unit EncodStr;

interface

uses
  Classes;

type
  TEncodedStream = class (TFileStream)
  private
    FKey: Char;
  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: Char read FKey write FKey default 'A';
  end;

implementation

constructor TEncodedStream.Create(
  const FileName: string; Mode: Word);
begin
  inherited Create (FileName, Mode);
  FKey := 'A';
end;

function TEncodedStream.Write(const Buffer;
   Count: Longint): Longint;
var
  pBuf, pEnc: PChar;
  I, EncVal: Integer;
begin
  // allocate memory for the encoded buffer
  GetMem (pEnc, Count);
  try
    // use the buffer as an array of characters
    pBuf := PChar (@Buffer);
    // for every character of the buffer
    for I := 0 to Count - 1 do
    begin
      // encode the value and store it
      EncVal := ( Ord (pBuf[I]) + Ord(Key) ) mod 256;
      pEnc [I] := Chr (EncVal);
    end;
    // write the encoded buffer to the file
    Result := inherited Write (pEnc^, Count);
  finally
    FreeMem (pEnc, Count);
  end;
end;

function TEncodedStream.Read(var Buffer; Count: Longint): Longint;
var
  pBuf, pEnc: PChar;
  I, CountRead, EncVal: Integer;
begin
  // allocate memory for the encoded buffer
  GetMem (pEnc, Count);
  try
    // read the encoded buffer from the file
    CountRead := inherited Read (pEnc^, Count);
    // use the output buffer as a string
    pBuf := PChar (@Buffer);
    // for every character actually read
    for I := 0 to CountRead - 1 do
    begin
      // decode the value and store it
      EncVal := ( Ord (pEnc[I]) - Ord(Key) ) mod 256;
      pBuf [I] := Chr (EncVal);
    end;
  finally
    FreeMem (pEnc, Count);
  end;
  // return the number of characters read
  Result := CountRead;
end;



end.

unit EncForm;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, ExtCtrls;

type
  TFormEncode = class(TForm)
    Memo1: TMemo;
    Memo2: TMemo;
    OpenDialog1: TOpenDialog;
    SaveDialog1: TSaveDialog;
    Panel1: TPanel;
    BtnLoadPlain: TButton;
    BtnSaveEncoded: TButton;
    BtnLoadEncoded: TButton;
    Splitter1: TSplitter;
    procedure BtnSaveEncodedClick(Sender: TObject);
    procedure BtnLoadEncodedClick(Sender: TObject);
    procedure BtnLoadPlainClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  FormEncode: TFormEncode;

implementation

{$R *.DFM}

uses
  EncodStr;

procedure TFormEncode.BtnSaveEncodedClick(Sender: TObject);
var
  EncStr: TEncodedStream;
begin
  if SaveDialog1.Execute then
  begin
    EncStr := TEncodedStream.Create(
      SaveDialog1.Filename, fmCreate);
    try
      Memo1.Lines.SaveToStream (EncStr);
    finally
      EncStr.Free;
    end;
  end;
end;

procedure TFormEncode.BtnLoadEncodedClick(Sender: TObject);
var
  EncStr: TEncodedStream;
begin
  if OpenDialog1.Execute then
  begin
    EncStr := TEncodedStream.Create(
      OpenDialog1.FileName, fmOpenRead);
    try
      Memo2.Lines.LoadFromStream (EncStr);
    finally
      EncStr.Free;
    end;
  end;
end;

procedure TFormEncode.BtnLoadPlainClick(Sender: TObject);
begin
  if OpenDialog1.Execute then
    Memo1.Lines.LoadFromFile (
      OpenDialog1.FileName);
end;

end.