Why blurring sensitive information is a bad idea • Hacker News Board

Re: Why blurring sensitive information is a bad idea

#21

Post by ovi256 » Thu, Nov 25, 2010, 10:34 AM UTC

panic wrote: post #18

erikpukinskis wrote: post #8

Earlier quoted context omitted.

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.

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?

Effort ? Way more work than putting a black box in.

Re: Why blurring sensitive information is a bad idea

#22

Post by yatsyk » Thu, Nov 25, 2010, 10:37 AM UTC

guynamedloren wrote: post #17

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

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

Re: Why blurring sensitive information is a bad idea

#23

Post by scrrr » Thu, Nov 25, 2010, 10:38 AM UTC

jbeluch wrote: post #6

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

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.

Re: Why blurring sensitive information is a bad idea

#24

Post by lwhi » Thu, Nov 25, 2010, 11:35 AM UTC

hboon wrote: post #13

nodata wrote: post #9

Earlier quoted context omitted.

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.

CCVs aren't always required for transactions.

CCV proves the cardholder was present at the time of the transaction. Online merchants are never allowed to store CCV numbers.

Re: Why blurring sensitive information is a bad idea

#25

Post by Cushman » Thu, Nov 25, 2010, 11:38 AM UTC

InclinedPlane wrote: post #15

nodata wrote: post #9

Earlier quoted context omitted.

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.

I haven't noticed any other bank which had the same practice. Also, not all online merchants use CCV. Also consider the risk of creating fake physical CCs, no address or CCV necessary.

Just checked— my credit union debit card number includes my account number.

Re: Why blurring sensitive information is a bad idea

#26

Post by lwhi » Thu, Nov 25, 2010, 11:39 AM UTC

yatsyk wrote: post #22

guynamedloren wrote: post #17

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

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

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

Re: Why blurring sensitive information is a bad idea

#27

Post by nodata » Thu, Nov 25, 2010, 11:54 AM UTC

lwhi wrote: post #24

hboon wrote: post #13

Earlier quoted context omitted.

CCVs aren't always required for transactions.

CCV proves the cardholder was present at the time of the transaction. Online merchants are never allowed to store CCV numbers.

They can ask for them though.

Re: Why blurring sensitive information is a bad idea

#28

Post by lwhi » Thu, Nov 25, 2010, 12:44 PM UTC

nodata wrote: post #27

lwhi wrote: post #24

Earlier quoted context omitted.

CCV proves the cardholder was present at the time of the transaction. Online merchants are never allowed to store CCV numbers.

They can ask for them though.

.. indeed, and they often do.

Re: Why blurring sensitive information is a bad idea

#29

Post by andraz » Thu, Nov 25, 2010, 1:08 PM UTC

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.

Re: Why blurring sensitive information is a bad idea

#30

Post by ig1 » Thu, Nov 25, 2010, 1:21 PM UTC

InclinedPlane wrote: post #5

jadedoto wrote: post #4

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 n…

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).

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