  The 21st century belongs to the world of computing, specially as a result of
the so-called cloud computing. This technology enables ubiquitous information
management and thus people can access all their data from any place and at any
time. In this landscape, the emergence of cloud storage has had an important
role in the last five years. Nowadays, several free-access public cloud storage
services make it possible for users to have a free backup of their assets and
to manage and share them, representing a low-cost opportunity for Small and
Medium Companies (SME). However, the adoption of cloud storage involves data
outsourcing, so a user does not have the guarantee about the way her data will
be processed and protected. Therefore, it seems necessary to endow public cloud
storage with a set of means to protect users' confidentiality and privacy, to
assess data integrity and to guarantee a proper backup of information assets.
Along this paper we discuss the main challenges to achieve such a goal,
underlining the set of functionalities already implemented in the most popular
public cloud storage services.
