Mobi and PRC are the default ebook formats used by the MobiPocket. In 2005, the company was aquired by Amazon, and now Amazon Kindle works with these files. Most of these encrypted files are a real pain if you want to throw them on different devices or use other software.

How to remove DRM from MOBI and PRC files so that you can read them on any device you have.
Note: Distributing copyrighted content is illegal!

Get the latest version of MobiDeDRM from darkreverser

Or

Download ebook DRM Tools Archive including MobiDeDRM:
ebook DRM removal tools archive

download MobiDeDRM v.0.16
download MobiDeDRM v.0.13
download MobiDeDRM v.0.09
download MobiDeDRM v.0.08

download MobiDeDRM v.0.07
download MobiDeDRM v.0.07 (works both as a Calibre plug-in and standalone)
Download the file and rename it to mobidedrm.py
(To create a Calibre plugin, rename this file so that the filename ends in '_plugin.py', put it into a ZIP file and import into Calibre using its plugin configuration GUI).

When properly run with your correct PID, it will remove the DRM from the book and allow you to use Calibre to convert it to whatever format you want.

MobiDeDRM.py is a tool that has been able to break nearly every mobi/prc encryption is MobiDeDrm, courtesy of Paul Durrant.

How to remove DRM from MOBI and PRC files

1. Download and install Python and wxPython
   
   MobiDeDrm scripts should run on any OS, given you've got the right files installed. So, before we can get started, you'll have to download Python and a Python extension module like wxPython.
   
   In both steps 1 and 2, choose the Python and wxPython module according to your operating system.
   
   After these applications are installed, Python scripts should run flawlessly on Windows, Mac and Linux.
    
2. Find Your PID
   
   Your PID is the personal key that your reader uses to encrypt and read the file. This key is needed to permanently strip the DRM from MOBI / PRC ebooks.
   
   Because MobiPocket is designed to be used with different devices, the PIDs are out in the open for the user to grab. If your ebook is hosted on another device (i.e. Kindle), simply sync it up with the software before you proceed.
   
   
   In MobiPocket on your desktop, go to Devices, and you'll see the PID of said reader in Details. Note that a PID is both device and ebook-specific. Upon transfering your ebook, the PID is also altered, so you'll need the PID of the right device or the decryption will fail.
   
   When buying a Mobi ebook online, you're most often asked for your PID before downloading. Obviously, this is the PID you'll need to use. However, sometimes the process is turned around.
   
   If you're asked for the store username and password upon adding the book, it hasn't and won't be matched with your PID. Instead, you'll first need to transfer the ebook to another device (e.g. Calibre or Windows Mobile) and use that PID.
    
3. Remove DRM protection
   
   Once we have the right PID, most of the work is already behind us. We'll now deploy the scripts to permanently remove the encryption. For accessibility, it's advised to put your eBook and the python scripts in the same folder.
   
   
   First, open the Terminal or command screen.
   
   In Windows XP, go to Start – Run, type cmd and hit enter. In Vista, go to Start, type cmd in the search box and hit enter.
   
   The Terminal application is located in /Applications/Utilities for Mac OS X.
   
   And if you're using Linux, you'll probably know the location. In most distributions, this will also be located at Applications/Utilities.
   
   
   Next, you navigate to the folder containing your files. To do this, type cd, followed by the full file path to your folder, and hit enter. In my case, that'll make for
   
   Code:

   cd /home/admin/ebook

   If your filepath contains spaces, put single quotes ( ‘ ) around it.
   
   
   Now, all you need to do is run the script. Type
   
   Code:

   python mobidedrm009.py infile.mobi outfile.mobi PID

   and hit enter. Substitute "infile.mobi", "outfile.mobi" and "PID" with the appropriate filenames, file extensions and the PID respectively. In my case:
   
   Code:

   python mobidedrm009.py ebook.prc free_ebook.prc 1234567$ab

   If you're experiencing trouble, try putting your PID between single quotes ( ' ) or using one of the previous script versions (e.g. mobidedrm006.py). Some ebooks refuse to work with the later versions (strangely) but work perfectly well with the first one.
    
4. If all went well, you'll now be the proud owner of another, unencrypted version of your ebook.
    
5. You can sync this with any device you want without too much of a hassle, or convert it to an easier to use format like pdf, rtf or html. Calibre is one of the best tool to convert ebooks.