[X4U] Secure Empty Trash & Paranoia

[Jeff Porten]

On May 20, 2007, at 5:34 PM, Jon Warms wrote:

> Here's the first question: Does File>Save delete the old file in a  
> secure way, or does it just delete the old file, then reuse part or  
> all or none of that space as Finder sees fit, which could leave  
> some or all of the old file deleted insecurely?

The latter.  Your files essentially exist in two segments: the series  
of blocks on your hard drive where it is stored, and a directory  
pointer that says that "file XYZ is in blocks NNN through NNN".  Many  
file operations don't touch the blocks themselves, just the pointer  
-- so when you insecurely trash a file, the blocks still hold the  
data, but there's no longer a way of getting there.

When you save a file over an existing file, you're doing two things:  
storing your data to a series of blocks, and changing the pointer to  
the new series.  This *might* overwrite the old data -- and it might  
not.  In general, the application just tells the disk system, "Save  
this -- I don't care how", and the disk system says, "Hmmm, what's  
the best way of doing this?"  Leftover blocks will eventually be  
overwritten by other disk activity, but that could be any time  
between dinnertime and the heat death of the universe.

Making your job even more fun, Microsoft software is even more fun  
for these purposes, since it creates new files to manage fast saves  
and temporary data storage.  Also, some very intelligent people have  
come up with methods of recovering data that's been overwritten a few  
times.

What I tell my clients who are concerned about this: most of these  
data recovery methods require physical access to the hard drive for  
lengthy periods of time.  Therefore, you only need to take extreme  
measures with data that needs to stay secure if the computer is  
stolen.  99 times out of 100, you're safe if you store these  
documents to an encrypted disk image -- it's possible that some data  
might be stored in the clear elsewhere, but it's unlikely.

Best,
Jeff