个人的BUUCTF

个人的BUUCTF

把文件下载，用010 Editor打开，看到：

 

 自然就知道这是个elf文件，而且是64位的，拖进ida看看：

int __cdecl __noreturn main(int argc, const char **argv, const char **envp)
{
  int v3; // eax
  char v4; // [rsp+Fh] [rbp-1h]

  while ( 1 )
  {
    while ( 1 )
    {
      printf("Welcome to CTF game!nPlease input d/D to start or input q/Q to quit this program: ");
      v4 = getchar();
      if ( v4 != 100 && v4 != 68 )
        break;
      Decry("Welcome to CTF game!nPlease input d/D to start or input q/Q to quit this program: ", argv);
    }
    if ( v4 == 113 || v4 == 0x51 )
      Exit();
    puts("Input fault format!");
    v3 = getchar();
    putchar(v3);
  }
}

观察得到dercy是重要的函数（或者通过shift+f12找到Congratulation的位置）

unsigned __int64 Decry()
{
  char v1; // [rsp+Fh] [rbp-51h]
  int v2; // [rsp+10h] [rbp-50h]
  int v3; // [rsp+14h] [rbp-4Ch]
  int i; // [rsp+18h] [rbp-48h]
  int v5; // [rsp+1Ch] [rbp-44h]
  char src[8]; // [rsp+20h] [rbp-40h] BYREF
  __int64 v7; // [rsp+28h] [rbp-38h]
  int v8; // [rsp+30h] [rbp-30h]
  __int64 v9[2]; // [rsp+40h] [rbp-20h] BYREF
  int v10; // [rsp+50h] [rbp-10h]
  unsigned __int64 v11; // [rsp+58h] [rbp-8h]

  v11 = __readfsqword(0x28u);
  *(_QWORD *)src = 'SLCDN';
  v7 = 0LL;
  v8 = 0;
  v9[0] = 'wodah';
  v9[1] = 0LL;
  v10 = 0;
  text = join(key3, (const char *)v9);//text[]="killshadow"
  strcpy(key, key1);
  strcat(key, src);//key[]={"adsfkndcls"}
  v2 = 0;
  v3 = 0;
  getchar();
  v5 = strlen(key);
  for ( i = 0; i < v5; ++i )
  {
    if ( key[v3 % v5] > 64 && key[v3 % v5] <= 90 )
      key[i] = key[v3 % v5] + 32;
    ++v3;
  }//可以知道这个是个大写变小写.
  printf("Please input your flag:");
  while ( 1 )
  {
    v1 = getchar();
    if ( v1 == 10 )
      break;
    if ( v1 == 32 )
    {
      ++v2;
    }
    else
    {
      if ( v1 <= 96 || v1 > 122 )
      {
        if ( v1 > 64 && v1 <= 90 )
        {
          str2[v2] = (v1 - 39 - key[v3 % v5] + 97) % 26 + 97;
          ++v3;
        }
      }
      else
      {
        str2[v2] = (v1 - 39 - key[v3 % v5] + 97) % 26 + 97;
        ++v3;
      }
      if ( !(v3 % v5) )
        putchar(32);
      ++v2;
    }
  }//就是将字母进行变化.
  if ( !strcmp(text, str2) )
    puts("Congratulation!n");
  else
    puts("Try again!n");
  return __readfsqword(0x28u) ^ v11;
}

因为在加密里面取模了，就难以反向运算出来，不过加密仅限于对字母加密，我们可以通过暴力的方法求得。

#include 
#include 

int main() {

    char text[]={"killshadow"};
    char key[]={"adsfkndcls"};

    int i;
    int v3=10;
    int len;
    len= strlen(text);
    for ( i = 0; i < len; i++){
        for (int j = 0; j <= 'z'; j++){
            if (j < 'A' || j > 'z' || j > 'Z' && j < 'a'){
                continue;
            }
            if ((j - 39 - key[v3 % 10] + 97) % 26 + 97 == text[i]){
                printf("%c",j);
                v3++;
                break;
            }
        }
    }
    return 0;
}