core /lib/util.php

Language PHP Lines 896
MD5 Hash 716de19c7bf491238cfcbabd60af6c01
Repository https://github.com/sezuan/core.git View Raw File
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<?php

require_once 'Patchwork/PHP/Shim/Normalizer.php';

/**
 * Class for utility functions
 *
 */

class OC_Util {
	public static $scripts=array();
	public static $styles=array();
	public static $headers=array();
	private static $rootMounted=false;
	private static $fsSetup=false;
	public static $core_styles=array();
	public static $core_scripts=array();

	// Can be set up
	public static function setupFS( $user = '' ) {// configure the initial filesystem based on the configuration
		if(self::$fsSetup) {//setting up the filesystem twice can only lead to trouble
			return false;
		}

		// If we are not forced to load a specific user we load the one that is logged in
		if( $user == "" && OC_User::isLoggedIn()) {
			$user = OC_User::getUser();
		}

		// load all filesystem apps before, so no setup-hook gets lost
		if(!isset($RUNTIME_NOAPPS) || !$RUNTIME_NOAPPS) {
			OC_App::loadApps(array('filesystem'));
		}

		// the filesystem will finish when $user is not empty,
		// mark fs setup here to avoid doing the setup from loading
		// OC_Filesystem
		if ($user != '') {
			self::$fsSetup=true;
		}

		$CONFIG_DATADIRECTORY = OC_Config::getValue( "datadirectory", OC::$SERVERROOT."/data" );
		//first set up the local "root" storage
		\OC\Files\Filesystem::initMounts();
		if(!self::$rootMounted) {
			\OC\Files\Filesystem::mount('\OC\Files\Storage\Local', array('datadir'=>$CONFIG_DATADIRECTORY), '/');
			self::$rootMounted=true;
		}

		if( $user != "" ) { //if we aren't logged in, there is no use to set up the filesystem
			$user_dir = '/'.$user.'/files';
			$user_root = OC_User::getHome($user);
			$userdirectory = $user_root . '/files';
			if( !is_dir( $userdirectory )) {
				mkdir( $userdirectory, 0755, true );
			}
			//jail the user into his "home" directory
			\OC\Files\Filesystem::init($user, $user_dir);

			$quotaProxy=new OC_FileProxy_Quota();
			$fileOperationProxy = new OC_FileProxy_FileOperations();
			OC_FileProxy::register($quotaProxy);
			OC_FileProxy::register($fileOperationProxy);

			OC_Hook::emit('OC_Filesystem', 'setup', array('user' => $user, 'user_dir' => $user_dir));
		}
		return true;
	}

	public static function tearDownFS() {
		\OC\Files\Filesystem::tearDown();
		self::$fsSetup=false;
        self::$rootMounted=false;
	}

	/**
	 * get the current installed version of ownCloud
	 * @return array
	 */
	public static function getVersion() {
		// hint: We only can count up. Reset minor/patchlevel when
		// updating major/minor version number.
		return array(5, 80, 05);
	}

	/**
	 * get the current installed version string of ownCloud
	 * @return string
	 */
	public static function getVersionString() {
		return '6.0 pre alpha';
	}

	/**
	 * get the current installed edition of ownCloud. There is the community
	 * edition that just returns an empty string and the enterprise edition
	 * that returns "Enterprise".
	 * @return string
	 */
	public static function getEditionString() {
		return '';
	}

	/**
	 * add a javascript file
	 *
	 * @param appid  $application
	 * @param filename  $file
	 */
	public static function addScript( $application, $file = null ) {
		if( is_null( $file )) {
			$file = $application;
			$application = "";
		}
		if( !empty( $application )) {
			self::$scripts[] = "$application/js/$file";
		}else{
			self::$scripts[] = "js/$file";
		}
	}

	/**
	 * add a css file
	 *
	 * @param appid  $application
	 * @param filename  $file
	 */
	public static function addStyle( $application, $file = null ) {
		if( is_null( $file )) {
			$file = $application;
			$application = "";
		}
		if( !empty( $application )) {
			self::$styles[] = "$application/css/$file";
		}else{
			self::$styles[] = "css/$file";
		}
	}

