Google Authenticator

To use Authenticator, the app is first installed on a smartphone. 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 username and password to the site, which computes (but does not display) the required six-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 not sufficient 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 computer used for the login process is compromised by a trojan, then username, password and one-time password can be captured by the trojan, which can then initiate its own login session to the site or monitor and modify the communication between user and site.

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).[4] This is transferred to the Authenticator app as a 16, 26 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 that is HMAC-ed can be:

• the number of 30-second periods since the Unix epoch (TOTP); 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-digit code.

The Google Authenticator app for Android was originally open source, but later became proprietary.[3] Google made earlier source for their Authenticator app available on its GitHub repository; the associated development page states:

"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."[6]

Following Google Authenticator ceasing to be open source, a free-software clone named FreeOTP[7][3] was created, predominantly a fresh rewrite but including some code from the original.

Google provides Android,[8] BlackBerry, and iOS[9] versions of Authenticator.

Several other versions of authentication software are available. Those that use TOTP and HMAC in addition to other two-factor authentication can authenticate with the same sites and processes as Google Authenticator. Some of the listed software is available in versions for several platforms.

• Multi-factor authentication
• HMAC-based One-time Password algorithm
• FreeOTP
• LinOTP

• 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
• Authenticator on F-Droid
• Django-MFA Implementation Using Google Authenticator - Django-mfa is a simple package to add extra layer of security to your django web application. It gives web app a randomly changing password as an extra protection.
• Source code of version 1.02 on GitHub