An alternative to Google Authenticator is a free app called Authy. It works with all the same sites as Google Authenticator, but it has a cool extra capability: it can sync accounts across all of your iOS devices automatically, and (with a free companion Mac app, which works on newer Macs with Bluetooth 4.0 support) can even send codes to your Mac and enter them for you automatically—although this doesn't work as often as I'd like.
Use one-time verification codes
When you set up two-factor authentication, there's always the worry that you could lose the iOS device you use for that second factor, thus making it impossible for you to access your own account. So most companies supply you with an extra code of some sort (Apple calls it a recovery key; Dropbox and Twitter refer to it as a backup code) during the setup process. You should either print this out and keep it in a safe place, or put it in a secure digital location (such as your password manager). If you ever need to get into your account without your secondary device, this code can save the day.
But some companies take this concept a step further. Evernote, Facebook, and Google, for example, supply you with a list of codes that you can use whenever you like, in place of SMS or a code from an authenticator app. Each code can be used only once, however; if you run out of codes, you have to go back to the appropriate page in the Settings portion of each site and generate another list. Again, keep this list in a safe place—and take it with you when traveling, just in case.
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