逆向攻防世界CTF系列50-testre

定位

跟进400D00

逻辑：接收输入，长度17，最后附上一个0符号(可能代表结束符)

跟进sub_400700

复杂

__int64 __fastcall sub_400700(void *a1, _QWORD *a2, __int64 a3, size_t a4){
  s = a1;
  v30 = a2;
  v29 = a3;
  v28 = a4;
  v27 = -559038737;
  v26 = malloc(0x100uLL);
  v25 = v29;
  v24 = v6;
  v22 = 0LL;
  v17 = 0LL;
  for ( i = 0LL; i < v28; ++i ){
    v13 = *(unsigned __int8 *)(v25 + i);
    *((_BYTE *)v26 + i) = byte_400E90[i % 0x1D] ^ v13;
    *((_BYTE *)v26 + i) += *(_BYTE *)(v25 + i);
  }
  while ( 1 ){
    v12 = 0;
    if ( v17 < v28 )
      v12 = ~(*(_BYTE *)(v25 + v17) != 0);
    if ( (v12 & 1) == 0 )
      break;
    ++v17;
  }
  n = 138 * (v28 - v17) / 0x64 + 1;
  v23 = ((v17 + v28) << 6) / 0x30 - 1;
  v11 = (unsigned __int8 *)v6 - ((138 * (v28 - v17) / 0x64 + 16) & 0xFFFFFFFFFFFFFFF0LL);
  memset(v11, 0, n);
  v20 = v17;
  v18 = n - 1;
  while ( v20 < v28 ){
    v21 = *(unsigned __int8 *)(v25 + v20);
    for ( j = n - 1; ; --j ){
      v10 = 1;
      if ( j <= v18 )
        v10 = v21 != 0;
      if ( !v10 )
        break;
      v22 = v11[j] << 6;
      v21 += v11[j] << 8;
      v9 = 64;
      v11[j] = v21 % 58;
      *((_BYTE *)v26 + j) = v22 & 0x3F;
      v22 >>= 6;
      v21 /= 58;
      v27 /= v9;
      if ( !j )
        break;
    }
    ++v20;
    v18 = j;
  }
  for ( j = 0LL; ; ++j ){
    v8 = 0;
    if ( j < n )
      v8 = ~(v11[j] != 0);
    if ( (v8 & 1) == 0 )
      break;
  }
  if ( *v30 > n + v17 - j ){
    if ( v17 ){
      c = 61;
      memset(s, 49, v17);
      memset(v26, c, v17);
    }
    v20 = v17;
    while ( j < n ){
      v4 = v11;
      *((_BYTE *)s + v20) = byte_400EB0[v11[j]];
      *((_BYTE *)v26 + v20++) = byte_400EF0[v4[j++]];
    }
    *((_BYTE *)s + v20) = 0;
    *v30 = v20 + 1;
    if ( !strncmp((const char *)s, "D9", 2uLL)
      && !strncmp((const char *)s + 20, "Mp", 2uLL)
      && !strncmp((const char *)s + 18, "MR", 2uLL)
      && !strncmp((const char *)s + 2, "cS9N", 4uLL)
      && !strncmp((const char *)s + 6, "9iHjM", 5uLL)
      && !strncmp((const char *)s + 11, "LTdA8YS", 7uLL)){
      v6[1] = puts("correct!");
    }
    v32 = 1;
    v14 = 1;
  }
  else{
    *v30 = n + v17 - j + 1;
    v32 = 0;
    v14 = 1;
  }
  return v32 & 1;
}

关键是最后得是s=D9cS9N9iHjMLTdA8YSMRMp

代码很长，硬着头看，第一部分：

for ( i = 0LL; i < v28; ++i )
{
  v13 = *(unsigned __int8 *)(v25 + i);
  *((_BYTE *)v26 + i) = byte_400E90[i % 0x1D] ^ v13;
  *((_BYTE *)v26 + i) += *(_BYTE *)(v25 + i);
}

v28是长度也就是a4 ，v13是输入，看了下后面v26好像没有怎么用到，并且这里的逻辑好像是v13=input,v26=input^a,v26=input，其实v26改成了input而已，跟踪也知道这暗示是个假的

while ( 1 ){
  v12 = 0;
  if ( v17 < v28 ) v12 = ~(*(_BYTE *)(v25 + v17) != 0);
  if ((v12 & 1) == 0 ) break;
  ++v17;
}

如果想break，v12必须为0，~(*(_BYTE )(v25 + v17) != 0)为1,((_BYTE )(v25 + v17) != 0)为0，((_BYTE *)(v25 + v17) ≠0

总而言之没啥用，你会发现这里只是做了判断，本身并不会改变值，程序总能break。没啥用

下面代码我去除了无用变量v9，v27，v22 >>= 6;

while ( v20 < v28 ){
  v21 = *(unsigned __int8 *)(v25 + v20);
  for ( j = n - 1; ; --j ){
    v10 = 1;
    if ( j <= v18 ) v10 = v21 != 0;
    if ( !v10 ) break;
    v22 = v11[j] << 6;
    v21 += v11[j] << 8;
    v11[j] = v21 % 58;
    *((_BYTE *)v26 + j) = v22 & 0x3F;
    v21 /= 58;
    if ( !j ) break;
  }
  ++v20;
  v18 = j;
}

看这里，之前有一个fake，然后下面有一个58长度的字符串，下面有个64长度的（无用），结合v11[j] = v21 % 58;，猜测可能是个Base58编码表

具体逻辑有点复杂，其实不用深究，看看下面,同样用不闪婚，直接过

for ( j = 0LL; ; ++j ){
  v8 = 0;
  if ( j < n ) v8 = ~(v11[j] != 0);
  if ( (v8 & 1) == 0 ) break;
}

if ( *v30 > n + v17 - j ){
    if ( v17 ){
      c = 61;
      memset(s, 49, v17);
      memset(v26, c, v17);
    }
    v20 = v17;
    while ( j < n ){
      v4 = v11;
      *((_BYTE *)s + v20) = byte_400EB0[v11[j]];
      *((_BYTE *)v26 + v20++) = byte_400EF0[v4[j++]];
    }
    *((_BYTE *)s + v20) = 0;
    *v30 = v20 + 1;
    if ( !strncmp((const char *)s, "D9", 2uLL)
      && !strncmp((const char *)s + 20, "Mp", 2uLL)
      && !strncmp((const char *)s + 18, "MR", 2uLL)
      && !strncmp((const char *)s + 2, "cS9N", 4uLL)
      && !strncmp((const char *)s + 6, "9iHjM", 5uLL)
      && !strncmp((const char *)s + 11, "LTdA8YS", 7uLL)){
      v6[1] = puts("correct!");
    }
    v32 = 1;
    v14 = 1;
}

先开辟空间，再在while循环里赋值，v26用不上

得flag,flag{base58_is_boring}