JPEG canaries: exposing on-the-fly recompression

[bensummers]

http://www.lightbluetouchpaper.org/2011/02/04/jpeg-copy-evidence/

[KingOfB]

to cause maximum quantization error on recompression at a chosen target JPEG quality factor

Neat, but pretty worthless in the domain explained though isn't it? Exposing re-compressing proxies? So if your float for re-compression matches their float for re-compression, you'll see a slightly blurry word!

[slavak]

Like someone commented on the site itself, I also can see a faint outline of the hidden message in the original (left) image. It changes in clarity if I change the angle I'm viewing my LCD screen at - most visible when I view it from below (which darkens the screen).

[jarin → slavak]

It also changes from visible to invisible if you scroll one pixel up or down (although this may depend on your monitor).

I suspect though that it would be almost impossible to detect on a photo (the typical use case for JPEGs).

[Sniffnoy → slavak]

I can see it too. But now the question is - would we have been able to notice it if we didn't already know it was there?

[dustingetz → slavak]

i think the intended application is when printed then photocopied.

[limmeau]

Just tried it out with Opera Turbo enabled: both pictures have a similar moiree-like pattern, with "VOID" on neither.

[TheAmazingIdiot]

Hmm.

There was an article (of all places) on /b/ on explicitly how to manipulate compression filters to display 2 completely different images within the error domains that make the word "void" express. 1 or 2 people were actually publishing code in /b/'s conversation explaining how to repeat and why this happened.

However, they were using this algorithm to hide animated child porn and other just plain nasty stuff, but they were discussing it all right. If prodded, I could provide links. But I wont be held responsible for the disgust resulting.

[pavel_lishin → TheAmazingIdiot]

    *prod*

(I hate the "markdown" used on HN, btw. Took me three iterations to figure out that I can't just write (asterisk)prod(asterisk) without prepending space.)

[eru → pavel_lishin]

And why would you? What does that mean anyway? We should not descent into a meme-hole of in-group speak.

[citricsquid → eru]

Read the parent conversation again, when you realise that the grandparent said "if prodded I can link" you'll see why your parent said "prod".

[eru → citricsquid]

Yes, I see what you mean. Us none native speakers would appreciate some more context, though.

[pavel_lishin → eru]

Context - in a lot of clients, you can type "/me did an action" and it will render as "pavel_lishin did an action". Other chat clients, though, just display the action surrounded by a pair of asterisks.

Hence (asterisk)prod(asterisk) would be me prodding him.

[coderdude → TheAmazingIdiot]

If you don't end up posting the links, please send them to me in an email. I'd like to see that code and the explanation behind it.

[vlaube → coderdude]

My impression was that it had to do with the gamma correction of PNG images. The 4chan thumbnail generator seems to ignore the gamma correction while the browser does not.

[TheAmazingIdiot → coderdude]

Unfortunately the link to the FAQ that specifically talked about 4chan's scaling is a dead link now.

I'm currently looking there and asking to see if anybody remembers what its called, and where the FAQ is. It was how to make these in photoshop

I WILL be posting here, to this thread when I have information about it.

Do not open these files unless you are in a secure location in a private area. It's nasty, but it's what links /b/ could provide.

     _______________________________________________________
     WARNING - WARNING - WARNING

     http://img831.imageshack.us/img831/7619/1297043379628.png
     http://img130.imageshack.us/img130/2484/1297043089626.png
     http://img703.imageshack.us/img703/318/1297042904730.png

[coderdude → TheAmazingIdiot]

Well if all that's left are the child porn pics... I think we can pass on them. Thanks for looking for the thread though.

[est → TheAmazingIdiot]

pic got removed. Did you still got the conversion code?

[WildUtah → TheAmazingIdiot]

Just what is /b/?

[eneveu → WildUtah]

A 4chan board: http://en.wikipedia.org/wiki/4chan#.2Fb.2F

[lwat → TheAmazingIdiot]

You will not be able to provide links to anything on /b/, all conversations are removed from it in a matter of minutes or hours.

[derleth]

I can see the 'VOID' in the original image as well. I wonder what kind of problems that's going to cause if this gets out into the wild.

[limmeau → derleth]

Are you behind a compressing proxy (e.g. Opera Turbo)?

[derleth → limmeau]

Not to the best of my knowledge; certainly, no such thing is running on my system.

The point is, most people would have absolutely no idea of how to even begin answering that question. If you teach them that 'VOID is Bad' and then your legitimate images have 'VOID' on them, it's going to cause problems and you're not going to be able to educate them well enough, fast enough.

[limmeau → derleth]

Agree. As for the proxying: Vodafone uses a transparent proxy on their UMTS network[1], other mobile companies might, too.

1. http://www.yetanotherblog.com/2007/08/20/vodafone-transparen...