How we encrypt the connectionStrings in file web.config

I fully followed the steps in this page(http://msdn.microsoft.com/en-US/library/33ws57y0%28v=VS.90%29.aspx)
please look at the step 5 at "Using the Example Provider in an ASP.NET Application" element ,
I entered the command "
aspnet_regiis.exe -pe "connectionStrings" -app "/SessionState" -prov "TripleDESProvider"
" at the windows command line, it asked me "The protection provider 'TripleDESProvider' was not found"
I make sure the files keys.snk and keys.txt are in the same folder and the file web.config is correct. Who can tell me the reason for the error and how do I handle with it? Thanks!
 
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