[Reverse] MoeCTF Base

咱就是说有史以来最牛大的reverse非这题莫属

Reverse / IDA / Base64 难度：有手就行

0x00 题目分析

附件为 exe 程序，运行后要求输入 flag。

直接丢 IDA 看 main 直接瞳孔地震

0x01 IDA 反编译源码

直接查看 main 函数：

int __fastcall main(int argc, const char **argv, const char **envp)
{
  FILE *v3; // rax
  __int64 v4; // rdx
  __int64 v5; // rax
  unsigned __int64 v6; // rax
  char *v7; // rbx
  int v8; // eax
  char *v9; // rcx
  char v11[16]; // [rsp+20h] [rbp-98h] BYREF
  char Buffer[112]; // [rsp+30h] [rbp-88h] BYREF

  sub_140001010(Format);
  sub_140001010((char *)&byte_1400032F0);
  v3 = _acrt_iob_func(0);
  fgets(Buffer, 100, v3);
  v4 = -1;
  v5 = -1;
  do
    ++v5;
  while ( Buffer[v5] );
  if ( v5 && v11[v5 + 15] == 10 )
  {
    v6 = v5 - 1;
    if ( v6 >= 0x64 )
      sub_140001448(Buffer);
    Buffer[v6] = 0;
  }
  do
    ++v4;
  while ( Buffer[v4] );
  v7 = (char *)sub_140001070(Buffer, v4, v11);
  v8 = strcmp(v7, "bW9lY3Rme1kwdV9DNG5fRzAwZF9BdF9CNDVlNjQhIX0=");
  v9 = (char *)&unk_140003300;
  if ( v8 )
    v9 = (char *)&byte_140003318;
  sub_140001010(v9);
  free(v7);
  return 0;
}

0x02 解题思路

一眼看到单纯套了个base64：

strcmp(v7, "bW9lY3Rme1kwdV9DNG5fRzAwZF9BdF9CNDVlNjQhIX0=");

0x03 Base64 解码

使用 Hackbar / CyberChef 一键解码：

bW9lY3Rme1kwdV9DNG5fRzAwZF9BdF9CNDVlNjQhIX0=

0x04 Flag

moectf{Y0u_C4n_G00d_At_B45e64!!}