Enables U2F PAM (pam-u2f) module.
If set, users listed in $XDG_CONFIG_HOME/Yubico/u2f_keys (or $HOME/.config/Yubico/u2f_keys if XDG variable is not set) are able to log in with the associated U2F key. The path can be changed using security.pam.u2f.authFile option.
File format is: username:first_keyHandle,first_public_key: second_keyHandle,second_public_key This file can be generated using pamu2fcfg command.
More information can be found here.