ABSTRACT
The world is becoming more and more concerned about both safety and security. There has been a long tradition of concern for safety going back to the development of railroads and more recently with aviation. Software now pervades all aspects of the workings of society; accordingly, it is important that software concerned with systems for which safety or security are a major concern should be correct. Vital software systems such as those that control aircraft navigation and landing have to meet well-established certification and validation criteria. More recently, there has been growing concern with security in systems such as banking and communications generally, which has been heightened by concerns about terrorist activities.
