【攻防世界】逆向666

666

64位程序，无壳，拖进ida，找到关键函数

int __cdecl main(int argc, const char **argv, const char **envp)
{
  char s; // [rsp+0h] [rbp-1E0h]
  char v5; // [rsp+F0h] [rbp-F0h]

  memset(&s, 0, 0x1EuLL);
  printf("Please Input Key: ", 0LL);
  __isoc99_scanf("%s", &v5);
  encode(&v5, (__int64)&s);
  if ( strlen(&v5) == key )                     // 长度18
  {
    if ( !strcmp(&s, enflag) )                  // izwhroz""w"v.K".Ni
      puts("You are Right");
    else
      puts("flag{This_1s_f4cker_flag}");
  }
  return 0;
}

有个加密函数，加密后的密文我们知道的，查看加密函数

int __fastcall encode(const char *a1, __int64 a2)
{
  char v3[32]; // [rsp+10h] [rbp-70h]
  char v4[32]; // [rsp+30h] [rbp-50h]
  char v5[40]; // [rsp+50h] [rbp-30h]
  int v6; // [rsp+78h] [rbp-8h]
  int i; // [rsp+7Ch] [rbp-4h]

  i = 0;
  v6 = 0;
  if ( strlen(a1) != key )
    return puts("Your Length is Wrong");
  for ( i = 0; i < key; i += 3 )
  {
    v5[i] = key ^ (a1[i] + 6);
    v4[i + 1] = (a1[i + 1] - 6) ^ key;
    v3[i + 2] = a1[i + 2] ^ 6 ^ key;
    *(_BYTE *)(a2 + i) = v5[i];
    *(_BYTE *)(a2 + i + 1LL) = v4[i + 1];
    *(_BYTE *)(a2 + i + 2LL) = v3[i + 2];
  }
  return a2;
}

非常简单，写一个逆算法脚本，得到flag

a = 'izwhroz""w"v.K".Ni'
key = 18

a_ = list(map(ord,list(a)))
flag = ''
for i in range(0,key,3):
    flag += chr((a_[i]^key)-6)
    flag += chr((a_[i+1]^key)+6)
    flag += chr(a_[i+2]^key^6)

print(flag)