<?php

/**
 * CodeIgniter
 *
 * An open source application development framework for PHP
 *
 * This content is released under the MIT License (MIT)
 *
 * Copyright (c) 2019 - 2022, CodeIgniter Foundation
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 *
 * @package CodeIgniter
 * @author  EllisLab Dev Team
 * @copyright   Copyright (c) 2008 - 2014, EllisLab, Inc. (https://ellislab.com/)
 * @copyright   Copyright (c) 2014 - 2019, British Columbia Institute of Technology (https://bcit.ca/)
 * @copyright   Copyright (c) 2019 - 2022, CodeIgniter Foundation (https://codeigniter.com/)
 * @license https://opensource.org/licenses/MIT MIT License
 * @link    https://codeigniter.com
 * @since   Version 3.0.0
 * @filesource
 */

defined('BASEPATH') or exit('No direct script access allowed');

/**
 * CodeIgniter Encryption Class
 *
 * Provides two-way keyed encryption via PHP's MCrypt and/or OpenSSL extensions.
 *
 * @package     CodeIgniter
 * @subpackage  Libraries
 * @category    Libraries
 * @author      Andrey Andreev
 * @link        https://codeigniter.com/userguide3/libraries/encryption.html
 */
class CI_Encryption
{
    /**
     * Encryption cipher
     *
     * @var string
     */
    protected $_cipher = 'aes-128';

    /**
     * Cipher mode
     *
     * @var string
     */
    protected $_mode = 'cbc';

    /**
     * Cipher handle
     *
     * @var mixed
     */
    protected $_handle;

    /**
     * Encryption key
     *
     * @var string
     */
    protected $_key;

    /**
     * PHP extension to be used
     *
     * @var string
     */
    protected $_driver;

    /**
     * List of usable drivers (PHP extensions)
     *
     * @var array
     */
    protected $_drivers = array();

    /**
     * List of available modes
     *
     * @var array
     */
    protected $_modes = array(
        'mcrypt' => array(
            'cbc' => 'cbc',
            'ecb' => 'ecb',
            'ofb' => 'nofb',
            'ofb8' => 'ofb',
            'cfb' => 'ncfb',
            'cfb8' => 'cfb',
            'ctr' => 'ctr',
            'stream' => 'stream'
        ),
        'openssl' => array(
            'cbc' => 'cbc',
            'ecb' => 'ecb',
            'ofb' => 'ofb',
            'cfb' => 'cfb',
            'cfb8' => 'cfb8',
            'ctr' => 'ctr',
            'stream' => '',
            'xts' => 'xts'
        )
    );

    /**
     * List of supported HMAC algorithms
     *
     * name => digest size pairs
     *
     * @var array
     */
    protected $_digests = array(
        'sha224' => 28,
        'sha256' => 32,
        'sha384' => 48,
        'sha512' => 64
    );

    /**
     * mbstring.func_overload flag
     *
     * @var bool
     */
    protected static $func_overload;

    // --------------------------------------------------------------------

    /**
     * Class constructor
     *
     * @param   array   $params Configuration parameters
     * @return  void
     */
    public function __construct(array $params = array())
    {
        $this->_drivers = array(
            'mcrypt'  => defined('MCRYPT_DEV_URANDOM'),
            'openssl' => extension_loaded('openssl')
        );

        if (! $this->_drivers['mcrypt'] && ! $this->_drivers['openssl']) {
            show_error('Encryption: Unable to find an available encryption driver.');
        }

        isset(self::$func_overload) or self::$func_overload = (! is_php('8.0') && extension_loaded('mbstring') && @ini_get('mbstring.func_overload'));
        $this->initialize($params);

        if (! isset($this->_key) && self::strlen($key = config_item('encryption_key')) > 0) {
            $this->_key = $key;
        }

        log_message('info', 'Encryption Class Initialized');
    }

    // --------------------------------------------------------------------

    /**
     * Initialize
     *
     * @param   array   $params Configuration parameters
     * @return  CI_Encryption
     */
    public function initialize(array $params)
    {
        if (! empty($params['driver'])) {
            if (isset($this->_drivers[$params['driver']])) {
                if ($this->_drivers[$params['driver']]) {
                    $this->_driver = $params['driver'];
                } else {
                    log_message('error', "Encryption: Driver '" . $params['driver'] . "' is not available.");
                }
            } else {
                log_message('error', "Encryption: Unknown driver '" . $params['driver'] . "' cannot be configured.");
            }
        }

