A Natural Steganography Embedding Scheme Dedicated to Color Sensors in the JPEG Domain

International audienceUsing Natural Steganography (NS), a cover raw image acquired at sensitivity ISO 1 is transformed into a stego image whose statistical distribution is similar to a cover image acquired at sensitivity ISO 2 > ISO 1. This paper proposes such an embedding scheme for color sensors in the JPEG domain, extending thus the prior art proposed for the pixel domain and the JPEG domain for monochrome sensors. We first show that color sensors generate strong intra-block and inter-block dependencies between DCT coefficients and that theses dependencies are due to the demosaicking step in the development process. Capturing theses dependencies using an empirical covariance matrix, we propose a pseudo-embedding algorithm on greyscale JPEG images which uses up to four sub-lattices and 64 lattices to embed information while preserving the estimated correlations among DCT coefficients. We then compute an approximation of the average embedding rate w.r.t. the JPEG quality factor and evaluate the empirical security of the proposed scheme for linear and non-linear demosaicing schemes. Our experiments show that we can achieve high capacity (around 2 bit per nzAC) with a high empirical security (P E 30% using DCTR at QF 95)

JPEG Steganography and Synchronization of DCT Coefficients for a Given Development Pipeline

This short paper proposes to use the statistical analysis of the correlation between DCT coefficients to design a new synchronization strategy that can be used for cost-based steganographic schemes in the JPEG domain. First, an analysis is performed on the covariance matrix of DCT coefficients of neighboring blocks after a development similar to the one used to generate BossBase. This analysis exhibits groups of uncorrelated coefficients: 4 groups per block and 2 groups of uncorrelated diagonal neighbors together with groups of mutually correlated coefficients groups of 6 coefficients per blocs and 8 coefficients between 2 adjacent blocks. Using the uncorrelated groups, an embedding scheme can be designed using only 8 disjoint lattices. The cost map for each lattice is updated firstly by using an implicit underlying Gaussian distribution with a variance directly computed from the embedding costs, and secondly by deriving conditional distributions from multivariate distributions. The covariance matrix of these distributions takes into account both the correlations exhibited by the analysis of the covariance matrix and the variance derived from the cost. This synchronization scheme enables to obtain a gain of PE of 5% at QF 95 for an embedding rate close to 0.3 bnzac coefficient using DCTR feature sets

