science

Google Authenticator

Google Authenticator is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Google Authenticator rather than just read about it. In short: Google Authenticator is a software-based authenticator by Google. It implements multi-factor authentication services using the time-based one-time password (TOTP; specified in RFC 6238) and HMAC-based one-time password (HOTP; specified in RFC 4226), for authenticating users of software applications.

Key takeaways

Google Authenticator belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Google Authenticator to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Google Authenticator from memory before moving on to harder problems.

Reference excerpt

Google Authenticator is a software-based authenticator by Google. It implements multi-factor authentication services using the time-based one-time password (TOTP; specified in RFC 6238) and HMAC-based one-time password (HOTP; specified in RFC 4226), for authenticating users of software applications. When logging into a site supporting Authenticator (including Google services) or using Authenticator-supporting third-party applications such as password managers or file hosting services, Authenticator generates a six- to eight-digit one-time password which users must enter in addition to their usual login details. Google provides Android, BlackBerry, and iOS versions of Authenticator. An official open source fork of the Android app is available on GitHub. However, this fork was archived in April 6, 2021 and is now read only. Current software releases are proprietary freeware.

Typical use case

The app is first installed on a smartphone to use Authenticator. It must be set up for each site with which it is to be used: the site provides a shared secret key to the user over a secure channel, to be stored in the Authenticator app. This secret key will be used for all future logins to the site. To log into a site or service that uses two-factor authentication and supports Authenticator, the user provides a username and password to the site. The site then computes (but does not display) the required six- to eight-digit one-time password and asks the user to enter it. The user runs the Authenticator app, which independently computes and displays the same password, which the user types in, authenticating their identity. With this kind of two-factor authentication, mere knowledge of username and password is insufficient to break into a user's account - the attacker also needs knowledge of the shared secret key or physical access to the device running the Authenticator app. An alternative route of attack is a man-in-the-middle attack: if the device used for the login process is compromised by malware, the credentials and one-time password can be intercepted by the malware, which then can initiate its login session to the site, or monitor and modify the communication between the user and the site.

Technical description During setup, the service provider generates an 80-bit secret key for each user (whereas RFC 4226 §4 requires 128 bits and recommends 160 bits). This is transferred to the Authenticator app as a 16, 24, or 32-character base32 string, or as a QR code. Subsequently, when the user opens the Authenticator app, it calculates an HMAC-SHA1 hash value using this secret key. The message can be:

The number of 30-second periods since the Unix epoch (TOTP) as 64-bit big endian integer; or A counter that is incremented with each new code (HOTP). A portion of the HMAC is extracted and displayed to the user as a six- to eight-digit code; The last nibble (4 bits) of the result is used as a pointer, to a 32-bit integer, in the result byte array, and masks out the 31st bit.

License The Google Authenticator app for Android was originally open source, but later became proprietary. Google made earlier source for their Authenticator app available on its GitHub repository; the associated development page stated:

"This open source project allows you to download the code that powered version 2.21 of the application. Subsequent versions contain Google-specific workflows that are not part of the project." The latest open-source release was in 2020.

See also Multi-factor authentication HMAC-based one-time password FreeOTP LinOTP Comparison of OTP applications

Notes

References

External links Google Authenticator on Google Help Google Authenticator (Android) and Google Authenticator (other) legacy source code on GitHub Google Authenticator PAM module source code on GitHub Google Authenticator implementation in Python on Stack Overflow Django-MFA Implementation Using Google Authenticator - Django-MFA is a simple package to add an extra layer of security to your Django web application. It gives your web app a randomly changing password as extra protection.

Illustrations

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Worked examples

Example 1 — a first encounter with Google Authenticator

Start with the simplest possible case. Write down what Google Authenticator claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Google Authenticator before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Google Authenticator ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Google Authenticator

In research: Google Authenticator appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Google Authenticator in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Google Authenticator is common in secondary-school and first-year university syllabi. It links to neighbouring topics Authentication, Google services, Internet properties established in 2010, so understanding it makes those chapters shorter.
In everyday life: Look for Google Authenticator outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Google Authenticator in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Google Authenticator means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Google Authenticator out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Google Authenticator in simple terms?

Why does Google Authenticator matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Google Authenticator?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Google Authenticator.