Pedro Comesaña

Watermarking security: a survey

Watermarking security has emerged in the last years as as a new subject in the watermarking area. As it brings new challenges to the design of watermarking systems, a good understanding of the problem is fundamental. This paper is intended to clarify the concepts related to watermarking security, provide an exhaustive literature overview, and serve as a starting point for newcomers interested in carrying out research on this topic.

Fundamentals of data hiding security and their application to spread-spectrum analysis

This paper puts in consideration the concepts of security and robustness in watermarking, in order to be able to establish a clear frontier between them. A new information-theoretic framework to study data-hiding and watermarking security is proposed, using the mutual information to quantify the information about the secret key that leaks from the observation of watermarked objects. This framework is applied to the analysis of a Spread-Spectrum data-hiding scheme in different scenarios. Finally, we show some interesting links between a measure proposed in previous works in the literature, which is based on Fisher Information Matrix, and our proposed measure.

Security of Lattice-Based Data Hiding Against the Known Message Attack

Security of quantization index modulation (QIM) watermarking methods is usually sought through a pseudorandom dither signal which randomizes the codebook. This dither plays the role of the secret key (i.e., a parameter only shared by the watermarking embedder and decoder), which prevents unauthorized embedding and/or decoding. However, if the same dither signal is reused, the observation of several watermarked signals can provide sufficient information for an attacker to estimate the dither signal. This paper focuses on the cases when the embedded messages are either known or constant. In the first part of this paper, a theoretical security analysis of QIM data hiding measures the information leakage about the secret dither as the mutual information between the dither and the watermarked signals. In the second part, we show how set-membership estimation techniques successfully provide accurate estimates of the dither from observed watermarked signals. The conclusion of this twofold study is that current QIM watermarking schemes have a relative low security level against this scenario because a small number of observed watermarked signals yields a sufficiently accurate estimate of the secret dither. The analysis presented in this paper also serves as the basis for more involved scenarios

Detection of video double encoding with GOP size estimation

Video forensics is an emerging discipline, that aims at inferring information about the processing history undergone by a digital video in a blind fashion. In this work we introduce a new forensic footprint and, based on it, propose a method for detecting whether a video has been encoded twice; if this is the case, we also estimate the size of the Group Of Pictures (GOP) employed during the first encoding. As shown in the experiments, the footprint proves to be very robust even in realistic settings (i.e., when encoding is carried out using typical compression rates), that are rarely addressed by existing techniques.

The return of the sensitivity attack

The sensitivity attack is considered as a serious threat to the security of spread-spectrum-based schemes, since it provides a practical method of removing watermarks with minimum attacking distortion. This paper is intended as a tutorial on this problem, presenting an overview of previous research and introducing a new method based on a general formulation. This new method does not require any knowledge about the detection function nor any other system parameter, but just the binary output of the detector, being suitable for attacking most known watermarking methods. Finally, the soundness of this new approach is tested by attacking several of those methods.

Information-theoretic analysis of security in side-informed data hiding

In this paper a novel theoretical security analysis will be presented for data hiding methods with side-information, based on Costas dirty paper scheme. We quantify the information about the secret key that leaks from the observation of watermarked signals, using the mutual information as analytic tool for providing a fair comparison between the original Costas scheme, Distortion Compensated – Dither Modulation and Spread Spectrum.

Putting Reproducible Signal Processing into Practice: A Case Study in Watermarking

In this paper the authors analyze how the description and presentation of results about an algorithm proposed in the literature should be modified in order to comply with the reproducible signal processing paradigm. We describe the problems one is faced with, by specifically focusing on how the description of the algorithm should be improved with respect to the classical approach.

On distortion-compensated dither modulation data-hiding with repetition coding

An exhaustive analysis of the distortion-compensated dither modulation (DC-DM) data-hiding method with repetition coding is presented. Two decoding strategies, maximum likelihood lattice decoding and Euclidean distance decoding, are discussed and some simplifications presented. An exact performance analysis in terms of the bit error rate (BER) is given; such an exact analysis is currently not available in the literature. Two methods for computing the exact BER and several approximations and bounds, most of them in closed form, are provided. These approximations are employed to propose two novel improvements on the standard DC-DM method with repetition: the use of a weighted Euclidean distance, with optimizable weights, and a vector form of the distortion compensation parameter. Both account for significant performance improvements. DC-DM is compared with quantization methods in the projected domain, showing worse performance against additive noise attacks but higher robustness to cropping attacks. A performance analysis of DC-DM under coarse quantization that can be specialized to JPEG compression is also supplied. All our results are validated with numerical simulations with both synthetic data and real images.

On the capacity of stegosystems

Among the different applications where data hiding techniques can be used, one that has received huge attention in the last years is steganography. In that scenario, not just the embedded message is hidden, but the communication process itself is tried to be concealed. In spite of the numerous works in this field, the capacity of a perfect stegosystem (meaning a system where it is impossible to know if a given signal is watermarked or not) is still an open question in the data hiding community. In this paper we deal with the capacity of a discrete perfect stegosystem using some optimization procedures, and also present a lower-bound to the capacity of a perfect Gaussian stegosystem; interestingly this bound approaches the capacity of an AWGN channel (without host signal or the perfect steganography constraint) for small (compared with the power of the host) values of the embedding power. Furthermore, we apply the methodology used in this Gaussian scheme to a lattice-based embedding structure, introducing some promising results.

Secret dither estimation in lattice-quantization data hiding : a set-membership approach

In this paper, security of lattice-quantization data hiding is considered under a cryptanalytic point of view. Security in this family of methods is implemented by means of a pseudorandom dither signal which randomizes the codebook, preventing unauthorized embedding and/or decoding. However, the theoretical analysis shows that the observation of several watermarked signals can provide sufficient information for an attacker willing to estimate the dither signal, quantifying information leakages in different scenarios. The practical algorithms proposed in this paper show that such information leakage may be successfully exploited with manageable complexity, providing accurate estimates of the dither using a small number of observations. The aim of this work is to highlight the security weaknesses of lattice data hiding schemes whose security relies only on secret dithering.