I ran into an odd problem when messing around with generating MD5 hashes in Unity Android recently.
When testing on my PC I was creating an MD5 object by using MD5.Create() and it worked fine. However I then published over to my Android phone to test to make sure it works, good thing I did, because I got a NullReferenceException. Looking at the code, the only thing that jumped out at me was the MD5 object wasn't being created, so I added a check for that condition, and indeed the MD5 object didn't exist. So for some reason MD5.Create() didn't work on the Android hardware.
A little bit of Googling landed me on unifycommunity.com, where I noticed the author, Matthew Wegner, wasn't using the MD5.Create() method but new'ing a MD5CryptoServiceProvider object. Curious, since they both seem to return the same object, I tried out this new method and to my amazement it worked! Horray!
Before I was about to close the book on this and forget MD5.Create() forever, I noticed my project had the Stripping Level set to use micro mscorlib. That turned on a light in my head, so out of curiosity I disabled stripping, re-enabled MD5.Create and re-published to my phone. This time MD5.Create worked.
MD5.Create() doesn't return an object on Unity Android when the Stripping Level is set to Micro mscorlib, but 'new MD5CryptoServiceProvider()' does. When Stripping is disabled MD5.Create() does work as expected. I haven't tested this on the iPhone but the results are probably the same.
Even Short Version:
It's seems safest to 'new MD5CryptoServiceProvider()' whenever you need an MD5 object as it appears to be the most portable.