The URL for Yubikey Validation server. OTP protocol specific part will be appended to the ValidationServerURL. Defaults to http://127.0.0.1:8003/yhsm/validate? which is compatible with Yubico's PyHSM validation server yhsm-val.

HTTPS requires LWP::Protocol::https Perl module. If it's not installed, Radiator will log an error without connecting to the server.

# Use Yubico's service over HTTPS
ValidationServerURL https://api.yubico.com/wsapi/2.0/verify?