How can i check sap user and password in my add on application?

Hi all,

I need to create one add on application for my customer.In my app,I have to create log in form like the picture below

When they click Log In, I want to check this user name and password are correct/exist in table OUSR or not. If it has, it can see other form.

So to check the user and password in sap, Anyone know how to do?

Note: I want to execute store procedure in sql server to check user in OUSR table but because of the password is encrypt i don't know how to decrypt.

Thanks

TONY