feat: encryption key rotation

app/Config/Encryption.php

@@ -23,6 +23,23 @@ class Encryption extends BaseConfig
      */
     public string $key = '';
 
+    /**
+     * --------------------------------------------------------------------------
+     * Previous Encryption Keys
+     * --------------------------------------------------------------------------
+     *
+     * When rotating encryption keys, add old keys here to maintain ability
+     * to decrypt data encrypted with previous keys. Encryption always uses
+     * the current $key. Decryption tries current key first, then falls back
+     * to previous keys if decryption fails.
+     *
+     * In .env file, use comma-separated string:
+     *   encryption.previousKeys = hex2bin:9be8c64fcea509867...,hex2bin:3f5a1d8e9c2b7a4f6...
+     *
+     * @var list<string>|string
+     */
+    public array|string $previousKeys = '';
+
     /**
      * --------------------------------------------------------------------------
      * Encryption Driver to Use

system/Config/BaseConfig.php

@@ -130,18 +130,39 @@ public function __construct()
         foreach ($properties as $property) {
             $this->initEnvValue($this->{$property}, $property, $prefix, $shortPrefix);
 
-            if ($this instanceof Encryption && $property === 'key') {
-                if (str_starts_with($this->{$property}, 'hex2bin:')) {
-                    // Handle hex2bin prefix
-                    $this->{$property} = hex2bin(substr($this->{$property}, 8));
-                } elseif (str_starts_with($this->{$property}, 'base64:')) {
-                    // Handle base64 prefix
-                    $this->{$property} = base64_decode(substr($this->{$property}, 7), true);
+            if ($this instanceof Encryption) {
+                if ($property === 'key') {
+                    $this->{$property} = $this->parseEncryptionKey($this->{$property});
+                } elseif ($property === 'previousKeys') {
+                    $keysArray  = is_string($this->{$property}) ? array_map(trim(...), explode(',', $this->{$property})) : $this->{$property};
+                    $parsedKeys = [];
+
+                    foreach ($keysArray as $key) {
+                        $parsedKeys[] = $this->parseEncryptionKey($key);
+                    }
+
+                    $this->{$property} = $parsedKeys;
                 }
             }
         }
     }
 
+    /**
+     * Parse encryption key with hex2bin: or base64: prefix
+     */
+    protected function parseEncryptionKey(string $key): string
+    {
+        if (str_starts_with($key, 'hex2bin:')) {
+            return hex2bin(substr($key, 8));
+        }
+
+        if (str_starts_with($key, 'base64:')) {
+            return base64_decode(substr($key, 7), true);
+        }
+
+        return $key;
+    }
+
     /**
      * Initialization an environment-specific configuration setting
      *

system/Encryption/Encryption.php

@@ -138,6 +138,10 @@ public function initialize(?EncryptionConfig $config = null)
         $handlerName     = 'CodeIgniter\\Encryption\\Handlers\\' . $this->driver . 'Handler';
         $this->encrypter = new $handlerName($config);
 
+        if (($config->previousKeys ?? []) !== []) {
+            $this->encrypter = new KeyRotationDecorator($this->encrypter, $config->previousKeys);
+        }
+
         return $this->encrypter;
     }
 

system/Encryption/Handlers/OpenSSLHandler.php

@@ -154,6 +154,12 @@ public function decrypt($data, #[SensitiveParameter] $params = null)
         // derive a secret key
         $encryptKey = \hash_hkdf($this->digest, $key, 0, $this->encryptKeyInfo);
 
-        return \openssl_decrypt($data, $this->cipher, $encryptKey, OPENSSL_RAW_DATA, $iv);
+        $result = \openssl_decrypt($data, $this->cipher, $encryptKey, OPENSSL_RAW_DATA, $iv);
+
+        if ($result === false) {
+            throw EncryptionException::forAuthenticationFailed();
+        }
+
+        return $result;
     }
 }

system/Encryption/KeyRotationDecorator.php

@@ -0,0 +1,111 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * This file is part of CodeIgniter 4 framework.
+ *
+ * (c) CodeIgniter Foundation <admin@codeigniter.com>
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+namespace CodeIgniter\Encryption;
+
+use CodeIgniter\Encryption\Exceptions\EncryptionException;
+use SensitiveParameter;
+
+/**
+ * Key Rotation Decorator
+ *
+ * Wraps any EncrypterInterface implementation to provide automatic
+ * fallback to previous encryption keys during decryption. This enables
+ * seamless key rotation without requiring re-encryption of existing data.
+ */
+class KeyRotationDecorator implements EncrypterInterface
+{
+    /**
+     * @param EncrypterInterface $innerHandler The wrapped encryption handler
+     * @param list<string>       $previousKeys Array of previous encryption keys
+     */
+    public function __construct(
+        private readonly EncrypterInterface $innerHandler,
+        private readonly array $previousKeys,
+    ) {
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * Encryption always uses the inner handler's current key.
+     */
+    public function encrypt(#[SensitiveParameter] $data, #[SensitiveParameter] $params = null)
+    {
+        return $this->innerHandler->encrypt($data, $params);
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * Attempts decryption with current key first. If that fails and no
+     * explicit key was provided in $params, tries each previous key.
+     *
+     * @throws EncryptionException
+     */
+    public function decrypt($data, #[SensitiveParameter] $params = null)
+    {
+        try {
+            return $this->innerHandler->decrypt($data, $params);
+        } catch (EncryptionException $e) {
+            // Don't try previous keys if an explicit key was provided
+            if (is_string($params) || (is_array($params) && isset($params['key']))) {
+                throw $e;
+            }
+
+            if ($this->previousKeys === []) {
+                throw $e;
+            }
+
+            foreach ($this->previousKeys as $previousKey) {
+                try {
+                    $previousParams = is_array($params)
+                        ? array_merge($params, ['key' => $previousKey])
+                        : $previousKey;
+
+                    return $this->innerHandler->decrypt($data, $previousParams);
+                } catch (EncryptionException) {
+                    continue;
+                }
+            }
+
+            throw $e;
+        }
+    }
+
+    /**
+     * Delegate property access to the inner handler.
+     *
+     * @return array|bool|int|string|null
+     */
+    public function __get(string $key)
+    {
+        if (method_exists($this->innerHandler, '__get')) {
+            return $this->innerHandler->__get($key);
+        }
+
+        return null;
+    }
+
+    /**
+     * Delegate property existence check to inner handler.
+     */
+    public function __isset(string $key): bool
+    {
+        if (method_exists($this->innerHandler, '__isset')) {
+            return $this->innerHandler->__isset($key);
+        }
+
+        return false;
+    }
+}