Stéganographie naturelle pour images JPEG

International audienc

Méthodes de stéganographie fondées sur la prise en compte du bruit de capteur

The Greek etymology of the term « steganography » is the concatenation of the words « stego »: to keep secret and « graphia »: writing.Steganography is therefore a term for the science of secret (or discreet) communication. The fundamental characteristic of steganography is by essence that it must be impossible for a detection system to distinguish innocuous objects from those containing a secret message.Similar to cryptography, whose dual discipline is cryptanalysis to decrypt the encrypted message, steganography also has its dual discipline: steganalysis. The objective of steganalysis is, basically, to detect the presence of a hidden message.In the context of digital images, the disturbance induced by the embedding of a secret message on its cover image can be defined as the addition of a specific signal. The undetectability of this message will thus be based on the fact that the added signal will not disturb the statistical properties of the initial image (the cover image). It is from this principle that the paradigm of Natural Steganography was born, this one is attached to use a noise inherent to the photographic sensors (the photonic noise) which can be modelled by a normal law distributed independently on each photo-site. The message is thus embedded by imitating this natural noise when capturing a digital image.Thus the image thus generated (the stego image) has the statistical properties of an anodyne image, which guarantees an important practical security. Work in this field has so far only shown results in the spatial domain.This manuscript partly presents contributions that extend this method into the JPEG domain by exploiting a very precise image development process. Based on considerations of the independence of some of the DCT coefficients of the image, we have also been able to contribute to the security of classical insertion schemes.In order to present our work in this manuscript, the basic concepts (steganography, steganalysis, the image development process) are presented through the first two chapters. The first one provides a state of the art in steganography and steganalysis, the next one introduces some concepts on image development.The three following chapters report on our contributions. Chapter 3 studies the origins of the dependencies between DCT coefficients, introducing a method to model these dependencies using a covariance matrix from which we have obtained an analytical expression.Chapter 4 exploits the results of chapter 3 to derive an embedding scheme which use this covariance matrix, allowing an embedding which preserves the statistical distribution of sensor noise in the DCT domain, and thus preserving the dependencies between the coefficients of an image.Finally, based on chapters 3 and 4, the exploitation of the development chain allowed us to develop an approach to secure classical insertion schemes using synchronized modifications.Our two approaches have shown superior results to the state of the art, allowing in the first case to elaborate a methodology to preserve sensor noise during embedding in the JPEG domain, and opening in the second case possibilities to improve existing schemes so that they generate images less detectable than before.L’étymologie grecque du terme « stéganographie » est la concaténation des mots « stego »: garder secret et « graphia »: l’écriture.La stéganographie est donc un terme pour désigner l’art de réaliser une communication secrète (ou discrète). La caractéristique fondamentale de la stéganographie est par essence qu’il doit être impossible pour un système de détection de distinguer les objets anodins de ceux qui contiennent un message secret.De manière analogue à la cryptographie, dont la discipline duale est la crypt- analyse visant à décrypter le message chiffré, la stéganographie a également sa discipline duale : la stéganalyse. L’objectif de la stéganalyse étant par essence, à minima, de détecter la présence d’un message caché.Dans le cadre des images digitales on peut définir la perturbation induite par l’insertion d’un message secret sur son image de couverture comme l’ajout d’un signal spécifique, l’indétectabilité de ce message va ainsi reposer sur le fait que le signal ajouté ne va pas perturber les propriétés statistiques de l’image initiale (l’image cover). C’est en partant de ce principe que le paradigme de la Stéganographie Naturelle est née, celle-ci s’attache à utiliser un bruit inhérent aux capteurs photographiques (le bruit photonique) qui peut être modélisé par une loi normale distribuée indépendamment sur chaque photo-site. Le message est ainsi inséré par l’imitation de ce bruit naturel lors de la capture d’une image digitale. De ce fait l’image ainsi générée (l’image stego) dispose des propriétés statistiques d’une image anodine, ce qui garantit une importante sécurité pratique. Les travaux dans ce domaine n’avaient montré jusqu’à présent que des résultats dans le domaine spatial.Ce manuscrit présente en partie des contributions qui prolongent cette méthode dans le domaine JPEG en exploitant un processus de développement des images très précis. À partir de considérations sur l’indépendance de certains coefficients DCT de l’image, nous avons également pû contribuer à sécuriser des schémas d’insertions classiques.Pour présenter nos travaux dans ce manuscrit, la présentation des concepts de base (la stéganographie, la stéganalyse, le processus de formation d’une image) est effectuée à travers les deux premiers chapitres. Le premier fournit un état de l’art en stégnographie et en stéganalyse, le suivant introduit des notions sur le de développement des images.Les trois chapitres qui les suivent font état de nos contributions. Le chapitre 3 étudie l’origines des dépendances entre coefficients DCT, proposant une méthode pour modéliser celles-ci à l’aide d’une matrice de covariance dont nous avons obtenus une expression analytique.Le chapitre 4 emploit les résultats du chapitre 3 pour dériver un schéma d’insertion utilisant cette matrice de covariance, permettant une insertion préservant la distribution statistique du bruit de capteur dans le domaine DCT, et préservant de ce fait les dépendances entre les coefficients d’une image.Enfin à la lumière des chapitres 3 et 4, l’exploitation de la chaîne de développe- ment nous a permis d’élaborer une approche de sécurisation de schémas d’insertion classiques en utilisant des modifications synchronisées.Nos deux approches ont montré des résultats supérieurs à l’état de l’art, permettant dans le premier cas d’expliciter une méthodologie pour préserver le bruit de capteur lors d’une insertion dans le domaine JPEG, et ouvrant dans le deuxième des possibilités d’amélioration de schémas déjà existants afin qu’ils produisent des images moins détectables qu’auparavant

An Empirical Study of Steganography and Steganalysis of Color Images in the JPEG Domain

