BUUCTF（34）特殊的 BASE64

news2025/1/2 0:03:53

使用pycharm时，如果想把代码撤销到之前的状态可以用 Ctrl+z

如果不小心撤销多了，可以用 Ctrl+Shift+Z 还原，别傻傻的重新敲了

BUUCTF在线评测 (buuoj.cn)

查看字符串，想到base64的变表

这里用的c++的标准程序库中的string，头文件是#include<string>

这是base64的加密函数

std::string __cdecl base64Encode(std::string *p_decode)
{
  std::string *v1; // rdx
  char *v2; // rax
  int v3; // ebx
  char *v4; // rax
  int v5; // ebx
  char *v6; // rax
  _BYTE *v7; // rax
  char *v8; // rax
  _BYTE *v9; // rax
  char *v10; // rax
  int v11; // ebx
  char *v12; // rax
  _BYTE *v13; // rax
  __int64 v15; // [rsp+0h] [rbp-80h] BYREF
  int pos_0; // [rsp+30h] [rbp-50h]
  int pos; // [rsp+34h] [rbp-4Ch]
  int len; // [rsp+38h] [rbp-48h]
  int i; // [rsp+3Ch] [rbp-44h]
  std::string *p_decodea; // [rsp+68h] [rbp-18h]

  p_decodea = v1;
  std::allocator<char>::allocator(&v15 + 47);
  std::string::string(p_decode, &unk_489084);
  std::allocator<char>::~allocator();
  len = std::string::length(p_decodea);
  for ( i = 0; len / 3 > i; ++i )
  {
    v2 = std::string::operator[](p_decodea, 3 * i);
    std::string::operator[](&baseKey, *v2 >> 2);
    std::string::operator+=(p_decode);
    v3 = 16 * (*std::string::operator[](p_decodea, 3 * i) & 3);
    v4 = std::string::operator[](p_decodea, 3 * i + 1);
    std::string::operator[](&baseKey, v3 | (*v4 >> 4));
    std::string::operator+=(p_decode);
    v5 = 4 * (*std::string::operator[](p_decodea, 3 * i + 1) & 0xF);
    v6 = std::string::operator[](p_decodea, 3 * i + 2);
    std::string::operator[](&baseKey, v5 | (*v6 >> 6));
    std::string::operator+=(p_decode);
    v7 = std::string::operator[](p_decodea, 3 * i + 2);
    std::string::operator[](&baseKey, *v7 & 0x3F);
    std::string::operator+=(p_decode);
  }
  if ( len % 3 == 1 )
  {
    pos = 3 * (len / 3);
    v8 = std::string::operator[](p_decodea, pos);
    std::string::operator[](&baseKey, *v8 >> 2);
    std::string::operator+=(p_decode);
    v9 = std::string::operator[](p_decodea, pos);
    std::string::operator[](&baseKey, 16 * (*v9 & 3));
    std::string::operator+=(p_decode);
    std::string::operator+=(p_decode, "==");
  }
  if ( len % 3 == 2 )
  {
    pos_0 = 3 * (len / 3);
    v10 = std::string::operator[](p_decodea, pos_0);
    std::string::operator[](&baseKey, *v10 >> 2);
    std::string::operator+=(p_decode);
    v11 = 16 * (*std::string::operator[](p_decodea, pos_0) & 3);
    v12 = std::string::operator[](p_decodea, pos_0 + 1);
    std::string::operator[](&baseKey, v11 | (*v12 >> 4));
    std::string::operator+=(p_decode);
    v13 = std::string::operator[](p_decodea, pos_0 + 1);
    std::string::operator[](&baseKey, 4 * (*v13 & 0xF));
    std::string::operator+=(p_decode);
    std::string::operator+=(p_decode, "=");
  }
  return p_decode;
}

脚本直接出