I'm not familiar with ASP.Net MembershipProvider, but an alternative would be to create an SA web service with AUTHORIZATION ON and then attempt do a web service call to the SA web service providing the user ID/PWD to verify that the credentials were correct.

E.g.

CREATE WEB SERVICE Validate_User
  TYPE 'raw' 
  AUTHORIZATION ON
  USER "public"
  AS select 'OK' from Dummy;

Then start the SQL Anywhere server using "-xs http" switch

Then send a request to http://username:password@yourhost.com/Validate_User (or equivalent using Ado.Net). If the credentials are correct, you will get an "OK" response back. If not, you will get an error back.

Of course, you should use HTTPS so that the credentials are transferred securely and therefore you should add "SECURE ON" to the web service definition. (Exercise left to the reader - see "-xs https" server switch in the docs :-).

If you talk about ASP.NET MembershipProvider it is not so much effort to do this. You have to implement only the ValidateUser function to get it working. Only in the case that you want to allow the user to do self-administration than you will have to implement the rest too.

In

public override bool ValidateUser(string Username, string Password)

you get the user and password in clear text and can use this e.g. in an

OdbcConnection con=new OdbcConnection(string.Format("dsn=...;uid={0};pwd={1}",Username,Password));

If you can afterwards open the connection without an exception you know that the credentials have been the right ones.