Wouldn't it be pretty easy to restrict transactions to a specific MAC address? You register a MAC address for your account via a transaction. Only if the MAC address is the specified one, the transaction is executed. Just an idea.
It's impossible.
why?
Coz it's unknown what MAC address a transaction was sent from.
No, you misunderstood me. I don't claim that other nodes have to verify the MAC address. It's just a test that the server on your computer locally performs before he releases the transaction to other nodes. The MAC address is a fingerprint of the device you are using to send nxt coins.
Edit: OK, I think I see your point.