Extra data starting at offset error showing in logs

It seems the used library adds extra NULL characters. See phpseclib_mcrypt_generic_helper(), the helper function called by other functions (phpseclib_mdecrypt_generic(), for example.)

    function phpseclib_mcrypt_generic_helper(Base $td, &$data, $op)
    {
        // phpseclib does not currently provide a way to retrieve the mode once it has been set via "public" methods
        if (phpseclib_mcrypt_enc_is_block_mode($td)) {
            $block_length = phpseclib_mcrypt_enc_get_iv_size($td);
            $extra = strlen($data) % $block_length;
            if ($extra) {
                $data.= str_repeat("\0", $block_length - $extra);
            }
        }