Implements a block AES cipher over a regular filestore.
With ECB mode and no padding, we just replace the
the bytes 1 block at a time with some special handling
for when not block aligned.
A great deal of the security comes from the encryption
key only being used on a temporary basis. We also xor
by the block to add a very simple CTR like mode so that
identical blocks don't result in the same storage bytes.
TODO: use masking for division