	/**
	 * @brief Add a custom element to the header
	 * @param string tag tag name of the element
	 * @param array $attributes array of attributes for the element
	 * @param string $text the text content for the element
	 */
	public static function addHeader( $tag, $attributes, $text='') {
		self::$headers[] = array('tag'=>$tag, 'attributes'=>$attributes, 'text'=>$text);
	}

	/**
	 * formats a timestamp in the "right" way
	 *
	 * @param int timestamp $timestamp
	 * @param bool dateOnly option to omit time from the result
	 */
	public static function formatDate( $timestamp, $dateOnly=false) {
		if(\OC::$session->exists('timezone')) {//adjust to clients timezone if we know it
			$systemTimeZone = intval(date('O'));
			$systemTimeZone=(round($systemTimeZone/100, 0)*60)+($systemTimeZone%100);
			$clientTimeZone=\OC::$session->get('timezone')*60;
			$offset=$clientTimeZone-$systemTimeZone;
			$timestamp=$timestamp+$offset*60;
		}
		$l=OC_L10N::get('lib');
		return $l->l($dateOnly ? 'date' : 'datetime', $timestamp);
	}

	/**
	 * check if the current server configuration is suitable for ownCloud
	 * @return array arrays with error messages and hints
	 */
	public static function checkServer() {
		$errors=array();

		$defaults = new \OC_Defaults();

		$web_server_restart= false;
		//check for database drivers
		if(!(is_callable('sqlite_open') or class_exists('SQLite3'))
			and !is_callable('mysql_connect')
			and !is_callable('pg_connect')
			and !is_callable('oci_connect')) {
			$errors[]=array('error'=>'No database drivers (sqlite, mysql, or postgresql) installed.',
				'hint'=>'');//TODO: sane hint
			$web_server_restart= true;
		}

		//common hint for all file permissons error messages
		$permissionsHint = 'Permissions can usually be fixed by '
			.'<a href="' . $defaults->getDocBaseUrl() . '/server/5.0/admin_manual/installation/installation_source.html#set-the-directory-permissions" target="_blank">giving the webserver write access to the root directory</a>.';

		// Check if config folder is writable.
		if(!is_writable(OC::$SERVERROOT."/config/") or !is_readable(OC::$SERVERROOT."/config/")) {
			$errors[] = array(
				'error' => "Can't write into config directory",
				'hint' => 'This can usually be fixed by '
					.'<a href="' . $defaults->getDocBaseUrl() . '/server/5.0/admin_manual/installation/installation_source.html#set-the-directory-permissions" target="_blank">giving the webserver write access to the config directory</a>.'
				);
		}

