feat: implement external rekeying via nix run ".#rekey"

apps/rekey-output-derivation.nix

@@ -0,0 +1,24 @@
+pkgs: config:
+with pkgs.lib;
+  pkgs.stdenv.mkDerivation rec {
+    pname = "host-secrets";
+    version = "1.0";
+    description = "Rekeyed secrets for this host.";
+
+    srcs = mapAttrsToList (_: x: x.file) config.rekey.secrets;
+    sourcePath = ".";
+    # Required as input to have the derivation rebuild if this changes
+    hostPubkey = let
+      pubkey = config.rekey.hostPubkey;
+    in
+      if isPath pubkey
+      then readFile pubkey
+      else pubkey;
+
+    dontMakeSourcesWritable = true;
+    dontUnpack = true;
+    dontConfigure = true;
+    dontBuild = true;
+
+    installPhase = ''cp -r "/tmp/nix-rekey/${builtins.hashString "sha1" hostPubkey}/." "$out"'';
+  }

apps/rekey.nix

@@ -0,0 +1,60 @@
+{
+  self,
+  nixpkgs,
+  ...
+}: system:
+with nixpkgs.lib; let
+  pkgs = self.pkgs.${system};
+  toPubkey = pubkey:
+    if isPath pubkey
+    then readFile pubkey
+    else pubkey;
+
+  rekeyCommandsForHost = hostName: hostAttrs: let
+    hostPubkeyStr = toPubkey hostAttrs.config.rekey.hostPubkey;
+    secretDir = "/tmp/nix-rekey/${builtins.hashString "sha1" hostPubkeyStr}";
+    rekeyCommand = secretName: secretAttrs: let
+      masterIdentityArgs = concatMapStrings (x: ''-i "${x}" '') hostAttrs.config.rekey.masterIdentityPaths;
+      secretOut = "${secretDir}/${secretName}.age";
+    in ''
+      echo "Rekeying ${secretName} for host ${hostName}"
+      ${pkgs.rage}/bin/rage ${masterIdentityArgs} -d ${secretAttrs.file} \
+        | ${pkgs.rage}/bin/rage -r "${hostPubkeyStr}" -o "${secretOut}" -e \
+        || { \
+          echo "Failed to rekey secret ${secretName} for ${hostName}!" ; \
+          echo "This is a dummy replacement value. The actual secret could not be rekeyed." \
+            | ${pkgs.rage}/bin/rage -r "${hostPubkeyStr}" -o "${secretOut}" -e ; \
+        }
+    '';
+  in ''
+    mkdir -p "${secretDir}"
+    # Enable selected age plugins for this host
+    export PATH="$PATH${concatMapStrings (x: ":${x}/bin") hostAttrs.config.rekey.agePlugins}"
+    ${concatStringsSep "\n" (mapAttrsToList rekeyCommand hostAttrs.config.rekey.secrets)}
+  '';
+
+  rekeyScript = pkgs.writeShellScript "rekey" ''
+    set -euo pipefail
+    ${concatStringsSep "\n" (mapAttrsToList rekeyCommandsForHost self.nixosConfigurations)}
+    nix run --extra-sandbox-paths /tmp "${../.}#rekey-save-outputs";
+  '';
+
+  rekeySaveOutputsScript = let
+    copyHostSecrets = hostName: hostAttrs: let
+      drv = import ./rekey-output-derivation.nix pkgs hostAttrs.config;
+    in ''echo "Stored rekeyed secrets for ${hostAttrs.config.networking.hostName} in ${drv}"'';
+  in
+    pkgs.writeShellScript "rekey-save-outputs" ''
+      set -euo pipefail
+      ${concatStringsSep "\n" (mapAttrsToList copyHostSecrets self.nixosConfigurations)}
+    '';
+in {
+  rekey = {
+    type = "app";
+    program = "${rekeyScript}";
+  };
+  rekey-save-outputs = {
+    type = "app";
+    program = "${rekeySaveOutputsScript}";
+  };
+}

flake.nix

@@ -55,7 +55,7 @@
       homeConfigurations = import ./nix/home-manager.nix inputs;
       nixosConfigurations = import ./nix/nixos.nix inputs;
     }
-    // flake-utils.lib.eachSystem ["aarch64-linux" "x86_64-linux"] (localSystem: {
+    // flake-utils.lib.eachSystem ["aarch64-linux" "x86_64-linux"] (localSystem: rec {
       checks = import ./nix/checks.nix inputs localSystem;
       devShells.default = import ./nix/dev-shell.nix inputs localSystem;
 
@@ -75,5 +75,7 @@
         ];
         config.allowUnfree = true;
       };
+
+      apps = {} // import ./apps/rekey.nix inputs localSystem;
     });
 }

hosts/nom/net.nix

@@ -1,7 +1,6 @@
 {
   networking = {
     hostId = "4313abca";
-    hostName = "nom";
     wireless.iwd.enable = true;
   };
 

hosts/ward/net.nix

@@ -1,7 +1,6 @@
 {
   networking = {
     hostId = "49ce3b71";
-    hostName = "ward";
     wireless.iwd.enable = true;
   };
 

modules/core/default.nix

@@ -28,7 +28,6 @@ in {
   security.sudo.enable = false;
 
   rekey.hostPubkey = ../../secrets/pubkeys + "/${config.networking.hostName}.pub";
-  rekey.hostPrivkey = lib.head (map (e: e.path) (lib.filter (e: e.type == "ed25519") config.services.openssh.hostKeys));
   rekey.masterIdentityPaths = [../../secrets/yk1-nix-rage.pub];
   rekey.agePlugins = with pkgs; [age-plugin-yubikey];
 

