Methods of compression-based text classification have proven their usefulness for various applications. However, in some classification problems, such as spam filtering, a classifier confronts one or many adversaries willing to induce errors in the classifier's judgment on certain kinds of input. In this thesis, we consider the problem of finding thrifty strategies for character-based text modification that allow an adversary to revert classifier's verdict on a given family of input texts. We propose three statistical statements of the problem that can be used by an attacker to obtain transformation models which are optimal in some sense. Evaluating these three techniques on a realistic spam corpus, we find that an adversary can transform a spam message (detectable as such by an entropy-based text classifier) into a legitimate one by generating and appending, in some cases, as few additional characters as 20% of the original length of the message.

<p>Despite the rapid development of adversarial attacks on machine learning models, many types of new adversarial examples remain unknown. Undiscovered types of adversarial attacks pose a</p><p>serious concern for the safety of the models, which raises the issue about the effectiveness of current adversarial robustness evaluation. Image semantic segmentation is a practical computer</p><p>vision task. However, segmentation networks’ robustness under adversarial attacks receives insufficient attention. Recently, machine learning researchers started to focus on generating</p><p>adversarial examples beyond the norm-bound restriction for segmentation neural networks. In this thesis, a simple and efficient method: AdvDRIT is proposed to synthesize unconstrained controllable adversarial images leveraging conditional-GAN. Simple CGAN yields poor image quality and low attack effectiveness. Instead, the DRIT (Disentangled Representation Image Translation) structure is leveraged with a well-designed loss function, which can generate valid adversarial images in one step. AdvDRIT is evaluated on two large image datasets: ADE20K and Cityscapes. Experiment results show that AdvDRIT can improve the quality of adversarial examples by decreasing the FID score down to 40% compared to state-of-the-art generative models such as Pix2Pix, and also improve the attack success rate 38% compared to other adversarial attack methods including PGD.</p>

This thesis aims to contribute to the field of neural networks by improving upon the performance of a state-of-the-art regularization scheme called MixUp, and by contributing to the conceptual understanding of MixUp. MixUp is a data augmentation scheme in which pairs of training samples and their corresponding labels are mixed using linear coefficients. Without label mixing, MixUp becomes a more conventional scheme: input samples are moved but their original labels are retained. Because samples are preferentially moved in the direction of other classes we refer to this method as directional adversarial training, or DAT. We show that under two mild conditions, MixUp asymptotically convergences to a subset of DAT. We define untied MixUp (UMixUp), a superset of MixUp wherein training labels are mixed with different linear coefficients to those of their corresponding samples. We show that under the same mild conditions, untied MixUp converges to the entire class of DAT schemes. Motivated by the understanding that UMixUp is both a generalization of MixUp and a scheme possessing adversarial-training properties, we experiment with different datasets and loss functions to show that UMixUp provides improves performance over MixUp. In short, we present a novel interpretation of MixUp as belonging to a class highly analogous to adversarial training, and on this basis we introduce a simple generalization which outperforms MixUp.

Image interpolation in the medical area is of vital importance as most 3D biomedical volume images are sampled where the distance between consecutive slices is significantly greater than the in-plane pixel size due to radiation dose or scanning time. Image interpolation creates a certain number of new slices between known slices in order to obtain an isotropic volume image. The results can be used for the higher quality of 2D and 3D visualization or reconstruction of human body structure. Semantic interpolation on the manifold has been proved to be very useful for smoothing the interpolation process. Nevertheless, all previous methods focused on low-resolution image interpolation, and most of which work poorly on high-resolution images. Besides, the medical field puts a high threshold for the quality of interpolations, as they need to be semantic and realistic enough, and resemble real data with only small errors permitted. Typically, people downsample the images into 322 and 642 for semantic interpolation, which does not meet the requirement for high-resolution in the medical field. Thus, we explore a novel way to generate semantically correct interpolations and maintain the resolution at the same time. Our method has been proved to generate realistic and high-resolution interpolations on the sizes of 5262 and 5122. Our main contribution is, first, we propose a novel network, High Resolution Interpolation Network (HRINet), aiming at producing semantically correct high-resolution CT image interpolations. Second, by combining the idea of ACAI and GANs, we propose a unique alternative supervision method by applying supervised and unsupervised training alternatively to raise the accuracy and fidelity of body structure in CT when interpolated while keeping high quality. Third, we introduce an extra Markovian discriminator as a texture or fine details regularizer to make our model generate results indistinguishable from real data. In addition, we explore other possibilities or tricks to further improve the performance of our model, including low-level feature maps mixing, and removing batch normalization layers within the autoencoder. Moreover, we compare the impacts of MSE based and perceptual based loss optimizing methods for high quality interpolation, and show the trade-off between the structural correctness and sharpness. The interpolation experiments show significant improvement on both sizes of 256 2 and 5122 images quantitatively and qualitatively. We find that interpolations produced by HRINet are sharper and more realistic compared with other existing methods such as AE and ACAI in terms of various metrics. As an application of high-resolution interpolation, we have done 2D volume projection and 3D volume reconstruction from axial view CT data and their interpolations. We show the great enhancement of applying HRINet for both in sharpness and fidelity. Specifically, for 2D volume projection, we explore orthogonal projection and weighted projection respectively so as to show the improved effectiveness for visualizing internal and external human body structure.

