mod_authn_otp is an Apache web server module for two-factor authentication using one-time passwords (OTP) generated via the HOTP/OATH algorithm defined in RFC 4226. This creates a simple way to protect a web site with one-time passwords, using any RFC 4226-compliant hardware or software token device.

mod_authn_otp supports both event and time based one-time passwords. It also supports "lingering" which allows the repeated re-use of a previously used one-time password up to a configurable maximum linger time. This allows one-time passwords to be used directly in HTTP authentication without forcing the user to enter a new one-time password for every page load.

mod_authn_otp supports both basic and digest authentication, and will auto-synchronize with the user's token within a configurable maximum offset (auto-synchronization is not supported with digest authentication).

mod_authn_otp is especially useful for setting up protected web sites that require more security than simple username/password authentication yet also don't require users to install special VPN software, and is compatible with software tokens that run on cell phones.