  Conventional online social networks (OSNs) are implemented in a centralized
manner. Although centralization is a convenient way for implementing OSNs, it
has several well known drawbacks. Chief among them are the risks they pose to
the security and privacy of the information maintained by the OSN; and the loss
of control over the information contributed by individual members.
  These concerns prompted several attempts to create decentralized OSNs, or
DOSNs. The basic idea underlying these attempts, is that each member of a
social network keeps its data under its own control, instead of surrendering it
to a central host; providing access to it to other members of the OSN according
to its own access-control policy. Unfortunately all existing DOSN projects have
a very serious limitation. Namely, they are unable to subject the membership of
a DOSN, and the interaction between its members, to any global policy.
  We adopt the decentralization idea underlying DOSNs, complementing it with a
means for specifying and enforcing a wide range of policies over the membership
of a social community, and over the interaction between its disparate
distributed members. And we do so in a scalable fashion.
