how to implement single sign on in c#.net

hi everyone,

I have to implement a web base application....

it is require to use single sign on....

DirectoryEntry objDE = new DirectoryEntry("LDAP://172.16.100.12", UserName, Password, AuthenticationTypes.Secure);

UserName can get from Request.ServerVariable("LOGON_USER");

the problem is how I get the Password??

Anyone can help.....

Thanks!!!
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