Two-step verification

 The Google Authenticator project includes implementations of one-time
 passcode generators for several mobile platforms, as well as a
 pluggable authentication module (PAM). One-time passcodes are generated
 using open standards developed by the Initiative for Open
 Authentication (OATH) (which is unrelated to OAuth).
 .
 These implementations support the HMAC-Based One-time Password (HOTP)
 algorithm specified in RFC 4226 and the Time-based One-time Password
 (TOTP) algorithm currently in draft.