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Chen Hajaj

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9 papers

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AAMAS Conference 2026 Conference Paper

Cleaner Adversarial CAPTCHAs: Intelligent Targets and Precise Noise for Usable Security

Meir Litman
Chen Hajaj

TraditionalCAPTCHAsareincreasinglyvulnerabletodeeplearningbasedsolversthatdecodetextandimageswithhighaccuracy. Inthis work, we propose methods to strengthen adversarial CAPTCHAs without compromising human usability. First, we introduce a Precise Gradient Method (PGM) that preserves gradient magnitude (rather than discarding it via a sign operator), producing adversarial perturbations with significantly lower perceptual noise. Second, we develop intelligent target class selection, using either dataset-level confusion structure (Class Relations Network) or image-specific softmax probabilities (Distance-Based Target), to steer adversarial perturbations more efficiently. Across multiple modern architectures (MobileNets, EfficientNets, ResNet, and Vision Transformer), our framework achieves faster convergence (fewer iterations), reduced visual distortion, and notably greater robustness under iterative adversarial retraining. Experiments show that our methods consistentlyreduceiterationcountsandperceptualdistortionwhile significantly increasing the difficulty for automated attacks. Our results offer a practical, scalable path toward the next generation of CAPTCHA systems and contribute new insights to the adversarial machine learning landscape focused on security and usability.

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AAMAS Conference 2019 Conference Paper

Adversarial Coordination on Social Networks

Chen Hajaj
Sixie Yu
Zlatko Joveski
Yifan guo
Yevgeniy Vorobeychik

Extensive literature exists studying decentralized coordination and consensus, with considerable attention devoted to ensuring robustness to faults and attacks. However, most of the latter literature assumes that non-malicious agents follow simple stylized rules. In reality, decentralized protocols often involve humans, and understanding how people coordinate in adversarial settings is an open problem. We initiate a study of this problem, starting with a human subjects investigation of human coordination on networks in the presence of adversarial agents, and subsequently using the resulting data to bootstrap the development of a credible agentbased model of adversarial decentralized coordination. In human subjects experiments, we observe that while adversarial nodes can successfully prevent consensus, the ability to communicate can significantly improve robustness, with the impact particularly significant in scale-free networks. On the other hand, and contrary to typical stylized models of behavior, we show that the existence of trusted nodes has limited utility. Next, we use the data collected in human subject experiments to develop a data-driven agent-based model of adversarial coordination. We show that this model successfully reproduces observed behavior in experiments, is robust to small errors in individual agent models, and illustrate its utility by using it to explore the impact of optimizing network location of trusted and adversarial nodes.

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IJCAI Conference 2018 Conference Paper

Adversarial Task Assignment

Chen Hajaj
Yevgeniy Vorobeychik

The problem of task assignment to workers is of long-standing fundamental importance. Examples of this include the classical problem of assigning computing tasks to nodes in a distributed computing environment, assigning jobs to robots, and crowdsourcing. Extensive research into this problem generally addresses important issues such as uncertainty and incentives. However, the problem of adversarial tampering with the task assignment process has not received as much attention. We are concerned with a particular adversarial setting in task assignment where an attacker may target a set of workers in order to prevent the tasks assigned to these workers from being completed. For the case when all tasks are homogeneous, we provide an efficient algorithm for computing the optimal assignment. When tasks are heterogeneous, we show that the adversarial assignment problem is NP-Hard, and present an algorithm for solving it approximately. Our theoretical results are accompanied by extensive simulation results showing the effectiveness of our algorithms.

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IJCAI Conference 2017 Conference Paper

Enhancing Crowdworkers' Vigilance

Avshalom Elmalech
David Sarne
Esther David
Chen Hajaj

This paper presents methods for improving the attention span of workers in tasks that heavily rely on their attention to the occurrence of rare events. The underlying idea in our approach is to dynamically augment the task with some dummy (artificial) events at different times throughout the task, rewarding the worker upon identifying and reporting them. The proposed approach is an alternative to the traditional approach of exclusively relying on rewarding the worker for successfully identifying the event of interest itself. We propose three methods for timing the dummy events throughout the task. Two of these methods are static and determine the timing of the dummy events at random or uniformly throughout the task. The third method is dynamic and uses the identification (or misidentification) of dummy events as a signal for the worker's attention to the task, adjusting the rate of dummy events generation accordingly.

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JAAMAS Journal 2017 Journal Article

Selective opportunity disclosure at the service of strategic information platforms

Chen Hajaj
David Sarne

Abstract This paper studies the strategic behavior of platforms that provide agents easier access to the type of opportunities in which they are interested (e. g. , eCommerce platforms, used cars bulletins and dating web-sites). We show that under four common service schemes, a platform can benefit from not necessarily listing all the opportunities with which it is familiar, even if there is no marginal cost for listing any additional opportunity. The main implication of this result is that platforms should take the subset of opportunities to be included in their listings as a decision variable, alongside the fees set for the service in their expected-profit “maximizing” optimization problem. We show that none of the four schemes generally dominates any of the others or is dominated by any. For the case of homogeneous preferences, however, several dominance relationships can be proved. Furthermore, the analysis provides a game-theoretic search-based explanation for a possible preference of buyers to pay for the service rather than receive it for free (e. g. , when the service is sponsored by ads). The paper shows that this preference can hold both for the users and the platform simultaneously in a given setting, even if both sides are fully strategic. Finally, the paper analyzes the potential improvement in the platform’s expected profit that can be achieved by considering hybrid service schemes that combine the basic ones. In particular, we focus in the Two-Part Tariff scheme that combines the two commonly used subscription and pay-per-click schemes.

