Time-based one-time password

The time-based one-time password algorithm, usually shortened to TOTP, generates a one-time password by using the current time as the source of uniqueness. It extends the HMAC-based one-time password algorithm, known as HOTP, by replacing the event counter with a value derived from the clock, so that a code is only good for a short window. It has been adopted as the Internet Engineering Task Force standard RFC 6238. TOTP is a cornerstone of the Initiative for Open Authentication, called OATH, and it is used in a number of two-factor authentication systems, where a user proves possession of a shared secret by entering the code it currently produces.