Two-factor authentication

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Two-factor authentication

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References:
https://www.tbray.org/ongoing/When/201x/2013/06/24/Two-Factor
https://code.google.com/p/google-authenticator/wiki/KeyUriFormat
http://en.wikipedia.org/wiki/Time-based_One-time_Password_Algorithm

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Fido U2F (Universal 2nd Factor) is a new standard that uses a hardware key and support from the browser itself (currently Chrome 38+) to ensure the OTP is site-specific and so can't be collected by a phishing site. Google, GitHub and Dropbox already support this standard. Lastuser should too.

Fido specs: https://fidoalliance.org/specifications/download/
Fido videos: https://fidoalliance.org/adoption/videos/
GitHub announcement: https://github.com/blog/2071-github-supports-universal-2nd-factor-authentication

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Authy provides a wrapper API for 2FA that covers most of our requirements. If the user has the Authy app installed, they get a code within the app instead of an SMS.

As Authy's API runs on Twilio, there's an important factor with delivering to Indian numbers: by default Twilio sends promotional SMSes, which won't deliver to DND numbers. Switching to transactional requires explicit approval via customer support. Twilio's billing may also be considerably more than Exotel (to be verified).

If the user has the Authy app installed, the app acts as a proxy for the phone, but can also be installed and logged into the same account from other devices. This means the phone number has not actually been verified. This impacts the verified_at column defined in #178.