Could I build a smartphone app so that if an RFID tag was in range of the phone's NFC reader, it would act as a key to make the app work, rather than a trigger—that is, the app would only work if the reader and tag were in range of each other?
Before I answer your question, let me point out that the Near Field Communication (NFC) reader incorporated in the new Apple iPhone 6 and iPhone 6+, as well as in many Android phones, is designed to have a read range of only a few inches. This is because it is designed primarily for financial transactions and device-to-device communication, and you would not want to link to dozens of nearby devices or pay for things other people are buying at nearby counters. So when you say the app would work only if the reader in the phone was in range of a tag, keep in mind that that we're talking about a very short distance. The phones do have Bluetooth capability, so you could possibly use Bluetooth Low Energy (BLE) beacons for your app, if you require a longer read range.
Your question really is a software or app-development question, not an RFID question. Apps interface with the NFC reader and Bluetooth based on the operating system, as well as restrictions put in place by the handset manufacturer. When you read a tag, the serial number is captured, so I do not see why this cannot be used as a password to make the application work. The thing is, you would need to define which serial numbers from which tag would serve as passwords.
If you want all NFC tags to enable the app to work, I think it would be a matter of creating a field that would be populated by a tag's serial number, and then writing code that says, "If this field is blank, stay locked. If this field is populated, unlock."
Perhaps some of our readers who have developed NFC applications can comment further. If so, they are invited to post information below.
—Mark Roberti, Founder and Editor, RFID Journal
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