UNIX and Linux operating systems rely on a common stack called Pluggable Authentication Module (PAM) for any user authentication. All UNIX services like SSH, FTP, SMTP, etc... and privilege-based systems like SUDO and terminal login rely on PAM.
RCDevs provides a PAM module for both OpenOTP and TiQR Servers which implements the authorization and authentication in a PAM process. PAM OpenOTP is easy to implement and is provided in open-source code and pre-compiled versions for most Linux OSes. A proper deployment of PAM OpenOTP should rely on a pre-established PAM-LDAP foundation where the UNIX/Linux users and groups are stored in an LDAP directory.