		// Check if there is a writable install folder.
		if(OC_Config::getValue('appstoreenabled', true)) {
			if( OC_App::getInstallPath() === null
				|| !is_writable(OC_App::getInstallPath())
				|| !is_readable(OC_App::getInstallPath()) ) {
				$errors[] = array(
					'error' => "Can't write into apps directory",
					'hint' => 'This can usually be fixed by '
						.'<a href="' . $defaults->getDocBaseUrl() . '/server/5.0/admin_manual/installation/installation_source.html#set-the-directory-permissions" target="_blank">giving the webserver write access to the apps directory</a> '
						.'or disabling the appstore in the config file.'
					);
			}
		}
		$CONFIG_DATADIRECTORY = OC_Config::getValue( "datadirectory", OC::$SERVERROOT."/data" );
		// Create root dir.
		if(!is_dir($CONFIG_DATADIRECTORY)) {
			$success=@mkdir($CONFIG_DATADIRECTORY);
			if ($success) {
				$errors = array_merge($errors, self::checkDataDirectoryPermissions($CONFIG_DATADIRECTORY));
			} else {
				$errors[] = array(
					'error' => "Can't create data directory (".$CONFIG_DATADIRECTORY.")",
					'hint' => 'This can usually be fixed by '
					.'<a href="' . $defaults->getDocBaseUrl() . '/server/5.0/admin_manual/installation/installation_source.html#set-the-directory-permissions" target="_blank">giving the webserver write access to the root directory</a>.'
				);
			}
		} else if(!is_writable($CONFIG_DATADIRECTORY) or !is_readable($CONFIG_DATADIRECTORY)) {
			$errors[]=array('error'=>'Data directory ('.$CONFIG_DATADIRECTORY.') not writable by ownCloud',
				'hint'=>$permissionsHint);
		} else {
			$errors = array_merge($errors, self::checkDataDirectoryPermissions($CONFIG_DATADIRECTORY));
		}
		// check if all required php modules are present
		if(!class_exists('ZipArchive')) {
			$errors[]=array('error'=>'PHP module zip not installed.',
				'hint'=>'Please ask your server administrator to install the module.');
			$web_server_restart=true;
		}
		if(!class_exists('DOMDocument')) {
			$errors[] = array('error' => 'PHP module dom not installed.',
				'hint' => 'Please ask your server administrator to install the module.');
			$web_server_restart =true;
		}
		if(!function_exists('xml_parser_create')) {
			$errors[] = array('error' => 'PHP module libxml not installed.',
				'hint' => 'Please ask your server administrator to install the module.');
			$web_server_restart =true;
		}
		if(!function_exists('mb_detect_encoding')) {
			$errors[]=array('error'=>'PHP module mb multibyte not installed.',
				'hint'=>'Please ask your server administrator to install the module.');
			$web_server_restart=true;
		}
		if(!function_exists('ctype_digit')) {
			$errors[]=array('error'=>'PHP module ctype is not installed.',
				'hint'=>'Please ask your server administrator to install the module.');
			$web_server_restart=true;
		}
		if(!function_exists('json_encode')) {
			$errors[]=array('error'=>'PHP module JSON is not installed.',
				'hint'=>'Please ask your server administrator to install the module.');
			$web_server_restart=true;
		}
		if(!extension_loaded('gd') || !function_exists('gd_info')) {
			$errors[]=array('error'=>'PHP module GD is not installed.',
				'hint'=>'Please ask your server administrator to install the module.');
			$web_server_restart=true;
		}
		if(!function_exists('gzencode')) {
			$errors[]=array('error'=>'PHP module zlib is not installed.',
				'hint'=>'Please ask your server administrator to install the module.');
			$web_server_restart=true;
		}
		if(!function_exists('iconv')) {
			$errors[]=array('error'=>'PHP module iconv is not installed.',
				'hint'=>'Please ask your server administrator to install the module.');
			$web_server_restart=true;
		}
		if(!function_exists('simplexml_load_string')) {
			$errors[]=array('error'=>'PHP module SimpleXML is not installed.',
				'hint'=>'Please ask your server administrator to install the module.');
			$web_server_restart=true;
		}
		if(floatval(phpversion())<5.3) {
			$errors[]=array('error'=>'PHP 5.3 is required.',
				'hint'=>'Please ask your server administrator to update PHP to version 5.3 or higher.'
					.' PHP 5.2 is no longer supported by ownCloud and the PHP community.');
			$web_server_restart=true;
		}
		if(!defined('PDO::ATTR_DRIVER_NAME')) {
			$errors[]=array('error'=>'PHP PDO module is not installed.',
				'hint'=>'Please ask your server administrator to install the module.');
			$web_server_restart=true;
		}
		if (((strtolower(@ini_get('safe_mode')) == 'on')
			|| (strtolower(@ini_get('safe_mode')) == 'yes')
			|| (strtolower(@ini_get('safe_mode')) == 'true')
			|| (ini_get("safe_mode") == 1 ))) {
			$errors[]=array('error'=>'PHP Safe Mode is enabled. ownCloud requires that it is disabled to work properly.',
				'hint'=>'PHP Safe Mode is a deprecated and mostly useless setting that should be disabled. Please ask your server administrator to disable it in php.ini or in your webserver config.');
			$web_server_restart=true;
		}
		if (get_magic_quotes_gpc() == 1 ) {
			$errors[]=array('error'=>'Magic Quotes is enabled. ownCloud requires that it is disabled to work properly.',
				'hint'=>'Magic Quotes is a deprecated and mostly useless setting that should be disabled. Please ask your server administrator to disable it in php.ini or in your webserver config.');
			$web_server_restart=true;
		}

		if($web_server_restart) {
			$errors[]=array('error'=>'PHP modules have been installed, but they are still listed as missing?',
				'hint'=>'Please ask your server administrator to restart the web server.');
		}