        if (empty($this->_driver)) {
            $this->_driver = ($this->_drivers['openssl'] === true)
                ? 'openssl'
                : 'mcrypt';

            log_message('debug', "Encryption: Auto-configured driver '" . $this->_driver . "'.");
        }

        empty($params['cipher']) && $params['cipher'] = $this->_cipher;
        empty($params['key']) or $this->_key = $params['key'];
        $this->{'_' . $this->_driver . '_initialize'}($params);
        return $this;
    }

    // --------------------------------------------------------------------

    /**
     * Initialize MCrypt
     *
     * @param   array   $params Configuration parameters
     * @return  void
     */
    protected function _mcrypt_initialize($params)
    {
        if (! empty($params['cipher'])) {
            $params['cipher'] = strtolower($params['cipher']);
            $this->_cipher_alias($params['cipher']);

            if (! in_array($params['cipher'], mcrypt_list_algorithms(), true)) {
                log_message('error', 'Encryption: MCrypt cipher ' . strtoupper($params['cipher']) . ' is not available.');
            } else {
                $this->_cipher = $params['cipher'];
            }
        }

        if (! empty($params['mode'])) {
            $params['mode'] = strtolower($params['mode']);
            if (! isset($this->_modes['mcrypt'][$params['mode']])) {
                log_message('error', 'Encryption: MCrypt mode ' . strtoupper($params['mode']) . ' is not available.');
            } else {
                $this->_mode = $this->_modes['mcrypt'][$params['mode']];
            }
        }

        if (isset($this->_cipher, $this->_mode)) {
            if (
                is_resource($this->_handle)
                && (strtolower(mcrypt_enc_get_algorithms_name($this->_handle)) !== $this->_cipher
                    or strtolower(mcrypt_enc_get_modes_name($this->_handle)) !== $this->_mode)
            ) {
                mcrypt_module_close($this->_handle);
            }

            if ($this->_handle = mcrypt_module_open($this->_cipher, '', $this->_mode, '')) {
                log_message('info', 'Encryption: MCrypt cipher ' . strtoupper($this->_cipher) . ' initialized in ' . strtoupper($this->_mode) . ' mode.');
            } else {
                log_message('error', 'Encryption: Unable to initialize MCrypt with cipher ' . strtoupper($this->_cipher) . ' in ' . strtoupper($this->_mode) . ' mode.');
            }
        }
    }

    // --------------------------------------------------------------------

    /**
     * Initialize OpenSSL
     *
     * @param   array   $params Configuration parameters
     * @return  void
     */
    protected function _openssl_initialize($params)
    {
        if (! empty($params['cipher'])) {
            $params['cipher'] = strtolower($params['cipher']);
            $this->_cipher_alias($params['cipher']);
            $this->_cipher = $params['cipher'];
        }

        if (! empty($params['mode'])) {
            $params['mode'] = strtolower($params['mode']);
            if (! isset($this->_modes['openssl'][$params['mode']])) {
                log_message('error', 'Encryption: OpenSSL mode ' . strtoupper($params['mode']) . ' is not available.');
            } else {
                $this->_mode = $this->_modes['openssl'][$params['mode']];
            }
        }

        if (isset($this->_cipher, $this->_mode)) {
            // This is mostly for the stream mode, which doesn't get suffixed in OpenSSL
            $handle = empty($this->_mode)
                ? $this->_cipher
                : $this->_cipher . '-' . $this->_mode;

            if (! in_array($handle, openssl_get_cipher_methods(), true)) {
                $this->_handle = null;
                log_message('error', 'Encryption: Unable to initialize OpenSSL with method ' . strtoupper($handle) . '.');
            } else {
                $this->_handle = $handle;
                log_message('info', 'Encryption: OpenSSL initialized with method ' . strtoupper($handle) . '.');
            }
        }
    }

