CpawCTF Writeup Level 3 Q23.[Reversing]またやらかした！ (注意<spoiler>)

Q23.[Reversing]またやらかした！

rev200をghidraで解析する

Windows10 で Ghidra を動かす　https://hi120ki.github.io/blog/posts/20200109/

fileから新規プロジェクトを選択。適当なディレクトリでプロジェクト作成する。

選択したディレクトリにrev200をコピーする。

rev200をクリックすると解析が始まる。

解析のフィルタからmainを入力し選択。

右の解析出力にｃのコードが出力される。

rev200.cpp

undefined4 main(void)

{
  int iVar1;
  uint *puVar2;
  int local_84;
  uint local_7c [14];
  uint local_44 [14];
  
  local_7c[0] = 0x7a;
  local_7c[1] = 0x69;
  local_7c[2] = 0x78;
  local_7c[3] = 0x6e;
  local_7c[4] = 0x62;
  local_7c[5] = 0x6f;
  local_7c[6] = 0x7c;
  local_7c[7] = 0x6b;
  local_7c[8] = 0x77;
  local_7c[9] = 0x78;
  local_7c[10] = 0x74;
  local_7c[11] = 0x38;
  local_7c[12] = 0x38;
  local_7c[13] = 100;
  puVar2 = local_44;
  for (iVar1 = 0xe; iVar1 != 0; iVar1 = iVar1 + -1) {
    *puVar2 = 0;
    puVar2 = puVar2 + 1;
  }
  for (local_84 = 0; local_84 < 0xe; local_84 = local_84 + 1) {
    local_44[local_84] = local_7c[local_84] ^ 0x19;
  }
  return 0;
}

エラーとなる文を修正し、コードを実行するとフラグが現れる。

rev200fix.cpp

#include <iostream>
#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)

int main(void)

{
  int iVar1;
  unsigned int *puVar2;
  int local_84;
  unsigned int local_7c [14];
  unsigned int local_44 [14];
  
  local_7c[0] = 0x7a;
  local_7c[1] = 0x69;
  local_7c[2] = 0x78;
  local_7c[3] = 0x6e;
  local_7c[4] = 0x62;
  local_7c[5] = 0x6f;
  local_7c[6] = 0x7c;
  local_7c[7] = 0x6b;
  local_7c[8] = 0x77;
  local_7c[9] = 0x78;
  local_7c[10] = 0x74;
  local_7c[11] = 0x38;
  local_7c[12] = 0x38;
  local_7c[13] = 100;
  puVar2 = local_44;
  for (iVar1 = 0xe; iVar1 != 0; iVar1 = iVar1 + -1) {
    *puVar2 = 0;
    puVar2 = puVar2 + 1;
  }
  for (local_84 = 0; local_84 < 0xe; local_84 = local_84 + 1) {
    local_44[local_84] = local_7c[local_84] ^ 0x19;
  }
  char c;
  rep(i,14){
    c = local_44[i];
    cout << c ;
  }
  cout << endl;
  return 0;
}

cpaw{vernam!!}