issuer.nix

# A NixOS module which can run a Ristretto-based issuer for PrivateStorage
# ZKAPs.
{ lib, pkgs, config, ... }: let
  cfg = config.services.private-storage-issuer;
  # Our own nixpkgs fork:
  ourpkgs = import ../../nixpkgs-ps.nix {};
in {
  imports = [
    # Give it a good SSH configuration.
    ../../nixos/modules/ssh.nix
  ];

  options = {
    services.private-storage-issuer.enable = lib.mkEnableOption "PrivateStorage ZKAP Issuer Service";
    services.private-storage-issuer.package = lib.mkOption {
      default = ourpkgs.zkapissuer.components.exes."PaymentServer-exe";
      type = lib.types.package;
      example = lib.literalExample "pkgs.zkapissuer.components.exes.\"PaymentServer-exe\"";
      description = ''
        The package to use for the ZKAP issuer.
      '';
    };
    services.private-storage-issuer.domains = lib.mkOption {
      type = lib.types.listOf lib.types.str;
      example = lib.literalExample [ "payments.example.com" ];
      description = ''
        The domain names at which the issuer is reachable.
      '';
    };
    services.private-storage-issuer.tls = lib.mkOption {
      default = true;
      type = lib.types.bool;
      description = ''
        Whether or not to listen on TLS.  For real-world use you should always
        listen on TLS.  This is provided as an aid to automated testing where
        it might be difficult to obtain a real certificate.
      '';
    };
    services.private-storage-issuer.issuer = lib.mkOption {
      default = "Ristretto";
      type = lib.types.enum [ "Trivial" "Ristretto" ];
      example = lib.literalExample "Trivial";
      description = ''
        The issuer algorithm to use.  Either Trivial for a fake no-crypto
        algorithm or Ristretto for Ristretto-flavored PrivacyPass.
      '';
    };
    services.private-storage-issuer.ristrettoSigningKeyPath = lib.mkOption {
      default = null;
      type = lib.types.path;
      description = ''
        The path to a file containing the Ristretto signing key to use.
        Required if the issuer is ``Ristretto``.
      '';
    };
    services.private-storage-issuer.stripeSecretKeyPath = lib.mkOption {
      type = lib.types.path;
      description = ''
        The path to a file containing a Stripe secret key to use for charge
        and payment management.
      '';
    };
    services.private-storage-issuer.stripeEndpointDomain = lib.mkOption {
      type = lib.types.str;
      description = ''
        The domain name for the Stripe API HTTP endpoint.
      '';
      default = "api.stripe.com";
    };
    services.private-storage-issuer.stripeEndpointScheme = lib.mkOption {
      type = lib.types.enum [ "HTTP" "HTTPS" ];
      description = ''
        Whether to use HTTP or HTTPS for the Stripe API.
      '';
      default = "HTTPS";
    };
    services.private-storage-issuer.stripeEndpointPort = lib.mkOption {
      type = lib.types.int;
      description = ''
        The port number for the Stripe API HTTP endpoint.
      '';
      default = 443;
    };
    services.private-storage-issuer.database = lib.mkOption {
      default = "Memory";
      type = lib.types.enum [ "Memory" "SQLite3" ];
      description = ''
        The kind of voucher database to use.
      '';
    };
    services.private-storage-issuer.databasePath = lib.mkOption {
      default = null;
      type = lib.types.str;
      description = ''
        The path to a database file in the filesystem, if the SQLite3 database
        type is being used.
      '';
    };
    services.private-storage-issuer.letsEncryptAdminEmail = lib.mkOption {
      type = lib.types.str;
      description = ''
        An email address to give to Let's Encrypt as an operational contact
        for the service's TLS certificate.
      '';
    };
    services.private-storage-issuer.allowedChargeOrigins = lib.mkOption {
      type = lib.types.listOf lib.types.str;
      description = ''
        The CORS "Origin" values which are allowed to submit charges to the
        payment server.  Note this is not currently enforced by the
        PaymentServer.  It just controls the CORS headers served.
      '';
    };
  };

  config =
    let
      certroot = "/var/lib/letsencrypt/live";
      # We'll refer to this collection of domains by the first domain in the
      # list.
      domain = builtins.head cfg.domains;
      certServiceName = "cert-${domain}";
    in lib.mkIf cfg.enable {
    # Add a systemd service to run PaymentServer.
    systemd.services.zkapissuer = {
      enable = true;
      description = "ZKAP Issuer";
      wantedBy = [ "multi-user.target" ];

