MDL-36580 backup: General support for encrypted contents in backups

- Built using standard backup custom fields.
- Can be applied potentially everywhere.
- Automatically addded 'encrypted' attribute.
- Defaults to site generated key.
- Enforces key robutness / provides authentication (hmac integrity)
- Covered with unit tests.

backup/backup.class.php

@@ -141,6 +141,15 @@ abstract class backup implements checksumable {
      * Usually same than major release zero version, mainly for informative/historic purposes.
      */
     const RELEASE = '3.3';
+
+    /**
+     * Cipher to be used in backup and restore operations.
+     */
+    const CIPHER = 'aes-256-cbc';
+    /**
+     * Bytes enforced for key, using the cypher above. Restrictive? Yes, but better than unsafe lengths
+     */
+    const CIPHERKEYLEN = 32;
 }
 
 /*

backup/moodle2/backup_custom_fields.php

@@ -97,6 +97,138 @@ public function set_value($value) {
     }
 }
 
+/**
+ * Implementation of {@link backup_final_element} that provides symmetric-key AES-256 encryption of contents.
+ *
+ * This final element transparently encrypts, for secure storage and transport, any content
+ * that shouldn't be shown normally in plain text. Usually, passwords or keys that cannot use
+ * hashing algorithms, although potentially can encrypt any content. All information is encoded
+ * using base64.
+ *
+ * Features:
+ *   - requires openssl extension to work. Without it contents are completely omitted.
+ *   - automatically creates an appropriate default key for the site and stores it into backup_encryptkey config (bas64 encoded).
+ *   - uses a different appropriate init vector for every operation, which is transmited with the encrypted contents.
+ *   - all generated data is base64 encoded for safe transmission.
+ *   - automatically adds "encrypted" attribute for easier detection.
+ *   - implements HMAC for providing integrity.
+ *
+ * @copyright 2017 onwards Eloy Lafuente (stronk7) {@link http://stronk7.com}
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class encrypted_final_element extends backup_final_element {
+
+    /** @var string cypher appropiate raw key for backups in the site. Defaults to backup_encryptkey config. */
+    protected $key = null;
+
+    /**
+     * Constructor - instantiates a encrypted_final_element, specifying its basic info.
+     *
+     * Overridden to automatically add the 'encrypted' attribute if missing.
+     *
+     * @param string $name name of the element
+     * @param array  $attributes attributes this element will handle (optional, defaults to null)
+     */
+    public function __construct($name, $attributes = null) {
+        parent::__construct($name, $attributes);
+        if (! $this->get_attribute('encrypted')) {
+            $this->add_attributes('encrypted');
+        }
+    }
+
+    /**
+     * Set the encryption key manually, overriding default backup_encryptkey config.
+     *
+     * @param string $key key to be used for encrypting. Required to be 256-bit key.
+     *               Use a safe generation technique. See self::generate_encryption_random_key() below.
+     */
+    protected function set_key($key) {
+        $bytes = strlen($key); // Get key length in bytes.
+
+        // Only accept keys with the expected (backup::CIPHERKEYLEN) key length. There are a number of hashing,
+        // random generators to achieve this esasily, like the one shown below to create the default
+        // site encryption key and ivs.
