Rijndael.CreateDecryptor() crashes the IIS worker thread (but only in IIS 7.5 64bit)

We have an app that works great on IIS 6 and IIS 7. As soon as we go to Windows 2008 R2 with IIS7.5 we get an IIS crash. The worker thread actually dies and leaves a message in the event viewer:

Fault bucket , type 0
Event Name: APPCRASH
...
Problem signature: 
P1: w3wp.exe
P2: 7.5.7600.16385
P3: 4a5bcd2b
P4: mscorwks.dll开发者_运维问答
...

I have traced it down to a single line of code that can produce the crash:

var b = Rijndael.CreateDecryptor();

Is there some specific way to use this in a 64-bit environment?

The problem was caused because it seems that in R2, the Machine Key's storage location changed. I had to explicitly specify that the key should be read from the machine's store, whereas in prior versions of windows, this was automatic.