Note that SSPI

Note that SSPI is for establishing secure network connections and supports single sign-on by using default user credentials. Most BackOffice applications use SSPI for basic user authentication. It offers data integrity services, including both the generation and verification of digital signatures, but it doesn't include data encryption or decryption services. Applications that need data confidentiality must invoke such services through CryptoAPI, which provides core cryptography services for key management, signatures, and encryption. (For more detailed information about SSPI and CryptoAPI, see the Network Strategy Overview "Interoperable Authentication.") SSPI, like CryptoAPI 2.0, is available with version 4.0 of NT.