Copyright 2010 Google Inc.
https://github.com/google/google-authenticator-android
This project is an older fork of the one on the Play store. It's an older version that doesn't get changes synced to it from the Play store version.
The Google Authenticator project includes implementations of one-time passcode generators for several mobile platforms, as well as a pluggable authentication module (PAM). One-time passcodes are generated using open standards developed by the Initiative for Open Authentication (OATH) (which is unrelated to OAuth).
This project contains the Android app. All other apps, and the PAM module, are in a separate project.
This implementation supports 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.
Further documentation is available in the Wiki.