  Security risk management can be applied on well-defined or existing systems;
in this case, the objective is to identify existing vulnerabilities, assess the
risks and provide for the adequate countermeasures. Security risk management
can also be applied very early in the system's development life-cycle, when its
architecture is still poorly defined; in this case, the objective is to
positively influence the design work so as to produce a secure architecture
from the start. The latter work is made difficult by the uncertainties on the
architecture and the multiple round-trips required to keep the risk assessment
study and the system architecture aligned. This is particularly true for very
large projects running over many years. This paper addresses the issues raised
by those risk assessment studies performed early in the system's development
life-cycle. Based on industrial experience, it asserts that attack trees can
help solve the human cognitive scalability issue related to securing those
large, continuously-changing system-designs. However, big attack trees are
difficult to build, and even more difficult to maintain. This paper therefore
proposes a systematic approach to automate the construction and maintenance of
such big attack trees, based on the system's operational and logical
architectures, the system's traditional risk assessment study and a security
knowledge database.
