I have an application where I want to be able to send an encrypted file, and then mete out "keys" that allow the receipient to decrypt the file from a certain point to the end of the file. Actually, it's from a certain point to the beginning, and I'm doing the above by reversing the file.
Looking at AES (and thus Rijandel) and all of the available block modes, it needs the key to decrypt every block, and so releasing this would allow the receipient to decrypt the entire file. Not useable.
However, if I was to make the Key public, much like the IV or a Salt is usually public, and instead mete out the IV necessary to decrypt block N through the end, then that would give me the functionality I need, since theoretically the IV in a mid-stream block does not let you derive the IV for previous blocks, but only subsequent blocks.
So, the question is, how less secure is AES if I switch the Key and IV around as inputs to the block cipher?