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I had some projects needs to put its own secret key when they needed, and the secret key should not be exist as an alternative file or stored in any secret place in order to have mobility - meaning that application having a secret key should be able to use in any PC/Laptop. The easiest approach can do that is adding an additional information at the end of the binary file.

Below code shows the way to save the necessary secret information in Delphi. For your information, if you want to increase the size of the message, you should increase the value of CK_MAX_MSG_LEN below.

const CK_SINGATURE = '!CK!';
const CK_MAX_MSG_LEN = 1024*8;

function CKReadPassword(sFileName:string):string;
var
  sFile: TFileStream;
  lenPWD, lenPWD_old: WORD;
  sEncryptedPWD: AnsiString;
  buff: PAnsiChar;
begin
  GetMem( buff, CK_MAX_MSG_LEN);
  ZeroMemory( buff, CK_MAX_MSG_LEN);

  if FileExists(sFileName) then begin
    lenPWD_old := 0;
    sFile := TFileStream.Create( sFileName, fmOpenRead);
    if sFile.Handle<>THandle(nil) then begin
      sFile.Seek( -4, soFromEnd);
      sFile.Read( PAnsiChar(buff)^, 4);
      if buff=CK_SINGATURE then
      begin
        // has master key, so need to erase it
        sFile.Seek( -6, soFromEnd);
        sFile.Read( lenPwd, 2);
        sFile.Seek( -(lenPWD + 6), soFromEnd);
        sFile.Read( PAnsIChar(buff)^, lenPWD);
        Result := trim(string(buff));
      end;
      sFile.Destroy;
    end;
  end;

  FreeMem(buff);
end;

function CKUpdatePassword(sFileName, sPWD:String):Boolean;
var
  sFile: TFileStream;
  lenPWD, lenPWD_old: WORD;
  sEncryptedPWD: AnsiString;
  buff: PAnsiChar;
begin
  GetMem( buff, CK_MAX_MSG_LEN);
  ZeroMemory( buff, CK_MAX_MSG_LEN);

  if not FileExists(sFileName) then begin
    Result := False;
  end else begin
    lenPWD_old := 0;
    sFile := TFileStream.Create( sFileName, fmOpenReadWrite);
    if sFile.Handle<>THandle(nil) then begin
      sFile.Seek( -4, soFromEnd);
      sFile.Read( PAnsiChar(buff)^, 4);
      if buff=CK_SINGATURE then
      begin
        // has master key, so need to erase it
        sFile.Seek( -6, soFromEnd);
        sFile.Read( lenPwd, 2);
        sFile.Seek( -(lenPWD + 6), soFromEnd);
        lenPWD_old := lenPWD;
      end else sFile.Seek( 0, soFromEnd);

      while Length(sPWD)<lenPWD_old do sPWD := sPWD + ' ';

      sEncryptedPWD := AnsiString(sPWD);
      lenPWD := Length(sEncryptedPWD);
      sFile.Write( PAnsiChar(sEncryptedPWD)^, lenPWD);
      sFile.Write( lenPWD, 2);
      sFile.Write( PAnsiChar(CK_SINGATURE)^, 4);
      sFile.Destroy;

      Result := True;
    end else Result := False;
  end;

  FreeMem(buff);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  if not FileExists(eMasterFile.Text) then begin
    ShowMessage( 'Please select program file you want');
  end else begin
    if (CKUpdatePassword( eMasterFile.Text, ePWD.Text)=True) then begin
      ShowMessage('Successfully Changed !!');
      Close;
    end;
  end;
end;

Below code provides better secure way to store secret key

uses IdCoder, IdCoderMIME, IdGlobal;

.
.
.

