Understanding the Security Risks in Digital Watermarking

Robustness and Security: Two Often Confused Subjects in Watermarking

Over a decade ago, the authors of this paper noticed a lack of emphasis on security compared to robustness in digital image watermarking research. This stems from a common misconception in the field where security is frequently conflated with robustness. However, while they share similarities, both aspects demand distinct evaluations and considerations.

Yearly published articles in digital image watermarking to robustness and security problems. Source : see references #2.

Attacks for digital image watermarking. Source : see references #2.

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Watermarking Key: The Core of Digital Watermark Security 

Central to the concept of digital watermarking is the 'key' used for embedding and detecting the watermark. Much like a password, this key is fundamental to the watermark's security. However, a common misstep in watermarking is using the same key across multiple pieces of content. This practice is akin to using a single password for different online accounts, presenting a significant security risk. An attacker who decodes the key from one piece of content can potentially access or alter the watermark in others. The security of a watermark, therefore, heavily depends on the uniqueness and secrecy of its key.

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Theoretical Insights into Watermarking Security 

Nearly two decades ago, researchers like Teddy Furon offered profound insights into the security of digital watermarking. They approached the topic using information theory, a branch of applied mathematics and electrical engineering involving the quantification of information. In particular, they utilized concepts like Shannon's mutual information and the Fisher information matrix to measure how much information about a key could be leaked through watermarked content. These mathematical tools helped quantify the vulnerability of watermarking systems, offering a theoretical framework to understand and improve their security.

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Common Watermarking Techniques and Their Vulnerabilities

Two prevalent watermarking techniques are the substitute scheme and the spread-spectrum technique. The substitute scheme involves altering certain bits in the content, while the spread-spectrum technique disperses the watermark across the host signal. Though effective, these techniques have vulnerabilities. For instance, if the same key is used across various contents, it becomes easier for attackers to identify and manipulate the watermark.

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Blind Source Separation: A Threat to Watermark Security

One advanced method that poses a threat to watermark security is blind source separation. This technique allows attackers to separate the watermark from the content without prior knowledge of the key. By analyzing the differences between watermarked and non-watermarked content, attackers can effectively 'crack' the watermark, rendering it useless for its intended purpose.

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Generative AI Watermarking: Security Still Overlooked

In generative AI, watermarking is divided into post-hoc methods (like DCT-DWT in Python’s “invisible-watermark” and StegaStamp) and generation-based methods (like StableSignature and Tree-ring for diffusion models). Both use invisible watermarking, and while robustness is being tested, security is still overlooked. Current AI-watermarking uses a static message, making it vulnerable to cracking. Effective security, however, demands a variable message to prevent easy detection and manipulation.

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The Role of Secret Keys in Watermarking Security

In a recent article “Release the Detector!", Imatag-Lab highlights the security aspects of watermarking technology in the context of generative AI. It points out that most current watermarking systems are symmetric, requiring the same secret key for both watermarking and detection. This symmetric nature presents risks, especially when the watermarker is released, allowing anyone to watermark content and potentially claim it as generated. Moreover, releasing the full detector enables the extraction of any watermark from content. This emphasizes the importance of secret keys for security and highlights the limitations of fixed key systems.

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Conclusion: The Future of Digital Watermark Security

The conclusions drawn from early research on watermarking security remain valid today. Despite its nearly 20-year history, the mathematical foundation of these findings is unshaken. However, as digital watermarking becomes increasingly crucial, especially for marking AI-generated images, A surprising number of users continue to misuse watermarks by not implementing the renewal of keys or by not using a unique key for each watermark. This practice undermines the technology's potential effectiveness. Properly implemented, with unique keys for each application, digital watermarking stands as a robust method of protecting digital content. The need for awareness and education in this field is paramount to prevent the undermining of an otherwise secure technology.

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References

-1- (PDF) Watermarking security: Theory and practice

-2- (PDF) On the Robustness and Security of Digital Image Watermarking

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