Comparison
SSPI is a proprietary variant of GSSAPI with extensions and very Windows-specific data types. It shipped with Windows NT 3.51 and Windows 95 with the NT LAN Manager Security Support Provider (NTLMSSP). For Windows 2000, an implementation of Kerberos 5 was added, using token formats conforming to the official protocol standard RFC 1964 (The Kerberos 5 GSSAPI mechanism) and providing wire-level interoperability with Kerberos 5 implementations from other vendors.
The tokens generated and accepted by the SSPI are mostly compatible with the GSS-API so an SSPI client on Windows may be able to authenticate with a GSS-API server on Unix depending on the specific circumstances. One significant shortcoming of SSPI is its lack of channel bindings, which makes some GSSAPI interoperability impossible.
Another fundamental difference between the IETF-defined GSSAPI and Microsoft's SSPI is the concept of "impersonation". In this model, a server can switch to and operate with the full privileges of the authenticated client, so that the operating system performs all access control checks, e.g. when opening new files. Whether these are less privileges or more privileges than that of the original service account depends entirely on which client connects/authenticates. In the traditional (GSSAPI) model, a server runs under a service account, cannot elevate its privileges, and has to perform access control in a client-specific and application-specific fashion. The obvious negative security implications of the impersonation concept are mitigated in Windows Vista by restricting impersonation to selected service accounts.
Read more about this topic: Security Support Provider Interface
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