		return $errors;
	}

	/**
	* Check for correct file permissions of data directory
	* @return array arrays with error messages and hints
	*/
	public static function checkDataDirectoryPermissions($dataDirectory) {
		$errors = array();
		if (stristr(PHP_OS, 'WIN')) {
			//TODO: permissions checks for windows hosts
		} else {
			$permissionsModHint = 'Please change the permissions to 0770 so that the directory'
				.' cannot be listed by other users.';
			$prems = substr(decoct(@fileperms($dataDirectory)), -3);
			if (substr($prems, -1) != '0') {
				OC_Helper::chmodr($dataDirectory, 0770);
				clearstatcache();
				$prems = substr(decoct(@fileperms($dataDirectory)), -3);
				if (substr($prems, 2, 1) != '0') {
					$errors[] = array('error' => 'Data directory ('.$dataDirectory.') is readable for other users',
						'hint' => $permissionsModHint);
				}
			}
		}
		return $errors;
	}

	public static function displayLoginPage($errors = array()) {
		$parameters = array();
		foreach( $errors as $key => $value ) {
			$parameters[$value] = true;
		}
		if (!empty($_POST['user'])) {
			$parameters["username"] = $_POST['user'];
			$parameters['user_autofocus'] = false;
		} else {
			$parameters["username"] = '';
			$parameters['user_autofocus'] = true;
		}
		if (isset($_REQUEST['redirect_url'])) {
			$redirect_url = $_REQUEST['redirect_url'];
			$parameters['redirect_url'] = urlencode($redirect_url);
		}

		$parameters['alt_login'] = OC_App::getAlternativeLogIns();
		OC_Template::printGuestPage("", "login", $parameters);
	}


	/**
	 * Check if the app is enabled, redirects to home if not
	 */
	public static function checkAppEnabled($app) {
		if( !OC_App::isEnabled($app)) {
			header( 'Location: '.OC_Helper::linkToAbsolute( '', 'index.php' ));
			exit();
		}
	}

	/**
	 * Check if the user is logged in, redirects to home if not. With
	 * redirect URL parameter to the request URI.
	 */
	public static function checkLoggedIn() {
		// Check if we are a user
		if( !OC_User::isLoggedIn()) {
			header( 'Location: '.OC_Helper::linkToAbsolute( '', 'index.php',
				array('redirect_url' => OC_Request::requestUri())));
			exit();
		}
	}

	/**
	 * Check if the user is a admin, redirects to home if not
	 */
	public static function checkAdminUser() {
		if( !OC_User::isAdminUser(OC_User::getUser())) {
			header( 'Location: '.OC_Helper::linkToAbsolute( '', 'index.php' ));
			exit();
		}
	}

	/**
	 * Check if the user is a subadmin, redirects to home if not
	 * @return array $groups where the current user is subadmin
	 */
	public static function checkSubAdminUser() {
		if(!OC_SubAdmin::isSubAdmin(OC_User::getUser())) {
			header( 'Location: '.OC_Helper::linkToAbsolute( '', 'index.php' ));
			exit();
		}
		return true;
	}

	/**
	 * Redirect to the user default page
	 */
	public static function redirectToDefaultPage() {
		if(isset($_REQUEST['redirect_url'])) {
			$location = OC_Helper::makeURLAbsolute(urldecode($_REQUEST['redirect_url']));
		}
		else if (isset(OC::$REQUESTEDAPP) && !empty(OC::$REQUESTEDAPP)) {
			$location = OC_Helper::linkToAbsolute( OC::$REQUESTEDAPP, 'index.php' );
		}
		else {
			$defaultpage = OC_Appconfig::getValue('core', 'defaultpage');
			if ($defaultpage) {
				$location = OC_Helper::makeURLAbsolute(OC::$WEBROOT.'/'.$defaultpage);
			}
			else {
				$location = OC_Helper::linkToAbsolute( 'files', 'index.php' );
			}
		}
		OC_Log::write('core', 'redirectToDefaultPage: '.$location, OC_Log::DEBUG);
		header( 'Location: '.$location );
		exit();
	}

    /**
     * get an id unique for this instance
     * @return string
     */
    public static function getInstanceId() {
        $id = OC_Config::getValue('instanceid', null);
        if(is_null($id)) {
            // We need to guarantee at least one letter in instanceid so it can be used as the session_name
            $id = 'oc' . OC_Util::generate_random_bytes(10);
            OC_Config::setValue('instanceid', $id);
        }
        return $id;
    }

