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Hi Guys,
I am new to the forum but a lurker... Anyway I was bored tonight so I had a look at the KFX file just for fun for a purchased book that is not supposed to be DRM free.
It looks like we are looking at an encrypted file not just obfuscation, it is using AES with SHA2 256Kb hashes, exact spec is AES/CBC/PKCS5 according to the header file.
Not 100% sure what this file is but there is a "voucher" referenced in the main KFX file, this separate file appears to be some kind of license file from what you can gleam from a hex dump as there are fields such as "client restrictions" mentioned in it and the word purchase.
I can't gleam anything out of the file about what form of compression they are using I suspect the KFX file itself is an encrypted container so first step would be to get through the container and seeing what it contains. Obviously brute force is impossible as AES 256 is not exactly an easy one to crack, not without supercomputers, a time machine and a spare few trillion years on your hands to kill!
Hopefully once the format is supported on the Kindle PC app there is the possibility to attach debug tools and starting trying to break through its security by looking for vunerabilities in the app to exploit.
Probably not much help to you guys and there are far more experienced security people out there so its just me 2pence worth.
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