Abstract
Different techniques for expressing an amount of information on secrete data which can be obtained by a process observation are presented. They are based on information theory and they express certainty about sets of private actions which execution is guaranteed by a given observation and sets of actions which execution is excluded by a given observation. Moreover, the case when an intruder has same preliminary belief on secrete data is discussed. It is shown how the presented technique could be applied for such case. As regards working formalism, probabilistic process algebra is used for description of systems as well as attacker's belief.