The AuthenticatePlain method performs plain-text authentication. This method can be used with HTTPS connection only.
string AuthenticatePlain( string loginName, string password )
Authentication token if login was successful
To protect the plain-text passwords from being seen by intermediate parties, this method only works over an HTTPS connection. An attempt to use plain-text authentication over a non-secure connection results in the E_CONNECTIONISNOTSECURE error.