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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.
iCloud Keychain: Using mobile to approve password syncing
Using the "send to iPhone" over-the-air option, an iPhone or iPad can approve a Mac to use passwords or not. This has to be performed on as many devices that you want to use your cloud-based passwords with.