    // --------------------------------------------------------------------

    /**
     * Create a random key
     *
     * @param   int $length Output length
     * @return  string
     */
    public function create_key($length)
    {
        if (function_exists('random_bytes')) {
            try {
                return random_bytes((int) $length);
            } catch (Exception $e) {
                log_message('error', $e->getMessage());
                return false;
            }
        } elseif (defined('MCRYPT_DEV_URANDOM')) {
            return mcrypt_create_iv($length, MCRYPT_DEV_URANDOM);
        }

        $is_secure = null;
        $key = openssl_random_pseudo_bytes($length, $is_secure);
        return ($is_secure === true)
            ? $key
            : false;
    }

    // --------------------------------------------------------------------

    /**
     * Encrypt
     *
     * @param   string  $data   Input data
     * @param   array   $params Input parameters
     * @return  string
     */
    public function encrypt($data, array $params = null)
    {
        if (($params = $this->_get_params($params)) === false) {
            return false;
        }

        isset($params['key']) or $params['key'] = $this->hkdf($this->_key, 'sha512', null, self::strlen($this->_key), 'encryption');

        if (($data = $this->{'_' . $this->_driver . '_encrypt'}($data, $params)) === false) {
            return false;
        }

        $params['base64'] && $data = base64_encode($data);

        if (isset($params['hmac_digest'])) {
            isset($params['hmac_key']) or $params['hmac_key'] = $this->hkdf($this->_key, 'sha512', null, null, 'authentication');
            return hash_hmac($params['hmac_digest'], $data, $params['hmac_key'], ! $params['base64']) . $data;
        }

        return $data;
    }

    // --------------------------------------------------------------------

    /**
     * Encrypt via MCrypt
     *
     * @param   string  $data   Input data
     * @param   array   $params Input parameters
     * @return  string
     */
    protected function _mcrypt_encrypt($data, $params)
    {
        if (! is_resource($params['handle'])) {
            return false;
        }

        // The greater-than-1 comparison is mostly a work-around for a bug,
        // where 1 is returned for ARCFour instead of 0.
        $iv = (($iv_size = mcrypt_enc_get_iv_size($params['handle'])) > 1)
            ? $this->create_key($iv_size)
            : null;

        if (mcrypt_generic_init($params['handle'], $params['key'], $iv) < 0) {
            if ($params['handle'] !== $this->_handle) {
                mcrypt_module_close($params['handle']);
            }

            return false;
        }

        // Use PKCS#7 padding in order to ensure compatibility with OpenSSL
        // and other implementations outside of PHP.
        if (in_array(strtolower(mcrypt_enc_get_modes_name($params['handle'])), array('cbc', 'ecb'), true)) {
            $block_size = mcrypt_enc_get_block_size($params['handle']);
            $pad = $block_size - (self::strlen($data) % $block_size);
            $data .= str_repeat(chr($pad), $pad);
        }

        // Work-around for yet another strange behavior in MCrypt.
        //
        // When encrypting in ECB mode, the IV is ignored. Yet
        // mcrypt_enc_get_iv_size() returns a value larger than 0
        // even if ECB is used AND mcrypt_generic_init() complains
        // if you don't pass an IV with length equal to the said
        // return value.
        //
        // This probably would've been fine (even though still wasteful),
        // but OpenSSL isn't that dumb and we need to make the process
        // portable, so ...
        $data = (mcrypt_enc_get_modes_name($params['handle']) !== 'ECB')
            ? $iv . mcrypt_generic($params['handle'], $data)
            : mcrypt_generic($params['handle'], $data);

        mcrypt_generic_deinit($params['handle']);
        if ($params['handle'] !== $this->_handle) {
            mcrypt_module_close($params['handle']);
        }

        return $data;
    }

    // --------------------------------------------------------------------

    /**
     * Encrypt via OpenSSL
     *
     * @param   string  $data   Input data
     * @param   array   $params Input parameters
     * @return  string
     */
    protected function _openssl_encrypt($data, $params)
    {
        if (empty($params['handle'])) {
            return false;
        }

        $iv = ($iv_size = openssl_cipher_iv_length($params['handle']))
            ? $this->create_key($iv_size)
            : '';

