{ config, pkgs, ... }:

{
  # Enable SSH server.
  services.openssh = {
    enable = true;
    # Forbid root login through SSH.
    settings.PermitRootLogin = "no";
    # Use authorized keys only.
    settings.PasswordAuthentication = false;
  };

  # Use NitroKey USB smartcard with SSH.
  # https://nixos.wiki/wiki/Nitrokey
  #
  # Restart gpg-agent after config change.
  # Otherwise there might be a gpg error about "no pinentry".
  # https://discourse.nixos.org/t/cant-get-gnupg-to-work-no-pinentry/15373/19
  # But how to restart it? Maybe this?
  # systemctl --user restart gpg-agent

  # Not sure if this is needed: Reload udev rules.
  # sudo -- udevadm control --reload-rules && udevadm trigger
  #
  # Not sure if this is needed:
  # killall gpg-agent

  # TODO: gpg-agent pinentry problem
  # https://github.com/NixOS/nixpkgs/issues/97861
  #
  # gpgconf --check-programs
  #=> gpgconf: error running '/nix/store/lvsbmqy4dmlri22145hbr6799hgbnpnf-gnupg-2.4.0/bin/pinentry': probably not installed
  #
  # ssh -v nas
  #=> OpenSSH_9.3p2, OpenSSL 3.0.10 1 Aug 2023
  #=> debug1: Reading configuration data /home/yoda/.ssh/config
  #=> debug1: /home/yoda/.ssh/config line 67: Applying options for nas
  #=> debug1: /home/yoda/.ssh/config line 180: Applying options for *
  #=> debug1: Reading configuration data /etc/ssh/ssh_config
  #=> debug1: Executing command: '/nix/store/8fv91097mbh5049i9rglc73dx6kjg3qk-bash-5.2-p15/bin/bash -c '/nix/store/lvsbmqy4dmlri22145hbr6799hgbnpnf-gnupg-2.4.0/bin/gpg-connect-agent --quiet updatestartuptty /bye >/dev/null 2>&1''
  #
  #=> USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
  #=> yoda        2752  0.0  0.0 444812  3040 ?        SLsl 16:09   0:00 /nix/store/lvsbmqy4dmlri22145hbr6799hgbnpnf-gnupg-2.4.0/bin/gpg-agent --supervised --pinentry-program /nix/store/8cvidvpwnwyxixlhqfaa5jlfndh2vir5-pinentry-1.2.1-curses/bin/pinentry

  # NITROKEY SSH WORKAROUND (I): Do all of this in one shell!
  # CREDITS: https://unix.stackexchange.com/a/250045/315162
  #
  # BEFORE: SSH_AUTH_SOCK=/run/user/1000/keyring/ssh
  # AFTER:  SSH_AUTH_SOCK=/run/user/1000/gnupg/S.gpg-agent.ssh
  #
  # systemctl --user stop gpg-agent
  # systemctl --user stop gpg-agent.socket
  # systemctl --user stop gpg-agent-ssh.socket
  # ps -aux | grep -v grep | grep gpg-agent
  # => NONE
  # eval $(gpg-agent --daemon --pinentry-program /nix/store/5j87jnmfh19xlq9ij0v3rh7cwssr4586-pinentry-1.2.1-curses/bin/pinentry --enable-ssh-support --sh)
  # echo $SSH_AUTH_SOCK
  #=> /run/user/1000/gnupg/S.gpg-agent.ssh
  # gpg -d ./passphrase.txt.gpg
  #=> Works!
  # ssh nas
  #=> Works!

  # NITROKEY SSH WORKAROUND (II)
  #
  # export SSH_AUTH_SOCK=/run/user/1000/gnupg/S.gpg-agent.ssh
  # ssh nas
  #=> Works!

  # TODO: What is the difference between programs.gnupg.agent.enableSSHSupport and
  #       services.gpg-agent.enableSshSupport = true;

  services.udev.packages = [ pkgs.nitrokey-udev-rules ];
  programs = {
    ssh.startAgent = false;
    gnupg.agent = {
      enable = true;
      # ... Also sets SSH_AUTH_SOCK environment variable correctly.
      enableSSHSupport = true;
      pinentryFlavor = "curses";
    };
  };

#  users.users.yoda = {
#    packages = with pkgs; [
#      pinentry-curses
#    ];
#  };
#  environment.systemPackages = with pkgs; [
#    pinentry-curses
#  ];

  # Smartcard daemon.
  services.pcscd.enable = true;

  home-manager.users.yoda = { osConfig, config, pkgs, ... }: {
    # SSH configuration.
    programs.ssh = {
      enable = true;
      userKnownHostsFile = "~/.ssh/known_hosts ${../assets/ssh/known_hosts}";
      matchBlocks = {
        "nas" = {
          hostname = "p1st.de";
          user = "yoda";
          port = 2222;
          compression = true;
        };
        "rootnas" = {
          hostname = "p1st.de";
          user = "root";
          port = 2222;
          compression = true;
        };
      };
    };

    # GnuPG configuration.
    # Examples:
    #   https://github.com/ioerror/duraconf
    #   https://gist.github.com/graffen/37eaa2332ee7e584bfda
    programs.gpg = {
      enable = true;
      settings = {
        # Display long key IDs
        keyid-format = "0xlong";

        # List all keys (or the specified ones) along with their fingerprints
        with-fingerprint = true;

        # Display the calculated validity of user IDs during key listings.
        list-options = "show-uid-validity";
        verify-options = "show-uid-validity";
      };
      publicKeys = [
        {
          source = "${../assets/gpg/pubkey_nitrokey.asc}";
          # ultimate
          trust = 5;
        }
      ];
    };
  };
}