9.5 BUUCTF reverse3

重点在于

注意到这个函数 是对str进行了加密处理的
查看这个函数：

void *__cdecl sub_411AB0(char *a1, unsigned int a2, int *a3)
{
  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;
  *a3 = v10;
  v12 = malloc(v10 + 1);
  if ( !v12 )
    return 0;
  j_memset(v12, 0, v10 + 1);
  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:
        *((_BYTE *)v12 + v4) = aAbcdefghijklmn[(int)(unsigned __int8)byte_41A144[0] >> 2];
        v5 = v4 + 1;
        *((_BYTE *)v12 + v5) = aAbcdefghijklmn[((byte_41A144[1] & 0xF0) >> 4) | (16 * (byte_41A144[0] & 3))];
        *((_BYTE *)v12 + ++v5) = aAbcdefghijklmn[64];
        *((_BYTE *)v12 + ++v5) = aAbcdefghijklmn[64];
        v4 = v5 + 1;
        break;
      case 2:
        *((_BYTE *)v12 + v4) = aAbcdefghijklmn[(int)(unsigned __int8)byte_41A144[0] >> 2];
        v6 = v4 + 1;
        *((_BYTE *)v12 + v6) = aAbcdefghijklmn[((byte_41A144[1] & 0xF0) >> 4) | (16 * (byte_41A144[0] & 3))];
        *((_BYTE *)v12 + ++v6) = aAbcdefghijklmn[((byte_41A144[2] & 0xC0) >> 6) | (4 * (byte_41A144[1] & 0xF))];
        *((_BYTE *)v12 + ++v6) = aAbcdefghijklmn[64];
        v4 = v6 + 1;
        break;
      case 3:
        *((_BYTE *)v12 + v4) = aAbcdefghijklmn[(int)(unsigned __int8)byte_41A144[0] >> 2];
        v7 = v4 + 1;
        *((_BYTE *)v12 + v7) = aAbcdefghijklmn[((byte_41A144[1] & 0xF0) >> 4) | (16 * (byte_41A144[0] & 3))];
        *((_BYTE *)v12 + ++v7) = aAbcdefghijklmn[((byte_41A144[2] & 0xC0) >> 6) | (4 * (byte_41A144[1] & 0xF))];
        *((_BYTE *)v12 + ++v7) = aAbcdefghijklmn[byte_41A144[2] & 0x3F];
        v4 = v7 + 1;
        break;
    }
  }
  *((_BYTE *)v12 + v4) = 0;
  return v12;
}

结合 aAbcdefghijklmn 的值：

可以判断出是base64加密
写个脚本反向解密即可：

if __name__ == "__main__":
    str2 = "e3nifIH9b_C@n@dH"
    des = ""
    for j in range(0,len(str2)):
        des += chr(ord(str2[j])-j)
    print(des)

然后base64解密即可得到flag{}:flag{i_l0ve_you}