        $data = openssl_encrypt(
            $data,
            $params['handle'],
            $params['key'],
            1, // DO NOT TOUCH!
            $iv
        );

        if ($data === false) {
            return false;
        }

        return $iv . $data;
    }

    // --------------------------------------------------------------------

    /**
     * Decrypt
     *
     * @param   string  $data   Encrypted data
     * @param   array   $params Input parameters
     * @return  string
     */
    public function decrypt($data, array $params = null)
    {
        if (($params = $this->_get_params($params)) === false) {
            return false;
        }

        if (isset($params['hmac_digest'])) {
            // This might look illogical, but it is done during encryption as well ...
            // The 'base64' value is effectively an inverted "raw data" parameter
            $digest_size = ($params['base64'])
                ? $this->_digests[$params['hmac_digest']] * 2
                : $this->_digests[$params['hmac_digest']];

            if (self::strlen($data) <= $digest_size) {
                return false;
            }

            $hmac_input = self::substr($data, 0, $digest_size);
            $data = self::substr($data, $digest_size);

            isset($params['hmac_key']) or $params['hmac_key'] = $this->hkdf($this->_key, 'sha512', null, null, 'authentication');
            $hmac_check = hash_hmac($params['hmac_digest'], $data, $params['hmac_key'], ! $params['base64']);

            // Time-attack-safe comparison
            $diff = 0;
            for ($i = 0; $i < $digest_size; $i++) {
                $diff |= ord($hmac_input[$i]) ^ ord($hmac_check[$i]);
            }

            if ($diff !== 0) {
                return false;
            }
        }

        if ($params['base64']) {
            $data = base64_decode($data);
        }

        isset($params['key']) or $params['key'] = $this->hkdf($this->_key, 'sha512', null, self::strlen($this->_key), 'encryption');

        return $this->{'_' . $this->_driver . '_decrypt'}($data, $params);
    }

    // --------------------------------------------------------------------

    /**
     * Decrypt via MCrypt
     *
     * @param   string  $data   Encrypted data
     * @param   array   $params Input parameters
     * @return  string
     */
    protected function _mcrypt_decrypt($data, $params)
    {
        if (! is_resource($params['handle'])) {
            return false;
        }

        // The greater-than-1 comparison is mostly a work-around for a bug,
        // where 1 is returned for ARCFour instead of 0.
        if (($iv_size = mcrypt_enc_get_iv_size($params['handle'])) > 1) {
            if (mcrypt_enc_get_modes_name($params['handle']) !== 'ECB') {
                $iv = self::substr($data, 0, $iv_size);
                $data = self::substr($data, $iv_size);
            } else {
                // MCrypt is dumb and this is ignored, only size matters
                $iv = str_repeat("\x0", $iv_size);
            }
        } else {
            $iv = '';
        }

        if (mcrypt_generic_init($params['handle'], $params['key'], $iv) < 0) {
            if ($params['handle'] !== $this->_handle) {
                mcrypt_module_close($params['handle']);
            }

            return false;
        }

        $data = mdecrypt_generic($params['handle'], $data);
        // Remove PKCS#7 padding, if necessary
        if (in_array(strtolower(mcrypt_enc_get_modes_name($params['handle'])), array('cbc', 'ecb'), true)) {
            $data = self::substr($data, 0, -ord($data[self::strlen($data) - 1]));
        }

        mcrypt_generic_deinit($params['handle']);
        if ($params['handle'] !== $this->_handle) {
            mcrypt_module_close($params['handle']);
        }

        return $data;
    }

    // --------------------------------------------------------------------

    /**
     * Decrypt via OpenSSL
     *
     * @param   string  $data   Encrypted data
     * @param   array   $params Input parameters
     * @return  string
     */
    protected function _openssl_decrypt($data, $params)
    {
        if ($iv_size = openssl_cipher_iv_length($params['handle'])) {
            $iv = self::substr($data, 0, $iv_size);
            $data = self::substr($data, $iv_size);
        } else {
            $iv = '';
        }

        return empty($params['handle'])
            ? false
            : openssl_decrypt(
                $data,
                $params['handle'],
                $params['key'],
                1, // DO NOT TOUCH!
                $iv
            );
    }

