The Google Authenticator 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).

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.

Homepage:
https://github.com/google/google-authenticator-libpam
