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information since the very early days of
social media, such as explored by Gross
and Acquisti. 10 However, most of these
efforts have focused on privacy from an
individual point of view. For instance,
advances include research9 and indus-
try16 efforts on helping individual users
better target their audience by model-
ing different relationships and social
circles beyond the binary friendship
model that is prevalent in most social
media. While this has indeed helped
to advance the state of the art on the
topic, the problem of content affecting
the privacy of more than one user at the
same time has received little attention.
Privacy is not just about what you
say or disclose about yourself. It is
also about what others say or disclose
about you. Evidence shows there are
privacy boundaries collectively held
and managed by individuals within re-
lationships, families, groups, and or-
ganizations. 22 With the massive growth
of social media, however, collectively
held privacy boundaries have become
extremely challenging to maintain, as
many of the hundreds of billions of
items uploaded are co-owned by mul-
tiple users, 14, 15 yet mainstream social
media only allow the user uploading
a co-owned data item to set its privacy
settings, which often leads to conflicts
and severe privacy violations. 33, 35 Multi-
party privacy (MP) aims to facilitate the
coordination of collectively held privacy
boundaries by all individuals that co-
own a data item online, as the privacy
of all of them may be at stake depend-
ing on with whom the co-owned data
item is shared.a MP particularly focuses
on supporting the detection and reso-
lution of multiparty privacy conflicts
(MPCs), when individuals whose privacy
may be affected by the same co-owned
data item have conflicting privacy pref-
erences. Take a simplified but illustra-
tive example of MPC: Alice takes a photo
of her and Bob. Mainstream social me-
dia would only allow Alice (assuming
she uploads the photo) to set the privacy
settings for the photo, but what if Bob
would not like to share it with some of
the friends Alice would like to share the
a MP is different from other collective approaches
that focus on protecting just one individual. 4, 6, 21