Tech companies and browser developers have spent years pursuing a seemingly ideal solution to one of the web’s thorniest problems: using advanced cryptography to make online advertising useful without sacrificing user privacy.
New research co-authored at the University of Maryland shows there is a fundamental limit to that goal. Perfect privacy and useful targeted advertising, the researchers found, simply cannot coexist.
The findings, presented last month at the Privacy Enhancing Technologies Symposium (PETS) in Canada, challenge a foundational hope in privacy engineering. Rather than treating ad targeting and campaign measurement as separate features, the researchers zoomed out to examine them as two sides of the same coin. In doing so, they revealed that trying to protect user privacy while running both mechanisms simultaneously creates systemic vulnerabilities—a big-picture conflict that previous studies had overlooked by analyzing each piece in isolation.
Gabe Kaptchuk, a study co-author and UMD assistant professor of computer science, said the work demonstrates why the security of an advertising system cannot be evaluated piece by piece.
“Our research shows that when you connect targeting and performance feedback, the interaction introduces a fundamental, unavoidable layer of information leakage,” he said.
Along with Kaptchuk, the research team includes lead author Kyle Hogan, an electrical engineering and computer science Ph.D. student at the Massachusetts Institute of Technology (MIT); Alishah Chator, assistant professor of mathematics at Baruch College; Mayank Varia, associate professor of computing and data sciences at Boston University; and Srinivas Devadas, the Edwin Sibley Webster Professor of electrical engineering and computer science at MIT.
Kaptchuk, who has an appointment in the University of Maryland Institute for Advanced Computer Studies (UMIACS), said the study grew out of efforts across academia and the technology industry to rethink online advertising amid mounting concern over the extensive tracking that underpins much of the digital ad ecosystem.
“Digital advertising is incredibly invasive, as it relies on gathering a tremendous amount of information about individuals,” he said.
That tracking can cause concrete harms. Commercial ad networks can build profiles around sensitive characteristics—identifying, for example, people believed to be “depression-prone” or “credit-reliant.” Such profiling has been shown to enable predatory marketing, discriminatory job or housing advertising, and manipulative political microtargeting.
But the researchers found that efforts to make advertising more private have overlooked a basic contradiction: The performance information advertisers need to evaluate and refine campaigns can itself reveal information about users.
Modern advertising systems connect two critical functions—deciding who receives an ad and reporting how that ad performs. Even if each function incorporates privacy protections, combining them can create new vulnerabilities.
An advertiser, for example, could repeatedly target carefully selected groups, such as people on a customer list, and analyze the resulting performance metrics. Over time, those results can allow the advertiser to statistically infer private information about users—even when cryptographic protections prevent their identities from being directly revealed.
For browser vendors and ad-tech companies developing the next generation of web advertising standards, the findings suggest that adding privacy protections to individual components will not be enough.
“Simply throwing privacy-enhancing technologies at the problem won't work—we need to be careful about designing interventions,” said Kaptchuk, who is also a core faculty member in the Maryland Cybersecurity Center. “If we’re unable to make advertising private on a technical level, then we require robust social mechanisms to ensure that advertisers use digital advertising platforms responsibly.”
Rather than pursuing complete technical secrecy, the researchers propose a design framework that accounts for how targeting choices interact with performance data. Future advertising systems, they argue, should be “targeting-aware,” applying group-level protections that limit what advertisers can learn about sensitive attributes such as health, finances or political affiliation.
The findings point toward a broader approach to online privacy—one that combines carefully designed technical protections with rules governing how advertising platforms and advertisers can use the information available to them.
For Kaptchuk, that means recognizing the limits of what cryptography alone can accomplish and designing protections around those limits.
—Story by Melissa Brachfeld, UMIACS communications group