I am a beautiful miui system, need to use Google authenticator

I use beautiful MIUI's own backup app to backup your files and data, after the backup to empty and flash the phone directly, but after recovery, other app data, authenticator without, at the beginning of the secret key and no backup, lead to can't logged in now, I now have Google authenticator bak files, also found a way to understand the package online, how can you find the former secret key from bak?