PHP functions to encrypt/decrypt arbitrary data (binary safe) that gets devalued and useless after a specified amount of time. Useful e.g. for authorization tokens.

timecrypt.php

<?php

// Copyright 1012 Gerald Schittenhelm
// http://schittenhelm.at
// MIT License

function timecrypt($string,$key,$lifetime=3600) {
	$key = md5($key);
	$result = '';
	for($i=0; $i<strlen ($string); $i++) {
		$char = substr($string, $i, 1);
		$keychar = substr($key, ($i % strlen($key))-1, 1);
		$char = chr(ord($char)+ord($keychar));
		$result.=$char;
	}
	$death = time() + $lifetime;
	return str_replace("=","",base64_encode($death.".".base_convert(md5($death.".".$key.".".$result),16,36).":".$result));
}

function timedecrypt($string,$key) {
	$key = md5($key);
	$result = '';
	$string = base64_decode($string);
	$string = explode(":",$string,2);
	list($death,$checksum) = explode(".",$string[0],2);
	$string = $string[1];
	if($death<=time()) return false;
	if($checksum!==base_convert(md5($death.".".$key.".".$string),16,36)) return null;
	for($i=0; $i<strlen($string); $i++) {
		$char = substr($string, $i, 1);
		$keychar = substr($key, ($i % strlen($key))-1, 1);
		$char = chr(ord($char)-ord($keychar));
		$result.=$char;
	}
	return $result;
}

?>