  Caching content at the network edge is a popular and effective technique
widely deployed to alleviate the burden of network backhaul, shorten service
delay and improve service quality. However, there has been some controversy
over privacy violations in caching content at the network edge. On the one
hand, the multi-access open edge network provides an ideal surface for external
attackers to obtain private data from the edge cache by extracting sensitive
information. On the other hand, privacy can be infringed by curious edge
caching providers through caching trace analysis targeting to achieve better
caching performance or higher profits. Therefore, an in-depth understanding of
privacy issues in edge caching networks is vital and indispensable for creating
a privacy-preserving caching service at the network edge. In this article, we
are among the first to fill in this gap by examining privacy-preserving
techniques for caching content at the network edge. Firstly, we provide an
introduction to the background of Privacy-Preserving Edge Caching (PPEC). Next,
we summarize the key privacy issues and present a taxonomy for caching at the
network edge from the perspective of private data. Additionally, we conduct a
retrospective review of the state-of-the-art countermeasures against privacy
leakage from content caching at the network edge. Finally, we conclude the
survey and envision challenges for future research.
