For modern critical systems, it is necessary to consider their ability to avoid catastrophic behavior following fortuitous events such as internal failures in hardware components, environmental disturbances or even involuntary human error in the design and operation, but also non fortuitous events such as malicious attacks. Unfortunately, in French the same word "sécurité" is used to cover two different problematics, what in English is expressed in two different words : safety and security. The interconnected modern information systems, such as rail traffic signalling systems point out the need to deal in an overall way with both safety and security