SECURITYIDENTIFIER

SECURITYIDENTIFIER, often abbreviated as SID, is a unique, variable-length string that identifies a security principal such as a user account, group, or computer account within a Windows security model. SIDs are used by Windows operating systems to manage access control to resources. Each security principal is assigned a unique SID when it is created. If a user account is deleted and a new one is created with the same username, the new account will receive a different SID, meaning it will not inherit the original account's permissions.

SIDs are structured and consist of several parts. The first part indicates the revision level. The next part, known as the authority, specifies the entity that issued the SID, such as Microsoft or a specific domain. The subsequent parts are called relative identifiers (RIDs). For well-known SIDs, such as those for built-in groups like Administrators or Everyone, the RIDs are pre-defined and consistent across all Windows systems. For domain accounts, the domain portion of the SID is constant for all principals within that domain, and the RID is unique to each individual principal.

When a user logs into a Windows system, their SID and the SIDs of the groups they belong to are placed into an access token. This access token is then used by the system to determine whether the user has the necessary permissions to access a particular object or perform a specific action. SIDs are a fundamental component of Windows security, ensuring that access to resources is correctly managed and enforced.