const CK_SINGATURE = '!CK!';
const CK_SINGATURE_KEY = 'ChunKang';
const CK_MAX_MSG_LEN = 1024 * 8;
const CK_ENC_VAR_RANGE=10;

function CKEncrypt(const Source:string; Salt:string): string;
var
  i, lenSalt, key, keyWeight: Word;
  InString: string;
begin
  Result := '';
  if (Salt = '') then begin
    Result := Source;
  end else begin
    InString := TIdEncoderMIME.EncodeString(Source, IndyTextEncoding_UTF8);

    lenSalt := Length(Salt);

    keyWeight := 0;
    for i := 1 to lenSalt do keyWeight := keyWeight + ord(Salt[i]);
    keyWeight := keyWeight mod CK_ENC_VAR_RANGE;

    for i := 1 to Length(InString) do
    begin
      key := ( ord(Salt[(i mod lenSalt) + 1]) mod CK_ENC_VAR_RANGE);
      Result := Result + CHAR(Byte(InString[i]) + key - keyWeight);
    end;
  end;
end;

function CKDecrypt(const InString:string; Salt:string): string;
var
  i, lenSalt, key, keyWeight: Word;
  Target: string;
begin
  if (Salt = '') then begin
    Target := InString;
  end else begin
    lenSalt := Length(Salt);

    keyWeight := 0;
    for i := 1 to lenSalt do keyWeight := keyWeight + ord(Salt[i]);
    keyWeight := keyWeight mod CK_ENC_VAR_RANGE;

    Target := '';
    for i := 1 to Length(InString) do
    begin
      key := ( ord(Salt[(i mod lenSalt) + 1]) mod CK_ENC_VAR_RANGE);
      Target := Target + CHAR(Byte(InString[i]) - key + keyWeight);
    end;
    Result := TIdDecoderMIME.DecodeString(Target, IndyTextEncoding_UTF8);
  end;
end;

function CKReadPassword(sFileName:string):string;
var
  sFile: TFileStream;
  lenPWD, lenPWD_old: WORD;
  sEncryptedPWD: AnsiString;
  buff: PAnsiChar;
  tmp: AnsiString;
begin
  GetMem( buff, CK_MAX_MSG_LEN);
  ZeroMemory( buff, CK_MAX_MSG_LEN);

  if FileExists(sFileName) then begin
    lenPWD_old := 0;
    sFile := TFileStream.Create( sFileName, fmOpenRead);
    if sFile.Handle<>THandle(nil) then begin
      sFile.Seek( -4, soFromEnd);
      sFile.Read( PAnsiChar(buff)^, 4);
      if buff=CK_SINGATURE then
      begin
        // has master key, so need to erase it
        sFile.Seek( -6, soFromEnd);
        sFile.Read( lenPwd, 2);
        sFile.Seek( -(lenPWD + 6), soFromEnd);
        sFile.Read( PAnsIChar(buff)^, lenPWD);
        tmp := ansiString(string(buff));
        tmp := CKDecrypt( tmp, CK_SINGATURE_KEY);
        Result := Trim(tmp);
      end;
      sFile.Destroy;
    end;
  end;

  FreeMem(buff);
end;

function CKUpdatePassword(sFileName, sPWD:String):Boolean;
var
  sFile: TFileStream;
  lenPWD, lenPWD_old: WORD;
  sEncryptedPWD: AnsiString;
  buff: PAnsiChar;
begin
  GetMem( buff, CK_MAX_MSG_LEN);
  ZeroMemory( buff, CK_MAX_MSG_LEN);

  if not FileExists(sFileName) then begin
    Result := False;
  end else begin
    lenPWD_old := 0;
    sFile := TFileStream.Create( sFileName, fmOpenReadWrite);
    if sFile.Handle<>THandle(nil) then begin
      sFile.Seek( -4, soFromEnd);
      sFile.Read( PAnsiChar(buff)^, 4);
      if buff=CK_SINGATURE then
      begin
        // has master key, so need to erase it
        sFile.Seek( -6, soFromEnd);
        sFile.Read( lenPwd, 2);
        sFile.Seek( -(lenPWD + 6), soFromEnd);
        lenPWD_old := lenPWD;
      end else sFile.Seek( 0, soFromEnd);

      while Length(sPWD)<lenPWD_old do sPWD := sPWD + ' ';

      sEncryptedPWD := CKEncrypt(sPWD, CK_SINGATURE_KEY);
      lenPWD := Length(sEncryptedPWD);
      sFile.Write( PAnsiChar(sEncryptedPWD)^, lenPWD);
      sFile.Write( lenPWD, 2);
      sFile.Write( PAnsiChar(CK_SINGATURE)^, 4);
      sFile.Destroy;

      Result := True;
    end else Result := False;
  end;

  FreeMem(buff);
end;