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The Time-based One-Time Password algorithm (TOTP) is an extension of the HMAC-based One-time Password algorithm (HOTP) generating a one-time password by instead taking uniqueness from the current time. It has been adopted as Internet Engineering Task Force[1] standard RFC 6238,[1] is the cornerstone of Initiative For Open Authentication (OATH), and is used in a number of two-factor authentication systems.

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Because of latency, both network and human, and unsynchronised clocks, the one-time password must validate over a range of times between the authenticator and the authenticated. Here, time is downsampled into larger durations (e.g., 30 seconds) to allow for validity between the parties. However, as with HOTP the decreased uniqueness requires additional countermeasures, such as rate limiting.


To establish TOTP authentication, the authenticated and authenticator must pre-establish both the HOTP parameters and the following TOTP parameters:

  • T0, the Unix time from which to start counting time steps (default is 0)
  • TX, an interval which will be used to calculate the value of the counter CT (default is 30 seconds)

Both the authenticator and the authenticatee compute the TOTPvalue, then the authenticator checks if the TOTPvalue supplied by the authenticated matches the locally generated TOTPvalue. Some authenticators allow values that should have been generated before or after the current time in order to account for slight clock skews, network latency and user delays.

TOTP value[edit]

TOTP uses the HOTP algorithm, substituting the counter with a non-decreasing value based on the current time.

TOTPvalue(K) = HOTPvalue(K, CT)

The time counter, CT, is an integer counting the number of durations, TX, in the difference between the current Unix time, T, and some epoch (T0; cf. Unix epoch); the latter values all being in integer seconds.

CT=TT0TX,{displaystyle C_{T}=leftlfloor {frac {T-T_{0}}{T_{X}}}rightrfloor ,}

Note that Unix time is not strictly increasing. When a leap second is inserted into UTC, Unix time repeats one second. But a single leap second does not cause the integer part of Unix time to decrease, and CT is non-decreasing as well so long as TX is a multiple of one second.

Practical considerations[edit]

For subsequent authentications to work, the clocks of the authenticated and the authenticator need to be roughly synchronized (the authenticator will typically accept one-time passwords generated from timestamps that differ by ±1 time interval from the authenticated's timestamp).

Weaknesses and vulnerabilities[edit]

TOTPvalues can be phished like passwords, though this requires attackers to proxy the credentials in real time.

Implementations that don't limit login attempts are vulnerable to brute forcing of values.

An attacker who steals the shared secret can generate new, valid TOTPvalues at will. This can be a particular problem if the attacker breaches a large authentication database.[2]

Because TOTP devices have consumable batteries and clocks that can de-synchronize, and can be lost and/or stolen, all real-world implementations have backup methods to bypass the protection (e.g.: printed recovery codes, SMS/email/phone call delivered recovery code or link), etc.). This can create significant support costs for services with large user-bases, and also provide weak points for attackers to try first.

TOTPvalues are valid for longer than the amount of time they show on the screen (typically twice as long). This is a concession that the authenticating and authenticated sides' clocks can be skewed by a large margin.


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.[3] In May, 2011, TOTP officially became RFC 6238.[1]


  1. ^ abc'RFC 6238 – TOTP: Time-Based One-Time Password Algorithm'. Retrieved July 13, 2011.
  2. ^Zetter, Kim. 'RSA Agrees to Replace Security Tokens After Admitting Compromise'. WIRED. Retrieved 2017-02-17.
  3. ^Alexander, Madison. 'OATH Submits TOTP: Time-Based One Time Password Specification to IETF'. Open Authentication. Retrieved 22 February 2010.

See also[edit]

  • Botan, C++ cryptography library with HOTP/TOTP support

External links[edit]

  • Designing Docker Hub Two-Factor Authentication, (section 'Using Time-Based One-Time Password (TOTP) Authentication').
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Time-based_One-time_Password_algorithm&oldid=945091650'

by Prakash Sharma

With the increase in cyber security threats, it has become more and more necessary to upgrade the security standards of your web applications. You need to make sure your users’ accounts are safe.

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Nowadays, a lot of online web applications are asking users to add an extra layer of security for their account. They do it by enabling 2-factor authentication. There are various methods of implementing 2-factor authentication, and TOTP (the Time-based One-Time Password algorithm) authentication is one of them.

This article explains what it is, and how and why to use it. But before understanding that, let’s first briefly take a look at what two-factor authentication means.

What is Two Factor Authentication?

Two-factor authentication (or multi factor authentication) is just an extra layer of security for a user’s account. That means that, after enabling two factor authentication, the user has to go through one more step to log in successfully. For example, the usual steps for logging in to an account are:

But after enabling 2-factor authentication, the steps look something like this:

So this adds one more step to the login process. This method is more secure, because a criminal cannot access the user’s account unless they have access to both the user’s regular password and one time password.

Currently, there are two widely used methods to get that one time password:

  1. SMS-based: In this method, every time the user logs in, they receive a text message to their registered phone number, which contains a One Time Password.
  2. TOTP-based: In this method, while enabling 2-factor authentication, the user is asked to scan a QR image using a specific smartphone application.
    That application then continuously generates the One Time Password for the user.