modules/core/rekey.nix

@@ -6,69 +6,15 @@
   ...
 }:
 with lib; {
-  # TODO add /run to sandbox only for THIS derivation
-  # TODO interactiveness how
-  # TODO yubikeySupport=true convenience option
-  # TODO rekeyed secrets should only strip store path, not do basename
-
   config = let
-    rekeyedSecrets = pkgs.stdenv.mkDerivation rec {
-      pname = "host-secrets";
-      version = "1.0.0";
-      description = "Rekeyed secrets for this host.";
-
-      allSecrets = mapAttrsToList (_: value: value.file) config.rekey.secrets;
-      hostPubkeyStr =
-        if isPath config.rekey.hostPubkey
-        then readFile config.rekey.hostPubkey
-        else config.rekey.hostPubkey;
-
-      dontMakeSourcesWritable = true;
-      dontUnpack = true;
-      dontConfigure = true;
-      dontBuild = true;
-
-      installPhase = let
-        masterIdentityArgs = concatMapStrings (x: ''-i "${x}" '') config.rekey.masterIdentityPaths;
-        rekeyCommand = secret: ''
-          echo "Rekeying ${secret}" >&2
-          ${pkgs.rage}/bin/rage ${masterIdentityArgs} -d ${secret} \
-            | ${pkgs.rage}/bin/rage -r "${hostPubkeyStr}" -o "$out/${baseNameOf secret}" -e \
-            || { echo 1 > $out/status ; echo "error while rekeying secret!" | ${pkgs.rage}/bin/rage -r "${hostPubkeyStr}" -o "$out/${baseNameOf secret}" -e ; }
-        '';
-      in ''
-        set -euo pipefail
-        mkdir "$out"
-        echo 0 > $out/status
-
-        # Enable selected age plugins
-        export PATH="$PATH${concatMapStrings (x: ":${x}/bin") config.rekey.agePlugins}"
-
-        ${concatStringsSep "\n" (map rekeyCommand allSecrets)}
-      '';
-    };
-    rekeyedSecretPath = secret: "${rekeyedSecrets}/${baseNameOf secret}";
+    drv = import ../../apps/rekey-output-derivation.nix pkgs config;
   in
     mkIf (config.rekey.secrets != {}) {
-      environment.systemPackages = with pkgs; [rage];
-
-      # This polkit rule allows the nixbld users to access the pcsc-lite daemon,
-      # which is necessary to rekey the secrets.
-      security.polkit.extraConfig = mkIf (elem pkgs.age-plugin-yubikey config.rekey.agePlugins) ''
-        polkit.addRule(function(action, subject) {
-          if ((action.id == "org.debian.pcsc-lite.access_pcsc" || action.id == "org.debian.pcsc-lite.access_card") &&
-              subject.user.match(/^nixbld\d+$/))
-            return polkit.Result.YES;
-        });
-      '';
-
-      age.secrets =
       # Produce a rekeyed age secret for each of the secrets defined in our secrets
-        mapAttrs (_:
-          mapAttrs (name: value:
-            if name == "file"
-            then rekeyedSecretPath value
-            else value))
+      age.secrets = mapAttrs (secretName:
+        flip mergeAttrs {
+          file = "${drv}/${secretName}";
+        })
       config.rekey.secrets;
 
       assertions = [
@@ -88,7 +34,12 @@ with lib; {
       warnings = let
         hasGoodSuffix = x: strings.hasSuffix ".age" x || strings.hasSuffix ".pub" x;
       in
-        optional (toInt (readFile "${rekeyedSecrets}/status") == 1) "Failed to rekey secrets! Run nix log ${rekeyedSecrets}.drv for more information."
+        # drv.drvPath doesn't force evaluation, which allows the warning to be displayed
+        # before the error occurs
+        optional (!pathExists (removeSuffix ".drv" drv.drvPath)) ''
+          The secrets have not yet been rekeyed!
+          Be sure to run `nix run ".#rekey"` after changing your secrets!
+        ''
         ++ optional (!all hasGoodSuffix config.rekey.masterIdentityPaths)
         ''
           It seems like at least one of your rekey.masterIdentityPaths contains an
@@ -115,15 +66,6 @@ with lib; {
       #example = "age159tavn5rcfnq30zge2jfq4yx60uksz8udndp0g3njzhrns67ca5qq3n0tj";
       example = /etc/ssh/ssh_host_ed25519_key.pub;
     };
-    rekey.hostPrivkey = mkOption {
-      # Str to prevent privkeys from entering the nix store
-      type = types.str;
-      description = ''
-        The age identity (private key) that should be used to decrypt the secrets on the target machine.
-        This corresponds to age.identityPaths and must match the pubkey set in rekey.hostPubkey.
-      '';
-      example = head (map (e: e.path) (filter (e: e.type == "ed25519") config.services.openssh.hostKeys));
-    };
     rekey.masterIdentityPaths = mkOption {
       type = types.listOf types.path;
       description = ''

nix/nixos.nix

@@ -20,10 +20,13 @@
     };
   };
 
-  genConfiguration = hostname: {hostPlatform, ...}:
+  genConfiguration = hostName: {hostPlatform, ...}:
     lib.nixosSystem {
       modules = [
-        (../hosts + "/${hostname}")
+        (../hosts + "/${hostName}")
+        # Set hostName to same value as key in nixosConfigurations
+        {networking.hostName = hostName;}
+        # Use correct pkgs definition
         {
           nixpkgs.pkgs = self.pkgs.${hostPlatform};
           # FIXME: This shouldn't be needed, but is for some reason