xctf attack and defense world web master advanced area web2

0x01. 进入环境,查看问题

如图,给了一串代码：

0x02. 问题分析

可以很清楚的看到,The problem is to decipher the ciphertextok了.Mainly code auditing,先解释一下phpfunction meaning in ：
strrev(string): 反转字符串
strlen(string): 返回字符串的长度
substr(string, start, length): 返回字符串的一部分
string: the desired string
start: where the string starts
length: 可选.规定被返回字符串的长度.默认是直到字符串的结尾
ord(string): Returns the first character of the string ASCII 值
chr(): 从指定的 ASCII 值返回对应的字符
str_rot13(string): 对字符串执行 ROT13 编码.
ROT13 编码把每一个字母在字母表中向前移动 13 个字母.数字和非字母字符保持不变
Both encoding and decoding are done by this function.If you take an encoded string as a parameter,then the original string will be returned
base64_encode(string): 使用 MIME base64 对数据进行编码.

进行反解密,Thrown into the online run：

<?php
	$miwen="a1zLbgQsCESEIqRLwuQAyMwLyq2L5VwBxqGA3RQAyumZ0tmMvSGM2ZwB4tws";
	$flag="";
	$a = base64_decode(strrev(str_rot13($miwen)));
	
	for($_0=0;$_0<strlen($a);$_0++){
    
	    $_c=substr($a,$_0,1);
	    $__=ord($_c)-1;
	    $_c=chr($__);
	    $flag=$flag.$_c;   
	}
	
	$b = strrev($flag);
	echo($b);
?>

into the online operating address：https://www.dooccn.com/php/
solve the final answer：flag:{NSCTF_b73d5adfb819c64603d7237fa0d52977}