+        if ($bytes !== backup::CIPHERKEYLEN) {
+            $info = (object)array('expected' => backup::CIPHERKEYLEN, 'found' => $bytes);
+            throw new base_element_struct_exception('encrypted_final_element incorrect key length', $info);
+        }
+        // Everything went ok, store the key.
+        $this->key = $key;
+    }
+
+    /**
+     * Set the value of the field.
+     *
+     * This method sets the value of the element, encrypted using the specified key for it,
+     * defaulting to (and generating) backup_encryptkey config. HMAC is used for integrity.
+     *
+     * @param string $value plain-text content the will be stored encrypted and encoded.
+     */
+    public function set_value($value) {
+
+        // No openssl available, skip this field completely.
+        if (!function_exists('openssl_encrypt')) {
+            return;
+        }
+
+        // No hmac available, skip this field completely.
+        if (!function_exists('hash_hmac')) {
+            return;
+        }
+
+        // Cypher not available, skip this field completely.
+        if (!in_array(backup::CIPHER, openssl_get_cipher_methods())) {
+            return;
+        }
+
+        // Ensure we have a good key, manual or default.
+        if (empty($this->key)) {
+            // The key has not been set manually, look for it at config (base64 encoded there).
+            $enckey = get_config('backup', 'backup_encryptkey');
+            if ($enckey === false) {
+                // Has not been set, calculate and save an appropiate random key automatically.
+                $enckey = base64_encode(self::generate_encryption_random_key(backup::CIPHERKEYLEN));
+                set_config('backup_encryptkey', $enckey, 'backup');
+            }
+            $this->set_key(base64_decode($enckey));
+        }
+
+        // Now we need an iv for this operation.
+        $iv = self::generate_encryption_random_key(openssl_cipher_iv_length(backup::CIPHER));
+
+        // Everything is ready, let's encrypt and prepend the 1-shot iv.
+        $value = $iv . openssl_encrypt($value, backup::CIPHER, $this->key, OPENSSL_RAW_DATA, $iv);
+
+        // Calculate the hmac of the value (iv + encrypted) and prepend it.
+        $hmac = hash_hmac('sha256', $value, $this->key, true);
+        $value = $hmac . $value;
+
+        // Ready, set the encoded value.
+        parent::set_value(base64_encode($value));
+
+        // Finally, if the field has an "encrypted" attribute, set it to true.
+        if ($att = $this->get_attribute('encrypted')) {
+            $att->set_value('true');
+        }
+    }
+
+    /**
+     * Generate an appropiate random key to be used for encrypting backup information.
+     *
+     * Normally used as site default encryption key (backup_encryptkey config) and also
+     * for calculating the init vectors.
+     *
+     * Note that until PHP 5.6.12 openssl_random_pseudo_bytes() did NOT
+     * use a "cryptographically strong algorithm" {@link https://bugs.php.net/bug.php?id=70014}
+     * But it's beyond my crypto-knowledge when it's worth finding a *real* better alternative.
+     *
+     * @param int $bytes Number of bytes to determine the key length expected.
+     */
+    protected static function generate_encryption_random_key($bytes) {
+        return openssl_random_pseudo_bytes($bytes);
+    }
+}
+
 /**
  * Implementation of backup_nested_element that provides special handling of files
  *

