PNG and Hidden Pixels

https://www.hackerfactor.com/blog/index.php?/archives/894-PNG-and-Hidden-Pixels.html

by pimterry • 6 years ago

143 42 6 years ago

tvb12• 6 years ago

I really like the explanation of the png file format. I'm not sure I'll ever use this knowledge, but I was happy to learn it.

est• 6 years ago

I remember there was a similar trick that hides data in GZip streams in its FCOMMENT block.

Could something similar be done with DEFLATE? Allocate some unused Huffman table? Put stuff in reserved bits?

ekimekim• 6 years ago

Specifically with PNG it has a concept of "private" chunks which you can use to store arbitrary data. Conforming decoders will ignore these chunks, and may or may not drop them if the file is modified. See https://en.wikipedia.org/wiki/Portable_Network_Graphics#%22C...

0823498723498725• 6 years ago

A simple technique would be to use different block sizes.

StavrosK• 6 years ago

This is very nitpicky, but does adding extra data at the bottom of the image count as steganography? I've always thought of steganography as something that is very hard to detect, like changing the LSB of each pixel to a part of your message.

onion2k2• 6 years ago

does adding extra data at the bottom of the image count as steganography

"steganography" is simply the act of hiding messages, so I don't see why this wouldn't count.

What term for hiding information outside of the bounds of the image would you propose instead?

0823498723498725• 6 years ago

I'd say communicating outside normal means would be "using a covert channel", and reserve "steganography" for such communication as has been whitened to resemble noise[1].

Obviously tastes may differ, so someone else might call the former "steganography" and distinguish the latter as "deniable steganography", or at least "near-noise steganography".

(with a little information theory, one can even communicate reliably via sub-noise steganography)

cryptography vs. steganography?

[1] compare Bacon (1605): https://archive.org/details/advancementofl00baco/page/256/mo...

"... a method of expressing and signifying one's mind to any distance by objects that are either visible or audible — provided only the objects are but capable of two differences ..."

mc32• 6 years ago

Does that mean ADS is a form of steganography since ordinarily you will not see the alternate stream?

HelloNurse3• 6 years ago

Steganography is usually meant to refer to covert, deniable information encoding, while extra data is too unsubtle to be hidden from an adversary. It's a simple and convenient trick to hide data from unsuspecting users who don't look for secrets, a weaker threat model.

Actual image-based steganography schemes, as used e.g. for watermarks, usually alter pixels in a way that is approximately (and hopefully) undistinguishable from plausible noise.

m4632• 6 years ago

It's invalid to HIde a Message that way, yOu aren't perMitted to do it.

ShaneMcGowan• 6 years ago

Spent too long thinking "What the hell is a himom"

kd5bjo• 6 years ago

The PNG format supports arbitrary data being included in private (nonstandardized) chunks, which will be ignored by decoders that don’t recognize them. It’s easier to extract and more reliably hidden, so why bother inserting the extra data as pixels?

cf. https://en.wikipedia.org/wiki/Portable_Network_Graphics#"Chu...

chrisseaton2• 6 years ago

[you can use] private (nonstandardized) chunks ... so why bother inserting the extra data as pixels?

Because these extra chunk are super-obvious? As soon as someone looks in your file they're going to wonder what you've added these for.

They will likely also not survive re-encoding.

The point of modern stenography is that you can't tell it's there, and it's just a part of the core image data, and that it's completely deniable.

How do you deny your nonstandard chunks?

yodelshady2• 6 years ago

If it's compressed (it is!) how would you know? You can't simply do width x height x depth.

jansan3• 6 years ago

PNG's IDAT chunk, which contains actual image data, is simply all lines of data compressed with the deflate algorithm. Once you inflate (uncompress) it, you have all data and can compare if width and height match. This way it is quite easy to detect if additional data was added.

deadalus• 6 years ago

https://web.archive.org/web/20200901074527/https://www.hacke...

fenomas2• 6 years ago

Sorry if this is a dumb question, but what does hiding information in images have to do with CTF-style challenges? Does that term not refer to "break into server X and retrieve file Y" sorts of tasks?

marcan_42• 6 years ago

CTF games come in two styles (attack and defense i.e. PvP, and jeopardy i.e. PvE), and within the jeopardy class there are many categories, one or which can be steganography, file formats, and related challenges.

