Why do identity apps ask you to tap your passport?
Because the chip will not answer until the reader shows it has the open document in front of it, and what it then returns is signed by the issuing authority. The tap turns a photograph of a page into something an application can verify rather than believe.
Why the tap needs the page
An identity document does not behave like a tag. It releases nothing until the reader demonstrates that it has the document physically present and open, using information printed on the page itself, which the identity document page sets out and which the mechanisms in Part 11 of ICAO Doc 9303 specify.
That is why the application asks you to photograph or type the printed line first and to tap afterwards. The first step is not a convenience, it is the key to the second.
It also settles the question people arrive with: a passport in a bag has nothing to say to a passing reader, because the precondition has not been met.
What the chip proves
The data in the chip is signed by the issuing authority, so an application that checks the signature learns that this data was issued by that country and has not been altered. A convincing forgery of the printed page does not produce a chip that passes that check.
It contains the portrait the authority issued, which is what a face comparison is run against, rather than the photograph on the page that a camera might have been shown.
What it does not prove is that the person holding the document is its holder, which is a separate step, or that the document has not been reported lost or stolen, which is a lookup the verifier makes elsewhere. A valid chip in the wrong hands is still a valid chip.
What the application keeps
Everything in the chip is available to the application once the tap succeeds: the full data set, not the part relevant to the check. What happens to it afterwards is a policy question rather than a technical one.
So the question worth asking of any identity application is what it stores, for how long, and who receives it, which the data protection page frames without naming a national rule.
The contrast with a wallet attestation is the point of the rest of this cluster: a verification that only needs to know whether you are over an age does not need a passport number, and the age page describes what asking the narrower question looks like.
Why the tap fails so often
The chip is in the booklet, usually in the cover, and the phone antenna is in one part of the phone, which the antenna page maps. Two small antennas have to overlap and stay still, and most failures are that simple.
A case, a card behind the phone, and a hand holding both loosely make it worse, and the read has to survive a long exchange rather than a moment, which the fast tap page explains for tags and which applies with more force here.
Lay the document flat on a table, lay the phone on it, and leave both alone. It is the least satisfying advice in identity verification and the one that works.
Frequently asked questions
Can my passport be read from a bag?
No. The chip answers only a reader that shows it has the open document, using information printed on the page.
Why does the app want the printed line first?
That printed information is what unlocks the chip. The photograph or the typed line is the key for the tap that follows.
What does the chip add over a photograph of the page?
A signature from the issuing authority, so the data can be verified rather than trusted, and the authority portrait to compare a face against.
Why does the tap keep failing?
The two antennas have to overlap and stay still for a long exchange. Lay the document flat, lay the phone on it, and do not move either.