	/**
	 * @brief Static lifespan (in seconds) when a request token expires.
	 * @see OC_Util::callRegister()
	 * @see OC_Util::isCallRegistered()
	 * @description
	 * Also required for the client side to compute the piont in time when to
	 * request a fresh token. The client will do so when nearly 97% of the
	 * timespan coded here has expired.
	 */
	public static $callLifespan = 3600; // 3600 secs = 1 hour

	/**
	 * @brief Register an get/post call. Important to prevent CSRF attacks.
	 * @todo Write howto: CSRF protection guide
	 * @return $token Generated token.
	 * @description
	 * Creates a 'request token' (random) and stores it inside the session.
	 * Ever subsequent (ajax) request must use such a valid token to succeed,
	 * otherwise the request will be denied as a protection against CSRF.
	 * The tokens expire after a fixed lifespan.
	 * @see OC_Util::$callLifespan
	 * @see OC_Util::isCallRegistered()
	 */
	public static function callRegister() {
		// Check if a token exists
		if(!\OC::$session->exists('requesttoken')) {
			// No valid token found, generate a new one.
			$requestToken = self::generate_random_bytes(20);
			\OC::$session->set('requesttoken', $requestToken);
		} else {
			// Valid token already exists, send it
			$requestToken = \OC::$session->get('requesttoken');
		}
		return($requestToken);
	}

	/**
	 * @brief Check an ajax get/post call if the request token is valid.
	 * @return boolean False if request token is not set or is invalid.
	 * @see OC_Util::$callLifespan
	 * @see OC_Util::callRegister()
	 */
	public static function isCallRegistered() {
		if(!\OC::$session->exists('requesttoken')) {
			return false;
		}

		if(isset($_GET['requesttoken'])) {
			$token=$_GET['requesttoken'];
		} elseif(isset($_POST['requesttoken'])) {
			$token=$_POST['requesttoken'];
		} elseif(isset($_SERVER['HTTP_REQUESTTOKEN'])) {
			$token=$_SERVER['HTTP_REQUESTTOKEN'];
		} else {
			//no token found.
			return false;
		}

		// Check if the token is valid
		if($token !== \OC::$session->get('requesttoken')) {
			// Not valid
			return false;
		} else {
			// Valid token
			return true;
		}
	}

	/**
	 * @brief Check an ajax get/post call if the request token is valid. exit if not.
	 * Todo: Write howto
	 */
	public static function callCheck() {
		if(!OC_Util::isCallRegistered()) {
			exit;
		}
	}

	/**
	 * @brief Public function to sanitize HTML
	 *
	 * This function is used to sanitize HTML and should be applied on any
	 * string or array of strings before displaying it on a web page.
	 *
	 * @param string or array of strings
	 * @return array with sanitized strings or a single sanitized string, depends on the input parameter.
	 */
	public static function sanitizeHTML( &$value ) {
		if (is_array($value)) {
			array_walk_recursive($value, 'OC_Util::sanitizeHTML');
		} else {
			$value = htmlentities((string)$value, ENT_QUOTES, 'UTF-8'); //Specify encoding for PHP<5.4
		}
		return $value;
	}
	
	/**
	 * @brief Public function to encode url parameters
	 *
	 * This function is used to encode path to file before output.
	 * Encoding is done according to RFC 3986 with one exception:
	 * Character '/' is preserved as is. 
	 *
	 * @param string $component part of URI to encode
	 * @return string 
	 */
	public static function encodePath($component) {
		$encoded = rawurlencode($component);
		$encoded = str_replace('%2F', '/', $encoded);
		return $encoded;
	}

	/**
	 * Check if the htaccess file is working by creating a test file in the data directory and trying to access via http
	 */
	public static function ishtaccessworking() {
		// testdata
		$filename='/htaccesstest.txt';
		$testcontent='testcontent';

		// creating a test file
		$testfile = OC_Config::getValue( "datadirectory", OC::$SERVERROOT."/data" ).'/'.$filename;

		if(file_exists($testfile)) {// already running this test, possible recursive call
			return false;
		}

		$fp = @fopen($testfile, 'w');
		@fwrite($fp, $testcontent);
		@fclose($fp);

		// accessing the file via http
		$url = OC_Helper::makeURLAbsolute(OC::$WEBROOT.'/data'.$filename);
		$fp = @fopen($url, 'r');
		$content=@fread($fp, 2048);
		@fclose($fp);