backup/moodle2/tests/backup_encrypted_content_test.php

@@ -0,0 +1,116 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Tests for the handling of encrypted contents in backup and restore.
+ *
+ * @package core_backup
+ * @copyright 2016 onwards Eloy Lafuente (stronk7) {@link http://stronk7.com}
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+
+global $CFG;
+require_once($CFG->dirroot . '/backup/util/includes/backup_includes.php');
+require_once($CFG->dirroot . '/backup/util/includes/restore_includes.php');
+require_once($CFG->dirroot . '/backup/moodle2/backup_custom_fields.php');
+
+class core_backup_encrypted_content_testscase extends advanced_testcase {
+
+    public function setUp() {
+        if (!function_exists('openssl_encrypt')) {
+            $this->markTestSkipped('OpenSSL extension is not loaded.');
+
+        } else if (!function_exists('hash_hmac')) {
+            $this->markTestSkipped('Hash extension is not loaded.');
+
+        } else if (!in_array(backup::CIPHER, openssl_get_cipher_methods())) {
+            $this->markTestSkipped('Expected cipher not available: ' . backup::CIPHER);
+        }
+    }
+
+    public function test_encrypted_final_element() {
+
+        $this->resetAfterTest(true);
+
+        // Some basic verifications.
+        $efe = new encrypted_final_element('test', array('encrypted'));
+        $this->assertInstanceOf('encrypted_final_element', $efe);
+        $this->assertSame('test', $efe->get_name());
+        $atts = $efe->get_attributes();
+        $this->assertCount(1, $atts);
+        $att = reset($atts);
+        $this->assertInstanceOf('backup_attribute', $att);
+        $this->assertSame('encrypted', $att->get_name());
+
+        // Using a manually defined (incorrect length) key.
+        $efe = new encrypted_final_element('test', array('encrypted'));
+        $key = 'this_in_not_correct_32_byte_key';
+        try {
+            set_config('backup_encryptkey', base64_encode($key), 'backup');
+            $efe->set_value('tiny_secret');
+            $this->fail('Expecting base_element_struct_exception exception, none happened');
+        } catch (exception $e) {
+            $this->assertInstanceOf('base_element_struct_exception', $e);
+            $this->assertEquals('encrypted_final_element incorrect key length', $e->errorcode);
+
+        }
+
+        // Using a manually defined (correct length) key.
+        $efe = new encrypted_final_element('test', array('testattr', 'encrypted'));
+        $key = hash('md5', 'Moodle rocks and this is not secure key, who cares, it is a test');
+        set_config('backup_encryptkey', base64_encode($key), 'backup');
+        $this->assertEmpty($efe->get_value());
+        $secret = 'This is a secret message that nobody else will be able to read but me 💩 ';
+        $efe->set_value($secret);
+        $atts = $efe->get_attributes();
+        $this->assertCount(2, $atts);
+        $this->assertArrayHasKey('encrypted', $atts); // We added it explicitly.
+        $this->assertTrue($atts['encrypted']->is_set());
+        $this->assertSame('true', $atts['encrypted']->get_value());
+        $this->assertNotEmpty($efe->get_value());
+        $this->assertTrue($efe->is_set());
+        // Get the crypted content and decrypt it manually.
+        $ctext = $efe->get_value();
+        $hmaclen = 32; // SHA256 is 32 bytes.
+        $ivlen = openssl_cipher_iv_length(backup::CIPHER);
+        list($hmac, $iv, $text) = array_values(unpack("a{$hmaclen}hmac/a{$ivlen}iv/a*text", base64_decode($ctext)));
+        $this->assertSame(hash_hmac('sha256', $iv . $text, $key, true), $hmac);
+        $this->assertSame($secret, openssl_decrypt($text, backup::CIPHER, $key, OPENSSL_RAW_DATA, $iv));
+
+        // Using the default site-generated key.
+        $efe = new encrypted_final_element('test', array('testattr'));
+        $this->assertEmpty($efe->get_value());
+        $secret = 'This is a secret message that nobody else will be able to read but me 💩 ';
+        $efe->set_value($secret);
+        $atts = $efe->get_attributes();
+        $this->assertCount(2, $atts);
+        $this->assertArrayHasKey('encrypted', $atts); // Was added automatcally, we did not specify it.
+        $this->assertTrue($atts['encrypted']->is_set());
+        $this->assertSame('true', $atts['encrypted']->get_value());
+        $this->assertNotEmpty($efe->get_value());
+        $this->assertTrue($efe->is_set());
+        // Get the crypted content and decrypt it manually.
+        $ctext = $efe->get_value();
+        $hmaclen = 32; // SHA256 is 32 bytes.
+        $ivlen = openssl_cipher_iv_length(backup::CIPHER);
+        list($hmac, $iv, $text) = array_values(unpack("a{$hmaclen}hmac/a{$ivlen}iv/a*text", base64_decode($ctext)));
+        $key = base64_decode(get_config('backup', 'backup_encryptkey'));
+        $this->assertSame(hash_hmac('sha256', $iv . $text, $key, true), $hmac);
+        $this->assertSame($secret, openssl_decrypt($text, backup::CIPHER, $key, OPENSSL_RAW_DATA, $iv));
+    }
+}