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.