International audienceThis paper tackles the problem of JPEG steganography and steganalysis for color images, a problem that has rarely been studied so far and which deserves more attention. After focusing on the 4:4:4 sampling strategy, we propose to modify for each channel the embedding rate of J-UNIWARD and UERD steganographic schemes in order to arbitrary spread the payload between the luminance and the chrominance components while keeping a constant message size for the different strategies. We also compare our spreading payload strategy w.r.t. two strategies: (i) the concatenation of the cost map (CONC) or (ii) equal embedding rates (EER) among channels. We then select good candidates within the feature sets designed either for JPEG or color steganography. Our conclusions are threefold: (i) the GFR or DCTR features sets, concatenated on the three channels offer better performance than ColorSRMQ1 for JPEG Quality Factor (QF) of 75 and 95 but ColorSRMQ1 is more sensitive for QF=100, (ii) the CONC or EER strategies are suboptimal, and (iii) depending of the quality factors and the embedding schemes, the empirical security is maximized when between 33% (QF=100, UERD) and 95% (QF=75, J-UNIWARD) of the payload is allocated to the luminance channel

JPEG Steganography and Synchronization of DCT Coefficients for a Given Development Pipeline

International audienceThis paper proposes to use the statistical analysis of the correlation between DCT coefficients to design a new synchronization strategy that can be used for cost-based steganographic schemes in the JPEG domain. First, an analysis is performed on the covariance matrix of DCT coefficients of neighboring blocks after a development pipeline similar to the one used to generate BossBase, and applied on a photonic noise. This analysis exhibits (i) a decomposition into 8 disjoint sets of uncorrelated coefficients (4 sets per block used by 2 disjoint lattices) and (ii) the fact that each DCT coefficient is correlated with 38 other coefficients belonging either to the same block or to connected blocks. Using the uncorrelated groups, an embedding scheme can be designed using only 8 disjoint lattices. The proposed embedding scheme relies on ingredients. Firstly, we convert the empirical costs associated to one each coefficient into a Gaussian distribution whose variance is directly computed from the embedding costs. Secondly we derive conditional Gaussian distributions from a multivariate distribution considering only the correlated coefficients which have been already modified by the embedding scheme. This covariance matrix takes into account both the correlations exhibited by the analysis of the covariance matrix and the variance derived from the costs. This synchronization scheme enables to obtain a gain of $P_E$ of at least $7\%$ at $QF95$ for an embedding rate close to 0.3 bnzac coefficient using DCTR feature sets for both UERD and JUniward

A Natural Steganography Embedding Scheme Dedicated to Color Sensors in the JPEG Domain

International audienceUsing Natural Steganography (NS), a cover raw image acquired at sensitivity ISO 1 is transformed into a stego image whose statistical distribution is similar to a cover image acquired at sensitivity ISO 2 > ISO 1. This paper proposes such an embedding scheme for color sensors in the JPEG domain, extending thus the prior art proposed for the pixel domain and the JPEG domain for monochrome sensors. We first show that color sensors generate strong intra-block and inter-block dependencies between DCT coefficients and that theses dependencies are due to the demosaicking step in the development process. Capturing theses dependencies using an empirical covariance matrix, we propose a pseudo-embedding algorithm on greyscale JPEG images which uses up to four sub-lattices and 64 lattices to embed information while preserving the estimated correlations among DCT coefficients. We then compute an approximation of the average embedding rate w.r.t. the JPEG quality factor and evaluate the empirical security of the proposed scheme for linear and non-linear demosaicing schemes. Our experiments show that we can achieve high capacity (around 2 bit per nzAC) with a high empirical security (P E 30% using DCTR at QF 95)

Stéganographie naturelle pour images JPEG

International audienc

An Empirical Study of Steganography and Steganalysis of Color Images in the JPEG Domain

International audienceThis paper tackles the problem of JPEG steganography and steganalysis for color images, a problem that has rarely been studied so far and which deserves more attention. After focusing on the 4:4:4 sampling strategy, we propose to modify for each channel the embedding rate of J-UNIWARD and UERD steganographic schemes in order to arbitrary spread the payload between the luminance and the chrominance components while keeping a constant message size for the different strategies. We also compare our spreading payload strategy w.r.t. two strategies: (i) the concatenation of the cost map (CONC) or (ii) equal embedding rates (EER) among channels. We then select good candidates within the feature sets designed either for JPEG or color steganography. Our conclusions are threefold: (i) the GFR or DCTR features sets, concatenated on the three channels offer better performance than ColorSRMQ1 for JPEG Quality Factor (QF) of 75 and 95 but ColorSRMQ1 is more sensitive for QF=100, (ii) the CONC or EER strategies are suboptimal, and (iii) depending of the quality factors and the embedding schemes, the empirical security is maximized when between 33% (QF=100, UERD) and 95% (QF=75, J-UNIWARD) of the payload is allocated to the luminance channel