<s>
Spatial	B-Application
cloaking	I-Application
is	O
a	O
privacy	O
mechanism	O
that	O
is	O
used	O
to	O
satisfy	O
specific	O
privacy	O
requirements	O
by	O
blurring	O
users’	O
exact	O
locations	O
into	O
cloaked	O
regions	O
.	O
</s>
<s>
This	O
technique	O
is	O
usually	O
integrated	O
into	O
applications	O
in	O
various	O
environments	O
to	O
minimize	O
the	O
disclosure	O
of	O
private	O
information	O
when	O
users	O
request	O
location-based	B-Application
service	I-Application
.	O
</s>
<s>
Since	O
the	O
database	B-Application
server	I-Application
does	O
not	O
receive	O
the	O
accurate	O
location	O
information	O
,	O
a	O
set	O
including	O
the	O
satisfying	O
solution	O
would	O
be	O
sent	O
back	O
to	O
the	O
user	O
.	O
</s>
<s>
With	O
the	O
emergence	O
and	O
popularity	O
of	O
location-based	B-Application
services	I-Application
,	O
people	O
are	O
getting	O
more	O
personalized	O
services	O
,	O
such	O
as	O
getting	O
the	O
names	O
and	O
locations	O
of	O
nearby	O
restaurants	O
and	O
gas	O
stations	O
.	O
</s>
<s>
Thus	O
,	O
multiple	O
solutions	O
have	O
been	O
proposed	O
to	O
preserve	O
and	O
enhance	O
users’	O
privacy	O
when	O
using	O
location-based	B-Application
services	I-Application
.	O
</s>
<s>
Among	O
all	O
the	O
proposed	O
mechanisms	O
,	O
spatial	B-Application
cloaking	I-Application
is	O
one	O
of	O
those	O
which	O
has	O
been	O
widely	O
accepted	O
and	O
revised	O
,	O
thus	O
having	O
been	O
integrated	O
into	O
many	O
practical	O
applications	O
.	O
</s>
<s>
For	O
example	O
,	O
Pokémon	B-Application
Go	I-Application
requires	O
a	O
consistent	O
user	O
identity	O
,	O
since	O
users	O
are	O
required	O
to	O
log	O
in	O
.	O
</s>
<s>
The	O
untrusted	O
third-party	B-General_Concept
model	O
is	O
adopted	O
in	O
peer-to-peer	O
environments	O
.	O
</s>
<s>
The	O
most	O
popular	O
model	O
right	O
now	O
is	O
the	O
trusted	O
third-party	B-General_Concept
model	O
.	O
</s>
<s>
Therefore	O
,	O
by	O
introducing	O
a	O
trusted-third-party	O
,	O
users’	O
private	O
information	O
is	O
not	O
directly	O
exposed	O
to	O
the	O
service	O
providers	O
.	O
</s>
<s>
Several	O
methods	O
have	O
been	O
investigated	O
to	O
enhance	O
the	O
performances	O
of	O
location-preserving	O
techniques	O
,	O
such	O
as	O
location	O
perturbation	O
and	O
the	O
report	O
of	O
landmark	B-Protocol
objects	O
.	O
</s>
<s>
Location	O
perturbation	O
is	O
usually	O
satisfied	O
by	O
using	O
spatial	B-Application
cloaking	I-Application
,	O
temporal	O
cloaking	O
,	O
or	O
location	O
obfuscation	O
.	O
</s>
<s>
Based	O
on	O
the	O
idea	O
of	O
landmark	B-Protocol
objects	O
,	O
a	O
particular	O
landmark	B-Protocol
or	O
a	O
significant	O
object	O
is	O
reported	O
to	O
the	O
service	O
provider	O
,	O
instead	O
of	O
a	O
region	O
.	O
</s>
<s>
These	O
steps	O
are	O
usually	O
achieved	O
through	O
spatial	B-Application
cloaking	I-Application
or	O
path	O
confusion	O
.	O
</s>
<s>
Therefore	O
,	O
every	O
time	O
after	O
user	O
request	O
a	O
location-based	B-Application
service	I-Application
and	O
before	O
the	O
exact	O
location	O
information	O
is	O
forward	O
to	O
the	O
server	O
,	O
the	O
auxiliary	O
users	O
registered	O
in	O
this	O
system	O
collaborate	O
to	O
hide	O
the	O
precise	O
location	O
of	O
the	O
user	O
.	O
</s>
<s>
However	O
,	O
the	O
complexity	O
of	O
the	O
data	O
structure	O
which	O
is	O
used	O
to	O
anonymize	O
the	O
location	O
could	O
result	O
in	O
difficulties	O
when	O
applying	O
