How do you share a car key from a phone?
Through the system that issued it, rather than by sending anything. The owner invites a person, that person gets a key issued to their own device, and the invitation can carry limits: how long it lasts, which vehicle functions it allows, and whether it opens the whole car or a single door.
An invitation, not a transfer
The Car Connectivity Consortium describes its digital key as a standardised ecosystem that lets mobile devices, on any operating system, store, authenticate and share keys for a vehicle. Sharing is part of the specification rather than something bolted on by an application.
What happens is an issuance: the owner invites a person, and that person's device is provisioned with its own key. Nothing is copied, nothing is sent, and the recipient's key is not a duplicate of the owner's, which is the same rule as every other credential in this section.
The consequence people notice is that the recipient needs a supported device and an account with the vehicle's system, and that the invitation goes through that system rather than through a messaging application.
What an invitation can carry
The consortium lists what an owner can decide when sharing: how long the access lasts, which vehicle features the person may use, and whether they get the whole vehicle or entry to the passenger door or the boot alone.
That granularity is the argument for a digital key over handing somebody a spare. A neighbour fetching something from the boot for an afternoon does not need to be able to drive the car, and a plastic key cannot express that.
The same page notes that functions remain usable remotely, including in a low battery state, and that backward compatibility is part of the design, so a key issued today is expected to keep working as the ecosystem moves.
Where NFC comes into it
A digital car key normally works at a distance, with ultra wideband measuring where the phone is so the car knows it is outside the door rather than in the cabin, which the comparison page explains.
NFC is the deliberate fallback: holding the phone against a marked spot on the handle or in the cabin. The same consortium states that NFC support is maintained for backward compatibility and for battery low mode, which is exactly when a hands free path is least reliable.
For a shared key that matters more than for the owner's own, because a borrower is unfamiliar with the car and is the person most likely to end up tapping the handle and wondering where.
Taking it back
Revocation happens in the issuing system, and it reaches the vehicle rather than the phone: the key stops being accepted, whatever the borrower's device still displays.
Because the shared key was issued rather than copied, revoking it does not touch the owner's. That is the property that makes lending to a stranger reasonable, and it is the same independence the two cards page describes for payment.
When a car changes hands, every key it issued has to go with the account rather than with the vehicle, and that is a question for the manufacturer's process rather than for the phone. Ask it before the sale, not after.
Frequently asked questions
Can I send a car key by message?
No. The invitation goes through the vehicle system, which issues a key to the other person device. Nothing is copied or transmitted as a file.
Can I limit what the borrower can do?
The consortium lists duration, which vehicle functions are allowed, and access limited to the passenger door or the boot among the owner choices.
Does the borrower need the same brand of phone?
The specification covers mobile devices on any operating system, but the vehicle and the device both have to support the scheme in question.
How do I get the key back?
Revoke it in the issuing system. The vehicle stops accepting it, and the owner key is unaffected because the shared one was issued separately.