|Initial release||September 20, 2010|
|Operating system||Android, iOS, BlackBerry OS|
|License||Proprietary freeware (earlier versions were under Apache License 2.0)|
Google Authenticator is a software-based authenticator by Google that implements two-step verification services using the Time-based One-time Password Algorithm (TOTP; specified in RFC 6238) and HMAC-based One-time Password algorithm (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.
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 a 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). This is transferred to the Authenticator app as a 16, 26 or 32 character base32 string or as a QR code.
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. 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."
These implementations support the HMAC-Based One-time Password (HOTP) algorithm specified in RFC 4226 and the Time-based One-time Password (TOTP) algorithm specified in RFC 6238.