this	O
mechanism	O
to	O
highly	O
dynamic	O
location-based	O
mobile	B-Application
applications	O
.	O
</s>
<s>
A	O
peer-to-peer	O
(	O
P2P	O
)	O
environment	O
relies	O
on	O
the	O
direct	O
communication	O
and	O
information	O
exchange	O
between	O
devices	O
in	O
a	O
community	O
where	O
users	O
could	O
only	O
communicate	O
through	O
P2P	O
multi-hop	B-General_Concept
routing	I-General_Concept
without	O
fixed	O
communication	O
infrastructures	O
.	O
</s>
<s>
In	O
this	O
environment	O
,	O
peers	O
have	O
to	O
trust	O
each	O
other	O
and	O
work	O
together	O
,	O
since	O
their	O
location	O
information	O
would	O
be	O
reported	O
to	O
each	O
other	O
when	O
a	O
cloaked	O
area	O
is	O
constructed	O
to	O
achieve	O
the	O
desired	O
K-anonymity	O
during	O
the	O
requesting	O
for	O
location-based	B-Application
services	I-Application
.	O
</s>
<s>
Mobile	B-Application
devices	I-Application
have	O
been	O
considered	O
as	O
an	O
essential	O
tool	O
for	O
communication	O
,	O
and	O
mobile	B-Application
computing	O
has	O
thus	O
become	O
a	O
research	O
interest	O
in	O
recent	O
years	O
.	O
</s>
<s>
From	O
online	O
purchase	O
to	O
online	O
banking	O
,	O
mobile	B-Application
devices	I-Application
have	O
frequently	O
been	O
connected	O
to	O
service	O
providers	O
for	O
online	O
activities	O
,	O
and	O
at	O
the	O
same	O
time	O
sending	O
and	O
receiving	O
information	O
.	O
</s>
<s>
Generally	O
,	O
mobile	B-Application
users	O
can	O
receive	O
very	O
personal	O
services	O
from	O
anywhere	O
at	O
any	O
time	O
through	O
location-based	B-Application
services	I-Application
.	O
</s>
<s>
In	O
mobile	B-Application
devices	I-Application
,	O
Global	O
Positioning	O
System	O
(	O
GPS	O
)	O
is	O
the	O
most	O
commonly	O
used	O
component	O
to	O
provide	O
location	O
information	O
.	O
</s>
<s>
Besides	O
that	O
,	O
Global	O
System	O
for	O
Mobile	B-Application
Communications	O
(	O
GSM	O
)	O
and	O
WiFi	O
signals	O
could	O
also	O
help	O
with	O
estimating	O
locations	O
.	O
</s>
<s>
There	O
are	O
generally	O
two	O
types	O
of	O
privacy	O
concerns	O
in	O
mobile	B-Application
environments	O
,	O
data	O
privacy	O
and	O
contextual	O
privacy	O
.	O
</s>
<s>
Usually	O
,	O
location	O
privacy	O
and	O
identity	O
privacy	O
are	O
included	O
in	O
the	O
discussion	O
of	O
contextual	O
privacy	O
in	O
a	O
mobile	B-Application
environment	O
,	O
while	O
the	O
data	O
transferred	O
between	O
various	O
mobile	B-Application
devices	I-Application
is	O
discussed	O
under	O
data	O
privacy	O
.	O
</s>
<s>
In	O
the	O
process	O
of	O
requesting	O
location-based	B-Application
services	I-Application
and	O
exchanging	O
location	O
data	O
,	O
both	O
the	O
quality	O
of	O
data	O
transferred	O
and	O
the	O
safety	O
of	O
information	O
exchanged	O
could	O
be	O
potentially	O
exposed	O
to	O
malicious	O
people	O
.	O
</s>
<s>
Since	O
there	O
is	O
a	O
tradeoff	O
relationship	O
between	O
quality	O
of	O
service	O
and	O
privacy	O
requirements	O
in	O
most	O
location-based	B-Application
services	I-Application
,	O
sometimes	O
a	O
maximum	O
area	O
size	O
is	O
also	O
required	O
.	O
</s>
<s>
The	O
cloaked	O
region	O
generated	O
by	O
the	O
method	O
of	O
spatial	B-Application
cloaking	I-Application
could	O
fit	O
into	O
multiple	O
environments	O
,	O
such	O
as	O
snapshot	O
location	O
,	O
continuous	O
location	O
,	O
spatial	O
networks	O
,	O
and	O
wireless	O
sensor	O
networks	O
.	O
</s>
<s>
Foursquare	B-Application
