Google Authenticator generates 2-Step Verification codes on your phone.
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Pro Simple and straightforward
All you do after installing is log in with your Google account, and then you can add almost any account (that supports 2-factor authentication of any kind) quickly.
Pro Works offline
The app requires an internet connection to set up, but once that's done, you can disconnect from wifi entirely and it will still work.
Pro Storage efficient
Uses less than 20 mbs.
Pro The first app of this kind
Con Too basic
If you have over 10 TOTP secrets, it gets really complex to track them as there are no folders or categories or labels or images