		// cleanup
		@unlink($testfile);

		// does it work ?
		if($content==$testcontent) {
			return(false);
		}else{
			return(true);
		}
	}

	/**
	 * we test if webDAV is working properly
	 *
	 * The basic assumption is that if the server returns 401/Not Authenticated for an unauthenticated PROPFIND
	 * the web server it self is setup properly.
	 *
	 * Why not an authenticated PROFIND and other verbs?
	 *  - We don't have the password available
	 *  - We have no idea about other auth methods implemented (e.g. OAuth with Bearer header)
	 *
	 */
	public static function isWebDAVWorking() {
		if (!function_exists('curl_init')) {
			return true;
		}
		$settings = array(
			'baseUri' => OC_Helper::linkToRemote('webdav'),
		);

		// save the old timeout so that we can restore it later
		$old_timeout=ini_get("default_socket_timeout");

		// use a 5 sec timeout for the check. Should be enough for local requests.
		ini_set("default_socket_timeout", 5);

		$client = new \Sabre_DAV_Client($settings);

		// for this self test we don't care if the ssl certificate is self signed and the peer cannot be verified.
		$client->setVerifyPeer(false);

		$return = true;
		try {
			// test PROPFIND
			$client->propfind('', array('{DAV:}resourcetype'));
		} catch(\Sabre_DAV_Exception_NotAuthenticated $e) {
			$return = true;
		} catch(\Exception $e) {
			OC_Log::write('core', 'isWebDAVWorking: NO - Reason: '.$e->getMessage(). ' ('.get_class($e).')', OC_Log::WARN);
			$return = false;
		}

		// restore the original timeout
		ini_set("default_socket_timeout", $old_timeout);

		return $return;
	}

	/**
	 * Check if the setlocal call doesn't work. This can happen if the right
	 * local packages are not available on the server.
	 */
	public static function issetlocaleworking() {
		// setlocale test is pointless on Windows
		if (OC_Util::runningOnWindows() ) {
			return true;
		}

		$result = setlocale(LC_ALL, 'en_US.UTF-8', 'en_US.UTF8');
		if($result == false) {
			return false;
		}
		return true;
	}

	/**
	 * Check if the PHP module fileinfo is loaded.
	 * @return bool
	 */
	public static function fileInfoLoaded() {
		return function_exists('finfo_open');
	}

	/**
	 * Check if the ownCloud server can connect to the internet
	 */
	public static function isInternetConnectionWorking() {
		// in case there is no internet connection on purpose return false
		if (self::isInternetConnectionEnabled() === false) {
			return false;
		}

		// try to connect to owncloud.org to see if http connections to the internet are possible.
		$connected = @fsockopen("www.owncloud.org", 80);
		if ($connected) {
			fclose($connected);
			return true;
		}else{

			// second try in case one server is down
			$connected = @fsockopen("apps.owncloud.com", 80);
			if ($connected) {
				fclose($connected);
				return true;
			}else{
				return false;
			}

		}

	}
	
	/**
	 * Check if the connection to the internet is disabled on purpose
	 */
	public static function isInternetConnectionEnabled(){
		return \OC_Config::getValue("has_internet_connection", true);
	}

	/**
	 * clear all levels of output buffering
	 */
	public static function obEnd(){
		while (ob_get_level()) {
			ob_end_clean();
		}
	}


	/**
	 * @brief Generates a cryptographical secure pseudorandom string
	 * @param Int with the length of the random string
	 * @return String
	 * Please also update secureRNG_available if you change something here
	 */
	public static function generate_random_bytes($length = 30) {

		// Try to use openssl_random_pseudo_bytes
		if(function_exists('openssl_random_pseudo_bytes')) {
			$pseudo_byte = bin2hex(openssl_random_pseudo_bytes($length, $strong));
			if($strong == true) {
				return substr($pseudo_byte, 0, $length); // Truncate it to match the length
			}
		}

		// Try to use /dev/urandom
		$fp = @file_get_contents('/dev/urandom', false, null, 0, $length);
		if ($fp !== false) {
			$string = substr(bin2hex($fp), 0, $length);
			return $string;
		}

