Kevin M. Whelan

Joint iterative decoding and estimation for side-informed data hiding

We present a previously unavailable study on a general procedure for joint iterative decoding and estimation of attack parameters in side-informed data hiding. This type of approach, which exploits iteratively decodable codes for channel identification purposes, has recently become a relevant research trend in many digital communications problems. An advantage is that estimation pilots are not strictly required, thus affording in principle the implementation of blind methods that are able to work close to the theoretically maximum achievable rate. Such a target naturally requires the use of both near-optimum side-informed data hiding methods (e.g., DC-DM) and near-optimum iteratively decodable channel codes (e.g., turbo codes). The attack channels considered in this study are additive independent random noise, amplitude scaling, and a particular case of fine desynchronization of the sampling grid, whose parameters are estimated by maximum likelihood at the decoder. The complexity of this task is tackled by means of the Expectation-Maximization (EM) algorithm, relying on the use of a priori probabilities produced by the iterative decoding process.

THE CATHOLIC PARISH, THE CATHOLIC CHAPEL AND VILLAGE DEVELOPMENT IN IRELAND

The Catholic Church in Ireland underwent profound transformations in the early modern period. Among the most significant of these was the need to reconstruct its parish network and to provide places of worship for its adherents. This reconstruction and rebuilding phase was largely compressed into the 1780–1860 period and gave rise to a radically new parish framework which was adapted to the demographic, social and economic conditions of that time. Chapels were also constructed in large numbers and their sites were determined by the attitudes of landlords, the wealth of the local Catholic community and the sponsorship of prominent Catholic families. The diffusion of chapel building shows pronounced regional biases. Associated with this reinvigorated parish and chapel network was the development of new villages, for which the chapel acted as a nucleus and focus. The term chapel-village is used to describe them and their economic, social and morphological characteristics are examined. Their distribution reve...

ML detection of steganography

Digital steganography is the art of hiding information in multimedia content, such that it remains perceptually and statistically unchanged. The detection of such covert communication is referred to as steganalysis. To date, steganalysis research has focused primarily on either, the extraction of features from a document that are sensitive to the embedding, or the inference of some statistical difference between marked and unmarked objects. In this work, we evaluate the statistical limits of such techniques by developing asymptotically optimal tests (Maximum Likelihood) for a number of side informed embedding schemes. The required probability density functions (pdf) are derived for Dither Modulation (DM) and Distortion-Compensated Dither Modulation (DC-DM/SCS) from an steganalysts point of view. For both embedding techniques, the pdfs are derived in the presence and absence of a secret dither key. The resulting tests are then compared to a robust blind steganalytic test based on feature extraction. The performance of the tests is evaluated using an integral measure and receiver operating characteristic (ROC) curves.

PLL-Based Synchronization of Dither-Modulation Data Hiding

A new approach to synchronization recovery for signals watermarked using the dither modulation data hiding scheme is presented. The strategy followed involves the use of a digital phase-locked loop to track the offsets applied by an attacker to the sampling grid of the watermarked signal. The main element in this synchronization loop is the timing error detector which is responsible for generating an error signal, used to update the estimates of the applied offsets. It is shown how a timing error detector which has been used in digital communications may be easily adapted to extract timing information from DM watermarked signals. The performance of the proposed synchronizer is evaluated using the probability of decoding error under different models for the sampling grid offsets

A Two-Dimensional Extension of the Mueller and Müller Timing Error Detector

We present a 2-D extension of a timing error detector (TED) originally developed by Mueller and Muller for timing recovery on multilevel pulse amplitude modulation (PAM) over 1-D channels. We provide accurate theoretical expressions of the performance of the proposed scheme, and we verify its applicability by studying a particular case. The 2-D TED thus obtained is used as the engine of a 2-D phase locked loop (PLL) for timing recovery.

Iterative decoding of scale invariant image data-hiding

The vulnerability of quantization-based data hiding methods to amplitude scaling requires the formulation of countermeasures to this relatively simple attack. Rational dither modulation (RDM) was recently proposed as a quantization-based data hiding scheme, which is scaling invariant. This is achieved by a simple modification of the well-known dither modulation (DM) data hiding method. The performance of RDM, asymptotically in the memory size of the system, can be made to approach that of DM on the Gaussian channel. In this work we examine the application of near-optimal channel coding to RDM. The channel model required to undertake iterative decoding is presented and the proposed method is applied to real image data.

Iterative estimation of amplitude scaling on distortion-compensated dither modulation

The vulnerability of quantization-based data hiding schemes to amplitude scaling requires the formulation of countermeasures to this relatively simple attack. Parameter estimation is one possible approach, where the applied scaling is estimated from the received signal at the decoder. This estimate can be used to correct the mismatch with respect to the quantization step assumed by the decoder prior to decoding. In this work we first review previous approaches utilizing parameter estimation as a means of combating the scaling attack on DC-DM. We then present a method for joint iterative decoding and maximum likelihood estimation of the scaling factor for this quantization-based method. The estimation method exploits the reliabilities provided by the near-optimal iterative decoding process in order to successively refine the estimate. The complexity of this problem is tackled using the expectation maximization algorithm. By performing estimation in cooperation with the decoding process, reliable estimation is possible at very low watermark-to-noise power ratios by using sufficiently low rate codes.

Error correction coding of nonlinear side-informed watermarking schemes

The application of error correction coding to side-informed watermarking utilizing polynomial detectors is investigated. The overall system is viewed as a code concatenation in which the outer code is a powerful channel code and the inner code is a low rate repetition code. For the inner code we adopt our previously proposed side-informed embedding scheme in which the watermark direction is set to the gradient of the detection function in order to reduce the effect of host signal interference. Turbo codes are employed as the outer code due to their near capacity performance. The overall rate of the concatenation is kept constant while parameters of the constituent codes are varied. For the inner code, the degree of non-linearity of the detector along with repetition rate is varied. For a given embedding and attack strength, we determine empirically the best rate combinations for constituent codes. The performance of the scheme is evaluated in terms of bit error rate when subjected to various attacks such as additive/multiplicative noise and scaling by a constant factor. We compare the performance of the proposed scheme to the Spread Transform Scalar Costa Scheme using the same rates when subjected to the same attacks.