Based on the comments from the accepted answer, it looks like you're wanting to backup the key used to encrypt the internal and/or external storage. It is likely possible that you can backup both of the encryption key(s) if you can get root using dmsetup
. If you have backups of the encrypted block devices and have the key, you can access the decrypted files on a linux system supporting dm-crypt.
In order to do a backup of the internal storage block device, you'll need to have the bootloader unlocked to use TWRP or be rooted. If you relock the bootloader, userdata will be wiped, and I suspect a new encryption key generated, so be careful of that.
There's also an alternative method for getting the encryption key for the external storage, root needed as well.