Windows authentification : HTTP header "AUTHORIZATION" ?

From: mikej (mikej@guyih.fg)
Date: 08/30/02 

From: "mikej" <mikej@guyih.fg>
Date: Fri, 30 Aug 2002 06:43:47 -0700

Hello
I have a windows authentification on a web site.
So, i have to check the login/password of the windows
prompt with a login/password in a database.
I can request the identity name with User.Identity.Name
but i need the password too.
So, i try to use the HTTP header "AUTHORIZATION" but i do
not manage to convert the base 64 like this :

byte[] bPlain = new byte[50];
bPlain = Convert .FromBase64String(Request .Headers
["AUTHORIZATION"] );

I have an error Invalid character in a Base-64 string.
what can i do ?