Hi,
We had a requirement to encrypt a particular node and its corresponding child elements in one of our xml file. While searching for the best and the simplest way to do so, I came across these wonderful posts which make use of EncrptedXml class.
http://www.devx.com/dotnet/Article/21564/1954
http://dotnetslackers.com/articles/xml/XMLEncryption.aspx
http://blogs.msdn.com/b/shawnfa/archive/2003/11/14/57032.aspx
Hope it helps