
Apple introduced an additional layer of security for iPhone, iPad and Mac users in 2013 by rolling out two-step verification for Apple ID accounts. Two-step verification prevents anyone but you from accessing your Apple ID account, even if they know the password, by requiring a four-digit verification code sent via SMS or Find My iPhone on trusted devices. When you enable two-step verification, you must register at least one trusted device capable of receiving SMS text messages.

Once activated, two-step authentication is required when managing your Apple ID through My Apple ID, signing into iCloud, or making iTunes, iBooks or App Store purchases from a new device. Apple has also expanded two-step authentication to iMessage and FaceTime, requiring users to input an authentication code from a verified device on accounts that have two-factor verification enabled to prevent unauthorized entry attempts through both services.
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Article Link: How to Enable Two-Step Verification for Apple ID