[C++知识库]buu-[ACTF新生赛2020]rome

无壳32位文件

32位ida打开查看main函数

_main();太乱了不想看
先看func()

int func()
{
  int result; // eax
  int v1; // [esp+14h] [ebp-44h]
  int v2; // [esp+18h] [ebp-40h]
  int v3; // [esp+1Ch] [ebp-3Ch]
  int v4; // [esp+20h] [ebp-38h]
  unsigned __int8 v5; // [esp+24h] [ebp-34h]
  unsigned __int8 v6; // [esp+25h] [ebp-33h]
  unsigned __int8 v7; // [esp+26h] [ebp-32h]
  unsigned __int8 v8; // [esp+27h] [ebp-31h]
  unsigned __int8 v9; // [esp+28h] [ebp-30h]
  int v10; // [esp+29h] [ebp-2Fh]
  int v11; // [esp+2Dh] [ebp-2Bh]
  int v12; // [esp+31h] [ebp-27h]
  int v13; // [esp+35h] [ebp-23h]
  unsigned __int8 v14; // [esp+39h] [ebp-1Fh]
  char v15; // [esp+3Bh] [ebp-1Dh]
  char v16; // [esp+3Ch] [ebp-1Ch]
  char v17; // [esp+3Dh] [ebp-1Bh]
  char v18; // [esp+3Eh] [ebp-1Ah]
  char v19; // [esp+3Fh] [ebp-19h]
  char v20; // [esp+40h] [ebp-18h]
  char v21; // [esp+41h] [ebp-17h]
  char v22; // [esp+42h] [ebp-16h]
  char v23; // [esp+43h] [ebp-15h]
  char v24; // [esp+44h] [ebp-14h]
  char v25; // [esp+45h] [ebp-13h]
  char v26; // [esp+46h] [ebp-12h]
  char v27; // [esp+47h] [ebp-11h]
  char v28; // [esp+48h] [ebp-10h]
  char v29; // [esp+49h] [ebp-Fh]
  char v30; // [esp+4Ah] [ebp-Eh]
  char v31; // [esp+4Bh] [ebp-Dh]
  int i; // [esp+4Ch] [ebp-Ch]

  v15 = 81;
  v16 = 115;
  v17 = 119;
  v18 = 51;
  v19 = 115;
  v20 = 106;
  v21 = 95;
  v22 = 108;
  v23 = 122;
  v24 = 52;
  v25 = 95;
  v26 = 85;
  v27 = 106;
  v28 = 119;
  v29 = 64;
  v30 = 108;
  v31 = 0;
  printf("Please input:");
  scanf("%s", &v5);
  result = v5;
  if ( v5 == 'A' )
  {
    result = v6;
    if ( v6 == 'C' )
    {
      result = v7;
      if ( v7 == 'T' )
      {
        result = v8;
        if ( v8 == 'F' )
        {
          result = v9;
          if ( v9 == '{' )
          {
            result = v14;
            if ( v14 == '}' )
            {
              v1 = v10;
              v2 = v11;
              v3 = v12;
              v4 = v13;
              for ( i = 0; i <= 15; ++i )
              {
                if ( *((_BYTE *)&v1 + i) > 64 && *((_BYTE *)&v1 + i) <= 90 )
                  *((_BYTE *)&v1 + i) = (*((char *)&v1 + i) - 51) % 26 + 65;
                if ( *((_BYTE *)&v1 + i) > 96 && *((_BYTE *)&v1 + i) <= 122 )
                  *((_BYTE *)&v1 + i) = (*((char *)&v1 + i) - 79) % 26 + 97;
              }
              for ( i = 0; i <= 15; ++i )
              {
                result = (unsigned __int8)*(&v15 + i);
                if ( *((_BYTE *)&v1 + i) != (_BYTE)result )
                  return result;
              }
              result = printf("You are correct!");
            }
          }
        }
      }
    }
  }
  return result;
}

ACTF{}不用管，直接看for循环
第一个for循环
65-90,97-122,对大小写字母分别加密
第二个for就是单纯的比较

变化后的值等于v15的值就行
能正着跑绝不逆向）

v15 = [81,115,119,51,115,106,95,108,122,52,95,85,106,119,64,108]
flag = ''
for i in range(0,16):
     for j in range(0,127):
          f=j
          if j > 64 and j <= 90:
               j = (j-51)%26 + 65
          if j > 96 and j <= 122:
               j = (j-79)%26 + 97
          if(j == v15[i]):
               flag += chr(f)
print(flag)

flag{Cae3ar_th4_Gre@t}