Which user authentication scheme could be used at following scenario?

1.Application A developed by my company is installed at user's local computer.

2.Users login at my company's website.

3.If login is successful, access to the application A in user's local machine is granted.

The development environment is .NET.

Is there a secure way to authenticate the user to access the application?

I've thought about encrypting the login information entered at the website, and then sending that information to the program at user's local machine for authentication.

but I think as long as the client program has the decrypting ability, it is vulnerabe to decompiling,etc.

Thanks in advance!