yubikey-agent: init at 0.1.3

This adds yubikey-agent as a package and a nixos module.

On macOS, we use `wrapProgram` to set pinentry_mac as default in PATH;
on Linux we rely on the user to set their preferred pinentry in PATH.
In particular, we use a systemd override to prefix PATH to select a
chosen pinentry program if specified.

On Linux, we need libnotify to provide the notify-send utility for
desktop notifications (such as "Waiting for Yubikey touch...").

This might work on other flavors of unix, but I haven't tested.

We reuse the programs.gnupg.agent.pinentryFlavor option for
yubikey-agent, but in doing so I hit a problem: pinentryFlavour's
default value is specified in a mkDefault, but only conditionally.  We
ought to be able to pick up the pinentryFlavour whether or not gpg-agent
is running.  As a result, this commit moves the default value to the
definition of programs.gnupg.agent.enable.

nixos/modules/module-list.nix

@@ -811,6 +811,7 @@
   ./services/security/torsocks.nix
   ./services/security/usbguard.nix
   ./services/security/vault.nix
+  ./services/security/yubikey-agent.nix
   ./services/system/cloud-init.nix
   ./services/system/dbus.nix
   ./services/system/earlyoom.nix

nixos/modules/programs/gnupg.nix

@@ -70,6 +70,7 @@ in
     agent.pinentryFlavor = mkOption {
       type = types.nullOr (types.enum pkgs.pinentry.flavors);
       example = "gnome3";
+      default = defaultPinentryFlavor;
       description = ''
         Which pinentry interface to use. If not null, the path to the
         pinentry binary will be passed to gpg-agent via commandline and
@@ -91,8 +92,6 @@ in
   };
 
   config = mkIf cfg.agent.enable {
-    programs.gnupg.agent.pinentryFlavor = mkDefault defaultPinentryFlavor;
-
     # This overrides the systemd user unit shipped with the gnupg package
     systemd.user.services.gpg-agent = mkIf (cfg.agent.pinentryFlavor != null) {
       serviceConfig.ExecStart = [ "" ''

nixos/modules/services/security/yubikey-agent.nix

@@ -0,0 +1,61 @@
+# Global configuration for yubikey-agent.
+
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.yubikey-agent;
+
+  # reuse the pinentryFlavor option from the gnupg module
+  pinentryFlavor = config.programs.gnupg.agent.pinentryFlavor;
+in
+{
+  ###### interface
+
+  meta.maintainers = with maintainers; [ philandstuff rawkode ];
+
+  options = {
+
+    services.yubikey-agent = {
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to start yubikey-agent when you log in.  Also sets
+          SSH_AUTH_SOCK to point at yubikey-agent.
+
+          Note that yubikey-agent will use whatever pinentry is
+          specified in programs.gnupg.agent.pinentryFlavor.
+        '';
+      };
+
+      package = mkOption {
+        type = types.package;
+        default = pkgs.yubikey-agent;
+        defaultText = "pkgs.yubikey-agent";
+        description = ''
+          The package used for the yubikey-agent daemon.
+        '';
+      };
+    };
+  };
+
+  config = {
+    environment.systemPackages = [ cfg.package ];
+    systemd.packages = [ cfg.package ];
+
+    # This overrides the systemd user unit shipped with the
+    # yubikey-agent package
+    systemd.user.services.yubikey-agent = mkIf (pinentryFlavor != null) {
+      path = [ pkgs.pinentry.${pinentryFlavor} ];
+    };
+
+    environment.extraInit = optionalString cfg.enable
+      ''
+        if [ -z "$SSH_AUTH_SOCK" -a -n "$XDG_RUNTIME_DIR" ]; then
+          export SSH_AUTH_SOCK="$XDG_RUNTIME_DIR/yubikey-agent/yubikey-agent.sock"
+        fi
+      '';
+  };
+}