The availability of powerful GPUs and the consequent development of deep neural networks, have brought remarkable results in videogame levels generation, image-to-image translation , video-to-video translation, image inpainting and video generation. Nonetheless, in conditional or constrained settings, unconditioned generative models still suffer because they have little to none control over the generated output. This leads to problems in some scenarios, such as structured objects generation or multimedia manipulation. In the manner, unconstrained GANs fail to generate objects that must satisfy hard constraints (e.g., molecules must be chemically valid or game levels must be playable). In the latter, the manipulation of complex scenes is a challenging and unsolved task, since these scenes are composed of objects and background of different classes. In this thesis , we focus on these two scenarios and propose different techniques to improve deep generative models. First, we introduce Constrained Adversarial Networks (CANs), an extension of GANs in which the constraints are embedded into the model during training. Then we focus on developing novel deep learning models to alter complex urban scenes. In particular, we aim to alter the scene by: i) studying how to better leverage the semantic and instance segmentation to model its content and structure; ii) modifying, inserting and/or removing specific object instances coherently to its semantic; iii) generating coherent and realistic videos where users can alter the object’s position.

<p>Deep learning plays an important role in various disciplines, such as auto-driving, information technology, manufacturing, medical studies, and financial studies. In the past decade, there have been fruitful studies on deep learning in which training and testing data are assumed to follow the same distribution to humans. Recent studies reveal that these dedicated models are vulnerable to adversarial attack, i.e., the predicting label may be changed even if the testing input has an unaware perturbation. However, most existing studies aim to develop computationally efficient adversarial learning algorithms without a thorough understanding of the statistical properties of these algorithms. This dissertation aims to provide theoretical understandings of adversarial training to figure out potential improvements in this area of research. </p> <p><br></p> <p>The first part of this dissertation focuses on the algorithmic stability of adversarial training. We reveal that the algorithmic stability of the vanilla adversarial training method is sub-optimal, and we study the effectiveness of a simple noise injection method. While noise injection improves stability, it also does not deteriorate the consistency of adversarial training.</p> <p><br></p> <p>The second part of this dissertation reveals a phase transition phenomenon in adversarial training. When the attack strength increases, the training trajectory of adversarial training will deviate from its natural counterpart. Consequently, various properties of adversarial training are different from clean training. It is essential to have adaptations in the training configuration and the neural network structure to improve adversarial training.</p> <p><br></p> <p>The last part of this dissertation focuses on how artificially generated data improves adversarial training. It is observed that utilizing synthetic data improves adversarial robustness, even if the data are generated using the original training data, i.e., no extra information is introduced. We use a theory to explain the reason behind this observation and propose further adaptations to utilize the generated data better.</p>

