I have a symmetric-key encryption scheme. I think the output from this scheme (MAC + IV + CBC ciphertext) should already be practically indistinguishable from random, but I would like to pad the output to an arbitrary number of bits and have it still look like noise (I will be using the output for steganography). It would also be fine to pad the input.
What is the best cipher to use for this?
It seems to be difficult to do with AES without putting a plaintext length somewhere in the output. Perhaps I could use a stream cipher where I can decrypt part of the message to find the length?