Securely delete files...

[Chris Walters]

David Shaw wrote:
> Let's simplify things this way: you have regular people and forensics
> lab people.  You have single file shredding, and whole-disk shredding. 
> Regular people only have access to the disk that a computer can have. 
> That is, they are reading the disk using the disk interface.  Forensics
> labs can do things like remove the platters and go for the electron
> microscopes.  Single file shredding is just that - you pick a particular
> file on the disk and shred it.  Other files are not affected. 
> Whole-disk shredding shreds the whole disk at a block level, and the
> entire contents of the disk are lost.  The disk would need a reformat /
> re-mkfs before using it again.  This is a simplification, of course, but
> it's a reasonable one.
> 
> Regular people + single file shred: This may or may not be safe
> depending on whether the filesystem you are using overwrites in place or
> not.  Many modern filesystems (Reiser, XFS) do not necessarily overwrite
> in place.  More primitive filesystems (like the FAT FS that is used on
> many external disks) do overwrite in place.  Linux systems most commonly
> use ext3, and that may or may not overwrite in place, depending on how
> it is configured.  Then there is the fact that many programs create temp

<snip>

> Regular people + whole disk: This is pretty safe.  The disk might be
> doing magic underneath you, but given that the attacker is only able to
> read the disk via the disk interface itself, the magic is not visible
> (or rather, it is equally visible or invisible to both you and your
> attacker).  Any temp files are blown away just like everything else is.
> 
> Bottom line is, yes, you can protect yourself from your roommate with
> shredding (assuming your roommate doesn't work for a computer forensics
> lab).  Of course, you'd better make sure your shred program is a good
> one.  I've seen some pretty silly bugs that make some of them not
> actually protect you particularly well...
> 
> David

As interesting as this discussion has become, I think it is time to let it end.
If anyone knows of any *good* shred utilities, I'd be interested.  I have one
final comment, XFS, Reiser and JFS are all journaling filesystems, and
shredding files on such systems is useless, since the file or a large part of
it can be recovered from the journal.  And if your data are so sensitive that
you need to totally destroy the disk to rid yourself of it, maybe you should
rethink what you are storing, eh?

Anyhow, I'd rather talk about gnupg.  The part of this thread talking about
whole disk encryption is actually closer to that topic than most of the other
things I've seen and said, since loop-aes can use gpg-encrypted passphrases and
that is one way to encrypt files and partitions on Linux-based systems.

Regards,
Chris

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