was	O
one	O
of	O
the	O
earliest	O
location-based	O
applications	O
to	O
enable	O
location	O
sharing	O
among	O
friends	O
.	O
</s>
<s>
Moreover	O
,	O
SCVNGR	B-Application
was	O
a	O
location-based	O
platform	O
where	O
users	O
could	O
earn	O
points	O
by	O
going	O
to	O
places	O
.	O
</s>
<s>
With	O
the	O
popularity	O
and	O
development	O
of	O
global	O
positioning	O
system	O
(	O
GPS	O
)	O
and	O
wireless	O
communication	O
,	O
location-based	B-Application
information	I-Application
services	I-Application
have	O
been	O
in	O
high	O
growth	O
in	O
recent	O
years	O
.	O
</s>
<s>
Many	O
practical	O
applications	O
have	O
integrated	O
the	O
idea	O
and	O
techniques	O
of	O
location-based	B-Application
services	I-Application
,	O
such	O
as	O
mobile	B-Application
social	O
networks	O
,	O
finding	O
places	O
of	O
interest	O
(	O
POI	O
)	O
,	O
augmented	O
reality	O
(	O
AR	O
)	O
games	O
,	O
awareness	O
of	O
location-based	O
advertising	O
,	O
transportation	O
service	O
,	O
location	O
tracking	O
,	O
and	O
location-aware	O
services	O
.	O
</s>
<s>
Usually	O
,	O
the	O
location-based	B-Application
services	I-Application
are	O
requested	O
either	O
through	O
snapshot	O
queries	O
or	O
continuous	O
queries	O
.	O
</s>
<s>
One	O
of	O
the	O
attacks	O
specific	O
to	O
location-based	B-Application
services	I-Application
is	O
the	O
space	O
or	O
time	O
correlated	O
inference	O
attacks	O
,	O
in	O
which	O
the	O
visited	O
location	O
is	O
correlated	O
with	O
the	O
particular	O
time	O
,	O
and	O
this	O
could	O
lead	O
to	O
the	O
disclosure	O
of	O
private	O
life	O
and	O
private	O
business	O
.	O
</s>
<s>
Some	O
of	O
the	O
popular	O
location-based	B-Application
services	I-Application
include	O
:	O
</s>
<s>
Continuous	O
location-based	B-Application
services	I-Application
require	O
a	O
constant	O
report	O
of	O
location	O
information	O
to	O
the	O
service	O
providers	O
.	O
</s>
<s>
During	O
the	O
process	O
of	O
requesting	O
a	O
continuous	O
location-based	B-Application
service	I-Application
,	O
pressure	O
has	O
been	O
recognized	O
on	O
privacy	O
leakage	O
issues	O
.	O
</s>
<s>
Therefore	O
,	O
many	O
types	O
of	O
research	O
have	O
been	O
conducted	O
addressing	O
the	O
location	O
privacy	O
issues	O
in	O
continuous	O
location-based	B-Application
services	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
a	O
privacy	O
grid	O
called	O
a	O
dynamic	O
grid	O
system	O
is	O
proposed	O
to	O
fit	O
into	O
both	O
snapshot	O
and	O
continuous	O
location-based	B-Application
service	I-Application
environments	O
.	O
</s>
<s>
For	O
example	O
,	O
tools	O
such	O
as	O
cryptography	O
,	O
anonymity	O
,	O
obfuscation	O
and	O
caching	B-General_Concept
have	O
been	O
proposed	O
,	O
discussed	O
,	O
and	O
tested	O
to	O
better	O
preserve	O
user	O
privacy	O
.	O
</s>
<s>
Even	O
though	O
the	O
effectiveness	O
of	O
spatial	B-Application
cloaking	I-Application
has	O
been	O
widely	O
accepted	O
and	O
the	O
idea	O
of	O
spatial	B-Application
cloaking	I-Application
has	O
been	O
integrated	O
into	O
multiple	O
designs	O
,	O
there	O
are	O
still	O
some	O
concerns	O
towards	O
it	O
.	O
</s>
<s>
First	O
,	O
the	O
two	O
schemes	O
of	O
spatial	B-Application
cloaking	I-Application
both	O
have	O
their	O
limitations	O
.	O
</s>
<s>
Third	O
,	O
the	O
credibility	O
of	O
a	O
trusted-third-party	O
has	O
also	O
been	O
identified	O
as	O
one	O
of	O
the	O
issues	O
.	O
</s>
<s>
Software	O
bugs	O
,	O
configuration	O
