The type or namespace md5 / sha1 does not exist

I am trying to build my game for Windows Store and Windows 8 with Unity, but I get an error stating "the type or namespace md5cryptoserviceprovider does not exist", and so I did a quick search which recommended using sha1, but I get the same error but for sha1. What alternative is there which will allow me to do the equivalent of md5 or sha1?

I have found here: http://stackoverflow.com/questions/10980811/sha-1-algorithm-in-windows-phone that SHA1Managed works for WindowsPhone, but still getting an issue when building for Windows Store.