diff --git a/modules/environment/files.nix b/modules/environment/files.nix
deleted file mode 100644
index e1c5cb0..0000000
--- a/modules/environment/files.nix
+++ /dev/null
@@ -1,30 +0,0 @@
-# Copyright (c) 2019-2020, see AUTHORS. Licensed under MIT License, see LICENSE.
-
-{ config, lib, pkgs, ... }:
-
-with lib;
-
-{
-
-  ###### interface
-
-  options = {
-
-  };
-
-
-  ###### implementation
-
-  config = {
-
-    environment.etc = {
-      "nix/nix.conf".text = ''
-        sandbox = false
-        substituters = https://cache.nixos.org https://nix-on-droid.cachix.org
-        trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= nix-on-droid.cachix.org-1:56snoMJTXmDRC1Ei24CmKoUqvHJ9XCp+nidK7qkMQrU=
-      '';
-    };
-
-  };
-
-}
diff --git a/modules/environment/nix.nix b/modules/environment/nix.nix
new file mode 100644
index 0000000..9938b7b
--- /dev/null
+++ b/modules/environment/nix.nix
@@ -0,0 +1,73 @@
+# Copyright (c) 2019-2020, see AUTHORS. Licensed under MIT License, see LICENSE.
+
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.nix;
+in
+
+{
+
+  ###### interface
+
+  options = {
+
+    nix = {
+      substituters = mkOption {
+        type = types.listOf types.str;
+        default = [];
+        description = ''
+          A list of URLs of substituters.  The official NixOS and nix-on-droid
+          substituters are added by default.
+        '';
+      };
+
+      trustedPublicKeys = mkOption {
+        type = types.listOf types.str;
+        default = [];
+        description = ''
+          A list of public keys.  When paths are copied from another Nix store (such as a
+          binary cache), they must be signed with one of these keys.  The official NixOS
+          and nix-on-droid public keys are added by default.
+        '';
+      };
+
+      extraConfig = mkOption {
+        type = types.lines;
+        default = "";
+        description = "Extra config to be appended to /etc/nix/nix.conf.";
+      };
+    };
+
+  };
+
+
+  ###### implementation
+
+  config = {
+
+    environment.etc = {
+      "nix/nix.conf".text = ''
+        sandbox = false
+        substituters = ${concatStringsSep " " cfg.substituters}
+        trusted-public-keys = ${concatStringsSep " " cfg.trustedPublicKeys}
+        ${cfg.extraConfig}
+      '';
+    };
+
+    nix = {
+      substituters = [
+        "https://cache.nixos.org"
+        "https://nix-on-droid.cachix.org"
+      ];
+      trustedPublicKeys = [
+        "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+        "nix-on-droid.cachix.org-1:56snoMJTXmDRC1Ei24CmKoUqvHJ9XCp+nidK7qkMQrU="
+      ];
+    };
+
+  };
+
+}
diff --git a/modules/module-list.nix b/modules/module-list.nix
index aa2392a..cbac88f 100644
--- a/modules/module-list.nix
+++ b/modules/module-list.nix
@@ -6,10 +6,10 @@
   ./build/activation.nix
   ./build/config.nix
   ./environment/etc
-  ./environment/files.nix
   ./environment/links.nix
   ./environment/login
   ./environment/networking.nix
+  ./environment/nix.nix
   ./environment/path.nix
   ./environment/session-init.nix
   ./home-manager.nix