How do i write an user authentication script?

hi,
i have a C# form as the client that sends login credentials (user / password) to SQL server. Im using SQL server 2008 r2 developer. The client encrypts user / password with Encryption provided with .net framework this includes a "salt" value. When SQL receives these credentials it should decrypt user/password and authenticate user.

my application is a windows application and not a web application, however, when create an authentication code or script it should fit with web and windows apps

1. How do write a script to authenticate user?

2. How should i write an industry level script to authenticate user?

3. Is it an industry standard to encrypt both user name and password, would just password encryption be enough?

4. could any one here, give me a industry level sample on how to encrypt the user/password from the client side?

thanks