  Among access control models, Role Based Access Control (RBAC) is very useful
and is used in many computer systems. Static Combination of Duty (SCD) and
Dynamic Combination of Duty (DCD) constraints have been introduced recently for
this model to handle dependent roles. These roles must be used together and can
be considered as a contrary point of conflicting roles. In this paper, we
propose several new types of SCD and DCD constraints. Also, we introduce strong
dependent roles and define new groups of SCD constraints for these types of
roles as SCD with common items and SCD with union items. In addition, we
present an extension for SCD constraints in the presence of hierarchy.
