...It plugs into PAM stacks for services like SSH, su, or desktop display managers, prompting users for a time-based (TOTP) or counter-based (HOTP) code after their password. A lightweight enrollment program bootstraps each account by generating a secret, printing a QR code, and writing per-user configuration with safe file permissions. Because verification happens locally, codes work offline and do not depend on any Google service, which suits high-security and air-gapped environments. Administrators can tune skew tolerance, rate limiting, emergency scratch codes, and which PAM control flags enforce 2FA for specific users or groups. ...