Diego Renza

Text Hiding in Images Based on QIM and OVSF

This paper presents an algorithm of digital image watermarking that uses a text as mark. Our proposal generates an optimal level of robustness according to the parameters considered for watermarking. The proposed algorithm consists in the insertion over an image of a text string, previously modified by permutation using a random key and an OVSF (Orthogonal Variable Spreading Factor) generator. The insertion is made in the wavelet domain and uses Quantization Index Modulation (QIM). Also, the conditions to reverse the process and restore the original information are set. Finally, the robustness of the proposed algorithm is evaluated implementing several attacks on the marked image.

Authenticity verification of audio signals based on fragile watermarking for audio forensics

Abstract This paper presents a new fragile watermarking method for digital audio authenticity for audio forensics purposes. The aim is to verify if an audio proof has been tampered and to locate the segments where the signal was modified. Our proposal is based on an embedding process of a text that is encoded through OVSF (Orthogonal Variable Spreading Factor) codes and spread into the entire signal using automatic adjustment. Several tests were performed in order to quantify the accuracy and the reliability of the tampering detection against four classical attacks (cropping, replacement, additive noise and amplitude reduction) by using kappa index, sensitivity and specificity. It was demonstrated that even if a small number of samples is modified, the system correctly labels the audio proof as manipulated, and locates both the start and end of the manipulation; the kappa index (reliability) is around 0.96, sensitivity is always 1, and specificity is around 0.995. The proposed algorithm could be used as a decision support tool for audio forensics verification purposes, that allows to identify if an audio proof has been modified, and the time segments in which it has been modified.

High Scrambling Degree in Audio Through Imitation of an Unintelligible Signal

A reversible scheme of audio scrambling based on the imitation of Gaussian unintelligible signals is presented in this paper. It is supported by the similarities it shares with Gaussian unintelligible signals in terms of the Probability Density Function and the entropy. It is feasible for an audio signal to imitate the behavior of a Gaussian noise signal, and then the residual intelligibility is zero. Our proposed scheme, termed ASGI (Audio Scrambling by Gaussian signal Imitation), is tested with four different music genres, and the experimental tests reveal that the scrambled audio signals look like Gaussian noise signals and have high scrambling degrees. Additionally, our scheme preserves the advantages of imitation-based scrambling schemes.

Highly Transparent and Secure Scheme for Concealing Text Within Audio

This paper presents a highly transparent and secure scheme for concealing text within audio, based on Quantization Index Modulation and Orthogonal Variable Spreading Factor. The audio signal is decomposed through the Discrete Wavelet Transform and the approximation coefficients are selected to embed the text. Every character of the text is represented by a 256-bit orthogonal code, through mapping operations between the ASCII integer representation of the character and an external key. For improving the quality of recovered data, a repetition code is applied in the embedding process. Several tests were performed in order to measure the transparency of the output audio signal (i.e. stego signal) and the security of the recovered one. The main advantage of our proposal is the good trade-off among transparency, security and hiding capacity.

A Semi-supervised Speaker Identification Method for Audio Forensics Using Cochleagrams

The general task in speaker identification for audio forensics is to identify the unknown speaker within an audio proof, who is suspected of a crime. Here, the voice of each person within a group of suspects is compared to the audio proof with the aim to determining which of them corresponds to the source. In this paper, a semi-supervised speaker identification method is proposed, which does not require a training stage. Also, the feature extraction is based on the use of cochleagrams for the previously selected words. The system can identify one or multiple suspects which have high similarity to the audio proof, or give a null response if none of the suspects satisfies a similarity threshold. The results of the proposed method are compared with the respective results of the same method but using spectrograms instead of cochleagrams. The performance of our system is measured through a confusion matrix (true and false positives, and true and false negatives) and global results are given in terms of overall accuracy and kappa index. According to several tests, our system has an overall accuracy higher than 0.97 and a kappa index around 0.78; this means a high confidence in the results of identification and rejection.

Detection of Changes in Natural Aquifer Reservoirs based on the Index of Drought

The purpose of this work is the benefit exploitation of spectral indices such as NDVI, NDWI and NDDI for obtaining drought maps. To this objective, the geographical region of Poopo Lake (Bolivia) is selected as a study area. A set of multitemporal multiband scenes of this geographical area, sensed by Landsat7-ETM+ and Landsat8-OLI/TIRS sensors have been chosen. Experiments on this images datasets provide encouraging results which may be further used for the analysis of change detection, the decision-making and implementing future action plans.

Highly Transparent Steganography Scheme of Speech Signals into Color Images Using Quantization Index Modulation

A highly transparent steganography scheme of speech signals into color images based on Quantization Index Modulation (QIM) is presented in this paper. The proposed method takes advantage of the low distortion in the host image introduced by the scalar quantization of its wavelet coefficients. The stego image is highly similar to the host image, and the secret content is imperceptible. The secret message is recovered at the receiver with high correlation to the original speech signal. For the purpose of increasing the security of the system, an external mask is used to encrypt the secret content before the embedding process. Several tests were carried out in order to quantify the influence of the size of the quantizer (delta) on the quality of the recovered secret content and the transparency of the stego image.

Comunicación encubierta de imágenes a escala de grises en imágenes a color

This manuscript shows a proposal of covert communication of grayscale images into color images. The main parameter taken into account in the design of the scheme is the transparency of the covert image; meaning that the covert image should be highly similar (perceptually and statistically) to the original color image. Several tests were conducted in order to measure both the transparency of the covert image and the quality of the recovered secret image (i.e. gray image)