The Ruby One Time Password Library

A ruby library for generating and validating one time passwords (HOTP &
TOTP) according to RFC 4226 and RFC 6238.

ROTP is compatible with Google Authenticator available for Android and
iPhone and any other TOTP based implementations.

Many websites use this for multi-factor authentication, such as GMail,
Facebook, Amazon EC2, WordPress, and Salesforce.  You can find a more
complete list here:
https://en.wikipedia.org/wiki/Google_Authenticator#Usage.