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Enabling iPhone, iPad device trust
Your Mac machine now requires that a trust relationship be built between your desktop or laptop, and your iOS-powered devices, such as your iPhone or iPad, before it is synced with iTunes or other desktop apps.
Approving trust on the iPhone, iPad
Once the Mac sends the message, it asks that the iPhone or iPad user accept the "trust" relationship. This method of two-factor authentication requires a device is "paired" to a Mac, and prevents data from synchronizing to the wrong device.
iCloud Keychain: Mobile two-factor authentication
Now for something interesting: To use iCloud Keychain, the so-called cloud-based password storage and security feature, you are required to build trust between your Mac and iPhone. Approving a Mac as a secure computer to use remembered passwords requires a two-factor authentication with an iPhone or iPad.