Since 2000 every version of Windows uses object reuse protection. When you create a file, the space on the hard drive for that file will be zeroed out.
Another way of looking at this is when a file is created, Windows will make it impossible to see what was there before.
But what about right after you delete a file? Nothing has been written to that space yet. What is on the physical hard drive? The entire file of course. Not good if the file is highly confidential.
What if you wanted to delete a file securely?
There are plenty of tools out there that can do this, and most of them cost quite a bit of money. But there is a free alternative.
It is called SDelete and it is from Microsoft:
http://download.sysinternals.com/Files/SDelete.zip
How does it work?
Simply download the program and run it from the command line. By default it will only complete 1 pass when erasing a file. I would suggest overriding that with a higher number – the higher the better, but it will be slower.
Lets delete a file with it, using 25 passes:
sdelete.exe -p 25 FileToDelete.zip
This will securely delete FileToDelete.zip
SDelete implements the Department of Defense clearing and sanitizing standard DOD 5220.22-M. This will give you confidence that once deleted with this tool, your file is gone forever. Note that it does delete the file, but the file name is still recoverable from the file system (Make sure the file name itself does not have any revealing information).
The full list of possible commands for SDelete are:
usage: sdelete [-p passes] [-s] [-q]
sdelete [-p passes] [-z|-c] [drive letter]
-c Zero free space (good for virtual disk optimization)
-p passes Specifies number of overwrite passes (default is 1)
-q Don’t print errors (Quiet)
-s Recurse subdirectories
-z Clean free space
It supports wildcards too. So if you wanted to securely delete an entire folder you can call it like this:
C:\>sdelete c:\temp\*.*
SDelete – Secure Delete v1.51
Copyright (C) 1999-2005 Mark Russinovich
Sysinternals – www.sysinternals.com
SDelete is set for 1 pass.
c:\temp\Eula.txt…deleted.
c:\temp\Import.csv…deleted.
c:\temp\readme.txt…deleted.
c:\temp\sdelete.exe…deleted.
c:\temp\VCdControlTool.exe…deleted.
c:\temp\VCdRom.sys…deleted.
If you have files that need to remain secure, SDelete is a great way to make sure they are removed from your hard drive.
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