(Source: I run a CTF, have written over 100 challenges, and the JPEG padding trick was one of them years ago)

fenomas2• 6 years ago

Ah, I had thought they were exclusively about security/penetration/etc. Thanks!

tenbino• 6 years ago

The pico8 console stores the entire game in a png file in its data area.

Lerc• 6 years ago

Doesn't it store it in the low bits of visible picture?

That's what I do for my own virtual console project. While the methods listed in this article are pretty neat, once you go browser based you are limited to what the browser gives you access to. The only alternative is doing a request for the file and doing the entire file format decoding in JavaScript. If you go that far then you might as well use a zTXt or custom chunk.

knolax• 6 years ago

Thanks for reminding me about pico8. What a lovely platform.

grenoire• 6 years ago

I actually developed a game which used this trick for an Easter egg. I added in a texture for the Companion Cube from Portal, and set the alpha to 1. In game, however, I loaded the textures with only a single bit of alpha, which meant that the alpha of 1 was the same as 255. Not entirely invisible, but was fun to do!

_urga• 6 years ago

I wrote something similar but for the ZIP format, called Pure, to detect hidden buffer bleeds outside and between and within structures, amongst other anomalies. It's also amazing what you can do with a ZIP file.

Pure: https://github.com/ronomon/pure

The goal of Pure is to reduce the attack surface available for zero-days by doing the simple yet critical buffer bounds checks that most end-user application software will not, and to do this for more and more popular file formats.

The vision is that one day you can deploy Pure at your email server and get much stronger guarantees on the safety of email attachments that you open... as well as on the attachments you send out... for example that your billing software is not leaking the contents of sensitive server memory into XLSX reports that you are mass mailing to clients (Pure actually found two cases of prominent investment firms that were doing just this).

It's like type checking or linting but for everyday file formats.

spicybright• 6 years ago

"Back in the day" there was an image board where people uploaded JPGs that actually were zip archives when you renamed the extension. People would post music albums like that where the image was the cover art.

jansan3• 6 years ago

This is one of my favorite presentations ever:

https://media.ccc.de/v/31c3_-_5930_-_en_-_saal_6_-_201412291...

Ange Albertini shows how a file can be a multiple files in different formats at the same time. Amazing stuff.

jasomill• 6 years ago

I ran into a very annoying case of this in the wild: a USB hard drive "pre-formatted for Windows and Mac".

Specifically, the entire disk was pre-formatted as both NTFS and HFS+.

The trick: as shipped, LBA 0 contained an MBR pointing to an NTFS-formatted partition spanning the free space of an empty HFS+ volume whose (slightly larger) partition was defined by an Apple Partition Map in LBA 1.

So Windows would read the MBR and find the NTFS volume, Mac OS would read the APM and find the HFS+ volume, and all was right with the world.

Right, that is, until you plugged the disk into a Mac after writing to it in Windows, because the simple act of auto-mounting the "empty" HFS+ volume caused OS X to fill enough of the volume's "free" space to corrupt the colocated NTFS filesystem beyond repair.

userbinator• 6 years ago

You can do something similar with video too --- the width and height are padded to macroblock boundaries (usually 16 pixels) so there's plenty of space for extra data at the right and bottom.

In the analog realm, something similar was used to add closed captioning to the video signal: https://en.wikipedia.org/wiki/EIA-608

jansan3• 6 years ago

Column padding can be done with .ico files, too. For 16 and 256 color images each line of data must be a multiple of 4 bytes. And since 16 and 256 color images are required to be included in ico images, the low color resolution does not even raise suspicion.

PNG and Hidden Pixels

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