errors	O
at	O
the	O
trusted-third-party	O
and	O
malicious	O
administrators	O
could	O
expose	O
private	O
user	O
data	O
under	O
high	O
risks	O
.	O
</s>
<s>
Based	O
on	O
a	O
study	O
from	O
2010	O
,	O
two-thirds	O
of	O
all	O
the	O
trusted-third-party	O
applications	O
in	O
the	O
Android	B-Application
market	O
are	O
considered	O
to	O
be	O
suspicious	O
towards	O
sensitive	O
information	O
.	O
</s>
<s>
Man-in-the-middle	O
attacks	O
usually	O
occur	O
in	O
the	O
mobile	B-Application
environment	O
which	O
assumes	O
that	O
all	O
the	O
information	O
going	O
through	O
the	O
transferring	O
process	O
from	O
user	O
to	O
the	O
service	O
provider	O
could	O
be	O
under	O
attacks	O
and	O
might	O
be	O
manipulated	O
further	O
by	O
attackers	O
revealing	O
more	O
personal	O
information	O
.	O
</s>
<s>
Video-based	O
attacks	O
are	O
more	O
prevalent	O
in	O
mobile	B-Application
devices	I-Application
usually	O
due	O
to	O
the	O
use	O
of	O
Bluetooth	O
,	O
camera	O
,	O
and	O
video	O
capacities	O
,	O
since	O
there	O
are	O
malicious	O
software	O
applications	O
secretly	O
recording	O
users’	O
behavior	O
data	O
and	O
reporting	O
that	O
information	O
to	O
a	O
remote	O
device	O
.	O
</s>
<s>
Under	O
this	O
situation	O
,	O
even	O
adversaries	O
do	O
not	O
have	O
physical	O
contact	O
with	O
the	O
mobile	B-Application
device	I-Application
,	O
users’	O
personal	O
information	O
would	O
still	O
under	O
risks	O
of	O
being	O
disclosed	O
.	O
</s>
<s>
Policy	O
approaches	O
have	O
also	O
been	O
discussed	O
in	O
recent	O
years	O
which	O
intend	O
to	O
revise	O
relevant	O
guidelines	O
or	O
propose	O
new	O
regulations	O
to	O
better	O
manage	O
location-based	B-Application
service	I-Application
applications	O
.	O
</s>
<s>
For	O
example	O
,	O
there	O
have	O
been	O
identified	O
much	O
malware	O
in	O
the	O
Android	B-Application
app	O
market	O
,	O
which	O
are	O
designed	O
to	O
carry	O
cyber	O
attacks	O
on	O
Android	B-Application
devices	O
.	O
</s>
<s>
GSMA	O
published	O
a	O
new	O
privacy	O
guideline	O
,	O
and	O
some	O
mobile	B-Application
companies	O
in	O
Europe	O
have	O
signed	O
it	O
and	O
started	O
to	O
implement	O
it	O
so	O
that	O
users	O
would	O
have	O
a	O
better	O
understanding	O
of	O
the	O
information	O
recorded	O
and	O
analyzed	O
when	O
using	O
location-based	B-Application
services	I-Application
.	O
</s>
<s>
Even	O
though	O
many	O
privacy	O
preserving	O
mechanisms	O
have	O
not	O
been	O
integrated	O
into	O
common	O
use	O
due	O
to	O
effectiveness	O
,	O
efficiency	O
,	O
and	O
practicality	O
,	O
some	O
location-based	B-Application
service	I-Application
providers	O
have	O
started	O
to	O
address	O
privacy	O
issues	O
in	O
their	O
applications	O
.	O
</s>
<s>
For	O
example	O
,	O
Twitter	B-Application
enables	O
its	O
users	O
to	O
customize	O
location	O
accuracy	O
.	O
</s>
<s>
It	O
has	O
been	O
stated	O
that	O
Google	B-Application
does	O
not	O
meet	O
the	O
European	O
Union	O
’s	O
data	O
privacy	O
law	O
and	O
thus	O
increasing	O
attention	O
has	O
been	O
placed	O
on	O
the	O
advocation	O
of	O
guidelines	O
and	O
policies	O
regarding	O
data	O
privacy	O
.	O
</s>
<s>
It	O
has	O
been	O
arguing	O
that	O
less	O
than	O
a	O
week	O
after	O
Facebook	B-Application
uses	O
its	O
“	O
Places	O
”	O
feature	O
,	O
the	O
content	O
of	O
that	O
location	O
information	O
has	O
been	O
exploited	O
by	O
thieves	O
and	O
are	O
used	O
to	O
conduct	O
a	O
home	O
invasion	O
.	O
</s>
