Several formulations of system security can be found in the literature. Many of them are based on a non-interference which assumes an absence of any information flow between private and public systems activities. More precisely, systems are considered to be secure if from observations of their public activities no information about private activities can be deduced. A quantification of process's security by differential privacy is defined and studied in the framework of probabilistic process algebras. The resulting (quantitative) security properties are studied and compared with other (qualitative) security notions.