Attack Detection Using Item Vector Shift in Matrix Factorisation Recommenders

This paper proposes a novel method for detecting shilling attacks in Matrix Factorization (MF)-based Recommender Systems (RS), in which attackers use false user-item feedback to promote a specific item. Unlike existing methods that use either use supervised learning to distinguish between attack and genuine profiles or analyse target item rating distributions to detect false ratings, our method uses an unsupervised technique to detect false ratings by examining shifts in item preference vectors that exploit rating deviations and user characteristics, making it a promising new direction. The experimental results demonstrate the effectiveness of our approach in various attack scenarios, including those involving obfuscation techniques.

Evaluating Impact of User-Cluster Targeted Attacks in Matrix Factorisation Recommenders

In practice, users of a Recommender System (RS) fall into a few clusters based on their preferences. In this work, we conduct a systematic study on user-cluster targeted data poisoning attacks on Matrix Factorisation (MF) based RS, where an adversary injects fake users with falsely crafted user-item feedback to promote an item to a specific user cluster. We analyse how user and item feature matrices change after data poisoning attacks and identify the factors that influence the effectiveness of the attack on these feature matrices. We demonstrate that the adversary can easily target specific user clusters with minimal effort and that some items are more susceptible to attacks than others. Our theoretical analysis has been validated by the experimental results obtained from two real-world datasets. Our observations from the study could serve as a motivating point to design a more robust RS.

Bandit Convex Optimisation Revisited: FTRL Achieves $\tilde{O}(t^{1/2})$ Regret

We show that a kernel estimator using multiple function evaluations can be easily converted into a sampling-based bandit estimator with expectation equal to the original kernel estimate. Plugging such a bandit estimator into the standard FTRL algorithm yields a bandit convex optimisation algorithm that achieves $\tilde{O}(t^{1/2})$ regret against adversarial time-varying convex loss functions.

Two Models are Better than One: Federated Learning Is Not Private For Google GBoard Next Word Prediction

In this paper we present new attacks against federated learning when used to train natural language text models. We illustrate the effectiveness of the attacks against the next word prediction model used in Google's GBoard app, a widely used mobile keyboard app that has been an early adopter of federated learning for production use. We demonstrate that the words a user types on their mobile handset, e.g. when sending text messages, can be recovered with high accuracy under a wide range of conditions and that counter-measures such a use of mini-batches and adding local noise are ineffective. We also show that the word order (and so the actual sentences typed) can be reconstructed with high fidelity. This raises obvious privacy concerns, particularly since GBoard is in production use.

Online Caching with no Regret: Optimistic Learning via Recommendations

The design of effective online caching policies is an increasingly important problem for content distribution networks, online social networks and edge computing services, among other areas. This paper proposes a new algorithmic toolbox for tackling this problem through the lens of optimistic online learning. We build upon the Follow-the-Regularized-Leader (FTRL) framework, which is developed further here to include predictions for the file requests, and we design online caching algorithms for bipartite networks with fixed-size caches or elastic leased caches subject to time-average budget constraints. The predictions are provided by a content recommendation system that influences the users viewing activity and hence can naturally reduce the caching network's uncertainty about future requests. We also extend the framework to learn and utilize the best request predictor in cases where many are available. We prove that the proposed {optimistic} learning caching policies can achieve sub-zero performance loss (regret) for perfect predictions, and maintain the sub-linear regret bound $O(\sqrt T)$, which is the best achievable bound for policies that do not use predictions, even for arbitrary-bad predictions. The performance of the proposed algorithms is evaluated with detailed trace-driven numerical tests.

Online Caching with Optimistic Learning

The design of effective online caching policies is an increasingly important problem for content distribution networks, online social networks and edge computing services, among other areas. This paper proposes a new algorithmic toolbox for tackling this problem through the lens of optimistic online learning. We build upon the Follow-the-Regularized-Leader (FTRL) framework which is developed further here to include predictions for the file requests, and we design online caching algorithms for bipartite networks with fixed-size caches or elastic leased caches subject to time-average budget constraints. The predictions are provided by a content recommendation system that influences the users viewing activity, and hence can naturally reduce the caching network's uncertainty about future requests. We prove that the proposed optimistic learning caching policies can achieve sub-zero performance loss (regret) for perfect predictions, and maintain the best achievable regret bound $O(\sqrt T)$ even for arbitrary-bad predictions. The performance of the proposed algorithms is evaluated with detailed trace-driven numerical tests.

Universal Algorithms: Beyond the Simplex

The bulk of universal algorithms in the online convex optimisation literature are variants of the Hedge (exponential weights) algorithm on the simplex. While these algorithms extend to polytope domains by assigning weights to the vertices, this process is computationally unfeasible for many important classes of polytopes where the number $V$ of vertices depends exponentially on the dimension $d$. In this paper we show the Subgradient algorithm is universal, meaning it has $O(\sqrt N)$ regret in the antagonistic setting and $O(1)$ pseudo-regret in the i.i.d setting, with two main advantages over Hedge: (1) The update step is more efficient as the action vectors have length only $d$ rather than $V$; and (2) Subgradient gives better performance if the cost vectors satisfy Euclidean rather than sup-norm bounds. This paper extends the authors' recent results for Subgradient on the simplex. We also prove the same $O(\sqrt N)$ and $O(1)$ bounds when the domain is the unit ball. To the authors' knowledge this is the first instance of these bounds on a domain other than a polytope.

Optimality of the Subgradient Algorithm in the Stochastic Setting

Recently Jaouad Mourtada and St\' ephane Ga\"iffas showed the anytime hedge algorithm has pseudo-regret $O(\log (d) / \Delta)$ if the cost vectors are generated by an i.i.d sequence in the cube $[0,1]^d$. Here $d$ is the dimension and $\Delta$ the suboptimality gap. This is remarkable because the Hedge algorithm was designed for the antagonistic setting. We prove a similar result for the anytime subgradient algorithm on the simplex. Given i.i.d cost vectors in the unit ball our pseudo-regret bound is $O(1/\Delta)$ and does not depend on the dimension of the problem.