All objects that are controlled by the security policy are also labeled by the security policy with a set of security attributes, referred to as a security context. A fundamental issue in the architecture is how the association between objects and security contexts is maintained. The simplest solution would be to define a single policy-independent data type which is part of the data associated with each object. However, no single data type is well-suited to all of the differing ways in which labels are used in a system. The Flask architecture addresses these conflicting needs by providing two policy-independent data types for labeling.
A security context, the first policy-independent data type, is a variable-length string which can be interpreted by any application or user with an understanding of the security policy. A security context might consist of several attributes, such as a user identity, a classification level, a role and a type enforcement  domain, but this depends on the particular security policy. As long as it is treated as an opaque string, a security context can be handled by an object manager without compromising the policy flexibility of the object manager. However, using security contexts for labeling and policy decision lookups would be inefficient and would increase the likelihood of policy-specific logic being introduced into the object managers.
The second policy-independent data type, the security identifier (SID), is defined by Flask to be a fixed-size value which can be interpreted only by the security server and is mapped by the security server to a particular security context. Possession or knowledge of a SID for a given security context does not grant any authorization for that security context. The SID mapping cannot be assumed to be consistent across executions (reboots) of the security server nor across security servers on different nodes. Consequently, SIDs may be lightweight; in the implementation, SIDs are simply 32-bit integers. There is no specified internal structure to a SID; any internal structure is known only by the security server. The SID allows most object manager interactions to be independent of not just the content but even the format of a security context, simplifying object labeling and the interfaces that coordinate the security policy between the security server and object managers. However, in some cases, such as labeling persistent objects or labeling objects which are exported to other nodes, object managers must handle security contexts. This is described further in the discussion of the file server and network server in Section A.1 and Section A.2.
When an object is created, it is assigned a SID that represents the security context in which the object is created. This context typically depends upon the client requesting the object creation and upon the environment in which it is created. For example, the security context of a newly created file is dependent upon the security context of the directory in which it is created and the security context of the client that requested its creation. Since the computation of a security context for a new or transformed object may involve policy-specific logic, it cannot be performed by the object manager itself. The labeling of a new object is depicted in Figure 2. For some security policies, such as an ORCON policy [19,34], the security policy may need to uniquely distinguish subjects and objects of certain classes even if they are created in the same security context. For such policies, the SID must be computed from the security context and a unique identifier chosen by the security server.