In computer security, discretionary access control is a type of access control defined by the Trusted Computer System Evaluation Criteria "as a means of restricting access to objects based on the identity of subjects and/or groups to which they belong. The controls are in the sense that a subject with a certain access permission is capable of passing that permission on to any other subject ". Discretionary access control is commonly discussed in contrast to mandatory access control. Occasionally a system as a whole is said to have "discretionary" or "purely discretionary" access control as a way of saying that the system lacks mandatory access control. On the other hand, systems can be said to implement both MAC and DAC simultaneously, where DAC refers to one category of access controls that subjects can transfer among each other, and MAC refers to a second category of access controls that imposes constraints upon the first.
Implementations
The meaning of the term in practice is not as clear-cut as the definition given in the TCSEC standard, because the TCSEC definition of DAC does not impose any implementation. There are at least two implementations: with owner and with capabilities.
With owner
The term DAC is commonly used in contexts that assume that every object has an owner that controls the permissions to access the object, probably because many systems do implement DAC using the concept of an owner. But the TCSEC definition does not say anything about owners, so technically an access control system doesn't have to have a concept of owner to meet the TCSEC definition of DAC. Users have under this DAC implementation the ability to make policy decisions and/or assign security attributes. A straightforward example is the Unix file mode which represent write, read, and execute in each of the 3 bits for each of User, Group and Others..
With capabilities
As another example, capability systems are sometimes described as providing discretionary controls because they permit subjects to transfer their access to other subjects, even though capability-based security is fundamentally not about restricting access "based on the identity of subjects" .
