Who verifies that a tag is genuine?
Something that holds the matching secret, and that something is never the phone in the customer's hand. The phone relays a challenge and an answer to a service that can judge them, which is why an authentication scheme is a service with a chip attached rather than a chip on its own.
The phone is a courier
A phone reading an authenticating tag carries the exchange between the chip and whoever can judge it. It is not the judge.
It cannot be, because judging requires holding the matching secret, and a secret placed on every customer's phone is not a secret.
Take NFC Tools. On iOS it requires an iPhone 7 or later running iOS 15.6 or later, and the app must be launched manually. Background reading is available from the iPhone XS and XR onwards.
So the useful mental model is a courier with a sealed envelope, and the question that follows is who opens it.
What the service must hold
The key material corresponding to what was programmed into the chips, or access to a party that holds it.
A record of which tags exist, so that a valid proof from a tag nobody ordered is recognised as the problem it is.
For counter based schemes, a record of what has already been seen, plus a policy for tags that arrive out of step.
And an answer for failure. A service that returns a red cross with no defined consequence has produced an alarm nobody acts on.
The consumer facing problem
A scheme aimed at the public has to work for somebody who has never heard of it, holding whatever phone they own, standing in a shop.
If verification needs an application installed first, most people will not verify, and the scheme protects the brand's reputation rather than the customer.
If it works from a plain tap into a web page, adoption is far better and the verification is weaker, because what a page can do with the exchange is more limited.
That trade is the central design decision of any consumer authentication programme, and it is made by the person who chooses the chip, usually without realising.
What has to keep running
The service, for as long as the products are in circulation, which for durable goods is longer than most software lasts.
The key material, protected and recoverable, through staff changes and supplier changes.
And the address the tags point at, which is fixed in silicon or in content and cannot be changed once the goods have shipped.
A scheme is therefore a commitment with a duration, and the duration should be stated before the first tag is ordered.
Frequently asked questions
Can my phone verify a tag by itself?
No. Verification needs the matching secret, and a secret held on every phone would not be secret.
Does the customer need an application?
It depends on the scheme. Requiring one gives stronger verification and far fewer people who actually verify.
What happens if the service shuts down?
The tags still read and prove nothing anybody can check. The commitment lasts as long as the goods do.
Who should run the verifier?
Whoever will still exist when the products are still in circulation. That is a longer horizon than most projects assume.