Anonymized data is highly valuable to both businesses and researchers. A
large body of research has however shown the strong limits of the
de-identification release-and-forget model, where data is anonymized and
shared. This has led to the development of privacy-preserving query-based
systems. Based on the idea of "sticky noise", Diffix has been recently proposed
as a novel query-based mechanism satisfying alone the EU Article~29 Working
Party's definition of anonymization. According to its authors, Diffix adds less
noise to answers than solutions based on differential privacy while allowing
for an unlimited number of queries.
This paper presents a new class of noise-exploitation attacks, exploiting the
noise added by the system to infer private information about individuals in the
dataset. Our first differential attack uses samples extracted from Diffix in a
likelihood ratio test to discriminate between two probability distributions. We
show that using this attack against a synthetic best-case dataset allows us to
infer private information with 89.4% accuracy using only 5 attributes. Our
second cloning attack uses dummy conditions that conditionally strongly affect
the output of the query depending on the value of the private attribute. Using
this attack on four real-world datasets, we show that we can infer private
attributes of at least 93% of the users in the dataset with accuracy between
93.3% and 97.1%, issuing a median of 304 queries per user. We show how to
optimize this attack, targeting 55.4% of the users and achieving 91.7%
accuracy, using a maximum of only 32 queries per user.
Our attacks demonstrate that adding data-dependent noise, as done by Diffix,
is not sufficient to prevent inference of private attributes. We furthermore
argue that Diffix alone fails to satisfy Art. 29 WP's definition of
anonymization. [...