		// Fallback to mt_rand()
		$characters = '0123456789';
		$characters .= 'abcdefghijklmnopqrstuvwxyz';
		$charactersLength = strlen($characters)-1;
		$pseudo_byte = "";

		// Select some random characters
		for ($i = 0; $i < $length; $i++) {
			$pseudo_byte .= $characters[mt_rand(0, $charactersLength)];
		}
		return $pseudo_byte;
	}

	/**
	 * @brief Checks if a secure random number generator is available
	 * @return bool
	 */
	public static function secureRNG_available() {

		// Check openssl_random_pseudo_bytes
		if(function_exists('openssl_random_pseudo_bytes')) {
			openssl_random_pseudo_bytes(1, $strong);
			if($strong == true) {
				return true;
			}
		}

		// Check /dev/urandom
		$fp = @file_get_contents('/dev/urandom', false, null, 0, 1);
		if ($fp !== false) {
			return true;
		}

		return false;
	}

	/**
	 * @Brief Get file content via curl.
	 * @param string $url Url to get content
	 * @return string of the response or false on error
	 * This function get the content of a page via curl, if curl is enabled.
	 * If not, file_get_element is used.
	 */

	public static function getUrlContent($url){

		if  (function_exists('curl_init')) {

			$curl = curl_init();

			curl_setopt($curl, CURLOPT_HEADER, 0);
			curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
			curl_setopt($curl, CURLOPT_CONNECTTIMEOUT, 10);
			curl_setopt($curl, CURLOPT_URL, $url);
			curl_setopt($curl, CURLOPT_FOLLOWLOCATION, true);
			curl_setopt($curl, CURLOPT_MAXREDIRS, 10);

			curl_setopt($curl, CURLOPT_USERAGENT, "ownCloud Server Crawler");
			if(OC_Config::getValue('proxy', '')<>'') {
				curl_setopt($curl, CURLOPT_PROXY, OC_Config::getValue('proxy'));
			}
			if(OC_Config::getValue('proxyuserpwd', '')<>'') {
				curl_setopt($curl, CURLOPT_PROXYUSERPWD, OC_Config::getValue('proxyuserpwd'));
			}
			$data = curl_exec($curl);
			curl_close($curl);

		} else {
			$contextArray = null;

			if(OC_Config::getValue('proxy', '')<>'') {
				$contextArray = array(
					'http' => array(
						'timeout' => 10,
						'proxy' => OC_Config::getValue('proxy')
					)
				);
			} else {
				$contextArray = array(
					'http' => array(
						'timeout' => 10
					)
				);
			}


			$ctx = stream_context_create(
				$contextArray
			);
			$data=@file_get_contents($url, 0, $ctx);

		}
		return $data;
	}

	/**
	 * @return bool - well are we running on windows or not
	 */
	public static function runningOnWindows() {
		return (substr(PHP_OS, 0, 3) === "WIN");
	}


	/**
	 * Handles the case that there may not be a theme, then check if a "default"
	 * theme exists and take that one
	 * @return string the theme
	 */
	public static function getTheme() {
		$theme = OC_Config::getValue("theme", '');

		if($theme === '') {
			
			if(is_dir(OC::$SERVERROOT . '/themes/default')) {
				$theme = 'default';
			}

		}

		return $theme;
	}

	/**
	 * Clear the opcode cache if one exists
	 * This is necessary for writing to the config file
	 * in case the opcode cache doesn't revalidate files
	 */
	public static function clearOpcodeCache() {
		// APC
		if (function_exists('apc_clear_cache')) {
			apc_clear_cache();
		}
		// Zend Opcache
		if (function_exists('accelerator_reset')) {
			accelerator_reset();
		}
		// XCache
		if (function_exists('xcache_clear_cache')) {
			xcache_clear_cache(XC_TYPE_VAR, 0);
		}
	}

	/**
	 * Normalize a unicode string
	 * @param string $value a not normalized string
	 * @return bool|string
	 */
	public static function normalizeUnicode($value) {
		if(class_exists('Patchwork\PHP\Shim\Normalizer')) {
			$normalizedValue = \Patchwork\PHP\Shim\Normalizer::normalize($value);
			if($normalizedValue === false) {
				\OC_Log::write( 'core', 'normalizing failed for "' . $value . '"', \OC_Log::WARN);
			} else {
				$value = $normalizedValue;
			}
		}

		return $value;
	}
}
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