The NFC seal publishes its own key. What does that actually buy?

[andrei-chernikov]

The NFC seal is the strongest binding ODP offers between a passport and a physical object. It is also the part where the specification is least comfortable, and it says so in its own text.

Here is the shape of it. The chip is an NXP NTAG 424 DNA. Its native security is symmetric — chip and phone share a secret key and prove to each other that they both know it. There is no per-chip private key that signs a challenge verifiable against a public key on-chain. So for the check to be public, the key has to be public: ODP publishes the 16-byte EV2 key inside the passport's nfc anchor, where everyone can read it.

Which means a forger who reads the passport can program another NTAG 424 with the same key, and it will pass.

What the check does still buy

• The chip is physically present and live — not a copied URL, not a photographed QR, not a replayed recording
• With TagTamper, the seal loop has never been opened — that flag is permanent and cannot be reset
• The chip is the right kind of thing, answering a fresh challenge

That blocks the cheap attacks, which are the common ones. It does not block a forger who is willing to buy chips and program them.

The questions

• Is a published-key seal worth having at all? It stops casual fakes and not determined ones. Is that a useful rung on the ladder, or a false sense of security that would be better replaced by an honest "this is a tamper-evident sticker with a serial number"?
• Is there a chip that does this properly and is actually buyable at run quantities? Something with a per-unit private key and an attestation path. If you have shipped with one, we want to hear the part number and what it cost.
• Does TagTamper carry most of the value here? The permanent tamper flag is the one property a forger cannot fake without physically defeating the loop. Perhaps the cryptography is the smaller half of this.
• What does the honest sentence sound like? A verifier saying "authentic" from a chip check alone is overselling. What should it say instead, in words a buyer standing in a shop understands?

Nothing here is a settled position. The current model is written down as the best available option, not as a good one — SPEC §6 states the limits in its own text.