The Shiva Password Authentication Protocol (SPAP) is a reversible encryption mechanism employed by Shiva remote access servers. A Windows 2000 remote access client can use SPAP to authenticate itself to a Shiva remote access server. A remote access client running Windows 32-bit operating systems can use SPAP to authenticate itself to a Windows 2000 remote access server. SPAP is more secure than PAP but less secure than CHAP or MS-CHAP. SPAP offers no protection against remote server impersonation.
The use of SPAP is negotiated during LCP negotiation by specifying the authentication protocol LCP option (type 3) and the authentication protocol 0xC0-27. Once LCP negotiation is complete, SPAP messages use the PPP protocol ID of 0xC0-27.
Like PAP, SPAP is a simple exchange of messages:
The remote access client sends an SPAP Authenticate-Request message to the remote access server containing the remote access client's user name and encrypted password.
The remote access server decrypts the password, checks the user name and password, and sends back either an SPAP Authenticate-Ack message when the user's credentials are correct, or an SPAP Authenticate-Nak message with a reason why the user's credentials were not correct.