Add your signing key as the GPG default and trusted key list

flake.nix

@@ -115,6 +115,7 @@
       homeConfigurations.josh = let
         profile = "desktop";
         username = "josh";
+        gpgSigningKey = null;
       in home-manager.lib.homeManagerConfiguration {
           extraSpecialArgs = {
             inherit inputs;
@@ -123,6 +124,9 @@
           };
         pkgs = pkgs;
         modules = [
+          {
+            modules.gpg.signingKey = gpgSigningKey;
+          }
           ./homes/josh/home.nix
           stylix.homeModules.stylix
         ];

homes/josh/home.nix

@@ -1,12 +1,13 @@
 {
   config,
-  gpgSigningKey,
   lib,
   pkgs,
   profile,
   ...
 }:
 with lib;
+let gpgCfg = config.modules.gpg;
+in
 {
   imports = [
     ../../modules/homes
@@ -34,7 +35,7 @@ with lib;
         name = "Joshua Yuen";
         email = "joshuayuen99@gmail.com";
         # We need to do this to pass through the (optional) signing key from the hosts/* file
-        signingkey = if gpgSigningKey != "" then gpgSigningKey else config.modules.gpg.signingKey;
+        signingkey = mkIf (gpgCfg.signingKey != null) gpgCfg.signingKey;
       };
       commit.gpgsign = true;
       init.defaultBranch = "main";

modules/core/user.nix

@@ -24,10 +24,10 @@ in
         profile
         username
         ;
-      gpgSigningKey = config.modules.gpg.signingKey;
     };
     users.${username} = {
       imports = [ ../../homes/${username}/home.nix ];
+      modules.gpg.signingKey = config.modules.gpg.signingKey;
     };
   };
 

modules/homes/gpg.nix

@@ -25,6 +25,11 @@ in
     programs = {
       gpg = {
         enable = true;
+
+        settings = {
+          default-key = mkIf (cfg.signingKey != null) cfg.signingKey;
+          trusted-key = mkIf (cfg.signingKey != null) cfg.signingKey;
+        };
       };
     };
   };

modules/options/gpg.nix

@@ -11,8 +11,8 @@ with lib;
     };
 
     signingKey = mkOption {
-      type = types.str;
-      default = "";
+      type = types.nullOr types.str;
+      default = null;
       description = "The key ID to sign with for git";
     };
   };