The security identifier (SID) is the object identifier of the user or security group when the user or group is used for security purposes. The name of the user or group is not used as the unique identifier within the system. The SID is stored in the objectSid property of user objects and security group objects. Active Directory generates the objectSid when the user or group is created. The system ensures that the SIDs are unique across a forest. Be aware that the objectGuid is the unique identifier of a user, group, or any other directory object. The SID changes if a user or group is moved to another domain; the objectGuid always remains the same.
When a user or group is given permission to access a resource, such as a printer or a file share, the SID of the user or group is added to the access control entry (ACE) defining the granted permission in the resource's discretionary access control list (DACL). In Active Directory, each object has an nTSecurityDescriptor property that stores a DACL defining the access to that particular object or properties on that object. For more information about setting access control on objects in Active Directory, see Controlling Access to Active Directory Objects.
When a user logs on to a Windows 2000 domain, the operating system generates an access token. This access token is used when determining which resources the user may access. The user access token includes the following data:
Every process executed on behalf of this user has a copy of this access token.
When the user attempts to access resources on a computer, the service through which the user accesses the resource impersonates the user by creating a new access token based on the access token created at user logon time. This new access token additionally contains SIDs for all domain local groups that, first, are in the target resource domain and, second, include the user. The service uses this new access token to evaluate access to the resource. If a SID in the access token appears in any ACEs in the DACL, the service gives the user the permissions specified in those ACEs.