Security Identifiers

A security identifier (SID) is a unique value of variable length used to identify a trustee. Each account has a unique SID issued by an authority, such as a Windows domain controller, and stored in a security database. Each time a user logs on, the system retrieves the SID for that user from the database and places it in the access token for that user. The system uses the SID in the access token to identify the user in all subsequent interactions with Windows security. When a SID has been used as the unique identifier for a user or group, it cannot ever be used again to identify another user or group.

Windows security uses SIDs in the following security elements:

• In security descriptors to identify the owner of an object and primary group
• In access control entries, to identify the trustee for whom access is allowed, denied, or audited
• In access tokens, to identify the user and the groups to which the user belongs

In addition to the uniquely created, domain-specific SIDs assigned to specific users and groups, there are well-known SIDs that identify generic groups and generic users. For example, the well-known SIDs, Everyone and World, identify a group that includes all users.

Most applications never need to work with SIDs. Because the names of well-known SIDs can vary, you should use the functions to build the SID from predefined constants rather than using the name of the well-known SID. For example, the U.S. English version of the Windows operating system has a well-known SID named "BUILTIN\Administrators" that might have a different name on international versions of the system. For an example that builds a well-known SID, see Searching for a SID in an Access Token in C++.

If you do need to work with SIDs, do not manipulate them directly. Instead, use the following functions.