diff --git a/admin/class-robotstxt-manager-installer.php b/admin/class-robotstxt-manager-installer.php
index e21be60..3ad4bb3 100644
--- a/admin/class-robotstxt-manager-installer.php
+++ b/admin/class-robotstxt-manager-installer.php
@@ -323,7 +323,11 @@ class Robotstxt_Manager_Installer {
 		$result   = $upgrader->install(
 			$tmp_file,
 			array(
-				'overwrite' => $overwrite,
+				// Option key differs across WP versions: 'overwrite' (5.5-era)
+				// vs 'overwrite_package' (current). Pass both; the unused key
+				// is ignored by wp_parse_args().
+				'overwrite'         => $overwrite,
+				'overwrite_package' => $overwrite,
 			)
 		);
 
@@ -358,7 +362,11 @@ class Robotstxt_Manager_Installer {
 		}
 
 		if ( ! class_exists( 'Plugin_Upgrader' ) ) {
+			// class-wp-upgrader.php bundles WP_Upgrader + the skins (incl.
+			// Automatic_Upgrader_Skin), but Plugin_Upgrader itself lives in
+			// its own file since the WP 5.3 split — both are required.
 			require_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';
+			require_once ABSPATH . 'wp-admin/includes/class-plugin-upgrader.php';
 		}
 	}
 
diff --git a/changelog.txt b/changelog.txt
index 24095ca..4925b1c 100644
--- a/changelog.txt
+++ b/changelog.txt
@@ -1,5 +1,42 @@
 == Changelog ==
 
+= 0.5.3 =
+
+_Release date: 2026-08-15_
+
+**Added**
+
+* Authenticated encryption for the stored API key (same encrypt-then-MAC scheme as Core 1.6.0): tampered payloads fail closed, and encryption refuses to run without real WordPress salts. Legacy-stored keys keep decrypting and re-encrypt on the next save.
+
+**Changed**
+
+* Plugin version 0.5.2 → 0.5.3. No database schema changes (no custom tables).
+
+**Compatibility**
+
+* WordPress: 4.4 - 7.1 (scan-verified: wp-compat clean from 4.4)
+* PHP: 8.0 - 8.5 (scan-verified: PHPCompatibility + manual feature audit)
+
+= 0.5.2 =
+
+_Release date: 2026-08-15_
+
+**Fixed**
+
+* Manager-panel Install/Update failed while the native WordPress updater worked. Two defects in the panel's upgrader path (`Robotstxt_Manager_Installer`):
+  1. `ensure_plugin_functions()` required only `class-wp-upgrader.php`, which since the WordPress 5.3 class split no longer defines `Plugin_Upgrader` (it lives in `class-plugin-upgrader.php`). In a stock `admin-post.php` context nothing else loads it → fatal error. Both files are now required.
+  2. The overwrite flag was passed to `Plugin_Upgrader::install()` as `'overwrite'`, but current WordPress reads `'overwrite_package'` — the unknown key was silently discarded, `clear_destination` stayed false, and the install failed against the existing folder. Both keys are now passed (the unused one is ignored by `wp_parse_args()`), keeping compatibility across WP versions.
+* Verified end-to-end on the live store: panel-path download → overwrite-install succeeds, plugin remains active.
+
+**Changed**
+
+* Plugin version 0.5.1 → 0.5.2. No database schema changes (no custom tables).
+
+**Compatibility**
+
+* WordPress: 4.4 - 7.1 (scan-verified: wp-compat clean from 4.4)
+* PHP: 8.0 - 8.5 (scan-verified: PHPCompatibility + manual feature audit)
+
 = 0.5.1 =
 
 _Release date: 2026-08-15_
diff --git a/includes/class-robotstxt-manager-encryption.php b/includes/class-robotstxt-manager-encryption.php
index 48e046f..ae772f7 100644
--- a/includes/class-robotstxt-manager-encryption.php
+++ b/includes/class-robotstxt-manager-encryption.php
@@ -1,6 +1,6 @@
 <?php
 /**
- * AES-256-CBC encryption helper for the account-level API key.
+ * AES-256-CBC + HMAC encryption helper for sensitive plugin options.
  *
  * @package Robotstxt_Manager
  */
@@ -12,9 +12,14 @@ if ( ! defined( 'ABSPATH' ) ) {
 /**
  * Class Robotstxt_Manager_Encryption
  *
- * Same symmetric AES-256-CBC pattern Plugins Core uses for its Forgejo
- * token. Encrypted values are base64-encoded strings with the IV prepended
- * to the ciphertext, safe for wp_options storage.
+ * Provides symmetric authenticated encryption (encrypt-then-MAC) using a
+ * key derived from WordPress secret keys. Since 1.6.0 new ciphertexts are
+ * authenticated with HMAC-SHA256: tampered or truncated payloads fail
+ * closed. Legacy payloads (unauthenticated AES-256-CBC, format
+ * base64(iv . ciphertext)) remain decryptable; values re-encrypt to the
+ * authenticated format the next time they are saved.
+ *
+ * Requires the PHP openssl extension (bundled with PHP 8.0+).
  */
 class Robotstxt_Manager_Encryption {
 
