ah, thought you want to use it for presence detection/follow me. I'd better not use RFID for door locks. Not without expensive active crypto tokens.
I don't follow you. I thought each tag had a unique ID. Are you saying for security applications, instead of just presence, not any RFID would do. Where's the problem - that they're easy to copy, or that they're not unique?
I also want to use it for a pet door, but in my case it's not just because it's geeky and cool. The door is for a big dog, and it's large enough for a man to get through. RFID on the dog's collar was the only idea I came up with.