      # Make sure we have a certificate the first time, if we are running over
      # TLS and require a certificate.
      requires = lib.optional cfg.tls "${certServiceName}.service";

      after = [
        # Make sure there is a network so we can bind to all of the
        # interfaces.
        "network.target"
      ] ++
        # Make sure we run after the certificate is issued, if we are running
        # over TLS and require a certificate.
        lib.optional cfg.tls "${certServiceName}.service";

      # It really shouldn't ever exit on its own!  If it does, it's a bug
      # we'll have to fix.  Restart it and hope it doesn't happen too much
      # before we can fix whatever the issue is.
      serviceConfig.Restart = "always";
      serviceConfig.Type = "simple";

      script =
        let
          # Compute the right command line arguments to pass to it.  The
          # signing key is only supplied when using the Ristretto issuer.
          issuerArgs =
            if cfg.issuer == "Trivial"
              then "--issuer Trivial"
              else "--issuer Ristretto --signing-key-path ${cfg.ristrettoSigningKeyPath}";
          databaseArgs =
            if cfg.database == "Memory"
              then "--database Memory"
              else "--database SQLite3 --database-path ${cfg.databasePath}";
          httpsArgs =
            if cfg.tls
            then
              "--https-port 443 " +
              "--https-certificate-path ${certroot}/${domain}/cert.pem " +
              "--https-certificate-chain-path ${certroot}/${domain}/chain.pem " +
              "--https-key-path ${certroot}/${domain}/privkey.pem"
            else
              # Only for automated testing.
              "--http-port 80";

          prefixOption = s: "--cors-origin=" + s;
          originStrings = map prefixOption cfg.allowedChargeOrigins;
          originArgs = builtins.concatStringsSep " " originStrings;

          stripeArgs =
            "--stripe-key-path ${cfg.stripeSecretKeyPath} " +
            "--stripe-endpoint-domain ${cfg.stripeEndpointDomain} " +
            "--stripe-endpoint-scheme ${cfg.stripeEndpointScheme} " +
            "--stripe-endpoint-port ${toString cfg.stripeEndpointPort}";
        in
          "${cfg.package}/bin/PaymentServer-exe ${originArgs} ${issuerArgs} ${databaseArgs} ${httpsArgs} ${stripeArgs}";
    };

    # Certificate renewal.  A short-lived service meant to be repeatedly
    # activated to request a new certificate be issued, if the current one is
    # close to expiring.
    systemd.services.${certServiceName} = {
      enable = cfg.tls;
      description = "Certificate ${domain}";
      serviceConfig = {
        ExecStart =
        let
          configArgs = "--config-dir /var/lib/letsencrypt --work-dir /var/run/letsencrypt --logs-dir /var/run/log/letsencrypt";
        in
          pkgs.writeScript "cert-${domain}-start.sh" ''
          #!${pkgs.runtimeShell} -e
          # Register if necessary.
          ${pkgs.certbot}/bin/certbot register ${configArgs} --non-interactive --agree-tos -m ${cfg.letsEncryptAdminEmail} || true
          # Obtain the certificate.
          ${pkgs.certbot}/bin/certbot certonly ${configArgs} --non-interactive --standalone --expand --domains ${builtins.concatStringsSep "," cfg.domains}
          '';
      };
    };

    # Periodically trigger the certificate renewal service.
    systemd.timers.${certServiceName} = {
      enable = cfg.tls;
      timerConfig = {
        # "Defines a timer relative to when the unit the timer unit is
        # activating was last deactivated."
        #
        # Trigger the renewal service periodically.  Since it will activate
        # and then deactivate each time this timer triggers, this timer will
        # trigger it repeatedly.  The delay specified here is relative to the
        # last time the target unit is deactivated and that advances to the
        # current time after each time the trigger fires.
        OnUnitInactiveSec = "3d";

        # "Defines a timer relative to the moment the timer unit itself is
        # activated."
        #
        # Since at the time this timer is activated we're not sure whether the
        # renewal service has ever been activated or deactivated we don't know
        # when if or when the other trigger will fire.  This ensures that
        # shortly after this timer is activated it will trigger.  Thereafter,
        # the other trigger will take over for periodic re-triggering.
        OnActiveSec = "5m";
      };
    };

    # Open 80 and 443 for the certbot HTTP server and the PaymentServer HTTPS server.
    networking.firewall.allowedTCPPorts = [
      80
      443
    ];
  };
}