Namespace error - how to fix?

I'm new to Visual Studio 2010, how do I fix the following error?

Error: The type or namespace name 'Xml' does not exist in the namespace 'System.Security.Cryptography' (are you missing an assembly reference?)

with code:

using System;
using System.Xml;
using System.Security.Cryptography;
u开发者_如何转开发sing System.Security.Cryptography.Xml;

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You need to add a reference to the "System.Security" assembly. (Right-click project - add reference).