Modern AI systems, particularly with the rise of big data and deep learning in the last decade, have greatly improved our daily life and at the same time created a long list of controversies. AI systems are often subject to malicious and stealthy subversion that jeopardizes their efficacy. Many of these issues stem from the data-driven nature of machine learning. While big data and deep models significantly boost the accuracy of machine learning models, they also create opportunities for adversaries to tamper with models or extract sensitive data. Malicious data providers can compromise machine learning systems by supplying false data and intermediate computation results. Even a well-trained model can be deceived to misbehave by an adversary who provides carefully designed inputs. Furthermore, curious parties can derive sensitive information of the training data by interacting with a machine-learning model. These adversarial scenarios, known as poisoning attack, adversarial example attack, and inference attack, have demonstrated that security, privacy, and robustness have become more important than ever for AI to gain wider adoption and societal trust. To address these problems, we proposed the following solutions: (1) FLARE, which detects and mitigates stealthy poisoning attacks by leveraging latent space representations; (2) MANDA, which detects adversarial examples by utilizing evaluations from diverse sources, i.e, model-based prediction and data-based evaluation; (3) FeCo which enhances the robustness of machine learning-based network intrusion detection systems by introducing a novel representation learning method; and (4) DP-FedMeta, which preserves data privacy and improves the privacy-accuracy trade-off in machine learning systems through a novel adaptive clipping mechanism. / Doctor of Philosophy / Over the past few decades, machine learning (ML) has become increasingly popular for enhancing efficiency and effectiveness in data analytics and decision-making. Notable applications include intelligent transportation, smart healthcare, natural language generation, intrusion detection, etc. While machine learning methods are often employed for beneficial purposes, they can also be exploited for malicious intents. Well-trained language models have demonstrated generalizability deficiencies and intrinsic biases; generative ML models used for creating art have been repurposed by fraudsters to produce deepfakes; and facial recognition models trained on big data have been found to leak sensitive information about data owners. Many of these issues stem from the data-driven nature of machine learning. While big data and deep models significantly improve the accuracy of ML models, they also enable adversaries to corrupt models and infer sensitive data. This leads to various adversarial attacks, such as model poisoning during training, adversarially crafted data in testing, and data inference. It is evident that security, privacy, and robustness have become more important than ever for AI to gain wider adoption and societal trust. This research focuses on building trustworthy machine-learning systems in adversarial environments from a data perspective. It encompasses two themes: securing ML systems against security or privacy vulnerabilities (security of AI) and using ML as a tool to develop novel security solutions (AI for security). For the first theme, we studied adversarial attack detection in both the training and testing phases and proposed FLARE and MANDA to secure matching learning systems in the two phases, respectively. Additionally, we proposed a privacy-preserving learning system, dpfed, to defend against privacy inference attacks. We achieved a good trade-off between accuracy and privacy by proposing an adaptive data clipping and perturbing method. In the second theme, the research is focused on enhancing the robustness of intrusion detection systems through data representation learning.

