{
  config,
  lib,
  pkgs,
  ...
}: let
  inherit
    (lib)
    mkIf
    mkEnableOption
    ;
  cfg = config.services.mathebau-borgbackup;
in {
  imports = [];

  options.services.mathebau-borgbackup = {
    enable = mkEnableOption "mathebau borgbackup service";
  };

  config = mkIf cfg.enable {
    services.borgbackup = {
      # repos are made available at ssh://borg@hostname and served according to the presented ssh-key

      # If you think about adding keys of nix machines:
      # Congratulations, you are the first person to make backups from a nixos machine.
      # Your won the task of automatizing this endeavor, so in future we don't need to hand copy any
      # ssh keys anymore.
      repos = {
        aphoom-zhah = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIA8pI6uinXezAMH4vG2yEbu/yOYU5vXcsZN74tYgV+Wj Aphoom-Zhah Backup"
          ];
          path = "/var/lib/backups/aphoom-zhah";
          # subrepos are allowed because each vm creates at least one repo below this filepath and yibb-tstll even more
          allowSubRepos = true;
        };
        azathoth = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGBEwllQ77ktoirXX6dJ6ET8TfK4lzq0aaq+X4rrX2Vk Azathoth Backup"
          ];
          path = "/var/lib/backups/azathoth";
          allowSubRepos = true;
        };
        cthulhu = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIMSJl1MvabUADTdOCgufsBzn1tIIpxMq4iDcYZsaW1lV Cthulhu Backup"
          ];
          path = "/var/lib/backups/cthulhu";
          allowSubRepos = true;
        };
        eihort = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHLoDxtY4Tp6NKxLt9oHmWT6w4UpU6eA1TnPU2Ut83BN Eihort Backup"
          ];
          path = "/var/lib/backups/eihort";
          allowSubRepos = true;
        };
        fsaccount = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIG+Y7fQTYdIWHehrKdk92CaJ0AisEux4OrS4nIyMstU4 FS Account Backup"
          ];
          path = "/var/lib/backups/fsaccount";
          allowSubRepos = true;
        };
        hastur = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILeDvTyOUdIPARatX0PPhHgrV1gjERWLt2Twa8E2GETb Hastur Backupsystem"
          ];
          path = "/var/lib/backups/hastur";
          allowSubRepos = true;
        };
        ithaqua = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIPJmBf8cz3FTDdeuxWbp1MO2yPT5rvH8ZIGUzfogjpXi Ithaqua Backup"
          ];
          path = "/var/lib/backups/ithaqua";
          allowSubRepos = true;
        };
        kaalut = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIFcAJkEXcvrDEQf1zRhBXLe1CSHOTooM3qy0KMfS9oug Kaalut Backup"
          ];
          path = "/var/lib/backups/kaalut";
          allowSubRepos = true;
        };
        lobon = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICEptjf1UWRlo6DG9alAIRwkSDUAVHwDKkHC6/DeYKzi Lobon Backup"
          ];
          path = "/var/lib/backups/lobon";
          allowSubRepos = true;
        };
        sanctamariamaterdei = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIH9Le5OI4ympQ0mQKYHmxgxGF598rzpD5VVpWK1mGfd8 Sanctamariamaterdei Backupsystem"
          ];
          path = "/var/lib/backups/sanctamariamaterdei";
          allowSubRepos = true;
        };
        tsathoggua = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKS9/1lFOhv+3sNuGcysM3TYh2xRrjMeAZX3K7CBx0QW Tsathoggua Backup"
          ];
          path = "/var/lib/backups/tsathoggua";
          allowSubRepos = true;
        };
        uvhash = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIB8DjIqgFgmYhQnTLpbqL0r7xBPb8TPy6SO5RhQ31OGj Uvhash Backup"
          ];
          path = "/var/lib/backups/uvhash";
          allowSubRepos = true;
        };
        yibb-tstll = {
          authorizedKeysAppendOnly = [
            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAINlnGOV58Ks9lu+WTI4F7QAHtDrJq2jY8ZocITZG8K0+ Yibb-Tstll Backup"
          ];
          path = "/var/lib/backups/yibb-tstll";
          allowSubRepos = true;
        };
      };
      # Configure backup of files on the department's fs account:
      # This job first copies the files to the local account 'fsaccount' in tmpfs
      # and then takes a regular backup of the mirrored folder.

      # See also https://borgbackup.readthedocs.io/en/stable/deployment/pull-backup.html
      # which does not work due to missing permissions.
      jobs.fsaccount = {
        preHook = ''
          mkdir -p /home/fsaccount/sicherung # Create if it does not exist
          ${pkgs.rsync}/bin/rsync --rsh='ssh -i /run/secrets/backupKey' --recursive --delete fachschaft@gw1.mathematik.tu-darmstadt.de:/home/fachschaft/* /home/fsaccount/sicherung
        '';
        paths = "/home/fsaccount/sicherung";
        encryption.mode = "none"; # Otherwise the key is next to the backup or we have human interaction.
        environment = {
          BORG_RSH = "ssh -i /run/secrets/backupKey";
          # “Borg ensures that backups are not created on random drives that ‘just happen’ to contain a Borg repository.”
          # https://borgbackup.readthedocs.io/en/stable/deployment/automated-local.html
          # We don't want this in order to not need to persist borg cache and simplify new deployments.
          BORG_UNKNOWN_UNENCRYPTED_REPO_ACCESS_IS_OK = "yes";
        };
        repo = "borg@localhost:fsaccount";
        startAt = "daily";
        user = "fsaccount";
        group = "users";
        readWritePaths = ["/home/fsaccount"];
      };
    };
    # Extra user for FS account backup
    users.users = {
      fsaccount = {
        description = "FS Account backup";
        isSystemUser = true;
        home = "/home/fsaccount";
        createHome = true;
        group = "users";
      };
    };
    environment.persistence.${config.impermanence.name} = {
      users.fsaccount.files = [
        {
          file = ".ssh/known_hosts";
          parentDirectory = {
            mode = "u=rwx,g=,o=";
            user = "fsaccount";
            group = "users";
          };
        }
      ];
    };
  };
}