  Although cloud storage platforms promise a convenient way for users to share
files and engage in collaborations, they require all files to have a single
owner who unilaterally makes access control decisions. Existing clouds are,
thus, agnostic to shared ownership. This can be a significant limitation in
many collaborations because one owner can, for example, delete files and revoke
access without consulting the other collaborators.
  In this paper, we first formally define a notion of shared ownership within a
file access control model. We then propose a solution, called Commune, to the
problem of distributively enforcing shared ownership in agnostic clouds, so
that access grants require the support of a pre-arranged threshold of owners.
Commune can be used in existing clouds without requiring any modifications to
the platforms. We analyze the security of our solution and evaluate its
scalability and performance by means of an implementation integrated with
Amazon S3.
