<?php
/**
 * Piwik - Open source web analytics
 *
 * @link http://piwik.org
 * @license http://www.gnu.org/licenses/gpl-3.0.html GPL v3 or later
 * @version $Id$
 *
 * @category Piwik
 * @package Piwik
 */

/**
 * Nonce class.
 *
 * A cryptographic nonce -- "number used only once" -- is often recommended as part of a robust defense against cross-site request forgery (CSRF/XSRF).
 * Desrable characteristics: limited lifetime, uniqueness, unpredictability (pseudo-randomness).
 *
 * We use a session-dependent nonce with a configurable expiration that combines and hashes:
 * - a private salt because it's non-public
 * - time() because it's unique
 * - a mix of PRNGs (pseudo-random number generators) to increase entropy and make it less predictable
 *
 * @package Piwik
 */
class Piwik_Nonce
{
	/**
	 * Generate nonce
	 *
	 * @param string $id Unique id to avoid namespace conflicts, e.g., ModuleName.ActionName
	 * @param int $ttl Optional time-to-live in seconds; default is 5 minutes
	 * @return string Nonce
	 */
	static public function getNonce($id, $ttl = 300)
	{
		// save session-dependent nonce
		$ns = new Zend_Session_Namespace($id);
		$nonce = $ns->nonce;

		// re-use an unexpired nonce (a small deviation from the "used only once" principle, so long as we do not reset the expiration)
		// to handle browser pre-fetch or double fetch caused by some browser add-ons/extensions
		if(empty($nonce))
		{
			// generate a new nonce
			$nonce = md5(Piwik_Common::getSalt() . time() . Piwik_Common::generateUniqId());
			$ns->nonce = $nonce;
			$ns->setExpirationSeconds($ttl, 'nonce');
		}

		return $nonce;
	}

	/**
	 * Verify nonce and check referrer (if present, i.e., it may be suppressed by the browser or a proxy/network).
	 *
	 * @param string $id Unique id
	 * @param string $cnonce Nonce sent to client
	 * @return bool true if valid; false otherwise
	 */
	static public function verifyNonce($id, $cnonce)
	{
		$ns = new Zend_Session_Namespace($id);
		$nonce = $ns->nonce;

		// validate token
		if(empty($cnonce) || $cnonce !== $nonce)
		{
			return false;
		}

		// validate referer
		$referer = Piwik_Url::getReferer();
		if(!empty($referer) && (Piwik_Url::getLocalReferer() === false))
		{
			return false;
		}

		// validate origin
		$origin = Piwik_Url::getOrigin();
		if(!empty($origin) &&
			($origin == 'null'
			|| !in_array($origin, Piwik_Url::getAcceptableOrigins())))
		{
			return false;
		}

		return true;
	}
}