feat: setup sops-nix

modules/nixos/core/sops.nix

@@ -0,0 +1,25 @@
+{ inputs, ... }:
+{
+  flake.modules.nixos.core =
+    {
+      config,
+      pkgs,
+      ...
+    }:
+    let
+      isEd25519 = k: k.type == "ed25519";
+      getKeyPath = k: k.path;
+      keys = builtins.filter isEd25519 config.services.openssh.hostKeys;
+    in
+    {
+      imports = [ inputs.sops-nix.nixosModules.sops ];
+
+      sops.age = {
+        sshKeyPaths = map getKeyPath keys;
+        keyFile = "/var/lib/sops-nix/key.txt";
+        generateKey = true;
+      };
+
+      environment.systemPackages = with pkgs; [ sops ];
+    };
+}