I just read about NFCRing. It has a public and a private ID. The public part you can use for sharing your address by touching a NFC phone. The private part you can use for unlocking doors and you phone.
NFC in phones is a wireless technology like bluetooth, but with a much smaller range (< 20 cm). When the range is so low no approval is needed (unlike bluetooth) when the receiver connects.
NFC also is compatible with RFID. The phones can both work as initiators and receivers. Initiators/senders must be powered (Active), but the receiver can be unpowered (Passive) – just like RFID chips. This is also true for the phones.
Read more in Professional NFC Application Development for Android, RFID Handbook and Getting Started with RFID.