I plan to sell my old Kindle Keyboard and I want to erase all the Kindle's data ( my books, my personal info, wi-fi passwords...) before. Is it enough to deregister the Kindle and execute a factory reset? Would it completely erase everything?
Every Kindle has a menu option "Reset to Factory Defaults", which does exactly what you want: It removes all user specific data from the device.
For the paranoid: Most devices will not actually erase your data, but just mark it as "erased". For the user both are the same thing - if you sell your device, the new owner will not see your stuff. For anybody with sufficient knowledge and interest in that piece of information - such as crime scene investigators etc. - it usually is possible to restore data. I have not checked if a Kindle will actually overwrite your data or leave traces behind.
After deregistering the device and executing factory reset the Kindle looks empty. It shows none of my earlier books.
I've connected the Kindle to a Linux computer and copied all the information to a huge (3.2GB) file:
One of the books I've read on this device is Zendegi ( by Greg Egan ) so I've searched for the string zendegi which appears quite a few times:
It was very easy to create a script which lists most of the books previously installed on the device:
It's not a perfect list but clearly shows that factory reset does not erase all the information.
There are several ways to wipe the empty space of the filesystem. A very simple (not that secure) way is to create a huge files filled with zeroes and delete it:
After that the script does not list the previously installed books.