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JAAMAS Journal 2016 Journal Article

Enhancing comparison shopping agents through ordering and gradual information disclosure

Chen Hajaj
Noam Hazon
David Sarne

Abstract The plethora of comparison shopping agents (CSAs) in today’s markets enables buyers to query more than a single CSA when shopping, thus expanding the list of sellers whose prices they obtain. This potentially decreases the chance of a purchase within any single interaction between a buyer and a CSA, and consequently decreases each CSAs’ expected revenue per-query. Obviously, a CSA can improve its competence in such settings by acquiring more sellers’ prices, potentially resulting in a more attractive “best price”. In this paper we suggest a complementary approach that improves the attractiveness of the best result returned based on intelligently controlling the order according to which they are presented to the user, in a way that utilizes several known cognitive-biases of human buyers. The advantage of this approach is in its ability to affect the buyer’s tendency to terminate her search for a better price, hence avoid querying further CSAs, without spending valuable resources on finding additional prices to present. The effectiveness of our method is demonstrated using real data, collected from four CSAs for five products. Our experiments confirm that the suggested method effectively influence people in a way that is highly advantageous to the CSA compared to the common method for presenting the prices. Furthermore, we experimentally show that all of the components of our method are essential to its success.

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AAAI Conference 2015 Conference Paper

Strategy-Proof and Efficient Kidney Exchange Using a Credit Mechanism

Chen Hajaj
John Dickerson
Avinatan Hassidim
Tuomas Sandholm
David Sarne

We present a credit-based matching mechanism for dynamic barter markets—and kidney exchange in particular—that is both strategy proof and efficient, that is, it guarantees truthful disclosure of donor-patient pairs from the transplant centers and results in the maximum global matching. Furthermore, the mechanism is individually rational in the sense that, in the long run, it guarantees each transplant center more matches than the center could have achieved alone. The mechanism does not require assumptions about the underlying distribution of compatibility graphs—a nuance that has previously produced conflicting results in other aspects of theoretical kidney exchange. Our results apply not only to matching via 2-cycles: the matchings can also include cycles of any length and altruist-initiated chains, which is important at least in kidney exchanges. The mechanism can also be adjusted to guarantee immediate individual rationality at the expense of economic efficiency, while preserving strategy proofness via the credits. This circumvents a well-known impossibility result in static kidney exchange concerning the existence of an individually rational, strategy-proof, and maximal mechanism. We show empirically that the mechanism results in significant gains on data from a national kidney exchange that includes 59% of all US transplant centers.

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AAAI Conference 2014 Conference Paper

Ordering Effects and Belief Adjustment in the Use of Comparison Shopping Agents

Chen Hajaj
Noam Hazon
David Sarne

The popularity of online shopping has contributed to the development of comparison shopping agents (CSAs) aiming to facilitate buyers’ ability to compare prices of online stores for any desired product. Furthermore, the plethora of CSAs in today’s markets enables buyers to query more than a single CSA when shopping, thus expanding even further the list of sellers whose prices they obtain. This potentially decreases the chance of a purchase based on the prices outputted as a result of any single query, and consequently decreases each CSAs’ expected revenue per-query. Obviously, a CSA can improve its competence in such settings by acquiring more sellers’ prices, potentially resulting in a more attractive “best price”. In this paper we suggest a complementary approach that improves the attractiveness of a CSA by presenting the prices to the user in a specific intelligent manner, which is based on known cognitive-biases. The advantage of this approach is its ability to affect the buyer’s tendency to terminate her search for a better price, hence avoid querying further CSAs, without having the CSA spend any of its resources on finding better prices to present. The effectiveness of our method is demonstrated using real data, collected from four CSAs for five products. Our experiments with people confirm that the suggested method effectively influence people in a way that is highly advantageous to the CSA.

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AAAI Conference 2013 Conference Paper

Search More, Disclose Less

Chen Hajaj
Noam Hazon
David Sarne
Avshalom Elmalech

The blooming of comparison shopping agents (CSAs) in recent years enables buyers in today’s markets to query more than a single CSA while shopping, thus substantially expanding the list of sellers whose prices they obtain. From the individual CSA point of view, however, the multi-CSAs querying is definitely non-favorable as most of today’s CSAs benefit depends on payments they receive from sellers upon transferring buyers to their websites (and making a purchase). The most straightforward way for the CSA to improve its competence is through spending more resources on getting more sellers’ prices, potentially resulting in a more attractive “best price”. In this paper we suggest a complementary approach that improves the attractiveness of the best price returned to the buyer without having to extend the CSAs’ price database. This approach, which we term “selective price disclosure” relies on removing some of the prices known to the CSA from the list of results returned to the buyer. The advantage of this approach is in the ability to affect the buyer’s beliefs regarding the probability of obtaining more attractive prices if querying additional CSAs. The paper presents two methods for choosing the subset of prices to be presented to a fully-rational buyer, attempting to overcome the computational complexity associated with evaluating all possible subsets. The effectiveness and efficiency of the methods are demonstrated using real data, collected from five CSAs for four products. Furthermore, since people are known to have an inherently bounded rationality, the two methods are also evaluated with human buyers, demonstrating that selective price-disclosing can be highly effective with people, however the subset of prices that needs to be used should be extracted in a different (and more simplistic) manner.

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