The SMS-based method does not need any explanation. It’s easy, but it has its own problems, like waiting for the SMS on every login attempt, security issues, and so on. The TOTP-based method is becoming popular because of it’s advantages over the SMS-based method. So let’s understand how the TOTP-based method works.

How the TOTP-based method works

Before understanding this, let’s first discuss what problems this method will solve for us.

By using the TOTP method, we are creating a one time password on the user side (instead of server side) through a smartphone application.

This means that users always have access to their one time password. So it prevents the server from sending a text message every time user tries to login.

Also, the generated password changes after a certain time interval, so it behaves like a one time password.

Great! Now let’s understand the workings of the TOTP-method and try to implement the above solution ourselves. Our requirement here is to create a password on the user side, and that password should keep changing.

The following could be a way to implement this solution:

This should work, but there are three main problems with it:

  1. How will the application generate a one time password using a secret key and counter?
  2. How will the counter update? How will the web server keep track of the counter?
  3. How will the server share the secret key with the phone’s application?

The solution to the first problem is defined in the HOTP algorithm.

Understanding HOTP:

HOTP stands for “HMAC-Based One-Time Password”. This algorithm was published as RFC4226 by the Internet Engineering Task Force (IETF). HOTP defines an algorithm to create a one time password from a secret key and a counter.

You can use this algorithm in two steps:

  1. The first step is to create an HMAC hash from a secret key and counter.

2. In this code, the output would be a 20 byte long string. That long string is not suitable as a one time password. So we need a way to truncate that string. HOTP defines a way to truncate that string to our desired length.

It might look scary, but it is not. In this algorithm, we first obtain offset which is the last 4 bits of hmacHash[19]. After that, we concatenate the bytes from hmacHash[offset] to hmacHash[offset+3] and store the last 31 bits to truncatedHash. Finally, using a simple modulo operation, we obtain the one time password that’s a reasonable length.

This pretty much defines the HOTP algorithm. The RFA4226 doc explains why this is the most secure way to obtain a one time password from these two values.

Great! So we have found a way to obtain a one time password using a secret key and counter. But what about the second problem? How to keep track of the counter?

The solution to second problem is found in the TOTP.

Understanding TOTP:

TOTP stands for “Time-Based One-Time Password”. This was published as RFC6238 by IETF.

A TOTP uses the HOTP algorithm to obtain the one time password. The only difference is that it uses “Time” in the place of “counter,” and that gives the solution to our second problem.

That means that instead of initializing the counter and keeping track of it, we can use time as a counter in the HOTP algorithm to obtain the OTP. As a server and phone both have access to time, neither of them has to keep track of the counter.

Also, to avoid the problem of different time zones of the server and phone, we can use a Unix timestamp, which is independent of time zones.

However the Unix time is defined in seconds, so it changes every second. That means the generated password will change every second which is not good. Instead, we need to add a significant interval before changing the password. For example, the Google Authenticator App changes the code every 30 seconds.

So we have solved the problem of the counter. Now we need to address our third problem: sharing the secret key with the phone application. Here, a QR code can help us.

Using a QR code

Though we can ask the users to type the secret key into their phone application directly, we want to make secret keys quite long for security reasons. Asking the user to type in such a long string would not be a user friendly experience.

Since the majority of smartphones are equipped with a camera, we can use it and ask the user to scan a QR code to obtain the secret key from it. So all we need to do is to convert the secret key in the QR code and show it to the user.

We have solved all three problems! And now you know how TOTP works. Let’s see how to implement it in an application.

How to implement TOTP

There are some free phone applications (like Google Authenticator App, Authy, and so on) available which can generate an OTP for the user. Therefore, in most cases, creating your own phone application is not necessary.

The following pseudo codes explain a way to implement TOTP-based 2-factor authentication in a web application.

The user is asked to scan that QR code. When the phone application scans the QR code, it gets the user’s secret key. Using that secret key, the current Unix time, and the HOTP algorithm, the phone application will generate and display the password.

We ask the user to type the generated code after scanning the QR code. This is required, because we want to make sure that the user has successfully scanned the image and the phone application has successfully generated the code.

Here we use the HOTP algorithm on the server side to get the OTP-based authentication on the secret key and current unix time. If that OTP is the same as the one typed by the user, then we can enable 2-factor authentication for that user.

Now after every login operation, we need to check if this particular user has 2-factor authentication enabled. If it is enabled, then we ask for the one time password displayed in the phone application. And if that typed code is correct, only then is the user authenticated.

What happens if the user loses the code?

Code For Time-based Dynamic Key Generation 1

There are a couple of ways to help the user to recover the code. Usually when they are enabling 2-factor authentication, we can show the secret key to them along with the QR code and ask them to save that code somewhere safely.

Applications like Google Authenticator App let you generate the password by directly entering the secret key. If the user loses the code, they can enter that safely saved secret key in the phone application to generate the OTP again.

If we have the user’s phone number, we can also use the SMS-based method to send an OTP to the user to help them recover the code.

Code For Time-based Dynamic Key Generation 2017

Wrapping Up

Two factor authentication is gaining popularity. A lot of web applications are implementing it for extra security.

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Unlike the SMS-based method, the TOTP method does not require a lot of extra effort either. So this feature is worth implementing for any application.

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