    // --------------------------------------------------------------------

    /**
     * Get params
     *
     * @param   array   $params Input parameters
     * @return  array
     */
    protected function _get_params($params)
    {
        if (empty($params)) {
            return isset($this->_cipher, $this->_mode, $this->_key, $this->_handle)
                ? array(
                    'handle' => $this->_handle,
                    'cipher' => $this->_cipher,
                    'mode' => $this->_mode,
                    'key' => null,
                    'base64' => true,
                    'hmac_digest' => 'sha512',
                    'hmac_key' => null
                )
                : false;
        } elseif (! isset($params['cipher'], $params['mode'], $params['key'])) {
            return false;
        }

        if (isset($params['mode'])) {
            $params['mode'] = strtolower($params['mode']);
            if (! isset($this->_modes[$this->_driver][$params['mode']])) {
                return false;
            }

            $params['mode'] = $this->_modes[$this->_driver][$params['mode']];
        }

        if (isset($params['hmac']) && $params['hmac'] === false) {
            $params['hmac_digest'] = $params['hmac_key'] = null;
        } else {
            if (! isset($params['hmac_key'])) {
                return false;
            } elseif (isset($params['hmac_digest'])) {
                $params['hmac_digest'] = strtolower($params['hmac_digest']);
                if (! isset($this->_digests[$params['hmac_digest']])) {
                    return false;
                }
            } else {
                $params['hmac_digest'] = 'sha512';
            }
        }

        $params = array(
            'handle' => null,
            'cipher' => $params['cipher'],
            'mode' => $params['mode'],
            'key' => $params['key'],
            'base64' => isset($params['raw_data']) ? ! $params['raw_data'] : false,
            'hmac_digest' => $params['hmac_digest'],
            'hmac_key' => $params['hmac_key']
        );

        $this->_cipher_alias($params['cipher']);
        $params['handle'] = ($params['cipher'] !== $this->_cipher or $params['mode'] !== $this->_mode)
            ? $this->{'_' . $this->_driver . '_get_handle'}($params['cipher'], $params['mode'])
            : $this->_handle;

        return $params;
    }

    // --------------------------------------------------------------------

    /**
     * Get MCrypt handle
     *
     * @param   string  $cipher Cipher name
     * @param   string  $mode   Encryption mode
     * @return  resource
     */
    protected function _mcrypt_get_handle($cipher, $mode)
    {
        return mcrypt_module_open($cipher, '', $mode, '');
    }

    // --------------------------------------------------------------------

    /**
     * Get OpenSSL handle
     *
     * @param   string  $cipher Cipher name
     * @param   string  $mode   Encryption mode
     * @return  string
     */
    protected function _openssl_get_handle($cipher, $mode)
    {
        // OpenSSL methods aren't suffixed with '-stream' for this mode
        return ($mode === 'stream')
            ? $cipher
            : $cipher . '-' . $mode;
    }

    // --------------------------------------------------------------------

    /**
     * Cipher alias
     *
     * Tries to translate cipher names between MCrypt and OpenSSL's "dialects".
     *
     * @param   string  $cipher Cipher name
     * @return  void
     */
    protected function _cipher_alias(&$cipher)
    {
        static $dictionary;

        if (empty($dictionary)) {
            $dictionary = array(
                'mcrypt' => array(
                    'aes-128' => 'rijndael-128',
                    'aes-192' => 'rijndael-128',
                    'aes-256' => 'rijndael-128',
                    'des3-ede3' => 'tripledes',
                    'bf' => 'blowfish',
                    'cast5' => 'cast-128',
                    'rc4' => 'arcfour',
                    'rc4-40' => 'arcfour'
                ),
                'openssl' => array(
                    'rijndael-128' => 'aes-128',
                    'tripledes' => 'des-ede3',
                    'blowfish' => 'bf',
                    'cast-128' => 'cast5',
                    'arcfour' => 'rc4-40',
                    'rc4' => 'rc4-40'
                )
            );

