the multi-user location correlation efficiently.
The main contributions of this paper are organized as
following.
∙ By using HMMs, we can achieve the probable tra-
jectories of each user, preparing for extending the
differential privacy on private candidate sets to protect
the location correlations.
∙ We quantify the location correlation between two users
through the similarity measurement of two hidden
Markov Models. Through such measurement, we can
not only analyze the historical multi-user location
correlations, but also predict the potential correlations.
∙ We propose a private trajectory releasing mechanism
which satisfies the differential privacy and can protect
the location correlations.
∙ The evaluations demonstrate that our approach can
achieve differentially private location correlation re-
leasing with high efficiency and utility.
The remainder of this paper is organized as follows. In
Section II, we review the related works in the literature. In Sec-
tion III, we give some basic concepts, such as HMMs, as well
as similarity of HMMs and the bounded differential privacy.
Then, we formulate an adversary model and discuss motivation
and basic idea to protect multi-user location correlation priva-
cy. In Section IV, we present the details of multi-user location
correlation protection and a trajectory releasing mechanism.
Furthermore, the security analysis, the utility analysis and the
analysis on the adversary’s knowledge are depicted in Section
V. The experimental performance evaluation is shown Section
VI. Finally, Section VII concludes the paper.
II. R
ELATED WORK
In the literature, most research works on location privacy
protection and be summarized into two categories, i.e., location
privacy and correlation protection with differential privacy.
A. Location Privacy
There are a lot of works about location privacy protection.
In order to protect users’ privacy, early solutions are proposed
to remove users’ identities or replace them with pseudonyms
[1]. However, such solutions cannot meet requirements of
privacy protection in the context of LBSs. Actually, adversaries
can use users’ published locations and related data with
background knowledge to de-anonymized users. As a result,
pseudonymizing is not good enough to preserve users’ privacy.
Furthermore, 𝑘-anonymity is also used to protect location
privacy. To achieve that a record of one user in the context
databases cannot be distinguished from 𝑘 − 1 other users. A
concept of “hide locations of users inside a crowd” is proposed
and adapted to preserving users’ locations by capacitating users
to request the LBSs using a spatial area called a “cloak” instead
of their precise locations. There are some works dealing with
this solution [2], [3]. And they also try to balance the trade-off
between the resulting utility and the offered privacy guarantee.
The resulting utility and privacy guarantee are qualified by the
cloak size and 𝑘 respectively. For example, the performance of
a 𝑘-anonymity approach depends on the quadtrees that produce
the smallest cloak. What’s more, researchers extend this model,
to allow users to customize their desired privacy levels.
In addition, to protect online distributed LBSs, the
𝑘-anonymity is also introduced. For instance, a privacy-aware
query processing framework is proposed, called Casper [4].
It disposes the users’ queries based on their cloak location,
i.e., anonymous location. The critical disadvantage of the
spatio-temporal variant 𝑘-anonymity is that the third party is
trusted undesirably and it plays a role of anonymity server.
When users request an LBS, the third party will obtain users’
location information and obfuscate it. If adversaries have the
background knowledge and the prior knowledge about users,
the 𝑘-anonymity may not prevent adversaries analysing the
users’ privacy.
In order to eliminate this drawback, location privacy quan-
tifying is proposed. Under this approach, when adversaries per-
forms inference attacks, the estimation error will be measured
[5], [6]. For example, the main intention of the inference attack
is to infer the observed users’ true locations or re-identify the
users based on published locations. But a strong assumption
should be made on the knowledge available to the adversaries
in this approach.
Also, some approaches are propose for preserving location
privacy with differential privacy [7], [8], [15], [16]. The first
approach is proposed to deal with the differentially private
computation of the points of interest from a geographical
database populated with the mobility traces of users based on
a quadtree algorithm [7]. And then a database is considered as
a containing commuting patterns one. In this database, each
record represents a user and there are origin and destination
for each user. A synthetic location information is generated
simulating the original location information, instead of re-
leasing the original location information. Recently, a concept
of geo-indistinguishability [11] was proposed, and it expands
differential privacy. But this technology does not consider
correlations among multiple users. And Markov models are
used to build users mobility and infer user location or trajectory
[12], [13]. There are several works that consider temporal
correlations of multiple locations about only one user [10],
[14], [15]. These technologies can protect location privacy in
some extent, but they still do not consider correlations among
multiple users. However, these technologies cannot protect
location privacy well [9], [10]. Most of them do not consider
the correlation among multiple users and this correlation will
be exposed by inference attacks.
B. Correlation Protection with Differential Privacy
Differential privacy has been studied for several years,
and there many variants or generalizations are proposed. But
applying differential privacy for protecting location privacy has
not been studied in depth. Recently there are several works ap-
plying differential privacy for releasing aggregate information
from a huge amount of locations, trajectories or spatiotemporal
data [17]–[20]. Also, a location protection extended differential
privacy is proposed [15]. This method proposed 𝛿-location set
of continual location of only one user and the user’s locations
are temporally correlated. And in order to protect correlated
data, there are several works about correlation protection with