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Time-based One-time Password Algorithm (TOTP) is an algorithm that computes a one-time password from a shared secret key and the current time. It has been adopted as Internet Engineering Task Force standard RFC 6238, is the cornerstone of Initiative For Open Authentication (OATH) and is used in a number of two factor authentication systems.
TOTP is an example of a hash-based message authentication code (HMAC). It combines a secret key with the current timestamp using a cryptographic hash function to generate a one-time password. The timestamp typically increases in 30-second intervals, so passwords generated close together in time from the same secret key will be equal.
In a typical two-factor authentication application, user authentication proceeds as follows: a user will enter username and password into a website or other server, generate a one-time password for the server using TOTP running locally on a smartphone or other device, and type that password into the server as well. The server will then also run TOTP to verify the entered one-time password. For this to work, the clocks of the user's device and the server need to be roughly synchronized (the server will typically accept one-time passwords generated from timestamps that differ by ±1 from the client's timestamp). A single secret key, to be used for all subsequent authentication sessions, must have been shared between the server and the user's device over a secure channel ahead of time. If some more steps are carried out, the user can also authenticate the server using TOTP.
TOTP is based on HOTP with a timestamp replacing the incrementing counter.
The current timestamp is turned into an integer time-counter (TC) by defining the start of an epoch (T0) and counting in units of a time step (TS). For example:
Then we define
A TOTP draft was developed through the collaboration of several OATH members in order to create an industry-backed standard. It complements the event-based one-time standard HOTP and offers end user organizations and enterprises more choice in selecting technologies that best fit their application requirements and security guidelines. In 2008, OATH submitted a draft version of the specification to the IETF. This version incorporates all the feedback and commentary that the authors received from the technical community based on the prior versions submitted to the IETF. In May, 2011, TOTP officially became RFC 6238.
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