            // Notes:
            //
            // - Rijndael-128 is, at the same time all three of AES-128,
            //   AES-192 and AES-256. The only difference between them is
            //   the key size. Rijndael-192, Rijndael-256 on the other hand
            //   also have different block sizes and are NOT AES-compatible.
            //
            // - Blowfish is said to be supporting key sizes between
            //   4 and 56 bytes, but it appears that between MCrypt and
            //   OpenSSL, only those of 16 and more bytes are compatible.
            //   Also, don't know what MCrypt's 'blowfish-compat' is.
            //
            // - CAST-128/CAST5 produces a longer cipher when encrypted via
            //   OpenSSL, but (strangely enough) can be decrypted by either
            //   extension anyway.
            //   Also, it appears that OpenSSL uses 16 rounds regardless of
            //   the key size, while RFC2144 says that for key sizes lower
            //   than 11 bytes, only 12 rounds should be used. This makes
            //   it portable only with keys of between 11 and 16 bytes.
            //
            // - RC4 (ARCFour) has a strange implementation under OpenSSL.
            //   Its 'rc4-40' cipher method seems to work flawlessly, yet
            //   there's another one, 'rc4' that only works with a 16-byte key.
            //
            // - DES is compatible, but doesn't need an alias.
            //
            // Other seemingly matching ciphers between MCrypt, OpenSSL:
            //
            // - RC2 is NOT compatible and only an obscure forum post
            //   confirms that it is MCrypt's fault.
        }

        if (isset($dictionary[$this->_driver][$cipher])) {
            $cipher = $dictionary[$this->_driver][$cipher];
        }
    }

    // --------------------------------------------------------------------

    /**
     * HKDF
     *
     * @link    https://tools.ietf.org/rfc/rfc5869.txt
     * @param   $key    Input key
     * @param   $digest A SHA-2 hashing algorithm
     * @param   $salt   Optional salt
     * @param   $length Output length (defaults to the selected digest size)
     * @param   $info   Optional context/application-specific info
     * @return  string  A pseudo-random key
     */
    public function hkdf($key, $digest = 'sha512', $salt = null, $length = null, $info = '')
    {
        if (! isset($this->_digests[$digest])) {
            return false;
        }

        if (empty($length) or ! is_int($length)) {
            $length = $this->_digests[$digest];
        } elseif ($length > (255 * $this->_digests[$digest])) {
            return false;
        }

        self::strlen($salt) or $salt = str_repeat("\0", $this->_digests[$digest]);

        $prk = hash_hmac($digest, $key, $salt, true);
        $key = '';
        for ($key_block = '', $block_index = 1; self::strlen($key) < $length; $block_index++) {
            $key_block = hash_hmac($digest, $key_block . $info . chr($block_index), $prk, true);
            $key .= $key_block;
        }

        return self::substr($key, 0, $length);
    }

    // --------------------------------------------------------------------

    /**
     * __get() magic
     *
     * @param   string  $key    Property name
     * @return  mixed
     */
    public function __get($key)
    {
        // Because aliases
        if ($key === 'mode') {
            return array_search($this->_mode, $this->_modes[$this->_driver], true);
        } elseif (in_array($key, array('cipher', 'driver', 'drivers', 'digests'), true)) {
            return $this->{'_' . $key};
        }

        return null;
    }

    // --------------------------------------------------------------------

    /**
     * Byte-safe strlen()
     *
     * @param   string  $str
     * @return  int
     */
    protected static function strlen($str)
    {
        return (self::$func_overload)
            ? mb_strlen((string) $str, '8bit')
            : strlen((string) $str);
    }

    // --------------------------------------------------------------------

    /**
     * Byte-safe substr()
     *
     * @param   string  $str
     * @param   int $start
     * @param   int $length
     * @return  string
     */
    protected static function substr($str, $start, $length = null)
    {
        if (self::$func_overload) {
            // mb_substr($str, $start, null, '8bit') returns an empty
            // string on PHP 5.3
            isset($length) or $length = ($start >= 0 ? self::strlen($str) - $start : -$start);
            return mb_substr($str, $start, $length, '8bit');
        }

        return isset($length)
            ? substr($str, $start, $length)
            : substr($str, $start);
    }
}