Presumably it's padded if it's not a multiple of 16 bytes, because that's the AES blocksize, and not just some off-the-wall requirement that the data be 1000 by

Presumably it's padded if it's not a multiple of 16 bytes, because that's the AES blocksize, and not just some off-the-wall requirement that the data be 1000 bytes long. I'm also hoping that your encrypted format has one more field, which is an IV that changes each time the data is encrypted.