I explained why manual decryption is not an option with Sony firmwares. Even Flashtool was doing it wrong from the very beginning. The decryption instructions are received from Sony servers when you're downloading the firmware. Once a new firmware is released and the older one gets deleted from Sony servers, the files cannot be decrypted anymore (again, the method Flashtool uses is incorrect and inconsistent, and doesn't get all files correctly).
oh.. so does that means a big no for a decryption tool? well.. if so what about pause and resume option?(well if you add that, please make an installable application with feature to download files to temp folder so that downloaded parts wont get deleted accidentally)
anyway thanks for taking interest in analyzing options.