Observing Bag Gain in JPEG Batch Steganography

The bag gain relates to a gain in security due to spreading payload among multiple covers when the steganographer maintains a positive communication rate. This gain is maximal for a certain optimal bag size, which depends on the embedding method, payload spreading strategy, communication rate, and the cover source. Originally discovered and analyzed in the spatial domain, in this paper we study this phenomenon for JPEG images across quality factors. Our experiments and theoretical analysis indicate that the bag gain is more pronounced for senders that assign payloads based on image content more aggressively and maintain a fixed payload per bag in terms of bits per DCT rather than per non-zero AC DCT. We also observe a larger bag gain for higher JPEG qualities.