Securely delete files...

[David Shaw]

On Aug 20, 2008, at 3:04 AM, Vlad SATtva Miller wrote:

> Robert J. Hansen (20.08.2008 09:12):
>> Bhushan Jain wrote:
>>> I wanted to know how could the file be deleted securely using PGP?
>>
>> Assuming you meant GnuPG, the answer is 'no'.
>>
>> Assuming you meant PGP, the answer is 'maybe'.  PGP provides a secure
>> deletion tool, but as far as I know there has never been any serious
>> independent study of its effectiveness.
>
> It employs Gutmann's methodology on secure file erasure, so there  
> *is* a
> study of its effectiveness.

Note, though, the postscript that Gutmann added on to his paper in  
later years:

> In the time since this paper was published, some people have treated  
> the 35-pass overwrite technique described in it more as a kind of  
> voodoo incantation to banish evil spirits than the result of a  
> technical analysis of drive encoding techniques. As a result, they  
> advocate applying the voodoo to PRML and EPRML drives even though it  
> will have no more effect than a simple scrubbing with random data.  
> In fact performing the full 35-pass overwrite is pointless for any  
> drive since it targets a blend of scenarios involving all types of  
> (normally-used) encoding technology, which covers everything back to  
> 30+-year-old MFM methods (if you don't understand that statement, re- 
> read the paper). If you're using a drive which uses encoding  
> technology X, you only need to perform the passes specific to X, and  
> you never need to perform all 35 passes. For any modern PRML/EPRML  
> drive, a few passes of random scrubbing is the best you can do. As  
> the paper says, "A good scrubbing with random data will do about as  
> well as can be expected". This was true in 1996, and is still true  
> now.

The operative phrase here is "A good scrubbing with random data will  
do about as well as can be expected".  The world of hard drives has  
evolved since 1996, and unless you're pulling your hard drives from  
10-15 year old machines, the only relevant parts of the 35-pass  
Gutmann methodology are going to be the random ones.

David