@@ -26,25 +31,52 @@ class Robotstxt_Manager_Encryption {
 	private const CIPHER = 'aes-256-cbc';
 
 	/**
-	 * Context string used to derive a plugin-specific key.
+	 * Context string used to derive the encryption key.
+	 *
+	 * Must never change: existing ciphertexts (including legacy ones)
+	 * depend on it.
 	 *
 	 * @var string
 	 */
 	private const CONTEXT = 'robotstxt_manager_encryption_v1';
 
 	/**
-	 * Encrypts a plaintext string and returns a base64-encoded payload.
+	 * Context string used to derive the MAC key (separate from the
+	 * encryption key by design).
+	 *
+	 * @var string
+	 */
+	private const MAC_CONTEXT = 'robotstxt_manager_encryption_v2_mac';
+
+	/**
+	 * Prefix marking authenticated (v2) payloads.
+	 *
+	 * @var string
+	 */
+	private const PREFIX_V2 = 'v2:';
+
+	/**
+	 * Encrypts a plaintext string and returns an authenticated payload.
+	 *
+	 * Format: 'v2:' . base64( iv . ciphertext . hmac_sha256( iv . ciphertext ) ).
 	 *
 	 * @param string $plaintext The value to encrypt.
 	 *
-	 * @return string Base64-encoded ciphertext, or empty string on failure.
+	 * @return string Payload string, or empty string on failure (including
+	 *                missing WordPress secret keys — encryption without the
+	 *                real salts would be recoverable by anyone).
 	 */
 	public static function encrypt( string $plaintext ): string {
 		if ( '' === $plaintext ) {
 			return '';
 		}
 
+		if ( ! defined( 'AUTH_KEY' ) || ! defined( 'AUTH_SALT' ) ) {
+			return ''; // Fail closed — never encrypt under fallback keys.
+		}
+
 		$key       = self::derive_key();
+		$mac_key   = self::derive_mac_key();
 		$iv_length = openssl_cipher_iv_length( self::CIPHER );
 
 		if ( false === $iv_length ) {
@@ -58,13 +90,20 @@ class Robotstxt_Manager_Encryption {
 			return '';
 		}
 
-		return base64_encode( $iv . $ciphertext ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_encode
+		$mac = hash_hmac( 'sha256', $iv . $ciphertext, $mac_key, true );
+
+		return self::PREFIX_V2 . base64_encode( $iv . $ciphertext . $mac ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_encode
 	}
 
 	/**
-	 * Decrypts a base64-encoded payload previously produced by encrypt().
+	 * Decrypts a payload previously produced by encrypt().
 	 *
-	 * @param string $encoded The base64-encoded ciphertext.
+	 * Authenticated (v2) payloads are verified with a constant-time MAC
+	 * comparison before decryption; tampered payloads return an empty
+	 * string. Legacy unauthenticated payloads are decrypted as before so
+	 * existing stored values keep working until re-saved.
+	 *
+	 * @param string $encoded The stored payload.
 	 *
 	 * @return string The original plaintext, or empty string on failure.
 	 */
@@ -73,28 +112,85 @@ class Robotstxt_Manager_Encryption {
 			return '';
 		}
 
+		if ( 0 === strpos( $encoded, self::PREFIX_V2 ) ) {
+			return self::decrypt_v2( substr( $encoded, strlen( self::PREFIX_V2 ) ) );
+		}
+
 		$decoded = base64_decode( $encoded, true ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_decode
 
 		if ( false === $decoded ) {
 			return '';
 		}
 
+		return self::decrypt_cbc( $decoded );
+	}
+
+	/**
+	 * Decrypts an authenticated v2 payload.
+	 *
+	 * @param string $b64 Base64 portion after the 'v2:' prefix.
+	 *
+	 * @return string Plaintext, or empty string on any failure.
+	 */
+	private static function decrypt_v2( string $b64 ): string {
+		$decoded = base64_decode( $b64, true ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_decode
+
+		if ( false === $decoded ) {
+			return '';
+		}
+
 		$iv_length = openssl_cipher_iv_length( self::CIPHER );
 
-		if ( false === $iv_length || strlen( $decoded ) <= $iv_length ) {
+		if ( false === $iv_length ) {
+			return '';
+		}
+
+		$mac_length = 32; // sha256.
+
+		if ( strlen( $decoded ) <= $iv_length + $mac_length ) {
+			return '';
+		}
+
+		$iv         = substr( $decoded, 0, $iv_length );
+		$ciphertext = substr( $decoded, $iv_length, strlen( $decoded ) - $iv_length - $mac_length );
+		$mac        = substr( $decoded, -1 * $mac_length );
+
+		$expected = hash_hmac( 'sha256', $iv . $ciphertext, self::derive_mac_key(), true );
+
+		if ( ! hash_equals( $expected, $mac ) ) {
+			return ''; // Tampered or corrupted — fail closed.
+		}
+
+		return self::decrypt_cbc( $iv . $ciphertext );
+	}
+
+	/**
+	 * Performs the raw CBC decryption over iv . ciphertext bytes.
+	 *
+	 * @param string $bytes Raw bytes: iv followed by ciphertext.
+	 *
+	 * @return string Plaintext, or empty string on failure.
+	 */
+	private static function decrypt_cbc( string $bytes ): string {
+		$iv_length = openssl_cipher_iv_length( self::CIPHER );
+
+		if ( false === $iv_length || strlen( $bytes ) <= $iv_length ) {
 			return '';
 		}
 
