sops: fix config for darwin

home/modules/sops.nix

@@ -1,7 +1,32 @@
-{ inputs, ... }:
 {
-  sops = {
-    age.keyFile = "/home/user/.age-key.txt"; # must have no password!
-    # deliberately not setting `defaultSopsFile` because there is no clear file-hostname-mapping. Each separate home config has to configure this explicitly.
-  };
-}
+  inputs,
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  homeKeys =
+    if pkgs.stdenv.isDarwin then
+      "/Users/${config.home.username}/Library/Application Support/sops/age/keys.txt"
+    else
+      "/home/${config.home.username}/.config/sops/age/keys.txt";
+in
+lib.mkMerge [
+  {
+    home.ensureDirs."${builtins.dirOf homeKeys}".mode = "0700";
+    sops = {
+      age.keyFile = "/home/user/.age-key.txt"; # must have no password!
+      # deliberately not setting `defaultSopsFile` because there is no clear file-hostname-mapping. Each separate home config has to configure this explicitly.
+    };
+  }
+  # linux machines: assumption: there is an OpenSSH server of which we are able to use the hostkey, like at the NixOS module. The `keyDir` is only used for the private admin key.
+  (lib.mkIf pkgs.stdenv.isLinux {
+    sops.age.sshKeyPaths = [ "/etc/ssh/ssh_host_ed25519_key" ];
+  })
+
+  # darwin: no SSH server, no hostkey => let's use the `keyDir` key both for encryption and decrpytion
+  (lib.mkIf pkgs.stdenv.isDarwin {
+    sops.age.keyFile = homeKeys;
+  })
+]