Google lets iOS users use their iPhone as a hardware security key. That option was already there for Android users. The key is part of the Advanced Protection program for the highest level of account security Google offers.
Click for animated gif of Smart Lock operation
Users do not need to plug in their iPhone for verification. It works via bluetooth, says Google. The computer on which users want to log in and the telephone must therefore be in close proximity for the login attempt to succeed. Also, bluetooth must be on on both for it to work. According to a Google employee the new feature takes advantage of the Secure Enclave in iPhones. Apple has put the data from Touch ID and Face ID there. The Secure Enclave is separated from the rest of the soc, so that malicious parties should not be able to access it. The feature works with the Fido standard.
The feature is available to Advanced Protection users where a physical device serving as the second factor must be in close proximity when logging in. The feature is for people who think they need the extra security, such as activists and politicians. It is the first time that Advanced Protection works with iPhones. Google also recommends a USB key as a backup in case the phone gets lost or breaks. On Android, the security key is built-in, on iOS users must use the Smart Lock app. For more than two years now, Google has offered Advanced Protection as a security layer on top of the standard two-factor authentication.