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Embedded files
An embedded file is a file that is stored or hidden inside another file, particularly inside an image which may then be posted to the *chans. For example, concatenating a JPEG file with a RAR file produces an embedded archive which can be read either as a JPEG or a RAR, depending on how it's opened.
File concatenation
One of the most common ways of embedding files into images is simple concatenation. It usually doesn't matter what the first file is, but it should be a GIF, JPEG, or PNG file if you want to post it to 4chan. If the second file should be one of the following types, it will still be readable when opened with the appropriate program:
In addition:
- OGG sound files appended to images and posted to 4chan can be played with the 4chan sounds userscript.
- Broken web pages occasionally append HTML to the end of the images they serve. In most cases, the contents are unremarkable. But several images from the diaper fetish website wetherbed.com contain the login credentials. These images are often reposted in diaper fetish threads on /b/ with the posters unaware of what's in them. You can find this information by opening the files in a text editor such as Wordpad, and searching for "password".
Examples
In Windows:
copy /B foo.jpg + bar.rar foobar.jpg
In *nix:
cat foo.jpg bar.rar > foobar.jpg
Both of these examples will create a file named foobar.jpg, that when viewed graphically is identical to foo.jpg, but when unrar'd contains the contents of bar.rar.
Why does it work?
Many types of compressed archives (7Z, RAR, ZIP) can be distributed as self-extracting files, which are composed of an executable file concatenated with the archive. So these types of files are designed to be readable even if they're appended to another file.
Blocked on 4chan
Embedded 7Z, RAR, and ZIP files are currently blocked on 4chan, giving posters the message "Image file contains embedded archive." RAR files in particular are now explicitly prohibited by lit rules. But as it was in the case of 4chan.js, moot's jpg-rar filter is easy to circumvent, since he isn't scanning the whole file, only the first and last 64 KB first 256 KB and last 64 KB. All you generally need to do to get around it is add padding after the image (using several copies of the image will do) to push the beginning of the RAR file past the 256 KB threshold. See instructions in the image to the right.
Some other ways of evading the filter are:
- Alter the magic number in the RAR file, for example by replacing "Rar!" with "Bar!". Use a hex editor to do this so you don't make other unintentional changes to the file.
- Concatenate the image and file without compressing the file. If file isn't an archive, it most likely won't be blocked. But if the file isn't one of the types listed above, you'll need to use a hex editor to extract it. If the image is a JPEG file, search for FF D9 to find the end of the image data, and delete it. Alternatively, those of you not versed in Computer Science III may want to try this Greasemonkey script, which can detect the added data in images on 4chan and split the image back up into its original pieces. Also useful for telling fake jpeg-rar books from real ones. Do not use this technique to upload source code or HTML files as this may trigger the anti-4chan.js filter and get you banned.
You can also try one of the other methods of embedding archives...
Cornelian archives
Archives embedded into the image data of a 24-bit Windows bitmap, then converted it to a PNG so you can post it on 4chan. This was the format used by Cornelia to post the dox of infected users. Moot never figured out how to filter out Cornelia's posts efficiently as he had done with previous incarnations of 4chan.js, and instead gave up and added CAPTCHA to 4chan. So even if moot decides to beef up his anti-JPEGRAR filter in the near future, we should expect Cornelia's embedded file format to remain unblocked for some time. Even now, a Cornelian archive can be made smaller than than a JPEG-RAR with 256 KB of moot-evading padding. And unlike simple JPEG-RARs, they can be posted on sites that strip off appended data.
To create one, create an image with enough blank space at the bottom to hold the archive data. The number of pixels needed is 1/3 the length of the archive. It's also important that the image width is a multiple of 4. Then on Linux / OS X you can do:
convert image.png ppm:- | convert ppm:- bmp:- | (head -c 54; cat archive.rar; tail -c +$(($(wc -c < archive.rar)+1))) | convert bmp:- output.png
Other methods for creating them can be found bundled in the image to the right.
To extract the files:
- Convert the image to a 24-bit BMP file. You can do this by copying it to or opening it in an image editor, and saving it as the correct type:
- In MSPaint: Make sure the save type is set to "24-bit Bitmap".
- In Mac OS X's Preview: Before saving the image, flip it vertically. Choose the format "Microsoft BMP". Make sure the "Alpha" box, if present, is unchecked, and that the "Rotate without modifying contents" box is checked.
- In The GIMP: Change the extension to ".bmp". In the next dialog, make sure "24 bits: R8 G8 B8" is selected under "Advanced Options".
- Open the .bmp file with 7-Zip or WinRAR.
File binders
A file binder is a program that appends files and their names to images in its own special format, and extracts files added to images by others.
- pFBind is one of the best-known file binders. It was created to get around 4chan's block on embedded RARs and save Lithursday, but it was eventually blocked from 4chan itself.
- ChanGrouper (v1:[2] v2:[3]) is another file binder, written in Java. It has not yet been blocked from 4chan. The ChanGrouper websites may be down; you can alternately download ChanGrouper here (v1:[4] v2:[5]). The original source code of the program is included in the JAR file; you can examine it by downloading the file and either renaming it to .zip or opening it in your favorite archiver.
Steganography
For most of the above methods, the embedded files are fairly easy to detect, whether by script or by eye. Steganography is the art of making embedded files hard to detect.
The 4chan Gold File Embedder is another Java program that embeds files in images, but will be harder for moot to block because it uses an LSB-based steganography scheme to attach files, rather than appending them after the end of the image. It also includes a web interface; if you run the 4chan Gold proxy on your machine, and connect to 4chan through it, you can embed and extract files whenever you post or view an image. Source code is included in the JAR file. The minimum required Java version is 5.0.
Google will find you all sorts of other steganography utilities, some of them much harder to block. One of the better ones is steghide. The downside is that the hardest-to-block steganography tools typically require you to use a cover image at least 5-20 times the size of the hidden file. This can run you up against 4chan's file size limit.
