some battery - powered devices , such as smartphones , can be unlocked with use of wearable nfc authentication devices . however , using conventional nfc authentication procedures is expensive in terms of power consumption because of continual rediscovery and reauthentication of the wearable authentication device . to address this , the present inventors , devised , among other things , more efficient methods for authenticating nfc - tagged authentication device , such as rings . for example , one embodiment uses two nfc polling loops or structures , instead of one as conventionally done . the first loop , used for initial authentication , entails scanning for two or more types of nfc tags . and the second loop , invoked after initial authentication , scans for a smaller subset of the two or more types to conserve battery power .