was printed by the printer used for redacted info doesn't tell you much.

The govt should write themselves a virtual printer that would take in the data stream and output a PDF made of individual images. A typical print-to-PDF virtual printer, but government-sanctioned, and they could also make it do something like store the document in an audit database or whatever, just to give people a strong incentive not to skip it or use anything else.

That would be interesting and not hard to setup. Install cups on some box, definitely not the host containing sensitive info, and you have a limited, auditable outbound channel.

It does open another risk of nation state style attacks that compromise the virtual print driver in a manner that it does not redact information, or encodes 'hidden' information into the format of the document. That seems easier to do with a digital print driver than with the messy digital to analog conversions that occur in a scanner/printer.

Well if you can blur an arbitrary section you can, even if not black-black, just choose an average and write over everything with that (so adaptive grey).

Not in US, and certainly not in any agency, but anytime I black box something in a pdf, I print it back as image to PDF printer.

I may misinterpret what you mean by “print it back as image to PDF printer”, but I doubt that removes the text. When printing the document, the program likely will just render the text first, and the black box over it. Reason? To be able to skip rendering the text, the computer must know:

- a black box will get drawn over it. - that box completely covers the text. - that box will be drawn opaquely.

Verifying that isn’t easy.

Save to .png is safer. It doesn’t support layers, so what you see is what you get.

> Save to .png is safer. It doesn’t support layers, so what you see is what you get.

Just make sure your document->png converter doesn't do anything silly with the alpha channel.

It can be worse. Apparently, Adobe’s FireWorks writes bitmap layers and vector data to .png custom data chunks.

PDF-xChange keeps text as text if printed normally, completely selectable, vectorized, but changes and rasterises it to image if printed as image, pixelated and jpeg-ized. At about 200% zoom it is clearly visible that it is pixelated, and nothing else.

JPEG also stores the thumbnail separately in the EXIF data. This might not be updated when the image is edited and can reveal the original data.

TIFF too; possibly PDF, and no doubt other formats as well

It's not even in the EXIF data, it's part of JFIF.

A good example of this (maybe it was the subject of the previous already that you mention):

https://petsymposium.org/2016/files/papers/On_the_(In)effect...

Which reminds me of the disturbing case of Chritopher Paul Neil.

[1] https://en.wikipedia.org/wiki/Christopher_Paul_Neil

Great paper. It reminded me of seeing this recently, which was a bit surprising:

https://pbs.twimg.com/media/DM2-8SrX4AAqc4o.jpg:large

Context:

http://www.huffingtonpost.ca/entry/dillon-baldridge-trump-ch...

Actually, the post above has a video where no blurring at all occurs...!

I love this 2008 underhanded C contest along that same vein: http://www.underhanded-c.org/_page_id_17.html

> Accidentally appending the original data to the image file.

Or you could just blur fake data in its place lol

An alternative to black out is to interpolate the pixels at the borders of the "blacked out" box. I've seen it done to suppress logos on images, in many cases it's barely intrusive.

Applied to that QR-code, the currently blurred out part would be entirely white —because the original QR-code was surrounded by a white background.

That should take care of many aesthetic concerns, without using a single bit of secret information for the final rendering.

Check out Deconvolution[1])

The technique has been done before and the text string of the private key would probably be the easiest point of attack.

WHY BLURRING SENSITIVE INFORMATION IS A BAD IDEA[2]

RESTORATION OF DEFOCUSED AND BLURRED IMAGES[3]

Take away: black out any and all sensitive information. Blurring just marginally reduces the independence of the bits presented in blurred output. To have privacy, all dependence must be eliminated, e.g. black out.

[1] https://en.wikipedia.org/wiki/Deconvolution

[2] https://dheera.net/projects/blur

[3] http://yuzhikov.com/articles/BlurredImagesRestoration1.htm

In addition to just blacking out, it's also important to screenshot the content if you are able to and then black it out. Otherwise you may leak meta data or pre-rendered thumbnails. The reason you screenshot first and black out second is that it reduces the chances of you mixing up which file to upload.

Also, even more basic: don't just redact a PDF by adding black boxes over the text as a new layer. The text data is still there. (Yes, this has been a source of leaks on numerous occasions.)

Shoulda told the FBI that years ago!

Similar thing happened in Australia recently. Every member of parliament and senators have their phone bills published to the government website as part of public disclosure.

What happened though is instead of deleting their phone numbers, the contractor who was in charge of this simply changed the font colour of their phone number to white (same as the background).

So if you looked at the PDF, it looked normal. If you highlighted the text - the phone number instantly showed up. Also viewing google "text" cache version showed the number too. As a result - hundreds of their personal phone numbers were available to anyone and everyone.

http://www.smh.com.au/federal-politics/political-news/privat...

A criminal was caught after they released their own picture with a swirl filter applied to their face. Unfortunately for the criminal, a swirl is reversible.

Not just any criminal but a big time pedo

Gaussian blur is mathematically reversible, but in practice you don't have enough resolution to successfully deconvolve the original image. However in the case of video you have multiple images and you can reconstruct a "super-resolution" image by combining frames. I'm sure there is a tool somewhere that can do this, there are too many secrets being shown blurred on TV to pass ...

Not sure how up to date the algorithms are, but OpenCV has an implementation: https://docs.opencv.org/3.0-beta/modules/superres/doc/super_...

Very curious to see if someone can solve this using this method. Would be as magical as ECC

If the original image had high frequency edges such as a matrix code or text then it can still get you close enough to decode the deconvolved result.

Congrats.

I had also started trying to decode the QR but as I was starting to look into the ECC I noticed someone had moved half of the coins and gave up.

Glad you explained your technique. I initially used a Picross editor to map the screenshot before moving to a full Python script.

Nice trick to use the Picross editor. We started with GIMP but quickly moved to Google spreadsheet.

This was a great story which previously didn't hit the front page. I'm glad for the re-submit policy. I enjoyed learning about the build structure of QR codes. Years ago I chose to learn how barcodes are constructed so that I could create custom barcodes for my college library. Having access to nothing more than Microsoft office, I naively decided that a whole lot of conditional formatting rules would be enough to build the thing in Excel. And I was right! But it's not a method I would recommend anyone ever attempt.

My biggest take-away from all this -- look at how much high quality engineering work was just crowdsourced for $1,000?!

It doesn't look very blurred. I expect you could do a blind deconvolution and just read it.

I was hoping for some application of Bayes' theorem.

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