  As an emerging concept, steganography without embedding (SWE) hides a secret
message without directly embedding it into a cover. Thus, SWE has the unique
advantage of being immune to typical steganalysis methods and can better
protect the secret message from being exposed. However, existing SWE methods
are generally criticized for their poor payload capacity and low fidelity of
recovered secret messages. In this paper, we propose a novel
steganography-without-embedding technique, named DF-SWE, which addresses the
aforementioned drawbacks and produces diverse and natural stego images.
Specifically, DF-SWE employs a reversible circulation of double flow to build a
reversible bijective transformation between the secret image and the generated
stego image. Hence, it provides a way to directly generate stego images from
secret images without a cover image. Besides leveraging the invertible
property, DF-SWE can invert a secret image from a generated stego image in a
nearly lossless manner and increases the fidelity of extracted secret images.
To the best of our knowledge, DF-SWE is the first SWE method that can hide
large images and multiple images into one image with the same size,
significantly enhancing the payload capacity. According to the experimental
results, the payload capacity of DF-SWE achieves 24-72 BPP is 8000-16000 times
compared to its competitors while producing diverse images to minimize the
exposure risk. Importantly, DF-SWE can be applied in the steganography of
secret images in various domains without requiring training data from the
corresponding domains. This domain-agnostic property suggests that DF-SWE can
1) be applied to hiding private data and 2) be deployed in resource-limited
systems.
