在Delphi中预定义了Tfilestream类，通过它可以对磁盘文件进行读写，笔者选定Tfilestream为基类，通过对其核心的两个读、写方法进行重载，编写定制的文件流，实现对文件的读、写进行加密。 首先，来看一下定制文件流（Tmystream）的声明： type Tmystream=class(Tfilestream) private fkey:string; public constructor create (const filename:string;mode:word); function read(var buffer;count:longint): longint;override; function write(const buffer;count:longint): longint;override; property key:string read fkey write fkey ; end; 在Tmystream的声名中，我们对read、write两个方法进行了重载，并添加了一个新的特性key，用以存储对文件进行加密时所需的密码。为实现文件读写的加密，在write方法中，将key的每个字符依次与buffer中的字符相加，将得到的结果写入文件，实现加密；在read方法中，将读出的内容依次与key的每个字符相减，实现解密。加密及解密的方法多种多样，读者可以通过改写相关代码，得到不同的加密方法。 程序清单如下：function Tmystream.write(const buffer; count:longint):longint; var Pbu,Pmy,mykey:pchar; i,enc:integer;begin getmem(pmy,count); //为pmy分配内存 mykey:=pchar(key); //将key转换为pchar指针 try pbu:=pchar(@buffer); //将buffer转换为pchar指针 for i:=0 to count-1 do //将key的每个字符以此与buffer的 每个字符循环相加,结果放入pmy指向的内存区 begin enc:=(ord(pbu[i])+ord(mykey [(i mod length(key))])) mod 256; Pmy[i]:=char(enc); end; result:=inherited write(Pmy^,count); //将pmy指向的内容写入文件 finally freemem(Pmy,count); end;end;function Tmystream.read(var buffer;count:longint): longint;var Pbu,Pmy,mykey:pchar; i,mycount,enc:integer;begin getmem(Pmy,count);//为pmy分配内存 mykey:=pchar(key);//将key转换为pchar指针 try mycount:=inherited read(Pmy^,count); //将文件内容读入pmy指向内存区 Pbu:=Pchar(@buffer);将buffer转换为pchar指针 for i:=0 to mycount-1 do//将key的每个字符依次 与pmy的每个字符循环相减，结果放入pbu指向的变量 begin enc:=(ord(Pmy[i])-ord(mykey [(i mod length(key))])) mod 256; Pbu[i]:=chr(enc); end; finally freemem(P count); end; result:=mycount;end; 完成定制文件流的编写后，便可在程序中应用，实现文件的读写加密，例程如下： unit Unit1;interfaceuses Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, ExtCtrls, StdCtrls,unit2,unit3;//unit2定义了Tmystream//unit3定义了输入密码对话框form3type TForm1 = class(TForm) Button1: TButton; Button2: TButton; Button3: TButton; OpenDialog1: TOpenDialog; SaveDialog1: TSaveDialog; Panel1: TPanel; Panel2: TPanel; Memo1: TMemo; Splitter1: TSplitter; Memo2: TMemo; procedure Button2Click(Sender: TObject); procedure Button3Click(Sender: TObject); procedure Button1Click(Sender: TObject); private { Private declarations } public { Public declarations } end;var Form1: TForm1;implementation{$R *.DFM}procedure TForm1.Button2Click(Sender: TObject); //将选定的加密文件解开，读入memo2var encstr:Tmystream;begin if opendialog1.Execute and (form3.showmodal=mrok) then begin encstr:=Tmystream.create (opendialog1.filename,fmopenread); encstr.key:=form3.Edit1.Text; try memo2.lines.LoadFromStream(encstr); finally encstr.Free; end; end;end;procedure TForm1.Button3Click(Sender: TObject); //将memo1中的内容加密，用指定文件名另存var encstr:Tmystream;begin if savedialog1.Execute and (form3.showmodal=mrok) then begin encstr:=Tmystream.create(savedialog1.filename, fmcreate); encstr.key:=form3.Edit1.Text; try memo1.lines.SaveToStream(encstr); finally encstr.Free; end; end;end;procedure TForm1.Button1Click(Sender: TObject); //将指定文件读入memo1var mystr:Tfilestream;begin if opendialog1.Execute then begin mystr:=Tfilestream.create (opendialog1.filename,fmopenread); try memo1.lines.LoadFromStream(mystr); finally mystr.Free; end; end;end;end. 以上程序均在windows 98、Delphi4.0环境中，通过编译，运行正常。
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