Learning Continuous-Time Dynamic Graphs (C-TDGs) requires accurately modeling spatio-temporal information on streams of irregularly sampled events. While many methods have been proposed recently, we find that most message passing-, recurrent- or self-attention-based methods perform poorly on *long-range* tasks. These tasks require correlating information that occurred "far" away from the current event, either spatially (higher-order node information) or along the time dimension (events occurred in the past). To address long-range dependencies, we introduce Continuous-Time Graph Anti-Symmetric Network (CTAN). Grounded within the ordinary differential equations framework, our method is designed for efficient propagation of information. In this paper, we show how CTAN's (i) long-range modeling capabilities are substantiated by theoretical findings and how (ii) its empirical performance on synthetic long-range benchmarks and real-world benchmarks is superior to other methods. Our results motivate CTAN's ability to propagate long-range information in C-TDGs as well as the inclusion of long-range tasks as part of temporal graph models evaluation.

Active measurements allow the collection of server characteristics on a large scale that can aid in discovering hidden relations and commonalities among server deployments. Finding these relations opens up new possibilities for clustering and classifying server deployments; for example, identifying a previously unknown cybercriminal infrastructure can be valuable cyber-threat intelligence. In this work, we propose a methodology based on active measurements to acquire Transport Layer Security (TLS) metadata from servers and leverage it for fingerprinting. Our fingerprints capture characteristic behavior of the TLS stack, primarily influenced by the server’s implementation, configuration, and hardware support. Using an empirical optimization strategy that maximizes information gained from every handshake to minimize measurement costs, we generated 10 general-purpose Client Hellos. They served as scanning probes to create an extensive database of TLS configurations to classify servers. We propose the Shannon Entropy to measure collected information and compare different approaches. This study fingerprinted 8 million servers from the Tranco top list and two Command and Control (C2) blocklists over 60 weeks with weekly snapshots. The resulting data formed the foundation for two long-term case studies: classification of Content Delivery Network and C2 servers. Moreover, the detection was fine-grained enough to detect C2 server families. The proposed methodology demonstrated a precision of 99% and enabled a stable identification of new servers over time. This study shows how active measurements can provide valuable security-relevant insights and improve our understanding of the Internet.

Graph Neural Networks (GNNs) are well-suited for learning on homophilous graphs, i.e., graphs in which edges tend to connect nodes of the same type. Yet, achievement of consistent GNN performance on heterophilous graphs remains an open research problem. Recent works have proposed extensions to standard GNN architectures to improve performance on heterophilous graphs, trading off model simplicity for prediction accuracy. However, these models fail to capture basic graph properties, such as neighborhood label distribution, which are fundamental for learning. In this work, we propose GCN for Heterophily (GCNH), a simple yet effective GNN architecture applicable to both heterophilous and homophilous scenarios. GCNH learns and combines separate representations for a node and its neighbors, using one learned importance coefficient per layer to balance the contributions of center nodes and neighborhoods. We conduct extensive experiments on eight real-world graphs and a set of synthetic graphs with varying degrees of heterophily to demonstrate how the design choices for GCNH lead to a sizable improvement over a vanilla GCN. Moreover, GCNH outperforms state-of-the-art models of much higher complexity on four out of eight benchmarks, while producing comparable results on the remaining datasets. Finally, we discuss and analyze the lower complexity of GCNH, which results in fewer trainable parameters and faster training times than other methods, and show how GCNH mitigates the oversmoothing problem.

In this paper, we present the first conditional generative adversarial network (cGAN) architecture that is specifically designed to fuse synthetic aperture radar (SAR) and optical multi-spectral (MS) image data to generate cloud- and haze-free MS optical data from a cloud-corrupted MS input and an auxiliary SAR image. Experiments on Sentinel-2 MS and Sentinel-l SAR data confirm that our extended SAR-Opt-cGAN model utilizes the auxiliary SAR information to better reconstruct MS images than an equivalent model which uses the same architecture but only single-sensor MS data as input.

Recently, sparse signal representation of image patches has been explored to solve the pansharpening problem. Although these proposed sparse-reconstruction-based methods lead to promising results, three issues remained unsolved: 1) high computational cost; 2) no consideration given to the possibility of mutually correlated information in different multispectral channels; and 3) requirement that the spectral responses of the panchromatic (Pan) image and the multispectral image cover the same wavelength range, which is not necessarily valid for most sensors. In this paper, we propose a sophisticated sparse image fusion algorithm, which is named “jointly sparse fusion of images” (J-SparseFI). It is based on the earlier proposed sparse fusion of images (SparseFI) algorithm and overcomes the aforementioned three drawbacks of the existing sparse image fusion algorithms. The computational problem is handled by reducing the problem size and by proposing a fully parallelizable scheme. Moreover, J-SparseFI exploits the possible signal structure correlations between multispectral channels by introducing the joint sparsity model (JSM) and sharpening the highly correlated adjacent multispectral channels together. This is done by exploiting the distributed compressive sensing theory that restricts the solution of an underdetermined system by considering an ensemble of signals being jointly sparse. J-SparseFI also offers a practical solution to overcome spectral range mismatch between the Pan and multispectral images. By means of sensor spectral response and channel mutual correlation analysis, the multispectral channels are assigned to primary groups of joint channels, secondary groups of joint channels, and individual channels. Primary groups of joint channels, individual channels, and secondary groups of joint channels are then reconstructed sequentially, by the JSM or by modified SparseFI, using a dictionary trained from the Pan image or previously reconstructed high-resolution multispectral channels. A recipe of how to choose appropriate algorithm parameters, including the most crucial regularization parameter, is provided. The algorithm is evaluated and validated using WorldView-2-like images that are simulated using very high resolution airborne HySpex hyperspectral imagery and further practically demonstrated using real WorldView-2 images. The algorithm's performance is compared with other state-of-the-art methods. Visual and quantitative analyses demonstrate the high quality of the proposed method. In particular, the analysis of the difference images suggests that J-SparseFI is superior in image resolution recovery.

The fusion of hyperspectral data with a corresponding higher resolution multispectral image has become an increasingly active research field. The goal is to create a hyperspectral image that has the spatial resolution of the multispectral image. This work aims at combining two established fusion algorithms, namely J-SparseFI-HM and CNMF, to a new method which features their individual advantages. The sparse representation based J-SparseFI-HM algorithm is used to pre-process those hyperspectral channels that have a strong spectral overlap with the multispectral instrument. Then, three modified versions of the matrix factorization and unmixing based CNMF method are used for post-processing. The results are assessed and compared to the individual products of J-SparseFI-HM and CNMF, revealing a great potential for performance improvement.

In this paper we apply the recently proposed J-SparseFI data fusion method to the fusion of a low-resolution hyperspectral image and a high-resolution multispectral image. The high correlation of signals in adjacent hyperspectral channels is exploited by assuming signals in different channels are jointly sparse in suitable dictionaries that are created from the multispectral image. First experimental results using airborne HySpex hyperspectral data and synthesized WorldView-2 imagery are presented.

In this paper, we extend the Sparse Fusion of Images (SparseFI, pronounced “sparsify”) algorithm, proposed by the authors before, to a Jointly Sparse Fusion of Images (J-SparseFI) algorithm by exploiting the possible signal structural correlations between different multispectral channels. The algorithm is evaluated using airborne UltraCam data. The superior performance of the proposed methods has been demonstrated by a statistic assessment. Moreover, first experimental result using airborne HySpex hyperspectral data is presented.