Lock the Layer DLL of My ASP.NET Project

Hi firends,
I create asp.net web application with 3Tier. each tire have dll to access other layer . when I publish  my project
To IIS , probably other people access to my dll and access to my code and its very bad . how can I set lock or private key
on My dll layers(Business_Layer,DataAccess_Layer,....).
 
Please give me a complete solution or complete reference.
Its very important for me. 
Thanks. 
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