!!! Overview
[{$pagename}] ([TOTP]) is defined in [RFC 6238] as an extension of the [One-time password|One-Time password] ([OTP]) algorithm, namely the [HMAC-based One-Time Password Algorithm] ([HOTP]), as defined in [RFC 4226], to support the [time]-based moving factor.  

[{$pagename}] is generated by HMAC([Shared Secret], timestamp), where timestamp typically change, by default, every 30 [seconds]. 

The [HMAC-based One-Time Password Algorithm] ([HOTP]) specifies an event-based [OTP] algorithm, where the moving factor is an event counter. The present work bases the moving factor on a time value. A time-based variant of the [OTP] algorithm provides short-lived OTP values, which are desirable for enhanced security.
   
[{$pagename}] has been adopted as Internet Engineering Task Force standard [RFC 6238] is the cornerstone of Initiative For Open Authentication [Initiative for Open Authentication] and is used in a number of [multi factor authentication systems|Multi-Factor Authentication].

[{$pagename}] is an example of a hash-based message authentication code [HMAC]. [{$pagename}] combines a secret key with the current timestamp using a [cryptographic Hash Functions|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.

The [{$pagename}], uses the counter for a value derived from the current [Unix Time]. 
The derived value T, is calculated using an initial time T0 and a step X as follows: 
%%prettify 
{{{
T = (Current Unix time - T0) / X
}}} /%
Each generated OTP is valid for X seconds, by default 30. 

[{$pagename}] is used by Google Authenticator and the e Yubico [Initiative for Open Authentication] applet.!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [Time-based One-time Password Algorithm|https://en.wikipedia.org/wiki/Time-based_One-time_Password_Algorithm|target='_blank'] - based on 2015-02-12