逆向攻防世界CTF系列27-200simple-check-100

巨难受的一题，gdb环境配了半天

32位无壳

这里要check，先跟进下面的interesting_function看看

putchar，这里就是输出，那我们只需要动调将check_key那修改为正确就行

下断点

右上角右键modify成1

但是输出乱码。

查阅资料后可以用gbd动调

环境配了半天

如果是2024-11-11左右安装gbd如果出现报错，可以参考[Kali 2024 逆向调试 GDB 13.2 安装插件 Peda 不兼容报错解决方案_kali安装gdb-CSDN博客](https://blog.csdn.net/qq_51886509/article/details/141227182?ops_request_misc=%7B%22request%5Fid%22%3A%22984C9D72-904C-4F18-B580-8A359D6C46D7%22%2C%22scm%22%3A%2220140713.130102334.pc%5Fall.%22%7D&request_id=984C9D72-904C-4F18-B580-8A359D6C46D7&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2~all~first_rank_ecpm_v1~times_rank-2-141227182-null-null.142^v100^pc_search_result_base4&utm_term=no module named six gdb&spm=1018.2226.3001.4187)

开始

gbd

file task9_x86_64_46d01fe312d35ecf69c4ff8ab8ace75d080891dc

设置断点在main函数入口处

b main

开始运行

r

到这里继续单步执行,一直到test eax,eax那

n

查看eax寄存器值

i r eax

修改eax寄存器值

set $eax=1

修改就行

直接运行至程序结束

c

flag:

flag_is_you_know_cracking!!!

到这，其实还有解法，那就是纯静态分析，来自攻防世界的wp

unsigned int *__cdecl interesting_function(int *a1)
{
  unsigned int *result; // eax
  unsigned int v2; // [esp+1Ch] [ebp-1Ch] BYREF
  unsigned int *v3; // [esp+20h] [ebp-18h]
  int *v4; // [esp+24h] [ebp-14h]
  int j; // [esp+28h] [ebp-10h]
  int i; // [esp+2Ch] [ebp-Ch]

  result = (unsigned int *)a1;
  v4 = a1;
  for ( i = 0; i <= 6; ++i )
  {
    v2 = v4[i] ^ 0xDEADBEEF;
    result = &v2;
    v3 = &v2;
    for ( j = 3; j >= 0; --j )
      result = (unsigned int *)putchar((char)(*((_BYTE *)v3 + j) ^ flag_data[4 * i + j]));
  }
  return result;
}

这里无非是对flag做处理，enc来自flag_data，都可以获取，要点耐心还是可以解密的

我们不用管check_key，它的作用只是验证，我们目的是输出flag

#include <stdio.h>

unsigned char flag_data[] = {
	220, 23, 191, 91, 212, 10, 210, 27, 125, 218,
	167, 149, 181, 50, 16, 246, 28, 101, 83, 83,
	103, 186, 234, 110, 120, 34, 114, 211 };
char v7[] = {
	84, -56, 126, -29, 100, -57, 22, -102, -51, 17,
	101, 50, 45, -29, -45, 67, -110, -87, -99, -46,
	-26, 109, 44, -45, -74, -67, -2, 106 };

unsigned int v2;
unsigned char* v3;

int main() {
	for (int i = 0; i <= 6; ++i) {
		v2 = ((v7[4 * i] & 0x000000FF) + ((v7[4 * i + 1] & 0x000000FF) << 8)
			+ ((v7[4 * i + 2] & 0x000000FF) << 16) + ((v7[4 * i + 3] & 0x000000FF) <<
				24)) ^ 0xDEADBEEF;
		v3 = (unsigned char*)&v2;
		for (int j = 3; j >= 0; --j) printf("%c", *(v3 + j) ^ flag_data[4 * i + j]);
	}
	printf("\n");
	for (int i = 0; i <= 6; ++i) {
		printf("%c", v7[4 * i + 3] ^ 0xDE ^ flag_data[4 * i + 3]);
		printf("%c", v7[4 * i + 2] ^ 0xAD ^ flag_data[4 * i + 2]);
		printf("%c", v7[4 * i + 1] ^ 0xBE ^ flag_data[4 * i + 1]);
		printf("%c", v7[4 * i] ^ 0xEF ^ flag_data[4 * i]);
	}
}