Are you trying to fingerprint the device externally (i.e., passive observation)? Or is this trusted computing, where you're trying to key a license or some such to the board it's running on? i.e., where your code is running on the chip and you're trying to solidly determine its identity.
I wouldn't trust a MAC address, as it's easily changed on most platforms. But if the platform can return a burned-in chip ID then that'd be the one to use.
Beyond that, realize that your code can be hacked to circumvent the validation process (or "read" any desired result). Trusted computing is a complex topic, generally requiring an on-board crypto locker that can only be challenged.
Cheers,
Richard
Trying to put together sufficient information over time to uniquelly identify the item. I'm not doing the cryptography bit - just data collection. So the MAC is based on the ID which does not change whereas the MAC can be updated. I have that. I have radio jitter, RSSI (signal strength) and the random number. I'm guessing I can check RTC ram on powerup, but I need more items that vary depending on the environment and use......