DSi firmware bin's are encrypted, I'm not entirely sure what is the technique, but I'm pretty sure it's not AES, seeing that it was implemented in the 3DS.
Nintendo software creates unique keys that are tied to specific consoles, so that DSi apps cannot migrate between consoles. This key however is a "master key", it can "decrypt" anything regardless of the hardware. We just can't use it effectively because we can't flash a "borked" firmware like that onto the system.
The key itself is not encrypted - it's a key, a result of the encryption.