phpmcrypt_encrypt后json_decode结果为null,该如何处理

$key = '1234567890123456';
    $iv = '1234567890123456';
    $os = array(
        'os' => php_uname('s'),
        'time' => time(),
        );
    $os = json_encode($os);
    $crypttext = base64_encode((mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $os, MCRYPT_MODE_CBC, $iv)));

    $crypttext = base64_decode($crypttext);

    $crypttext = (mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $crypttext, MCRYPT_MODE_CBC, $iv));
    $crypttext = json_decode($crypttext);
    var_dump($crypttext);

$key = '1234567890123456';
$iv = '1234567890123456';
$os = array(
  'os' => php_uname('s'),
  'time' => time(),
);
var_dump( $os = json_encode($os));
$crypttext = base64_encode((mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $os, MCRYPT_MODE_CBC, $iv)));
 
$crypttext = base64_decode($crypttext);
 
var_dump( $crypttext = (mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $crypttext, MCRYPT_MODE_CBC, $iv)));
echo bin2hex($crypttext), PHP_EOL;//尾部多了11个空字符
$crypttext = json_decode(trim($crypttext));//删掉就对了
var_dump($crypttext);

string(37) "{"os":"Windows NT","time":1428450826}"
string(48) "{"os":"Windows NT","time":1428450826}"
7b226f73223a2257696e646f7773204e54222c2274696d65223a313432383435303832367d0000000000000000000000
object(stdClass)#1 (2) {
  ["os"]=>
  string(10) "Windows NT"
  ["time"]=>
  int(1428450826)
}