$C^2$-Steg: Generative Image Steganography for Integrated Covert Communication and Copyright Protection

Generative steganography and copyright watermarking, both subsumed under information hiding, have long been studied as separate paradigms. No single model supports covert communication and copyright authentication simultaneously. Direct integration of the two tasks confronts three fundamental challenges: (1) **Steganography capacity zero-sum game**: Simultaneously hiding secret images and watermarks makes secret residuals block watermark extraction, and watermark features leak steganographic traces, jointly compromising both tasks’ security. (2) **Semantic representation deficiency**: Neither task is modeled at a semantic level, and joint two tasks in low-level numerical space causes mutual feature interference, degrading recovery fidelity. (3) **Conflicting security boundaries**: A single key for both streams makes secret data and watermarks indistinguishable under coercion: implausible secret denial (weakening steganography) or invalid ownership proof (invalidating authentication). To systematically address these issues, this paper makes the first attempt at a unified diffusion-based generative image steganography framework, **$C^2$-Steg**, which integrates covert communication and copyright protection in a single pipeline and is supported by a rigorous theoretical foundation. Specifically, the proposed $C^2$-Steg framework conducts a **shared Discrete Semantic Space** to elevate steganography from low-level numerical space to the high-level semantic domain. Feature mutual interference and insufficient semantic representation induced by low-level steganography are thereby eliminated. Moreover, a **Disjoint Dual-key Mask Decoupling mechanism** is designed, by which the concealing space is spatially decoupled to overcome the zero-sum capacity trade-off caused by constrained steganography resources. Furthermore, the dual-key design resolves the conflicting security boundaries: **plausible deniability for steganography** under coercion (the watermark key alone recovers a legitimate watermark while secret images remain undetectable) and verifiability of copyright watermarks. Experiments demonstrate the $C^2$-Steg’s outstanding performance in steganography fidelity, security, and copyright verifiability.