Why blurring sensitive information is a bad idea

by soundsop • 16 years ago

100 41 16 years ago

makecheck• 16 years ago

I'm wondering if the copyright and year are accurate...I recall reading something like this a few years ago, complete with pictures of sample checks.

_delirium• 16 years ago

It looks like this is the original source, but it's from 2007, not 2010. This 2007 Slashdot article links to the same URL: http://it.slashdot.org/article.pl?sid=07/01/07/1352242. Maybe the current year gets auto-added by whatever CMS he's using? Either that or it's been updated.

Incidentally, while I was looking for that link, I found an implementation in the form of a Photoshop filter: http://tlrobinson.net/blog/2008/10/08/recovering-censored-te...

jadedoto• 16 years ago

I dont think he is stressing the ease of doing this with credit card numbers. The sample space he is suggesting generating is far too large... You can usually identify the first several digits simply by the issuing organization, as they all use standardized numbers, the remaining digits must pass a certain checksum algorithm. So really generating a bunch of valid cc numbers is quite trivial. Matching exp dates with numbers and ccv numbers.. Different story.

But i wonder what the limits to effectiveness is on this attack. I usually randomly swirl around with a smear tool to blur out things...

InclinedPlane2• 16 years ago

Bank of America uses a horrible method for generating debit card numbers. It's a standard prefix + account number + sequence number + check digit. If you have stolen someone's BofA debit card number then you can easily guess the replacement card's number (just increment the sequence number and recalculate the check digit). From there you just need to guess the expiration date (a comparatively trivial task).

nodata2• 16 years ago

But that's for debit cards - I think most banks include the account number in a debit card number. You would still need the CCV number from the back of the card for the attack to work.

ubernostrum• 16 years ago

Either you're skipping over a lot of information in the process of how that number's generated, or that's not how they do it anymore (and not how they've done it for at least the last couple of years).

ig1• 16 years ago

Also they generate your PIN by running your account number through a DES encryption round. This method is known to be vulnerable to attack:

http://www.cl.cam.ac.uk/~mkb23/research/PIN-Cracking.pdf

cookiecaper• 16 years ago

I'd expect random swirls to be reliable, or any obfuscation that introduces a reasonable amount of entropy. This whole attack relies on an almost-exact replication of the original blurred image. If you do something a computer can't easily reproduce over and over again, or something that looks the same no matter what the obfuscated content is (like blacking out), this attack cannot work.

Robin_Message• 16 years ago

Actually swirls are very reversible - http://news.bbc.co.uk/1/hi/7384834.stm (Paedophile caught from video with swirled obfuscation of his face.)

You're definitely right about adding entropy though, but why bother? Just blacking it out guarantees how much information is available - zero.

jbeluch• 16 years ago

Also be sure to strip EXIF data since it can contain an original thumbnail. Not all image editors update the thumbnail with changes.

scrrr• 16 years ago

good point. i usually just display the pic, snap a screenshot and all metadata is gone. if it is scaled it obfuscates blurred things, too.

sliverstorm• 16 years ago

You don't even have to color over, or blur, or do any of that hard stuff. Just select the region, and press "CTRL-X", save and quit. No reason to do it any other way.

erikpukinskis2• 16 years ago

People blur to maintain the general look of the original image. Having black boxes everywhere is jarring.

Honestly, I don't think the lesson has to be "don't blur"... it can just be "blur enough". If I blur something out, I just use a radius big enough to erase all of the information.

JoachimSchipper• 16 years ago

So, how much is "enough"?

panic• 16 years ago

People blur to maintain the general look of the original image. Having black boxes everywhere is jarring.

In that case, why not erase the numbers and replace them with random digits?

skew• 16 years ago

Deconvolution is quick with Fourier transforms. In infinite precision, convolution by any nonzero function (which includes Gaussian blurs) is invertible. In practice, quantization and stopping the convolution at the edge of the blur region complicates things. The blur itself removes no information, if you want to convince yourself it's safe you need to argue numerical analysis.

Check out "High Quality Motion Deblurring from a Single Image", for some quite impressive photo reconstruction done by actually fitting a spatially varying set of blur kernels to an image.

andrewreds• 16 years ago

WHAT... why would you completely black out the number, where you could instead use random coloured squares, that look like it is a blurring, so someone can go through all the effort, decoding your white noise, and thinking in the end they have your number... when they don't ;)

guynamedloren• 16 years ago

Here's a tip: if you do blur, don't use mosaic. Use the blur tool.

yatsyk• 16 years ago

Usually it's easier to extract exact numbers from blured image.

lwhi3• 16 years ago

Yep, mosaic (often drastically) reduces the resolution of a portion of the image. The information that's been removed can't easily be recovered.

pbhjpbhj• 16 years ago

Unless the blur tool is randomised then it may yield to the same tactic, no?

Natsu• 16 years ago

No, don't use blur. Ever.

Black it out. And I don't mean the stupid PDF trick where they draw a black box over it (but you can still copy/paste the number from underneath). I mean actually black it out. Print it out, draw on it with a marker, and scan it again if you have to make sure.

But I've seen so many blurred out numbers that I could just about figure out with my eyes, let alone a computer program that could decode it algorithmically. And yes, standard (and non-standard, i.e. Photoshop) algorithms are fairly well known and can be tested against known data. Also, people really can tell what font was used. That gives them more than enough information to decode it.

But why are you giving people information in the first place? Never, ever try to distort information you should be destroying.

DrJokepu• 16 years ago

As seen in the brilliant 2008 Underhanded C Contest, sometimes even masking isn't enough: http://underhanded.xcott.com/?p=12

andraz• 16 years ago

Actually when you really need to decode a blurred or mosaiced image you can do even more tricks. Especially when they are screenshots. Since you can take a pattern (digits) and blur them with all possible options of certain most popular image editing software, you can then do massive number of comparisons to see what comes out right. It's massively cpu intensive, but I am sure people that need it can do it.

Blacking out the section entirely is the only proper way, since you really want to be sure you are destroying the information in the image, not just dispersing it.

Even then, if you are removing a single digit it can be partially recovered by observing kerning statistics, etc.

lelele• 16 years ago

What? Are you talking about proportional fonts?

andrewmu• 16 years ago

"Identify the exact size and offset, in pixels, of the mosaic tiles used to blur the original image (easy)"

I don't see that this is easy. Surely you have to test a number of offsets and sizes of text? And without knowing the digits, this is not going to be totally accurate.

tptacek• 16 years ago

This attack is, for what it's worth, at least 4 years old (probably older).

gus_massa• 16 years ago

According to SearchYC this has been posted twice

3 years ago (3 comments) http://news.ycombinator.com/item?id=79405

9 months ago (no comments) http://news.ycombinator.com/item?id=1115919

DrJosiah• 16 years ago

The attack came out at least while I was in college, so 1998-2002.

Why blurring sensitive information is a bad idea

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