Delete a file

Edited text of question

Here is an example.////

DeleteFile('C:\windows\desktop\something.txt');

Regards,
Viktor Ivanov

All deleted file may be recovered if you know how to crack it. so the question is how secure do you want it to be ?

Hi,
This is not an easy question.
To be able to "shred" a file in order to not be able to read it afterwards is a tough one. DeleteFile will only remove/hide the allocation table entry and not really shred/wipe the data.
To wipe a file, you will have to replace it's sectors data with patterns or pseudorandom data.
Here's an example on how to do it. Do NOT trust this code, since it is not properly tested. The problems are:
1. If you modify the file, does the OS move the data physically so that the old data resides, unchanged, in it's old position?
2. If you shred the data and delete the file afterwards, do you know if the new data is physically changed on the disk or are the changes just made in the cache?

Anyway, here's something that MIGHT work. You can select pattern or pseudorandom shredding and number of passes:


function ShredFile( sFilename: string; cPasses: Integer; bPR, bDelete: Boolean ): Boolean;
type
  PBuf = ^TBuf;
  TBuf = array[ 0..0 ] of Byte;
const
  BLOCKSIZE = 65536;
  PATTERN: array[0..3] of Byte = ( $A, $A, $5, $5 );
var
  fs              : TFileStream;
  iIndex,
  iBytesToWrite,
  iBytesWritten,
  iBufSize        : Integer;
  p               : PBuf;
begin
  Result := False;
  p      := nil;
  try
    if FileExists( sFilename ) then begin
      fs := TFileStream.Create( sFilename, fmOpenReadWrite or fmShareExclusive );
      if fs.Size < BLOCKSIZE
      then iBufSize := fs.Size
      else iBufSize := BLOCKSIZE;
      try
        GetMem( p, iBufSize );
        if not bPR then begin
          for iIndex := 0 to iBufSize - 1 do begin
            p^[ iIndex ] := PATTERN[ iIndex and 3 ];
          end;
        end else Randomize; // initiate pseudorandom series
        while cPasses > 0 do begin
          fs.Seek( 0, soFromBeginning ); // always from the beginning
          repeat
            if bPR then for iIndex := 0 to iBufSize - 1 do p^[ iIndex ] := Random( 255 );
            if ( fs.Size - fs.Position ) <= BLOCKSIZE
            then iBytesToWrite := fs.Size - fs.Position
            else iBytesToWrite := BLOCKSIZE;
            iBytesWritten := fs.Write( p^, iBytesToWrite ); // speeds up things
          until ( iBytesToWrite < BLOCKSIZE ) or ( iBytesWritten <> iBytesToWrite );
          if iBytesWritten <> iBytesToWrite then begin
            Result := False;
            raise Exception.Create( 'Unable to write sufficient #bytes to file' );
          end;
          Dec( cPasses );
        end;
        fs.Free;
        if bDelete then Result := DeleteFile( sFilename );
      finally
        FreeMem( p );
        fs.Free;
      end;
    end;
  except end;
end;

Usage:

ShredFile( 'c:\mysecret.dat', 255, True, False ); // 255 passes, pseudorandom, keep file
ShredFile( 'c:\mysecret.dat', 1, False, True ); // 1 pass, pattern, delete file
ShredFile( 'c:\mysecret.dat', 0, False, True ); // just delete file

/// John

Thanks.

Yes. This is security problem.
erojoj is right.
'DeleteFile' just deletes the allocation table.
If it's a text type file, it will be restored.  
Now I am testing erojoj's code.

Sakya

Sakya

Hello.

The erojoj's code works on windows95.
But I don't test it on OSR2 or NT.
I've heard Shredder code is different between win95 ver.1 and OSR2 before.

Anyway thanks.

I want to give my points to erojoj's excellent code.

Sakya

Thanks

Sakya

Thanks Motaz.
My comments fade to oblivion in the shine of your excellent answers.

/// John

Thanks John

Motaz.