The presence of noise and corruption in real-world data can be inevitably caused by accidental outliers, transmission loss, or even adversarial data attacks. Unlike traditional random noise usually assume a specific distribution with low corruption ratio, the data collected from crowdsourcing or labeled by weak annotators can contain adversarial data corruption. More challenge, the adversarial data corruption can be arbitrary, unbounded and do not follow any specific distribution. In addition, in the era of data explosion, the fast-growing amount of data makes the robust models more difficult to handle large-scale data sets. This thesis focuses on the development of methods for scalable robust models under the adversarial data corruption assumptions. Four methods are proposed, including robust regression via heuristic hard-thresholding, online and distributed robust regression with adversarial noises, self-paced robust learning for leveraging clean labels in noisy data, and robust regression via online feature selection with adversarial noises. Moreover, I extended the self-paced robust learning method to its distributed version for the scalability of the proposed algorithm, named distributed self-paced learning in alternating direction method of multiplier. Last, a robust multi-factor personality prediction model is proposed to hand the correlated data noises. For the first method, existing solutions for robust regression lack rigorous recovery guarantee of regression coefficients under the adversarial data corruption with no prior knowledge of corruption ratio. The proposed contributions of our work include: (1) Propose efficient algorithms to address the robust least-square regression problem; (2) Design effective approaches to estimate the corruption ratio; (3) Provide a rigorous robustness guarantee for regression coefficient recovery; and (4) Conduct extensive experiments for performance evaluation. For the second method, existing robust learning methods typically focus on modeling the entire dataset at once; however, they may meet the bottleneck of memory and computation as more and more datasets are becoming too large to be handled integrally. The proposed contributions of our work for this task include: (1) Formulate a framework for the scalable robust least-squares regression problem; (2) Propose online and distributed algorithms to handle the adversarial corruption; (3) Provide a rigorous robustness guarantee for regression coefficient recovery; and (4) Conduct extensive experiments for performance evaluations. For the third method, leveraging the prior knowledge of clean labels in noisy data is actually a crucial issue in practice, but existing robust learning methods typically focus more on eliminating noisy data. However, the data collected by ``weak annotator" or crowd-sourcing can be too noisy for existing robust methods to train an accurate model. Moreover, existing work that utilize additional clean labels are usually designed for some specific problems such as image classification. These methods typically utilize clean labels in large-scale noisy data based on their additional domain knowledge; however, these approaches are difficult to handle extremely noisy data and relied on their domain knowledge heavily, which makes them difficult be used in more general problems. The proposed contributions of our work for this task include: (1) Formulating a framework to leverage the clean labels in noisy data; (2) Proposing a self-paced robust learning algorithm to train models under the supervision of clean labels; (3) Providing a theoretical analysis for the convergence of the proposed algorithm; and (4) Conducting extensive experiments for performance evaluations. For the fourth method, the presence of data corruption in user-generated streaming data, such as social media, motivates a new fundamental problem that learns reliable regression coefficient when features are not accessible entirely at one time. Until now, several important challenges still cannot be handled concurrently: 1) corrupted data estimation when only partial features are accessible; 2) online feature selection when data contains adversarial corruption; and 3) scaling to a massive dataset. This paper proposes a novel RObust regression algorithm via Online Feature Selection (textit{RoOFS}) that concurrently addresses all the above challenges. Specifically, the algorithm iteratively updates the regression coefficients and the uncorrupted set via a robust online feature substitution method. We also prove that our algorithm has a restricted error bound compared to the optimal solution. Extensive empirical experiments in both synthetic and real-world data sets demonstrated that the effectiveness of our new method is superior to that of existing methods in the recovery of both feature selection and regression coefficients, with very competitive efficiency. For the fifth method, existing self-paced learning approaches typically focus on modeling the entire dataset at once; however, this may introduce a bottleneck in terms of memory and computation, as today's fast-growing datasets are becoming too large to be handled integrally. The proposed contributions of our work for this task include: (1) Reformulate the self-paced problem into a distributed setting.; (2) A distributed self-paced learning algorithm based on consensus ADMM is proposed to solve the textit{SPL} problem in a distributed setting; (3) A theoretical analysis is provided for the convergence of our proposed textit{DSPL} algorithm; and (4) Extensive experiments have been conducted utilizing both synthetic and real-world data based on a robust regression task. For the last method, personality prediction in multiple factors, such as openness and agreeableness, is growing in interest especially in the context of social media, which contains massive online posts or likes that can potentially reveal an individual's personality. However, the data collected from social media inevitably contains massive amounts of noise and corruption. To address it, traditional robust methods still suffer from several important challenges, including 1) existence of correlated corruption among multiple factors, 2) difficulty in estimating the corruption ratio in multi-factor data, and 3) scalability to massive datasets. This paper proposes a novel robust multi-factor personality prediction model that concurrently addresses all the above challenges by developing a distributed robust regression algorithm. Specifically, the algorithm optimizes regression coefficients of each factor in parallel with a heuristically estimated corruption ratio and then consolidates the uncorrupted set from multiple factors in two strategies: global consensus and majority voting. We also prove that our algorithm benefits from strong guarantees in terms of convergence rates and coefficient recovery, which can be utilized as a generic framework for the multi-factor robust regression problem with correlated corruption property. Extensive experiment on synthetic and real dataset demonstrates that our algorithm is superior to those of existing methods in both effectiveness and efficiency. / Doctor of Philosophy / Social media has experienced a rapid growth during the past decade. Millions of users of sites such as Twitter have been generating and sharing a wide variety of content including texts, images, and other metadata. In addition, social media can be treated as a social sensor that reflects different aspects of our society. Event analytics in social media have enormous significance for applications like disease surveillance, business intelligence, and disaster management. Social media data possesses a number of important characteristics including dynamics, heterogeneity, noisiness, timeliness, big volume, and network properties. These characteristics cause various new challenges and hence invoke many interesting research topics, which will be addressed here. This dissertation focuses on the development of five novel methods for social media-based spatiotemporal event detection and forecasting. The first of these is a novel unsupervised approach for detecting the dynamic keywords of spatial events in targeted domains. This method has been deployed in a practical project for monitoring civil unrest events in several Latin American regions. The second builds on this by discovering the underlying development progress of events, jointly considering the structural contexts and spatiotemporal burstiness. The third seeks to forecast future events using social media data. The basic idea here is to search for subtle patterns in specific cities as indicators of ongoing or future events, where each pattern is defined as a burst of context features (keywords) that are relevant to a specific event. For instance, an initial expression of discontent gas price increases could actually be a potential precursor to a more general protest about government policies. Beyond social media data, in the fourth method proposed here, multiple data sources are leveraged to reflect different aspects of the society for event forecasting. This addresses several important problems, including the common phenomena that different sources may come from different geographical levels and have different available time periods. The fifth study is a novel flu forecasting method based on epidemics modeling and social media mining. A new framework is proposed to integrate prior knowledge of disease propagation mechanisms and real-time information from social media.

