Hi-Stega: A Hierarchical Linguistic Steganography Framework Combining Retrieval and Generation

Due to the widespread use of social media, linguistic steganography which embeds secret message into normal text to protect the security and privacy of secret message, has been widely studied and applied. However, existing linguistic steganography methods ignore the correlation between social network texts, resulting in steganographic texts that are isolated units and prone to breakdowns in cognitive-imperceptibility. Moreover, the embedding capacity of text is also limited due to the fragmented nature of social network text. In this paper, in order to make the practical application of linguistic steganography in social network environment, we design a hierarchical linguistic steganography (Hi-Stega) framework. Combining the benefits of retrieval and generation steganography method, we divide the secret message into data information and control information by taking advantage of the fact that social network contexts are associative. The data information is obtained by retrieving the secret message in normal network text corpus and the control information is embedded in the process of comment or reply text generation. The experimental results demonstrate that the proposed approach achieves higher embedding payload while the imperceptibility and security can also be guaranteed. (All datasets and codes used in this paper are released at https://github.com/wanghl21/Hi-Stega.)