Two-factor authentication adds an extra layer of security by asking you to provide two forms of identification before you can access your personal information. The first is your usual user ID and password. The second is typically a unique code that is sent to your phone or some other physical device, that you then enter during the second stage of the login process.
Because the second identifier is sent to something that you physically carry with you, it makes it harder for hackers to access your information.
Apple sends a four-digit code via SMS or Find My iPhone to register your machine as a trusted device. Anytime you try to access your iCloud or iTunes account from a new device, you’ll be required to enter a new four-digit passcode. Instructions on how to enable two-step verification for your Apple ID are here
Google can send verification codes
in several different ways: SMS, voice call, or through its Google Authenticator app. During initial sign-in, you can tell Google to remember your device for 30 days. But a new code is required anytime someone tries to log in from a new machine.
Once two-step authentication is activated
, Yahoo will send a passcode via text or voice call. The verification process will only occur whenever you’re logging in from a new computer or mobile device, or if you’ve cleared your browser’s cache.
Facebook offers login approvals
, which require you to enter a passcode anytime you try to log in from an unrecognized computer or mobile phone. Codes are sent via text message.
Twitter will send a verification code
via text or as a push notification on iOS and Android devices. The company also provides a backup code, so that in the event that you lose your phone, you can enter the backup code to log in to your account. This isn’t saved anywhere, so be sure to write it down someplace.
Like the others, you’ll receive a code
anytime you try to access Dropbox from a new machine. They can be sent via text, or you can use an app like Google Authenticator or Duo Mobile. The company also provides a 16-digit backup code in case you lose your phone or for some reason can’t receive a PIN using the aforementioned methods. Again, write it down somewhere safe.
You can receive codes from Microsoft via text, email or authenticator app. A PIN is only required when you sign in from an untrusted device. Instructions on how to enable two-step verification are available here