[GXYCTF2019]luck_guy

IDA快速定位关键函数get_flag()

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unsigned __int64 get_flag()
{
  unsigned int v0; // eax
  int i; // [rsp+4h] [rbp-3Ch]
  int j; // [rsp+8h] [rbp-38h]
  __int64 s; // [rsp+10h] [rbp-30h] BYREF
  char v5; // [rsp+18h] [rbp-28h]
  unsigned __int64 v6; // [rsp+38h] [rbp-8h]

  v6 = __readfsqword(0x28u);
  v0 = time(0LL);
  srand(v0);
  for ( i = 0; i <= 4; ++i )
  {
    switch ( rand() % 200 )
    {
      case 1:
        puts("OK, it's flag:");
        memset(&s, 0, 0x28uLL);
        strcat((char *)&s, f1);
        strcat((char *)&s, &f2);
        printf("%s", (const char *)&s);
        break;
      case 2:
        printf("Solar not like you");
        break;
      case 3:
        printf("Solar want a girlfriend");
        break;
      case 4:
        s = 0x7F666F6067756369LL;
        v5 = 0;
        strcat(&f2, (const char *)&s);
        break;
      case 5:
        for ( j = 0; j <= 7; ++j )
        {
          if ( j % 2 == 1 )
            *(&f2 + j) -= 2;
          else
            --*(&f2 + j);
        }
        break;
      default:
        puts("emmm,you can't find flag 23333");
        break;
    }
  }
  return __readfsqword(0x28u) ^ v6;
}

可以看出flag由f1和f2组成，f1可以直接得到值f1=GXY{do_not_
f2值为空，但经过case4完成赋值（注意这里是小端序所以要反过来），经过case5加密得到，我们直接根据流程写py脚本即可

f2=[0x69,0x63,0x75,0x67,0x60,0x6F,0x66,0x7F]
flag='GXY{do_not_'

for j in range(0,8):
    s=''
    if(j%2==1):
        s=chr(int(f2[j])-2)
    else:
        s=chr(int(f2[j])-1)
    flag+=s
print(flag)