Since encryption deals rather with the incomming signal then node itself, I would say that you dont need to use Decryption to Spoof. Unless the node is only accepting commands encrypted with proper key, which is also described in Unwired. In such case, you will need to break encrytion on the signal. Which, on the other side, you will also need to Intercept the traffic and Trace the commanding node AcessID, so you will need it this way or another. At least signal cannot be Encrypted hard.