 		$key        = self::derive_key();
-		$iv         = substr( $decoded, 0, $iv_length );
-		$ciphertext = substr( $decoded, $iv_length );
+		$iv         = substr( $bytes, 0, $iv_length );
+		$ciphertext = substr( $bytes, $iv_length );
 		$plaintext  = openssl_decrypt( $ciphertext, self::CIPHER, $key, OPENSSL_RAW_DATA, $iv );
 
 		return false !== $plaintext ? $plaintext : '';
 	}
 
 	/**
-	 * Derives a 32-byte encryption key from WordPress secret constants.
+	 * Derives the 32-byte encryption key from WordPress secret constants.
+	 *
+	 * The deterministic fallback only applies while WordPress constants are
+	 * undefined (early install context); encrypt() refuses to run there.
 	 *
 	 * @return string 32-byte raw key.
 	 */
@@ -108,4 +204,23 @@ class Robotstxt_Manager_Encryption {
 			32
 		);
 	}
+
+	/**
+	 * Derives the 32-byte MAC key from WordPress secret constants.
+	 *
+	 * Derived with a different context string than the encryption key, so
+	 * knowing one reveals nothing about the other.
+	 *
+	 * @return string 32-byte raw key.
+	 */
+	private static function derive_mac_key(): string {
+		$auth_key  = defined( 'AUTH_KEY' ) ? AUTH_KEY : 'auth_key_not_defined';
+		$auth_salt = defined( 'AUTH_SALT' ) ? AUTH_SALT : 'auth_salt_not_defined';
+
+		return substr(
+			hash_hmac( 'sha256', self::MAC_CONTEXT, $auth_key . $auth_salt, true ),
+			0,
+			32
+		);
+	}
 }
diff --git a/readme.txt b/readme.txt
index 2766b7c..c89c1b7 100644
--- a/readme.txt
+++ b/readme.txt
@@ -3,9 +3,9 @@ Contributors: javiercasares, robotstxt
 Tags: dashboard, catalog, updates, subscriptions, management
 Requires at least: 4.4
 Tested up to: 7.1
-Stable tag: 0.5.1
+Stable tag: 0.5.3
 Requires PHP: 8.0
-Version: 0.5.1
+Version: 0.5.3
 License: GPL-3.0-or-later
 License URI: https://www.gnu.org/licenses/gpl-3.0.txt
 
@@ -89,6 +89,18 @@ Encrypted at rest using AES-256-CBC with a key derived from your site's WordPres
 
 == Changelog ==
 
+= 0.5.3 =
+
+_Release date: 2026-08-15_
+
+* Authenticated encryption (encrypt-then-MAC, matching Core 1.6.0) for the stored API key: tampered payloads fail closed; legacy-stored keys keep working and upgrade on next save.
+
+= 0.5.2 =
+
+_Release date: 2026-08-15_
+
+* Fixed: updating (and installing) a plugin from the Manager catalog page failed with a fatal or a bare "Installation failed." Two defects in the panel's upgrader path: `Plugin_Upgrader` was never loaded in the `admin-post` context (it lives in its own file since the WordPress 5.3 class split, and only `class-wp-upgrader.php` was required), and the overwrite option was passed under the key `overwrite`, which current WordPress reads as `overwrite_package` — so the existing folder was never cleared. Verified end-to-end against the live store.
+
 = 0.5.1 =
 
 _Release date: 2026-08-15_
diff --git a/robotstxt-manager.php b/robotstxt-manager.php
index dbf7d6d..c07cb03 100644
--- a/robotstxt-manager.php
+++ b/robotstxt-manager.php
@@ -3,7 +3,7 @@
  * Plugin Name:       Manager (by ROBOTSTXT)
  * Plugin URI:        https://git.robotstxt.es/ROBOTSTXT/robotstxt-manager
  * Description:       Client-side dashboard for the ROBOTSTXT plugin ecosystem. Lists the catalog from a remote Plugins Core install, resolves local install/update state, and installs, activates, and updates plugins directly from the store.
- * Version:           0.5.1
+ * Version:           0.5.3
  * Requires at least: 4.4
  * Requires PHP:      8.0
  * Author:            ROBOTSTXT
@@ -21,7 +21,7 @@ if ( ! defined( 'ABSPATH' ) ) {
 }
 
 /** Plugin version. */
-define( 'ROBOTSTXT_MANAGER_VERSION', '0.5.1' );
+define( 'ROBOTSTXT_MANAGER_VERSION', '0.5.3' );
 
 /** Absolute path to the plugin directory, with trailing slash. */
 define( 'ROBOTSTXT_MANAGER_DIR', plugin_dir_path( __FILE__ ) );