Discretionary Access Control (DAC):
Unlike Mandatory Access Control (MAC) where access to system resources is controlled by the operating system (under the control of a system administrator), Discretionary Access Control (DAC) allows each user to control access to their own data. DAC is typically the default access control mechanism for most desktop operating systems.
Instead of a security label in the case of MAC, each resource object on a DAC based system has an Access Control List (ACL) associated with it. An ACL contains a list of users and groups to which the user has permitted access together with the level of access for each user or group. For example, User A may provide read-only access on one of her files to User B, read and write access on the same file to User C and full control to any user belonging to Group 1.
It is important to note that under DAC a user can only set access permissions for resources which they already own. A hypothetical User A cannot, therefore, change the access control for a file that is owned by User B. User A can, however, set access permissions on a file that she owns. Under some operating systems it is also possible for the system or network administrator to dictate which permissions users are allowed to set in the ACLs of their resources.
Discretionary Access Control provides a much more flexible environment than Mandatory Access Control but also increases the risk that data will be made accessible to users that should not necessarily be given access.
Mandatory Access Control (MAC):
Mandatory Access Control (MAC) is the strictest of all levels of control. The design of MAC was defined, and is primarily used by the government.
MAC takes a hierarchical approach to controlling access to resources. Under a MAC enforced environment access to all resource objects (such as data files) is controlled by settings defined by the system administrator. As such, all access to resource objects is strictly controlled by the operating system based on system administrator configured settings. It is not possible under MAC enforcement for users to change the access control of a resource.
Mandatory Access Control begins with security labels assigned to all resource objects on the system. These security labels contain two pieces of information - a classification (top secret, confidential etc) and a category (which is essentially an indication of the management level, department or project to which the object is available).
Similarly, each user account on the system also has classification and category properties from the same set of properties applied to the resource objects. When a user attempts to access a resource under Mandatory Access Control the operating system checks the user's classification and categories and compares them to the properties of the object's security label. If the user's credentials match the MAC security label properties of the object access is allowed. It is important to note that both the classification and categories must match. A user with top secret classification, for example, cannot access a resource if they are not also a member of one of the required categories for that object.
Mandatory Access Control is by far the most secure access control environment but does not come without a price. Firstly, MAC requires a considerable amount of planning before it can be effectively implemented. Once implemented it also imposes a high system management overhead due to the need to constantly update object and account labels to accommodate new data, new users and changes in the categorization and classification of existing users.