Adversarial network coding studies the transmission of data over networks affected by adversarial noise. In this realm, the noise is modeled by an omniscient adversary who is restricted to corrupting a proper subset of the network edges. In 2018, Ravagnani and Kschischang established a combinatorial framework for adversarial networks. The study was recently furthered by Beemer, Kilic and Ravagnani, with particular focus on the one-shot capacity: a measure of the maximum number of symbols that can be transmitted in a single use of the network without errors. In this thesis, both bounds and capacity-achieving schemes are provided for families of adversarial networks in multiple transmission rounds. We also demonstrate scenarios where we transmit more information using a network multiple times for communication versus using the network once. Some results in this thesis are joint work with Giuseppe Cotardo (Virginia Tech), Gretchen Matthews (Virginia Tech) and Alberto Ravagnani (Eindhoven University of Technology). / Master of Science / We study how to best transfer data across a communication network even if there is adversarial interference using network coding. Network coding is used in video streaming, autonomous vehicles, 5G and NextG communications, satellite networks, and Internet of Things (IoT) devices among other applications. It is the process that encodes data before sending it and decodes it upon receipt. It brings advantages such as increased network efficiency, improved reliability, reduced redundancy, enhanced resilience, and energy savings. We seek to enhance this valuable technique by determining optimal ways in which to utilize network coding schemes. We explore scenarios in which an adversary has partial access to a network. To examine the maximum data that can be communicated over one use of a network, we require the intermediate parts of the network process the information before forwarding it in a process called network decoding. In this thesis, we focus on characterizing when using a network multiple times for communication increases the amount of information that is received regardless of the worst-case adversarial attack, building on prior work that shows how underlying structure influences capacity. We design efficient methods for specific networks, to communicate at capacity.

In order to interact with the real world, humans need to perform several tasks such as object detection, pose estimation, motion estimation and distance estimation. These tasks are all part of scene understanding and are fundamental tasks of computer vision. Depth estimation received unprecedented attention from the research community in recent years due to the growing interest in its practical applications (ie robotics, autonomous driving, etc.) and the performance improvements achieved with deep learning. In fact, the applications expanded from the more traditional tasks such as robotics to new fields such as autonomous driving, augmented reality devices and smartphones applications. This is due to several factors. First, with the increased availability of training data, bigger and bigger datasets were collected. Second, deep learning frameworks running on graphical cards exponentially increased the data processing capabilities allowing for higher precision deep convolutional networks, ConvNets, to be trained. Third, researchers applied unsupervised optimization objectives to ConvNets overcoming the hurdle of collecting expensive ground truth and fully exploiting the abundance of images available in datasets. This thesis addresses several proposals and their benefits for unsupervised depth estimation, i.e., (i) learning from resynthesized data, (ii) adversarial learning, (iii) coupling generator and discriminator losses for collaborative training, and (iv) self-improvement ability of the learned model. For the first two points, we developed a binocular stereo unsupervised depth estimation model that uses reconstructed data as an additional self-constraint during training. In addition to that, adversarial learning improves the quality of the reconstructions, further increasing the performance of the model. The third point is inspired by scene understanding as a structured task. A generator and a discriminator joining their efforts in a structured way improve the quality of the estimations. Our intuition may sound counterintuitive when cast in the general framework of adversarial learning. However, in our experiments we demonstrate the effectiveness of the proposed approach. Finally, self-improvement is inspired by estimation refinement, a widespread practice in dense reconstruction tasks like depth estimation. We devise a monocular unsupervised depth estimation approach, which measures the reconstruction errors in an unsupervised way, to produce a refinement of the depth predictions. Furthermore, we apply knowledge distillation to improve the student ConvNet with the knowledge of the teacher ConvNet that has access to the errors.