https://otp2.org/
OTP2 - RFC 6238: Time-based one-time password tokens
rfc 6238timebasedonepassword
https://openvpn.net/as-docs/tutorials/tutorial--hardware-tokens-on-access-server-mfa.html
Tutorial: Use TOTP Hardware Tokens for MFA (RFC 6238) in Access Server (YubiKey)
Use hardware tokens such as YubiKey for TOTP-based multi-factor authentication (MFA) in OpenVPN Access Server. Learn how to enable MFA and enroll a hardware...