truecrypt:DDD

function XOR_File(const AFileName: string; const key: string): boolean;
const

  PREFIX : string = '__';
var
  F1, F2: File;
  W: Int64;
  ReadBufer, WriteBufer: array[1..1024] of byte;
  i, j, h, BytesRead: Integer;
  LongKey: string;
begin
  if (fileexists(AFileName)) then
  begin
    AssignFile(F1, AFileName);
    Reset(F1, 1);
    AssignFile(F2, PREFIX + AFileName);
    Rewrite(F2, 1);

    for i := 0 to Length(key) do
    begin
      LongKey := LongKey + key;
    end;
    LongKey := LongKey + IntToStr(Length(LongKey)) + IntToStr(Length(Key));

    while (BytesRead <> 0) do
    begin
      BlockRead(F1, ReadBufer, sizeof(ReadBufer), BytesRead);
      for i := 0 to bytesRead do
      begin
        WriteBufer[i] := ReadBufer[i];
        for j := 0 to 7 do
        begin
          for h := 0 to Length(LongKey) - 1 do
            WriteBufer[j] := WriteBufer[j] xor Ord(LongKey[h]);
        end;
      end;
      BlockWrite(F2, WriteBufer, BytesRead);
    end;

    CloseFile(F1);
    CloseFile(F2);

    DeleteFile(PChar(AFileName));
    repeat
      if (not FileExists(AFileName)) then
      begin
        RenameFile(PREFIX + AFileName, AFileName);
        exit;
      end;
    until (not FileExists(AFileName));
  end;

  result := true;
end;