A JPEG that becomes a PNG after AES encryption and a PDF after 3DES decryption

https://code.google.com/p/corkami/source/detail?r=1906

by soundsop • 12 years ago

240 27 12 years ago

code.google.com

tiles• 12 years ago

The raw file:

* (JPG) https://corkami.googlecode.com/svn-history/r1906/trunk/src/a...

nathancahill• 12 years ago

The file in question: https://corkami.googlecode.com/svn-history/r1906/trunk/src/a... (direct download, 173kb)

Imgur mirror: http://i.imgur.com/jUEAygp.jpg

nwh• 12 years ago

People are going to be mighty confused if they try to use that, imgur re-encodes all of it's images uploaded.

gpvos2• 12 years ago

Even JPEGs? That would explain some of the bad quality images on there. Or do they re-encode without recompressing, à la jpegtran?

josu• 12 years ago

Yep, 45KB vs 143KB.

eik3_de• 12 years ago

Lol that's funny: I took my math classes exactly in that building. University of Applied Sciences, Darmstadt, Germany. www.h-da.de

diafygi• 12 years ago

Previous discussion: https://news.ycombinator.com/item?id=7775015

pushedx• 12 years ago

Does anyone know what block size is needed, IV, cypher mode?

shuzchen• 12 years ago

The python file in that commit shows you everything you need: https://code.google.com/p/corkami/source/browse/trunk/src/an...

gear54rus• 12 years ago

How in the hell does one discover that kind of thing? :) I don't really know much about crypto and symmetric ciphers in general, but aren't the odds of discovering something that converts from JPEG to PNG by applying AES extremely small? Let alone something that ALSO goes from JPEG to PDF by applying 3DES.

I presume there is some algorithm based on the nature of ciphers themselves?

andrewchambers• 12 years ago

Only the file headers need to be valid, and generating a new file is as simple brute forcing encryption with random keys and then checking for a minimal correct file header.

gizmo686• 12 years ago

I didn't check this demo in particular, but I have seen this trick before. The first observation needed is that you can insert an arbitrary block of data into the picture without changing how it renders. The second observation is that AES is a symmetric algorithm, meaning decryption is the same operation as encryption. Using this, you can encrypt the PNG, and embed the result in the unencrypted JPEG. Then, when you encrypt the JPEG, you encrypt the embedded PNG.

There are a couple of complications to this. The major one is that both formats require correct data near the begging of the file. The size of this region is small enough that it is feasible to brute force passwords until you find one that works.

nightcracker• 12 years ago

AES encryption is not the same as decryption.

mehrdada• 12 years ago

You get the definition of "symmetric" wrong. Symmetric encryption refers to using the same key for encryption and decryption, as opposed to asymmetric, aka public-key encryption. By no means AESEncrypt and AESDecrypt are the same functions.

That said, some encryption modes employing AES, like AES in CTR mode, are based on generating a pseudo-random keystream and XORing it with cleartext to get the ciphertext. The inverse of that operation is itself, but that has nothing to do with the term "symmetric encryption". This particular case uses AES in CBC mode, which is not symmetric in that sense.

singlow• 12 years ago

The trick is that the encrypted JPEG data is not PNG data. The file is usually crafted in such a way that the container formats are tolerant of the junk data that becomes the second file after encryption. The second file's format is also tolerant of the junk data that used to be a JPEG. Some parts may actually be used on both sides of the transformation, but not most of the bits.

gpvos2• 12 years ago

PDF is particularly lenient, so I'm not surprised about that. But the JPEG-to-PNG trick must involve some brute forcing.

tedunangst• 12 years ago

The slides in the same repo may help.

https://code.google.com/p/corkami/source/browse/trunk/src/an...

zrm• 12 years ago

See also The International Journal of Proof-of-Concept Or Get The Fuck Out.

https://archive.org/details/Pocorgtfo00

https://archive.org/details/Pocorgtfo01

https://archive.org/details/Pocorgtfo02

https://archive.org/details/pocorgtfo03

https://archive.org/details/pocorgtfo04

https://archive.org/details/pocorgtfo05

voltagex_• 12 years ago

For the uninitiated, some of these files are bootable (on x86), or contain other goodies hidden somehow. These are written (or collated) by Travis Goodspeed - travisgoodspeed.blogspot.com

snake_plissken• 12 years ago

This blog is amazing! Very motivating. Thanks for sharing.

dmead• 12 years ago

GOD WHERE IS THE PICTURE IM FREAKING OUT

sprkyco• 12 years ago

tl:dr I'm a fanboy of Corkami and just wanted to show appreciation.

Ange Albertini does some amazing research and I was orginally introduced to his works after reading POC||GTFO from a Talk Travis Goodspeed gave. Recently Corkami did a giveaway on twitter for a signed mini-poster (Corkami's Posters are amazing as well highly recommended!) Fortunately I have a signed mini-poster en route to my mailbox cannot wait to have that thing framed.

A JPEG that becomes a PNG after AES encryption and a PDF after 3DES decryption

Related Stories