What is card emulation, and what does it require?
Card emulation is the phone taking the role of a contactless card, answering an external reader that supplies the field. It is how phone payment and phone based door access work. What a phone may present is controlled by the operating system and by the credential issuer, not by the user.
The role reversal
In reader writer mode the phone is the power source. In card emulation everything flips: an external terminal produces the field, and the phone answers within it, exactly as a plastic card would.
The terminal cannot tell the difference, and that is the point. A payment terminal talking to a phone runs the same exchange it would run with a card, because the phone is presenting a card application rather than pretending in some looser sense.
What it requires
Three things, and the third is the one people overlook.
Hardware capable of the role, which any phone with NFC has. A place for the credential to live, either a secure element or a software path such as host card emulation. And permission, which is not a technical component at all.
A payment credential is issued by a bank into a wallet the platform controls. A transport pass is issued by an operator. A corporate badge is issued by whoever runs the access system. In every case somebody has decided your phone may present that credential.
Why you cannot copy your office badge
This is the single most common expectation about NFC on phones, and the answer is no, for two independent reasons. Either alone would be enough.
The first is that applications cannot freely choose what to emulate. Platforms restrict which credentials may be presented, and an arbitrary application announcing itself as your building's badge is not something either operating system permits.
The second is that the badge itself usually cannot be copied. If it relies on a proprietary scheme, the phone has no way to reproduce it. If it authenticates cryptographically, the secret never leaves the card, so there is nothing to copy even in principle. And if it is a bare identifier, the phone may still be unable to present that identifier, because the value used during selection is not freely settable.
The exception worth knowing
Some access systems now issue phone credentials officially, through the platform wallet or a vendor application. That is not copying a badge, it is being issued a second credential. If you want your phone to open your office door, the route is the facilities team, not a utility.
What legitimately works
Payment, through the platform wallet with a card your bank has provisioned. Transport, where the operator supports it. Building access, where the vendor supports it. Loyalty and ticketing, through the wallet mechanisms both platforms provide.
What these share is an issuer who agreed. That is the actual requirement, and it is why card emulation feels closed compared with the freedom of reader writer mode.
Frequently asked questions
Can any app on Android emulate a card?
An application can register to handle specific application identifiers through host card emulation, which is a documented mechanism. That is not the same as being free to impersonate an arbitrary existing credential, and payment grade uses have further requirements.
Does card emulation work when the phone is switched off?
Some devices keep a payment function available for a period after the battery is exhausted, using reserve power. Behaviour varies by manufacturer and model, so it should be verified rather than assumed.
Can an iPhone emulate an arbitrary NFC card?
No. Apple controls what may be presented: legitimate uses go through its wallet or, since iOS 17.4 and 18.1, through its NFC and SE Platform to apps granted an entitlement under a commercial agreement. A general purpose application still cannot present a credential of its choosing.
Why does my phone open one door and not another?
Because the credential was issued for one system. A phone credential is not a universal key: it is a specific credential for a specific access system, exactly like the plastic card it replaced.