References were really helpful in encryption and decryption of a key, following them now under linux I'm able to generate a key blob that is used for encryption of the file system.
But I'm still lost on my second question. There is a way to decrypt an encrypted-kernel? because at the moment I can encrypt u-boot and final file system, but not the images in the middle (boot-script,device tree and kernel). Do you have something useful also for this problem? Thank you