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Embedded files
Combining different filetypes into a single file, such that the appropriate data is preserved with respect to how the file is being read. This is usually accomplished by concatenating the files together.
Compatible file types
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. The second file should be one of these types:
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 (ZIP, 7ZIP, RAR) 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 7ZIP, RAR, and ZIP files are currently blocked on 4chan, with RAR files specifically prohibited by /lit/ rules. But as 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 below.
If or when moot fixes his filter, a number of fallbacks are available:
- 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. Since the file isn't an archive, it most likely won't be blocked. Upon receiving such an image, you may need to do some work to extract it. MP3 files will still work in some players even when attached to an image. For other files, it's possible to extract the original file using a hex editor. 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.
- ChanGrouper (v1:[2] v2:[3]) is a program similar to pFBind that is not currently blocked. The ChanGrouper websites are often down; if you can't reach them now, try again later.
- The 4chan Gold File Embedder also offers similar functionality to the now-blocked pFBind, 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. Also includes a web interface designed for 4chan which you can use by connecting to a proxy on localhost. The original source code of the program is included; you can examine it by downloading the file and either renaming it to .zip or opening it in your favorite archiver. Requires Java 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.
Accidental file synthesis
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".