[BUUCTF]reverse3

发表于 更新于 分类于 本文字数： 697 阅读时长 ≈ 3 分钟

无壳, 直接拖进IDA分析, F5查看伪代码

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int __cdecl main_0(int argc, const char **argv, const char **envp)
{
  size_t v3; // eax
  const char *v4; // eax
  size_t v5; // eax
  char v7; // [esp+0h] [ebp-188h]
  char v8; // [esp+0h] [ebp-188h]
  signed int j; // [esp+DCh] [ebp-ACh]
  int i; // [esp+E8h] [ebp-A0h]
  signed int v11; // [esp+E8h] [ebp-A0h]
  char Destination[108]; // [esp+F4h] [ebp-94h] BYREF
  char Str[28]; // [esp+160h] [ebp-28h] BYREF
  char v14[8]; // [esp+17Ch] [ebp-Ch] BYREF

  for ( i = 0; i < 100; ++i )
  {
    if ( (unsigned int)i >= 0x64 )
      j____report_rangecheckfailure();	//应该是VS调试器加的代码,不管
    Destination[i] = 0;
  }
  sub_41132F("please enter the flag:", v7);
  sub_411375("%20s", (char)Str);
  v3 = j_strlen(Str);
  v4 = (const char *)sub_4110BE(Str, v3, v14);	//注意这个sub_4110BE()函数, 作用未知
      strncpy(Destination, v4, 0x28u);	//将sub_4110BE()返回值指向的字符拷到Destination数组
  v11 = j_strlen(Destination);
  for ( j = 0; j < v11; ++j )
    Destination[j] += j;		//字符移位
  v5 = j_strlen(Destination);
  if ( !strncmp(Destination, Str2, v5) )	//Str2="e3nifIH9b_C@n@dH"
    sub_41132F("rigth flag!\n", v8);		
  else
    sub_41132F("wrong flag!\n", v8);
  return 0;
}

关键在于sub_4110BE()这个函数, 菜鸡一时没看出来它是干啥的. 于是把它从IDA抠出来, 略作修改让它能跑起来, 命名为unknown(), 看看这个它的作用

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#include <cstdio>
#include <cstdlib>
#include <cstring>

using namespace std;

void* __cdecl unknown(char* a1, unsigned int a2)	//第三个参数无用, 可删去
{
    int v4; // [esp+D4h] [ebp-38h]
    int v5; // [esp+D4h] [ebp-38h]
    int v6; // [esp+D4h] [ebp-38h]
    int v7; // [esp+D4h] [ebp-38h]
    int i; // [esp+E0h] [ebp-2Ch]
    unsigned int v9; // [esp+ECh] [ebp-20h]
    int v10; // [esp+ECh] [ebp-20h]
    int v11; // [esp+ECh] [ebp-20h]
    void* v12; // [esp+F8h] [ebp-14h]
    char* v13; // [esp+104h] [ebp-8h]

    if (!a1 || !a2)
        return 0;
    v9 = a2 / 3;
    if ((int)(a2 / 3) % 3)
        ++v9;
    v10 = 4 * v9;
    v12 = malloc(v10 + 1);
    if (!v12)
        return 0;
    memset(v12, 0, v10 + 1);
    char byte_41A144[4];
    char aAbcdefghijklmn[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=\x00";
    //后来才知道,很明显这是一个base64的码表
    v13 = a1;
    v11 = a2;
    v4 = 0;
    while (v11 > 0)
    {
        byte_41A144[2] = 0;
        byte_41A144[1] = 0;
        byte_41A144[0] = 0;
        for (i = 0; i < 3 && v11 >= 1; ++i)
        {
            byte_41A144[i] = *v13;
            --v11;
            ++v13;
        }
        if (!i)
            break;
        switch (i)
        {
        case 1:
            *((unsigned char*)v12 + v4) = aAbcdefghijklmn[(int)(unsigned __int8)byte_41A144[0] >> 2];
            v5 = v4 + 1;
            *((unsigned char*)v12 + v5) = aAbcdefghijklmn[((byte_41A144[1] & 0xF0) >> 4) | (16 * (byte_41A144[0] & 3))];
            *((unsigned char*)v12 + ++v5) = aAbcdefghijklmn[64];
            *((unsigned char*)v12 + ++v5) = aAbcdefghijklmn[64];
            v4 = v5 + 1;
            break;
        case 2:
            *((unsigned char*)v12 + v4) = aAbcdefghijklmn[(int)(unsigned __int8)byte_41A144[0] >> 2];
            v6 = v4 + 1;
            *((unsigned char*)v12 + v6) = aAbcdefghijklmn[((byte_41A144[1] & 0xF0) >> 4) | (16 * (byte_41A144[0] & 3))];
            *((unsigned char*)v12 + ++v6) = aAbcdefghijklmn[((byte_41A144[2] & 0xC0) >> 6) | (4 * (byte_41A144[1] & 0xF))];
            *((unsigned char*)v12 + ++v6) = aAbcdefghijklmn[64];
            v4 = v6 + 1;
            break;
        case 3:
            *((unsigned char*)v12 + v4) = aAbcdefghijklmn[(int)(unsigned __int8)byte_41A144[0] >> 2];
            v7 = v4 + 1;
            *((unsigned char*)v12 + v7) = aAbcdefghijklmn[((byte_41A144[1] & 0xF0) >> 4) | (16 * (byte_41A144[0] & 3))];
            *((unsigned char*)v12 + ++v7) = aAbcdefghijklmn[((byte_41A144[2] & 0xC0) >> 6) | (4 * (byte_41A144[1] & 0xF))];
            *((unsigned char*)v12 + ++v7) = aAbcdefghijklmn[byte_41A144[2] & 0x3F];
            v4 = v7 + 1;
            break;
        }
    }
    *((unsigned char*)v12 + v4) = 0;
    return v12;
}

int main()
{
    char a[] = "Hello World!";
    printf("%s", (char*)unknown(a, sizeof(a)));
}
//SGVsbG8gV29ybGQhAA==
//看起来是进行base64编码的函数

知道了这个函数的作用, 接下来只需要处理一下字符的移位就可以还原出原文了

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import base64

ciphertext = "e3nifIH9b_C@n@dH"
plaintext = ""
for i in range(0,len(ciphertext)):
    plaintext += chr(ord(ciphertext[i])-i)
print(base64.b64decode(plaintext).decode())
#{i_l0ve_you}

验证一下, 没毛病

image-20220121230115616